- . •7 o a5 ^ \" -^ v ">> <° ^ "'/-% -> "'o X ? % '. A ,0 c, V ^ -v, • °o N ; v 0o. \' * -^ ^ -\- A O. • <, ., ^ * ^ '<$, \ O ^ V i?.°* ^ C^ ^ " v g i ' (i \V «<■"■. ■ <3- " «-* * c- v ' ^ ^ .\*\, A V .** N ^ y * -v e, A ^. y o. <^ - V o, ^ "^. ^v xv ,0 o^ http://www.archive.brg/details/cementconcreteOOsabr> s c & - ""■ &'■ CEMENT AND CONCRETE BY LOUIS CARLTON SABIN, B. S., C. E. Assistant Engineer, Engineer Department, U. S. Army; Member of the American Society of Civil Engineers NEW YORK McGRAW PUBLISHING COMPANY 1905 3 A f3$-~ c jubrary oToewasEssf " fwu Copies rtecBiveti MAR 15 1905 Ooiyrujm Entry &&•&•/ f*f- IsUiSS A Ms, Not 97S2 t> COPY B. Copyright, 1904 BY L. C. SABIN Stanbopc fl>reos F. H. GILSON COMPANY BOSTON. U.S.A. PREFACE That the use of cement has outstripped the literature on the subject is evidenced by the number and character of the inquiries addressed to technical journals concerning it. This volume is not designed to fill the proverbial "long felt want," for until within a few years the number of engineers using cement in large quantities was quite limited. These American pioneers in cement engineering, under one of whom the author received his first practical training in this line, needed no formal introduction to the use and properties of cement; their knowl- edge was born and nurtured through intimate association and careful observation. To-day the young engineer frequently finds a good working knowledge of cement one of the essentials of success, and the gaining of this knowledge by experience alone is likely to be too slow and expensive, judged by twentieth century standards. In fact, the variety and extent of the uses to which cement is applied, and the knowledge concerning its properties, have of late increased so rapidly that even the older engineer, whose practice may have directed his special attention along other channels for a few years, finds it difficult to follow its progress. One who wishes only a catechetical reply to any question that may arise concerning cement and its use will be somewhat disappointed in these pages; on the other hand, he who would devote special attention to the subject must, of course, go far beyond them. The author has attempted to take a middle course, avoiding on the one hand a dogmatic statement of facts, and on the other too detailed and extended series of tests, but giving, where practicable, sufficient tests to support the state- ments made, and endeavoring to show the connection between theory and practice, the laboratory and the field. The original investigations forming the basis of the work were made in connection with the construction of the Poe Lock at St. Marys Falls Canal, Michigan, under the direction iv PREFACE of the Corps of Engineers, U. S. Army. To the late General 0. M. Poe, the Engineer officer in charge of the district at that time, and to Mr. E. S. Wheeler, his chief assistant engineer, may be credited a very large share of the value of the results obtained, since the accomplishment of a series of experiments of so comprehensive a character was made possible only through the broad views held by them as to the value of thorough tests of cement. The author wishes to express his appreciation of the courtesy of General G. L. Gillespie, Chief of Engineers, U. S. A., in grant- ing permission to use the data collected, and of the kindness of Major W. H. Bixby in presenting a request for this per- mission. When not otherwise stated, the tables in the work are con- densed from the results of the above mentioned investigations. In supplementing this original matter, much use has been made of the experiments of others as published in society transac- tions, technical journals, etc., to all of whom credit has been given in the body of the work. If this attempt to place in one volume a connected story of the properties and use of cement serves to make the road to this knowledge a little less devious than that followed by the writer, the latter will be rewarded. L. C. S. Sault Ste. Marie, Mich. January 3, 1905. CONTENTS PART I. CEMENT: CLASSIFICATION AND MANUFACTURE CHAPTER I. DEFINITIONS AND CONSTITUENTS Page Art. 1. General Classification of Hydraulic Products .... 1 Art. 2. Lime: Common and Hydraulic 3 Art. 3. Portland Cement 4 Art. 4. Slag Cement 7 Art. 5. Natural Cement 8 CHAPTER II. MANUFACTURE Art. 6. Manufacture of Portland Cement 10 Materials. — Wet Process. — Dry Process. — Semi-dry Process. — Details of the Manufacture: Burning, Grinding. — Sand-Cement. Art. 7. Other Methods of Manufacture of Portland .... 22 Art. 8. Manufacture of Slag Cement 23 Art. 9. Manufacture of Natural Cement 24 PART II. PROPERTIES OF CEMENT AND METHODS OF TESTING CHAPTER III. INTRODUCTOEY Desirable Qualities. — Uniform Methods of Testing 28 CHAPTER IV. CHEMICAL TESTS Art. 10. Composition and Chemical Analysis 31 CHAPTER V. THE SIMPLER PHYSICAL TESTS Art. 11. Microscopical Tests. — Color 36 Art. 12. Weight per Cubic Foot, or Apparent Density .... 37 Art. 13. Specific Gravity, or True Density 39 CHAPTER VI. SIFTING AND FINE GRINDING Akt. 14. Fineness 45 Importance of Fineness. — Sieves. — Methods. — Specifications. vi CONTENTS Page Art. 15. Coarse Particles in Cement 52 Effect on Weight, Time of Setting and Tensile Strength. Art. 16. Fine Grinding 58 Effect on Weight, Time of Setting and Tensile Strength. CHAPTER VII. TIME OF SETTING AND SOUNDNESS Art. 17. Setting of Cement 65 Process of Setting. — Rate. — Variations in Rate. Art. 18. Constancy of Volume 76 Causes of Unsoundness. — Tests. — Discussion of Methods. — Hot Tests for Natural Cements. — Conclusions. CHAPTER VIII. TESTS OF THE STRENGTH OF CEMENT IN COMPRESSION, ADHESION, ETC. Art. 19. Tests in Compression and Shear 89 Art. 20. Tests of Transverse Strength 90 Art. 21. Tests of Adhesion and Abrasion 92 CHAPTER IX. TENSILE TESTS OF COHESION Art. 22. Sand for Tests 95 Value of Tests of Sand Mortars. — Uniformity in Sand. — Com- parison of Different Kinds. — Tests with Natural Sand. — Fineness. Art. 23. Making Briquets 97 Proportions. — Consistency. — Temperature. — Gaging: Hand and Machine. — Methods. — Amount of Gaging. — Form of Briquets. — Molds. — Molding. — Briquet Machines. — Approved Methods of Hand Molding. — Marking the Briquets. Art. 24. Storing Briquets 117 Time in Air before Immersion. — Moist Closet. — Water of Im- mersion. — Storing in Air; in Damp Sand. Art. 25. Breaking the Briquets Testing Machines. — Clips. — Clip-breaks. — Comparative Tests of 123 Clips. — Requirements for a Perfect Clip. — Form Recommended. — Rate of Applying Tensile Stress. — Treatment of Results. Art. 26. Interpretation of Tensile Tests of Cohesion .... 137 CHAPTER X. RECEPTION OF CEMENT AND RECORDS OF TESTS Art. 27. Storing and Sampling 144 Storage Houses. — Percentage of Barrels to Sample. — Method of Taking and Storing the Sample. Art. 28. Records of Tests 146 Value of Records. — Marking Specimens. — Records at St. Marys Falls Canal. CONTENTS vn PART III. THE PREPARATION AND PROP- ERTIES OF MORTAR AND CONCRETE CHAPTER XI. SAND FOR MORTAR Page Art. 29. Character of the Sand 154 Shape and Hardness of the Grains. — Siliceous vs. Calcareous Sands. — Slag Sand. ; — Sand for Use in Sea Water. Art. 30. Fineness of Sand 159 Relation Volume and Superficial Area. — Effect of Fineness. " Art. 31. Voids in Sand 162 17 Conditions Affecting Voids: Shape of Grains; Granulometric Com- position. — Effect on Tensile Strength of Mortar. — Moist Sand. Art. 32. Impurities in Sand 168 Art. 33. Conclusions. — Weight and Cost of Sand 170 CHAPTER XII. MORTAR: MAKING AND COST Art. 34. Proportions of the Ingredients 172 Capacity of Cement Barrels. — Equivalent Proportions by Weight and Volume. — Richness of Mortars. — Effect of Pebbles. — Con- sistency. Art. 35. Mixing the Mortar 177 Hand Mixing. — Machine Mixing. Art. 36. Cost of Mortars 179 Ingredients Required. ■ — Tables of Quantities. — Estimates of Cost. — Tables of Cost of Portland and Natural Cement Mortars. CHAPTER XIII. CONCRETE: AGGREGATES Art. 37. Character of Aggregates 186 Proper Materials. — Screenings in Broken Stone. — Foreign In- gredients. Art. 38. Size and Shape of Fragments and Volume of Voids . . 188 Conditions Affecting Voids. — Effect on Strength of Concrete. — Gravel vs. Broken Stone. Art. 39. Stone Crushing and Cost of Aggregate 194 Breaking Stone by Hand. — Stone Crushers. — Cost of Aggregate. — Examples. CHAPTER XIV. CONCRETE MAKING: METHODS AND COST Art. 40. Proportions of the Ingredients 200 - Theory of Proportions. — Determination of Amount of Mortar Required. — Aggregates Containing Sand. — Required Strength. Art. 41. Mixing Concrete by Hand 203 Hand vs. Machine Mixing. — Method of Hand Mixing; Number of Men and Output; Examples. viii CONTENTS Page Art. 42. Concrete Mixing Machines 207 General Classification. — Description of Machines. — Basis of Comparison. Art. 43. Concrete Mixing Plants and Cost of Machine Mixing 212 Art. 44. Cost of Concrete . 218 Ingredients Required for a Cubic Yard. — Examples of Actual Cost. CHAPTER XV. THE TENSILE AXD ADHESIVE STRENGTH OF CEMENT MORTARS AND THE EFFECT OF VARIATIONS IN TREATMENT Art. 45. Tensile Strength of Mortars of Various Compositions and Ages . 227 Art. 46. Consistency of Mortar and Aeration of Cement . . 232 Art. 47. Regaging of Cement Mortar 236 Art. 48. Mixtures of Cement with Lime, Plaster Paris, etc. . 243 Mixtures of Portland and Natural. — "Improved" Cement. — Ground Quicklime with Cement; Slaked Lime; Plaster of Paris. — Conclusions. Art. 49. Mixtures of Clay and Other Materials with Cement. 253 Effect of Powdered Limestone, Brick, etc. ; Sawdust ; Terra Cotta. Art., 50. Use of Cement Mortars in Freezing Weather . . . 260 Effect of Frost on Set Mortars. — Effect of Salt; Heating Materials; Consistency ; Fineness of Sand. — Conclusions. Art. 51. The Adhesion of Cement 270 Adhesion between Portland and Natural. — Adhesion to Stone and Other Materials. — Effect of Consistency; Regaging; Character of Surface of Stone. — Effect of Plaster of Paris. — Adhesion to Brick; Effect of Lime Paste. — Adhesion to Rods of Iron and Steel. CHAPTER XVI. COMPRESSIVE STRENGTH AND MOD- ULUS OF ELASTICITY OF MORTAR AND CONCRETE Art. 52. Compressive Strength of Mortars 288 Ratio of Compressive to Tensile Strength. Art. 53. Concretes with Various Proportions of Ingredients 291 Effect of Consistency; Amount and Richness of Mortar; Methods of Storage. Art. 54. Concretes with Various Kinds and Sizes of Aggregates 298 Art. 55. Cinder Concrete and Effect of Clay 302 Art. 56. Modulus of Elasticity of Cement Mortar and Concrete 306 CHAPTER XVII. THE TRANSVERSE STRENGTH AND OTHER PROPERTIES OF MORTAR AND CONCRETE Art. 57. Transverse Strength 313 Transverse Strength of Mortars Compared to Tensile and Com- pressive Strength. — Richness of Mortar; Consistency. — Transverse Tests of Concrete Bars: Variations in Mortar Used; Consistency; Mixing ; Aggregate ; Screenings. — Deposition in Running Water. — Use in Freezing Weather. CONTENTS ix Page Art. 58. Resistance to Shear and Abrasion 328 Art. 59. Expansion and Contraction op Cement Mortar, and the Resistance of Concrete to Fire 331 Change in Volume during Setting. — Coefficient of Expansion of Mortar and Concrete. — Fire-Resisting Qualities of Concrete. — Aggregate for Fireproof Work. Art. 60. Preservation of Iron and Steel by Mortar and Concrete 336 Action of Corrosion. — Tests of Effect of Concrete. Art. 61. Porosity, Permeability, etc 340 Porosity. — Permeability. — Waterproof Mortars and Concretes. — Washes for Exteriors of Walls. — Efflorescence. — Pointing Mortar. — Cements in Sea Water. PART IV. USE OF MORTAR AND CONCRETE CHAPTER XVIII. CONCRETE: DEPOSITION Art. 62. Timber Forms or Molds 351 Sheathing. — Lining. — Posts and Braces. Art. 63. Deposition of Concrete in Air 358 Transporting, Depositing, Ramming. — Rubble Concrete. — Fin- ish ; Plastering ; Facing ; Bushhammering ; Colors for Concrete Finish. Art. 64. Placing Concrete under Water 369 Laitance. — Tremie, Skip, etc. — -Depositing in Bags; Cost. — Block System: Molds; Cost. CHAPTER XIX. CONCRETE-STEEL Art. 65. Monier System 381 Art. 66. Wunsch, Melan, and Thacher Systems 383 Art. 67. Other Systems of Concrete-Steel 385 Hennebique, Kahn, Ransome, Roebling, Expanded Metal. Art. 68. The Strength of Combinations of Concrete and Steel 387 Art. 69. Beams with Single Reinforcement 390 Formulas for Constant Modulus Elasticity; for Varying Modulus. — Excessive Reinforcement. — Tables of Strength. Art. 70. Beams with Double Reinforcement 403 Art. 71. Shear in Concrete-Steel Beams 405 CHAPTER XX. SPECIAL USES OF CONCRETE: BUILD- INGS, WALKS, FLOORS, AND PAVEMENTS Art. 72. Buildings 410 Roof ; Floor System ; Columns. — Building Forms. — - N. Y. Build- ing Regulations. Art. 73. Walks 420 Foundation; Base; Wearing Surface; Construction; Cost. Art. 74. Floors of Basements, Stables, and Factories .... 426 x CONTENTS Page Art. 75. Pavements and Driveways 428 Pavement Foundations. — Concrete Wearing Surface. — Construc- tion. — Example. Art. 76. Curbs and Gutters 431 Art. 77. Street Railway Foundations 433 CHAPTER XXI. SPECIAL USES OF CONCRETE (Continued): SEWERS, SUBWAYS, AND RESERVOIRS Art. 78. Sewers 436 Methods and Cost. — Forms . Art. 79. Subways and Tunnel Lining 443 Waterproofing. — Subways. — Tunnels in Firm Earth ; in Soft Ground; in Rock. — Examples; Methods; Cost. Art. 80. Reservoirs: Linings and Roofs 453 Details of Construction. — Groined Arch. — Forms. — Examples; Cost. CHAPTER XXII. SPECIAL USES OF CONCRETE (Continued): BRIDGES, DAMS, LOCKS, AND BREAKWATERS Art. 81. Bridge Piers and Abutments and Retaining Walls . . 464 Bridge Piers; Steel Shells. — Repair of Stone Piers. — Retaining Walls and Abutments: Coping; Rules for Use of Concrete. Art. 82. Concrete Piles 471 Building in Place. — Concrete-Steel Piles: Molding; Driving. Art. 83. Arches 474 Design; Centers; Construction; Finish and Drainage. — Examples and Cost. Art. 84. Dams 484 Concrete vs. Bubble. — Quality of Concrete. — Construction. — Examples. Art. 85. Locks 488 Methods of Building. — Examples. Art. 86. Breakwaters 493 PART I CEMENT CLASSIFICATION AND MANUFACTURE CHAPTER I DEFINITIONS AND CONSTITUENTS Art. 1. General Classification of Hydraulic Products 1. The use of a cementitious substance for binding together fragments of stone is older than history, and it is known that the ancient Romans prepared a mortar which would set under water. So far as our present knowledge of cement manufac- ture is concerned, however, the credit of demonstrating that a limestone containing clay possessed, when burned and ground, the property of hardening under water, is due to Mr. John Smeaton, who announced this as the result of his experiments made in 1756 in seeking a material with which to build the Eddystone Lighthouse. After this discovery by Smeaton nearly sixty years elapsed before M. Vicat gave the true explanation of this action, namely, that the lime during burning combined with the silica to form silicate of lime, the essential ingredient of hydraulic limes and cements. In 1796, Parker, of London, obtained a patent for the manu- facture of a cement from septaria nodules, and aptly named his product "Roman Cement." In 1824, Joseph Aspdin of Leeds, England, patented a process of manufacture of "Portland Cement." 2. The cements in general use in the United States to-clay are of two kinds, Portland cements and natural cements, and in what follows our attention will be directed almost entirely to these two products. Common limes were formerly used largely in engineering construction, but have of late been almost entirely superseded, 2 CEMENT AND CONCRETE for this purpose, by cements. Since the hardening of lime mortar depends on the absorption of carbonic acid from the atmosphere, these limes are sometimes called "air limes," while the hydraulic products which set under water are, for a similar reason, styled "water limes." Hydraulic limes, though playing an important role in foreign countries, are not manufactured or used to any extent in the United States. The European prod- uct known as "Roman" or "Vassy" cement, somewhat re- sembles our natural cement, but is usually inferior to the Ameri- can article. Our chief interest in these products, which are used only abroad, is to know what relation they bear to the cements with which we are familiar. The following classifications are selected as being authoritative: 3. The conferences of Dresden (1886) and Munich (1884) on Uniform Methods of Testing for Materials of Construction, clas- sified the hydraulic products as follows: — (1) Hydraulic limes: made by roasting either argillaceous or siliceous limestones. They slake partially or wholly on the ad- dition of water. (2) Roman cements: made from argillaceous limestones hav- ing a large proportion of clay. They do not slake by the addi- tion of water and hence must be mechanically ground to powder. (3) Portland cements: obtained by burning to the point of insipient vitrification either hydraulic limestones or mixtures of argillaceous materials and limestones, and afterward grinding the product to fine powder. (4) Hydraulic gangues: natural or artificial materials which do not harden alone, but which furnish hydraulic mortars when mixed with quicklime. (5) Pozzolana cements produced by an intimate mixture of powdered hydrate of lime and finely pulverized hydraulic gangues. (6) Mixed cements: the products of intimate mixtures of manufactured cement with certain materials proper for such a purpose. Mixed cements should always be designated as such and the materials entering into the composition should be stated, but it may be added parenthetically that these things are seldom done. 4. MM. Durand-Claye and Debray divide cements into six classes, namely, (1), Grappier cements — obtained by grinding LIME 3 the pieces of hydraulic lime which do not slake; (2), quick-set- ting (Vassy) cements — formed by burning very argillaceous limestones at a low temperature; (3), natural Portland cements, or those cements made from natural rock which correspond to artificial Portland in character; (4), mixed cements; (5), arti- ficial Portlands; and (6), slag cements. M. H. LeChatelier, an eminent French authority, divides hydraulic products into four classes, namely: 1 — Portland ce- ments, hydraulic limes, natural cements, and mixed cements. Hs subdivides the third class, natural cements, into quick-setting, slow-setting and grappier cements, and includes natural Port- lands among the slow-setting natural cements. Slag cements, which are put in a separate class by MM. Durand-Claye and Debray, are included in "mixed cements" by M. LeChatelier. 5. Prof. I. 0. Baker gives a classification that is better adapted for use in this country than any of the above. 2 He divides the products obtained by burning limestone, either pure or impure, into lime, hydraulic lime and hydraulic cements. He then sub-divides cement into Portland, Rosendale (preferably called natural) and Pozzolana. Art. 2. Lime: Common and Hydraulic 6. Common lime is the product obtained by burning a pure, or nearly pure, carbonate of lime. On being treated with water it slakes rapidly, evolving much heat and increasing greatly in volume. It is now seldom used in engineering construction and will not be considered further. 7. Prof. M. Tetmajer has thus defined hydraulic limes: Hy- draulic limes are the products obtained by the burning of argil- laceous or siliceous limestones, which, when showered with water, slake completely or partially without sensibly increasing in volume. According to local circumstances, hydraulic limes may be placed on the market either in lumps, or hydrated and pul- verized. The following table gives a classification of hydraulic limes according to M. E. Candlot 3 who states that the first 1 "Tests of Hydraulic Materials," by H. LeChatelier. Trans. Am. Inst. Mining Engrs., 1893. 2 "Masonry Construction," p. 48. 3 "Ciraents et Chaux Hydrauliques," par E. Candlot. CEMENT AND CONCRETE class is seldom used for important work and that the fourth class is quite rare. TABLE 1 Classification of Hydraulic Limes. E. Candlot Class. Feebly Hydraulic Lime Ordinary " " Real Eminently " " Per Cent, of Clay in Limestone. 5 to 8 8 to 15 15 to 19 19 to 22 Per Cent, of Silica and Alumi- na in Fin- ished Prod- uct. 9 to 14 14 to 24 24 to 30 80 to 33 Hydraulic Index, or Ratio of Silica and Alumina to Lime. .10 to .16 .16 to .31 .31 to .42 .42 to .50 Approx. Time to Set, Days. 16 to 30 10 to 15 5 to 9 2 to 4 Hydraulic limes should be burned slowly, and at such a tem- perature that sintering does not take place. The best hydraulic limes have a composition very similar to that of Portland cement. The comparatively low temperature at which they are burned permits them to slake on the addition of water. They gain strength much more slowly than cements. Having considered the classification of hydraulic products as a whole, we may proceed to the discussion of Portland and nat- ural cements, the hydraulic products which have by far the greatest importance here, and the only varieties which will be taken up in detail in the present work. Art. 3. Portland Cement 8. As the classification of hydraulic products varies, so do opinions vary as to what shall be included under the name Port- land cement. There seems to be agreement on at least one point, namely, that the burning shall be carried to a point just short of vitrification. Ideas concerning other points are crys- tallizing rapidly. The Association of German Portland Cement Manufacturers has given a definition of Portland cement in a practical manner by binding its members "to produce under the name of Portland cement only such an article as is made by calcining a thorough mixture, consisting essentially of calcare- ous and clayey substances, and then grinding the same to the fineness of flour;" and they further declare that "any article made in a manner differing from the above method, or to which during or after burning any foreign substances have been added," PORTLAND CEMENT 5 is not recognized by them as Portland cement, and the sale of such products under the designation "Portland Cement" is re- garded by them as defrauding the purchaser. This declaration does not apply to such minor additions as are made to regulate the setting time of Portland cement, and which are permitted to an extent of 2 per cent." 9. M. LeChatelier has given the following limits for the amounts of the materials usually contained in good commercial Portland cements : l — Silica 21 per cent, to 24 per cent. Alumina 6 8 " Oxide of Iron 2 " 4 Lime 60 6.5 " Magnesia 5 " 2 " Sulphuric Acid 5 . " 1.5 " Water and Carbonic Acid .1 3 ". The upper limit for lime (65 per cent.) is being exceeded in re- cent years. These substances occur as "(1) Si0 2 , 3CaO, the essentially cementitious ingredient; (2) A1 2 3 , 3CaO, the substance mainly active during setting and contributing somewhat to the subse- quent hardening; and (3) a fusible calcium silico-aluminate whose chief function is that of a flux during burning to promote the necessary chemical reactions." 2 M. LeChatelier further holds that in good Portland cements the following formulas should be true: CaO, Mg O < Si0 2 + Al 2 03 = 3 > and CaO, MgO > SiC-2 - (A1 2 3 , Fe 2 O s ) = 3 " in each case the quantities in the formulas being equivalents of the substances, not weights. The ratio of the acid constitu- ents, silica and alumina, and the basic constituents, lime and magnesia, is called the hydraulic index. Although these form- ulas have been quite generally accepted as properly fixing the limit? of the ingredients it maybe noted that they are based on the assumption that Si0 2 , and A1 2 3 , are equally capable of dispos- 1 "Tests of Hydr, Materials," Tr. Am. Inst. Mining Engrs., 1893. 2 Jour. Soc. Ch. Ind., Mar. 31, 1891, p. 256. CEMENT AND CONCRETE ing of a given quantity of lime and magnesia, and it is thought by some authorities that the assumption is not warranted. In the Journal Society Chemical Industry, 1897, Messrs. S. B. and W. B. Newberry give the results of some investigations in this line from which they concluded that the essential in- gredient of Portland cement is a tri-calcium silicate, but that the alumina occurs as a dicalcic aluminate. They therefore considered that the per cent, of lime should equal 2.8 times the per cent, of silica plus 1.1 times the per cent, of alumina. 10. The following analyses of brands in the market are se- lected from the various sources indicated in the table. They are given here merely to illustrate the proportions obtaining in commercial products. TABLE 2 Analyses of Portland Cements Brand. Si0 2 . Ai 2 O s . Fe 2 3 . CaO. MgO. Na»0 K 2 0. so 3 . H 2 0& Loss. 1. Alpha 2. Atlas 3. Bronson 4. Buckeye 5. Empire 0. Wyandotte 7. Omega 8. Yankton 9. Giant 10. Medusa 11. Dyckerhoff 12. Germauia 13. Alsen's 14. Alsen's 20.38 21.30 22.90 21.30 22.04 23.20 22.24 23.36 23.20 19.35 21.14 24.90 23.30 7.65 6.80 6.95 6.45 8.00 7.26 7.70 8.07 7.03 7.00 6.30 8.00 5.85 2.85 3.60 2.00 3.41 2.40 2.54 4.30 4.83 2.41 4.50 2.50 3.22 4.65 63.30 60.95 63.90 62.30 60.92 02.10 64.96 60 00 58.93 64.19 63.75 66.04 59.38 60.90 2.86 2.95 0.70 1.20 3.53 2.00 2.26 0.80 1.00 0.97 1.11 0.38 0.90 1.15 1.10 1.20 0.50 0.75 0.30 1.13 1.81 0.40 0.98 2.25 0.41 0.50 0.98 2.43 1.75 1.41 0.60 4.62 0.80 0.33 2.46 2.20 5.40 2.91 2.16 1.40 Brand. Authority. Raw Materials. Location. 1 2 4 5 6 7 8 9 10 11 12 13 14 " Directory Amer'n Cement Industries" u u (i a (( i( (i u U (I (( (i (« u U. Cummings " Amer'n Cements" (( u c< (i tt it (( it Richard K. Meade, "Exam, of P. Cem.' Cement Rock and Limestone (I (< (( c< Marl and Clay 11 U [( (i u t< Soda Ash Waste and Clay Marl and Clay Chalk and Clay CementRockand Limestone Marl and Clay Limestone, Marl and Clay Marl and Clay Chalk and Clay Chalk and Thames Mud Alpha, N.J. Northampton, Pa. Bronson, Mich. Bellefontaine, Ohio. Warners, N.Y. Wyandotte, Mich. Jonesville, Mich. Yankton, S. Dakota. Egypt, Pa. Sandusky, Ohio. Amoeneburg, Ger . Lehrte, Germany. Itzehoe, Germany. England. SLAG CEMENT 7 Art. 4. Slag Cement 11. Slag cement is manufactured to a considerable extent in Europe and is beginning to assume some importance in the United States. It is a pozzolana cement in which the silica ingredient is supplied by blast furnace slag. Pozzolana ce- ments have been defined as " products obtained by intimately and mechanically mixing, without subsequent calcination, pow- dered hydrates of lime with natural or artificial materials which generally do not harden under water when alone, but do so when mixed with hydrates of lime (such materials being pozzo- lana, Santorin earth, trass obtained from volcanic tufa, furnace slag, burnt clay, etc.), the mixed product being ground to ex- treme fineness." 1 Slag cement somewhat resembles Portland in its properties, but is more like some of the natural cements in its constituents, while the manner of occurrence of these constituents and the method of manufacture are quite different than in either of these classes. 12. As this cement is a mixture of lime and pozzolanic ma- terials, its value depends largely upon its extreme fineness and the intimate mixture of the ingredients. Its specific gravity is low, about 2.7 to 2.8, and it sets very slowly, although the setting may be hastened by the addition of certain substances such as caustic soda. On account of the sulphide present, most slag cements are not suited to use in air, as they crack and soften in this medium; neither are they suitable for use in sea water, nor in freezing weather, but when mixed with two or three parts sand and kept constantly wet with fresh water, they give quite satisfactory results. Slag cement has an approximate composition of silica, 20 to 30 per cent., alumina, 10 to 20 per cent., and lime, 40 to 50 per cent. It usually contains calcium sulphide, the amount some- times reaching three or four per cent. The characteristic green- ish tint which slag cements exhibit when they harden in water is due to this ingredient, as is the odor of hydrogen sulphide sometimes given off by a briquet when broken, especially if it 1 " Report of Board of Engineers on Steel Portland Cement," Washing- ton, 1900. 8 CEMENT AND CONCRETE has hardened in sea water. Some slag cements have also quite a percentage of magnesia. 1 Art. 5. Natural Cement 13. Natural cement, as its name implies, is made from rock as it occurs in nature. Argillaceous limestones, magnesian lime- stones, or argillo-magnesian limestones, having the proper pro- portion of clay, magnesia and lime, may be used for the production of natural cement. The burning is not carried so far as in the manufacture of Portland cement, and the resulting TABLE 3 Analyses of Natural Cements Refer- ence. Silica. "A s hi < Iron Oxide. Lime. GO < Potash and Soda. Carbonic Acid, Water and Loss. C d e / (J h i 1 24.30 2.61 6.20 39.45 6.16 5.30 15.23 2 34.66 5.10 1.00 30.24 18.00 6.16 4 84 3 23.16 6.33 1.71 36.08 20.38 5.27 7.07 4 26.40 6.28 1.00 45.22 9.00 4.24 7.86 5 27.30 7.14 1.80 35.98 18.00 6.80 2.98 6 27.98 7.28 1.70 37.59 15.00 7.96 2.49 7 27.69 8.64 2.00 42.12 14.55 2.00 3.00 8 27.60 10.60 0.80 33.04 7.26 7.42 2.00 9 28.02 10.20 8.80 44.48 1.00 0.50 7.00 Refer- ence. Brand. Place of Manufacture. a b 1 Buffalo Buffalo, N. Y. 2 Utica Utica, 111. o •J Vlilwauke e Milwaukee, Wis. 4 Louisville Louisville, Ky. 5 6 Hoffman Norton H igh Falls Rosendale, N. Y. Rosendale, N. Y. 7 Akron Akron, N. Y. 8 9 Utica Round T( >P LaSalle, 111. Hancock, Md. Selected from table compiled by Mr. U. Cummings, " Brickbuilder," May, 1895. 1 For an excellent resume of the qualities and distinguishing character- istics of slag cements, the reader is referred to " Report of Board of Engi- neers on Steel Portland Cement as used in United States Lock at Plaque- uiiiic, La." Washington, 1900. NATURAL CEMENT 9 product is of lighter weight and usually quicker setting, though some natural cements are quite slow setting. The properties of these cements, coming from different localities, vary greatly. In fact, it is difficult to distinguish some natural cements from Portland, and they may be considered to grade into the natural Portlands. Light burning in manufacture, light weight per cubic foot, and slower rate of acquiring strength, may be considered the distinguishing characteristics from a physical point of view. 14. Analyses. — Table 3 gives the results of a number of analyses of natural cement, selected from a table, compiled by Mr. U. Cummings. Comparing these analyses with those given for Portland ce- ment in Table 2, it is seen that natural cements have a higher percentage of silica, about the same amount of alumina, and a much smaller content of lime, than have Portlands. Many natu- ral cements have a large percentage of magnesia, but the mag- nesia and lime together of natural cements usually do not equal the percentage of lime in Portlands. In other words the hy- draulic index is usually higher than in Portland cements. CHAPTER II MANUFACTURE Art. 6. The Manufacture of Portland Cement 15. Historical. — It is said that as early as 1810 a patent was obtained in England for the manufacture of an artificial product by calcining a mixture of carbonate of lime and clay. This, however, was not called cement, and it was not until 1824 that Joseph Aspdin, of Leeds, England, in obtaining a patent for the manufacture of a similar material, called his product "Portland Cement." This name was probably suggested by the fact that the color of the hardened product resembled that of a limestone quarried on the Island of Portland. The industry was introduced into Germany about thirty years later, and has since grown to very substantial proportions in both of these countries, as well as in France, Austria and Russia. David 0. Saylor was the first to manufacture Portland ce- ment in the United States, at Coplay, Pa., about 1872, and works were established at that point in 1875. These were soon followed by other factories in Pennsylvania and Indiana, and at present cement is successfully manufactured in nearly half of the states of the Union, the production having steadily increased. 16. Materials Required. — The materials requisite for the manufacture of Portland cement are carbonate of lime and silica. The former may be in the form of limestone, chalk, or calcareous marl, the last two being preferable on account of greater ease of working. The silica may be in the form of shale or clay, the latter to be preferred. The clay need not be entirely free from impurities, but it should not contain any con- siderable amount of sand, for although silica is the most useful constituent of the clay, it must not be in this insoluble form. Although formerly authorities did not agree as to whether the alumina in the clay was an unwelcome constituent for Portland cement manufacture, it is now considered that the dicalcic or 10 PORTLAND CEMENT 11 tricalcic aluminate formed plays a role in the setting of the cement, and possibly also in the subsequent hardening. A few analyses of materials suitable for Portland cement manufacture are given in Table 4. TABLE 4 Analyses of Cement Materials Materials. White Marl, Empire 1 . Clay, Empire l . . . Gray Marl Clay Limestone, Glens Falls 1 Clay, Glens Falls 1 . . Gray Chalk, Medway, Eng. River Mud, Medway, Eng. 2 SiO„ .26 40.48 7.26 53.5 3.30 55.27 5.45 71.71 AloO., and Fe,0, .10 20.95 1.49 24.20 1.30 28.15 3.87 16.70 CaCO, 94.39 25.80 84.10 5.15 93.13 10.43 88.72 4.05 MgCOs .38 .99 .91 2.15 1.58 2.25 SO, 0.30 0.12 Water and Loss. 3.10 8.50 3.98 14.10 1 " Manufacture Portland Cement in New York State," by Mr. Edwin C. Eckel, C. E. 2 " Cement for Users," Mr. Henry Faija. 17. The materials for Portland cement manufacture, lime- stone, marl, clay, shale, etc., are widely disseminated, but the suitability of a certain locality for successful commercial manu- facture depends upon the manner of occurrence of these requi- sites. In England the clay is dug from the old beds of the Thames and Medway Rivers, and chalk, which occurs in abun- dance, furnishes the carbonate of lime in most cases, though limestone is sometimes used. In Germany both chalk and marl are used; the chalk being a soft white marl similar to the deposits in this country, and the marl a "more or less hard limestone rock containing clay." In the United States both limestones and marls are used. The most important cement producing egion in the United States is in the Lehigh Valley, where an argillaceous limestone is employed. The factories using marl are situated in New York, Ohio, Indiana, Michigan, etc., where the marl is found overlying beds of clay suitable for cement making. In the Lehigh Valley region many advan- tages are combined. The cement rock of that locality has nearly the correct composition for Portland cement manufac- 12 CEMENT AND CONCRETE ture. The supply of this rock is almost inexhaustible, the man- agers of the works have had long experience in the production of cement from these materials, and a market for the product is near at hand. Deposits of cement materials are of value only when the limestone or marl, and clay or shale, are found in large quanti- ties and near together, when the physical character of the ma- terials is such as to render them easy of comminution and mix- ture, when coal or other suitable fuel may be had at low prices, and when the market is not too far removed. The following estimate of the relative quantities of cement made in the United States in 1902 from the several classes of materials has been made by Mr. E. C. Eckel : 1 Argillaceous limestone and pure limestone .... 68 Per cent. Marl and clay 14 " Soft limestone and clay 4J " Hard limestone and clay . . 13^ " 18. General Description of Processes. The essentials of any method of Portland cement manufacture are that the materials shall be correctly proportioned, very finely comminuted and thoroughly mixed, that the mixture shall be carefully burned to just the proper degree of calcination and the result- ing clinker ground to extreme fineness. How these essentials can be best accomplished depends upon the character of the raw materials and the cost of fuel and labor, so that the de- tails of the method vary with the materials used and with the local conditions. In order that the proportions may be accurately determined, it is usually necessary to dry one or both of the raw materials. The ingredients may be ground separately and afterward mixed, though with certain materials the grinding and mixing may be done at the same time. In this mixing, a large amount of water may be used, as in the "wet process," giving a very thin slurry; a moderate amount may be used, as in the semi-wet process, giv- ing a slurry of creamy consistency; or the dry process may be employed, where the amount of water used is no more than sufficient to dampen the materials. The burning may be ac- ' Engineering News, April 16, 1903. PORTLAND CEMENT 13 complished in any one of several styles of kiln, the selection depending upon the relative cost of labor and fuel, the relative necessity of economy and rapid production, and, perhaps we should add, the rigidity of the specifications which the finished product must fulfill. The grinding is a simple mechanical prob- lem, to secure the required degree of fineness with least cost. 19. THE WET PROCESS. Although an excess of water may be used to mix materials that require previous grinding, the wet process is particularly adapted to such raw materials as are easily acted upon by water. This method was developed in England, where it is still employed to some extent and it has been used in this country as well. Proper amounts of the raw materials, previously ground if necessary, are placed in a wash mill with a large amount of water. The wash mill is a circular trough in which teeth or arms are made to revolve, agitating the mass. When the mate- rials are so finely divided as to be held in suspension, the thin slurry is run off into "backs," or shallow settling reservoirs, where the solid matter settles; the clear liquid is then rim off, the slurry being allowed to dry further until it can be cut into bricks and placed on drying floors artificially heated. The bricks are then taken to the burning kilns and finally ground to form the finished product. The disadvantages of this method are that much space is required for the settling floors, the amount of heat required to dry the brick is excessive, and the process is necessarily slow. These disadvantages are so great that the method above outlined is rapidly falling into disuse. Materials particularly adapted to wet mixing are still treated by this process, but the wet mixture is run directly into very long rotary kilns and is dried in passing through the first half of the length, which is heated by the gases from the lower portion where the burning is completed. 20. THE DRY PROCESS. This method of manufacture is best adapted to materials, such as limestone and shale, that must be dried and ground before they can be mixed. The rock as it comes from the quarry is first passed through a rock crusher, reducing it to the size of broken stone used for con- crete; then to some other form of crusher, such as heavy rolls, until it is reduced to pieces about one-half inch or less in size. It is then dried by artificial heat. 14 CEMENT AND CONCRETE The materials may now be combined in proper proportions and ground together to extreme fineness, thereby becoming thoroughly mixed. If the mixture is to be burned in the old style kiln, it must now be dampened so that it may be pressed into bricks to be charged in the kiln. If a rotary kiln is used, however, the dry mixture may be fed directly into it, or it may be moistened enough so that it will form into little lumps the size of wheat grains, and these fed to the rotary. 21. THE SEMI-DRY PROCESS. The two processes briefly de- scribed above are extremes admitting many modifications which will not be entered into in detail. What may be called the semi-dry process, however, has been so widely used in the United States that it deserves some special mention, and it may perhaps be best explained by giving the method formerly em- ployed in a well-known American factory which, until a few years ago, was using the vertical kiln. The carbonate of lime in the form of marl was found above the clay in beds varying in thickness up to 20 feet. The clay in general contained little sand, and the beds were of such thickness that whenever too much sand was present, the clay might be wasted. The materials were delivered to the factory, about three-quarters of a mile from the deposit, by small cars running on a narrow gage railroad. When the clay reached the factory it was put in shallow wooden pans and run into dry kilns on light cars. After dry- ing, which required 36 to 48 hours, the clay was ground and de- livered in weighed quantities to the mixer. The main object of drying the clay was to be able to control the amount added to a given quantity of marl, and the grinding was to facilitate the mixing of the two ingredients. As the cars of marl entered the building, they were brought to a given weight by means of a scale, which was set and locked by the manager. The marl was then dumped directly into the wet pan or mixer. The latter consisted of an iron pan, about 12 feet in diameter, in which revolved two cast iron rollers weighing three tons each. These rollers were on opposite ends of a horizontal axis which was attached to a vertical shaft in the center of the pan. This shaft being driven from below, the rollers traveled in a circular path; as the rollers were hung loose on the horizontal axis, they revolved about the latter only when sufficient fric- PORTLAND CEMENT 15 tion was developed between their peripheries and the floor of the pan. In front of each roller traveled two blades, one of which pushed the material under the roller from the center, while the other did the same from the circumference. A weighed amount of dry, powdered clay was admitted at the side of the mixer, from a hopper scale, at the same time as the marl was dumping into it, and sufficient water was added through a hose to bring the contents of the pan to a pasty mass. Five minutes were allowed for mixing each charge, when a slide was drawn, leaving two holes in the path of the wheels and on opposite sides of the pan. The material, or "mix," was delivered on a belt conveyor and carried to a pug mill, whence it issued in the form of rough bricks, partially cut by wires into six-inch cubes. These cubes, being loaded on cars, were run into the dry kiln, where they remained from two to four days, and were then taken to the kiln room to be filled, by hand, into the burning kilns, which were of the dome type. In charging, layers of cement-brick and coke were alter- nated. For convenience, as well as to prevent the bricks being crumbled by a fall, the charging was done from three levels. From 36 to 72 hours were required for burning a charge. The kiln was then opened at the mouth, and the clinker, which had shrunk in volume about three-quarters, and in weight about one-half, was drawn off as fast as it cooled. The clinker was shoveled from the kilns to a pan conveyor and sorted as shov- eled, only that which appeared properly burned being allowed to pass; the underburned portion was stored for further burn- ing, and the overburned, wasted. Further sorting was done by two men stationed in the kiln room, who watched the clinker as it passed on the conveyor and picked out any pieces defective in burn that might have passed the hands of the shovelers. The conveyor delivered the clinker to a Blake crusher, which broke it into pieces the size of pebbles; thence it passed to horizontal millstones, or, to what replaced these, ball and tube mills, for final reduction. The material was then deliv- ered into cylindrical screens having about 2,500 meshes per square inch, that portion retained in the screen being returned to a stone supplied almost entirely with these screenings. The cement was then conveyed to the stock house, which was divided 16 CEMENT AND CONCRETE into bins of 1,500 barrels capacity, and finally packed in barrels by means of a screw blade fitting the interior of the barrel. 22. DETAILS OF THE MANUFACTURE: Preparation and Mix- ing of the Raw Materials. The main points in the preparation of the raw materials for burning are : first, the proper amount of each ingredient must enter the mixture; second, the materials must be reduced to an extremely fine state of division, with no lumps; and third, the mechanical mixing must be as perfect as possible. Unless the ingredients are dried, the first require- ment is difficult to accomplish, especially with marl and clay, as the absorptive power of the materials renders it difficult to properly apportion them. More than three-fourths of the Portland cement manufactured in the United States is made from limestones. These must be ground before they can re- ceive the required addition of clay or of purer limestone, as the case may be, and they are usually dried to facilitate the grind- ing as well as to permit of determining the correct proportions of the ingredients. These hard materials are first crushed in an ordinary stone crusher or between heavy rolls, then dried in rotary driers, or otherwise; next, mixed and ground together to an extreme fineness in ball or tube mills. When rotary kilns are employed, the mix may be burned dry, but with fixed kilns, it is moistened to form bricks which are charged in the kilns with alternate layers of coke. Soft materials, such as marl and clay, are easy of reduc- tion in water, and are naturally treated by the wet or semi- dry process, although they may be prepared by the dry process. In the former method the grinding and mixing are accom- plished by edge runners, pug mills or wash mills. If the ma- terials have not been dried before mixing, the mix or slurry .should be sampled and analyzed before it is passed to the kilns. When fixed kilns are employed, it is desirable that the bricks should be as porous as possible, that the fire may more readily reach the interior of the brick. It is claimed by some manu- facturers that by spreading the slurry on a floor to dry, and then cutting into rough cubes when dry enough to be taken to the dry kiln, more porous bricks are obtained. 23. BURNING: STYLES OF KILNS. The various styles of kilns in use may be divided into four classes, namely: (1) Common dome kilns, (2) Continuous kilns, (3) Chamber and PORTLAND CEMENT 17 ring kilns, and (4) Rotary kilns. The dome kiln is the simplest type. The chamber is usually egg shaped. Cement- brick and coke are piled in alternate layers, the use of the proper amount of the latter requiring much skill, as it is a matter of experience. As the draft in the kiln varies with the weather, this method of burning is more or less at the mercy of the winds. When the burning is complete, the kiln is al- lowed to cool before removing the clinker, and thus much heat is lost, and the lining of the kiln is destroyed by alternate heat- ing and cooling. The amount of underburned and over- burned clinker is likely to be large. The output is small, and fuel expense high. The Dietsch kiln is one of the best examples of the second type, or continuous kiln. The slurry, in the form of bricks, is introduced at the base of the stack, into what may be called the heating chamber. Below this there is a right angle with a short horizontal section, over which the hot slurry is raked, to fall into the burning chamber. The clinker in the lower part of the latter is cooled by the air entering through the grates, while the slurry in the upper chamber is heated by the gases from the burning zone. At intervals a portion of the clinker, partially cooled, is removed at the bottom; this causes a general settlement in the kiln and leaves a space at the top of the burn- ing chamber, into which the dried clinker from above is raked, and more fuel added. This kiln uses small coal for fuel and is more economical than the dome type. The distinguishing feature of the Scliofar kiln is the con- traction of the dome at the point where combustion takes place, concentrating the draft at this point. The air entering the shaft at the bottom cools the clinker already burned, while the gases from the clinker burning in the central section serve to dry the raw bricks above. Several kilns of this type are in successful operation in this country. 24. Chamber kilns are used largely in England with coke as fuel. The gases from the kiln are made to pass over the slurry spread on brick floors, the kiln proper being at one end of this chamber and the stack at the other. These kilns are inter- mittent, have a comparatively small output, and require con- siderable labor. The Hoffman ring kiln consists of a series of compartments 18 CEMENT AND CONCRETE built around a large central stack. The chambers communicate by means of flues in such a way that the smoke and hot gases from one may be passed through other chambers before reach- ing the chimney. The kiln may be either "up draft" or "down draft," according to the direction in which the heat is drawn through the chamber. The compartments are charged from the sides, and when the moisture has been driven off from the material in the chamber first fired, the gases from this chamber are passed through the adjacent chambers, which have in the meantime been filled with raw materials. Although this kiln is economical of fuel if run continuously, much labor is re- quired to charge and empty it. This type is not used in the United States, though it has been employed to some extent in Germany. 25. Rotary Kilns. — Although rotary kilns for other purposes had been in use for some time, the first patent for a process of manufacture of cement by their use was issued in 1877 to Mr. T. R. Crampton. The method, apparently, did not pass beyond the stage of laboratory experiment until 1885, when Frederick Ransome of England patented a rotary kiln, which, however, required many important modifications to make it a success. About 1888 Mr. J. G. Sanderson and Dr. Geo. Duryee made some successful experiments with the rotary kiln for wet mix- tures, and in the following year experiments were begun at the works of the Atlas Portland Cement Co. under Mr. P. Giron, which resulted in the construction of a practical kiln for burning dry mixtures. Prof. Spencer B. Newberry, at about the same time, perfected the rotary process for wet materials at Warners, N. Y., and Sandusky, Ohio. A rotary kiln as used for the burning of cement consists of a steel cylinder five feet to six and a half feet in diameter and about sixty feet in length. This cylinder is lined with fire- brick, rests- on rollers with its axis slightly inclined to the hori- zontal, and is revolved slowly by means of gearing. The mix- ture to be burned is introduced at the upper end of the cylinder, while a jet of gas, crude oil, or more frequently, powdered coal, is injected through a special burner at the lower end. As the cylinder revolves, the material works slowly toward the lower end, the clinkering temperature being maintained throughout about the lower third of the length. In some of the more elab- PORTLAND CEMENT 19 orate styles, the clinker is passed through one or more cooling cylinders before it is conveyed to the grinding machinery. In the Hurry and Seaman rotary, the clinker, after it leaves the first cooling cylinder, is passed between rolls that serve to break any large lumps, and is moistened with water before its passage through the second cooling cylinder, which delivers the clinker warm, moist, and in small pieces. The lining of rotary kilns has given much trouble, as the clinker acts upon fire brick lining to form a fusible compound at the high temperatures required in the burning. One method of overcoming this difficulty is to fuse upon the fire brick a coat- ing of clinker which is beaten down while still plastic, so that it adheres to the brick and protects them more or less successfully from further injury. The kind of fuel and the burner giving the best result have also received much attention; while petro- leum was first tried and is still used to some extent, powdered coal is now more commonly employed, and one of the most suc- cessful forms of burner is constructed like an injector, the pul- verized coal being drawn in with the blast of air. 26. Output and Fuel Consumption of Different Kilns. — A comparison of the average output of the several styles of kilns described above, and the approximate fuel consumption, are given in the following table. Where it is necessary to dry the materials before introducing them into the burning kiln, the fuel required in drying is not included. Style. Intermittent dome Hoffman (per chamber) Dietsch and Schofer . Chamber Rotary Barrels per Day. 30 25 50 to 75 30 120 to 150 Fuel as Per Cent. of Weight of Clinker 20 to 30 15 to 20 15 to 20 40 to 50 30 to 40 27. Advantages of the Rotary Kiln. — Although the burning of cement in a rotary kiln requires a somewhat larger fuel con- sumption than with some other types, the ability to use a cheaper form of fuel, and the saving in the amount of labor required, much more than offset this disadvantage. Either wet or dry materials may be fed to the kiln, thereby eliminating the neces- sity of forming the slurry into bricks, drying and stacking 20 CEMENT AND CONCRETE them in the kilns. By the rotary process it is possible to so arrange a plant that the material is handled entirely by ma- chinery from raw material to finished product. The control possible in burning with the rotary is much better than with any other style of kiln, as the intensity of the flame and the speed of revolution of the cylinder may both be regulated. On this account, as well as because the pieces of clinker are much smaller, the cement is more uniformly burned. The remarkable development of the Portland cement industry in the United States is due in no small measure to the adoption and perfection of the rotary kiln, for the labor expense in manufacture has been so reduced thereby that we are able to successfully compete with cements made abroad where lower wages prevail. 28. GRINDING. — In grinding it is not sufficient that the cement be so reduced that a certain percentage of it will pass a sieve having, say, 10,000 holes per square inch; but it is de- sired that as large a proportion as possible shall be of the finest floury nature. To accomplish this result it has been claimed that French buhr millstones are the best, but their great consumption of power has led to the introduction of other forms of grinding machinery, so that at present millstones find their chief use in natural cement manufacture. It is usually considered that the greatest economy results from a gradual reduction of the clinker as it passes from one form of grinder to another, each machine being supplied with the size of pieces it is best adapted to handle. Large pieces of clinker are first passed through an ordinary rock crusher, such as the Gates or Blake. Where rotary kilns are in use, this step in the process may be omitted, as the clinker comes from the kiln in small, nut-like pieces. 29. Ball mills may also be used for the first reduction. The ball mill is a short cylinder of large diameter which is partially filled with flint or steel balls. When the cylinder revolves, the balls and the clinker fall upon hard metal surfaces, and as the material is ground to the size of sand grains, it falls through screens in the periphery into a hopper, where it is delivered to a conveyor, or to another form of pulverizer for further reduction. 30. Tube mills may be used in connection with millstones, but are usually employed for final reduction of the product of PORTLAND CEMENT 21 the ball mill. The tube mill is a steel cylinder, about 4 or 5 feet in diameter and 15 to 25 feet long, with axis horizontal or nearly so, and revolving on trunnions. . The cylinder is lined with hard iron or porcelain, and is half filled with flint pebbles. The material is fed in at one end and is gradually pulverized as it works toward the other end. Some styles are not continuous in their action, but are charged and closed, the material being removed after a certain number of revolutions. 31. Griffin Mills. — The Griffin mill is an American invention that has found much favor, especially in grinding tailings from other mills. A heavy steel roller is attached to the bottom of a steel shaft, which is provided at its upper end with a ball-and- socket joint. When the shaft is given a gyratory motion, the roller presses by centrifugal force against the inside surface of a heavy steel ring where the grinding takes place. The material which drops below the roller is thrown up again by steel blades that are also attached to the shaft, and when finally of sufficient fineness, the powder escapes through screens above the ring into a hopper. 32. The method of grinding to be adopted at any mill de- pends upon the size and hardness of the particles of clinker, but usually the clinker is passed through at least two machines. It has been stated 1 that the power consumed in grinding one ton of cement by the different principles is as follows: For millstones .... 30 to 32 I.H.P. per ton per hour. For ball principle ... 16 to 18 I.H.P. For edge runners . . . 12 to 14 I.H.P. " The sifting of the product, which formerly required special revolving or shaking screens of wire cloth, is now usually done by the sieves attached to the grinding machinery. 33. SAND-CEMENT. — This product, which is also called silica cement, is composed of Portland cement and silicious sand mixed in any desired proportion and then ground to extreme fineness. This product is placed on the market by dealers, but rights to use the process may be purchased. In the construction of Lock and Dam No. 2, Mississippi River, between Minneapolis and St. Paul, Major F. V. Abbot 2 used the process, grinding with 1 Mr. Henry Faija, in Trans. A. S. C. E., Vol. xxx, p. 49. 2 Report of Mr. A. O. Powell, Asst. Engineer, Report Chief of Engineers, U. S. A„ 1900, p. 2779. 22 CEMENT AND CONCRETE a tube mill one part of Portland cement with one part fine sand. The cost, exclusive of plant, is estimated as follows: £ barrel of Portland cement at $2.85 $1.42 J " " sand at .05 03 Cost of grinding 50 Cost of royalty 05 Cost of one barrel Silica cement $2.00 This cement has given remarkably high tests considering the adulteration with sand, and is claimed to be specially useful in making impervious mortar and concrete. Art. 7. Other Methods of Manufacture of Portland Cement 34. Portland Cement from Blast Furnace Slag. — The prep- aration of a true Portland cement from blast furnace slag has been followed in Germany and elsewhere in Europe for several years, and recently has been introduced in the United States. As this process utilizes a waste product, its popularity is likely to increase. Whereas, for the manufacture of slag cement only the slag from gray pig iron is available, it is found that in most . cases the slag from white pig iron may be used for the produc- tion of Portland cement from slag. The method of manufacture is briefly as follows: The slag as it comes from the blast furnace is subjected to the action of a stream of water, which granulates it and changes it chemi- cally, the water combining with the calcium sulphide, which is injurious to cement, to form lime and sulphuretted hydrogen. The granulated slag is then dried, mixed with the correct proportion of dried limestone, and ground to extreme fineness. The mixture is next burned in rotary kilns, the remainder of the process being the same as that employed when ordinary raw materials are used. While a cement made from slag by this method may have some peculiarities due to the nature of the raw materials used, and should be very carefully tested before it is used in important work, it should not be confounded with slag cement, which is a mixture of granulated slag and hydrated lime subsequently ground, but not burned together. 35. Portland Cement from By-Products of Soda Manufacture. — The Michigan Alkali Company has installed at Wyandotte, Mich., a cement plant to utilize the large amount of limestone SLAG CEMENT 23 which they have as waste in the manufacture of soda products. The limestone which has served its purpose in the soda manu- facture is in a finely divided and semi-fluid state; to this is added the proper percentage of clay, which has been dried and pulverized. The two are then very thoroughly mixed by pug mills and wash mills, the slurry corrected by small additions of one or the other of the ingredients, and finally burned in rotary kilns. Art. 8. The Manufacture of Slag Cement 36. Slag cement is made by adding calcium hydrate to a granulated basic slag resulting from the manufacture of gray pig iron. The slag must be carefully selected as to its chemical composition, Prof. Tetmajer having found by extended experi- ments that slags containing silica, alumina, and lime in the ratio 30 to 16 to 40 are best adapted to the purpose. As the molten slag runs from the blast furnace it is suddenly chilled by being run into water, or is partially disintegrated by being treated with a strong current of water, air, or steam. It is thus reduced to coarse particles resembling sand, or to a spongy or fibrous mass which, after drying, is readily ground to a fine powder. The process of chilling results in a certain chemico- physical change that renders the powder capable of combining more readily with the slaked lime which is subsequently added. Slag which has been allowed to cool slowly will not form an hydraulic product when mixed with the lime, although the chemical composition of the slag may be identical in the two cases. The lime is dipped into water, or treated with steam, until slaked to a fine dry powder, and is then added to the powdered slag in proportions of about one part of the former to three parts of the latter, this proportion depending upon the composition of the slag used. The powdered slag and lime are sifted, then mixed and reground together to an extreme fine- ness, thus insuring an intimate incorporation of the ingredients. Since there is no burning in the process, it is evident that the finished product is merely a mixture, not a chemical compound as is the case with Portland cement. 37. One of the largest mills for the manufacture of slag cement in the United States is conducted by the Illinois Steel Company, and the following description of the process is con- 24 CEMENT AND CONCRETE densed from a statement of Mr. Jasper Whiting/ manager of the cement department, and patentee of the process: Slag of the proper composition is chilled as it comes from the furnace by the action of a large stream of cold water under high pres- sure. The slag is thereby broken up, about one-third of its sulphur is eliminated, and it is otherwise changed chemically. A sample of the slag thus granulated is mixed with a proportion of prepared lime, and ground in a small mill whereby actual slag cement is produced. If the tests upon this trial cement are satisfactory, the slag is dried and then ground, first in a Griffin mill and then in a tube mill, where it is mixed with the proper amount of prepared lime and the two materials ground and intimately mixed together. The resulting product is said to be so fine that but 4 per cent, is retained on a sieve having 200 meshes per linear inch. The lime is burned from a very pure limestone and stored in bins, beneath which are two screens of different mesh, the coarser at the top. A quantity of lime being drawn on the upper screen is slaked by the addi- tion of water containing a small percentage of caustic soda. The lime passes through the two screens as it slakes ami is then heated in a dryer; the slaking being thus completed, the lime may be incorporated with the slag. The purpose of the caustic soda added in the above process is to render the cement quicker setting. Art. 9. The Manufacture of Natural Cement 38. History. — The American product called natural cement was first manufactured at Fayetteville, Onondaga County, N. Y., in 1818, and used in the construction of the Erie Canal. Other early dates of manufacture are given as 1823, near Rosen- dale, N.Y., and 1824 at Williamsville, Erie County, N.Y., the products being used in the construction of the Erie and the Delaware & Hudson Canals. Factories were boon started in other states,, and at present nearly every State in the Union has one or more natural cement factories, the total annual produc- tion being now about nine million barrels. 39. Materials Required. — The composition of rock from which natural cement may be made, varies within wide limits. As stated in §13, an argillaceous limestone, a magnesian lime- 1 " Report of Board of Engineers on Steel Portland Cement," Appendix I. NATURAL CEMENT 25 stone or an argillo-magnesian limestone may be used. Argilla- ceous limestone makes what is sometimes called an aluminous natural cement, its essential ingredient being a bisilicate, or sili- cate of alumina and lime, while the product made from mag- nesian limestone is called magnesian cement and is composed of a triple silicate of lime, magnesia and alumina. The Maryland cements are typical of the former or alumi- nous variety, containing only one to five per cent, of magnesia, while the Rosendale and the Milwaukee are magnesian cements containing 15 to 25 per cent, magnesia. (See Table 3.) With a given raw material, the silica and alumina should bear a certain proportion to the lime and magnesia, but close limits cannot be stated for this proportion, as it varies with the chemical and physical character of the rock. The silica should be combined with the alumina, not in the form of sand. The materials found at any locality may vary considerably as to chemical composition, especially among the several strata. In some cases the different strata are utilized to make two or more brands, which differ somewhat in their characteristics as to time of setting, etc. It is common also to mix two or more layers together in the manufacture, with the idea that the in- gredients lacking in one stratum will be supplied by the others. 40. DESCRIPTION OF PROCESS. — As the proper ingredients to produce the cement have been incorporated by Nature, that part of the process of Portland cement manufacture preliminary to the burning is unnecessary. The rock occurs in strata and is either quarried in open cut where the stripping is light, or by means of tunnels. ~ In open cut, a face of twenty feet or more is sometimes worked. As has already been stated, the strata vary in chemical composition, and while two or more brands are sometimes made at the same mill, it is a more general practice to mix the rock from several strata in the production of one brand. The idea is that if one layer contains too much silica, it may be corrected by another containing too much lime or mag- nesia. As the rock is not finely pulverized before it enters the kiln, each lump burns by itself and makes a certain cement; the piece of rock next it must make as distinct a product as though burned in a separate kiln. What is obtained, then, by this method is a mixture of several cements, and it is questionable whether the mere mechanical mixing of an over-limed cement 26 CEMENT AND CONCRETE with an over- clayed one will make a well balanced product. This practice may account, in a great degree, for the large vari- ations that occur in the cement from a single factory, variations which are often, however, more noticeable in short-time tests than in the longer ones. 41. The rock, as quarried, is broken by an ordinary rock crusher or otherwise, into pieces varying in size up to six inches, and is then conveyed, usually by tramway, directly to the kilns. These are of the cylindrical continuous type, built of stone or steel, and lined with fire brick. The kilns are commonly about 45 feet high and 16 feet in diameter; the tramway leads to a loading platform on top of the kiln. According to the locality, the fuel may be either bituminous or anthracite coal of about pea size. The rock and fuel are spread in the top of the kiln in alternating layers, the proportion of fuel being usually regulated by the man in charge of the burning, but sometimes a machine is employed which automatically governs the amount of coal used. The temperature in the kilns is much below that required in Portland cement manufacture, but varies of course with the materials. 42. The calcined rock is conveyed first to some sort of a stone crusher; a common form is known as a "pot-cracker," and consists of a corrugated conical shell in which works a cast iron core, also corrugated. After passing the cracker, the material may be screened, giving a certain proportion of finished product, and another portion which may go directly to the finishing stones, while the coarsest pieces are conveyed to another form of cracker, such as iron edge runners, which prepares it for the millstones. In many factories ordinary under-run millstones are used, in others rock emery stones are employed, while in some factories stones found locally prove satisfactory. There have been recently installed in some of the natural cement fac- tories, ball and tube mills for grinding as used for Portland cement clinker, and in several factories special forms of grinding machinery are in use that have been perfected by the managers of the works. The product passes from the reducing mills to the "mixers," by means of which the material is thoroughly mixed to promote uniformity. It is now ready for packing, and may be conveyed directly to the chute from which the barrels or bags are filled. NATURAL CEMENT 27 In packing, the barrel rests upon a circular disc which is given a vertical jarring motion, and thus the cement is thoroughly settled in the 'barrel. It is seen that the manufacture of natural cement is very similar to that portion of Portland cement manufacture suc- ceeding the preparation of the raw material for burning. In general, less care is requisite with natural cement, the burning is carried on at a lower temperature, and the calcined rock is softer, so that less expense is incurred in grinding. PART II THE PROPERTIES OF CEMENT AND METHODS OF TESTING CHAPTER III INTRODUCTORY 43. In the tests of such structural materials as wood and steel it will not usually be difficult to determine the suitability of the material for the intended purpose, provided the test pieces truthfully represent the members to be used. It is known that so long as these members are protected from oxidation and over-loading they will retain their qualities, and there is always a reasonably clear understanding of what these qualities should be. On the other hand, in the testing of cement, one may be perfectly sure that from the moment the cement is manufac- tured until long after it has been in service in the structure its properties will be ever changing; and, further, the qualities which it is desirable the cement should possess are not always clearly in mind. 44. Desirable Qualities in Cement. — The desirable elements in a cement may be stated as follows: 1st, That when treated in the proposed manner it shall develop a certain strength at the end of a given period. 2d, That it shall contain no compounds within itself which may, at any future time, cause it to change its form or volume, or lose any of its previously acquired strength. 3d, That it shall be able to withstand the action of any exterior agency to which it may be subjected that would tend to decrease its strength or change its form or volume. When it is deter- mined that a cement has these three qualities, it is certain that it is safe to use it, but it is further desirable to know that the 28 UNIFORM METHODS 29 cement in question will accomplish the given object as cheaply as any other cement. The cohesive and adhesive strengths of cement are not usu- ally considered in the design of the structure into which cement enters. The design of a masonry arch does not comprehend any adhesive strength in the cement, except as it may be recognized as an additional factor of safety, and a masonry dam is so de- signed that there shall be no tension at the heel. These facts are due in a large measure to the very imperfect knowledge we have of the behavior of cements in various contingencies. With the increasing use of concrete, as in arches, locks, floors, roofs, etc., the tensile and transverse strengths of cement are coming to be relied on to a certain extent; and as its properties become better known, and as means of recognizing these properties become more certain and widespread in their application, ce- ment will be more extensively employed in a scientific and eco- nomical manner. Cement may be compared in one sense to timber and cast iron. A large factor of safety is employed when dealing with these materials because of hidden defects that may exist. The defects which lie hidden in cement may be even greater than these in proportion to its possible strength, and defects in ce- ment are often more treacherous because their development may be deferred for some time. The importance of knowing whether the cement fulfills the second and third requirements noted above is therefore evident. 45. Having considered the qualities a cement should have, we may proceed to the detailed consideration of the various tests employed to disclose the presence or absence of these qual- ities. The strength a given cement will develop is investigated by chemical analysis, by obtaining the specific gravity and fine- ness, and by actual rupture tests, whether they be tensile, com- pressive, transverse, or shearing. By tests for change of volume and by chemical analysis, it is sought to determine whether a cement has within itself elements of destruction. For the power to withstand external agencies there are no adequate tests, though chemical analysis is considered an aid. The methods of use, the proportions of the materials, their incorporation and deposition are of great importance in insuring against external causes of injury. 30 CEMENT AND CONCRETE 46. Uniform Methods of Cement Testing. — In order that uniformity should prevail in the methods employed in testing cements, various societies have discussed the subject in detail, usually through committees, and much valuable work has been done along this line. The engineers of public works in many European countries have adopted specifications and laid down more or less detailed rules for testing. The Corps of Engineers, U. S. A., has recently adopted a similar code of rules. The International Society for Testing Materials, with which the American Society for Testing Materials is affiliated, has con- sidered the subject and still has committees at work upon it. The New York section of the Society of Chemical Industry has recently formulated a method for analysis of materials for the Portland cement industry. The American Society of Civil En- gineers received a report in 1885 from a committee appointed to consider methods of cement testing, and in order to keep the subject abreast of the latest developments in the manufacture and use of cement, a second committee was appointed several years ago, which has been making a thorough discussion of the subject, and has submitted a preliminary or progress report. 47. Notwithstanding that so much has been done toward unification of methods, it may never be possible to determine accurately the value of one cement as compared with another tested in a different laboratory; though in tests of iron and steel no such difficulty is experienced. Certainly, as at present carried out, strength tests of cement are purely relative tests and do not show the absolute strength which may be developed in the structures; nor can the results be compared with the re- sults obtained in other laboratories and any fine distinctions of cpiality drawn. To attempt to carry out acceptance tests in such a way as to show directly the strength which will be de- veloped in actual construction, is only to introduce causes of irregularity in the tests. CHAPTER IV CHEMICAL TESTS Art. 10. Composition and Chemical Analysis 48. Value of Chemical Tests. — The definite aid which chem- ical analysis may render in determining the quality of a cement is limited by the following considerations. It is not definitely known just what part is played by each of the compounds that go to make up commercial cement, and chemical analysis does not tell the manner of the occurrence of these compounds. A cement may have a chemical composition that is thought to be perfect, but if the burning has not been properly accomplished, it may be a dangerous product and analysis would show no de- fect. Some of the best authorities say that chemical analysis is useful principally in tracing the cause of defects which, by other tests, have been found to exist. However, there are some constituents which it is fairly well known a cement should not contain in any considerable quantities. An analysis may be of value in estimating quantitatively such constituents, while it may also be of service in detecting adulterations. It is not im- possible, then, that chemical tests may yet play a more impor- tant role in cement testing, especially if the method of analysis can be made more simple and rapid, without too great a sacri- fice of accuracy. 49. Lime. — The proportion of lime in Portland cement may vary from 59 to 67 per cent. A much greater range than this is allowable in natural cement, the percentage usually being from 30 to 45, according to the amount and character of the other active constituents. An analysis of Portland cement which shows a percentage of lime far outside of the limits mentioned above, should be regarded with suspicion and submitted to very thorough tests before acceptance. As already stated, the ratio of the silica and alumina to the lime in a cement is called the hydraulic index. The value of this ratio is usually between .42 and .48 for Portland cement. 31 32 CEMENT AND CONCRETE Cement mixtures containing a large percentage of lime re- quire a high temperature for calcination, are difficult to grind, and yield a slow-setting product. The danger in highly limed cements is that they will not be properly calcined and a por- tion of the lime will be left in a free state. The demand for high strength in short-time tests has led manufacturers to make a heavily limed product, and in some cases the limits of safety have probably been overstepped. The introduction of the rotary kiln, however, has so improved the facilities for burning cement that a higher percentage of lime is now possible. There is no method known at present for determining quanti- tatively the amount of free lime in a cement, and it seems doubt- ful whether its presence can be detected with certainty by chemi- cal analysis. The method usually employed for this purpose depends on the hydration of the lime and subsequent absorption of carbonic acid. 50. Magnesia. — The detection of magnesia in several con- crete structures that had failed, led to the conclusion that mag- nesia, in quantities exceeding two or three per cent., was a dangerous element in Portland cement. In 1886-87 Mr. Har- rison Hayter x mentioned several failures of masonry and con- crete which he considered were due to magnesia, and concluded that cement should not contain more than one per cent. Later investigations, however, indicated that such failures could be explained in other ways, and that the magnesia found in the failing structure had come from the sea water and replaced the lime in the cement. Mr. A. E. Carey 2 has considered that "an excess of caustic lime or magnesia causes first, disintegration by expansion due to hydration, and second, being soluble, when conditions permit of their washing out, leave the concrete in a honeycombed state." Notice that this refers to caustic mag- nesia, and Prof. S. B. Newberry 3 has stated that "it is doubtful if magnesia is ever combined in Portland cement. Our own experiments tend to confirm the opinion of many German authorities that magnesia remains free in cement and does not combine with the constituents of clay after 'the manner of lime." 1 Proc. Inst. C. E., Part 1, Session of 1886-87. 2 Ibid., 1891-92. 3 Municipal Engineering, October, 1896. COMPOSITION AND ANALYSIS 33 On the other hand, M. H. LeChatelier 1 says that the " acci- dents occasioned by certain magnesian elements, and the similar results obtained in laboratory experiments, have been clue to the employment of badly proportioned cements, containing free uncombined magnesia and too small a quantity of clay. Cor- responding mixtures containing lime instead of magnesia would have caused still more serious accidents, yet it would not be con- cluded that there must be no lime in cement." Again, Dr. Erdmenger characterizes magnesia as an adulterant only, and considers that its effect is nil if a greater percentage of lime is added in the manufacture. Some authoritative information on the amount of magnesia allowable in Portland cement is contained in the report of the magnesia commission of the Association of German Cement Makers, 1895: Three members of this committee, Messrs. Schott, Meyer and Arendt concluded that "the presence of magnesia up to ten per cent, causes no harmful expansion or cracking of the cement, even after several years." Mr. Dyckerhoff, how- ever, presented a minority report, in which he pointed out that while a large amount of magnesia, not sintered, may not have an injurious effect, yet a content of more than four per cent, of sintered magnesia, whether added or substituted for part of the lime, has an injurious effect after long periods. The committee continued the ruling of 1893 that "a magnesia content of five per cent, in burnt cement is harmless," but held the question open for further investigation, indicating that this limit might be raised. In view of the disagreement among such eminent authori- ties it is impossible to arrive at a satisfactory conclusion, but if the effect of magnesia depends upon the manner of its occur- rence, whether free or combined, sintered or unsintered, then chemical analysis can be of but limited value as a test of quality in this regard. Natural cements frequently contain large pro- portions of magnesia replacing lime, and in this case an analysis is of the same value as an analysis for lime. 51. Alumina and Iron Oxide. — The amount of alumina which a cement should contain is not well established. Its presence tends to facilitate the burning, and it renders the prod- 1 Trans. Amer. Inst. Mining Engrs., 1893. 34 CEMENT AND CONCRETE uct quicker setting. Cements containing large percentages of alumina are inferior for use in air or sea water, and it is probable that the percentage of alumina should not exceed eight or ten to obtain the best results in all media. A slag cement may be detected by its large content of alumina. Oxide of iron acts as a flux in burning, but in the finished product is little more than an adulterant. 52. Sulphuric Acid. — French specifications say that Port- land cements shall not contain more than one per cent, of sul- phuric acid or sulphides in determinable proportions. This is doubtless intended for cement to be used in sea water. Adul- terations with blast-furnace slag may sometimes be detected by the amount -of sulphides present, but small quantities of sul- phuric acid in the cement may be derived from the coke used in burning and have no injurious effect for use in fresh water. A content of 1.75 per cent, of sulphuric anhydride, S0 3 , is now con- sidered the maximum permissible. Sulphates mixed with the raw materials and burned with the cement may be harmless, while the same amount added after burning would not be per- missible. [For tests on the effect of adding sulphate of lime to cement, see Art. 48.] 53. Water and Carbonic Acid. — The determination of these may give some idea of the deterioration of a product by storage, and they may also indicate defective burning. M. Candlot con- siders that in the case of Portland cement, a loss on ignition (water and carbon dioxide) exceeding three per cent, "indicates that the cement has undergone sufficient alteration to appre- ciably diminish its strength." Natural cements may, however, contain considerable proportions of these ingredients and still give good results. 54. Conclusions. — Finally, then, the determination of silica, alumina, magnesia and lime may be of value, first, in classify- ing a product, and second, as indicating whether the proportions contained in it are such that if properly manufactured it is capable of giving good results. What these proportions should be for Portland cement has already been stated, § 9. The de- termination of certain injurious ingredients is also of some value, but it must be remembered that the dangerous elements most commonly occurring, namely, free lime and magnesia, are not determinable by chemical analysis. It has been stated by COMPOSITION AND ANALYSIS 35 M. LeChatelier that "neither complete nor partial chemical analysis of the constituents of hydraulic materials can be ranked among normal tests. But chemical analysis may render real service in controlling the classification of a product concerning which there is reason to doubt the declaration of the manufac- turer. Thus, a slag cement can be distinguished from a Port- land by its tenor in alumina and water; certain natural cements, by their contents of sulphuric acid, etc." 1 The methods of analysis for Portland cement are given in considerable detail in a little book, "The Chemical and Physical Examination of Portland Cement," by Richard K. Meade. The method of analysis suggested by the New York Section of the Society of Chemical Industry is published in the Engineering Record of July 11, 1903, and in Engineering News of July 16, 1903. 'Tests of Hydraulic Materials," H. LeChatelier. CHAPTER V THE SIMPLER PHYSICAL TESTS Art. 11. Microscopical Tests. Color 55. Microscopical examinations are of some interest and value to those who are thoroughly versed in the chemistry of the burning and hardening of cements, as an aid in determining the part played by each compound in the hardening. Examinations may be made either of the dry powder, or of thin sections of hardened cement, or clinker. Dry powder of Portland cement appears to be made up of scaly particles, many of which are clearly defined and semi-transparent, while natural cement particles are more nearly opaque and less angular. Thin sections of Portland cement clinker have been found to exhibit colorless crystals somewhat cubical in structure, which are thought to form the essential hardening constituent; thin sec- tions of hardened Portland cement show a clear crystalline structure. Prof. Hayter Lewis found that the particles in good Portland cement were angular in form, consisting of scales and splinters, while the particles of cement of poor quality were rounded or nodular. Microscopic examinations have no place at present in ordi- nary tests of quality. 56. Significance of Color. — - The color of cement is chiefly derived from its impurities, such as oxides of iron and manga- nese, rather than from its essential ingredients, and the color is therefore of minor importance. Other things being equal, a hard burned Portland cement will be darker in color than an underburned product. An excess of lime may be indicated by a bluish cast, and excess of clay or underburning may give a brownish shade. Gray or greenish gray is usually considered to be indicative of a good Portland. 57. The colors of natural cements have a wide range, vary- ing from a light yellow to a very dark brown, without reference to quality. Owing to a popular idea that dark color indicated 30 WEIGHT PER CUBIC FOOT 37 strength, some manufacturers have been said to add coloring matter to their product, but although this may have been true at one time, the correction of this false idea has doubtless ren- dered such a practice quite unnecessary now. Variations in shade in different samples of the same brand of natural cement may indicate differences in burning or in the composition of the rock; but the interpretation of color for any given brand must be the result of close study, for some cements become lighter on burning and others become darker, while in some cases no variation in shade can be detected for different degrees of burning. Art. 12. Weight per Cubic Foot or Apparent Density 58. Significance. — Since a hard burned Portland cement will usually be heavier than a light burned one, a test of the weight per cubic foot was once thought to be of great value in judging of the degree of burning. But it has been shown re- peatedly that the weight per cubic foot depends quite as much on the fineness as on the burning. It also depends on the age of the cement, and its chemical composition. As a test for quality, the determination of the apparent density has therefore been discarded. However, it is an aid in classifying a product, since Portland cements weigh from 70 to 90 pounds per cubic foot when loosely rilled in a measure, while natural cements weigh from 45 to 65 pounds. A knowledge of the weight per cubic foot is also useful in reducing proportions given by weight to equivalent volumetric proportions, and vice versa. 59. Method. — This test may be made with a very simple apparatus, and the results obtained, though not strictly accu- rate, are sufficient for all practical purposes. A metal tube, 2 feet 4 inches long, about 6 inches in diameter at the top, and 3 or 4 inches at the bottom, is supported by a frame resting on four legs. A metal cylinder, 6 inches in diameter and 6^ inches deep, holding one-tenth cubic foot, is placed on the floor below the tube. A coarse sieve, through which all of the ce- ment will pass, is placed on top of the tube and three feet above the bottom of the measure. The cement passes through the sieve, falling freely to the cylinder below, which is struck off level when full. The cement must not be heaped too much, and great care must be taken that the measure is not jarred 38 CEMENT AND CONCRETE while it is being filled or struck off. The cement is in such a light condition that a very slight jar is sufficient to cause it to settle. The above apparatus is on the same plan as that used by- Mr. E. C. Clarke on the Boston Main Drainage Works, and is described here for general use when it is desired to compare the results obtained by operators at different points. Should one wish simply to obtain a series of results on different cements which are to be compared among themselves, it is quite suf- ficient to sift each sample through a coarse sieve, and then with an ordinary scoop carefully fill a measure of any known capac- ity, without other apparatus. Mr. Henry Faija has described an apparatus consisting of a funnel with a screw at the mouth which carries the cement horizontally to the point where it falls freely into the measure. Various other devices have been employed, but none seems to have met with universal favor. 60. To determine the relative accuracy obtainable with the simple form of apparatus first described, the author made a series of tests which may be summarized as follows: — 1st Method. — Cement passed a wire mesh sieve, holes .033 inch square and fell freely two feet through a 6-inch tube into a measure holding £ cu. ft. Five trials with a sample of Dycker- hoff Portland, highest weight per cubic foot, 81 lbs. 4 oz., lowest, 79 lbs. 2 oz., difference, 2 lbs. 2 oz. Three trials with Alsen's Portland, highest weight, 73 lbs., lowest, 72 lbs., dif- ference, 1 lb. 2d Method. — Measure same size filled with scoop without other apparatus, and cement not shaken or jarred in measure. Five trials with Alsen's Portland, highest result, 73 lbs. 8 oz. per cu. ft., lowest result, 72 lbs. 12 oz., difference, 12 oz. Five trials with different sample of same cement, highest, 72 lbs. 4 oz., lowest, 72 lbs., difference, 4 oz. 3d Method. — Measure filled with scoop, and cement well shaken down as filling proceeded. Five trials with Alsen's Portland, highest result, 100 lbs. 8 oz., lowest, 97 lbs. 14 oz., difference, 2 lbs. 10 oz. It appears from these tests that when the measure is filled with the scoop, the results are about as uniform as when the apparatus is used, provided the filling is always done by the SPECIFIC GRAVITY 39 same person. But the results obtained by different operators with the same sample of cement would probably vary less, one from the other, when the apparatus is employed. In other words, the personal factor is more nearly eliminated when the cement is passed through a sieve and allowed to fall freely from a given height. 61. As to the effect of age on the weight per cubic foot, it was found in one case that cement which weighed 93^ pounds per cubic foot when freshly ground, weighed but 88 pounds when a few days old, and 78 and 74 pounds after six months and one year, respectively. 1 Many experiments have been made to show the effect of fineness on the weight per cubic foot, but as this subject will be taken up again under "fineness," it will suffice to quote one series of tests made by Mr. E. C. Clarke, 2 giving the "weight per cubic foot of the same sample of German Portland cement containing different percentages of coarse particles as deter- mined by sifting through the No. 120 sieve." Samples containing 0, 10, 20, 30, and 40 per cent, of coarse particles retained on No. 120 sieve gave the following weights per cubic foot: 75, 79, 82, 86 and 90 pounds, respectively. It may be repeated that the weight per cubic foot is no longer considered an indication of quality, but should it be desired to specify a given weight, the method by which the test is to be made should also be stated. Art. 13. Specific Gravity or True Density 62. The apparent density or weight per cubic foot is in- fluenced to such an extent by the degree of fineness of the cement that this test has been almost superseded by the test for specific gravity. Although the true density, or specific gravity, is not affected by the fineness, it is influenced by the composition, the degree of burning, and the age, or amount of aeration of the sample. The method commonly employed in this test consists in de- termining the absolute volume of a given weight of the cement 1 "Cement for Users," by H. Faija, p. 54. 2 " Record of Tests of Cements for Boston Main Drainage Works," Trans. A. S. C. E., Vol. xiv, p. 144. 40 CEMENT AND CONCRETE powder by measuring the amount of liquid which it will dis- place. A simple form of apparatus may be constructed in any laboratory as follows: In a wide mouth bottle, having straight sides and holding 200 c.c. or more, fit a perforated cork. Through the cork slip a burette graduated in cubic centimeters from to 50, placing the zero end down. Fill the bottle and the tube up to the zero mark, with some liquid such as turpentine, benzine or kerosene oil, but preferably benzine (62° Baume naptha). By means of a funnel in the top of the burette, add slowly 100 grams of cement; then jar the bottle to remove air bubbles and read the burette. This reading, x, represents the volume of 100 grams of cement; and 100, the volume of 100 grams of water, divided by x gives the specific gravity of the sample. 63. Among other forms of apparatus which are also of sim- ple construction and tend to facilitate the test, may be men- tioned the following: — M. Candlot x devised an apparatus consisting of a graduated tube terminating in a bulb at the upper end, the lower end of the tube being ground to fit the neck of a flask. The tube and flask being disconnected, sufficient liquid is placed in the bulb so that when connected with the flask and placed upright, the level of the liquid will be at or near the zero mark on the tube. The actual level of the liquid is read after standing a few minutes; the apparatus is again inverted and the flask disconnected to allow of the introduction of 100 grams of cement. The flask is then replaced and the contents of the apparatus well shaken to expel air-bubbles. When the latter have been completely ex- pelled, the flask is placed upright, and after standing a short time, the level of the liquid is again read, the difference between the two readings indicating the absolute volume of 100 grams of the cement powder. The apparatus devised by M. H. LeChatelier 2 consists of a flask of a capacity of about 120 c.c, and having a neck some 20 c. in length, halfway up which is a bulb having a capacity 1 "Ciments et Chaux Hydrauliqiies," par. E. Candlot. 2 "Report of Commission des Methods -d'Essai des Materiaux de Con- struction," The Engineer (London); Illustrated also in Meade's "Examina- tion of Portland Cement," Spaulding's "Hydraulic Cement," and Engineer- ing News, January 29, 1903. SPECIFIC GRAVITY 41 S -40- -30- /ns/c/s afta 8mm. of 20 c.c. Near the bottom of the tube, or flask, is the zero mark, and above the bulb the tube is graduated for a length corresponding to a capacity of 3 c.c, each graduation repre- senting .1 c.c. The diameter of the tube is about 9 mm. The zero mark on the tube is below the bulb. The method of opera- tion is similar to that described above. 64. The following style of apparatus (see Fig. 1) is sug- gested as a very convenient form, and one which may be used for another test soon to be described. In this form, the flask, of a capacity of about 200 c.c, has straight sides and a flat bottom. The lower part of the burette is of large diame- ter, about 15 mm., to allow the cement to pass readily, while the upper portion is made smaller, about 8 mm., to per- mit more accurate reading, and is graduated from 30 c.c. to 40 c.c, the divisions being 0.1 c.c. Half divisions may be esti- mated. The zero mark is in the larger part of the burette, but it is less difficult to make an ac- curate reading at the zero mark, since at the time of taking this reading the liquid is clear; this mark should entirely surround the burette. The mouth of the bottle and the lower end of the burette should be ground to fit, and a ground glass stopper should form a part of the apparatus. A long pipette will be found convenient for adjusting the level of the liquid to the zero mark. 65. Turpentine is frequently employed for this test, but it is somewhat inconvenient to use, since its volume is so sensi- 1. — SPECIFIC GRAVITY APPA- RATUS 42 CEMENT AND CONCRETE tive to changes in temperature. This sensitiveness renders it imperative that the temperature at the time of taking the final reading be the same as when the initial reading is taken, or that a correction be applied. To assure this condition the ap- paratus should be immersed in a water bath, and the tempera- ture of the cement should be the same as that of the turpentine. The use of water in the apparatus does not offer this inconven- ience, but it is possible that the hydration of the cement during the experiment might be sufficient to so affect the volume as to change the result, especially with quick-setting cements. Light oils, such as benzine and kerosene, are rather volatile, but the former (62° Baume naptha) is recommended in the preliminary report of the Committee of the American Society of Civil Engineers. With the precautions mentioned above, turpentine may be used with good results; that which has been dried by standing over cement or quicklime is to be preferred. 66. This test may be extended to give interesting and valu- able results, in the following manner: When the cement has settled in the bottle, leaving the liquid clear, pour off a portion of the latter and replace the burette by a glass stopper. Thor- oughly agitate the remaining liquid and cement until the latter is in suspension; allow the cement to settle again without dis- turbance, and it will be found that it is graded in the bottle according to its fineness, the coarsest particles being at the bottom. With Portland cement, if a portion of the sample is underburned it will appear as the top layer, and be indicated by its yellow color. It will also be interesting to note what proportion of the cement is so fine that the separate grains are indistinguishable. That the bottle should have straight sides and a flat bottom is to accommodate this part of the test, which also dictates the use of some other liquid than water. 67. Effect of Composition, Aeration, Etc. — It has been said above that the composition of a cement affects its specific gravity, a highly limed cement having a higher density. On this account an analysis for lime is valuable in connection with this test, in order to determine whether a high specific gravity is (hie to a high percentage of lime or to hard burning. The age, or aeration of a sample affects its specific gravity SPECIFIC GRAVITY 43 because of the absorption of water from the atmosphere. The absorption of two per cent, of water is sufficient to lower the specific gravity from 3.125 to 3.000. The following may be given as illustrating this point: a certain sample of natural cement when taken from the barrel had a specific gravity of 3.106; after it had been spread out in the air for two months its specific gravity was 3.000. A quantity of this aerated cement weighing 120 grams was placed in an iron vessel and heated over an oil stove for about one hour; at the end of this time the cement had lost two grams in weight. The specific gravity of the fresh cement being 3.106, 118 grams would have an ab- solute volume of 33 c.c; two grams of water would occupy 2 c.c., hence 120 grams of the aerated cement would occupy 40 c.c, and 120 -h 40 = 3.00, the specific gravity of the aerated cement as found above. It is not always possible to thus drive off all of the water absorbed, since a portion of it may enter into combination with the cement; but a sample should always be heated for at least thirty minutes at a temperature of 100° C. before making the test for specific gravity, and should any appreciable loss of weight occur, it is an indication of aeration. 68. A determination of the specific gravity is primarily a test for burning, but it may also be of much value in detecting adulterations, as with blast furnace slag or ground limestone. An admixture of 10 per cent, of either of these substances would suffice to lower the specific gravity from 3.15 to about 3.10. The specific gravity of Portland cement ranges from 2.90 to 3.25, but a first-class product should not show a lower specific gravity than 3.05. If fresh Portland gives a result below this it is probably either underburned or underlimed, or, perhaps, has been adulterated. The specific gravity of natural cements has been found to vary from 2.82 to 3.25. The specific gravity of one sample of underburned natural cement was found to be lower than a sample of the same brand which was overburned, but it seems very doubtful whether this is true of other brands made from rock of a different character. It was also found that the spe- cific gravity of the coarse particles of some natural cements is lower than that of the fine particles (see Table 10, Art. 15), while the opposite is true in the case of Portland cements. 44 CEMENT AND CONCRETE No general rules can be given at present for the interpreta- tion of this test that are applicable to all natural cements; it is thought that the test will be of value in comparing samples of the same brand, though it seems doubtful whether it will prove of value in comparing one brand of natural cement with another, since it is quite probable that the interpretation may vary with the variety of rock used in the manufacture. The value of the test for Portland cements is, however, well established. CHAPTER VI SIFTING AND FINE GRINDING Art. 14. Fineness 69. Importance of Fineness. — The fineness of cement is al- ways conceded to be one of its most important qualities, and the determination of fineness is omitted in none but the very crudest tests. Unfortunately, however, sieves that are so coarse as to give delusive results are usually employed. It is very easy to show that grains of cement as large a*s one-fiftieth of an inch in diameter are practically valueless, but much more difficult to determine the point of fineness at which the particles begin to have cementitious value. 70. A moderately coarse sieve is easier to operate than a very fine one, less time being consumed in sifting. The impres- sion seems to be quite general also that there is a fixed relation between the proportions of the different sized grains in different samples. Many specifications require that a certain percentage "shall pass a sieve having 2,500 holes per square inch." Now, there is little doubt that grains of cement larger than .005 inch in one dimension have very little cementitious value, and hence a cement, all of which would pass holes .015 inch square, while but 50 per cent, of it would pass holes .005 inch square, is little better than one which leaves a larger residue on the coarser sieve but the same residue on the finer. In America and Germany it is the usual practice in the pro- cess of manufacture to pass the cement through a screen which will reject particles larger than about .015 inch in diameter; the futility in, attempting to determine, with a sieve no finer than this, the proportion of the particles which are fine enough to be of value, is therefore apparent. Since the English cement makers have not been so progressive in the practice of screen- ing, they have obtained the reputation of producing a coarse product. In many cases this reputation is probably a just one, but when tested with a very fine meshed sieve, some of the 45 46 CEMENT AND CONCRETE English cements do not compare so unfavorably with those of German manufacture. It is a curious fact in this connection that the English are the most conservative in holding to the use of the coarse sieve in testing, which makes their cement appear so very much coarser than the American or German product. 71. SIEVES. — Sieves for cement testing may be made either of wire or silk gauze, set in metal or wood frames. Sieves of perforated metal plate are sometimes employed for sifting sand, but seldom for cement. It is with considerable difficulty that accurate gauze sieves are obtained. They are usually desig- nated by numbers corresponding to the number of meshes per linear inch; this is in some respects an unsatisfactory method, for the size of the wire, which is quite as important as the number of meshes, is frequently not given at all, or stated in terms of some wire gage which is capable of various interpretations. As usually supplied by different manufacturers, sieves pur- porting to have the same number of meshes per linear inch may vary in this regard as much as 10 or 15 per cent. Likewise the size of wire used by different makers, in sieves having the same number of meshes per inch, may vary quite as much. Again, on account of irregularities in the gauze, the holes in a given sieve vary one from another; in some cases an opening may be but 60 or 70 per cent, as large in one dimension as an adjacent one. An ideal sieve should conform to the following requirements: (1) holes to be of uniform size and shape throughout, (2) sides of the holes to be very smooth, and (3) the spaces between the holes to be of such size and shape that particles will not easily rest there. It is evident that the largest holes determine the character of the sieve. For example, a sieve having half its holes 0.01 inch square and the other half 0.02 inch square, would, if used long enough, separate the cement exactly as it would if all the holes had been 0.02 inch square. Hence, if a very small percentage of the holes are larger than the normal, it seriously impairs the accuracy of the sieve by introducing an indeter- mination; but holes smaller than the normal have no greater objection than that, as the sifting proceeds, they become spaces FINENESS 47 between the real or larger holes, and as such do not fulfill the third requirement mentioned above. The shape of the holes, whether round, square or hexagonal, seems of minor importance so long as uniformity is maintained. The second requirement is necessary, because, should particles adhere to the sides of the hole, the size of the latter would be decreased to that ex- tent. The third requirement is for convenience, but would require consideration if the style of the sieve were changed to a punched metal plate. 72. The Committee of the American Society of Civil Engi- neers, in their report on "A Uniform System for Tests of Cement " in 1885, recommended three sizes of sieves for cement: No. 50 (2,500 meshes to the square inch) wire to be of No. 35 Stubbs' wire gage ; No. 74 (5,476 meshes to the square inch) wire to be of No. 37 Stubbs' wire gage; No. 100 (10,000 meshes to the square inch) wire to be of No. 40 Stubbs' wire gage. For sand, two sieves were recommended, No. 20 and No. 30 (400 and 900 meshes per square inch) wire to be of No. 28 and No. 31 Stubbs' TABLE 5 Sieves : — Number of Meshes per Linear Inch and Sizes of Openings, as Found by Measurement o « H H !> 2 3 NO. OF Meshes PEE LINEAR Inch. Diameter of Wire in Decimals of an Inch. Mean Size of Opening in Decimals of an Inch. .a H \% a < SH O o ti < Web. Woof. 6 5 S 5 P CO ® a ? D s> a> CD Portland ingj pound. ) Natural 270 102 247 90 225 84 196 72 175 60 158 54 135 55 43 111. Amount of Gaging. ' — If a cement paste containing a moderate amount of water be insufficiently gaged, it will appear dry, when a more thorough working might make it plastic. Thus an insufficient gaging may make a cement appear quicker setting. It is also the case that when a cement is regaged after having begun to set, the second setting will take place more slowly; this, however, is a somewhat different matter. 112. The temperature and character of the medium in which the pat is kept during the setting process will have a decided influence on the rate of setting. This is clearly shown by the following table, given by M. TABLE 24 Time of Setting as Affected by Temperature of the Water and of the Medium in which Cement Sets Sample. TEMPERATURE Tjme Kequired to Of water at time of gaging. Of air (hiring setting. Begin to set. Set. Degrees C. Degrees C. Hr. Min. Hr. Min. 1 ! 1 47 11 10 10 20 2 23 2 I 1 5 30 8 8 16 10 52 5 13 5 3 12 20 3 \ 15 15 '0 43 o o 4 { 3.5 24 1 3 15 17 20 45 74 CEMENT AND CONCRETE Paul Alexandre/ from which it appears that different samples are affected in very different degrees. It is seen that the higher the temperature, the more rapid the setting. 113. At temperatures below 32° F. (0° C), setting seems to be entirely suspended. If a cement paste, which has been submitted to such low temperatures since gaging, is brought into a warm room, the setting process begins as though the mortar had just been gaged. It must not be concluded, however, that freezing has no evil effect on mortars. (See Art, 50.) 114. Setting in Air and Water. — A cement paste sets much quicker in air than in water. This is due to the percolation of water to the interior of the pat, when it is immersed as soon as made, being analogous to using an excess of water in gaging. When a pat sets in dry air, the evaporation of water from the surface hastens the hardening of that portion. If immersed directly after it has set in air, it re-softens, and this is also true of some briquets immersed when twenty-four hours old. The time of setting of cements that are to be deposited under water may well be tested in that medium, when they should be protected by a mold of some form to retain their shape. Ordinarily the time of setting should be tested in moist air. Cements are said to set more quickly in compressed air than in free air; this may be partially due to the higher temperature usually existing in the former. 115. Requirements as to Time of Setting. — What is desir- able as to time of setting will, of course, depend on the work in hand; certain purposes requiring that the cement shall be able to retain its shape soon after deposition, while in other cases ability to mix large quantities at a time, without fear of the cement setting before it is in place, may be very convenient. An extremely quick setting cement should be regarded with suspicion until it has proved itself of good quality. It is some- times stated that where a quick setting mortar is desired, nat- ural cement must be used, but this is not true; either Portland or natural may be found with almost any rate of setting de- sired. As a general rule, however, among cements that have been stored several months, the Portlands are slower settins;. 1 " Recherches Experimentales sur les Mortiers Hydrauliques ," par Paul Alexandre. SETTING OF CEMENT 75 Portland cement will ordinarily begin to set in from twenty minutes to six hours, and natural cement in from ten minutes to two hours, though there are many cements the time of setting of which is outside of these limits. 116. Conclusions. — The purpose aimed at in the test for time of setting will, to a certain extent, regulate the method to be employed. The pressure of the finger nail will be suf- ficient to determine (after a little experience) whether a cement will answer a certain purpose in this regard. But, if one is working to rigid specifications, or pursuing investigations as to the effect of different treatment on time of setting, it becomes very desirable to have a method of determining and defining the consistency of the mortar, and an accurate method of de- termining the rate of setting. In the author's experience, the Vicat consistency apparatus as modified by M. LeChatelier (see § 99) has proved unsatisfac- tory except for thin pastes of neat cement or mortars contain- ing less than two parts of sand. If the paste is not of such a consistency as to run freely into the ring, or "vase," an error may be introduced in the method of filling the latter. In oper- ating with a natural cement it was found that a neat paste, in which the water used was 32 per cent, of the dry cement, re- quired a gross weight of 640 grams to make the disc (1cm. diam- eter) penetrate midway in the vase; with 33 per cent, water, a weight of 410 grams was required; 34 per cent., about 250 grams; 35 per cent., 175 grams; 37 per cent., 155 grams. It would seem that some modification of this apparatus might be made which would not only indicate when a thin, neat cement paste has the assumed "normal" consistency, but which would also define the consistency of a given mortar, wmether of neat cement or of sand mixture. General Gilmore's wires are very simple, and will perhaps answer the purpose of obtaining the time of setting as well as any method in use. They can be used somewhat more accu- rately if the wires are made to slide vertically in a frame, than when held in the hand. The necessity of care in all of the details of this test, tem- perature and amount of water, amount of gaging, character of medium, etc., has been sufficiently emphasized in the preced- ing paragraphs. 76 CEMENT AND CONCRETE Art. 18. Constancy of Volume 117. That a cement should not contain within itself ele- ments which may lead to its destruction, is evidently a most important quality. It is probable that nearly all cements un- dergo a slight change in volume during induration, contracting in air and expanding in water. But it is the detection of those larger changes, which result from bad proportions or defective manufacture, and which cause deterioration or even complete disintegration, that is the object of the tests for soundness. 118. Causes of Unsoundness. — The most frequent cause of unsoundness is considered to be the presence of free lime or magnesia. (See §§49 and 50.) Any one of the following causes may account for the presence of free lime in cement: (1) An excessive percentage of lime may have been used in proportion- ing the raw materials; (2) the raw materials may not have been sufficiently mixed to render the mass homogeneous; (3) hard particles of lime, such as shells, may not have been ground fine enough in making the mix to permit them to enter into com- bination with the other ingredients during burning; or (4) the cement may have been underburned, so that part of the lime did not enter into combination. The particles of free lime which occur in cements are nat- urally rather difficult to slake on account of their impurity and the high temperature at which they have been calcined, and the same thing is probably true of magnesia. It may thus require weeks or months of exposure to the atmosphere to cor- rect tendencies to expand due to the presence of free lime or magnesia. Likewise when such defective cements are immersed in water of ordinary temperature, the expansion may not occur for a considerable period. This fact has led to the use of hot tests of various kinds to detect such faults, but before touch- ing on these so-called "accelerated tests," the ordinary cold- water test will be described. 119. TESTS FOR SOUNDNESS.— The Committee of the Amer- ican Society of Civil Engineers on a "Uniform System for Tests of Cement" recommended, in 1885, the following test for soundness: "Make two cakes of neat cement two or three inches in diameter, about one-half inch thick, with thin edges. One of these cakes, when hard enough, should be put in water and CONSTANCY OF VOLUME 77 examined from day to day to see if it becomes contorted, or if cracks show themselves at the edges, such contortions or cracks indicating that the cement is unfit for use at that time. In some cases the tendency to crack, if caused by the presence of too much unslaked lime, will disappear with age. The re- maining cake should be kept in air and its color observed, which for a good cement should be uniform throughout, yellowish blotches indicating a poor quality; the Portland cements being of a bluish-gray, and the natural cements being light or dark, according to the character of the rock of which they are made." For the ordinary cold test this method will probably give as valuable results as any of the forms that are suggested. 120. The German regulations require a very similar test, except that in the case of slow setting cements the pat is not immersed until twenty-four hours old. While a cement that is decidedly bad may show its defects in from one day to one week by this cold water test, it may be the case that cracks will ap- pear only after several months' immersion. It has therefore been proposed to hasten the destructive action of the free lime or magnesia by submitting the cakes of cement to steam, hot water, or dry heat. 121. The Kiln Test, recommended by Prof. Tetmajer in 1S90, consists in placing in an air bath, pats which have been kept in moist air for twenty-four hours; and then gradually raising the temperature of the air bath to 120° C. This temperature is maintained for at least one-half hour after the disengage- ment of steam has ceased. The pats should show no tendency to expand under this treatment, but if cements fail to pass the test, the results of the ordinary cold water treatment are to be awaited. This test is intended for cements that are to be used in air. 122. The Boiling Test, which was also recommended by Prof. Tetmajer, consists in placing the pats, twenty-four hours after made, in water of ordinary temperature, and gradually heating the water to bring it to the boiling point in about an hour; five or six hours in the boiling water should develop no defects. This is a severe test, and has been objected to on the ground that cements which have been well proportioned, but which are a trifle underburned, will fail to pass this test while giving good results in mortars to be used in the air. This test, how- 78 CEMENT AND CONCRETE ever, is steadily gaining in favor, and is used in many cement works as a test of quality. 123. The Warm Water Test. — Mr. H. Faija was an early experimenter in accelerated tests for soundness, and about 1882 he began the use of a "steamer," using a temperature of about 110° Fahr. After eleven years' use he still believed this tem- perature to be high enough to detect tendencies to expand in faulty cements. The apparatus 1 "consists of two vessels, one within the other, a water space being thus maintained between them, which assists in equalizing the temperature of the inner or working vessel." The latter is partially filled with water and is provided with a rack or shelf near the top. A ther- mometer is inserted through the cover of the inner vessel, and the water within is kept constantly at 110° Fahr. As soon as the pat is gaged, it is placed on the rack in the vapor, which will be at about 100° Fahr. After six or seven hours in this moist heat, the pat is immersed in the warm water. "In the course of twenty-four hours it is taken out and examined, and if then found to be quite hard and firmly attached to the glass, the cement may at once be pronounced sound and perfectly safe to use; if, however, the pat has come off the glass and shows cracks or friability on the edges, or is much curved on the under side, it may at once be decided that the cement in its present condition is not fit for use." Mr. Faija also recom- mended, in case of failure in the first test, that the cement be spread out in a thin layer for a few clays and a second test made. If the cement passes this second test, it is pronounced sound and fit for use after being stored a sufficient length of time. 124. The Hot Water Test. — The temperature to be used in accelerated tests for soundness is a point which has received much attention and is still under discussion. In 1890 M. Deval described a series of experiments he had made, in which he employed a temperature of 80° C. While this is much more severe than the temperature used by Mr. Faija, it is still mild in comparison to some temperatures that have been advocated. 125. Mr. W. W. Maclay, who was probably the first engi- neer in this country to introduce a hot test requirement in 1 "Portland Cement Testing," by H, Faija, Trans. A. S. C. E., Vol. xvii, p. 222. CONSTANCY OF VOLUME 79 specifications, gave the results of his experiments in a paper presented to the American Society of Civil Engineers in 1892. The method used "consists in molding six pats of pure cement and water, about one-half inch thick and about three inches in diameter, on thin glass plates, and of the same consistency as for the briquets for tensile strength." The treatment to which these pats are submitted is as follows: — No. 1, in steam (vapor) bath, temperature 195° to 200° F., as soon as made. No. 2, in same vapor bath when set hard (bear ^ inch wire weighing one pound). No. 3, ditto, after twice the length of time in air allowed the second pat. No. 4, ditto, after 24 hours. No. 5, in water of temperature about 60° F. when set hard. No. 6, kept in moist air at temperature of about 60° F. "The first four pats are each kept in the steam bath three hours, then immersed in water of a temperature of about 200° Fahr. for twenty-one hours each, when they are taken out and examined. To pass this test perfectly, all four pats, after being twenty-one hours in hot water, should, upon examination, show no swelling, cracks, nor distortions, and should adhere to the glass plates. The latter requirement, while it obtains with some cements nearly free from uncombined lime, is not insisted upon; the cracking, swelling and distortion of the pats being much the more important features of this test. The cracking or swelling of No. 1 pat alone can generally be disregarded." 126. Deval's Method. — Making tests of mortar briquets, which have been kept in hot water, seems to be the most rational accelerated test for soundness. This method was used in Ger- many several years ago, when it was claimed that a definite relation existed between the results thus obtained and the longer time cold water tests. This theory being disproved, threw dis- credit on the hot test, but M. Deval * has since made many experiments showing that it is of much value in detecting bad products. The method consists in making briquets with three parts sand to one of cement, and after twenty-four to seventy-two 1 "Hot Tests for Hydraulic Cements," M. Deval, Bull. Soc. d'Encourage- ment, etc., 1890, pp. 560-583. 80 CEMENT AND CONCRETE hours in moist air, according to the rate of setting, immersing them in water maintained at 80° C, the briquets being broken after an immersion of from two to seven days. These hot water briquets are to be compared with briquets stored in water of the ordinary temperature and broken at seven and twenty-eight days after immersion. 127. Among other tests M. Deval compared the results ob- tained with six samples of Portland cement as follows: — No. 1. Good finely ground cement of modern make. No. 2. Coarsely ground cement of good quality, but partially aerated. No. 3. Quick setting cement with low per cent, lime and lighter burn. No. 4. Made from clinker having property of disintegrating spontaneously while cooling; large proportion of inert material. No. 5. Under-burnt cement; contains free lime. No. 6. Over-limed cement. The results of the tests are given in the following table: — TABLE 25 Cold and Hot Tests on Six Samples of Portland Cement (M. Deval) Cement. Tensile Strength in Kilos per Sq. Cm. Cold. Hot. 7 clays. 28 days. 2 days. 7 days. 1 2 3 4 5 6 15.0 6.7 6.2 2.9 6.1 7.6 23.3 13.7 16.5 3.9 12.2 20.2 17.2 7.6 7.3 Disiiite 24.3 11.0 16.2 grated. No. 4, when allowed forty-eight hours to set, gave 3.2 kilos at two days, and 4.3 kilos at seven days, when tested hot. Among the cements which disintegrated in the hot water, the only one that gave a high result cold was No. 6, and this sam- ple, it is stated, would crack and swell badly even in cold water CONSTANCY OF VOLUME 81 if mixed neat. It is quite possible, however, that a sample might be found which, not having quite as flagrant defects as No. 6, would pass all the cold tests but be condemned by the hot test. 128. The conclusions drawn from these experiments have been stated as follows: — "(1) Tests made cold do not indicate the quality of the cement, inasmuch as cement containing excess of lime, and, in consequence, deplorably bad, may give excellent results." "(2) Portland cement of good quality, mixed with normal sand in the proportion of one to three, resists water at 80° C. Its strength at two and seven days after setting is about equal to that which it would have at seven and twenty-eight days in the cold." "(3) Poor cement containing much inert material does not resist the action of water at 80° C. unless the setting be allowed to proceed for some clays before immersion." "(4) Cements containing free lime do not withstand the ac- tion of water at 80° C. if immersed twenty-four hours after setting." Comparison of the strength hot and cold will suffice for the detection of even small quantities of free lime. 129. Before passing to the comparison of the tests for sound- ness already outlined, a few other tests which have been sug- gested for use may be briefly mentioned. The Chloride of Calcium Test depends on the fact that slak- ing of free lime is hastened by feeble solution of chloride of calcium. (See § 107.) Concerning this test, Prof. F. P. Spald- ing 1 says he "has found it to give true indications in a number of cases, including some unsound magnesian cements. It con- sists in mixing the mortar for the cakes with a solution of 40 grammes chloride of calcium to one liter of water, allowing them to set, immersing them in the same solution for twenty- four hours, and then examining them for checking and soften- ing as in other tests." 130. M. H. LeChatelier's Method. — The method recom- mended by M. H. LeChatelier for testing soundness requires the use of a cylindrical mold, about 1| inches in diameter and of about the same height, which is made of thin metal and 1 "Notes on the Testing and Use of Hydraulic Cement," by Fred. P. Spalding. 82 CEMENT AND CONCRETE slit along a generatrix. The mortar is to be placed in the mold as soon as made, and immersed at once in cold water; the mold is held firmly by a clamp, and a flat plate at either end of the mold retains the mortar in shape until set. When setting has taken place, the mold is undamped and the widen- ing of the slit indicates the expansion of the mortar. If de- sired, the swelling may be increased and hastened by transfer- ring the mold and its contents to hot water as, soon as the ce- ment is set. The same writer has suggested a modification of the hot test by placing briquets in cold water and gradually heating to near the boiling point, this temperature being main- tained for six hours. Various other tests have been suggested, such as the effect of regaging; withstanding immersion as soon as gaged; allow- ing large thin cakes to harden in air and striking them to obtain a musical sound. Most of these tests, however, are worthy of passing notice only. 131. Discussion. — There are but few experiments to show that a cement which will actually fail and disintegrate when properly used, may still pass the cold water neat pat test; yet there is no doubt that inferior cements may pass this test per- fectly, "inferior cements" being those which will not give the best results in practice, though they do not disintegrate. Cement is at present used in a very crude way, and it is only in exceptional cases that a poor quality of material may be detected in the completed structure. This is sufficient reason why so few failures can be found in cement work which may be attributed to a poor quality of cement. But in the more economical manner in which this material is, even now, begin- ning to be used, it is absolutely essential to know what its fu- ture behavior will be. That the cement will never be exposed to hot water in actual use, is a weak argument against hot water tests. It must be remembered that the chief object of testing cement is to arrange the various products in their true order of merit, and any system which will effect this result is perfectly legitimate. On the other hand, it is due to the man- ufacturers that a test which will occasionally reject perfect cements should not be adopted when it is possible in any other way to detect poor products with certainty. 132. It is possible that the temperature used and recom- CONSTANCY OF VOLUME 83 mended by Mr. Faija is sufficiently high to detect unsoundness or a tendency to "blow." It has never been clearly proved that it is not, but the higher temperature of 70° to 100° C. has appeared to meet with greater favor. The writer made a few experiments to compare results obtained with mixtures of Portland cement and lime when using the temperature of 110° Fahr. (43° C.) with those obtained in water at 190° Fahr. (8S° C), and in water at the ordinary temperature of 60° to 65° Fahr. (16° to 18° C). Quicklime, in proportions varying from one to ten per cent., was added to the cement, and seven pats were made from each mixture of cement and lime. These pats were subjected to the following treatment: — Pat No. 1, placed in vapor of water at 110° F. when made. 2, " " " 110° F. when set. 3, " " " 110° F. after 24 hours. 4, " " " 190° F. when made. 5, " " " 190° F. when set. 6, " " " 190° F. after 24 hours. Above six pats immersed in the hot water after three hours in vapor. Pat No. 7, placed in cool water when set. When no lime was added, pats 1, 2 and 3 revealed no defects; pats 4 and 5 showed small cracks in two days, but pat No. 6 still adhered to the glass after eight days. Pat No. 7 was perfect after two months. With 2 per cent, lime added to the cement, pat No. 1 was slightly warped and cracked, and Nos. 2 and 3 were off glass; Nos. 4 and 5 were cracked and warped; No. 6 was off glass, and No. 7 became detached from glass after two months, but was otherwise perfect. With 4 per cent, lime, all the pats failed, the one in cool water being off glass, cracked and warped after one day. It must be remembered that the free lime occurring in cement is of a different character from the quicklime added in these tests, because the former contains impurities and has been cal- cined at a very high temperature, and would therefore slake more slowly. It has been said that as small an amount as 1 per cent of free lime in cement is dangerous. If this is true, and it probably is, the temperature of 110° Fahr. would seem to be inadequate to quickly indicate a tendency to "blow." 84 CEMENT AND CONCRETE 133. Some of the results obtained by M. Deval have already been given (§ 127). Mr. Maclay made similar tests on several samples of Portland cement, using a temperature of 200° Fahr., but these tests only permit of comparing the strength acquired in cold water in seven and twenty-eight days with the strength in hot water at ages of from two to seven days. Long time tests, showing that the cements which give low results in hot water and normal results in cold water on short time tests, give in reality a low 'strength at the end of six months or more, have been almost entirely lacking until very recently. Table 40, § 226, gives some of the results obtained by the author in hot tests and long time cold tests on Portland cement. It is seen that the hot test at 80° C. indicated, in seven days, TABLE 26 Cold and Hot Tests on Samples of One Brand of Portland Cement Cement. Parts. Sand Date Made. 1894. Age. Tensile Strength. Briq uets Stored. Mo. Da. Moist air. Water. B' 2 4 10 5 da. 8 1 da. 80° O. 4 da. A 2 7 2 5 da. 235 1 " 4 " \Y 2 4 16 7 da. 13 1 " " (i " A 2 7 2 7 da. 221) 1 " " " B :; 7 2 7 da. 197 1 " 15 to 18° 0. 6 da. A '•> 7 2 7 da. 108 1 " 6 " B 3 7 2 28 da. 298 1 " " 27 " A 3 7 2 28 da. 11*8 1 " 27 " IV 2 4 16 7 mo. 411 1 " " 7 mo. A 2 7 2 6 mo. 405 1 " 6 " Behavior of Pats Made July 2, 1S94 No. 1 in No. 2 in No. 3 in No. 4 in Cement : Immersed in water cS()° C. after three hours in vapor. vapor, when held \# wire, vapor, when held 1# wire, tank, when held 1# wire, tank ; two hours after held 1# wire. A, No. 1 off glass in two days; No. 2 warped some in two days. A, No. 3 O.K. after twenty-one days; off glass and warped in fifty-two days. A, No. 4 loose on glass in twenty-one days; off glass and warped in fifty-two days. B, No. 1 off glass and warped some in two days; No. 2 entirely disintegrated in two days; No. 3 loose on glass in twenty- one days; off glass and warped in fifty-two days; No. 4 loose on glass in twenty-one days ; off glass and warped in fifty-two days. CONSTANCY OF VOLUME 85 the inferior quality of sample W, although it gave normal re- sults in cold water up to twenty-eight days; the two year tests with mortars containing two parts or more sand, show it to be inferior. If we attempt to carry the analogy too far, however, we fall into the error which placed the hot test in disrepute for several years, that is, we must not expect that the strength in cold water after a long time will be exactly proportional to the strength developed in hot water in a few days. 134. In Table 26 are given the results of tests by the author, on samples of a single brand of Portland cement. The por- tion marked "A" had been spread out in open air for seventy- seven days in a thin layer. The portion marked "B" was taken directly from the barrel July 2d, and B ' was taken from the same barrel April 16th. Samples B and B' are not identical, because the cement had undergone some change, though stored in the barrel. Each result is the mean of five briquets. In the short time cold tests there was nothing to indicate that the cement directly from the barrel was not good, except the very small evidence in the fact that pat No. 3 was loose on glass plate after twenty-one days. In fact, the cold water briquet tests at seven and twenty-eight days unmistakably declare in favor of the sample B. On the other hand, how sharply did the hot tests bring out the defects, two days in hot water being sufficient to entirely disintegrate one of the pats. Although sample B' showed a considerable tensile strength at seven months with two parts sand, yet the pats of neat cement failed, even in cold water, after two months, altogether too late a date to be of any value in preventing the use of the cement. 135. In a paper read before the American Society for Testing Materials, July, 1903/ Mr. W. P. Taylor of the City Testing Laboratory, Philadelphia, gives some very interesting data con- cerning the behavior of cements that failed to pass the boiling test. The method employed was to make cakes of cement in the form of a small egg, keep them in moist air for twenty-four hours, then place them in cold water which is gradually raised to the boiling point and maintained at that temperature for three hours. The results cited show that some unsound ce- 1 Proceedings Amer. Soc. for Testing Materials, 1903. 86 CEMENT AND CONCRETE merits may be much improved by sifting out the coarse parti- cles, and that a cement failing in the boiling test when fresh may pass it satisfactorily after four or five weeks. Examination of the results showed that 96 per cent, of a large number of specimens which did not pass the hot water test failed within three hours, and 99 per cent, in four hours. This fixes a practical limit to the time necessary to continue the test. Some very valuable tests are cited to show the ultimate failure in cold water of samples that failed in the hot tests. Ten cements which passed the cold water pat test of twenty-eight days' duration, but which failed in the boiling test above described, gave normal results in one-to-three mor- tars at twenty-eight days, showing a tensile strength of 217 to 252 pounds per square inch, but gave only 47 to 147 lbs. per square inch at four months. Another valuable comparison is given by Mr. Taylor: A compilation of data, covering over a thousand tests on many varieties of cements, showed that "of those samples that failed in the boiling test but remained sound at twenty-eight clays (in cold water), 3 per cent, of the normal pats showed checking or abnormal curvature in two months; 7 per cent, in three months; 10 per cent, in four months; 26 per cent, in six months and 48 per cent, in one year; and of these same samples, 37 per cent, showed a falling off in tensile strength in two months; 39 per cent, in three months; 52 per cent, in four months; 63 per cent. in six months and 71 per cent, in one year." 136. It may be of interest to introduce here some of the opinions that have been expressed concerning hot tests. M. Candlot * says that cements of normal composition, the burning of which has not been carried to the point of vitrification, would be condemned by the hot test of neat cement, although mor- tars made with them show no signs of alteration in sea water, and, when preserved in air, give entirely satisfactory results. Referring to the tests of one-to-three mortar briquets in water at 80° C, he considers that "cements containing free lime give in hot water, lower resistances than in cold water; cements of good quality give resistances at least equal and nearly always greater in hot water than in cold. Cements well proportioned 'Ciments et Chaux Tlydrauliques," par il/. Candlot, pp. 144—145. CONSTANCY OF VOLUME 87 and homogeneous, but not having obtained the maximum burn- ing, give satisfactory results with this test." In using the slit cylinders mentioned in § 130, M. H. Le Chatelier found * that the addition of 5 per cent, of lime could be detected by cold tests in a few hours, while 5 per cent, of magnesia could not be detected in twenty-eight days. The cement containing 5 per cent, lime disintegrated almost at once in hot water, while the sample to which 5 per cent, of mag- nesia had been added, swelled considerably in one day. Mr. A. Marichal 2 found that "the percentage of water en- tered in combination, after ten days in hot water, was the same as for six months in cold water, and that the strength of the cement was increasing with the amount of water entered in combination. It was discovered incidentally, that cement con- taining over 5 per cent, of magnesia, or 3 per cent, of uncom- bined lime, would not stand the boiling test." 137. Hot Tests for Natural Cements. — All that has pre- ceded concerning hot tests refers to their use for testing Port- land cements. Very little is known concerning the value of hot tests for natural cements. There are comparatively few natural cements that are absolutely bad, but to distinguish between the first and second quality of this variety of products is much more difficult than to make a similar distinction with Portlands. One point is certain, natural cements must not be expected to with- stand boiling water. Mr. de Smedt experimented with fifteen brands of natural cement, and found that thirteen of them went to pieces in boiling water in two hours, although none of them was thought to contain caustic lime. Prof. Tetrnajer has stated that for Roman cements, boiling water, or even 75° C, is not at all conclusive, and recommends 50° C. for trial, but our natural cements are not strictly comparable with Roman ce- ments. 138. The author has experimented with three temperatures, namely, 50°, 60°, and 80° C, and is inclined to consider that 80° C. is likely to give the most useful information for sand mortar briquets but not for neat cement pastes. Table 41, § 227, gives the results of hot briquet tests on six brands of 1 " Tests Hydr. Materials," by H. LeChatelier. 2 Trans. Amer. Soc. C. E., Vol. xxvii, p. 438. 88 CEMENT AND CONCRETE natural cement. It is seen that, with two parts sand, brands Jn, Hn, and Bn, give very low results at 80° C, and these brands are really inferior cements as shown by the two-year cold tests. Brand Jn is the only one that gave a lower result at seven days than at five days when tested at 80° C, and this brand failed entirely at two years, though it gives normal results in cold water up to six months. Neat cement pats of this brand, after being stored in cold water for nearly one year, were found to be cracked, although they had been perfect after one month in cold water. It was also found that neat cement pats of this brand warped and cracked in two days when placed in water of 60° C. when set. 139. CONCLUSIONS. — It may be said that although the limits within which the hot tests are reliable have not been well established, and although a strict adherence to them may at times reject a usable product, yet it is believed that sufficient experiments have been made to indicate that they are of much value, and should be made in all cases where the quality of the cement is of high importance. The present indications seem to be that Portland cements may well be tested in the form of neat cement pats and sand mortar briquets at a temperature of about 80° C. Natural ce- ments in the form of neat paste should not be called upon to resist a temperature above 60° C, but 80° C. will probably give the most useful information with sand mortars. In either case, the mortar should be allowed to set in moist air of ordinary temperature, then transferred to the vapor, to remain two or three hours before immersion in the hot water. It is not rec- ommended that these hot tests should replace the ordinary cold tests, but simply that in cases where the extra work in- volved is not prohibitive, the hot tests should be made in con- nection with the cold tests. CHAPTER VIII TESTS OF THE STRENGTH OF CEMENT IN COMPRES- SION, ADHESION, ETC. 140. In testing the strength of cement the object is three- fold : 1st, to obtain an idea of the strength that may be ex- pected from the cement as used in the structure; 2d, to obtain a basis for comparing the value of different cements in this regard ; and 3d, to determine the ability of the cement to withstand destructive agencies, whether these agencies be due to exterior causes or emanate from the character of the cement itself. To illustrate the last point it is only necessary to mention such de- stroying agents as free lime (interior) and frost (exterior). It is evident that the stronger the cement the more effectually will these agencies be resisted. The strength of cement may be tested by compression, shearing, bending, adhesion, abrasion and tension. The tensile test is the one most frequently used, but the tests will be con- sidered in the order named. Art. 19. Tests in Compression and Shearing 141. Value of Test. — In practically all forms of masonry- construction, cement is called upon to resist compression. In consequence of this fact, the opinion is somewhat general that the greatest amount of information would be obtained by com- pressive tests. But the compressive strength of cement is so much greater than its tensile strength, that when failures occur, they are likely to be due to other forms of stress. In short, the ratio of the compressive strength to the crushing force it is likely to be called upon to resist, is usually much greater than the corresponding ratio in tensile strength. 142. There is no doubt that compressive tests are of much interest and value, especially so since the use of concrete and steel in combination has become general, but as yet the facili- ties, for making the test are not available without considerable expense. This is on account of the larger force required (the 89 90 CEMENT AND CONCRETE compressive strength being six to ten times the tensile) and be- cause the uniform distribution of the stress over the surface of the specimen, and the accurate recording of the force exerted, are even more difficult than the corresponding operations in tensile tests. Prof. Sondericker, 1 in a paper read before the Boston Society of Civil Engineers, describes an apparatus in which he seems to have overcome a part of these difficulties. A convenient specimen for compressive tests is a cube meas- uring two inches on a side. The specimens are prepared and treated in the same way as briquets for tensile tests. Before testing, two opposite faces of the cubes are usually ground so as to be true planes, parallel to each other, or the opposite sides may be faced with plaster of Paris, though this is not recom- mended. Grinding two surfaces to true planes increases very much the work involved in testing, so that several tensile tests may be made in the time required to make one compressive test. 143. Conclusions. — Although tests of compressive strength are of interest from a scientific point of view, it is not considered that they would give much greater information concerning the relative qualities of cements than is given by tensile tests, and therefore they need not be included in an ordinary series of acceptance tests. 144. Tests of Shearing Strength. — Although cement is' fre- quently called upon to withstand a shearing stress, tests of this kind are very seldom made. Some of the difficulties encoun- tered in compressive tests are also present in tests of shearing. Prof. Cecil B. Smith made quite an extended series of shearing tests by cementing together three bricks, the middle one pro- jecting above the other two, and the pressure being so applied as to avoid any transverse stress. It is evident that by this method the adhesive strength is also brought into play. Shear- ing tests need not be included in normal tests of quality. Art. 20. Tests of Transverse Strength 145. It is probable that the earliest rupture tests of cement were made by submitting rectangular prisms to a bending stress; but such tests have long held a place subordinate to trials of tensile strength. A mass of masonry, taken as a whole, is very apt to be subjected to a bending stress, but it is a ques- 1 Jour. Assoc. Engr. Soc, Vol. vii, p. 212. TRANSVERSE TESTS 91 tion whether a transverse test on a small specimen gives any better idea of the ability of a large beam to carry its load, than do simple tensile and compressive tests. In Engineering News of December 14, 1893, appeared an article giving the comparative results obtained in tensile and transverse tests. The tensile specimens had an area of one square inch at the smallest place, and the transverse specimens also had an area of cross-section of one square inch. It was found that the modulus of rupture computed by the common , , . 3WI formula/ = ^- 2 was from 1.1 to 3.8 times the tensile strength developed by the briquets. Some comparative tests made at St. Mary's Falls Canal are discussed in Art. 56. 146. The objections to transverse tests are: 1st, if the speci- mens are made but one inch in cross-section, it is difficult to handle them without injuring them, and if the section is made much larger than one inch square, a much greater amount of cement is required to make the specimens and more room re- quired to store them; 2d, it would seem that the results ob- tained might be less trustworthy than those in tensile tests because of the greater influence of the outside layers, which are subjected to the greatest accidental variations, on the apparent strength of the specimen. On the other hand, it may be said that, when no testing machine is at hand, the apparatus requi- site to make a crude test may easily be improvised. All that is required is a rectangular wooden mold, three knife edges, and a pail with a quantity of sand or water. 147. When it is a question of making tests of transverse strength accurately and rapidly, the apparatus required is no more simple than the apparatus for tensile tests. In the con- struction of metal molds in large quantities it makes little dif- ference whether the form requires curved or straight lines. As far as breaking is concerned, there is a certain force to be applied, and a machine that will answer for one test may also be used for the other. In the matter of clips, there may be a slight advantage as to simplicity in a clip designed for transverse breaking. In making transverse tests the author has used a form two inches square and eight inches long. By placing the end sup- ports five and one-third inches apart, the modulus of rupture 92 CEMENT AND CONCRETE SWl by the formula / = , , 2 becomes equal to W, the center load applied. 148. Finally, it may be said that there is little objection to substituting transverse tests for tensile tests, although no evi- dent advantage would be gained. It would also seem that there is no object in making tests for quality by both trans- verse and tensile tests, though from a scientific standpoint comparative tests of transverse and tensile strength are of great interest. Art. 21. Tests of Adhesion and Abrasion 149. ADHESION. — The test for adhesion is also one of long standing, being used during that time when engineers were con- tent with an approximate idea of what might be expected of an hydraulic product. It has been stated above that when failure occurs in a mass of masonry, it is more frequently a failure in tension than in compression; it may be added, that it is also more likely to fail in adhesion than in cohesion. Hence, an adhesive test is a very proper one to make, and will give most valuable results. In fact, it is perhaps the most rational rup- ture test, and were it not for the difficulties involved in its ap- plication, it would doubtless come into general use. 150. One of the greatest difficulties experienced in making adhesive tests is the preparation of the specimens of stone or other material to which the mortar is to adhere. In early ex- periments common brick were used, or pieces of stone were cut to the same shape as brick, and two or more pieces cemented together. In later methods the flat surfaces of two specimens are sometimes joined with their axes at right angles, thus mak- ing the cemented surface square. The upper brick being held on two supports, a load is applied to the lower brick. 151. Mr. I. J. Mann, in a paper presented to the Institution of Civil Engineers, 1 described a method of testing adhesion in which are used test pieces 1^ inches long by 1 inch wide by \ to | inch thick. These are cemented together in a cruciform shape, and a simple spring balance machine with properly arranged levers pulls them apart. The upper block is supported at its ends and an inverted U-shaped piece bears upon the ends of Proo. last. C. E., Vol. lxxi, p. 251. TRANSVERSE TESTS 93 the lower block. The stress is applied through a conical shaped pivot bearing on the U-shaped saddle. Mr. Mann states that test pieces may be made either of plate glass or close grained limestone, the latter being sawn into pieces of the right size. 152. Another method is to make test pieces to fit one end of the mold used for tensile tests, and after placing the piece of stone in the mold, to fill the other end with the mortar to be tested. The objection to this method is the expense of pre- paring pieces of this form. It has been suggested to substitute artificial stone for the cut stone samples. Thus, suppose it is required to test the adhesion of a certain mortar to granite: mold half briquets of a mixture of ground granite with cement, and after these have well hardened, replace them in the mold and fill the other end of the mold with the mortar m be used. It is quite certain that the same result would not be obtained in this way as though the specimens were cut from a piece of solid granite. 153. One of the simplest methods of applying this test is one which the author has used for some time. The test pieces are in the form of flat plates one inch square and one-fourth inch or less in thickness. These plates being placed in the center of a briquet mold, the ends of the mold are filled with mortar. The plates may be improved by cutting shallow grooves in two opposite sides to make a more perfect fit with the sides of the mold. This may easily be done with a round file. Besides the simple form of the test pieces and consequent ease of making them, this method has the further advantage that a test may be made almost as readily and accurately as a tensile test of cohesion. Also, since the adhesive area is one square inch, the results may be compared with cohesive tests on specimens having the same area of cross-section. 154. The experiments on adhesive strength made by Mr. Mann were probably more extensive than any others published. His results are useful mainly as showing the lack of cementitious properties in the coarser grains of cement, and this point he proves very clearly by quite a large number of experiments. It was also developed that cement that had been rendered slow setting by aeration or "cooling" gave a lower adhesive strength than samples directly from the makers, which set more rapidly. But the method followed by Mr. Mann, of immersing the speci- 94 CEMENT AND CONCRETE mens as soon as cemented together, may have had something to do with this result; the quicker setting samples would earlier resist the injurious action which is likely to follow the immer- sion of such small quantities of mortar before they have set. 155. All of the things which influence the results in testing the cohesive strength must also be considered as affecting the adhesive test. The consistency of the mortar, the method of gaging, the pressure applied in cementing the specimens, and the conditions of storage until the time of breaking, will all have an influence on the result obtained. In addition to these, the character of the samples as to the kind of stone used, its structure, the physical condition of the surfaces, etc., must all be considered. It is therefore clear that many difficulties must be met before the test for adhesion can ever be included in standard tests. 156. Special tests directed toward ascertaining the compara- tive adhesion of cement to different varieties of stone, the effect of the various differences in manipulation, the comparative ad- hesion of mortars containing various proportions of sand, etc., are of undoubted value. But, before the adhesive test can be considered a normal one for cement, much of this experimental work will be required. The results of a number of adhesive tests made under the author's direction are given in Art. 51. 157. TESTS OF ABRASION. — Abrasion tests of cement are not at all common, and for the ordinary uses to which cement is put, its resistance to such action is of little interest except as it may imply other kinds of strength. Occasionally, however, it may be desired to have a mortar which will withstand wear, as, for instance, in making concrete walk. In such cases, tests for resistance to abrasion have some interest and value. The test is usually made on a sample prepared as for' tensile or compressive tests, by submitting it to the wearing action of an emery or grindstone, or a cast iron disc covered with sand. The number of revolutions of the stone or disc is recorded, automatically if possible, and the loss of weight is determined after a given number of revolutions. A few tests of this kind made at St. Mary's Falls Canal are given in Art. 58. CHAPTER IX TENSILE TESTS OF COHESION 158. The testing of cement by applying tensile stress to a previously prepared briquet, containing definite proportions of cement and water, or of cement, sand and water, is the strength test which is now in most general use. The value of this method in comparison with that of other forms of rupture tests has al- ready been briefly discussed. That cement fails oftener in tension than in compression is one reason for preferring the tensile test. Its ready applica- bility is a still more important point in its favor. Art. 22. Sand for Tests 159. Whether the tensile test should be applied to neat ce- ment briquets or to those prepared from sand mortars has been a disputed point, but there are now but few authorities who recommend the use of the neat test exclusively. When tests for soundness are not carefully made, the behavior of the cement in neat briquets gives, perhaps, a better idea as to the reliability of the cement than do sand tests, but otherwise the sand test is a better index of the value of the cement. The principal ob- jection to the sand test is that the use of sand introduces another cause of variation in the results obtained by different experi- menters. This objection has considerable weight, because it is impracticable to find sand in widely separated localities which is absolutely the same in composition and physical properties; but two cements which appear to be of equal value when tested neat may exhibit quite different characteristics when used with sand, and it is believed that this fact far outweighs the objec- tion noted. As soon as regularity in sieves is established, the size of the sand grains may be regulated. The chemical and physical properties of the sand and the shape of the grains is a more difficult matter. The crushed quartz that is used in the manufacture of sandpaper was recommended by the Committee 95 96 CEMENT AND CONCRETE of the American Society of Civil Engineers of 1885, and if some care is taken to select that which is clean and made from pure quartz, there is little difficulty in obtaining a uniform product of this kind. 160. The German Normal Sand is obtained by washing and drying a natural quartz sand. In various parts of Germany sand answering the purpose may be found. Some tests made in this country to compare the "normal" German sand with American crushed quartz have shown the sand to give a some- what higher strength, while other tests have shown an opposite result. 1 A few of these tests are given in Table 27. TABLE 27 Results Obtained with German "Normal" and American aid " Sand in Three Laboratories Stand- Sand. Age. Days. Strength of Mortar, 1 Cement, 3 Sand, Obtained at Laboratory Number 3 4 5 7 7 28 28 218 253 317 334 173 219 341 300 201 211 281 283 Per Cent, of Water Used. 8 9 10 Mr. Max Gary has stated that "the Russian standard sand gives markedly lower, and the Swiss sand considerably higher, strength than the German." 161. Tests with Natural Sand. — It is not to be concluded from what has preceded that one must make mortar tests with a "standard" sand only. On the contrary, one may obtain valuable results by using in tests the sand which it is proposed to use on the work. The only point to be insisted upon is that a cement shall not be rejected on account of the poor quality of the sand used in testing. It is thus very desirable that a certain proportion of the tests be made with a pure quartz sand, and by making parallel tests with the natural sands, the Article by Clifford Richardson, Engineering Record, Aug. 4, 1894. MAKING BRIQUETS 97 coefficient of the latter may be obtained. In any case it is necessary, in order to obtain comparable results, to sift the sand used for tests. 162. Fineness of Sand for Tests. — The American practice in using crushed quartz is to reject the coarser particles by a sieve having 20 meshes per linear inch (holes about .03 inch square) and to reject the finer particles by a sieve of 30 meshes per linear inch (holes about .02 inch square). The size of grain of German normal sand is practically the same. In using a natural sand it is not necessary to use this size of grain, but it is better to do so, or at least to use some definite size or definite combination of sizes; as, for instance, one-half of 20 to 30 (passing holes .03 inch square and not passing holes .02 inch square) and one-half 30 to 50 (passing holes .02 inch square and failing to pass holes .012 inch square). Such a method will permit of duplicating a given size of grain at any time, while if the sand is used as it occurs in nature, considerable variations will be found. The effect of the quality of sand on the strength obtained is dis- cussed in Chapter XL Art. 23. Making Briquets 163. Proportions. — The proportions of the ingredients should always be determined by weight rather than by measure. It will be found more convenient to use metric weights for the dry ingredients. The water should then be measured in cubic centimeters, which is equivalent to weighing it in grams. The proportion of sand to be used for mortar briquets will depend upon circumstances, but for short time (seven day) tests good results are not usually obtained with natural cement if more than two parts of sand by weight are added to one part of ce- ment. Portland cement may be tested at seven days with three parts of sand to one of cement. If too large a proportion of sand is used, the briquets are liable to be injured in handling, and very low strengths are not as accurately recorded by the testing machine. 164. CONSISTENCY: DETERMINATION. — The consistency of the mortar has such a marked influence on the strength obtained that its importance can hardly be overestimated. The difficul- ties attendant upon specifying the consistency of a given mortar have already been touched upon in § 116. The Committee of 98 CEMENT AND CONCRETE the American .Society of Civil Engineers of 1885 recommended the use of a "stiff plastic" mortar, but this phrase has had va- rious interpretations. The present Committee in its progress report 1 recommended the use of the Vicat apparatus: "In making the determination, 500 gr. (17.64 oz.) of cement are kneaded into a paste, and quickly formed into a ball with the hands, completing the oper- ation by tossing it six times from one hand- to the other, main- tained six inches apart; the ball is then pressed into the rubber ring (§ 98) through the larger opening, smoothed off, and placed (on its large end) on a glass plate, and the smaller end smoothed off with a trowel; the paste, confined in the ring resting on the plate, is placed under the rod bearing the cylinder, which is brought in contact with the surface and quickly released. The paste is of normal consistency when the cylinder (1 cm. in di- ameter and loaded to weigh 300 grams) penetrates to a point in the mass 10 mm. (0.39 in.) below the top of the ring. Great care must be taken to fill the ring exactly to the top." The following simple test taken from French specifications will determine a good consistency of mortar to use for briquets. It should be capable of being easily molded into a ball in the hands, and when dropped from a height of one and a half feet on a hard slab, this ball should retain its rounded form without cracking. The mortar should also leave the trowel clean when allowed to drop from it. Were a smaller quantity of water used, the mortar would be crumbly and the ball would crack when dropped on the slab, while a larger amount of water would cause the mortar to adhere to the trowel and the ball would be flattened by striking the slab. 165. Another method of determining the proper consistency, which the author believes will prove very satisfactory, is to make several batches of mortar containing the same weights of cement and sand, but having different percentages of water. As each batch is mixed, the volume of the resulting mortar is measured by pressing it lightly into a metal cylinder (a small tin pail will answer the purpose), taking pains to fill the cylinder in the same manner each time. That batch of mortar which 1 Proc. Amer. Soc. C. E., Jan. 1903; also Engineering News, Jan. 29, 1903, and Engineering Record, Jan. 31, 1903. CONSISTENCY OF MORTAR 99 occupies the least volume, when thus lightly packed, is the one in which the amount of water used is most nearly correct. Should either the mortar which contained the least water or that which contained the most water chance to have the least measured volume, then more trials must be made until such a consistency is obtained that either more or less water will in- crease the bulk of the mortar. This method will give a con- sistency somewhat more moist than that which gives the highest results on short time cohesive tests, but it is believed that where briquets are made by hand, more uniform results will be ob- tained when the mortar is a trifle moist. This method is not suited to daily use, as it requires too much time, but is valuable as a check on one's ideas of proper consistency. 166. EFFECT OF CONSISTENCY ON TENSILE STRENGTH.— Tables 28 and 29 give a few of the results obtained by the author TABLE 28 Variations in Consistency of Mortar. — Effect on Tensile Strength, Neat Natural Cement Tensile Strength , Pounds per Square Ixih. Cem ENT. Water Used Expressed as Age of Per Cent, of Dry Ingredients by Weight. Briquets. Brand. Sample. 25% 30% 35% 40% 45% Gn 83 R 7 clays 1136 205d 122/ "f2g 51A Gn 84 R EC 926 72cZ 58/ 54f/ 39ft An G " 162c 165e 108/ 75c/ 54A An N ft 1526 194c 204e 134/ 79c/ Hn 26 S " 226d 176/ 99c/ 567i 35 i Gn 83 R 28 clays 1896 2Ud 211/ 182c/ 135/i Gn 84 R u 1496 168d 114/ 107p 108/i An G u 210c 228e 165/ 102c/ 80/i An N 4 t 1736 286c 254e 208/ 150c; Hn 26 S t< 333cZ 309/ 217c; 1217i 89 i Ln 31 S 1 day 162c 148e 97/ 63(7 S6h Ln " 7 clays 178c 177e 124/ 71c/ 45/i Ln " 28 clays 207c 257e 202/ 140(7 88A Ln a 3 mos. 300c 389e 333/ 264(7 197/4 Significance of Letters a — barely damp. 6 — very dry ; no moisture shown on surface briquets. c — dry ; slight moisture shown on surface briquets. d — trifle dry. e — about right consistency. / — trifle moist. g — moist. h — very moist; would just hold shape. % — extremely moist ; would not hold shape. Lof 100 CEMENT AND CONCRETE in tests to determine the effect of consistency on the tensile strength of natural cement mortars. All of the briquets were made in the usual manner and stored in fresh water until time of breaking. Each result given is the mean of from two to ten briquets. The letters affixed to each result indicate the degree of moisture which the mortar appeared to have when mixed, varying from "a," barely damp, to "i," so wet that the mortar could not hold its shape when laid on a glass slab. The results in Table 28 were obtained with neat cement mor- tars of several brands of natural cement. The first point to be noted is the variation in the amount of water required by dif- ferent samples to give the same consistency; thus, Brand An, sample N, when mixed with 35 per cent, water, appeared to have about the same consistency as did sample G of the same brand mixed with 30 per cent. It is also apparent that the strength of all samples is not affected alike by given variations in the amount of water used in mixing; comparing the results obtained when 45 per cent, water is used with that given when 25 per cent, water is used, it is seen that at seven clays the wet mortar gives 42 per cent, of the strength obtained with dry mor- tar for sample 84 R, Brand Gn, while with the sample of Brand Hn the strength of the wet mortar briquet is but 16 per cent. of that given by the dry mortar. Of the six samples tested at seven days and twenty-eight days, three gave the highest strength at seven days when mixed with 25 per cent, water, and five gave the highest strength at twenty-eight days when 30 per cent, water was used. The results on Brand Ln show the greater proportionate gain with age of the wet briquets. Table 29 shows similar results for mortars made with one, two and three parts sand. With one part sand the wet mortar made from Gn, 21 R, which gave but 22 pounds per square inch at seven days, gave 429 pounds, or nearly the highest strength, at six months. A similar result is shown for sample 15 R of the same brand when mixed with two parts sand, the highest strength at one year and two years being given by the mortar containing the greatest per cent, of water. That mor- tars containing three parts sand to one cement may be more easily damaged by an excess of water, is indicated by the re- sults on Brand Ln in this table. 167. The effect on the strength of Portland cement mortars, CONSISTENCY OF MORTAR 101 10 in o CO H o £ IN o N U V [fl 4-1 w a SO of o JO a 0J •- c a Oi co t« oi cm 1— I UO O TjH ess <55 c*> cs> ►«; i-s; i^S -si CS i— ( ■* © Oi CO CO i— l X'OlOO © © CM 00 co co co i— i co co cq **-.m^«<^m^ S 22 S S «■+©» 00 t-i Oi CO N00[,h O U tj Cj i-i iO CM t- to cq o) t© i-i (M CO CO 0> Q3 (^ Qi OfflOO IMOOO i— l CO CO CO «) ID (» H) CO t~ 00 t^ CD CO 00 00 i-i oq * CO rr> o ™ * "° rt « 3 cj ft 00 CO o CO 0> 5 QQ 5 05 co u Dry . . Ordinary . Wet . . 14 22 30 31 25 16 56 46 35 73 74 55 77 116 89 69 153 126 67 170 136 88 162 180 Disintegrated 190 189 From " Recherches Experiment ales sur Les Mortiers Hydrauliques," par M. Paul Alexandre, Annales des Ponts et Chaussees, Sept., 1890 It is seen that the highest strength at three days and seven days is given by the dry est mortar, at twenty-eight clays to two years by that of the ordinary consistency, and at three years by that containing the highest per cent, of water. All of the sam- ples exhibited white spots in the broken section at three years, and at four years the dry mortar briquets had lost their cohe- rence on account of their porosity permitting the sea-water to permeate them. 168. Conclusions. — It may be concluded, then, that the consistency of the mortar has a very marked effect on the ten- sile strength obtained; that different samples of cement are not affected in the same degree by given variations in consistency ; that the effect of consistency is usually shown most plainly in short time tests; and that while the dryer or stiff er mortars give the highest results on short time tests, the moist mortars attain a greater strength after a certain time. 1 "Records of Tests of Cement made for Boston Main Drainage Works. : Trans. A. S. C. E., Vol. xiv. 2 Annates des Ponts et Chaussees, Sept., 1890. TEMPERATURE 103 169. Temperature of the Ingredients and of the Air where the Briquets are Made. — The temperature of the mortar and of the air in which the briquets are prepared is a matter of some moment. In 1877, Mr. Maclay 1 reported a series of experi- ments on Portland cements from which conclusions may be drawn concerning the effects of the temperature of the mortar. These experiments indicate that mortar having a temperature of 40° Fahr. when gaged, will attain greater strength in from seven days to three weeks than a mortar having an initial temperature of 70° Fahr. One is most likely to work some- where between these two temperatures, but it may be mentioned that according to Mr. Maclay's experiments, it appears that mortars gaged at a temperature of 90° or 100° Fahr. also at- tain a higher strength than those gaged at 70° Fahr. Similar experiments made by M. Candlot 2 indicate that mor- tars gaged with cold water give but feeble resistance at first, but in from two weeks to one month, such mortars surpass in strength those gaged with warm water. M. P. Alexandre 3 im- mersed some briquets at a temperature of about 90° C. (194° Fahr.) for forty-eight hours and then at 15° to 18° C. (60° to 65° Fahr.) until broken, while other briquets were maintained at the latter temperature from the time of molding. The bri- quets that were broken at the age of four days showed that the highest strength had been obtained by the briquets which had been kept hot for forty-eight hours, but at twenty-eight days and three months those briquets which had not been sub- jected to this high temperature gave the highest strength. 170. Table 31 gives a few of the many experiments on this point made under the author's direction. It appears that the briquets made in a low temperature (34° to 37° Fahr.) are usu- ally stronger than those made in the ordinary temperature of 65° to 68° Fahr. In some cases the difference was not very great, and in some of the tests the briquets made in the ordi- nary temperature gave higher results at one day and seven days than those made in the cold; but at twenty-eight days the cold-made briquets were nearly always in the lead, and in many 1 "Notes and Experiments on the Use and Testing of Portland Cement," Trans. A. S. C. E., Vol. vi, p. 311. 2 "Ciments et Chaux Hydrauliques." 3 "Les Mortiers Hydrauliques." 104 CEMENT AND CONCRETE (D 0) g -=1 £ M * <% mh cd o S 0) Eh • • • -H • ■ S - PI°0 . CO • • . N . . co 1* • t- • • 1-1 OTJBjW . CO • • 00 N • © 1 •ptoa CO i-H . N i* CO . *o w o 8 CO iN • © % •UI.I'BAY -* i-H . I— M o "* CO . -* MOOOhjm 'PPO © -* hH -* -* i-i CO N CO !M t^ O (B S CO o 1 CONOCO •hub AY h+i 00 hH O © CO ffl CO (NnWNl-^ 1 ^ I © t)<00-*COh - f •ppo O O h H 't CO NNNH IT- CO § © CO iO iO OS iN CO •Ul.I'B U i-i © 00 CO i-H -f h M N N N i-H t~ CO © i-H r- • CO i-l C. •ppo (N3t- ■ t~ lO £ >i CN i-h . © IN IS bfl cS b- CO t— • lO r-H 02 l - • "U^AY co »© © • r H i-l T-H . © N H • • • • i^ • Jq w ■ppo . i- . . . "O . H H • • • • lO • CO •UWBjW . . . . o • . . . . © . # 00 e.xaqAV J IV rl T3 !H © CO *0 s s ^3 II II g -S^S ft o3 o3 £3+ "— -^ half inch in length, having an inclination of nearly 45 degrees with the axis of the briquet. The total length of the briquet is four inches, the ends be- ing formed by straight lines tangent to the curves forming the corners. If the clip is so formed that the gripping points bear at the centers of the one- half inch tangents form- ing the sides of the briquet, the distance between op- posite gripping points will be If inches. 180. Comparison with other Forms. — Compar- ing this briquet with the forms in common use, the German and the form shown in Fig. 5 both have an area between opposite gripping points about If times the area of the smallest section, but in the form shown in Fig. 4 this ratio is too small to fulfill the second specification. The unequal distribution of stress over the breaking section of the briquet has already been mentioned as a probable partial cause why briquets of small cross-section show a greater strength Fig. -FORM OF BRIQUET SUGGESTED FOR USE 112 CEMENT AND CONCRETE per unit area than those having a larger area of cross-section. In Johnson's "Materials of Engineering" is given the theory of the distribution of stress over the breaking section of a briquet, as developed by M. Durand-Claye, and published in Annates des Ponts et Chaussces of June, 1895. Applying the formulas there given to three styles of briquet, the A. S. C. E. form of 1885 the German standard, and the form shown in Fig. 5, it is found that the ratios of the maximum stress to the mean stress are, for the three forms respectively, 1.54, 1.52 and 1.22. From a theoretical point of view, this means that with a total pull of 100 pounds on each briquet, the outer fiber of the briquet shown in Fig. 4 would be subjected to a stress of 154 pounds per square inch, while with the form suggested above, the stress on the outer fiber would be but 122 pounds per square inch ; briquets of the latter form should, therefore, theoretically, show a breaking strength 1.27 times the strength given by briquets of the same mortar made in the A. S. C. E. form of 1885. The German form has too sharp a curve at the sides to fulfill the fourth requirement given above. All of the forms comply with the first, fifth and sixth requirements. As to the volume of the briquet, the author's form having a total length of four inches, has about 50 per cent, greater volume than the A. S. C. E. form of 1885. 181. MOLDS. — In the early tests of cement, wooden molds were employed, but they absorb water from the mortar and soon warp out of shape. Iron molds have also been used to a considerable extent, but these are apt to become rusted if not in constant use. Brass, bronze or some similar metal not easily corroded should be used, and molds of this character can be obtained of dealers in testing apparatus. The molds may be made single, or in "nests" or "gangs" of three to five. The two halves of the mold may be entirely separable, or may be hinged at one end and fastened by a clip at the other end. The gang molds are somewhat cheaper than the single ones. The hinged molds and those held with patent clip are rather difficult to clean, while the gang molds, if made heavy enough to prevent spreading, are unwieldy, and briquets are removed from them with greater difficulty than from the single molds. It is considered, therefore, that the most con- MOLDING BRIQUETS 113 venienfc form is the single mold, in which the two halves are held together by a screw clamp of simple design. 182. To clean these molds, place ten in a row with clamps removed ; scrape the upper faces with a piece of zinc, brush with a stiff "horse-brush," and wipe with oily waste. Turn them over and repeat the process. Then separate the two halves of each mold, place the twenty halves in line with inner surfaces up, forming a trough twenty inches long. Wipe this trough thoroughly with oily waste, finishing with some that is only slightly oiled. 183. MOLDING. — Methods of molding briquets vary widely and have a considerable effect on the results obtained by differ- ent operators. The mold may be placed on a glass or marble slab, or on a porous bed. This difference in treatment will affect the results chiefly because a porous bed will extract moisture from the briquet, and, unless it is already mixed very dry, will make it give a higher result on a short time test. The use of a porous bed probably originated with a desire to more closely imitate the use of mortar in actual work, but it intro- duces another source of variation in results and should not be followed. 184. In hand work the whole mold may be filled at once, or small amounts of mortar may be added at a time, and each layer packed; the mortar may be tamped into the mold with a rod, in which case the pressure used may vary widely; or the mortar may be pressed in with the fingers, or with the point of a small trowel; and, finally, the pressure applied on the top of the whole briquet may be light or heavy. It is evident that it is almost impossible to so describe all these details of manipula- tion that another operator may follow the same system and obtain the same results. The practice of ramming the mortar into the mold by means of a metal rod or a stick faced with zinc is objectionable, because of the possible wide variation in the force thus applied. This method is sometimes used by manufacturers, since by making the mortar quite dry and ram- ming it into the molds very hard, a high initial strength is obtained. But the foremost cement makers are now eschewing such methods and are aiming to make fair tests. Some experi- ments made under the author's direction indicate that the pressure applied to the top of the briquet is the salient point in 114 CEMENT AND CONCRETE the process of molding, and that the other details are of minor importance. In Germany a heavy trowel or iron plate weighing about 250 grams, and provided with a handle, is used in making one- to-three mortar briquets. The mortar is made rather dry (about 10 per cent, water), and after the mold is filled and heaped, the mortar is beaten with the trowel until it becomes elastic, and water appears on the surface. The excess of mor- tar is then scraped off with an ordinary trowel or spatula. 185. Several machines have been devised for making bri- quets, some of which are said to give good results. Among these the most prominent is the Bohme hammer apparatus, which is much used in Germany, although not employed to any extent in the United States. It consists of a plunger which fits the mold and upon which a given number of blows are struck by a hammer. The mortar is first gaged as for hand molding, and placed in the form. A pinion, turned by a hand crank, is geared to a wheel provided with ten cams. These cams operating on the wrought iron handle of the hammer cause a certain number of blows to be delivered to the plunger. The mechanism is automatically shut off after the proper number of blows has been delivered. The following results were ob- tained by Professor Bohme with his apparatus: — TABLE 32 Comparison of Hand Made Briquets with Those Made by Bohme Hammer No. Method. Weight of Briquets. Mean Tensile Strength at 7 Davs in Kgs. per Sq. Cm. 1 2 3 4 5 By hand Hammer, 75 blows 100 " " 125 " 150 " 160.0 158.0 159.5 159.5 159.0 16.06 12.75 13.25 14.56 1556 186. Several American engineers have devised machines for briquet-making, but none of them has been generally adopted. An apparatus designed by Prof. Charles Jameson, of Iowa University, is said to work very rapidly. The mortar is packed in the mold by a plunger of the form of the briquet. This MOLDING BRIQUETS 115 plunger works in a chamber of the same shape as the briquet mold. The mortar is placed in a hopper at the side of this chamber, and is delivered to the mold automatically when the plunger is raised. The force is applied to the plunger by hand, but it should be so arranged that this be done by a weight, to prevent variations in pressure. In this method the briquet is removed from the mold as soon as made, and this would appear to be an objectionable feature. Professor Spalding, of Cornell University, in his excellent little book on "Hydraulic Cement," states that he has found that "a pressure of about 500 pounds upon the surface of the briquet is sufficient to produce a compact and homogeneous briquet, and a crude appliance consisting of a lever arranged to bring a pressure upon the mortar in the mold by means of a weight suspended at the end of the lever, has been found to increase both the rapidity and the regularity of the work, and especially to diminish the variations in results obtained by dif- ferent men." A machine which would give more uniform results and work more rapidly than hand molding, would commend itself for general use. 187. Method Recommended. — In making briquets by hand, the mortar may well be packed into the molds by the fingers, which should be protected by rubber tips. When the mold is filled and slightly heaped, the trowel should be placed on top, and the molder put about 60 pounds pressure on the trowel. The excess mortar is then cut off by the trowel and the top of the briquet is smoothed by drawing the trowel across the face. The results obtained by four molders using this method in the same laboratory are given in Table 33. 188. The recent progress report of the Committee of the American Society of Civil Engineers on uniform tests of cement contains the following, under "Molding": — "Having worked the paste or mortar to the proper consist- ency, it is at once placed in the molds by hand. "The Committee has been unable to secure satisfactory re- sults with the present molding machines; the operation of machine-molding is very slow, and the present types permit of molding but one briquet at a time, and are not practicable with the pastes or mortars herein recommended. 116 CEMENT AND CONCRETE TABLE 33 Results Obtained by Different Molders when Using Similar Mortar a o to w M K ft o . t» to to to 02 ft, Q OH h a Age. Me S *.n Tensile fRENUTH. a « n s ° Dp Date. « w w H << a « a j o Q H 3 a b c d e / 9 h i 1 31.6 62-65 7 days 81 92 89 5 10-22 Clear 2 " " 28 " 197 213 220 5 " 3 1 18.7 67-62 7 " 79 91 89 5 " 4 1 t l " 28 " 235 257 259 5 K 5 1 ci 63-68 3 mo. 515 541 519 5 4 t 6 1 u • " 1 year 558 569 555 5 " 7 2 152 70-65 28 days 196 186 197 " 8 2 " " 3 mo. 423 383 406 5 " 9 3 13.3 65-61 3 mo. 253 263 239 5 LI 10 3 " " 1 year 260 232 236 5 U 11 Su m of Means 2797 2827 2809 Molder S. Molder T. 12 13 316 62-65 7 days 28 " 60 145 60 167 5 5 10-28 Cloudy 14 1 18.7 65 7 " 67 71 5 " 15 1 u 28 " 223 211 5 " 10 1 " " 3 mo. 435 449 5 " 17 1 " " 1 year 504 491 5 t i 18 2 15 2 67 28 days 182 179 5 " 19 Su m of M eans 1616 1628 Cement, Brand Gn, Sample 21 R. Sand, Crushed Quartz 20 to 40. All briquets in same line received same treatment after made and were immersed in same tank until broken. 1 Mean of ten specimens. "Method. The molds should be filled at once, the material pressed in firmly with the fingers and smoothed off with a trowel without ramming; the material should be heaped up on the upper surface of the mold, and, in smoothing off, the trowel should be drawn over the mold in such a manner as to exert a moderate pressure on the excess material. The mold should be turned over and the operation repeated. "A check upon the uniformity of the mixing and molding is afforded by weighing the briquets just prior to immersion, or STORING BRIQUETS 117 upon removal from the moist closet. Briquets which vary in weight more than 3 per cent, from the average should not be tested." 189. Marking the Briquets. — The briquets made in a given laboratory should be numbered consecutively, so that no con- fusion can arise, and this one number is all that should be placed on the briquet. The record of the brand of cement, the pro- portions used, etc., should be placed in a book opposite the briquet number. The briquets should be numbered on the face, near the end. Steel stamps furnish a ready means of numbering, and when mortar contains more than two parts of sand to one of cement a thin strip of neat cement paste plastered across one end of the briquet will aid in making the numbers legible. Art. 24. Storing Briquets 190. The Time in Air before Immersion. — As soon as the briquets are molded they should be covered with a damp cloth TABLE 34 Variations in Length of Time Briquets are Left in Moist Air before Immersion — Natural Cement Tensile Strength, Pounds PER Sq. Inch. Cement. Parts Crushed Quartz, 20-30° Age When Hours in Moist Air BEFORE TO 1 Cement. Broken. Immersion. 8 12 24 48 72 168 Brand. Sample. Gn 15 R 7 days 123 139 151 161 237 k " 1 7 clays 91 106 114 114 182 " 16 R 28 days 110 106 109 89 113 u " 1 28 days 142 138 139 152 175 u u 2 28 days 102 105 112 113 115 An G 7 days 168 181 194 185 238 ct " 28 days 200 210 224 241 243 (i a 1 7 days 108 137 141 157 160 u i( 1 28 days 278 283 297 297 301 i i " 3 28 days 120 130 137 139 152 Note : — All briquets made by same molder. Each result is mean of ten specimens. until they are ready to be removed from the molds, when they should be transferred to a "damp closet," lined with zinc or other non-corroding metal. It was formerly the practice to immerse the briquets as soon as they were considered to be 118 CEMENT AND CONCRETE sufficiently set; but for the sake of uniformity, they are now left in moist air for twenty-four hours before immersion, whether the cement is quick or slow setting. Briquets which are to be broken at twenty-four hours, however, are usually immersed as soon as set hard. Table 34 gives the results obtained by allowing natural cement briquets to remain in moist air different lengths of time before immersion. In general, the strength is greater for seven and twenty-eight day tests the longer the briquets are allowed to remain in the moist air. It appears that, while the time in moist air should be made as nearly uniform as possible, a varia- tion of a few hours will not cause an important difference in strength. TABLE 35 Gain or Loss in Strength of Natural Cement Briquets by Immersion Tensile Strength, Pounds Time in Moist Air. Age When Broken. per Sq . Inch. Time in Tank. One Part Stand- Neat Cement. ard Sand to One Cement. 20 hours 20 hours 151 94 18 hours 6| days 7 days 147 153 2 days 2 days 192 126 2 days 5 days 7 days 160 158 3 days 3 days 205 141 3 days 4 days 7 days 177 155 4 days 4 days 218 165 4 days 3 days 7 days 191 165 5 days 5 days 230 175 5 days 2 days 7 days 192 169 Note: — All briquets made by same molder. Each result is mean of five specimens. Table 35 shows the early action of the water on the briquets. These tests were made in sets of ten; five briquets of a set were immersed after twenty hours, forty-eight hours, etc., while the other five of the same set were broken at the time the first five were immersed. With this sample of natural cement, it appears that the briquets lose part of their strength by immersion, and that some time is required to regain this lost strength." Thus, with neat cement mortar the briquets broken at twenty hours without immersion were as strong as those broken at seven days which had been immersed the last six and one-fourth days. With briquets of one-to-one mortar, it appears that if immersed STORING BRIQUETS- 119 at the end of four days, the gain, in strength during the last three days (in water) is about equal to the loss of strength due to immersion. If immersed earlier than this, the gain is greater than the loss, but if immersed later, the loss is greater than the gain. 191. For storing briquets the required twenty-four hours before immersion a moist closet is very convenient, tends to promote uniformity of treatment, and may be very easily made. The use of a damp cloth for covering briquets is- incon- venient, as the cloth may dry out. If it is used, the end of the cloth should rest in a pail of water, so it will keep wet by capillarity; it should also be kept from touching the briquets by a wire screen or by wooden slats. A moist closet may be made of slate, glass or soapstone, or of wood lined with metal. In the bottom of the box is a pan of water, or a sponge kept constantly wet. The shelves may well be of glass, and should be so arranged that any shelf may be removed without disturbing the others. 192. Water of Immersion. — When the briquets are ready to be immersed, i.e., usually, twenty-four hours after made, they are placed in a tank, containing water that is kept fresh by frequent renewals. The water in the tank should also be maintained at a nearly constant temperature. It is sometimes the case that briquets are subjected to considerable variations of temperature while in storage. It also frequently occurs that the water is allowed to become stale. A few of the many experiments made at St. Marys Falls Canal to show the effect, on the tensile strength of natural cement briquets, of variations in the tem- perature of the water of immersion, are given in Table 36. The details of these experiments, as well as other tests on the same point, may be found In the Annual Report, Chief of Engineers, U. S. A., for 1894, page 2314. The very marked effect which the temperature of the water may have on the rate of hardening of natural cements is clearly shown. When broken at the age of one clay or seven days, the effect on the strength may not be evident, or the briquets stored in cold water may develop a greater strength, but the more rapid hardening of the briquets stored in warm water is usually very evident at twenty-eight days, and increases up to two or three months. Some samples of cement are affected 120 CEMENT AND CONCRETE less than others, and a few experiments indicated that the differences in strength due to the temperature of water of im- mersion decrease after three months and become almost nil at one year. 193. The conclusion drawn from these tests may be briefly stated as follows: Between certain limits the early strength of natural cement mortars is usually developed faster in cool TABLE 36 Variations in Temperature of Water in which Briquets are Immersed 6 2 Tensile Strength, Pounds per Square "A Natural tl -*j 3 Inch, When Immersed in H Cement. ^OHB w«; Water of Approximate Temperature, O 3 o * 2 r H Degrees Fahr. ft hh|s ft y w Brand. Sample. 2h <1 38° 146 40° 50° 137 55° 125 60° C5° 70° 80° 1 Gn 15 R 7 days 126 154 2 41 14 days 144 131 125 131 150 168 208 3 I* 28 days 166 178 184 247 280 4 " 1 7 days 83 88 '84 89 98 97 121 5 U 1 14 days 84 111 123 150 191 6 it 1 28 days 96 156 187 221 243 288 7 Ln 31 S 1 day 143 124 120 109 109 8 " 7 days 204 201 183 193 186 9 t i. 14 days 184 203 204 229 245 10 " 28 days 221 245 254 281 303 11 u 2 mos. 261 292 348 382 429 12 An G 1 7 days 134 140 150 154 158 13 " 1 14 days 149 162 189 182 216 14 " 1 28 days 198 223 250 281 296 15 u 1 2 mos. 251 286 337 386 403 16 u Q 14 days 50 58 69 73 100 17 u Q 28 days 67 87 100 . . . 102 157 18 " 3 2 mos. 104 127 147 ... 194 231 water, but after the first seven clays, and sometimes after a shorter time, the strength is developed more rapidly in warm water, and the strength at any time between seven days and three months is approximately proportional to the temperature. After three months, the effect of the temperature seems to diminish, and may entirely disappear in time. M. Paul Alexandre 1 made quite a number of experiments on this point with Portland cement. In these experiments the 1 " Recherches Experimentales sur les Mortiers Hydrauliques," STORING BRIQUETS 121 gaging was done in about the same temperature as that at which the water of immersion was maintained, so that a double cause of variation was present. However, it was found that in all cases the higher strength was attained at seven days by the briquets made and stored in the higher temperature (15° to 18° C, 60° to 65° Fahr.) while at twenty-eight days the briquets of the lower temperature (0° to 5°C, 32° to 40° Fahr.) were ahead in the case of neat cement, and nearly as high as the warm briquets in the case of mortar. At three months the differences seemed to disappear. 194. Stale Water. — ■ Some experiments made to compare the strength of briquets which were alike in all other respects, but were immersed in different tanks in which the water had not been frequently renewed, showed very clearly the possible varia- tions from this source. Natural cement briquets, neat, and with one and two parts sand, gave, when immersed in one of the tanks, only from 40 to 60 per cent, of the strength attained in another tank by briquets entirely similar. • To store briquets in running water is going to the other extreme; this appears to be the best method, at least for short- time acceptance tests, provided the temperature can be regu- lated. However, in some cases where this has been adopted, the strength of the briquets is said to have fallen off very much after four or five years. Whether this is clue to the action of running water is a very interesting point, and a valuable one from the practical standpoint of the use of cement, but it has not yet been thoroughly investigated. 195. It appears from the foregoing that variations in the temperature and freshness of the water in which the briquets are immersed is an uncertain contingent, and therefore that all such variations should be carefully avoided. As a matter of convenience, the tanks may well be maintained at 60° to 70° Fahr., but if one does not care for a comparison of his results with those obtained in other laboratories, then any other con- stant temperature between 40° and 75° may be adopted. The water in the tanks should be renewed at least once a month, and preferably once a week. 196. Storing Briquets in Sea Water. — When the cement under test is to be used for constructions in the sea, some of the briquets should be stored in sea water to indicate the be- 122 CEMENT AND CONCRETE havior in this medium. Many tests have been made in this way by several experimenters, but the varied results obtained only indicate the different effects of such treatment on different samples of cement. One of the effects of storing in sea water has been touched upon under the head of consistency of mortar, where it is shown that porous briquets may disintegrate in this medium. A small specimen like a briquet will of course be more quickly affected than a large mass of concrete, but on the other hand, the concrete in work is likely to be more porous than the briquet. The effect of sea water upon cement will be taken up in another place. 197. Other Methods of Storing Briquets. — It has been thought that briquets, made to test cement that is to be used in air, should be hardened in the same medium in order that the tests should more nearly approach the conditions of use. Several points, however, should be borne in mind in interpret- ing the results obtained with air-hardened specimens. In actual work the mortar is usually in a large mass, or is protected from the influence of a warm, dry atmosphere, so that it remains moist for a long time, whereas a briquet placed in the open air is much more affected by changes in atmospheric condi- tions. If the briquets are allowed to harden in a room, such a small quantity of mortar may become quite dry in a few days, and, unless the amount of moisture in the air is regulated, an- other source of variation is introduced in the tests. It has been found impossible to obtain uniform results from briquets made as nearly alike as possible and stored side by side in the air of the laboratory. The regular acceptance tests should, therefore, it is thought, be made in the ordinary man- ner, but if cement is to be used in locations where it is likely to become very dry, a few special tests should be made to assure one that the brand of cement in question is one that will yield good results in such exposure. It may be found that certain kinds or brands should be entirely avoided for use in such lo- cations. A few tests of this character are given in Tables 72 and 73, §§ 359, 360. The results in any given line of the table are from briquets made the same way but treated differently in the method of storing. It is seen that these brands harden well in dry air. The effect of the amount of water used in gaging appears to follow somewhat the same law, whether the briquets are stored in air or water. BREAKING THE BRIQUETS 123 A method more nearly approaching conditions that fre- quently prevail in practice is to bury the briquets in damp sand. Table 120, §409, gives the results obtained with a large number of briquets stored in this way. While the results are somewhat more irregular than those for water-hardened speci- mens, since the conditions cannot be made so nearly uniform, yet this method gives better results than dry air storage. Art. 25. Breaking the Briquets 198. the Testing Machine. — The function of the testing machine is simply to furnish a means of applying the tensile stress, and of measuring the amount of force required to break the briquet. Aside from the clips, which hold the briquet, any contrivance which may be conveniently operated, and which will accurately measure the force applied, may be used for this purpose. There are several forms of testing machines on the market, all designed on the lever principle, though differing slightly in the method of application. The force is applied either by al- lowing water or shot to run into or out of a vessel suspended at the end of the longer arm of a lever, or a weight is made to run along the lever arm, which is graduated so that the force applied may be read from the beam. 199. In machines of the first class the delivery of shot is cut off automatically the instant the briquet breaks. The ad- vantage of this style is that the flow of shot may be so adjusted as to approximately regulate the rate of applying the stress; but little skill is required to operate it, and, since in its best form two levers are used, the shorter arm of one acting on the longer arm of the other, the machine occupies but little space. This machine does not permit rapid operation, since the shot must be weighed each time a briquet is broken. One of the main disadvantages of this form has been that in the case of strong briquets, a certain initial strain had to be applied in order that the stretch of the briquet and the slipping of the clips should not allow the shot to be cut off before the briquet broke. This objection, however, has recently been met by the makers, who have provided means of taking up this slip by a hand crank. 200. Another objection urged against the short-lever shot 124 CEMENT AND CONCRETE machines is the fact that as the stream of shot flowing into the scale pan is cut off by the breaking of the briquet, a certain amount of shot on its way to the pan falls into the pan after the briquet breaks, and is weighed, although not acting on the briquet at the time of the break. A form of shot machine is now on the market, however, in which this objection has been overcome. The load is applied by means of a weight hanging from one end of a lever. This weight is at first counterbalanced by a pail of shot at the other end of the lever, but as the shot is allowed to run out of the vessel, the unbalanced portion of the weight acts, through suitable levers, upon the briquet. The flow of shot is shut off automatically by the breaking of the briquet, and the shot that has escaped is weighed on a special scale to determine the load acting on the briquet. 201. In the other form of machine the weight is made to move along the arm by means of a cord and hand-wheel. This style may be operated much more rapidly, but some skill is required to use it properly, and as now made it occupies too much space. These machines are preferable for laboratories, while the shot machines may well be used in cement factories and small works where a foreman does the testing. 202. It would seem that a machine could easily be made which would combine the desirable features of both of these forms, by placing a heavy weight provided with rollers upon the upper lever arm of the shot machine, and using it in the same way that the hand power machine is now used. This would involve placing a hand wheel and cord upon the machine to operate the moving weight, the shot attachment being re- moved. Such a machine would combine the compactness of the shot machine, with the accuracy and speed of the single lever machine; the graduations on the beam could represent five pounds each, instead of two pounds, the value of the grad- uations now on the single lever machines. 203. FORMS OF CLIP. — Since cement has been tested by tensile strain, it has ever been a problem to obtain a clip which would give a perfectly true axial pull on the briquet. Various forms of clips have been used from time to time, but none of them has proved satisfactory in all respects. To trace the his- tory of the development of the clip is not warranted by its interest, but it may be said that in some of the early forms the BREAKING THE BRIQUETS 125 head of the briquet was held between two plates and clamped tight enough to develop sufficient friction to transmit the stress. The later forms of briquets are made with a shoulder or with wedge-shaped ends to allow the clip to grasp them. Mr. John Grant, Mr. Alfred Noble, General Gilmore, Mr. J. Sondericker and Mr. D. J. Whittemore have each designed or adapted dif- ferent forms, and more recently Mr. S. Bent Russell and Mr. 1 1 n hi u lEEE I" 3 M Vi tt I Z 3//r. FIG. 6. — RIEHLE "ENGINEERS' STANDARD" CLIP W. R. Cock have each devised a clip which will be mentioned below. 204. Form in Most General Use. — The clip in most general use in the United States is of the general style shown in Fig. 6. It differs only in detail from the form recommended by the Amer. Soc. C. E. Committee of 1SS5, which has been called 126 CEMENT AND CONCRETE the "Engineers' Standard." The general form is pear shaped; the briquet is grasped at the points of reverse curve at the side of the briquet, giving an area between opposite gripping points of about one and a quarter square inches. The gripping points are rather too sharp, when new, as they have a tendency to crush the briquet locally. The width of the bearing increases with the amount of wear the clip sustains. The clip is provided with a conical pivot, which rests in a cone-shaped cavity at- tached to the machine, so that the two parts of the clip are free to swing. In a form which was previously used to a con- siderable extent, each bearing surface was designed to be about an inch square, the jaw being made to conform to the outline of the briquet. This form, however, did not give satisfactory results; a particle of sand between the briquet and the bearing surface of the clip would give an eccentric pull, and strong briquets would sometimes break in the head of the briquet transverse to the axis, in several curved layers joining opposite gripping surfaces. 205. CLIP-BREAKS. — When a briquet is inserted in the or- dinary clip, the gripping points will not, in general, grasp the briquet symmetrically. The gripping points have a tendency to slide on the surface of the briquet in order to assume a sym- metrical position; there is friction to resist this sliding, and when this resistance overcomes the tendency to motion, the two clips and the briquet become a rigid system, and bending strains may be introduced. Again, if the briquet is not too badly ad- justed in the clips, it is apt to break in a line joining two op- posite gripping points, instead of at the smallest section; this is called a " clip-break." The tendency to form clip-breaks is greater if the gripping points are very narrow or have sharp edges; neat cement briquets exhibit this tendency much more than briquets from sand mortars, and some samples of cement are much more likely to give clip-breaks than others. 206. Cause of Clip-breaks. — When a briquet breaks in this manner, the broken section is usually about normal to the side of the briquet at the point where the jaw was in contact. This indicates that a clip-break is caused by compression at that place; there is evidently compression along the plane joining the two opposite gripping points, and tension at right angles to that plane, and the briquet fails here as a result of the two BREAKING THE BRIQUET 'S 127 stresses. If the briquet is not properly adjusted in the clips, but is so placed that its longest axis is at one side of the line joining the points of application of the forces (in the "Engi- neers' Standard" clip, the line joining the pivot points), then the bending strain that is introduced is greatest at the central section of the briquet; this may cause the briquet to break at the smallest section, when if it were properly adjusted in the clips it would develop a clip-break. The bearing surfaces of the clip should not be too small, as this increases the intensity of pressure, but on the other hand there appears to be no prac- tical advantage in making this area more than - r \ to J inch wide (the length being limited by the thickness of the briquet, one inch). 207. Prevention of Clip-breaks. — The method most fre- quently adopted to prevent clip-breaks is to cushion the grip- ping points with some compressible material, such as thin rub- ber or blotting-paper. This device prevents clip-breaks, but the result of about three hundred tests made under the author's direction showed clearly that it also lowered the apparent strength very materially. 1 Briquets broken with the bare clips showed a mean strength of 606 pounds per square inch, while the cushioned clips gave an apparent strength of but 521 pounds, or 86 per cent, of the strength without the cushion; of the bri- quets broken with the bare clips, 33 per cent, were clip-breaks; with the cushioned clips no clip-breaks occurred. The rubber was applied by slipping two rubber bands over each end of the briquet, giving cushions about T l % inch thick. 208. Strength of Briquets that Develop Clip-breaks. — It was also found in breaking 277 briquets with two styles of clips without cushions that 129 of them that gave clip-breaks aver- aged 611 pounds per square inch, while 148 which did not de- velop clip-breaks had a mean strength of 590 pounds. This result is easily accounted for by saying that some of the bri- quets that broke in the small section were made to do so by the cross-strain introduced by imperfect adjustment in the clips. When a briquet breaks at other than the smallest section, it is certain that the smallest section has a greater strength per 1 For a report of these tests in detail, see Annual Report Chief of Engi- neers, U.S.A., 1S95, p. 2913. Also "Municipal Engineering," Dec, 1896, Jan., Feb., 1897. 128 CEMENT AND CONCRETE square inch than is shown by the result obtained; how much greater cannot be told. But it follows that if clip-breaks could be eliminated in a proper way, one which would not cause center breaks by the introduction of cross-strains or other undesirable conditions, the strengths thus obtained would be greater than when clip-breaks occur. The fact that the use of a rubber cushion gives lower strengths, shows that this is not the proper method of preventing clip-breaks. 209. Mr. W. R. Cock has devised a clip, with rubber-covered gripping points, which has attracted some attention. It has 1 1 1 u i i 1 1 1 1 1 1 I S A '/z l A Fig. 7.— RUSSELL CLIP 3//7. sometimes been assumed that because this clip eliminated clip- breaks it must give a higher apparent strength than the rigid form. No extensive series of experiments have been published which permit of comparing this clip with other forms, but from the results obtained above, in using rubber cushions, it would appear that the Cock clip may give lower apparent strengths. BREAKING THE BRIQUETS 129 210. The form of clip designed by Mr. S. Bent Russell is constructed on the "evener" principle, each clip having free- dom of motion imparted by four pin-connected joints (see Fig. 7). It is sought to prevent any but an axial pull being ap- plied to the briquet. On account of details of construction, into which it is not necessary to enter here, the clip must be in its normal position when the briquet is inserted, in order that the possibility of cross-strain shall be effectually removed. As a result of many tests with this form and the ordinary "En- I I i I i i I i i rrrt- l" 3 M v z % l 2 3 in. Fig. 8. — SIXGLE GIMBAL CLIP gineers' Standard," it was found that they gave very nearly the same strength. But that the evener motion itself was of some value was shown by a series of experiments in which part of the briquets were broken by this form of clip without modi- fication, while part were broken by the same clip when it had been changed to a rigid form by means of a clamp that elimi- nated the evener motion. It is believed that with some modifi- cations this clip will give good results, and it may be used al- most as rapidly as the ordinary rigid form. 130 CEMENT AND CONCRETE 211. Several experiments were made with a clip in which the gimbal principle wa,s applied, the stress passing from the machine to the gripping points through knife edges placed in the line joining opposite gripping points and midway between them 1 (Fig. 8). Higher results were obtained with this form, the " Single Gimbal," than with any of the styles with which it was compared, but it was made only for experimental purposes, and unless modified is not convenient enough to be recom- mended for general use. 212. In the course of these experiments it was shown that to increase the distance between gripping points, grasping the bri- quet nearer the head, increased the apparent strength and diminished the number of clip-breaks. With the Russell clip, increasing this distance from 1{\ inches to 1 T 7 ^ inches gave an increase of about six per cent, in the apparent strength; and a similar increase in the width between jaws of the Gimbal clip, from ly\ to \ T % inches, gave an increase in apparent strength of about five per cent. It was found later that Mr. J. Son- dericker had previously arrived at similar results, 2 and as the form of briquet used by the latter had permitted extending the experiment, he found that when the points were about If inches apart (making the area of the briquet about If square inches between opposite gripping points), nearly all the frac- tures occurred at the smallest section. 213. Effect of Improper Adjustment. — The effect of not properly adjusting the briquets in the clip was also investigated. In some cases the briquets were placed in the proper position as nearly as possible. In the other cases they were in a de- cidedly distorted position, much worse than they would be placed with the most careless manipulation. It was found that if the briquet was so placed in the "Engineers' Standard" clip that the gripping points on one side of the briquet were farther apart than those on the other side, the decrease in break- ing strength was very marked (about 35 per cent.), while if the planes determined by the lines of contact of the gripping points of each clip were parallel, there appeared to be no effect. The 1 This clip was devised by the author at the suggestion of Mr. E. S. Wheeler, M. Am. Soc. C. E. 2 Jour. Assoc. Eng. Soc, Vol. vii, p. 212. BREAKING THE BRIQUETS 13i reason of this is evident: in the former case the line of force, joining the two pivot points, does not pass through the center of the smallest section of the briquet, and transverse stresses are introduced, while in the latter case the line of force does pass through the center of the smallest section, though not at right angles to its plane. With the Russell and Gimbal clips the distortion seemed to have little effect, provided, that in the case of the former, the clip was itself in its normal position when the briquet was inserted. 214. Conclusions Derived from Tests of Several Styles of Clips. — From the tests described above, 1 the following conclu- sions may be drawn: — 1st. When using the ordinary form of clips with metal gripping points, the briquets which break at the places of con- tact of the jaws give higher apparent strengths than those which break at the smaller sections. 2d. A rubber cushion between the briquet and the jaw of the clip prevents clip-breaks, but materially lowers the stress required to break the briquet. 3d. The form of clip designed by Mr. S. Bent Russell gives somewhat less irregular results than are obtained with the Riehle "Engineers' Standard" rigid clip. Although the results given by the Russell clip in its present form are a trifle lower than those given by the Riehle, it- seems probable that these lower results are due to defects in detail which may readily be eliminated. 4th. By the application of the gimbal principle to cement testing clips, higher, as well as more nearly uniform, results may be obtained. 5th. In using the rigid form of clip, careless manipulation in adjusting the briquet may result in serious error due to the introduction of cross-strains, while with either the Single Gim- bal or Russell clip slight deviations in adjustment are not im- portant. 6th. With the form of briquet recommended by the commit- tee of the American Society of Civil Engineers in 1885, the break- ing stress may be somewhat increased, and the number of clip- 1 These tests were described in greater detail and discussed by the writer in "Municipal Engineering," Dec, 1896, Jan. and Feb., 1S97. 13: CEMENT AND CONCRETE breaks may be very materially decreased, by such a modifica- tion of the clip as to allow grasping the briquet nearer the head. 215. Requirements for a Perfect Clip. — As a logical result of these conclusions, the ideal clip should fulfill the following requirements : — 1st. It should impart a true axial pull to the briquet with- out subjecting it either to cross-strains or to compressive forces li 1 1 1 ' i q I » 1 1 I" 3 A Vt [ A 3//7. FIG. 9. — FORM OF ARTICULATED CLIP SUGGESTED FOR USE sufficient to cause it to break at other than the smallest section. 2d. The bearing surfaces of the gripping points should not be more than about one-fourth of an inch wide, since this is sufficient to prevent crushing the briquet at these places, and too wide a jaw will not usually bear uniformly over its whole surface. BREAKING THE BRIQUETS 133 3d. Its parts should have sufficient strength and stiffness, so that they will not bend appreciably when in use. 4th. It should permit rapid operations, and 5th. It should be as light as consistent with the above requirements. 216. Form Suggested. — Fig. 9 shows a style of clip which closely conforms to the above specifications. The evener form devised by Mr. Russell has been selected for modification. The S. G. clip would more nearly meet some of the requirements, and, so far as the principle is concerned, this form is considered quite the equal of the evener clip. But no method of applying the gimbal principle has commended itself as affording such rapid manipulation as does the evener motion, and since it is thought that either form will obviate cross-strains in a plane parallel to the face of the briquet, the evener form has been adopted on account of convenience. The defects in detail of the Russell clip which have already been mentioned have been obviated in the present form. The gripping points are made one-fourth of an inch wide, and a little more material has been used between the gripping points and the first pin to stiffen the clip. This form is designed for use with the briquet shown in Fig. 5 (see § 179). 217. Rate of Applying the Tensile Stress. — Table 37 gives the results of several hundred experiments made by Mr. TABLE 37 Relation of Apparent Tensile Strength to Rate of Applying Stress Rate of Applying Tensile Strength Stress, Obtained, Pounds pee Minute. Pounds per Sq. Inch. 50 400 100 415 200 430 400 450 6,000 493 Henry Faija 1 to show the effect on tensile strength of varying the rate of applying the stress. A few of the results obtained from nearly 900 tests, made 1 "Cement for Users," by Mr. Henry Faija. 134 CEMENT AND CONCRETE under the author's direction to illustrate this point, are given in Table 38. Some of these results accord very well with those given in Table 37, but the results in the latter table were doubt- less obtained from neat Portland briquets only, while the ex- periments given in Table 38 were made with briquets neat and with two parts sand, and on natural as well as Portland cement mortars. TABLE 38 Relation of Apparent Tensile Strength to the Rate of Applying the Stress Tensile Strength, Pounds PER Square Inch, for Stress Applied at Age of Briquets. Rate of Pounds per Minute. Cement. Proportions. 100 300 500 700 900 Portland Neat cement 7 and 14 days 453 485 521 520 528 " Neat cement 3 months 590 617 622 640 u 1-2 3 months 445 467 487 507 510 Natural Neat cement 7 days 150 169 186 202 " Neat cement 3 months 309 351 363 378 390 1-2 3 months 255 299 327 329 354 218. It appears from all these results that the increase in the breaking strength due to increasing the rate of applying the stress is considerable in the case of low rates of speed, but when a rate of 500 or 600 pounds per minute has been reached, a further increase in rapidity does not make a material increase in the apparent strength. Since certain variations in rate are sure to occur, until some device is used to automatically regu- late it, a rate should be adopted which would allow of slight variations without materially changing the result of the test. A rate of 600 pounds per minute would fulfill this requirement, and, with certain machines at least, would be still more con- venient than the rate of 400 pounds per minute which has here- tofore been quite generally used. An analysis of the experiments made to determine the de- gree of uniformity obtained by using each of the given rates, showed there was but little difference in this regard, but if any choice could be made on this basis it seemed to lie with the more rapid rate. 219. With the shot machines it is not difficult to approxi- BREAKING THE BRIQUETS 135 mately regulate the rate at which the stress is applied. In operating a machine in which a handwheel moves a weight along the graduated beam, it must be remembered that the rate at which the weight moves is the controlling factor, and not the movement of the lower wheel, which simply serves to take up lost motion, the stretch of the briquet under strain, and the slipping of the briquet in the jaws of the clip. A mistaken idea concerning this matter has sometimes led to the adoption of a device to regulate the motion of this lower wheel. Until one is accustomed to applying the stress at a given uniform rate, he will find it an aid to hang near the machine a pendulum of such a length that a certain number of vibrations correspond to a complete revolution of the handwheel. 220. Treatment of the Results. — The number of briquets which are made to test the strength of a given sample of cement will depend on the accuracy which it is desired to attain. If but two briquets are made, neither of the results may be re- jected; however widely they may differ one from the other, the mean of the two must be considered the result of the experi- ment when nothing is known as to their comparative value. But if several briquets are made from the same sample, and they vary one from another, the final result is sometimes ob- tained by rejecting certain of the observations. In some cases if five or six specimens are made, the highest and the lowest ones are omitted, while sometimes the two lowest are rejected, and the mean of the three or four highest is taken. 221. While the absolute mean of all of the observations will ordinarily be quite sufficient, and should usually be considered the result of the test, yet where tests are very carefully made to compare two samples, or two methods of manipulation, it may be desired to reject certain observations that appear to be abnormal. The beginner in cement testing, unfamiliar with observations of this character, may not feel confidence in his own judgment as to what observations may be rejected, and the criteria sometimes used in more accurate work are entirely too complicated for this purpose. To serve as a guide in such cases, the writer would suggest the following simple method which, though entirely arbitrary, is more justifiable than either of the methods mentioned above. As the experimenter be- comes more familiar with the work, he will doubtless prefer to 13G CEMENT AND CONCRETE depend on his own judgment in the rejection of observations, taking into account the general accuracy of the work. First obtain the absolute mean and the difference between this mean and each individual result; let us call this difference the "error" for each result. Reject any observations whose error is, say, ten per cent, of the absolute mean, and obtain the mean of the remaining observations as the true result. 222. For example, suppose that we have broken ten bri- quets obtaining the strengths given below, and wish to deter- mine the result of the test. The absolute mean is found to be 213.9 pounds, or, the nearest whole number, 214 pounds. TABLE 39 Rejection of Observations Number of Briquets. Observed Strength. Error. Observed Strength. New Error. 1 2 3 4 5 6 7 8 9 10 209 226 227 184 217 252 200 195 193 236 5 12 13 30 q 38 14 19 21 22 209 226 227 217 200' 195 193 1 16 17 7 lb' 15 17 Sum . . . 2,139 177 1,467 83 Mean . . 213.9 17.7 209.6 11.9 The "errors" are given in the third column, and it is seen that three of them are greater than ten per cent, of the mean. Omitting the results having these large errors, we obtain a new mean of 209.6 pounds, which is to be considered the result of the test. An inspection of the first column of errors shows that the mean of the errors is 17.7 pounds; if we divide this by the mean of the tensile strengths, we obtain 17.7 -3- 213.9 = .0827. Expressing this as a percentage, we may call 8.27 per cent, the "average error." The same result is, of course, obtained by dividing the sum of errors by the sum of the strengths. Now if we consider column five, we see that the new average error will be but 83 -s- 1467 = 5.66 per cent. INTERPRETATION 137 223. In giving the results of a series of tests, it is a common practice to state only the absolute mean, but it is of considerable interest to know the variations that occurred in breaking in order that one may judge of the reliability of the results, or, in other words, to make a rough approximation as to the prob- able error. For this purpose the highest and lowest result may be given, but a much better index to reliability would be to give the "average error" as explained above. However, in reporting a large number of tests, the extra labor involved in obtaining this "average error" is usually considered too great to be attempted, and in such cases the absolute mean and the highest and lowest results must serve the purpose. 224. Accuracy Obtainable. — When an operator has become expert and is working under good conditions, he may expect to obtain results within the following limits: The extreme varia- tions between the results in a set of ten briquets (the difference between the highest and lowest) not exceeding 20 per cent, of the mean strength of the set, the maximum variation from the mean not exceeding 12 per cent, of the mean, and the "aver- age error," as explained above, not exceeding 8 per cent. Art. 26. The Interpretation of Tensile Tests of Cohesion 225. One of the problems presented in the inspection of cement is to foretell the ultimate relative strengths of two samples from the results of short time tests. Formulas have been presented purporting to solve this problem, such formulas being based on the assumption that the strength gained at the end of months or years is a function of that developed in a few days. In fact, the raison d'etre of tensile or other short-time strength tests for the acceptance of cement, rests, in a sense, upon this same assumption. The value of strength tests as one of the guides in determin- ing in a short time the probable quality of a cement is unques- tioned. One is apt, however, to seek too close an agreement between the results of such tests and the actual quality of the cement. It would be easy to select examples illustrating the harmony between short and long time tests; but it will be of greater value to show, rather, some of the many exceptions to such a rule, and thereby emphasize the fact that it is only by 138 CEMENT AND CONCRETE a close analysis of all of the information obtainable concerning a sample, and a general knowledge of the behavior of the dif- ferent grades of cement, that one may hope to arrive at a tol- erably accurate opinion, 226. Comparative Tests of Portland Cements. — In Table 40 are given the results of tests on four brands of Portland cement at seven clays, twenty-eight clays and two years. From the tests at two years it appears that T and U are the best cements, V is nearly as good, but W gives a much lower result. Turning now to the seven and twenty-eight day tests of bri- quets maintained at the ordinary temperature, it is seen that W gave in every case higher results than T, and nearly as high as U or V. Among the short time tests it is only the results of briquets maintained at 80° C. that indicate the inferiority of Brand W. TABLE 40 Interpretation of Short Time Tests of Portland Cement, Several Brands Te s t siue Strength, Pounds Parts Sand to 1 Cement BY Tempera- tube Water of Age of Briquets. per Square Inch. Brand. Immersion. Weight. T U V W 2 Hot, 80° C. 7 (lays 339 278 284 222 3 It u 4 t 221 191 180 134 3 " 60° C. " 144 142 169 144 Ordinary " 426 510 487 565 1 a. " 327 425 400 396 2 u " 172 275 256 236 3 C( " 73 150 160 150 1 " 28 days 526 577 557 556 2 " " 312 394 387 332 O o " C( 142 241 223 247 i " 2 years 719 753 763 654 2 u " 554 553 513 407 o " " 380 Ol o 340 287 227. Comparative Tests of Natural Cements. — From the nature of natural cements a much greater variation in strength among different brands, and even among different samples of the same brand, is to be expected. With Portland cements made in accordance with ordinary methods, the variations in strength among ten or twenty brands will usually be compara- tively small. One of them may possibly prove unsound, and INTERPRETATION 139 one or tAvo others may give inferior strength, but the variations in strength among three-fourths of the samples will not gener- ally exceed 20 per cent. With the same number of brands of natural cements, variations of 50 to 200 per cent, may be expected. TABLE 41 Interpretation of Short Time Tests of Natural Cement, Several Brands Tensile Strength, Pounds per Square Inch. Parts Sand to 1 Cement. Tempera- ture Water of Immersion. Age of Briquets. Brand. Ju He Bn Mn Nn Kn 2 Hot, 50° C. 7 days 152 192 84 133 160 277 2 Hot, 60° C. " 170 270 79 154 164 254 2 Hot, 80° C. it 58 136 128 179 166 221 Ordinary " 174 203 130 189 210 189 1 " " 125 198 103 164 169 164 it 28 days 208 344 293 203 316 289 1 " " 237 342 247 247 252 385 2 " " 132 223 148 158 184 217 3 £ t ; ' 64 113 85 93 104 101 1 " 2 years 177 271 358 631 665 532 2 " 1 1 106 157 195 515 550 561 3 ' ' u 99 130 117 340 328 372 In Table 41 six brands of natural cement are compared by tests at seven days, twenty-eight days and two years. These six brands have been arranged in the table according to their value as shown by the two year tests, and it is seen that the first three, Jn, Hn and Bn, are especially poor, while the last three, Mn, Nn and Kn, are exceptionally good. In the short time tests of briquets maintained at ordinary temperature, Jn and Bn gave low results and Nn and Kn gave fairly high results, in harmony with the long time tests; but Hn, which proved to be one of the poorest samples, gave in every case the highest, or next to the highest, result in seven and twenty-eight day cold tests. In this table we find again that the results of the briquets maintained at 80° C. for seven days gave, in a general way, the best indication of the relative values of the six brands. 228. Several Samples of One Brand. — To show that short time tests do not always indicate the relative values of several samples of cement, even when all of the samples are of the 140 CEMENT AND CONCRETE same brand, Tables 42 and 43 are given. All of the results in these tables are from samples of the one brand of natural ce- ment. TABLE 42 Comparison of Short and Long Time Tests of Samples of One Brand of Natural Cement 3 H Sand. Age. Kind. Parts to 1 Cement. per Square Inch. A B C 28 days 6-7 mouths N umber of Samples Tested. 3 7 2 3 5 84 121 123 186 177 241 220 301 297 381 Std. 1 1 7 days 6 months Num her Samples. 17 20 17 1G 62 402 74 468 86 442 146 367 P.P. 1 1 and 2 2 7 days 6 months x 7 days 2 Number Samples. 50 50 19 19 49 473 273 54 426 249 73 381 26-3 128 321 283 D P.P. 2 2 7 days 1 year 7 days 2 Number Samples. 13 48 38 18 66 473 257 80 422 234 95 377 277 147 325 215 E 2 7 days 6 months Number Samples. 12 21 18 9 74 535 83 477 120 424 167 373 F ( Cr.Qtz. \ \ 20 to 40 J * 2 28 days j 6 months ) j and 1 year J Number Samples. 287 170 41 135 565 191 454 235 367 Mean one-to-one and one-to-two mortars. Briquets immersed six days in water maintained at 60° C. INTERPRETATION 141 In Table 42 the results are selected from a large number of tests of this brand, and are arranged in groups according to the strength shown at a certain age. For instance, in Series A the results of twenty samples are given, arranged according to the strength at twenty-eight days. Three of the samples gave less than 100 pounds per square inch, neat, at twenty-eight days; the same three samples gave a mean strength of 121 pounds per square inch, neat, six to seven months. Seven samples, the strength of which fell between 100 and 150 pounds at twenty-eight days, gave a mean strength of 186 pounds at six to seven months. The results of this series show the harmony between short and long time tests when it is a question of comparing neat cement mortars. In Series D of this table the samples are arranged in order according to the strength developed by one-to-two mortars one year old. Thirteen samples had a strength at this age of between 450 and 500 pounds, average 473 pounds. The same samples gave but 66 pounds, neat, seven days. Forty-eight samples, giving between 400 and 450 pounds, average 422 pounds, gave but 80 pounds, neat, seven days, while eighteen samples that developed only 300 to 350 pounds mean, 325 pounds at one year, showed a mean strength of 147 pounds, neat, seven days. A little study of this table will show that the samples which were comparatively weak in seven and twenty-eight day tests, either neat or with sand, gave the best results in the long time tests of sand mortars. Series A shows that the neat tests at seven days and at six months are consistent, but in all cases where sand mortars are tested at six months to one year, the highest results are given by the samples showing the lowest strength in the short time tests in cool water. It is very sel- dom that this conclusion has not been indicated by the author's tests of this brand. It is not invariably true, however, for some samples which were selected as being defective in burn, gave low results both in short and long time tests. The con- clusion stated above must therefore be understood to have limits even for this brand, and may not apply at all to many brands. As to the results of short time tests of briquets stored in hot water, Series C and D indicate that such results are more nearlv 142 CEMENT AND CONCRETE consistent with the long time tests, yet it is evident that even with hot tests one could not readily and accurately differen- tiate the best from the mediocre samples. TABLE 43 Natural Cement : Rate of Increase in Strength, Hardening in Water and Dry Air Sand, Parts to One Age of Briquets Tensile Strength per Sq. In., of Samples. Hardened in Water. Hardened in Air of itoom. Cement. When Broken. 84 IP 0' 84 IT' 0' 1 7 days. 74 53 103 107 68 187 1 28 days. 228 189 228 188 95 256 1 3 in os. 415 345 331 158 100 248 1 mos. 506 381 307 425 161 359 1 2 years. 446 383 209 151 147 403 3 28 days. 99 97 64 112 61 180 3 3 mos. 244 241 129 153 81 194 3 6 mos. 255 232 162 92 69 173 3 1 year. 274 264 186 229 70 144 q 2 years. 258 268 167 274 152 228 Sample 84 U' O' Fineness : Per cent, passing Sieve No. 120, Holes .0046 inch square 80.5 87.8 89.7 Time Setting — to bear -fa" \ lb. Wire, min. ... 54 23 97 Specific Gravity 3.012 2.950 3.145 U', underburned, 0', overburned. All samples same brand, Gn. 229. The results in Table 43 will serve to illustrate the same point by showing the very different rates of increase in strength of three samples when the briquets are stored in water and in dry air. One of these samples, 84, was taken at random from a shipment, while U' and O r were supposed to be defective in burn. Of the water-hardened specimens, No. 84 gained in strength up to six months or one year and then suffered only a slight falling off. The underburned sample showed a con- tinuous gain, but the overburned cement showed a marked decrease in strength after six months or one year. The air- hardened specimens were very irregular in strength, but the underburned sample gave very low results throughout. Table 44 gives similar results obtained with several samples, the briquets being hardened in water as usual. 16 R is a fair INTERPRETATION 143 sample of the best cement of this brand, and its rate of increase in strength with one to three parts sand is shown. Samples M and L were tested together, as were CC and DD. M and CC are of the class giving comparatively high results at seven clays, while L and DD give high results at seven days, but develop only a moderate ultimate strength. TABLE 44 Natural Cement: Difference in Rates of Increase in Strength of Several Samples of the Same Brand Cement. Sand. Tensile Strength, Pounds per Sq. In. at Age of •6 M o> 2 cS m Kind. V - -^ 75 g » cS >> ea oo q s IM S CO M O S so cS 1*. cS cS 03 CO 1 2 Gn 16 R Crushed Qtz. 20-30 1 2 94 59 142 101 334 289 399 341 430 335 500 386 445 354 8 u a 3 73 204 243 252 268 262 248 4 M 118 199 256 248 300 5 L 40 88 148 146 167 6 M Point aux Pins 2 63 155 216 241 252 7 L " 2 30 150 296 415 8 CC " 1 123 232 276 269 317 9 DD u 1 77 218 327 337 474 10 CC " 2 185 268 242 279 279 11 DD l( 2 189 326 303 373 359 230. Conclusions. — From the above tables one should not draw the conclusion that all strength tests are valueless be- cause likely to be misleading. Some lessons, however, seem to be plain; conclusions drawn from the results of short time tests of strength alone are likely to be far from infallible. This is especially true of natural cements. The correctness of one's conclusions concerning the value of a sample is likely to de- pend very much upon his knowledge of the behavior of that particular brand, and the beginner in cement testing should not have too great confidence in his early conclusions. Samples under inspection should be tested in comparison with other samples of known quality, and the results of the strength tests studied in connection with all the information obtainable from the other tests of quality already outlined. CHAPTER X THE RECEPTION OF CEMENT AND RECORDS OF TESTS Art. 27. Storing and Sampling 231. STORAGE. — The storage houses provided for the ce- ment should be such as will effectually preserve it' from damp- ness, the floor being dry and strongly built. A circulation of air under the floor will insure dryness. In building houses for storage, due regard should be given to the ease of getting the cement in and out, and facilities pro- vided for the use of block and tackle in tiering. When the cement is received, whether in sacks or barrels, it should, if possible, be so tiered in the warehouse that any pack- age is accessible for sampling. In the case of barrels this may readily be attained by tiering in double rows, the barrels lying on the side. It has been found that ordinary cement barrels will withstand the pressure if tiered five high with a "binder" row on top; and when so piled, a warehouse 32 feet wide and 100 feet long will readily hold 2,200 barrels, an allowance of about one hundred fifty square feet of floor space for one hun- dred barrels. 232. Where storage space is limited, the barrels may be numbered and sampled before they are placed in the warehouse, and they may then be piled solid, but this should be avoided if practicable. Sacks cannot be quite so neatly stored, and since a smaller quantity is contained in a sack, they may be tiered so that every third or fourth sack is accessible. It is desirable where work is executed with the greatest care that every package be numbered for future identification, but this may sometimes prove impracticable, especially when the ce- ment is in sacks, and in such cases the sampled packages only may receive numbers. 233. Percentage of Barrels to Sample. — The amount of ce- ment which shall be accepted _on the test of a single sample must be determined by each user of cement according to his STORING AND SAMPLING 145 knowledge as to the uniformity and reliability of the brand in use, and according to the character of the work in which the cement is to be used. In a few isolated cases every barrel is tested, while sometimes several tons of cement are accepted on a single test. As the improvements in methods have decreased the work involved in making the simpler tests, the tendency has been to test a larger percentage of the packages. 1 The report of the committee of the Amer. Soe. C. E. in 1885, contains the following concerning sampling: "There is no uniformity of practice among engineers as to the sampling of the cement to be tested, some testing every tenth barrel, others every fifth, and others still every barrel delivered. Usu- ally, where cement has a good reputation, and is used in large masses, such as concrete in heavy foundations, or in the back- ing or hearting of thick walls, the testing of every fifth barrel seems to be sufficient; but in very important work, where the strength of each barrel may in great measure determine the strength of that portion of the work where it is used, or in the thin walls of sewers, etc., every barrel should be tested, one briquet being made from it." 234. Taking the Sample. — The sample should be taken in such a manner as to fairly represent the package, and for this purpose a "sugar trier" may be used, by which is obtained a core of cement about one inch in diameter and eighteen inches long. As any tool used for boring cement barrels soon becomes dull, and as a sugar trier is somewhat difficult to sharpen, the author prefers to use an ordinary bit and brace to penetrate the barrel head, and then extract the sample with a "trier," or a long, slender scoop of similar form provided with a handle. For storing the sample until it is tested, it has been found convenient to use covered tin cans holding about one pint, the cover of the can being labeled with the number of the pack- age from which the sample is taken. 1 In a paper read before the Institution of Civil Engineers in 1865-66, Mr. John Grant states that "after using, during the last six years, more than 70,000 tons of Portland cement, which has been submitted to about 15,000 tests, it can be confidently asserted that none of an inferior or dangerous character has been employed in any part of the work in question." (The Metropolitan Main Drainage, London.) This is an average of one test to twenty-five barrels. 146 CEMENT AND CONCRETE Art. 28. Records of Tests 235. Value of Records. — In conducting work in which the use of cement enters as a prominent factor, it is not only neces- sary to know that the cement used is of a good quality, but also to be able to show at any future time what tests were made to establish its value. This fact, as well as the convenience of the work, demands that a record shall be kept of all the tests made. These records may be more or less elaborate, according to the kind and amount of the work in hand, but in any case, enough detail should be given to make them intelligible to other engineers. 236. Marking Specimens. — There is sometimes a tempta- tion, in making tensile specimens, to stamp upon them many details of the test, and for this purpose an elaborate cipher system has sometimes been used. But this method is to be strongly deprecated. Each briquet should receive its proper consecutive number, as mentioned in §189, and the details concerning it should be placed in the record book. 237. records Kept at St. Marys Falls Canal.— In the tests of cement at St. Marys Falls Canal, during the construc- tion of the Poe Lock, a system of records was used that gave entire satisfaction. At the time the largest amount of cement was being used three molders were employed, each making fifty briquets per day of eight hours. Over one hundred thou- sand briquets were made in five and one-half years. Although the system of records used at this point may be more elaborate than is often necessary, yet the system will be described, and certain modifications will be suggested for places requiring less complete records. 238. Barrel Records. — The barrels receive consecutive num- bers after they are tiered up in the warehouse. The " receiv- ing book" is a simple transit book in which are entered the date of the receipt of each cargo, the name of the boat (or the car number, if shipped by rail), the brand of cement, the num- ber of barrels, the first and last barrel number of the cargo and the warehouse in which the cement is placed. The next book to be used is the "barrel book," in which the numbers of the barrels are entered consecutively in a column at the left, each barrel being given one line. This book is also of transit size, but might well be larger. The headings are given below. RECORDS OF TESTS 147 SAMPLE PAGE OF "BARREL RECORD" No. Bbl. Sampled. Defects. Ac- cepted. Re- jected. Issued. Remarks. 88251 2 4 5 6 7 8 9 88260 1 a S 4 5 M. D. 5 16 M. 1). 6 5 S7 = 48$ M. D, 6 13 M. D. July 25 July 25 July 25 Sept. 27 July 25 July 25 July 25 July 25 July 25 87 = 65$ S7 = 102$ \ Removed by \ Contractor 87 = 105$ \ Removed by \ Contractor 5 19 6 6 6 5 ( 87= 32$ I \ S7= 35$ \ 6 13 6 12 5 19 5 26 5 19 5 26 July 25 July 25 July 25 Sept. 28 July 25 July 25 5 19 6 6 6 5 ( 87= 40$ I I S7= 32$ \ 6 13 6 12 When the barrels are sampled and briquets made, the date sampled is entered in the second column of the barrel record book. The other columns will be explained later. 239. Molders' Records. — Separate sheets of paper ruled and headed as shown on page 148 are used by the molders to record the details concerning the making of briquets. Separate sheets properly ruled and headed are also given to the assistants who test time of setting and fineness. These record sheets, when filled in by the assistants, are copied the following day by the bookkeeper, into the permanent "record book." At the end of the month these separate sheets, con- taining original records of work done, are folded and filed for future reference. 240. Briquet Record. — The briquets are made in sets of ten for convenience. Each set is given a page in the "record book," as is indicated on page 149 where the form for this book is given. The size of page is 9 by 12 inches. Paper having the same ruling and column headings is convenient for reporting tests to the chief engineer. 241. Summary Book. — The data for each set of briquets are copied from the record book, in a condensed form, into the "summary book," one line of the latter containing a page of the former. In the summary book each brand is given a few 148 CEMENT AND CONCRETE HamvaAV SPh -ixirnoiaa •OX •s'rug ■ox O X »> ®o ^> '^ y, 4 "?! SQ ^t fcq fej fa fc* ^5 '-i '8681 "9 'ffiS RECORDS OF TESTS 149 M o o n p « o o H « W a H « pq Em O w o < Oh H CO EG * is =q to^- X » -K [■=1 ^! h K S c-s > ^o^> s a, ^ co H r 1 ~-£ a © «2 P •< 02 -s "^ co co cd e 2 2 £ S'C-c m ^fefqpq. ^ £ g .'£.~a 93 S CD t, -=c ~? 53 S 2 S * s3 0J 3 E " CD ^ ^ o tc ■77 W ^ ^ X H fipnop jaifivajfr Q gg '•xd%X)«i dJLnpyjadutdj, •sai 'nx'jjtaaig HTTSKaX to*>.*<*-'Ototo«oOo'<50ol l *5l . J-^toto^^GOtotoOO^I^I^l M H c a g 02 3 « H Eh ■4 p SO go 5 K D 5 to 3 GO £ a H 03 CO « iW §3 5 3 ^ to . Co • • To Bear Heavy Wire, Mill. . . ** To Bear Light Wire, Min. ; ***■ CO "StAKIS 00T -"ViOXlSSYd CO •kahis "lENso aaj s a O O 2 ft 2 >-3 H o 43 •xanSiag .40 •ojvt , -~H^(SOVJ.lo~| to to •Tag jio 'ojs[ to S >^ 150 CEMENT AND CONCRETE M "sKOixvaijiiaaag jo sxNamaHino -aa AOiag - o>j totototo-sSoto^t 1 ®* 1 — 1 """ — 1 1 — 1 "■ — ( T — ( 1— | W H ^ '-( UiSstoS-h i-iCTs^l ©^ S\j Sx> ^i Co . ^ l>(i ft C5 O Cs "I Ui Hi Hi Itj Hi •aavn sxaaorag o^Qto to to to ■* °i °i Oi ^ »- o» a ^ *i *-< ^ <3s) 00 . °0 "ijacs oi Oi ci *■< hs hs its nj ^i Q . 93 W o o 3 03 be to 1 ~H©lS >- H ^ f-?-^i-*so6ov}.v}-vs. -H i-l <-l *1 d b - - 3 ^ ' - - - - •sxanoiaji jo 'ok NCNON5 > •< — i c^> ■■ — i ~h t — i <^> i — i c^j OiC i ^i- i > -iSxjS;}s5. ^} ®* ^ SO So CiOid3iOoC»;0^|OoCo ^f-?^e-i xi Z ~ ^ ^ •^saAvoq; ©I ^O v). to to °o c "^ »:oo So ^J- So <^j >~h to 00 to «S ^ Oi 00 t^ Co lo Q\( "5-J ^ •*saqStH Si ^( q> -<*-to vf-to> to '^s c-h ■irB8J\[ to to *-h ©} to S.( So vj. V}. Oi v|. v). so S* >-H SO to ^So to SO So S^ *N >-H *^H *~H ^)->Q to i^ S>0 to to to to to VJ.XJ to ?^ oo CO RECORDS OF TESTS 153 243. The Diary. — When the bookkeeper has copied the data contained on the record blanks into the record book and summary book, he turns to the proper page in the diary and records the briquets to be broken. Thus, if briquets made May 17th are to be broken at three months, he enters the num- bers of these briquets and the tank in which they have been placed under the date Aug. 17th. This leaves no chance of allowing briquets to go beyond the proper time of breaking. 244. Acceptance or Rejection. — If all of the tests on a given sample are satisfactory, the date of acceptance is placed in the proper column of the barrel book. It only remains then to mark the barrels "0. K.," and issue them when needed, placing the date of issue in the column indicated. If, however, some of the tests have given unsatisfactory results, the failure is noted in the "defects" column of the barrel book, and the barrel is resampled to determine whether the failure was due to faulty manipulation. If finally rejected, the barrel is promi- nently marked to prevent its being issued for use. It is seen from the above that the history of each barrel is given in the barrel book, and the record of any brand is given in a condensed form in the summary book. 245. Special Tests. — When special tests are made to inves- tigate the effects of variations in manipulation, or for any other special purpose, such as to test the value of certain kinds of sand, it becomes convenient to have still another form which may be called a "series book." In this the results are so ar- ranged that they may be studied for conclusions, and tables for reports may be copied directly from it. A sample form is given on preceding page. Should extra rulings be needed, they may be placed at the right in the "remarks" column. PART III PREPARATION AND PROPERTIES OF MORTAR AND CONCRETE CHAPTER XI SAND FOR MORTAR 246. Mortar. — When cement is mixed with sand and water, the resulting paste is called mortar. The term "neat cement mortar" is sometimes used to designate a cement paste with- out sand, but when the term mortar is not qualified, it refers to the mixture containing sand. The primary function of mor- tar is to bind together pieces of stone of greater or less size, though it is sometimes used alone to prevent the percolation of water, to make a smooth exterior finish, or in places too confined to permit of placing concrete. There are comparatively few cases in which it is judicious to use cement without the addition of sand, for such an ad- mixture not only cheapens the mortar, but actually improves it for nearly all purposes. The quality of sand used is only second in importance to the quality of the cement. Indeed, if one does not know how to select either a good cement or a good sand, he is in greater danger of going amiss in the selection of the latter than the former; for the cement has been placed upon the market by a manufacturer who has a reputation to estab- lish or maintain. Art. 29. Character of the Sand. 247. Various kinds of rock are capable of producing sand of good quality. The natural sands are usually siliceous in char- acter, but calcareous sands are also met with and may give excellent results in mortar. Good artificial sand may be made from almost any kind of rock that is not liable to chemical 154 CHARACTER OF SAND 155 decay, even though it be only moderately hard. One of the most essential features of a good sand is that the grains should be perfectly sound. Evidences that chemical decay is going on in the grains would indicate that the sand is of very inferior quality. 248. SHAPE AND HARDNESS OF THE GRAINS.— It is gener- ally believed that the grains of sand should be angular in order to give the best results; this is probably true, although in test- ing three varieties of calcareous sand, M. Paul Alexandre 1 ob- tained results which seemed to indicate that if rounded grains are disadvantageous, the other properties of the sand may readily counterbalance this disadvantage. M. Alexandre used three sands which were reduced to the same fineness by sifting into different sizes and then remixing them in fixed proportions (equal parts of five sizes). The three sands were, 1st, white marble, very hard with sharp corners; 2d, moderately hard limestone; and 3d, chalk, very soft with rounded grains. The proportions used were 400 kg. of cement to one cubic meter of sand, the amount of water varying from twenty-five to thirty per cent, of the sand, according to the amount required to produce plasticity. The tensile strength of the mortars, in pounds per square inch, is given in Table 45. TABLE 45 Results Obtained -with Three Varieties of Calcareous Sand Character of Sand. Tensile Strength, Pounds Per Square Inch at 7 da. 28 da. 6 mo. H yrs. 2. Limestone ... ... 3. Chalk 45 72 86 107 148 12!) 171 222 20-3 220 256 252 As these sands varied in the structure and hardness as well as in the shape of the grains, it cannot be concluded that rounded grains are as good as sharp and angular ones for mortar-making. There is little question that if two samples of pure quartz sand, '■Recherches Experimentales sur Les Mortiers Hydrauliques.' n 156 CEMENT AND CONCRETE differing in sharpness but alike in all other respects, including the percentage of voids, were tested side by side, the rounded grains would be found inferior. (See also § 253.) M. Alexandre also made tests on sands differing both in chemical and physical characteristics, but having the same fine- ness, namely, twenty per cent, each of five sizes of grain. Some of the results are given in Table 46. TABLE 46 Results Obtained with Various Sands Sand. Water Per Cent, of Volume of Sand. Tensile Strength, in Lbs. per Sq. In., of Mortars Contain- ing 400 Kg. of Cement to 1 Cu. Meter Sand, at Ages of 7 da. 1 yr. 3 yrs. Calcareous (Renville stone) . . . Granitic . Siliceous (Cherbourg Quartzites) . Coke 21 28 21 20 20 28 69 78 65 63 79 35 165 198 158 174 178 99 245 267 201 215 244 132 249. Siliceous vs. Calcareous Sands. — The above tests would seem to show that sand to be used in mortar need not be siliceous. In experimenting on different varieties of sand, both natural and artificial, the author has obtained results that point to a similar conclusion. Some of these tests are given in Tables 47 to 50. Table 47 gives the results obtained with four varieties of siliceous sand. The first was an artificial sand made by crush- ing sandstone, the second and third were natural sands con- taining a large percentage of quartz grains, and the fourth appeared to be almost pure quartz. Only the fine particles of the sands were used in the tests given in this table. The dif- ferences in strength at the end of two years are not great, but the two natural sands appear to give somewhat lower results. In Table 48 the two natural sands were again compared, but this time in connection with a calcareous sand formed by crushing limestone. The latter gave the best results. Only the finer grains were used in these tests. 250. Tables 49 and 50 are more valuable in this connection, CHARACTER OF SAND 157 TABLE 47 Values of Different Varieties of Fine Siliceous Sand for Use in Portland Cement Mortar Two Parts Sand to One Cement by Weight a o -A a Si a a Sand. Fineness. Water, Per Cent. Tensile Strength, Lbs. per Sq. In. at 6 Mo. 2Tr. a b c d e 1 2 3 4 5 ( Screenings from ( ) crushing Pots- < ( dam sandstone ( Bank sand, siliceous River sand, siliceous Clean quartz Pass 40 sieve . Pass 40 sieve, retained on 100 Pass 40 sieve . Pass 40 sieve . Pass 40 sieve . 18.5 17.5 13.3 12.1 13.3 388 478 433 382 398 470 539 445 437 506 Note. — Holes in No. 40 sieve 0.015 inch square, holes in No. 100 sieve about 0.0065 inch square. TABLE 48 Different Varieties of Fine Sand for Portland Cement Mortar Tensili Strength, Pounds per Square Inch. o "A Per Cent. Water. Sand. Fineness. 1 Part Sand to 1 2 Parts San 1 to « a a a Cement by Wt. Cement by Wt. 1 to 1 1 to 2 6mo. 13 mo. 3yr. C mo. 18 mo. 3yr. a b c d e / 9 h i 3 1 River sand, sili- ceous . . . Pass 40 sieve 14.0 12.4 715 725 776 491 575 581 2 Bank sand, sili- ceous . . . Pass 40 sieve 14.5 12.6 664 699 759 442 502 524 3 Calcareous sand from crushing limestone . . Pass 40 sieve 18.2 17.7 721 770 788 531 632 680 4 Calcareous sand Pass 40, re- limestone . . tained on 100 17.5 17.0 753 783 844 597 659 727 since the coarser particles of the sand were used with the fine. The sand was separated into four sizes by sifting, and then remixed in equal proportions. Table 49 gives the results ob- 158 CEMENT AND CONCRETE tained with natural cement, and Table 50 refers to Portland. The superiority of the screenings is very clearly shown, the limestone giving especially good results. Indeed, the strength obtained with three parts limestone screenings to one part of either Portland or natural cement is remarkably high. The mortar made from such sand is peculiarly plastic when fresh, and soon gains a high strength which it appears to maintain. TABLE 49 Values of Different Varieties of Sand for Natural Cement Mortar Sand. CO H 'A H El H Tensile Strength, Lbs. per Sq. In., 3 Parts Sand to 1 Cement by Wt. 28 Da. 6M<5s. 1 Yr. 2 Yrs. 1 2 3 4 5 a b c d e / 9 Clean crushed quartz . . . River sand, siliceous . . . Limestone screenings . Potsdam sandstone screenings Clean crushed quartz . Mx. Mx. Mx. Mx. 20-30 15.4 13.3 16.7 18.2 12.5 117 1 93 143 113 118 344 297 4(57 316 330 356 339 526 416 342 332 308 601 462 324 1 13.6 per cent, water, trifle dry. Note. — Fineness Mx. means 25 per cent, each of 20-30, 30-40, 40-50 and 50-80. Expression 20-30 means passing No. 20 sieve and retained on No. 30 sieve. TABLE 50 Values of Different Varieties of Sand for Portland Cement Mortar Sand. in W 'A W Tensile Strength, Lbs. per Sq. In., 3 Parts Sand to 1 Cement by Wt. 28 Da. 6Mos. 1 Yr. 2 Yrs. a b c d e / 9 1 2 3 4 5 Clean crushed quartz . . River sand, siliceous . . . Limestone screenings . Sandstone screenings . Clean crushed quartz . Mx. Mx. Mx. Mx. 20-30 12.5 11.1 12.5 1 12. 5 1 11.1 255 206 407 321 259 327 284 574 438 344 359 329 667 495 369 335 324 665 2 492 3 335 1 Trifle dry, plastic. 2 13.3 per cent, water. 3 14.3 per cent, water. Note. — Fineness Mx. means 25 per cent, each of 20-30, 30-40, 40-50 and 50-80. FINENESS OF SAND I 59 251. Slag Sand. — To turn to good account some of the immense quantities of blast furnace slag produced yearly, the use of granulated slag in place of ordinary sand has been ad- vocated. In a paper read before the Engineers' Society of Western Pennsylvania, in March, 1904, Mr. Joseph A. Shinn described some experiments he had made, in which it was shown that "slag sand," with Portland cement, natural cement, or common lime, gave a higher strength than the sample of river sand used in the comparison. The "slag sand" is produced by projecting two flat jets of water into the stream of molten slag, the resulting sand being heavier, finer and more nearly uniform in size of grain than the ordinary slag granulate. 252. Sand for Use in Sea Water. — It has been said that granitic sands when used in sea water do not give good results on account of the felspar of the granite being attacked by the cement when the concrete is impregnated with sea water. M. Paul Alexandre would proscribe the use of argillaceous sands in sea water, but he found that sands containing calcareous marl gave excellent results in the sea, and others have stated that the mixture of crushed limestone with concrete has been known to hinder the action of sea water upon it. Since porous and permeable mortars are most liable to disintegration by sea water, it is evident that it is especially desirable to employ a sand in which the proportion of voids is small. Art. 30. Fineness of Sand 253. The size and shape of the grains are important ele- ments in the quality of sand. Considering grains of the same shape but differing in size, the larger grain will have a smaller surface area in proportion to the volume than the smaller grain, since the volume varies approximately as the cube of one di- mension while the surface varies as the square. Since, in order to obtain the best results in mortar, each grain of sand must be coated with cement, it follows that, other things being equal, the coarser grained sands will give the best results, because they will be more thoroughly coated; this will be especially true when the amount of sand in the mortar is relatively large. Following the same reasoning given above as to the relative volume and superficial area of sand grains, it would appear 160 CEMENT AND CONCRETE that spherical grains would be better than cubical or angular ones (see § 248). This, however, is not thought to be the case, for the better bond obtained with angular grains seems to coun- terbalance the advantage which the small superficial area would appear to give to the spherical grains. For this reason a len- ticular shaped grain, while having a very large area relative to its volume, will give excellent results in mortar if otherwise suited to the purpose. It is usually desirable to have all of the voids in the sand filled by the cement paste, as this renders the mortar less por- ous, and makes it more certain that all the grains are coated with cement. On this account a mixture of fine and coarse particles is excellent. TABLE 51 Effect on Tensile Strength of Varying Fineness of Limestone Screenings Used -with Portland Cement Age Tensile Strength, Pounds per Square Inch Fineness of Screenings. Briquets when Broken. 10-20. 20-30. 30-40. 40-50. 40-80. Pass 50. 6 months . 718 657 633 516 403 2 years . 812 754 656 516 488 4 years . 845 782 714 571 516 Significance of Fineness Designation. Sieve Number. Approximate Mean Size of Grain. Passing. Retained on. 10-20 20-30 30-40 40-50 40-80 Pass 50 10 20 30 40 40 50 20 30 40 50 80 Inch. .057 .028 .020 .015 .012 .008 Notes. — Three parts screenings to one cement by weight. All briquets made by one molder and immersed in one tank. Variations in consistency were slight, the largest percentage of water being used for the finest particles. FINENESS OF SAND 161 254. TESTS ON EFFECT OF FINENESS OF SAND. — Many of the experiments made to show the effect of the fineness of sand on the strength of the mortar are defective, because the sand used varies in the shape of the grains and in chemical charac- teristics as well as in fineness. The experiments given in Table 51 were made with screenings obtained in crushing limestone, and thus all causes of variation aside from the fineness of the sand were absent, except the differences in consistency of the mortar, the uniformity in consistency depending on the judg- ment of the operator. The results show quite clearly the su- periority of the coarser sand. 255. The Relative Effect of Fine Sand on Portland and Nat- ural Cement. — The tests in Table 52 were made to determine Coarse and Fine TABLE 52 Sand, — Relative Effects -with Portland and Natural Cement Age of Briquets when Broken. -ij . asp a, ■*< « o Tensile Strength, Pounds per Sq. In. when Sand is Percentage Strength, Fine to Coarse. 'A . 8 i-l X 2 f- 1 H < a 2 & o Tensile Strength, Pounds per Sq. In. avhen Sand is % & & ^ 20-30 40-80 20-30 40-80 28 days . . \ 6 mouths . . < 2 years . . j Bn In Bn In Bn In 197 89 216 364 256 450 145 57 188 267 250 419 74 64 87 73 98 93 A U A U A U 406 352 520 499 546 567 337 . 275 446 415 451 496 83 78 86 83 83 89 Notes. — Sand, limestone screenings; three parts to one cement by weight. 20-30 means sand passing sieve with 20 meshes per linear inch, and retained on sieve with 30 meshes per linear inch. Columns 5 and 9 show percentage that strength with finer sand is of the strength with coarser sand. the relative effects of fine sand on Portland and natural cements. Limestone screenings of two sizes of grain were used in con- nection with two brands of each kind of cement. At twenty- eight days the natural cement shows the decrease in strength due to the use of fine sand more than Portland cement does. 162 CEMENT AND CONCRETE At six months the fine sand seems to have about the same effect on Portland and natural, but the two-year results in- dicate that the ultimate effect is less on the natural cement than on the Portland; the mean ratio of the strength obtained with fine sand to that given by coarse sand being ninety-six in the case of natural, and only eighty-six in the case of Port- land. The effect of fine sand appears to decrease with age, especially with natural cement. The fineness of sand will be treated further in the following article relating to voids. Art. 31. Voids in Sand 256. Conditions Affecting Voids. — The voids present in a given mass of sand will depend upon the shape of the grains, the degree of uniformity in size of grains, the amount of moisture present, and the amount of compacting to which the mass has been subjected. If all of the grains in a given mass of sand are of uniform size, the percentage of voids will be independent of what that size may be. In other words, the percentage of voids in a cubic foot of buckshot will be the same as in a cubic foot of bird shot; but if we take a cubic foot of a mixture of buck and bird shot we will find that the voids are much less. 257. Effect of Shape of Grain. — M. Feret has published in France the results of a large number of experiments made by him as to the voids in sand and broken stone. 1 Table 53 gives the results he obtained concerning the effect of the shape of the grains on the percentage of voids present. He first divided each sand into three parts by means of three sieves, which we will call A, B and C. Sieve A had four meshes per sq. cm. (about five meshes per linear inch), sieve B had 36 meshes per sq. cm. (about fifteen meshes per linear inch), and sieve C had 324 meshes per sq. cm. (about forty-five meshes per linear inch). The grains that passed A and were retained on B were designated G, the grains that passed B and were retained on C were designated M, and the grains that passed C were desig- nated F. These different sizes were then recombined by tak- ing five parts of- G, three parts of M and two parts of F, and Abstracted in Engineering News, Vol. XXVII, p. 310. VOIDS IN SAND 163 the resulting sand was designated G 5 M 3 F 2 . Thus, all of the sands tested had the same "granule-metric" composition. TABLE 53 Voids in Sands Having Different Shaped Grains From M. Feret Nature of Sand. Volume of Voids Remaining in One Liter of Sand. Unshaken. O.C. Shaken to Refusal. C.C. Natural sand with rounded grains. Cherbourg quartzite, angular grains. Crushed shells, flat grains. Residue of Cherbourg quartzite crushed between jaws, laminated grains. 359 421 44;! 475 256 274 318 34(3 It is seen that the rounded grains have the smallest percent- age of voids, or about thirty-six per cent, unshaken, while the laminated grains gave the largest percentage. It may also be noticed that the angular grains were compacted more by shak- ing than any of the others. 258. Effect of Granulometric Composition of Sand on the Percentage of Voids. — To determine the effect of uniformity of size of grain upon the percentage of voids and the strength of mortars, the author has experimented with an artificial sand formed by crushing limestone. That portion of the product that passed the coarse screen of the crusher varied in fine- ness from particles three-eighths of an inch in one dimension to a very fine powder, the particles of which were less than .0065 inch in one dimension. Such material admits of division into parts that differ widely in fineness, but which are essentially of the same composition, and it is therefore excellent for an experiment of this kind. The four sieves used in first separating the material into parts had, respectively, 10, 20, 40 and 80 meshes per linear inch, the sizes of the holes being, respectively, about as follows: 0.08 inch, 0.033 inch, 0.017 inch, and 0.007 inch square. The sev- eral sizes of grain are designated as follows: — "C," Coarse, passing No. 10, retained on No. 20. "M," Medium, " " 20, " " 40. "F," Fine, " " 40, " " 80. "V," Very fiae, " " 80. 104 CEMENT AND CONCRETE M. Feret's method of designating the granulo metric compo- sition, namely, to represent by exponents the number of parts of each size of grain, has been adopted. 259. The voids were obtained by first weighing a given volume of the sand; dividing the weight by the specific gravity of the limestone, as previously determined, gives the amount of solid material in the measure, and this subtracted from the volume of the measure, gives the voids. This method is con- sidered more nearly accurate than the usual one of measuring the amount of water required to fill the voids in a measure of sand, especially so for a sand of uniform character and one which absorbs water quite freely. TABLE 54 Voids in Limestone Screenings, Showing Effect of Variations in Granulometric Composition Weight of "olume Solid Sand in Per Cent. Fineness op Granulometric one Liter of Sand, Drv, ,< Grams. One Liter 3P. Gr. = 2.667) IN ''OIDS Sand. Composition. Loose. Shaken. Loose. Shaken. I iOOS« . Shaken. a b c d e / g C = Coarse 10 to 20 1126 1358 422 509 37.8 49.1 M = Medium 20 to 40 1140 1362 428 511 37 2 48.9 F = Fine 40 to 80 1150 1392 431 522 56 .9 47.8 V = Very fine, pass 80 1165 1609 437 603 56 .3 39.7 C 1395 523 47.7 M 1439 540 46.0 F 1459 547 45.3 V 1656 621 37.9 C 55 , M 25 , F 15 , V 5 1606 602 39.8 C 40 , M 30 F 20 V 10 1732 649 35.1 C 25 , M 25 , F 25 , V 25 1912 717 28.3 C 30 , M 25 , F1 5 , V 3 ° 1850 694 30.6 C 50 , M°, F°, V 50 1991 746 25.4 The results obtained are given in Table 54. Comparing the voids in C, M, F and V, it is seen that the first three have nearly the same percentage, but V has less voids than the others. This is explained by the fact that this sample was made up of all sizes smaller than the holes in No. 80 sieve, down to the fine powder. Comparing the mixed sands, it is seen that the sample made up of equal parts of coarse and very fine had VOIDS IN SAND 161 the least voids, the percentage being only a little more than half of that obtained with coarse particles alone. The next lowest percentage was given by the sample having equal parts of four sizes. It is apparent that the granulo metric composition has a very important effect on the percentage of voids. When one desires to make a compact mortar with as small a quantity of cement as possible,, similar tests might well be made with the materials available for use. 260. Effect on Strength of Mortars of Varying the Granulo- metric Composition of Sand. — Table 55 gives the results of tensile tests of mortars made with limestone screenings of vari- ous granulometric compositions. The differences in strength are not very great, but it appears that with one-to-three mor- tars the highest strength is developed at six months, with the coarse grains alone, but when poorer mortars are in question the result is affected by the percentage of voids in the sand. TABLE 55 Limestone Screenings -with Portland Cement. Effect on Tensile Strength of Variations in Granulometric Composition of Sand Tensile Strength at (tRasdlometeic Composition 6 Mos. Pounds per Weight of of Sand. Per Sq. In. with Parts Sand Briquets in Cent. of each Size Grain. Voids. to One Cement by Grams. % Weight. C M F V 3 5 3 5 100 46 509 324 1465 1438 40 30 20 10 35 505 392 1466 1480 25 25 25 25 81 470 356 1445 1455 30 25 15 30 28 496 391 1448 1470 50 50 25 487 349 1455 1460 Cement. see text. Portland, Brand R. For significance of composition of sand, 261. Table 56 gives the results of similar tests of both Port- land and natural cement with Point aux Pins sand dredged from St. Marys River and containing a very large percentage of quartz grains. The sand was divided into but three parts by sifting, and was then remixed, the proportion of each size being indicated in the table. The results verify the conclusions 166 CEMENT AND CONCRETE already drawn that the coarser sands give the higher strength. It appears that not more than one-half of the grains should be very fine if the best results are desired. TABLE 56 Varying the Granulometric Composition of River Sand. Effect on Value of, for Use in Cement Mortar Composition of Sand as to Fineness. Tensile Strength, Pounds per Square Inch. Parts Used that Passed No. 20 Sieve and Re- tained on No. 30. Parts Used, 30-40 Parts Used that Passed No. 40 Sieve. Portland Cement with Two Parts Sand to One Cement by Weight, at age of Natural Cement with Three Parts Sand to One Cement by Weight, at age of M F V 28 da. 6 mo. 1 yr. 2 yr. 28 da. 6 mo. lyr. 2yr. 10 4 2 1 1 1 4 3 2 5 4 6 7 .142 300 290 246 271 471 448 425 384 366 560 515 494 455 456 591 507 503 442 438 77 77 79 46 67 267 237 278 222 226 348 304 291 234 247 341 319 325 251 251 Note. — River sand, mostly quartz, obtained at Point aux Pins. Each result mean of five briquets, all made by one molder. 262. Effect of Moisture. — The effect of a small amount of moisture on the bulk of a given weight of sand is not usually appreciated, but it may easily be shown that it is very marked. The results in Table 57 were obtained by adding small amounts of water to a given bulk of dry sand. Each time, after the water was added, the sand was stirred up and the weight of a given volume of the moist sand was obtained. It appears that the finer sands are affected more than coarse ones. In the case of the limestone screenings 40-80, if we add but 3.7 per cent, water to a given quantity of dry sand, the bulk of the sand is so increased that if we take 1,000 c.c. of the moist sand it will contain but 720 c.c. of dry sand. The voids are, of course, correspondingly increased from 54.5 per cent, to 67.2 per cent. The cause of this increase in bulk is that each grain of sand is surrounded by a film of water which prevents the grains from lying close together after they have been disturbed. A large amount of air is also imprisoned in the mass. It may be noticed that the difference in bulk between moist and dry sand is greater when the measurements are made "loose." VOltiS IN SAND 167 TABLE 57 Volume of Sand and Voids as Affected by the Addition of Water Volume of o •A w H Sand. H t. « . ft 8 & 5 ft o c3 * 2 0h M £ Parts Loose Damp Sand to One of Packed Cement. oO 3 o oO on -a % § " = Parts Loose Damp Sand to One of Loose Cement. 1 1.10 1.38 85 1.06 0.75 0.94 0.60 0.75 2 2.20 2.75 1.70 2.12 1.50 1.88 1.20 1.50 3.30 4 12 2.55 3.19 2.25 2.81 1 80 2.25 4 4.40 5.50 3.40" 4 25 3.00 3.75 2.40 3.00 5 5.50 6.88 4.25 5.31 3.75 4.69 3.00 3.75 6.60 8.25 5.10 6.38 4.50 5.62 3.60 4.50 In preparing the above table the following assumptions are made : Material. Weight IN A Barrel. Volume OF A Barrel. Weight per Cubic Foot. Packed. Loose Dry. Loose Damp. Portland cement . Natural cement . Sand .... 380 280 3.45 cu. ft. 3.75 cu. ft. 110 75 85 60 100 80 usually give about the same tensile strength neat and with one part sand by weight. Tests showing the rate of decrease of strength with added sand are discussed in §§363 to 365. Portland cements are usually mixed with from one to three parts sand by weight, and natural cements are mixed with from one to four parts by weight (or three-fourths part to three parts by measure). For certain special purposes poorer mortars are sometimes employed. To arrive at the proper proportion to use in mortar for a given purpose, the tables of strength given in Chapter XV will be of value. 277. Effect of Pebbles. — If the sand contains pebbles, the proportions should be considered in a little different way. Suppose we make a one-to-three mortar with sand that con- tains ten per cent, of pebbles. We have in reality, then, 3 X .90 = 2.7 parts of sand to one of cement, and .3 part pebbles em- bedded in this richer mortar. This point is of special signifi- cance in making concrete from gravel containing some sand, or 176 CEMENT AND CONCRETE from broken stone from, which the fine particles or screenings have not been removed. Such fine particles serve to weaken the mortar by increasing the dose of sand, while the pro- portion of aggregate is diminished. In using aggregates con- taining some fine material, then, or in using sand containing pebbles or fine gravel, one should not permit himself to be de- ceived as to the actual richness of the resulting mortar or concrete. 278. AMOUNT OF WATER FOR MORTAR. — The amount of water required for mortar will vary with the proportion of sand to cement, the character and condition of the ingredients, the weather, and the purpose which the mortar is to serve. If the water is stated as such a percentage of the combined weight of cement and sand, the amount required for a rich mortar will be greater than for a poor one, since the cement requires more water than the sand. Fine cement will require more water than coarse; the same is true of sand. Sand from ab- sorbent rock will require a larger amount of water. On a hot, dry day, more water must be used to allow for evaporation; and again, if the mortar is to be placed in contact with brick or porous stone, the mortar must be more moist than when used in connection with metal, or with hard rocks such as granite. All of these points must be borne in mind when determining the proper consistency for a given purpose. 279. We may arrive at the approximate amount of water required in the following manner: find what proportion of water is required for the neat cement. This will vary among different samples, and especially between Portland and natural cements; the former requiring twenty to twenty-eight per cent, of water (by weight), and the latter thirty to forty per cent. Then find the amount of water required to bring the sand alone to the consistency of mortar. This will vary considerably, fine sand requiring much more water than coarse, etc., as men- tioned above. Having these two quantities, we may find the amount of water required for a mortar having any given pro- portions of these samples of cement and sand. Thus, suppose we find that the neat cement requires twenty-five per cent, water and the sand ten per cent, water to bring them to the proper consistency. If we wish to make a one-to-three mortar from these ingredients, using one hundred pounds of cement, the MIXING 111 required amount of water is (100 X .25) + (100 X 3 X .10) = 25 + 30 = 55 pounds. 280. However, it will usually be better to experiment di- rectly upon the mixture which it is proposed to use, and for this purpose the following rule will be found of value. For ordinary purposes, that amount of water should be used which for given weights of the dry ingredients will give the least volume of mortar with a moderate amount of packing. In the actual use of mortars it is not practicable to state that a cer- tain definite amount of water shall always be used with given quantities of the dry materials. It is the resulting consistency of the mortar that must be specified and insisted upon, while the amount of water required to produce this consistency will var}'' from day to day and must be left to the discretion of the inspector or foreman. For a discussion of the relation of con- sistency to the tensile strength of the mortar, see Art. 46. Art. 35. Mixing the Mortar 281. Having decided upon the proportions of cement, sand and water, it remains to incorporate these into a plastic, homo- geneous mass. The size of the batch should be so adjusted, if possible, that a full barrel of cement shall be used, and for careful work the amount of sand should be weighed instead of measured. Where this is impracticable, the condition of the sand from day to day, as regards the amount of moisture con- tained, should be taken into account (see §§ 262 and 263). Mortar is usually mixed by hand, but where large amounts are to be used, machine mixers may profitably be introduced. 282. HAND MIXING. — For hand mixing, a water tight plat- form or shallow box should be used, of such a size that the given batch will not cover the bottom more than four inches deep. If the sand is measured, a bottomless box, provided with two handles at each end, will be found more convenient than the bottomless barrel which is often employed for this purpose. When the sand is delivered on the mixing platform in barrows, the latter may be fitted with rectangular boxes to avoid re- measuring. A two-wheel cart, the box of which may be in- verted to discharge .the contents on the mixing platform, will also be found very serviceable when the runway is suited to such a vehicle. 178 CEMENT AND CONCRETE The proper amount of sand is evenly spread on the plat- form, the cement is then dumped on top of the sand and spread out over it to an even thickness. With either hoes or shovels the dry materials are then thoroughly mixed, until, when a small amount is taken in the hand, it will appear of uniform color throughout. From two to five turnings of the materials, according to the expertness of the workmen, will be required to produce this result. The dry mixture is then drawn to the edges of the platform to form a ring, and the requisite amount of water is added at one time in the center. The mixture is then gradually incorporated with the water, and the mass is thoroughly worked until plastic and homogeneous. Should it be found that too little water has been used, a small amount may be added from a sprinkling pot or rose nozzle, but the mass should always be worked over again after such addition. Four shovels may be used at one platform, but if the mixing is done by hoes, not more than two can be used to advantage with a batch of ordinary size. Some engineers prefer one method and some the other, but in whatever manner done, the mixing should not be stinted. From two to four turnings of the mass are usually considered sufficient, but as a general rule it will be found that further mixing, beyond that required to just give the mass a uniform appearance, will be amply repaid in the strength of the result- ing mortar. (See Art. 47.) 283. MACHINE MIXING. — Where large quantities of mortar are required, machine mixers are sometimes used. A very complete plant for mortar-making was used in building the Titicus Dam. 1 In this case machinery was used in measuring the proportions of cement and sand as well as in making the mortar. The measuring apparatus consisted of two cylindrical troughs, one for cement and one for sand. Each trough was divided, by means of six radial vanes and four discs, into eigh- teen equal compartments. These cylinders revolved in cast iron boxes which were so constructed as to serve as hoppers for filling the compartments. Three compartments were pre- sented to the hoppers at once, and slides were provided by which any of the hoppers could be cut off at will. The cylin- 1 Engineering Record, August 3, 1895. INGREDIENTS REQUIRED 179 ders being geared to the same pinion, it was possible, by means of the slides, to make any desired proportion of cement and sand from neat cement, to three parts sand to one cement. The mixing machine "consisted essentially of a semi-cylindri- cal wronght-iron trough with extended flaring sides, with ele- ments slightly inclined to the horizontal, and in its axis a re- volving shaft with oblique radial blades set at an incline of ninety degrees to each other and of a length to just clear the bottom of the trough." 284. Another form of machine that is sometimes employed consists of a semi-cylindrical trough in which rotates an axis carrying a blade in the form of a screw. The materials are fed to the mixer at one end and the screw mixes them while working the mass toward the other end. Art. 36. Cost of Mortar 1 285. INGREDIENTS REQUIRED FOR ONE CUBIC YARD OF MORTAR. — The character of the ingredients used in making cement mortar varies so much that it is difficult to accurately deter- mine the quantities of materials required for a proposed mortar except by experimenting with the materials that are to be employed. It has been shown that the weights per cubic foot of both cement and sand vary greatly according to the condi- tions of packing, the moisture, etc. The percentage of voids in the sand is one of the most important variations affecting the amount of mortar made with certain materials mixed in given proportions. The consistency of the mortar also has a marked effect, and different cements show a considerable varia- tion in the volume of mortar that a given weight will yield. In any general treatment of the question, then, we may expect only approximate results, and the tables given in this connec- tion must be considered in this light. 286. Results of Experiments. — The tests from which Tables 60 and 61 were derived, were made with a natural sand weigh- ing about one hundred pounds to the cubic foot, dry, and having about three-eighths of the bulk voids. The grains varied in size from 0.01 in. to 0.1 in. in diameter with a few grains out- 1 Portions of this article were contributed to " Municipal Engineering," and appeared in that magazine, Feb., 1899. 180 CEMENT AND CONCRETE o u C Proportions by Volume Dry Loose Sand to Loose Cement, Loose Cement Assumed at 85 lbs. per Cu. Ft. si* CO s o § 0-t o: H t~' CO CM' T-I T-I T-i 5 o ^ © © © © iffl uO ' ' H -t « C H (M . . CO -t © © "3 T(H IMH •■ . Proportions by Volume Dry Loose Sand to Packed Cement, Cement Assumed at 114 lbs. per Cu. Ft. or 380 lbs. per Bbl. of 3.33 Cu. Ft. --I'd !8 . O '=■* ©-*-*'Nr-©'— 'CO O LO N OO X 00 Ol C35 ©©©©©©©© 53 S3 O o5 ® « o CO ■ca © CM © -* >0 i© ■<* CO 13 a 3 o * ©©©©»©©©© -H -+l -fl I© © Iffi t~ I— 1 CO © i-i CO © i-O -# •* CM i-H i-H Proportions by Volume Dry Loose Sand to Packed Cement, Packed Cement Assumed at 104 lbs. per Cu. Ft. or 3S0 lbs. per Bbl. of 3.65 Cu. Ft. CO S o C» ©or- h CO © © i© t~- CO CO © - © © © © © © ■ 4^ 53 53 O cq o CO V. © r~ -* © cm co Tjn ■<*! i-l CO © © TO i— 1 t^-*(N(MHrHH - •2 5a © i©> © i© *© l© »© r-i CO CO CO i—l © CO 00 i£> © l— © "3 •* 1MHH Yt V. Tti cq cq i-i i-i i-J o Ph ■o © i.O © © © o © • i-H •© IO © © © CM CO >© © t~ © ■«* -* ' CM i-H ^H •xxaitao .IO T OX adVS SXHVtl e 0^ > Proportions by Volume Dry Loose Sand to Loose Cement, Loose Cement Assumed at 60# per Cubic Foot. OQO^- 1 - O t 1 i^ O CO 00 00 ©'©'©'© 3 3 D Bbls. of 3O0# Net. • r^ © © l^ Tf O 'N O i^ vi o- CO CO CO CO ©' ©' ©" ©' O © © 3 Bbls. of 300# Net. W C— © CO O © l— CO *rt< p CN -^ 05 O CO t^-* WCqr-irHH Bbls. of 280# Net. -a O CO CO t- CO CO CO © © tP >0 © © "*< oo' "■#' CO Ol CM' r-< i-H Bbls. of Net. o "3 HO(M us t~- r-i ■* C-) "O 1- r-< t- iO oo' id co cm oi r-i rt ■spunoj rO © © © © © © o rt CD l^ 7) t^ i^ © CM CO © Irt O -ft rt. CM ^ ■isaiiag jo i ox aitvs sxhvj e o rt ?) co -t io a 182 CEMENT AND CONCRETE side of these limits. The consistency of the mortar was such that when struck with the shovel blade the moisture would glisten on the smooth surface thus formed. In the experiments the proportions were determined by weight, and the results for proportions by volume were deduced from them. The results for neat natural cement mortar and for the natural cement mortars containing more than four parts sand by weight were derived by analogy. 287. Explanation of Tables. — The first section of Table 60 gives the amount of materials required for Portland cement mortar when the proportions are stated by weight; the second and third sections refer to proportions by volume of loose sand to packed cement when the size of the cement barrel is as- sumed at 3.65 cu. ft. and 3.33 cu. ft., respectively. The fourth section gives the materials required when the proportions are given in terms of loose sand to loose cement. Likewise, the first section of Table 61 for natural cement refers to proportions by weight; the second, third and fourth sections, to propor- tions by volume of loose sand to packed cement when the cement weighs 265 pounds, 280 pounds and 300 pounds, net : per barrel, respectively; while the fifth section refers to propor- tions of loose sand to loose cement. As has been shown, the method of stating proportions by weight is the most accurate, but when the sand does not ap- proximate the weight of 100 pounds per cubic foot when shoveled dry into a measure, the sections of the tables referring to weight proportions may require a correction, and it may be simpler to use the sections giving proportions by volume of loose sand to packed cement. The method of stating proportions by vol- umes of loose sand to loose cement is to be deprecated, but since it is occasionally used, provision is made for it in the tables. In using those portions of the tables where the proportions are stated by volume, it should be borne in mind that if the sand is damp when used it will weigh less per cubic foot, and hence more, by measure, will be required to make a cubic yard of mortar. 288. Estimating Cost of Mortar. — With the data given in Tables 60 and 61 and a knowledge of unit prices of the mate- rials used in the mortar, one may estimate the cost of the ma- terials in a given quantity of mortar. The cost of the mixing COST OF MORTAR 18. f will, of course, depend upon the cost of labor, the method em- ployed, etc., and may vary from fifty cents to a dollar and fifty cents per cubic yard. If we assume, for illustration, that natural cement can be delivered on the mixing platform for SI. 10 per barrel of 2S0 pounds net, that sand costs 60 cents per cubic yard, and the mixing costs $1.00 per yard of mortar, then we have for the cost of a mortar composed of one part cement to two parts sand by weight: — ■ 3.46 bbls. cement at $1.10 $3.80 0.72 cu. yd. dry sand at .60 . .43 Cost of mixing per cu. yd 1.00 Total cost of one cu. yd. of mortar $5.23 289. For approximate results, Tables 62 and 63 give the cost of the materials used in a cubic yard of mortar for different prices of cement. In Table 62 the proportions by weight only are indicated, since for Portland the proportions by volume of loose sand to packed cement vary so little from proportions by weight. TABLE 62 Cost of Portland Cement Mortar Cost of Cement and Sand in One Cubic Yard of Portland Cement Mortar. Sand, 75 Cents per Cubic Yard Cost of Port- Cost of Ingredients in Mortar, in Dollars. land Cement per Proportions in Mortar by Weight, — Parts Sand to One of Cement. Barrel Net. 1 2 3 4 5 6 $1.20 8.90 5.33 3.89 3.03 2.56 2.23 2.02 1.30 9.62 5.73 4.17 3.23 2.72 2.36 2.13 1.40 " 10.36 6.14 4.44 3.43 2.88 2.49 2.24 1.50 11.10 6.55 4.72 3.63 3.03 2.62 2.35 1.60 11.84 6.96 5.00 3.83 3.19 2.75 2.46 1.70 12.58 7.37 5.27 4.03 3.35 2.88 2.57 1.80 13.32 7.77 5.55 4.23 3.51 3.01 2.68 1.90 14.06 8.18 5.82 4.43 3.67 3.13 2.79 2.00 14.80 8.59 6.10 4.63 3.82 3.26 2.90 2.10 15.54 9.00 6.38 4.83 3.98 3.39 3.01 2.20 16.28 9.41 6.65 5.03 4.14 3.52 3.12 2.30 17.02 9.81 6.93 5.23 4.30 3.65 3.23 2.40 17.76 10.22 7.20 5.43 4.46 3.78 3.34 2.50 18.50 10.63 7.48 5.63 4.61 3.91 3.45 2.60 19.24 11.04 7.76 5.83 4.77 4.04 3.56 2.70 19.98 11.45 8.03 6.03 4.93 4.17 3.67 2.80 20.72 11.85 8.31 6.23 5.09 4.30 3.78 2.90 21.46 12.26 8.58 6.43 5.25 4.42 3.89 3.00 22 20 12.67 8.86 6.63 5.40 4.55 4.00 184 CEMENT AND CONCRETE TABLE 63 Cost of Natural Cement Mortar Cost of Cement and Sand in One Cubic Yard of Natural Cement Mortar. Sand, 75 Cents per Cubic Yard Method of Stating Cost of Cement per Barrel, Dollars. Proportions, and "2" DO a Weight of Cement in One Barrel. H 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 ( 5.07 5.92 6.76 7.60 8.45 9.30 10.14 10.98 11.83 12.68 1 3.50 4.02 4.54 5.06 5.59 6.11 6 63 7.15 7.67 8.19 Proportions by weight. J Barrel 265 lbs. net. "i 2 2.74 3.10 3.47 3.83 4.20 4.57 4.93 5.30 5.66 6.03 3 2.23 2.50 2.78 3.05 3.32 3.59 3.86 4.14 4.41 4.68 4 1.90 2.12 2.33 2.54 2.76 2.98 3.19 3.41 3.62 3.83 ( 4.48 5.23 5.98 6.72 7.47 8.22 8.96 9.71 10.46 11.20 1 3.14 3.60 4.06 4.52 4.98 5.44 5.90 6.36 6.82 7.28 Proportions by weight. J Rarre] Slid lbs net \ 2 2.48 2.80 3.12 3.45 3.77 4.09 4.42 4.74 5.06 5.38 Bariel3001bs.net. ^ 3 2.04 2.28 2.52 2 76 3.00 3.24 3.48 3.72 3.96 4.20 I 4 1.75 1.94 2.13 2.32 2.51 2.70 2.89 3.08 3.27 3.46 / 5.07 5.92 6.76 7.60 8.45 9.30 10.14 10.98 11.83 12.68 By volume ; parts dry loose sand to packed 1 3.13 3.58 4.02 4.46 4.91 5.36 5.80 .6.24 6.69 7.14 cement. Cement as-- 2 2.29 2.57 2.85 3.14 3.42 3.70 3.99 4.27 4.55 4.84 sumed 265 lbs. per bbl. Q 1.83 2.04 2.24 2.45 2.65 2.86 3.06 3.26 3.47 3.67 of 3.75 cu. ft. V 4 1.63 1.79 1.95 2.11 2.27 2.43 2.59 2.75 2.91 3.07 By volume; parts dry ( 1 2.94 3.36 3.77 4.19 4.61 5.03 544 5.86 6.28 6.70 loose sand to packed cement. Cement as--! sumed 300 lbs. per bbl. 1 2 3 2.22 1.82 2.50 2.02 2.77 2.22 3.05 2.42 3.32 2.62 3 60 2.82 3.87 3.02 4.15 3.22 4.42 3.42 4.70 3.60 of 3.75 cu. ft. J, 4 1.59 1.75 1.91 2.06 2.22 2.38 2.53 2.69 2.85 3.00 In Table 63 the cost of materials in one cubic yard of natural cement mortar is given, 1st, for various parts of sand to one of cement by weight when the cost of cement refers to a barrel of 265 pounds; 2d, when this cost is for a barrel of 300 pounds net; 3d, for various parts sand to one cement when the propor- tions are expressed as parts by volume, dry loose sand to one volume of packed cement weighing 265 pounds per barrel; and 4th, when the proportions are expressed as parts of dry loose sand to one volume of packed cement weighing 300 pounds per barrel. The quantities in the table are based upon the as- sumption that the sand used is similar to that used in the ex- periments from which Tables 60 and 61 were derived, and that the cost of sand is seventy-five cents per cubic yard. COST OF MORTAR 185 290. Example. — To indicate the use of these tables, let us determine the cost per cubic yard of natural cement mortar composed of one volume of packed cement to three volumes of loose dry sand when the cement weighs 300 pounds per barrel, net, and costs SI. 25 per barrel, while sand costs $1.00 per cubic yard. In the fourth section of Table 63, opposite three parts sand and under $1.20 and $1.30, we find, respectively, $3.02 and $3.22; then with cement costing $1.25 and sand $0.75, we should have cost of mortar per cubic yard $3.12. But in our example sand is assumed to cost $1.00 per cubic yard, or twenty-five cents more than the price for which the tables are computed, and from Table 61 we find that for this mortar 0.83 cubic yard of sand is required. We must therefore add to $3.12, .83 X 25 = 21 cents, giving $3.33 as cost of materials in one cubic yard of the mortar. The cost of mixing the mortar must be added to obtain the total cost per cubic yard. CHAPTER XIII CONCRETE : AGGREGATE 291. Cement concrete is composed of a mixture of cement mortar and fragments of stone, brick or other moderately hard substances to which the mortar may adhere. Put in place while plastic, it soon obtains a strength and hardness equal to good building stone. This property, combined with its cheapness and adaptability to monolithic construction, renders it one of the most useful of engineering materials. Art. 37. Character of Aggregate 292. MATERIAL FOR AGGREGATE. — Many of the points men- tioned concerning the selection of a good sand are also applicable to broken stone. The latter may be produced from almost any moderately hard rock, provided it is not subject to decay. The best material for broken stone is a rather hard and tough rock, which breaks into angular fragments with surfaces that are not too smooth. Gravel makes a good aggregate, although its surfaces are too smooth and rounding to give the best results. Coarse gravel may be improved by running it through a rock crusher to render some of the fragments angular and rough. A mixture of gravel and broken stone gives excellent results (see § 454). The gravel assists the compacting of the mass, and the fragments of broken stone furnish a good bond. A mixture of this kind also leaves but a small percentage of voids in the mass, and this decreases the amount of mortar required. 293. Sandstones are sometimes said to be better than lime- stones, but this will depend on their relative hardness and structure, and the use to which the concrete is to be put; no general rule will apply. Some limestones seem to be particu- larly adapted to concrete-making, as the cement adheres to the surface so firmly. Granite, syanite and trap are excellent for the purpose. Fragments of brick and of other burnt clay ISO CHARACTER OF AGGREGATE 187 products give good results up to the limit of the strength of the pieces, but this limit is not high. Table 155 gives the results of transverse tests of concrete bars made under the author's direction, to show the comparative value of different kinds of stone. The results of these tests are discussed in § 454. Mr. E. L. Ransome l has pointed out that "for fireproof work, care should be taken to avoid such aggregates as contain feldspar," and that limestone should not be used if the con- crete is likely to be subjected to a long continued red heat. The same writer mentions the fact that finely crushed granite may be inferior to finely crushed limestone for use in concrete; one reason for this being that, ''owing to the brittle quality of granite, in crushing it is not only broken into small pieces, but many of these pieces are so bruised or contused that upon a little pressure being exerted upon them, such, for instance, as can be applied by the finger or thumb, they will crumble." 294. The care required in the selection of a proper quality of broken stone or gravel will depend upon the required strength of the concrete. If a strong concrete is required, rich mortar will not be able to make up a deficiency in the strength of the stone ; but if a low strength is sufficient, and consequently a poor mor- tar is to be used, but little will be gained by having a very strong rock from which to obtain broken stone. In this case a rock which presents a good surface to which mortar may adhere is the principal requirement, and a very hard rock need not be insisted upon. 295. PRESENCE OF SCREENINGS IN BROKEN STONE. — It is frequently required that the broken stone shall be freed from all fine material, resulting from the crushing of the stone, before the mortar is added to form concrete. The wisdom of this requirement is not always clear and depends upon the kind of stone. It has already been stated that some forms of crusher dust or screenings give, if not too fine, most excellent results in mortar; this is especially true of limestone screenings. Again, to retain in the broken stone all of the screenings, will result in diminishing the percentage of voids in the aggregate, and thus decrease the amount of mortar necessary. On the other hand, if a stone is covered with a layer of Engineering Record, Nov. 17, 1894. 188 CEMENT AND CONCRETE moistened, floury dust, it cannot be so readily brought in direct contact with the mortar, and if the mortar does reach the stone it is made less rich by the dust, which acts as so much fine sand. It must be said, however, that so far as our ex- periments go, they do not confirm this latter theory when a moderate amount of fine material is in question, especially with crushed limestone. There is a reason, however, in some cases why the very fine material which acts as sand should be screened out of broken stone, even if it is again used in the mortar for the concrete; the fine material collects in certain parts of the bin or pile, making the proportions irregular, so that one batch of concrete may have a rich mortar with a comparatively large amount of stone, while another may have a poor mortar with but little stone. If, therefore, all of that portion of the broken stone finer than, say, one-eighth of an inch, be screened out and used as so much sand in making the mortar, the resulting concrete will be better and more nearly uniform in quality. 296. Impurities. — Material that is really foreign, such as vegetable mold or loam, will be detrimental to the strength of the concrete. Even clay is not permissible here if it adheres to the stone, because if the surface of a piece of stone is smeared with clay, the mortar will not be able to adhere as well to that surface. Clay in a granulated form and not adhering to the stone may be permitted, however, in small amounts, possibly as much as ten per cent., without seriously injuring the concrete for many uses. When old masonry is torn down, the stones are sometimes crushed for use in concrete, but such stones, having particles of mortar adhering to the surfaces, will not be of first quality for the purpose; their cheapness, however, will frequently out- weigh such objections. Art. 38. Size and Shape of the Fragments and the Volume of Voids 297. As in the case of sand, the shape of the fragments and the degree of uniformity in size have an important effect on the proportion of voids in the mass, and all of these elements affect the value of broken stone for use in concrete. As in mortar each grain of sand should be completely covered with cement, so in concrete should each piece of stone be completely SIZE OF FRAGMENTS 189 covered with mortar. As the pieces in a given volume of broken stone will have a smaller total superficial area when the frag- ments are large than when they are small, we should conclude that the larger fragments will require less mortar or be more thoroughly coated with a limited amount. From the same point -of view we should expect that round fragments would require less mortar than those of irregular shape. It is found however, in practice, that these theoretical con- siderations must be modified to correspond with the facts. TABLE 64 Voids in Broken Stone and Gravel Varying in Granulometric Composition Weight of Character Stone. Granulometric Composition. Broken Stone, Lbs. per Per Cent. Voids. Cu. Ft. Limestone . . . K 83 47 u V 89 44 " F 90 43 " M 91 42 " C 85 46 !< pa) m 70 91 42 U F 5( \ C 50 94 40 " K 30 , F 20 , M 50 102 35| " jy2o v 20 F 20 M 25 C 20 104 34 £( C, 120i i DSo R, 3U lbs. \\\\ 29 Potsdam sandstone V 86 45 tl u M 84 44 cc a V33, F67 88 43 it ti y33 ]?33 JV133 92 40 U ![ j^25 v 25 F 25 M 25 971 36 l Gravel .... V 110" 32" i& F 108 33 u M 106 34 l( y33 p33 ]y[33 112 30 it yso ]y[5o 114 29 Potsdam sandstone! P. 6 cu. ft. on \ in. screen G. 2 " " \ in. to Jin. " ! 100 39 and gravel . . j P. 4 cu. ft. on i in. screen G. 4 " " \ in. to \ in. " ! 109 33 Note . — Stone jarred down in measure for all trials. K passed holes \ inch square, failed to pass holes y 1 ^ inch square. V . " \ \ Y " 1 " " \ " M "2 " " 1 C " 3 " " 2 " 190 CEMENT AND CONCRETE When the pieces of broken stone are too large, they do not bed themselves well in the matrix of mortar, but become wedged one against another, leaving voids in the concrete. While round fragments have a small superficial area in relation to their volume, have a small percentage of voids, and pack to- gether readily, yet they are lacking in ability to form a good bond, and hence do not give the best results. 298. Relation of Size of Stone to Volume of Voids. — As illus- trating the effect of the size of fragments and granulometric com- position of stone on the volume of voids, Table 64 gives a number of results obtained at St. Marys Falls Canal. Table 65 gives some of the results obtained by M. Feret in similar tests. 1 TABLE 65 Size of Stone and Volume of Voids Composition, by Weight, of Small Stone. Pee cent, of Voids by Volume. Fragments Passing a Ring of 90 mm. 1 60 mm. 40 mm. 20 mm. Rounded Pebbles. Broken Stone. and Retained on a Ring of 60 mm. 40 mm. 20 mm. 10 mm. 1 41.4 52.1 1 40.0 53.4 1 38.8 51.7 1 41.7 52.1 1 1 35.6 47.1 ] 4 1 1 33.5 48.8 1 1 1 4 35.6 46.4 The percentage of voids in a mass^of broken stone of uniform size should be independent of what the size may be, and the first few lines in Table 65 show this to be nearly the case with the four samples tested. It is seen from both tables that the more complex mixtures give smaller percentages of voids, and that for all sizes the voids are much less in the gravel than in the broken stone. 1 " Sand and Stone Used for Cement Mortar and Concrete, Abstracted in Engineering News, March 26, 1892. by M. Feret. SIZE OF FRAGMENTS 191 299. M. Feret's Experiments. — ■ To show the effect of the variation in sizes of fragments on the strength of the concrete made, M. Feret experimented with four mixtures of three sizes. The proportions used in the mortar were one part by weight of Portland cement to three parts of Boulogne gravel, gaged with an amount of water equal to seventeen per cent, of the total weight of cement and sand. The volume of mortar used in each case was made equal to the volume of the voids in the stone. The concrete was thoroughly mixed and then rammed into a large cylindrical mold. After four months' exposure to the air, twelve cubes were cut from the cylindrical block, four cubes being cut from each of three consecutive horizontal layers. These cubes were placed in sea water and crushed after one month, being then five months old. The results of the tests are given in Table 66. TABLE 66 Strength of Concrete. Varying Size Stone Vol. of Voids per Cu. Meter. Vol. of Mortar per Cu. Meter of Stone. Weight of Con- crete PER CU. Meter. Mean Resistance in Kg. per Sq. Cm. Granulometric Composition of Broken Stone. Of 4 Cubes from $2 S S i CO -a £ o G M F Cu. Meter. Cu. Meter. Kg. 4 1 1 2 1 4 1 2 1 1 4 2 0.402 0.404 0.486 0.478 402 0.404 0.486 0.478 2296 2272 2276 2264 144 141 106 115 143 141 121 132 173 154 133 151 153 145 120 133 Size "G" of broken stone passed a ring 60 mm. (2.4 inches) in diameter and was held by a ring 40 mm. (1.6 inches) in diameter; "M" passed 40 mm. (1.6 inches) ring and was held by a ring 20 mm. (0.8 inch) in diameter; while "F" passed the 20 mm. (0.8 inch) ring and would not pass a ring 10 mm. (0.4 inch) in diameter. The following conclusions are drawn from this table: (1) In each block the lower layers, which had been submitted to longer continued ramming than the upper layers, offered a 192 CEMENT AND CONCRETE greater resistance. (2) The mean resistance varied according to the granulometric composition of stone used, and was greater with the increasing proportion of large stone in each block. Since the amount of mortar used was in all cases equal to the volume of voids in the stone, the effect of voids on the strength was not noticeable. 300. Further Experiments. — Tables 153 and 155 give the results of some experiments made under the author's direction to test the effect of size and character of broken stone. In these tests the proportions are generally 35 pounds of cement to 105 pounds of sand and 3.75 cubic feet of broken stone, the stone being measured after jarring it down in the vessel. The amount of mortar made was sufficient to fill the voids in the stone when the latter did not exceed about thirty-three per cent (§§ 452, 454). It is seen that, in general, a higher result was given by mix- tures of various sizes than by any one size alone, and the fine stone gave higher results than the coarse. In these tests the effect of voids is shown, since in some cases there was not suf- ficient mortar to fill the voids. 301. GRAVEL VS. BROKEN STONE AS AGGREGATE. — The ele- ments entering into the analysis of the superiority of one kind of aggregate over another are given above, but since the ques- tion of the relative merits of gravel and broken stone is so frequently discussed, a word may be added here to show the special points involved in such a comparison. Gravel is composed of hard, rounded pebbles, the surfaces of which are usually quite smooth. On account of the manner of its formation and occurrence, the sizes of the pebbles are usually graded from coarse to fine. Occasional beds of gravel arc found, however, in which the sizes of the several fragments are nearly the same. In broken stone the fragments are angular and usually have rough surfaces, though the degree of rough- ness depends upon the kind of stone. The sizes of the frag- ments as they come from the stone crusher vary from coarse to fine, but by regulating the crusher jaws and by screening, any desired size may be obtained. 302. In determining the value of a certain material for aggregate, at least six characteristics are to be considered, — the strength and durability of the stone, the size and shape of the GRAVEL VS. BROKEN STONE 193 fragments, the volume of the voids, and the character of the sur- face to which the cement must adhere. As gravel is usually from the igneous rocks, its strength and durability are not often open to question. This may or may not be so in the case of broken stone, but the question of relative value of gravel and broken stone, which is so frequently conclusively settled either one way or another, seldom hinges on this point. As to the average size of the fragments, it is evident that as a general proposition it must be allowed that by proper screening either broken stone or gravel may be obtained of any desired size. Of the three remaining characteristics, the shape of the fragments, volume of voids, and character of surface, the first is probably the least important and the third of the greatest moment. The round pebbles of the gravel slide readily one on another, and do not interlock to give a good bond. The angular fragments of broken stone give a better bond, but on the other hand, if not thoroughly tamped, are likely to bridge, or arch, and thus leave holes in the mass. On account of the shapes of the fragments and because the sizes are usually more varied in gravel, the latter has generally a smaller percentage of voids; thirty to thirty-seven per cent, voids in gravel, and forty to fifty per cent, in broken stone, may be considered to give, in a general way, some comparative figures. Coming now to the character of surface, cement will not usually adhere so firmly to the smooth surface of the gravel as to the freshly broken surface of the fragments of stone, but this cannot be con- sidered a universal rule, for the strength in adhesion is not simply a matter of smoothness or roughness as it appears to the eye or the touch. The adhesion to limestone may be very much stronger than to a sandstone which has a rougher appearance. 303. Summing up the relative advantages, we find that the gravel is suitable for concrete because, first, it is not likely to bridge and leave holes in the concrete; if mixed rather wet, very little tamping is required to compact it; and second, the usual smaller percentage of voids makes it possible to secure a com- pact concrete with a smaller amount of mortar than would be required for broken stone. On the other hand, the angular fragments of broken stone will knit together, as it were, to form a strong concrete if properly tamped, and the very important 194 CEMENT AND CONCRETE question of a suitable surface for adhesion is usually in favor of the broken stone. It is evident, then, that this matter must resolve itself into a question of relative cost and suitabil- ity, and a general statement that either gravel or broken stone is superior, is not tenable. One experimenter using a small percentage of mortar in the concrete, so that the voids in the broken stone are not nearly filled, may conclude that gravel is the better, while another experimenter using a larger amount of mortar, filling the voids in the broken stone but giving a large excess of mortar for the gravel, will conclude that broken stone is much to be preferred. Art. 39. Stone Crushing and Cost of Aggregate 304. Breaking Stone by Hand. — When but a small quantity of concrete is to be made, and broken stone cannot be pur- chased in the vicinity, the stone for concrete may be broken by hand. This is an extremely tedious process, however, and is generally avoided, since broken stone prepared in this way will cost from two dollars and a half to four dollars per cubic yard. In the reconstruction of the breakwater at Buffalo, the cost of breaking stone by hand was two dollars and eighty-six cents per cubic yard, and loading on boat cost thirty-nine cents, making total cost about three dollars and twenty-five cents per cubic yard. 1 305. STONE CRUSHERS. — The most common forms of rock crushers are the gyratory and the movable jaw types. The jaw breaker, or Blake crusher, consists of one fixed plate or jaw and one movable one. The latter is hinged at the upper end and the lower end is moved backward and forward through a short space by means of a toggle joint or other mechanism. The jaws are several inches apart at the upper end, depending on the size of the machine, and converge toward the bottom. The distance between the jaws at the bottom regulates the size of fragments delivered, and this distance may be adjusted at will. The Gates crusher is of the gyratory type and consists of a corrugated cone of chilled iron, called the breaking head, 1 Report of Capt. F. A. Mahan in Report Chief of Engineers, U. S. A., 1888, p. 2034. STONE CRUSHING 195 within a larger inverted cone, or shell, which is lined with chilled iron pieces. The vertical shaft bearing the breaking head is pivoted at the upper end while the lower end travels in a small circle ; an eccentric motion is then imparted to the head, so that it approaches successively each element of the shell. The size of opening can be regulated by raising or lowering the breaking head. Stone crushers are made of various sizes having capacities up to one hundred tons per hour. The cost of running a stone crusher is not great, the principal expense being incurred in breaking the stone into pieces of proper size to feed the crusher, the delivery of the stone to the crusher, and taking it away when broken. Crushing plants are usually provided with revolving screens into which the broken stone is delivered from the crusher. These screens are usually made of perforated steel plate, the holes being such as to separate the material into the sizes desired. Where large amounts of concrete are required, and the stone is to be crushed on the work, the arrangement of the crusher plant should receive careful study to facilitate the transporta- tion of the rock to and from the crusher. The broken stone should be discharged from the crusher into bins, from which the carts or cars may be filled by gravity, or from which the material may be led directly to the mixer through a chute or other form of conveyor. In quarries preparing aggregate for sale, and on important works, very complete stone crushing plants are erected. 1 306. COST OF AGGREGATE. — The cost of aggregate varies greatly according to the proximity of the stone to the crusher, the character of the stone, and the amount required. In ex- ceptional cases gravel suitable for use in concrete is so near at hand that it may be delivered on the mixing platform for from twenty-five to forty cents per cubic yard. When it must be brought from a distance, the cost is correspondingly in- creased. Where a considerable quantity of stone is to be broken, the cost of crushing, aside from transportation of the materials 1 The stone crushing and sand and gravel washing plant used in the con- struction of the Canal at the Cascades of the Columbia, Ore., is described and illustrated in Report of Chief of Engineers, 1891, p. 3332. 196 CEMENT AND CONCRETE to and from the site of the work, would usually be from thirty to forty cents per cubic yard. In one case where the stone was delivered to the crusher in carts after having been sorted from spoil banks containing much poor stone that had to be handled over, the cost per cubic yard of crushed stone was approximately as follows for about six thousand cubic yards crushed in one season: Labor, including sorting and delivering to crusher, per cubic yard of crushed stone $.67 Rent of power plant 04 Fuel 05 Tools, supplies, breakages, etc 12 Interest and depreciation of plant 12 Total cost per cubic yard $1.00 307. The following data concerning the cost of breaking a large amount of stone for road material are given by Messrs. Spielman and Brush. 1 "The stone was broken by a ten-inch Blake stone crusher at the rate of about twenty cubic yards in ten hours. The size of the stones as they came from the crusher was: fifty per cent., two inches size; twenty-five per cent., one and one-half to one inch size; twenty-five per cent., screenings and pea dust. The cost of the crusher, engine, boiler, etc., set up complete, was about twenty-five hundred dollars. The cost of working per day independent of the original cost of the machinery and interest thereon, and also independent of any royalty on the stone, was found by the contractor to be as follows: — Repairs, lubricants, wear and tear on crusher and engine, about $6.00 1 engineman, $2.50; 1 feeder, $1.50; 1 screener, $1.50; 5 laborers quarrying and breaking up stone at $1.00 10.50 1 team hauling stone 5.00 I ton coal 2.50 Cost of preparing and crushing 20 cu. yds. of stone, $24.00 Cost of one cubic yard, $1.20. 308. The cost of breaking trap on the Palisades is given as follows: 2 "Two crushers deliver thirty-five cubic yards of two- 1 Trans. Am. Soc. C. E., April, 1S79. 2 "Construction and Maintenance of Roads," by Mr. Edward P. North, M. Am. Soc. C. E., Trans. A. S. C. E., April 16, 1879. COST OF CRUSHING STONE 197 inch stone per day. when working well, the stone being sledged to go into the jaws readily; fifteen per cent, of the time is lost by breakdowns: — 1 engineman and fireman $2.50 2 laborers feeding, at $1.25 . 2.50 2 laborers screening, at $1.25 2.50 Coal, 1 ton 3.50 Oil and waste 1.00 Breakages 5.00 $17.00 or about fifty-seven cents per cubic yard. "On Snake Island, three crushers were arranged in a row, and the broken stone was carried by an endless belt to the revolving screen, whence it fell into the bins, so that no screen- ers were employed. The engine had one cylinder, eight inches by twenty-four inches, and was running with eighty pounds of steam. The product was said to be one hundred eighty cubic yards per day when there was no breakdown." The cost was as follows: 1 — 1 engineman and fireman $2.50 3 laborers feeding, at $1.25 3.75 2\ tons coal, at $3.50 8.75 Oil, etc 2.00 Breakages 15.00 $32.00 "Allowing for the fifteen per cent, lost by breakdowns, the cost would be about twenty-one cents per cubic yard." At another place on the Hudson, two crushers, set face to face, nine-inch by fifteen-inch jaws, could deliver at the rate of one hundred twenty cubic yards per day when no trouble occurred, but one hundred cubic yards was a fair average. Cost. 1 engineman and fireman $2.50 3 feeders 3.75 2 screeners 2.50 l\ tons coal, at $4.00 6.00 Oil, etc 2.50 Repairs 10.00 $27.00 or twenty-seven cents per cubic yard." 1 "Construction and Maintenance of Roads," by Mr. Edward P. North, M. Am. Soc. C. E. Trans. A. S. C. E., April 16, 1879. 198 CEMENT AND CONCRETE It is noticeable that in all the above cases the item for repairs is very large. The wages paid are lower than at present. 309. The following data concerning the cost of quarrying and crushing about five thousand six hundred yards of broken stone at Baraboo, Wis., is taken from an article by Mr. W. G. Kirchoffer, C. E. 1 Cost per Cubic Yard of Crushed Rock Items. 1901. 1902. Dynamite, at 24 to 27 cents pound . Tools, repairs, depreciation, supplies and improvements • Labor, quarrying and tending crusher . Fuel, at $4.60 per ton, and oil ... Superintendence, including livery Hauling stone to city $ .040 .056 .200 .714 .078 .085 .086 .500 $ .027 .110 .218 .544 .053 .066 .165 .500 Total cost per cubic yard . ■$1.76 §1.68 The cost of common labor was fifteen cents an hour, quarry- men and drill runners, seventeen and one-half to twenty cents, engineers and engine, thirty-five cents, and team and driver, thirty cents. 310. The cost of crushing cobble stone with a rented plant at Port Huron, Mich., is given by Mr. Frank F. Rogers, C. E., from which the following data have been derived. 2 July and August. October and November. Hours run 171.5 1145. 94 522.0 Average cubic yards crushed per hour . 6.67 5.55 Average rental cost per cubic yard . . . 11.6 cents 16.1 cents Average fuel cost per cubic yard. 3.7 " 7.1 " Average labor cost per cubic yard . 22.2 " 27.9 " Average total cost of crushing per cu. yd. 37.5 " 51.1 " 1 Engineering News, Jan. 15, 1903. 2 Michigan Engineers' Annual, 1902, abstracted in Engineering News, March 6, 1902. COST OF CRUSHING STONE 199 In the construction of the defenses at Portland, Me., 1 a No. 5 Champion Crusher was used, driven by a thirty horse-power portable engine. Granite was purchased at one dollar per ton on the wharf. Hauling to crusher cost thirteen cents per ton. Cost of crushing, twenty cents per cubic yard of crushed stone, making total cost of crushed stone in bin at crusher one dollar eighty-three cents per cu. yd. 1 Report of Charles P. Williams; Officer in charge, Maj. Solomon W. Roessler, Corps of Engineers,U.S.A.; Report Chief of Engineers, 1900, p. 757. CHAPTER XIV CONCRETE MAKING: METHODS AND COST Art. 40. Proportions of the Ingredients 1 311. Concrete is simply a class of masonry in which the stones are small and of irregular shape. The strength of the concrete largely depends upon the strength of the mortar; in fact, this dependence will be much closer than in the case of other classes of masonry, since it may be stated as a general rule, that the larger and more perfectly cut are the stone, the less will the strength of the masonry depend upon the strength of the mortar. In deciding, then, upon the proportions of ingredients to use in a given case, the quality of the mortar should first be con- sidered. If the concrete is to be subject to but a moderate compressive stress, the mortar may be comparatively poor in cement; but if great strength is required, the mortar must be of sufficient richness; while if imperviousness is desired, the mortar must also possess this quality and be sufficient to thor- oughly fill the voids in the stone. 312. Theory of Proportions. — The usual method of stat- ing proportions in concrete is to give the number of parts of sand and aggregate to one of cement. These parts usually refer to volumes of sand and stone, measured loose, to one vol- ume of packed cement. However, there is no established prac- tice in regard to this and a "1-2-5 concrete" may mean five volumes of loose stone to two volumes loose sand to one volume loose cement, or any one of several combinations. This method of stating proportions leads to confusion unless one is careful to explain what is meant by such an expres- sion as " 1-3-6 concrete." The evils of similar methods of stating proportions in mortars, and the desirability of fixing upon some standard system of weight or volume, have already 1 Portions of this article were contributed to Municipal Engineering by the author, and appeared in that magazine, May, 1899. 200 PROPORTIONS OF THE INGREDIENTS 201 been pointed out. The only circumstances under which such expressions as the above may be used with propriety are when one wishes to give only an approximate idea of the character of concrete used. From tests of strength it is known that to obtain the strong- est concrete with a given quality of mortar the quantity of the latter should be just sufficient to fill the voids in the aggregate. The strength is notably diminished if the mortar is deficient, and is also impaired by a large excess of mortar. This last statement is subject to one exception: if the mortar is stronger than the stone, then an excess of mortar does not weaken the concrete. This case, however, should never be allowed to occur, since it is evident that the strength of the stone should be at least equal to the required strength of the concrete. Further, the ordinary use? of concrete are generally best served by a compact mixture containing as few voids as possible. For these reasons, then, one should consider concrete not as a mixture of cement, sand and stone, but rather as a volume of aggregate bound together by a mortar of the proper strength. The volume of voids in the aggregate, the per cent, of this volume filled with mortar, and the strength of this mortar be- come then the important considerations in proportioning con- crete. When thus considered, it is an easy matter to determine the required volume of mortar for a given volume of stone, and the amount of cement and sand required for a given volume of mortar has already been considered. 313. Determination of Amount of Mortar to Use. — The bulk of a given quantity of broken stone is not so variable as the volume of sand. The volume of the stone, and consequently the voids, will vary with the degree of packing, but the packing- is not influenced appreciably by the amount of moisture present. The proportion of voids in the broken stone may be obtained as follows: Find the weight per cubic foot of the broken stone in the condition in which the volume of voids is sought, being careful to use a measure holding not less than two or three cubic feet. Also obtain the specific gravity, and hence the weight per cubic foot of the solid stone. Then one, less the quotient obtained by dividing the weight per cubic foot of the broken stone by the weight per cubic foot of the solid stone, will be the proportion of voids in the aggregate. 202 CEMENT AND CONCRETE For example, suppose the weight per cubic foot of the broken stone is 102 pounds. The specific gravity of the solid stone determined in the ordinary manner is found to be 2.724. Then weight per cubic foot of solid stone is 62.4 X 2.724 = 170 102 pounds and 1 — r^ = .40, voids in stone. Another method is to fill a vessel of known capacity with the stone to be used, and to pour in a measured quantity of water until the vessel is entirely filled. The volume of water required indicates the necessary amount of mortar to use. The stone should be moistened before placing in the vessel, to approxi- mate more nearly its condition when used for concrete, and to avoid an error from absorption of the water used to measure voids. 314. As to the degree of jarring or packing to which the stone should be subjected in filling the measure, if the stone is filled in loose, and it is proposed to ram the concrete in place, the amount of mortar indicated will be a little more than the required quantity. If the concrete is to be placed without ramming (as in submarine construction), the amount of mortar indicated will not be too great. On the other hand, if the stone is shaken down in the vessel to refusal, the voids obtained will be less than the amount of mortar which should be used, be- cause it is not possible to obtain a perfect distribution of mor- tar in a mass of concrete, and because the concrete will usually occupy a greater space than did the stone when shaken down. And again, for perfect concrete, pieces of stone should be sepa- rated one from another by a thin film of mortar, and hence the volume of the concrete will be greater than the volume of the stone measured in a compact condition without mortar. A deficiency of mortar is usually more detrimental than an excess. It is safer, therefore, to measure the voids in the stone loose, or when but slightly packed, and make the amount of mortar equal to, or a trifle in excess of, the voids so obtained. 315. Aggregates Containing Sand. — If in the case of broken stone all of the fine particles are used, or if gravel which con- tains a considerable amount of sand is employed, then this fine material or sand must be considered as forming a part of the mortar. This will not change the method of obtaining the amount of mortar required for such broken stone or gravel, but it will change the composition of the mortar used. Thus, MIXING BY HAND 203 suppose we have a gravel ten per cent, of which is sand (grains smaller than one-tenth inch in diameter) and we find the voids to be thirty-three and one-third per cent. To three cubic yards of this gravel we will add one cubic yard of a one-to-three mortar. The voids will be filled, but instead of having three cubic yards of stone imbedded in one cubic yard of a one-to- three mortar, we will in reality have a little less than that amount of stone imbedded in a mortar composed of one part of cement to about three and three-tenths parts sand. 316. Required Strength. — In the paragraphs just preced- ing, an attempt has been made to indicate the general principles to be applied in proportioning the materials in concrete. To decide on the actual proportions of the ingredients to use for a given purpose, one must have clearly in mind the strength that will be demanded and any special condition to which the con- crete is to be subjected. A reference to Art. 57 concerning the strength of concrete, will be of service in deciding on the proper proportions to use in a given case. Art. 41. Mixing Concrete by Hand 317. Necessity of Thorough Mixing. — Too much stress can hardly be laid upon the necessity of thoroughly mixing the concrete if the best results are to be attained. It has already been shown that thoroughness in mixing mortar is repaid by greatly increased strength, and the result is even more marked in the case of concrete. Every grain of sand should be coated with cement, and every piece of stone should be covered with mortar. In general, the cost of mixing is from one-tenth to one-fifth of the total cost of the concrete in place. If by doub- ling the cost of mixing we can increase its strength more than one-tenth or one-fifth in these respective cases, or permit a corresponding decrease in the amount of cement necessary for a given result, the additional labor in mixing is justified. 318. Concrete may be mixed by hand or by machine. Opin- ions vary as to the comparative merits of the two systems, but as a machine properly installed usually furnishes much the cheaper method of mixing, it is usually employed. The saving by this method, however, will evidently depend upon the cost of labor, the total amount of work to be done, and the degree of concentration of the work, or facilities for distributing the 204 CEMENT AND CONCRETE concrete. In certain sections where cheap labor is abundant, the cost of hand mixing may be as low as machine mixing. With proper supervision, hand mixing may be thorough, and the chief argument against it, aside from its cost, is that such hard work is likely to be slighted. The best forms of mixers now on the market, however, give results quite equal to the best hand work. 319. Method of Hand Mixing. — We will assume that the materials have been brought within easy reach of the mixing place. If the concrete is to be mixed near the point where it is to be deposited, the mixing platform must be made portable. Three platforms, each 8 by 14 feet, built of two-inch plank or of two layers of one-inch boards, nailed to four 2x6 inch longi- tudinal scantlings laid flat, will be suitable for such a case. The platforms should be made without vertical sides, though if desired a narrow piece of one-inch board may be laid flat around the edges and nailed. A short piece of rope attached to each corner of the platforms, or to the ends of the longitudinal scant- lings, wall be found convenient in moving them. These mixing boards are placed side by side. The sand, which may be delivered to the mixing platform in wheelbarrows, is first dumped on the board and spread evenly over the surface. If the sand is measured, the barrows should be so arranged as to hold the required amount after "striking" with a straight edge. This will make the measure- ment independent of the judgment of the shoveler. If the sand is delivered in cars, bottomless boxes of two or three barrels capacity, according to the proportions used, will be found more convenient for measuring than barrels. If the sand is determined by weight, which as has been shown is the more accurate method, the scales should be set at a weight which is a factor of the total weight, and but little time will be required to bring the scales to a balance for each barrow. If it is possible, the batch should be of such a size as to take either one or two full barrels, or a certain number of full sacks of cement. This will obviate the necessity of measuring or weighing the cement. The sand having been spread over the surface of the mixing board, the cement is dumped upon it and spread evenly over the sand. These ingredients are then mixed dry, the required amount of water is added at one time in the MIXING BY HAND 205 center of a ring formed of the dry materials, and the whole is thoroughly mixed as described under the head of mortar-making. 320. The mortar having been spread evenly over the board, the broken stone is dumped upon it and evenly distributed over the surface. Four shovelers then mix the concrete. Each shoveler starts at a corner of the board and turns each shovel- ful completely over, casting toward the end and spreading the mortar a little as he draws the shovel toward him. The two shovelers at each end work toward each other, and meeting at the axis of the platform, return to the side and repeat. When the four shovelers meet at the center of the board, they turn the mass again by casting toward the center in a similar manner. If in mixing the concrete it is found that sufficient water has not been used, more may be added from a rose nozzle, or sprink- ling pot, previous to the last turning of the mass. The shovel should always be used at right angles to either the side or the end of the board, never diagonally; and it should always scrape the mass clean from the board, never cut it at mid-depth. From three to five turnings are required to thoroughly mix the concrete. The mode of mixing has been thus minutely described, be- cause if a gang of men are started properly they will soon be- come expert, working in unison; whereas if each man is allowed to mix according to his notion, confusion is sure to result. It is sometimes preferred to spread the stone on a separate board and cast the mortar upon it, but this necessitates one handling of the mortar which does not appear to contribute much to the incorporation of the ingredients. While the shovelers are engaged in mixing the concrete on one platform, the mortar mixers have proceeded to the next platform to mix another batch of mortar, and the cement and sand are being placed upon the third platform. Thus the work proceeds in regular progression without delays. The shoveling of concrete is hard work, and it will be found necessary not only to pick good men for this duty, but to cull them until the evolution results in the proper men for the work. An extra compensation for men who perform satisfactory service in the mixing of concrete will usually be repaid in the character and quantity of the output. 321. With the method described above, a working gang would consist of the following men under ordinary conditions: — 206 CEMENT AND CONCRETE Measuring and supplying cement and sand 1 Mixing mortar 2 Delivering stone from bin, one man with horse and cart, or two men with barrows 2 Shovellers to mix concrete and cast or wheel to place .... 4 Water boy 1 Spreading and tamping concrete 1 Total men required 11 if it is found impracticable to mix the concrete near the place of deposition, it may be necessary to put on two or more extra men to wheel the concrete to place. This gang of eleven men may be doubled and still work on the same three platforms when so desired. With a moderate length of wheel for the materials and the finished concrete ; a gang of eleven picked men, working ac- cording to system, will be able to make from twenty-five to thirty cubic yards per day of ten hours, or about two and a half yards per man. The double gang of twenty-two men may not work to quite as good advantage, and will probably not put in more than from forty to fifty cubic yards per day. It would therefore be somewhat more economical to work two gangs of eleven men each on separate sets of platforms, espe- cially as in this way a rivalry is created. Lack of room, however, will frequently preclude this arrangement. 322. COST OF MIXING BY Hand. — The amount of concrete stated above, two and a half yards per man, may be taken as a maximum. With wages at SI. 75 per day this would corre- spond to a cost of about seventy cents per yard, exclusive of the wages of a foreman. Numerous examples might be cited where the mixing costs more. Colonel Mendell, in writing of the fortifications at Fort Point, California, 1 states that a fore- man (at $4 per clay) and twenty laborers (at $2 per day) made forty-five cubic yards per day of eight hours, the cost of mixing being thus about $1 per cubic yard. It is stated that "the circumstances were exceptionally favorable." As an instance where hand mixing was done at a very low cost, the Lonesome Valley Viaduct 2 may be mentioned. At 1 Jour. Assn. of Engr. Soc, March, 1895. 2 Construction of Substructure for Lonesome Valley Viaduct, Gustave R. Tuska, Trans. A. S. C. E., Vol. xxxiv, p. 247, MACHINES FOR MIXING 207 this point colored labor was used at a cost of $1 for eleven hours' work. A gang of men, distributed as follows, would mix and lay forty cubic yards of concrete per day: — Filling sand barrows and handling water 1 Filling rock barrows 2 Mixing sand and cement 4 Mixing stone and mortar 4 Wheeling concrete 2 Spreading concrete in the molds 1 Tamping concrete in the molds 1 Foremen I Total 16 Fifteen men at $1 per day, and foreman at $2.50 per day, makes a cost of $17.50 for forty yards of concrete, or at the rate of forty-four cents a yard for mixing. Had the laborers received $1.75 per day, however, the cost would have been 72 cents per yard. 323. In the construction of the Forbes Hill Reservoir and standpipe at Quincy, Mass., 1 all concrete was mixed and placed by hand. "The ordinary concrete gang was made up of a sub-foreman, two men gaging materials, two men mixing mor- tar, three men turning the concrete, three men wheeling con- crete, one man placing, and two men ramming. Two gangs were ordinarily employed, placing about twenty cubic yards per day each, or about 1.43 cubic yards per man. The con- crete was turned at least three times before placing." With labor at $1.75 per clay, this would give the cost of mixing and placing $1.22 per cubic yard. The actual cost of mixing and placing varied from $0.97 to $1.53, according to the character of the work. Art. 42. Concrete Mixing Machines 324. General Classification. — Concrete mixing machines may be divided into two general classes, batch mixers and continu- ous mixers. In the former, sufficient materials are proportioned to make a convenient sized batch for the mixer. They are then charged into the machine at once, given a certain amount of mixing, and then discharged at once. In the continuous mixers 1 Described by C. M. Saville, M. Am. Soc. C. E., Engineering News, Mar. 13, 1902. 208 CEMENT AND CONCRETE the materials are clumped on a platform, and after being prop- erly proportioned, are delivered gradually to the mixer, and if fed uniformly, the concrete is discharged continuously by the machine. In the latter method care must be taken to feed the cement, sand and stone together and at a uniform rate. If one man shovels cement, two men shovel sand and four men handle the stone, and the cement man stops to fill his pipe, there is likely to be a poor streak of concrete. It is therefore desirable in feeding a continuous mixer to spread the measured quantity of stone on the platform, and on top of this place the weighed quantities of sand and cement. Then if each shoveler gets his shovel blade under the whole mass, he will have some of each ingredient. 325. There are many styles of concrete mixers of both classes on the market. One of the oldest, as well as one of the best, is the cubical box mixer which consists of a box four or five feet on a side, supported by trunnions at opposite corners, and made to revolve about this axis. A hinged door is provided near one corner of the box by which the latter is charged and emptied. The dry materials may be first charged and mixed and the water added later, either through the door or through a perforated pipe in the axis, or the water may be added with the dry materials; after from ten to thirty revolutions of the box, the mixed concrete is discharged into a skip or on a car, to be conveyed to the place of deposition. The great merit of this mixer is that the materials are thrown back and forth from one side of the cube to another and a thorough commingling results. The chief disadvantage is the difference in elevation between the receiving hopper and point of delivery, making it necessary to elevate the materials; one other defect is that the batch is not in view while being mixed, so that the amount of water cannot be regulated according to slight variations that may occur in the moisture of the sand and stone when charged. 326. To obviate this latter difficulty as well as to facilitate to some extent the charging and clumping of the batch, a form of box mixer is made in which the corners of the box in the axis of revolution are truncated, and the trunnions are replaced by collars which support the box, and through which the materials may be fed and discharged. The collars are supported in a MACHINES FOR MIXING 209 tilting cradle which permits the delivery end to be depressed after the batch is mixed. The advantage of having the batch visible during mixing is perhaps somewhat offset by the greater difficulty of thoroughly cleaning the box when discharged. Mixers working on the same principle are sometimes made in other forms than the cube. One of these is the cylindrical mixer, which is made of boiler plate and may be four or five feet in diameter and five or six feet long. This is rotated about a diagonal axis. It is said to be more easily and cheaply made than the cubical mixer, and dumps more quickly and cleanly, while the cost of operation is about the same, and the mixing is as satisfactorily done as in the cubical form. 327. The so-called " Dromedary Mixer " 1 is a batch mixer specially designed for use on street work. The mixing chamber is a cylindrical steel drum with closed ends, mounted between two wheels. It is hinged along an element of the cylinder so that it opens into two halves like a clam shell bucket, to dis- charge. A trap door is provided for filling. The cart is drawn by a horse, and the chamber may be thrown in or out of gear with the cart wheels. The cement and sand being first added and the trap door closed, the horse draws the cart to the stone pile. The stone and water are here added and the cart is drawn to the work; the concrete, mixed on the way, is dumped by the driver, who merely raises a lever which not only separates the two halves of the mixer, but throws it out of gear so that it stops revolving. The chamber may be thrown out of gear at any time without dumping if desired. 328. The Ransome Concrete Mixer 2 "consists of a hollow rotary dome, having upon the inner surface of its periphery directing guides or flanges, and hinged shelves, by means of which the materials are thrown together and perfectly com- mingled. A discharge chute, or spout, is arranged to deliver the material into the barrow or cart when properly mixed." The mixer is also provided with an automatic device for proportion- ing the materials, and a conveyor to carry them to the mixer. Water is supplied to the mixer through a pipe with facilities for regulating the supply. 1 Fisher and Saxton, 123 G St., N. E., Washington, D. C. 2 Ransome Concrete Machinery Co., 11 Broadway, N. Y. 210 CEMENT AND CONCRETE 329. The Smith Mixer 1 is a batch machine made of two truncated cones placed base to base, and provided on the in- terior with deflecting plates designed to throw the materials from one end of the mixer to the other as the machine is re- volved. At the junction of the two cones, on the outer cir- cumference, is a spur gear by which the chamber is actuated. The latter rests upon rollers in a swinging frame, so arranged that the machine may be tilted for dumping while the drum is revolving. In operating this mixer it has been found advanta- geous to charge the broken stone or gravel first, and give one or two revolutions before adding the oement and sand, as this cleans the mortar from the corners. This form seems to be particularly adapted for a portable machine. They may be had mounted on trucks, with or without an engine, as desired. 330. The McKelvey Mixers 2 are made in two styles, con- tinuous and batch. Both styles are cylinders revolving on friction rollers, and having, on the interior, deflecting blades and a patent "gravity shovel" which lies against the rising side of the drum and casts the materials downward when the cylinder has revolved far enough to overturn the blade. The batch mixer has a shorter cylinder and can be discharged at will. These mixers may be fed by shovels, or they may be provided with a hopper into which the materials may be dumped from carts or barrows. They discharge directly into wheel- barrows. The mixer, and an engine and boiler to run it, are mounted compactly on a truck, or the mixer is furnished on a steel frame without an engine. 331. The pan mixer 3 consists of a large shallow pan into which may be lowered a framework carrying a series of plows. The materials are spread in the pan in layers, the plows are lowered into it, and the pan is revolved about its vertical axis, the plows remaining stationary. The plows are so arranged as to move the materials radially toward and away from the cen- ter of the pan. The water may be added from a rose nozzle. For dumping, an opening is made in the bottom of the pan by withdrawing a slide. Were the plows made to revolve in a 1 Contractors' Supply Co., 232 Fifth Ave., Chicago. 2 McKelvey Concrete Machinery Co., N. Y. Life Bldg., Chicago. 3 Clyde Iron Works, Duluth, Minn. MACHINES FOR MIXING 211 stationary pan, the concrete would be more conveniently dumped in a pile, or in a car, instead of being scattered about under the pan. 332. The Cockburn, 1 a continuous mixer, is in the form of a long box square in cross-section, surrounded at either end by circular rings supported on friction rollers. By suitable gear- ing the mixer is revolved about its longest axis, which has a slight inclination toward the discharge end. The materials are added through a hopper at one end, and fall from one side of the box to the adjacent side as the machine revolves, working gradually toward the delivery end, which is open. The water is added through a pipe at about one-third of the length of the box from the feed end. While this machine has no complicated system of blades to become clogged, the mortar has a tendency to stick in the corners of the mixer, making the interior cylin- drical, and thus much less effective in mixing. Striking the sides of the box with a heavy hammer will detach the mortar, and this requires occasional attention. 333. A common form of continuous mixer consists of a screw working in a cylinder. The materials are fed to the cylinder near one end and are mixed while being gradually worked toward the other end by the screw. The water is added through a fixed perforated pipe at a point about one-third of the distance from the feed end of the cylinder, and the mixed concrete falls from the outlet at the other end. This style is frequently made in a light form and mounted on wheels, and is then con- venient in the laying of concrete for pavements. A modification of the screw mixer consists of a semi-cylin- drical trough, in which revolves a shaft carrying blades set at right angles to the shaft and to each other. The trough is sometimes given a slight inclination to the horizontal, and the blades are so shaped as to assist in working the materials toward the delivery end. 334. The Drake Mixer 2 is of the general form just described. One of the machines made by this company is a semi-cylindrical trough in which revolve in opposite directions two shafts, each carrying some thirty blades. Most of the blades are straight, Cockburn Barrow and Machine Co., Jersey City, N. J. Drake Standard Machine Works, 29S-302 W. Jackson Boul., Chicago. 212 CEMENT AND CONCRETE but some of them are curved to work the material toward the delivery end. 335. Gravity Mixer. — An appliance recently devised, which is called a concrete mixer, consists of a steel trough provided with staggered pins and deflecting plates. The trough is sup- ported in an inclined position and has a hopper at its upper end. Water is supplied through spray pipes at the side of the trough. The materials, stone, sand and cement, are spread in layers on the mixing platform, with the stone at the bottom. The materials are then thrown into the hopper; they are mixed as they descend through the pins, and the product is caught in barrows or carts at the bottom. 336. In a very able article on concrete mixers, 1 Mr. Clarence Coleman, M. Am. Soc. C. E., makes an analytical discussion of the relative efficiencies of the several forms. In this analysis he gives the following weights to the several requirements for a perfect mixer. That the entire mass of concrete shall be so commingled that the cement shall be uniformly distributed throughout the batch is given a weight of forty; that the amount of water shall be subject to control is given a weight of twenty- five; perfect dry mixing and relative time of mixing, each ten; and receiving materials, discharging concrete and self-cleaning, are each given a weight of five. The first three requirements, with a combined weight of seventy-five, relate to the production of good concrete, while the remaining requirements, with a combined weight of twenty- five, pertain to economy in use. In short, the first requisite is that a machine shall be capable of producing a perfect mix- ture; then the machine that accomplishes this result at the lowest cost per cubic yard is the best. The choice of a machine will depend frequently on the character of the work to be done, as some machines can only be used economically where large quantities of concrete are to be used in a restricted area, while others are particularly adapted for portable plants. Art. 43. Concrete Mixing Plants and Cost of Machine Mixing 337. Coosa River Improvement. — The concrete plant used at Lock No. 31, Coosa River Improvement, 2 was erected in a 1 Engineering News, Aug. 27, 1903. 2 Major F. A. Mahan, Corps of Engineers, U. S. A., in charge. CONCRETE PLANTS 213 three-story shed. The top story served as a cement storage room and two hoppers were arranged in the floor to receive the cement for the mixers below. Level with the floor of the second story were two other hoppers immediately below the cement hoppers, to receive the sand and broken stone, while in the first story or basement the mixers were suspended at a height sufficient to allow concrete cars to pass under them. The following description is from the report of the designer, Mr. Charles Firth, U. S. Asst. Engineer: * "The cars used in handling the sand and broken stone are of the side dump pattern and are brought into the charging room on either side of the hoppers. The cement is drawn from the cement room overhead in proper quantities, through verti- cal chutes arranged somewhat on the principle of the old- fashioned powder flask. "The water is added to the materials as they enter the mix- ers, and the quantity, which will probably be variable with the temperature, is controlled by valves on the mixing floor, the operators being governed by indicators, which show the quantity used. The mixers are cubical boxes four feet on each side, inside measurement, made of steel plate five-sixteenths of an inch thick, with 2^ by 2^-inch angle irons. Each mixer is provided with a door in one corner, twenty-two inches square, fastened with a tempered steel spring catch, and held open when required with a hinged screw bolt. The shaft which revolves the mixers is three inches square. It is securely fastened to them by trunnion castings at diagonally opposite corners. The whole is driven by a 10 by 16 inch horizontal engine, and thrown in and out of gear by ordinary friction gearing with friction and brake levers. "After a sufficient number of revolutions in the mixers, the concrete is dumped into the concrete cars below, which are of the center dump pattern." The method given of measuring the cement is not recom- mended, as the charge of cement, if not a full barrel, should always be weighed. The three-story arrangement by which the materials were handled almost entirely by gravity was made possible by the high bank at the side of the lock pit. Annual Report, Chief of Engineers, U.S.A., 1894, p. 1292. 214 CEMENT AND CONCRETE The total cost of the plant, exclusive of the boilers, is stated to have been about $8,000, and the average output about two hundred cubic yards of concrete per day of eight hours. The cost of mixing, depositing and ramming 8,710 cubic yards of concrete in the construction of lock walls was at the rate of $0,884 per cubic yard. 338. Portland, Maine, Defenses. — In the construction of the defenses at Portland, Maine, 1 a five-foot cubical mixer was used. Sand and stone were delivered, by bucket conveyors, in bins directly over the mixer. "Immediately under these bins were two measuring hoppers for stone and sand, respec- tively, and an additional hopper for cement. From these meas- uring hoppers the charge was dumped into the mixer and thence, when mixed, into a car immediately under it. This car delivered the mixed batch by means of a hoisting engine and an inclined track to the site of the battery under a fifty-five foot derrick, which placed it in the work at the point required. Two barrels of cement, sixteen cubic feet of sand, and thirty- two cubic feet of stone constituted a batch. * * * The usual number of men engaged in the operation of mixing and placing was as follows: — Two master laborers, three steam engineers, two stokers and twenty-five laborers." It is said that 200 barrels of cement, or 100 batches, could be mixed and placed in a day of eight hours. This would make the labor cost of this portion of the work 50 or 60 cents per cubic yard. The cost stated, however, varies greatly according to the amount of detail in construction, and the lowest cost given for " labor of mixing and placing" is $1.15 per cubic yard. 339. San Francisco Defenses. — A cubical mixer used in the construction of the defenses at San Francisco 2 mixed 250 cubic yards per day with seven men, engineer, fireman, and five men to feed and dump mixer, at a labor cost of $14.67 per day, or about six cents per cubic yard, exclusive of cost of transporta- tion and ramming. The materials and concrete were handled on cars run almost entirely by gravity. 340. Buffalo Breakwater. — In the construction of the Buf- 1 Report of Charles P. Williams to Maj. Solomon W. Roessler, Corps of Engrs., U. S. A., in charge. Report Chief of Engineers, 1900, p. 745. 2 Maj. Charles E. L. B. Davis, Corps of Engineers, U. S. A., Report Chief of Engrs., 1900, p. 980. CONCRETE PLANTS 215 falo Breakwater/ the mixing plant, consisting of a cubical mixer with necessary engines and boilers and two derricks, was mounted on a dismantled lake schooner which could be placed beside the section of the breakwater under construction. The broken stone was delivered in a canal boat which could be tied up alongside the schooner, and outside of the canal boat lay the material scow. The latter was made from an old dump scow, the decked pockets serving as bins for cement, sand and gravel. Into a steel bucket on the scow were loaded, by wheelbarrows, the following materials: 5.4 cu. ft. (H bbls.) cement. 10.8 cu. ft. sand. 5.4 cu. ft. gravel. 21.6 cu. ft. total. Into a similar bucket on the canal boat 21.6 cubic feet of broken stone were shoveled. As these buckets were filled, they were hoisted by one of the derricks and dumped into the cubical mixer. The latter discharged the mixed concrete into a skip and a derrick deposited the concrete in place. The cost of labor per cubic yard of concrete is as follows: Items. No. Men. Cost PER Hour. Cu. Yds. PER Hour. Cost of Labor per Cu. Yd. Loading material into buckets from scows Mixing, including engine men and derrick men 18 11 13 $3.17£ 2.35 2.65 18.2 18.2 18.2 $0,174 0.129 0.146 Placing, including foreman Total labor . . . 42 $8.17J 18.2 $0,449 The above does not include cost of fuel, nor of transporting materials from the storehouses or yards to the site of the work. 341. Quebec Bridge. — The plant used in the construction of the Quebec Cantilever Bridge 2 consists of a No. 5 rotary stone crusher, with a maximum capacity of thirty cubic yards per hour, discharging into a bucket conveyor which delivered the crushed stone in a small storage bin directly over the con- crete mixer. The latter was of the cubical form, five feet on a side, with a capacity of two cubic yards of concrete per batch. 1 Emile Low, U. S. Asst. Engr., Engineering News, Oct. 8, 1903. 2 Engineering News, Jan. 29, 1903. 21G CEMENT AND CONCRETE The cement warehouse and the sand supply were near the mixer. Cement and sand were hoisted to the top of the ma- chine in boxes, with bottoms inclined at forty-five degrees, each holding a batch, and dumped into the charging hopper of the mixer as required. The mixer was elevated sufficiently to permit dumping the concrete directly in a skip on a car, the latter being run to the work. The skip was handled by guy derricks. This plant made the remarkable record of two hun- dred eighty-five batches in ten hours, and on one occasion turned out one hundred fifty batches in five hours, or, if all were two-yard batches, at the rate of sixty yards per hour. 342. Galveston. — For the construction of the Galveston sea wall two concrete mixing and handling machines were designed, 1 each consisting of a double-deck car, on eight wheels, with two revolving derricks, one on either side for handling materials and concrete, respectively. The materials are delivered on tracks beside the mixer car track which is parallel to the sea wall. One derrick hoists the loaded skips from the material cars and deposits them on the upper deck of the mixer car, whence they are delivered in measured quantities to the Smith Rotary Mixer located on the lower deck. When mixed, the concrete is clumped into a skip, which is handled by the second derrick and dumped into the forms. 343. For work having similar requirements to that just described, namely, for retaining walls on track elevation, Chicago & Western Indiana R. R. at Chicago, the problem was met in a somewhat different manner. 2 An ordinary flat car was double decked and the space between decks inclosed to protect the machinery, including the Drake Concrete Mixer. Cars contain- ing cement, sand and stone were coupled in the rear of the mixer car. These material cars were fitted with removable wheeling platforms, making a complete runway along the sides of the cars. The materials were delivered at the mixer car in wheel- barrows and dumped into measuring boxes, and thence fed to the mixer. The concrete was delivered on a belt conveyor mounted on a boom with turntable permitting nearly half of a revolution. The outer end of the conveyor could be raised or lowered as desired, and the concrete was thus deposited where 1 Engineering News, Jan. 15, 1903. 2 Ibid., Feb. 28, 1901. COST OF CONCRETE 21: needed in the work. To permit the mixer train to move along the track, the two ends of a cable were made fast to anchorage.-; placed about a thousand feet apart, one in front of, and the other behind, the train. As this cable had about eight turns around a winding drum on the mixer car, the train could be propelled forward or backward at will. A somewhat similar form has been used for street work, where the mixer and electric motor are mounted on a truck with a swinging conveyor for the delivery of concrete anywhere between the curbs. A pair of wheels in the rear serve to carry an inclined runway for wheelbarrows by which the materials are delivered to the mixer. 344. The data for the following items concerning the cost of mixing concrete for culverts on railroad work are taken from an article in Engineering News. 1 "The plant is located on a hillside with the crusher bins above the loading floor or platform that extends over the top of the mixer, so that crushed stone can be drawn directly from the chutes of the bins and wheeled to the mixer. The sand is hauled up an incline in one-horse carts and dumped on the floor, and is also wheeled in barrows to the. mixer." The capa- city of the cubical mixer used was seven-eighths cubic yard. The cost of mixing and placing was as follows: Items. Cost pee Day. Cost PER Cu.Yd. Three men supplying mixer at $1.50 per day ..... Fuel and supplies assumed at Cost of mixing 40 cu. yds Two men loading wheelbarrows at 81.50 Four men wheeling wheelbarrows at §1.50 Cost of wheeling 40 cu. yds. 100 feet Four men ramming at $1.50 Four men wheeling in and bedding large stone in concrete at $1.50 $? 50 4.50 2 00 2.00 $0,275 0.225 0.150 150 $11.00 $3.00 6.00 $9.00 $6.00 6.00 Total cost mixing and placing $0,800 1 Location and Construction of the Ohio Residency, Pittsburg, Carnegie & Western R.R., Engineering News, May 21, 1903, 218 CEMENT AND CONCRETE It is not explained why six men are required to load and wheel forty cubic yards one hundred feet in ten hours, but it may be that these men assisted in other operations. Another contractor on the same work used a different form of mixer with much lower loading platform and handled the mixed concrete with skips and derrick. The cost is estimated as follows: 4 1 man feeding mixer $1.50 1 engineman assumed at 2.50 1 derrick man assumed at 2.50 2 tagmen swinging boom and dumping 3.00 6 barrowmen supplying mixer 9.00 2 men tamping 3.00 Fuel, supplies, etc 1.50 Cost of mixing and placing 50 cu. yds. . . $23.00 Cost per cu. yd., 40 cents. Art. 44. Cost of Concrete 345. Quantities of Ingredients in a Cubic Yard. —As has already been indicated, the rational method of proportion- ing concrete is to use just sufficient mortar to fill the voids in the stone, or possibly a very small excess to allow for imperfect mixing; and in ordinary practice this rule should not be de- parted from unless it be for some special reason. When so pro- portioned, a cubic yard of concrete will contain approximately a cubic yard of stone, depending on the method of measure- ment. If we know the percentage of voids in the broken stone or gravel, and consequently the percentage of mortar which should be found in a cubic yard of the finished concrete, we may readily obtain the approximate cost per cubic yard of the latter for a given quality of mortar and given unit prices. Thus, suppose we have stone in which the voids are such that the mortar will amount to forty per cent, of the finished concrete, and we wish to have the mortar composed of three volumes of loose sand to one volume packed natural cement, unit prices being as follows: Cement, $1.25 per barrel of 300 pounds net, 3.75 cubic feet. Sand, SI. 00 per cubic yard. Stone, $1.75 per cubic yard. As in § 290, we find the ingredients in one cubic yard of COST OF CONCRETE 219 mortar to cost $3.33. Since forty per cent, of the concrete is to be composed of mortar, the mortar in one cubic yard of concrete will cost forty per cent, of $3.33, or $1.33, and one yard of stone at $1.75 will make the total cost of the materials in the concrete $3.08 per cubic yard. The diagram herewith may be used to get the approximate cost of the concrete after having obtained the cost of the mortar as before. Thus, if we enter the diagram with the cost of mor- tar $3.33, and follow it to the diagonal line marked forty per cent., we find this is on the ordinate $2.33, the cost of the in- gredients in one cubic yard of concrete when the stone costs one dollar per cubic yard. Hence, $2.33 plus $0.75 equals $3.08, the approximate cost of the materials in a cubic yard of the concrete as desired. 346. The usual method, however, of stating proportions in concrete is to give the volumes of sand and stone to one volume of cement. Thus, one of cement, three of sand and six of stone would usually mean one volume of packed cement, three vol- umes of loose sand and six volumes of loose broken stone. To arrive at the cost of concrete when proportions are thus ar- bitrarily stated, involves a greater amount of work. From the tables already given (Art. 36), we can determine the amount of mortar which a given quantity of dry ingredients will make, and the consequent cost of the mortar per cubic yard. Then a knowledge of the voids in the broken stone will permit of a close estimate of the amount of concrete made, whence we can determine the cost of the latter. For example, suppose it is desired to determine the cost of the materials in a cubic yard of natural cement concrete under the following conditions: 1 bbl. cement containing 280 pounds net, at $1.00 per bbl. 3 bbls. sand weighing 100 pounds per cu. ft., at $.75 per cu. yd. 6 bbls. loose broken stone, having 45 per cent, voids, at $1.25 per cu. yd. 1 bbl. cement = 3.75 cu. ft. = .139 cu. yd., cost $1,000 3 bbls. sand = 11.25 cu. ft. = .417 cu. yd., cost .313 6 bbls. stone = 22.50 cu. ft. = .833 cu. yd., cost 1.041 Total cost $2,354 From Table 61, § 286, we find that it requires 2.03 barrels of cement to make one cubic yard of one-to-three mortar, when 220 CEMENT AND CONCRETE CONCRETE MAKING Cost of Concrete, Dollars per Cu. Yd. Stone Assumed to Cost $1.00 per Cu. Yd. EXAMPLES OF COST 221 proportions are stated as above; then one barrel of cement would make ~-^ = .493 cu. yd. As forty-five per cent, of the stone is voids, the amount of solid stone in six barrels would be 7^^ x .55 = .458 cu. yd. Then the mortar plus solid 27 stone would be .493 + .458 = .951 cu. yd. It has been found by experiment that the amount of concrete will exceed the sum of the mortar and solid stone by from two to five per cent.; hence we may assume in this case that the amount of concrete made with the above materials would be .95 + .03 = .98 cu. yd., and 2.354 -s- .98 = $2.40, the cost of materials in one cubic yard of finished concrete. To obtain the actual cost of con- crete in place, the cost of mixing and deposition must be added (see Arts. 41 and 43). When the volume of mortar used is not greater than the voids in the loose stone, then the amount of rammed concrete made may be less than the volume of loose broken stone. 347. EXAMPLES OF ACTUAL COST OF CONCRETE. — The fol- lowing data are given concerning the cost of concrete on several works where sufficient details have been published to be of value. Defenses Staten Island. 1 Cubical box mixer ; proportions by vol- ume, 1 cement, 3 sand, 5 broken stone; 5,609 cu. yds. of concrete. Items. Cost pee Cu. Yd. Concrete in Place. Cement, Portland, at $1.98 per bbl Sand drawn from beach Superintendence and miscellaneous .... 12.546 1.041 0.225 0.149 0.879 0.477 0.190 85.507 It is stated that hand mixing for a portion of the concrete used in another emplacement cost fifty-six cents more per cubic yard than machine mixing. 1 Major M. B. Adams in charge. Report Chief of Engineers, U.S.A., 1900, p. 837. 222 CEMENT AND CONCRETE 348. Defenses Tampa Bay, Florida. 1 — Cockburn-Barrow mixer, with cableway for placing concrete. Shell concrete made up of 1 cement, 3^ sand and 5|- shell. 1.31 bbls. cement, at $2.42 per bbl $3.17 .71 cu. yd. sand, at 21 cents per cu. yd .15 1.08 cu. yd. shell, at 50 cents per cu. yd 5.4 ■ $3.86 Labor mixing . . $0.28 Labor placing and tamping .33 Labor on forms .155 .765 Total cost per cubic yard $4,625 The above does not appear to include costs of running ma- chinery, fuel, repairs, and depreciation of plant. At the same battery 2 in the following year the cost of broken stone and shell concrete was as follows: .9 bbl. cemsnt at $2.46 (including $0.59 per bbl. storage) $2,214 .28 cu. yd. shell, at $0.45 per cu. yd 128 .47 " sand, at 0.12 " " 056 .80 " stone, at 2.95 " " 2.360 Total materials $4,758 Mixing and placing $0,623 Forms .370 Total .993 Total cost per cubic yard $5,751 349. Defenses San Francisco, Cat. 3 — Cubical mixer; ma- terials drawn from bins into measuring cars, hoisted by elevator and dumped into hopper of mixer. Mixer given twelve to four- teen turns and concrete dumped into cars, pushed by hand out on trestles, and clumped in place. Average capacity plant, 280 cubic yards per day of eight hours. Itemized cost of 8,328 cubic yards of concrete in place was as follows: 1 Report Lieut. Robert P. Johnson, Corps of Engineers, U. S. A., Report Chief of Engineers, 1899, p. 906. 2 Report Lieut. Frank C. Boggs, Corps of Engineers, U. S. A., Report Chief of Engineers, 1900, p. 931. 3 Maj. Charles E. L. B. Davis, Corps of Engineers, in charge. Report Chief of Engineers, 1900, p. 980. EXAMPLES OF COST 223 .758 bbl. Portland cement, at $3.03 per bbl . . . $2,298 .887 cu. yd. rock, at $1.80 per cu. yd 1.597 .41 cu. yd. sand, at 0.73 per cu. yd .299 Water .016 Cost of materials $4,210 Concrete plant, erection per cu. yd. concrete . . $0,269 Concrete plant, running expenses per cu. yd. . . .022 Concrete plant, taking down .020 Cost for plant, exclusive of purchase price .311 Forms — materials $0,272 Forms — labor in erecting .346 Forms — labor taking down .079 Cost forms .697 Labor mixing, placing and ramming .626 Total cost per cubic yard $5,844 350. In the building of a concrete dam for the enlargement of the head of the Louisville and Portland Canal, 1 comparison of cost of hand and machine mixing is given by Asst. Engr. J. H. Casey. 1.63 bbls. natural cement, at $0,635 per bbl. . . $1,034 2 volumes sand, .47 cu. yd., at .87 per cu. yd. . . .408 5 volumes broken stone, .89, cu. yd. at $.84 . . .756 Cost of testing cement • . . .081 Forms, material for .107 Forms, labor making and setting up .168 Cost materials and forms $2,554 Hand mixed concrete: Cost of mixing $1,917 Cost of placing and tamping .791 Cost of mixing and placing $2,708 Total cost hand mixed concrete per cu. yd. in place $5,262 Machine mixed concrete: Charging and running mixer $0,864 Placing and tamping .585 Cost mixing and placing $1,449 Total cost machine mixed concrete per cu. yd. in place $4,003 Difference in favor machine mixing $1.26 1 Capt. George A. Zinn, Corps of Engineers, U. S. A., in charge. Report Chief of Engineers, 1900, p. 3467. 224 CEMENT AND CONCRETE Since the above concrete was placed in large masses, the costs of labor are considered high, and it is probable the work was done with exceptional care. 351. In the construction of the lock at the Cascades Canal 1 the concrete plant was so arranged that the materials did not have to be elevated, but much of the work of transportation was done by gravity. The mixing of about eighteen hundred yards by hand permits a comparison to be made with machine mixing by which method about seven thousand eight hundred yards were made. The costs were as follows: Cost per Op. Yd . op Concrete. Hand Mixed and Machine Mixed and Items. Placed by Derrick. Placed by Chute. Amounts. Total. Amounts. Total. .805 bbl. Portland cement at $4.08 $3.29 $3.29 .450 cu. yd. sand at $1.04 . . .47 .47 .579 cu. yd. gravel at $1.04 . . .60 .60 .317 cu. yd. broken stone at $1.70 .54 .54 Cost materials in concrete . $4.90 $4.90 Timbering .15 .15 Testing cement and general .22 .22 Forms and tests .37 .39* .37 Mixing, labor 1.07 " repairs and fuel . . . .02 .04 1.09 .43 Placing, labor .60 .41 " fuel, tramways, etc. .19 .05 Total cost placing .... Total cost concrete per cu. yd. .79 .46 $7.15 $6.16 352. In the construction of the retaining walls for the Chicago Drainage Canal, 2 a special plant was designed for the work on account of the large quantities of concrete required, and this, combined with the low cost of materials and the char- acter of the work, resulted in a very low cost concrete. On 1 Maj. Thomas H. Handbury, Corps of Engineers, U. S .A., in charge. Report Lieut. Edward Burr, Report Chief of Engineers, 1891, Vol. v. Ab- stracted, Engineering News, June 2, 1892. " Construction of Retaining Walls for the Sanitary District of Chicago," by Mr. James W. Beardsley, and discussion by Mr. Charles L. Harrison. Jour. W. Soc. Engrs., Dec, 1898. EXAMPLES OF COST 225 Section 14 the stone was selected from the spoil banks along the canal and could usually be obtained within one hundred feet. This stone, which was delivered to the crusher by wheelbarrows, required some sledging to reduce it to crusher size. An Austin jaw crusher was mounted on a flat car with the Sooysmith mixer. "The cement, sand and stone were raised from their respective bins by means of belt conveyors running at the same rate of speed, but carrying buckets spaced proportional to the required ingredients." "The cost of a second hand plant used on this section was estimated at $9,600, including two crushers and two mixers at $1,500 for each machine. Common labor cost $1.50 per day; firemen, enginemen, and carpenters from $2.00 to $3.00 per day. The itemized cost is as follows: Items. Cost, Cents PER Cq.YD. General, including superintendent, blacksmith, water boys, etc. Quarrying, i. e., delivering stone to crusher Crushing 7.8 30.3 7.3 14.2 15.0 12.1 10.8 Transportation, delivering sand and cement to mixer by teams Forms, exclusive of lumber Mixing Placing and tamping Total 97.5 40.7 103.3 Cost of plant (no salvage allowance) . . . Cost of cement and sand Total cost concrete per cubic yard $3,015 The amount of concrete used on this section was 23,568 cu. yds. 353. On Section 15 of the same work the conditions were somewhat different. The stone had to be quarried within about a thousand feet of the crusher. The stone, after being broken to crusher size, was delivered on the tipping platform of the No. 7 Gates crusher in cars drawn by a cable hoist. "The average output of the crusher for a day of ten hours was about 210 cubic yards." The materials were transported to the mixer in four and one-half yard dump cars drawn by a light locomotive. The mixer was of the spiral screw type and de- posited the materials on a rubber belt conveyor. The mixer and operating machinery were mounted on a car which pro- pelled itself by means of rope and winch. The plant for this section was new and estimated to cost $25,420, including $12,000 for one crusher. 226 CEMENT AND CONCRETE The detailed cost is as follows: Items. Cost per Cu. Yd. of Concrete, Cents. General, including superintendent, blacksmith, teams, etc. Quarrying (exclusive of 8.3 cents for explosives) Crushing Transportation, delivering cemeut, sand and stone on a platform beside the mixer Forms, exclusive of timber Mixing, including shoveling materials from platform to mixer Placing and tamping Total Cost of plant (no salvage allowance) ....... Powder for quarrying Cement and sand Total cost concrete per cu. yd 8.2 19.2 12.8 8.1 14.2 25.0 11.6 99.1 56.7 8.8 158.6 $3,227 The amount of concrete used on this section was 44,811 cubic yards. CHAPTER XV THE TENSILE AND ADHESIVE STRENGTH OF CEMENT MORTARS AND THE EFFECT OF VARIATIONS IN TREATMENT Art. 45. The Tensile Strength of Mortars of Various Compositions and Ages 354. THE PROPORTION OF SAND. — The rate of change in the strength of mortars as the proportion of sand is increased varies greatly for different cements. The fineness and chemical composition of the cement, and the quality of the sand, are the most important factors influencing this rate of change upon which the question of the relative economies of different mor- tars is so largely dependent. Table 67 gives the results of tests with two brands of Port- land cement mixed with from two to ten parts of river sand, the age of briquets being six months and two years. It is of interest to notice that the strengths of the mixtures are ap- proximately in the inverse ratio of the number of parts of sand used. Thus the strength with six parts sand is approximately two-sixths of the strength with two parts, while with ten parts sand 7 the .strength is nearly two- tenths of that with mortar containing two parts. TABLE 67 Rate of Decrease in Strength with Addition of Sand Portland Cement; River Sand, "Point aux Pins" Parts Sand TO 1 Cement by Weight. Tensile Strength, lbs. per Sq. In. Proportionate Strength, Two Years, if 1 to 2=100. 6 Months. 2 Years. H R H R Mean. 2 3 4.09 6 8 10 512 390 295 175 113 64 504 335 261 144 96 74 534 363 296 191 132 104 548 355 288 174 132 116 541 359 292 182 132 110 100 66 54 35 24 20 227 228 CEMENT AND CONCRETE 355. In Table 68 similar results are given for two samples of Portland cement and two kinds of sand, neat cement speci- mens being included in the comparison. The one-to-one mor- tars give a higher strength than neat cement, and even the mortar containing two parts of the limestone screenings is stronger than the neat specimens. From the one-to-one mor- tars the strengths decrease rapidly as more sand is added, until five parts sand are used, but the strengths then decrease less rapidly as larger additions of sand are made. TABLE 68 Rate of Decrease in Strength with Addition of Sand Portland Cement, Brand Ii; Sand, Crushed Quartz and Limestone Screenings Parts Sand TO 1 Cement by Weight. Tensile Stre PEE S ngth, Pounds q. In. Proportionate Strength if Strength 1 to 1 Mortar = 100. Sample Cement H H, Crushed Quartz Sand, 20-30. Age Briquets, 6£ Months. Sample Cement II, Limestone Screenings, 20-30. Age Briquets, G Months. Crushed Quartz. Limestone Screenings. 689 686 82 78 1 840 881 100 100 2 521 703 62 80 o 368 508 44 58 4 236 335 28 38 5 203 267 24 30 G 156 178 19 20 8 104 138 12 15 10 78 98 9 11 356. In Table 69 two samples of natural cement are treated in a similar manner, from one to eight parts river sand being used in the mortars. With Sample II the strength is dimin- ished rapidly until five parts sand have been added, but with further additions of sand, the strength is decreased more slowly. Sample 18 S gives quite a different curve, as the one-to-two mortar is stronger, and the one-to-three mortar is but little weaker than the one-to-one. With four parts sand the mortar shows a marked falling off in strength, but further additions of sand diminish the strength more slowly. 357. Increase in Tensile Strength with Time. — In Table 70 are given the results obtained in tests of tensile strength COMPOSITION AND AGE 229 TABLE 69 Rate of Decrease in Strength with Addition of Sand. Natural Cement, Brand Gn; River Sand, "Point aux Pins" Parts Sand to 1 Cement by Weight. Tensile Strength, Pounds per Square Inch. Age. G Months. 2 Years. Proportionate Strength, Two Years if 1 to 2 = 100. Sample Cement II. 18 S. 18 S. 1 2 Q O 4 5 7 8 380 297 260 183 128 81 69 56 53 308 314 280 193 161 142 119 101 280 324 294 187 165 172 156 114 ' 86 100 91 58 51 53 48 35 with twelve samples of Portland cement, illustrating the rates of increase in strength from seven days to three years. It is seen that rich mortars gain strength rapidly, neat and one-to- one mortars showing usually eighty to ninety per cent, of their ultimate strength in twenty-eight days. Mortars containing not more than four parts sand to one cement give practically their ultimate strength at six months. It is also of interest to notice that the variations in strength among the several sam- ples are not very great. The lowest strength at the end of two to three years is seventy-five to eighty per cent, of the highest. 358. In the case of natural cements, results for ten brands of which are given in Table 71, only fifty to seventy per cent. of the ultimate strength is gained in the first twenty-eight days; with mortars containing three parts sand to one cement the average result at twenty-eight days is less than forty per cent, of the strength at two years. Most of the samples gain some strength after six months, but two samples fail at two years which had given a fair result at six months. The varia- tions in strength among the several samples are very much greater than with Portland cements; even omitting the two samples that failed, the strength of the highest is two or three times the strength of the weakest sample at two years. 230 CEMENT AND CONCRETE zn H Hi w n < 0) H a id m oi pnug e •SJB8X g oo ■* lo co -* (NMH Ci CO (NNIM i-l i-H l.sraao I oipu-BSf •s.iue^ . CO O .CO 00 CO co • CM CM ' CM O 55 O C B 3 Q S •< b. ^h fa g o •sxbqK z >*OOO'-0^««00MNH CO CM KiOl^OOOOOMH^NH CM iO COCOCOCOCOCOCOCOCOCOCOCO CM CO -.rea^ x OOLO-tOJCCKOCHOOW lO tH a. o o cc co co i' l- a io o co co t- CO CO CO CO CO CO CO CO CO CO CO CO -# CO CO '*-tNrti>i!5iNH(N'*00 1-1 i-H lOCOCOCOOOCDCOCO — i Ci t~ CO t- CO -#-* CO CO CO CO CO CO CO CO CO CO lO CO •stop 8S •stop i t^OiOiOiOMHHOOOfN-f r~ -^ CM CM CM CM CM CM CM CM i-i CM CM CM CO CM •s.res^ s O C) O i-O X M CO © C. 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H3 10 !0 «J »T)( 10 CO (0 ») « t- 00 as © -* co ■— iosoos CMCMOICOCCCMCOCO'* 1 1 of 10 results of 4, 8 and 9 of 2,5, 7 and 10 of 1, 3 and 6 3 Pq _ Hj J^ C+J ft H? g fc fl ■30&5 raaian HiNOO-HHOOt-OOOO I— 1 e8 - - - a> - - - 232 CEMENT AND CONCRETE 359. Table 72 shows in some detail the rate of increase in strength of a sample of natural cement when the specimens are maintained in air and in water. This table has several points of interest. When hardened in water, the cement gained steadily in strength up to six months, when it began to fall off, and at two years this cement failed, as is shown in Table 71. The neat cement specimens hardened in air are very irregular, as usual. These specimens showed high strength at three months, suffered a marked falling off at six months and one year, but showed a remarkable strength, equal to neat Portland cement, at two years. The strength developed at one year by specimens of this sample containing two parts sand to one cement and hardened in air, is also equal to that shown by similar mortars of Portland cement. TABLE 72 Rate of Increase in Strength in Water and Air - Age of Briquets. Tensile Strength, Pounds per Square Inch. Neat Cement. Mortar Two Parts Sand by Weight to One Cement. Water. Air. Water. Air. 1 day 28 days 3 mos 6 mos 1 year 81 192 232 305 390 437 432 395 152 254 315 473 551 372 314 731 135 142 232 367 409 249 271 459 475 537 All cement, Brand Hn, Sample 26 S, which fails in water after two years, see Table 71. Art. 46. Consistency of Mortar and Aeration of Cement 360. Effect of Consistency of Mortar on Tensile STRENGTH. — The results in Table 73 are from briquets of Portland cement with two parts "Standard" crushed quartz. The consistency of the mortars varied from a "trifle dry," in which water rose to the surface only after continued tamping, to a wet mortar which would just hold its shape when placed in a heap on the slab. Half of the briquets were immersed, CONSISTENCY 233 while the remainder were stored in the air of the laboratory. The air hardened specimens gave higher results in all cases than those hardened in water. The highest strength was given in general by the dryest mortar, but the differences in strength decrease as the age of the specimen increases. TABLE 73 Effect of Consistency on the Strength of Portland Cement Mortar Hardened in Water and Air Age of Briquets. Tensile Strength, Pounds per Square Inch. Consistency of Mortar. Briquets Hardened in Fresh Water. Briquets Hardened in Air of Laboratory. a b c d e a b c d e 7 days . . . 28 days . . 3 months . 340 383 515 310 378 535 226 314 514 191 291 429 158 249 411 407 506 665 341 463 593 263 345 638 230 393 597 202 302 451 ___ Cement: Brand R, Sample 18 R, with two parts "Standard" sand. Consistency: a, trifle dry; b, O.K.; c, moist; d, very moist; e, would just hold shape. 361. Tables 74 and 75 give similar results for Portland and natural cement mortars, respectively, all specimens having hardened in water for three months. The amount of water used in gaging had a wide range, giving mortars of all consis- tencies from very dry to very moist. The richness of the mor- tar was also varied, from neat cement to five parts sand. A comparison of the results in these two tables indicates that the highest strength is usually given by mortars a trifle dryer than that considered right for briquets; that an excess of water is less deleterious to rich mortars than to lean ones, and to Port- land cement than to natural cement. 362. Conclusions. — Although all of these tests indicate the superiority of dry mortars, in considering the effect of consis- tency from a practical standpoint, one must not fail to consider the difference between the conditions existing in the actual use of mortars and in laboratory tests. When mortar is used in 234 CEMENT AND CONCRETE TABLE 74 Variations in Consistency of Mortar Effect on Strength of Portland Mortar at Three Months Tensile Strength, Pounds per Square Inch, for Parts Sand to Consistency Number. 1 Cement by Weight. 1 2 3 4 5 6 7 8 9 608 635 763 744 708 707 729 085 1 513 543 618 588 594 613 566 566 538 2 429 447 398 393 382 3 289 322 329 310 279 5 208 230 201 189 167 1 — Very dry ; little or no moisture appeared on Consistency- | surface of briquets. Significance of numbers : J 5 — About proper consistency for briquets. Increasing per cent, water j 9 — Very moist; mortar would barely hold shape used for higher numbers. ^ and shrank in molds in hardening. TABLE 75 Variations in Consistency of Mortar Effect on Strength of Natural Cement Mortar at Three Months Parts Sand to 1 Cement by Weight. Tensile Strength, Pounds per Square Consistency Number. Inch 'OR 1 2 3 4 5 6 7 8 9 1 2 5 372 312 373 314 283 208 155 277' 206 125 305 286 258 183 101 281 242 268 241 204 139 74 263 207 176 239 255 217 150 Consistency — Significance of numbers: 1 — Very dry ; little or no moisture appeared on surface briquets. 5 — About proper consistency for briquets. 9 — ■ Very moist ; mortar would barely hold shape and shrank in hardening. masonry, the stones or bricks, even though they be dipped, or sprayed with a hose before setting, are very likely to press out or absorb considerable moisture from the mortar. To realize this one has only to raise a heavy stone just after it has been bedded; and the greater ease of setting either stone or AERATION OF CEMENT 235 brick, and obtaining a full mortar bed, with a rather wet mor- tar, is appreciated by all masons. In the laying of concrete, the difficulties of obtaining a compact mass with a dry mortar are also not to be overlooked, but this point is discussed elsewhere. 363. Effect of Aeration on the Tensile Strength of Cement. — Portland cements that are not perfect in composition and burning, and that therefore contain- free lime, may sometimes be rendered sound by exposing them to air, and such exposure was at one time considered almost essential in Portland cement manufacture. Fresh Portland cements that are slightly defective may have their properties quite radically changed by such treatment; their rate of setting becoming first more rapid, and then, by further aeration, slower, and their tendency to expand over- come or ameliorated. Portland cements that are perfectly sound suffer some loss in specific gravity by the absorption of carbonic acid and water from the atmosphere, but moderate aeration has no radical effect upon their strength, and Port- lands deteriorate but very slowly by storage, provided the cement is kept dry and does not cake in the package. Natural cements, however, usually suffer by aeration, and this is illustrated by tests on several samples of one brand given in Tables 76 and 77. Of the four samples in Table 76, TABLE 76 Effect of Aeration on Four Samples of Same Brand Natural Cement Number Weeks Cement Aerated. Tensile Strength, Pounds per Square Inch. Age of Briquets, 6 Months to 7 Months. Age Briquets, 2 Years. Sample QQ ss NN 00 NN OO 2 5 7 10 11 13 242 237 256 268 .313 183 269 403 358 279 343 357 225 213 340 506 212 218 316 368 246 260 306 432 284 258 Cement: Brand Gn; Sand, two parts crushed quartz to one cement. All briquets of one sample were made by one molder and same percentage water used. 236 CEMENT AND CONCRETE NN and 00 showed an improvement by two weeks' exposure to air, spread out in a thin layer, but longer exposure resulted in a serious loss of strength. Of the other two samples, SS was greatly improved by five weeks' aeration, but longer exposure was detrimental, while sample QQ showed a continuous im- provement up to the limit of eleven weeks' exposure to air. In Table 77 the effect of aeration on five samples of the same brand is shown. One of these samples was overburned and was rendered practically worthless by fourteen weeks' exposure to air. Nearly all of the samples in this table were seriously affected by six weeks' aeration. TABLE 77 Natural Cement. Effect of Aeration Cement. Parts Sand to 1 Age of Bri- Tensile Strength, Pounds per Square Inch, Cement Aerated. b fa o ° a * g £ O h fa fa Brand. Sam- ple. MEST. 4 to 5 days. 11 to 12 days. 45 to 51 days. 99 days. Oh -H(M«O0 O •C 7-1 CO t~ t- os uo co o o t — t< co co n e co o o o os 7-i r — t< co cm i-H co -T^-^O-OCOI^COt-L^CC'NCN fe '/ zc to a: in w - H O , • O ,■ O „ ?>, o . • o . • 3f & s >,s >,: >> a>>a>-i -< a CD H M H O CM CO H M H +; «(NlSSlCOOOHHTfTf &4 S o O Q fc O • • • • r G 03 i. t? • • • • S cc--- . . . . 'O - - -. co - - - fl - - - ce . . . . g ■ • ■ -co Pn Ph £ 02 „ ia H ~ si co w Ul a xi o B : W:«::::::: H N M ■* iO t- 1^ « C. O H « " M 77-1 -^2 >. I' S PI CD H% 2 fc4 7 ^2 5 S3 CD fl c ^ P c B *- ° CD CD -3 +3 S- « C3 O c3 si S3 a>~ .^ i( bt „ Oj fc T3 e3 CD CD -TO (L'^3^ S3 ^3 C bJD M &£-a tC "S "C £ CD 7- MM a £ S3 CD CD r/j > CC CO bC O g IB dTP-3 ^ ^ 3 " ^S=3s3^0^!-c52 ^~ O g|^^^^ 3 ^-^-o goo I |, -C -j- co i_o p . iia -S3 43 j s s| i g a g'c On" 31 " „^=^^^ 7— -Ti+^^f CDCD-7 2 3o3 "3 r c3 tc i:3 ajccc3o3 £cooo go g g o o 03^' C3 ,0 WtS »V, CT---C ' REG AGING 230 even neat cement mortar does not suffer a great decrease in strength by three to six hours standing. Rich mortars, con- taining one part sand, are not seriously affected by standing three hours if regaged frequently. Poorer mortars, with two to four parts sand, show an actual increase in strength as the effect of such severe treatment as standing five hours, if re- tempered with more water once an hour. These two brands were slow setting Portlands, beginning to set in forty minutes to two hours. The increase in strength of the regaged mortars is doubtless due, at least in part, to the more thorough gaging which they received. Table 80 gives similar results of briquets one year old made at another time with two parts river sand. The fact that dur- ing the delay between the making and use of the mortar it should be frequently retempered with water to make up for the loss by evaporation, is plainly shown. TABLE 80 Regaging Portland Cement Mortar Tensile Strength, Pounds t-er Square Inch, for Varying Treatment. a c d e / h i J 579 554 565 579 569 570 568 627 624 560 Cement: Portland, Brand R, Sample 42 M. Sand: 2 parts "Point aux Pins " passing No. 10 sieve. Age of briquets, 1 year. Treatment: — a — Molded as soon as gaged. c — Mortar let stand 1 hour, regaged and briquets made. d — Mortar let stand 3 hours, regaged each hour. h — Mortar let stand 3 hours, regaged each hour and water added to restore original consistency. e — Mortar let stand 5 hours, regaged each hour. i — Mortar let stand 5 hours, regaged each hour and water added to restore original consistency. / — Mortar let stand 5 hours, regaged and briquets made. j — Mortar let stand 5 hours, regaged and briquets made ; water added to restore original consistency. 367. Similar tests with natural cements are shown in Table 81, and it appears that cements of this class, especially if mixed neat, will not stand the same severe treatments without injury. 240 CEMENT AND CONCRETE CO 0) H U 3 w lH <4 d H id & a nj K I"""" - (M O CO UO 0-1 O H CM 00 CM fc _H CO 00 rr- -91 i— iO CM CO CM 02 H N O t- 5D M CO -f 66 -t 00 X » 1- C OS CM Ol CM CO CM —1 i-i « M o co cm W 03 00 ■* «5 s=< CI o CO gs ,, 1© c to W Jei o* CO El -H > ^1 M >c o rl W t (N CO 0? CO ? S CO^ — o H P5 o 05 --i d 3 cm; oo IK E-* w o r— CO EH£ .o CO X CM CM « o N ■*Hiau5MONfflH e iOffiOH-*'tO'jiOO CM CM -# ^*< CO CO CM i— 1 CM CD CD 03 CO CD ■— 43 +J CC ^ +i f +3 a o icDziaji.z.'li^cD ffl COH©cq©MHMH 0-2 •£ 2 CNCMOOOCMCNTh-* a o y A 1—1 00 ►? ^ • « 1 1 ll 2 CO - 1 1 1 •• ■■ •■ w re; d . 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Mortars containing two parts sand that had stood sixty to ninety minutes with intermediate retempering, showed a slight increase in tensile strength, but more severe treatment was deleterious. In Table 82 the mortars all contain two parts sand to one of cement by weight. The only cases of any serious results of retempering are for mortars standing four hours and regaged, at intervals of one-half hour or one hour, with no water added to tha original mortar. Briquets made from mortar that had been gaged every half hour, and was molded two hours after first mixed, showed a somewhat higher strength than briquets made of fresh mortar. Table 83 shows that the behavior of regaged natural cement mortars, as shown in the preceding tables, is not an eccentricity of one or two brands. Mortars containing two parts sand do not appear to suffer in tensile strength by being allowed to stand two hours if regaged hourly. TABLE 83 Effect of Regaging on Tensile Strength. Five Brands Natural Cement o M Tensile Sti ENGTH, Pounds PER H ffl ■ Square Inch. f ■aj g GO <- Age Briquets. Elapsed BETWEEN First Gag- So Interval Between Suc- cessive Brands 8<=> Molding. Gagings. <( u Damp sand 430 445 442 332 171 7 Nat. An Dry aii- 3 mo. 310 338 359 251 69 8 " " Damp sand u 267 344 327 318 93 9 K (i Water 222 301 319 293 79 In all of the above tests the mortars containing much lime paste were not only more plastic, but somewhat wetter than the corresponding mortars of cement and sand alone, on ac- count of the water contained in the paste. 376. The conclusion to be drawn from these tests appears to be that the addition of a small amount, ten to twenty per cent., of slaked lime to cement mortars containing as much as three parts sand, not only renders them more plastic, but actually increases the tensile strength, especially if the mortars are kept damp during the hardening. It also appears that for PLASTER PARIS WITH CEMENT 249 TABLE 89 Use of Lime Paste in Cement Mortars Containing Four Parts Sand to One Cement Composition of Mortar. Tensile Strength of Mortar, Pounds per Square Inch. Cement. Lime Paste, Grams. Lime in Paste, Grams. Sand, Grams. Stored in Damp Sand. Stored in Dry Air. Kind. Grams. Fresh Lime Paste. Old Lime Paste. Fresh Lime Paste. Old Lime Paste. Portland, f Brand X, J Sample 41 S I Natural, BrandAn, -i Sample L 240 240 200 180 240 240 200 180 00 80 120 180 00 80 120 180 00 27 40 00 00 27 40 60 900 900 960 960 960 960 960 960 176 212 198 204 150 160 106 140 180 200 212 194 133 154 173 166 254 280 227 232 127 162 131 124 244 250 237 184 142 150 170 154 Note. — All briquets three months old when broken. mortars exposed to the open air the lime should be in the form of slaked powder rather than paste. It may be added, that in all cases care should be taken that the lime is thoroughly slaked before use, and all lumps should be removed by straining or sifting. Further results on this subject are given in connection with the tests on adhesion of cement mortar to brick (Art. 5). 377. EFFECT OF PLASTER OF PARIS ON THE COHESIVE STRENGTH OF MORTARS. The use of plaster of Paris, or calcium sulphate, in the man- ufacture of cement to regulate the time of setting, has already been mentioned. The amount of such additions at the factory are usually small, the German Cement Makers' Association limit- ing it to two per cent. Tests on three brands of Portland cement, showing the effect of small additions of plaster Paris, are given in Table 90. All of these mortars hardened in water. It is not known whether any of the cements had received additions of plaster Paris be- fore leaving the factory. It is probable that brands R and X had been so treated, since they are German cements, but it is not probable that the other brands of Portland had received any addition of plaster. It appears that with these brands the addition of from one 250 CEMENT AND CONCRETE to three per cent, of plaster Paris hastens the hardening and increases the strength of the mortar at ages of six months to two years. Six: per cent, plaster sensibly retards the hardening, but, in all cases except one, Brand S, neat, six months, the mortars containing six per cent, plaster, gave higher results on long time tests than did the corresponding mortars to which no plaster had been added. TABLE 90 Plaster of Paris in Portland Cement Mortars, Hardening in Water w a ft « H o « o K g Tempera- ture Water IN WHICH Briquets Stored. ft w w . ° X P sag M < ID B 0) w V z m « D O 01 a H ft 05 a is P o Ph K & C? K K h M H H P5 E-< CC H iJ Z m H 3 5 o o 02 s CO s o S 8 CO o 715 498 416 445 410 442 408 492 o CI s CO 1~ .-* .00 .CO .00 ."# .CO ■<# O 05 CO CO -* CM t^- " LO • -^ " rj< " -^ ' LO • t*i 05 o LO o s> CO CO . LO . LO . t— .CO .05 .CO CO CO CO GO 1^ CO t~ 1- 'CO ' LO - LO - ■* 'CD • ■* CM LO o s 00 •^ . LO .00 .CM . 1— . O .05 O Ol CD l^ CO i-H O CO 'CD ' LO ' LO " -^H "CO ' "0 Ol 00 LO lO ^ 1 OS LO .CD . i-i .CO . O . TH . ■* CO CO I-H i-H LO O -* CO - CO 'CO " t- "CO "l© "LO 05 Ol CO CO CO CO .CO . LO .CO .CO . O .CM 01 i—l CO CO 05 05 05 00 "CD ' LO 'CO ' lO ' lO • lO CD o 5- I— 1 CM CO . -+ .© .05 .CO .CO .CO LO 05 C5 GO T— 1 i—l i—l CO ' LO ' LO ' LO "CO ' LO ' LO 05 05 lO S n o a e3 o a) u o 05 0) p CS . ■^ a c'Sb •~ * SO cS oc S o 3 Ph 8 & LO CO . ■>* .CO .CM .00 .OS .00 . CM -tn LO O CM ■* CO U0 " •* ' •<* " LO ' LO • ■* - Tt< - CO o ei «s CC <-H . t- . I~- .i-l . Tl< .CM .CO . -* Ol CM CJ ri -+ LO LO "."* ■ LO ' "* " LO ■ •<* • -* 05 lO o CC -1- -* .CO . t- .CO .t— .t^ .CM CO CO CC CM i— i CO CO lO ■ T ' ~f " LO ' lO " rj< ' ->* 00 o « o: LO " co .lo .-—I . r— . -m .co oo . t- * • -* - ^ ■ Tfi - CM CO LO s co t- CC t— .1-- . t— .1—1 .CO .OS .CO 00 Oi t^ CD CO O -# -tH 'CO ' ■* - Tjl • T« ■ Tfl • -# ■ o CO - CC CM -* •* . i— 1 .05 .CD .CM ,i—l .CO i-l OS CO CO lO o 00 lo "co • -ji 'co •-* • -* ■ rP • i—l CO o * CC CC o .t^ .i—i .co . o .t— .oo . 05 CO CO I— I LO 05 CO ■«* - C0 • Tt( • -* ■ -^ -CO "tJ( - t^ n -* •N33 roiig NaHM aoy ., H* _*, Hfl H]N rtt-1 ^tN . JiJ^ ®J C5^ CD" 05^ CO" 05 aaa •aavj^ * cScO ^ ^co^i'co^^H"^: ■aavjtj axvQ S3 rfi^ oo i^ » « s oo © ® !N © « a o a o O CO CO CO CO Ol CM CM Ol Ol CM Ol ■ 1 1 1 1 1 1 1 1 1 1 1 1 LOO5'tO5 00Tt CM ~ COCDCOCOOOt^OOt^OS C5 CM i— i CN i— i i— iCMi— iCMt— i C5 i— iC - " 1 1 1 1 1 1 1 1 1 1 1 1 OlNOHOGOOOTf©^ CMl— INHHHHi- It— 1 ,— 1 H z 3 H O •8[dureg F K S^a Mean Tensile Strength, Brand. rt Q Ph H £ a PL, W H a Gn. An. Kn. Hn. Ju. a b c d e / h 1 J k 1 Mo. Da. Deg. Days l 2 20 N 1 9-11 18 Canal 234 322 285 423 284 2 2 22 N 1 16-19 " 201 327 326 302 219 3 2 20 S 1 9-11 18 Open air 344 416 412 321 292 4 2 22 S 1 16-19 " 367 305 480 360 311 5 2 20 s 1 9-11 18 " 7 274 306 413 244 304 6 2 22 s 1 16-19 tt 7 292 338 426 311 304 7 2 21 N 2 7-14 19 Canal 161 318 329 348 238 8 2 23 N 2 9-9 t c 160 217 355 258 186 9 2 21 S 2 9-14 19 Open air 288 289 382 282 319 10 2 23 S 2 9-9 a 338 275 422 423 367 11 2 21 s 2 9-14 19 C( Q 268 271 340 240 295 12 2 23 s 2 9-9 i ( 7 317 333 414 345 356 Note. — All briquets broken when six and a half months old. 266 CEMENT AND CONCRETE ment are given in Table 106. The briquets were made in a temperature of nine to nineteen degrees Fahr. Half of the briquets were made with fresh water, and half with water con- taining enough salt to lower its freezing point below that of the TABLE 107 Portland Cement Mortar in Low Temperatures Effect of Heating Materials o . a « aZ Tensile Strength, Pounds V z Is & X a p < « • £% &8 per Square Inch a £ at? j a. a ^ o y £ Where Stored. at> IS B ««' a z Cold Ma- Warm Ma- Cold Ma- Warm Ma- H §o o a a » 3* terials, terials, terials, terials, « r w a^ a fin « 40°. 110°. 40°. :io°. Mo. 1 1 1-5 23 Canal Wet 6 582 598 2 1 8-9 tc tt " 590 593 3 1 1-5 23 " '« 181 71l' 734' 4 1 8-9 " ' t " 770 737' 5 2 14-16 14 " '' 6i 542 550 6 2 23-24 " " tc 460 476 7 2 14-16 11 CC (1 181 549' 597' 8 Means 9 2 23-24 cc (t " 467' 54l' 572 587 596 469 620 450 1 4-6 23 Open air Dry 61 10 1 9-10 " " it 711 724' 11 1 4-6 23 i t Wet 1 1 487' 470 12 1 9-10 tc it " 628' 614 13 2 15-18 14 tt Dry tc 507' 542' 14 2 24-25 " (( 673 657 15 2 15-18 14 tt Wet " 422' 453 10 Means Grand 2 24-25 i c U 11 543' 495 639 605 622 605 471 534 479 549 Means Notes. — Cement, Portland. Sand, " Point aux Pins." When warm materials used, the temperature mortar after briquets finished, 63° to 71° Fahr. When cold materials used, the temperature mortar after briquets finished, 32° to 39° Fahr. When salt water used for mixing, water was 23 per cent, salt for 1 to 1 mortars and 14 per cent, salt for 1 to 2 mortars. Briquets stored in canal were left in cold air three days before immersion. Part of briquets stored in open air were immersed in tank in labo- ratory one week just before breaking, while others were broken dry as indicated. In general, each result is mean of five briquets. EXPOSURE TO FROST 267 air where the briquets were made. The results are chiefly of interest as showing the strength that may be attained by natural cement mortars under these severe conditions. Higher results are usually given by the air-hardened speci- mens than by those immersed in cold water, though this de- pends somewhat upon the brand. Salt is usually beneficial if the briquets are immersed, and detrimental for open air ex- posure. 394. Effect of Heating the Materials. — The tests in Table 107 were made to determine the effect of heating the materials when working in low temperatures, and thus delaying for a time the freezing of the mortars. The details of the tests are fully given in the table. The conclusion indicated is that the ingredients may be used cold or warm indifferently. A gain of only four per cent, is indicated for warm materials in mortars mixed with salt water and hardened in cold fresh water. In practical work, however, the use of warm materials may so delay the freezing as to permit thorough tamping before the mortar freezes. Table 108 gives similar results with one brand of natural cement, from which it appears that warm materials have a slight advantage for either cold water or cold air hardening:. TABLE 108 Natural Cement Mortars in Freezing Weather Effect of Heating Materials o . to si O B 1* Tensile Strength, Pounds per Sq. In. £ « a a a 05 a * (B r H a Ha 5 £ Tempera- ture Air Where Briquets Molded. Stored in Canal. Stored in Open Air. Materials. 32° F. Materials. 100° F. Materials. 32° F. Materials. 100° F. . Deg. Fahr. 1 3 15 to 16 6 in os. 140 151 311 372 2 3 15 to 19 9 " 175 203 3 2 22 to 24 9 " 1(37 204 355 361 Notes. — Cement, Brand Gn, Natural. All mortars made with fresh water. Briquets made with warm materials were frozen in from 15 to 24 minutes after made. Each result, mean of ten briquets. 268 CEMENT AND CONCRETE 395. Consistency of Mortars to Withstand Frost. — Since the injury due to frost is caused by the expansion of the water used in gaging, it would be expected that mortars mixed wet would suffer most. This conclusion is confirmed by the tests in Table 109. The superiority of dry mortars is especially shown in mortars that harden in the air. The treatment to which these briquets were subjected was very severe, yet the results are excellent. TABLE 109 Consistency of Mortars as Affecting Ability to Withstand Low- Temperatures Age of Bhiquets When Broken. Tensile Strength, Pounds per Square Inch. Stored in Canal. Stored in Open Air. a b c d e / g h 6 mos. 9 mos. 414 474 414 468 372 431 501 527 601 727 571 (522 521 525 674 563 Notes. — Cement, Portland, Brand R;sand, " Point aux Pins," passing holes .08 inch sq. Two parts sand to one cement by weight. Each result, mean of five briquets. Temperature, air where briquets were molded, 13° to 14° Fahr. ; materials used, 40° Fahr. Temperature mortar when molding completed 32° to 36° Fahr. Briquets made with fresh water had frozen after 30 minutes. Treatment briquets: — a to d, stored in canal (under ice). e to h, stored in open air, January, North- ern Michigan. Water used: — a and e, 10.4 per cent, fresh water. b and /, 11.9 per cent, fresh water. c and g, 13.3 per cent, fresh water. d and h, 11.9 per cent, water containing 15 per cent. salt. 396. Fineness of Sand and Effect of Frost. — The briquets reported in Table 110 were made from mortar containing one and two parts limestone screenings to one cement, the screen- ings varying from coarse to fine. In general, the results follow the rule applicable to mortars used in ordinary temperatures, namely, that the coarse sands give the best results; but it appears that the briquets made with fresh water and exposed EXPOSURE TO FROST 269 a m a > 0) W +-> a J a m .Q ..t-H ^r* -g ^ ^ ^ ? s? SP - P 2 P' "" S o a .1? i-]" arUt H iO CO "M CO t- OS i-H -<# 5. "M 00 CO HiOMHXOO >a ?j (fl tO O O '.O ■* o ^ o o w o ifflTHSDtONOOIMCO^HCOcg LO O 0 lO M O O « lO jo^,— { r -t'Ni'Nt^c0-t# CO -# O g CM CM tlj - s a +J — ■** r* 0) o / — OS r. 03 £ X 7. CD 0> co rr| a; JEJ £ fee CD — — — X o — J2 S-t S3 - cu T3 O O -r a; ^ 03 — CD — +j Q3 03 ... - - a 'C m CO - a 2 "3 CD 03 "o 5 — r- — O) - bl) C 3 pq PQ << 270 CEMENT AND CONCRETE in open air reverse this rule, either the finest sand, f#, or the |g giving the best result. 397. CONCLUSIONS. — The following conclusions concerning the use of cement mortars in freezing weather appear to be indi- cated by the foregoing tests: 1st, Mortars should not be mixed wet for use in low tem- peratures. 2d, Portland cement mortars made in cold weather usually develop a good tensile strength, especially when exposed to the open air. 3d, Portland cement mortars for open air exposure may be benefited by the use of from three to seven per cent, salt in the water used in gaging, and from ten to twenty per cent, salt in the gaging water may prove beneficial for mortars hard- ening in cold water. 4th, Warming the materials for Portland cement mortar appears to have but little effect on its frost resisting qualities. 5th, Coarse sand usually gives the best results in Portland mortars made in cold weather, but fresh water briquets ex- posed in open air appear to give better results with fine sand. 6th, Some natural cements give fairly good results in freez- ing weather, while others are practically destroyed by severe exposure. The effect of variations in treatment on different brands of natural cement is so varied that no general conclu- sions can be drawn from the above tests, but the indications are that salt water for gaging is beneficial if the mortar hardens in cold water, but detrimental for mortars exposed to the open air. Art. 51. The Adhesion of Cements 398. the Adhesion between Portland and Natural CEMENTS. — The question sometimes arises as to whether Port- land cement will adhere to natural cement already set, and whether fresh natural and Portland cement mortars may be used together, as in the case of a Portland facing mortar used with natural cement concrete. Tests bearing on these points are given in Tables 111, 112 and 113. In the tests in Table 111 fresh Portland cement mortar was applied to natural cement mortar that had set seven days. Natural cement briquets, made neat and with one to four parts sand, as seen in the headings of the columns of the table, ADHESION PORTLAND AND NATURAL 271 were broken at the age of seven days. The fresh Portland mortar was applied to the half briquets on the same day that the latter were broken, by placing the half briquet in one end of the mold, and filling the other half of the mold with fresh Portland mortar of the composition shown in the second column. TABLE 111 The Adhesion of Portland Cement to Hardened Natural Cement Mortar Ref. Parts Sand to One Part Portland Cement in Fresh Mortar. Adhesion of Portland Mortar to Half Briquets of Hardened Natural Cement Containing Parts Sand. l 2 3 4 1 2 3 1 2 240 185 63 255 210 75 197 186 00 104 152 84 101 120 85 Notes. — Briquets of natural cement, containing parts sand indicated at top of columns, were broken at seven days. The half briquets were then placed in one end of briquet mold and the other end of mold was filled with fresh Portland mortar. Fresh mortar made of Portland cement, Brand R. Sand, " Point aux Pins," passing No. 10. sieve. In general, each result is mean of ten briquets. Nearly all briquets broke at juncture of Portland and natural mortars. It is seen that the neat Portland gave the highest results in adhesion, the one-to-one mortar giving a comparatively low ad- hesive strength. It is also seen that the neat and one-to-one- mortars adhered best to the richer natural cement briquets, but the one-to-two Portland gave the greatest adhesive strength with the poorer natural cement mortars. All of the tests gave very irregular results. 399. To make the briquets the results of which are recorded in Table 112, a plate was placed in the center of the mold, one- half of the mold was filled with fresh natural cement mortar, the plate was then withdrawn and the other half of the mold filled with fresh Portland mortar. Briquets in line 1 were made with Portland cement alone, while those in line 2 con- tained only natural cement, these briquets being made for pur- poses of comparison. The briquets containing both Portland Si Ji CEMENT AND CONCRETE and natural were made neat and with from one to three parts sand. By noting the number of briquets that broke at the juncture between Portland and natural, it was found that, in general, the adhesion of rich Portland mortar to rich natural cement mortar is greater than the strength of the natural, but that with the poorer mortars the adhesion is less than the strength of the natural. TABLE 112 Adhesion between Fresh Mortars of Portland and Natural Cement Parts Sand to One op Adhesive or Cohesive Strength, Pounds Ref. Cement. per Square Inch. In Po rtland Mortar. In Natural Mortar. 28 days. 3 mouths. 6 months. 1 year. 1 2 278 372 410 464 2 2 164 243 268 308 3 318 358 323 380 4 1 1 252 326 376 383 5 1 229 331 356 357 6 2 1 226 196 339 298 7 2 2 128 235 265 285 8 1 2 145 213 259 271 9 1 3 103 160 185 206 10 2 3 95 176 206 197 11 3 3 63 162 197 193 ■ Notes. — Portland Cement, Brand R. Natural Cement, Brand An. Sand, " Point aux Pins," passing No. 10 sieve. Both mortars mixed fresh and filled in opposite ends of mold. 400. In Table 113 the natural cement mortar contained three parts sand to one of cement, while the richness of the Portland mortars varied from neat to four parts sand. Four combinations of different brands were used. Brand R, Port- land, and brand An, natural, appear to give the best results together. It is also seen from this table that the adhesion of the rich Portland mortar is greater than the cohesive strength of the natural cement, but when the Portland mortar contains three or four parts sand to one cement, the adhesion is less than the strength of the natural cement mortar. 401. The Adhesion to Stone and Other Materials. — Since cement mortars are usually employed to bind other ma- terials together, it follows that the adhesive strength is of the ADHESION TO VARIOUS MATERIALS 273 greatest importance. On account of the difficulty of making tests of adhesive strength, however, the data concerning it are very meager. Two methods have been employed by the au- thor in making such tests. One method, used for brick, is to cement two bricks together in a cruciform shape. The other method consists in placing small blocks of the substance to be used in the center of a briquet mold, and filling the ends of the mold with the desired mortar. TABLE 113 Adhesion between Fresh Mortars of Portland and Natural Cement o z H X H K H £ S a HO £ 3* ~ ffl PL, Natural Cement Mortar. Eh E P a E ffi H O < Adhesive Strength Pounds per Square Inch. Parts Sand to One Cement in Portland Mortar. Brand Cement. Parts Sand to One Cement. l 2 3 4 1 R An o 3 mo. 162 N 173 N 177 N 162 J 127 J 2 R Gn 3 " 117 N 175 N 167 N 156 151 3 A En 3 " 156 N 157 N 143 N 136 N 134 4 G Bn 3 ct 88 N 106 N 115 N 94 91 J fi R An 3 1 yr. 214 206 N 206 N 201 N 183 J 6 R Gn 3 t 4 167 N 165 N 180 N 175 N 169 7 A En 3 &l 157 N 158 N 173 N 166 N 160 8 G Bn 3 It 108 J 127 J ' 126 J 130 J 114 J Notes: — In general, each result is mean of ten briquets. Results marked N, briquets broke through the natural cement. Results marked J, briquets broke at juncture of Portland and natural. The small blocks were made one inch square and about one-fourth inch thick, two opposite edges of each piece being very slightly hollowed to fit, approximately, the side of the mold. These blocks being placed transversely in the center of the mold, and the ends of the latter filled with the mortar to be tested, formed two joints between the mortar and the block. 402. Table 114 shows the adhesion of a rich Portland ce- ment mortar to various materials. The mortar adheres most strongly to brick, the adhesion exceeding the strength of the brick itself. A very high result is also obtained with terra cotta, and the adhesion to Kelleys Island limestone is high. The latter is a dolomitic limestone of the corniferous group, which is soft enough to be worked quite easily. The adhesion 274 CEMENT AND CONCRETE to Drummorid Island limestone, which is a much harder stone belonging to the Niagara group, is considerably less, and the adhesion to the Potsdam sandstone is very low. A higher re- sult than would be expected is obtained with ground plate glass, but the hammered bar iron gives the lowest result of any of the substances tried. TABLE 114 Adhesion of Portland Cement Mortar to Various Materials w o a K K H U W Kind or Sand. < ^ S CO Age of Speci- mens. o . 02 « W u Adhesion, Pounds per Square Inch, to Materials. a b c d e / a 1 2 Cr. Qtz. 20-30 1 1 28 days 6 mos 742 775 91 103 78 122 211 201 100 252 241 284 223 310 290 395 Notes: — Cement, Portland, Brand 11. Adhesion Blocks. 1 in. X 1 in. X i in. inserted in center mold. Materials: — a — Hammered bar iron. b — Potsdam sandstone, c'eavage surface. c — Drummond Id. limestone, cleavage surface. d — Ground plate glass. e — Kelleys Id. limestqne, sawn surface. / — Soft terra cotta, filed surface. g — Soft red building brick, sawn surface. 403. The Adhesion of Neat and Sand Mortars. —Table 115 shows the cohesive and adhesive strengths of different mortars, the adhesion blocks being all of the same material, Kelleys Island limestone. The Portland mortar giving the highest ad- hesive strength at six months is that containing one-half part sand to one part cement, though the greatest cohesive strength is given by the one-to-one mortar. With natural cement the one-to-one mortar gives the highest strength, both in adhesion and cohesion. The ratio of the adhesive strength to the co- hesive strength is greater for natural than for Portland. It also appears that between twenty-eight days and six months the adhesive strength increases more than the cohesive strength. 404. Effect of Consistency on Adhesion. — Table 116 gives the results of tests to show the relative effects of the consis- tency of the mortar on the adhesive and cohesive strength. It EFFECT OF CONSISTENCY 275 TABLE 115 Adhesion of Mortars Containing Different Amounts of Sand o "A B « Cement. Age op Speci- mens. Cohesion or Adhesion. Cohesive or Adhesive Strength, Lbs. per Square Inch, of Mortars with Sand, Parts by Weight. None. One- Half Part Sand. One Part. Two Parts. Kind. Brand. 1 2 3 4 5 7 8 Port. U ( C Nat. R (t An u 4 t 28 days 6 nios. 28 days 6 nios. Cohesion Adhesion Cohesion Adhesion Cohesion Adhesion Cohesion Adhesion 686 270 631 335 183 94 263 228 710 233 787 346 198 104 334 222 747 221 816 287 218 116 383 233 467 169 551 209 186 66 376 171 Notes : — Sand, crushed quartz, 20 to 30. Adhesion blocks, 1 in. x 1 in. x \ in., Kelleys Id. limestone, sawn surface, saturated before used. is seen that the effect of consistency on the adhesive strength is less than on the cohesive strength, but that the best results in adhesion are given by a mortar that is considerably more moist than that which gives the highest strength in cohesion. The practical bearing of this point on the use of mortars is evi- dent. TABLE 116 Adhesion of Mortars. Varying Consistency Cohf.sive or Adhesive Strength, Lbs. a z H K B h H Cem ent. Age of Speci- mens. Cohesion or Adhesion. per Square Inch, Mortar of Consistency: Trifle Dry. Trifle Moist. Quite Moist. Very Moist. Kind. Brand. 1 Port. R 28 days Cohesion 541 502 443 372 2 It " EC Adhesion 148 160 145 136 3 U " 6 nios. Cohesion 697 660 616 539 4 i t (i it Adhesion 191 209 228 192 5 Nat. An 28 days Cohesion 239 212 151 112 6 " 4 t " Adhesion 96 96 87 70 7 u U - 6 mos. Cohesion 397 385 314 285 8 i i. I . ' Adhesion 146 165 164 126 Notes: — Sand," Point aux Pins," pass No. 10 sieve, one part to one cement by weight. Adhesion blocks, 1 in. X 1 in. X J in., Kelleys Id. limestone, surfaces filed smooth, saturated with water before used. 270 CEMENT AND CONCRETE 405. Effect of Regaging on Adhesive Strength. — The tests given in Table 117 were designed to show the effect of regaging on the adhesion of cement mortar to stone. A comparison is made between mortars used fresh and those that were allowed to stand three hours and gaged once an hour. There are but few tests from which to draw conclusions and the treatment is very severe, but it appears that while the regaging to which these mortars were subjected usually resulted in a slight in- crease in cohesive strength, the adhesive strength was consid- erably impaired. The decrease in adhesive strength was greater for natural cement than for Portland, and greater for rich than for poor mortars. The effect of regaging on the cohesive strength is treated in Art. 47. TABLE 117 Effect of Regaging on Adhesive Strength Cement. Adhesion or Adhesion or Cohesion, Lbs. per So.. In. One Part Sand to One Three P arts Sand to Cohesion. Cement. One Cement. Fresh. Regaged. Fresh. Regaged. Portland, Brand X Adhesion 178 141 02 41 " " " u 202 170 50 61 .. u u Cohesion 718 764 327 343 Natural, " An Adhesion 142 no 17 " " " 1 1 180 120 31 28 it ;c if Cohesion 352 301 235 227 Notes: — Sand, crushed quartz, §§. Each result, mean of two to five speci- mens, broken at age of six months. In adhesive tests, pieces Kelleys Id. limestone, 1 in. X 1 in. X \ in., placed in center mold and two ends mold filled with mortar. Results in columns headed "Fresh" from mortar treated as usual. Results in columns headed ' ' Regaged ' ' mortar allowed to stand three hours before use, mortar being regaged each hour. 406. Character of Surface of Stone. — In the tests recorded in Table 118 all of the adhesion blocks were of Kelleys Island limestone, but part of them were finished with smooth filed surfaces, while the others were grooved with a coarse rasp. In the twenty-eight-day tests there is but little difference in the adhesion to the different surfaces, but at six months the adhe- sion to the smooth surfaces appears to be slightly greater, ex- cept in the case of one-to-two natural cement mortar. EFFECT OF PLASTER PARIS 277 TABLE 118 Adhesion of Mortars. Effect of Character of Surface of Stone Cohesion or Adhesion and Character of Surface. Age of Specimens. Adhesion or Cohesion, Lbs. per Sq. In. Portland Brand R. Natural Brand D. Parts Sand to One Cement. 1 2 l 2 Cohesion Adhesion, smooth surface . . " grooved surface Cohesion Adhesion, smooth surface . . " grooved surface 28 days ct 6 mos. 539 151 152 714 238 223 377 . 85 115 503 176 154 343 138 129 387 141 115 289 113 98 304 68 96 407. The Effect of Plaster of Paris on the Adhesion of Mortar to Stone. — The results in Table 119 show the effect on the adhesive strength of adding small percentages of plaster of Paris to cement mortars of Portland and natural cement. The Portland cement used was a quick setting sample, neat cement pats of which began to set in eighteen minutes. The effect of plaster of Paris on the cohesive strength of mortars from these samples hardened in dry air, is shown in Table 92, § 378. It is seen that the addition of from one to three per cent, plaster TABLE 119 Effect of Plaster of Paris on the Adhesive Strength of Cement Mortars Ref. Cement. Parts P.P. Sand to One Cement. Age of Specimens. Adhesive Strength, Lbs. per Sq. In., of Mortars in which Per Cent, of Cement Replaced by Plaster of Paris. Kind. Brand. Sample. l 2 3 6 1 2 q 4 Port. Nat. R R An An 26 R L 2 2 1 year u 263 130 88 64 311 107 97 74 376 144 87 89 291 157 133 82 89 34 a 93 Notes: — Adhesion pieces between two halves of briquet were of Kelleys Id. limestone, sawn surfaces, saturated with water before used. Cement and plaster Paris passed through No. 50 sieve. All briquets stored in tank in laboratory. Each result, mean of four to ten briquets. a Found badly cracked and separated from limestone prisms after three days. 278 CEMENT AND CONCRETE has no deleterious effect on the adhesive strength of these samples at one year. Six per cent, plaster, however, ruins the Portland and the neat natural cement. 408. The Adhesion of Cement Mortar to Brick. — Tests of the adhesion of cement mortar to brick were made by cementing pairs of brick in a cruciform shape, with a one-fourth inch joint of mortar. The brick were placed together flatwise, with the bed down, so that in the case of stock brick, one stock mark, or depression in one side, was filled with mortar. The mortar was made more moist than was ordinarily used for briquets, but not so moist as would be used in brickwork. The top brick of each pair was slightly tapped to place with the handle of a pointing trowel, and the excess mortar cut away. About forty-eight hours after cemented, the pairs of brick were packed in damp sand in a large box prepared for the purpose, and the sand was kept in a moist condition by a thorough daily sprinkling. For pulling the bricks apart, a special clip was de- vised to equalize the pull on the two ends of each brick, and a simple lever machine was used to measure the force required. 409. Tensile tests were made of briquets from mortars similar to those used in the adhesive tests and stored in damp sand, and the results are used for comparison with the adhesive tests. The cohesive strength given by the briquets is not strictly comparable with the adhesive strength shown in the tests with brick, because of the great difference in the area of the breaking sections in the two cases. It has been well estab- lished in tensile tests of cohesion that briquets of large cross- section break at a lower strength than those of small section. It is quite possible also that even with the special clip devised, cross-strains were more likely to occur in the adhesive tests than in the briquet tests. An opportunity was furnished of comparing the tensile strength of neat natural cement mortar under the two conditions, for in one case six joints broke di- rectly through the mortar, the adhesion being greater than the cohesion. It was found that the strength per square inch given by the briquets was at least six times that given by the large joint. This difference should be kept in mind in making comparisons in the tables between the cohesion and adhesion as given. It should also be noted that some of the highest Jesuits of adhesive strength represent in reality the strength ADHESION TO BRICK 279 of the brick rather than the adhesive strength of the mortar, as chips were pulled from the brick, leaving the mortar joint undisturbed. The brick were of a rather poor quality, but selected with a view to obtaining those of a uniform degree of burning-. TABLE 120 Adhesion of Cement Mortar to Brick. of Mortai- Variations in Richness Tensile Strength, Pounds per Square Ince, of Mortars Containing Cement. Age of Mortar. Adhesion or Cohesion. Parts Sani to One Cement. None. § 1 2 3 Portland, X, 41 S 28 clays Cohesion 632 596 589 40!) 270 u a a " Adhesion 48 42 24 20 11 ,i (i .. 3 months Cohesion 676 728 694 423 325 u (t u " Adhesion 64 52 41 24 12 '• " " 6 months Cohesion 723 764 679 521 374 u u .. i t Adhesion 50 56 39 20 14 Natural, Gn, KK 3 months Cohesion 180 240 317 279 181 " " " " Adhesion 46 52 42 28 15 " " " 6 months Cohesion 276 444 388 331 236 U U u ii Adhesion 44 52 50 38 18 Notes: — Bricks were cemented together in pairs in cruciform shape and kept in damp sand until time of test. Briquets for cohesion tests stored in same manner. Each result in cohesion, mean of five briquets. Each result in adhesion is in general mean of six results, three with common die cut brick and three with sand molded stock brick. When adhesion exceeded 50 pounds per square inch, bricks were about as likely to break as the joint between brick and mortar. 410. Adhesion of Neat and Sand Mortars of Portland and Natural. — Some of the results of these tests are given in Table 120. The most noteworthy point developed is that for mor- tars containing more than one-half part of sand to one part cement, the adhesion of the natural cement is greater than that of the Portland with the same proportion of sand, al- though the Portland mortar was much the stronger in cohesion. The mortars giving the highest adhesive strength are those containing not more than one-half part sand to one part cement. The addition of sand lowers the adhesive strength more rapidly than it does the cohesive strength. This point would 280 CEMENT AND CONCRETE be shown still more clearly if the true adhesive strength of the richest mortars was obtained, as we may be certain that the adhesion of these mortars would be shown to be considerably greater if the brick were strong enough to allow this strength to be developed. With natural cement mortars containing not more than two parts sand to one cement, the adhesion is one- sixth to one-ninth the cohesion, and with Portland mortars containing not more than one part sand, the adhesion is about one-fifteenth the cohesion. (But see § 409 in this connection.) 411. Effect of Lime Paste on Adhesive Strength of Cement Mortars. — A number of tests were made to determine the effect, on the adhesive and cohesive strength of mortars, of mixing lime paste with the cement. Tables 121 and 122 give the results of a few preliminary tests on this subject. For the tests recorded in Table 121 the mortars were al- lowed to harden in dry air. From the cohesive tests it is seen that lime in form of paste to the amount of ten per cent, of TABLE 121 Adhesion of Cement Mortar to Brick. Effect of Lirne Paste in Mortar Hardened in Dry Air Tensile Strength, Pounds PER Age Cohesion Square Inch. Cement. of Mortar. or Adhesion. Composition of Mortar. A B C D E Portland, X, 41S 3 months Cohesion 97 99 101 46 59 IS 11 11 4 " Adhesion 18 29 20 22 13 u 11 u (3 " " 24 20 19 11 Natural, Gn, LL 3 " Cohesion 18 38 21 22 68 " " " 4 " Adhesion 39 32 36 28 11 " " " o " " 26 31 25 27 Notes: — Brick, sand molded stock. All briquets and brick stored in dry air. Composition of mortars: Grams P. P. river sand, Grams cement, Grams lime paste, Grams lime contained in lime paste, Lime in paste expressed as per cent, of cement plus lime, A B C D E 480 480 480 480 480 120 120 90 60 40 30 60 120 14 10 20 41 10 10 25 100 Consistency about same as mason's mortar. EFFECT OF LIME PASTE 281 the cement had little effect on one-to-four Portland mortars, but that a larger amount of lime was very deleterious for dry air exposure. The sample of natural cement used did not harden well in dry air, and the highest result is given by the lime mor- tar without cement. It appears that the adhesive strength of the Portland mortar was slightly increased by the addition of a small amount of lime paste, but the adhesive strength of natural was not greatly affected. The adhesive strength of the natural cement is, in general, higher than the Portland. The nat- ural cement appeared to harden better in the joints than in the briquets, and we have, as a peculiar result, the adhesive strength exceeding the cohesion. This illustrates a statement already made, that to store briquets in dry air does not approach very nearly the ordinary conditions of use. 412. In Table 122 are given a few tests of mixtures of Port- TABLE 122 Adhesion of Cement Mortar to Brick. Effect of Lime Paste Portland Cement Tensile Strength, Pounds per Square Inch. Mortar Hardened in Cohesion or Adhesion. Composition' of Mortar. A B C D E Tank in Laboratory Dry air, " Damp sand, " Dry air, Damp sand, " Coh'n. k u Adh'n. 177 167 173 15 15 203 180 198 36 33 183 107 171 40 35 158 150 154 33 32 82 81 88 26 27 Notes: — Bricks cemented together in pairs in cruciform shape. Age of all mortars when tested, three months. Cement, Portland, Brand R, Sample 14 R. Sand, " Point aux Pins." Lime paste slaked about six months before use. Each result in cohesion, mean of five to ten briquets. Each result in adhesion, mean of eight to sixteen pairs of bricks. Half of pairs were hard burned brick and half soft burned. Composition of mortars: Grams P. P. (river) sand, Grams cement, Grams lime paste, Grams lime contained in paste, Lime as per cent, lime plus cement, A B c D E 960 960 960 960 960 240 240 200 180 120 80 120 180 360 27 40 60 120 10 16.7 25 50 282 CEMENT AND CONCRETE land cement and lime paste, the mortars being hardened in dry air and in damp sand. Cohesive tests are also given of briquets hardened in damp sand, water and dry air. It appears that the addition of ten per cent, of lime in the form of paste to mortars of this sample of Portland increases the tensile strength, the effect being least when the mortars harden in dry air. The substitution of lime for one-sixth of the cement in a one-to-four mortar has little effect on the tensile strength. Larger propor- tions of lime result in decreased strength, and if one-half of the cement is replaced by lime, the resulting strength is only about one-half that given by the cement mortar without lime. The results of the adhesive tests show that if half of the cement in the mortar is replaced by an equal weight of lime in the form of paste, the resulting strength is increased by nearly 100 per cent., and that if smaller amounts of lime are used, the adhesive strength is increased by about 150 per cent, over that given by the cement mortar without lime. 413. The results of a more complete set of tests on this sub- ject are given in Table 123. The mortars used included one made with four parts sand to one of cement by weight; one in which about ten per cent, of lime by weight, which had pre- viously been made into lime paste, was added to the mortar; a third in which lime, in the form of paste, was substituted for one-sixth of the weight of the cement used in the first mortar; a fourth, in which lime was substituted for one-fourth of the cement; and finally, a mortar composed of lime paste and sand only. The adhesive strengths of the mortars are given in the table. The difference in the adhesion of Portland cement mortar to hard brick and to soft brick is not clearly brought out. Neither is the strength of air-hardened specimens much different from that of the mortars stored in damp sand. The use of lime paste with Portland cement in the amounts tried here more than doubles the adhesive strength of the mortar. The first point to notice in the case of natural cement is that the adhesive strength of this mortar without lime is nearly double the adhesive strength of Portland mortar without lime. The adhesive strength of mortars hardened in damp sand is somewhat greater than the strength of similar mortars hard- ened in dry air. 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CM CO CO CD -* CD ■># "* CO CO co o CO CS CO CS CO tJH CD ^ -t< CO Q z CO Ck s co DON CS tH t^ CM CO CO CM OS CO ■* CS -* ■* CO CO 03 a5 S CM lO CM CO -* CM O^H 00 tO to CO CM o lr- © tO CO -t CO A » a) £.11 ^ CD CM t-oot- CO O 00 CO © ■* ©. i—' ©' os •* Tt< CO •SWIQ 'aNVg ■e o CD OS = r = o CD CS i s - •ewo 'axsvj aivi^; o © o o o o CO CM CO I— 1 T-l O O © © © CO CM CO a a H O •siu-B-io •=> o CM o o o -* O CO (M CM !-H o ■* CM © © © -* © CO CM CM .-H M 8 • -co JS'S a - - - -g 3"E. j - - OiH g 02 t« 3 s "" •aoNaHaaa'jj I—H CM CO -^ lO © t- 00 O o T3 p3 rS S co O J2 « =3 £ O 284 CEMENT AND CONCRETE increases the adhesive strength somewhat, and when as much as twenty-five per cent, of the cement is replaced by lime in the form of paste, the adhesive strength of the natural cement mortar is not usually diminished. The effect of lime paste, however, on the adhesive strength is not nearly so great as it is in the case of Portland mortars. The following conclusions may be briefly stated: The ratio of adhesive to cohesive strength is much greater with natural cement than with Portland. If a high adhesive strength is desired, Portland cement should not be mixed with more than two parts sand unless lime paste is added to the mortar, as the use of lime paste materially increases the adhesive strength of lean mortars. Tests of cohesion of similar mortars contain- ing lime paste are given in Art. 48. 414. THE ADHESION OF CEMENT TO RODS OF STEEL AND IRON. — The tests recorded in Tables 124 and 125 were made to determine- the adhesion of cement mortar to iron rods, or the strength of a bolt anchorage secured with cement mortar, and the style of rod and kind of mortar which would give the best results. The bars were made in an ordinary concrete mold, ten inches by ten inches by four and one-half feet. The rods or bolts were placed in a row along the center of the box, being spaced about nine inches apart, and the mortar was rammed about them. After being allowed to set in a warm room for twenty-eight days, the rods were pulled by means of two hydraulic jacks, a special grip being used to grasp the free end of the rod, and an hydraulic weighing machine serving to measure the pull required to start it. The supports against which the hydraulic jacks were braced bore at points on the concrete bar about three or four inches from the center of the rod which was being tested. 415. The rods given in Table 124 were imbedded in mortar composed of one part of Portland cement to two parts lime- stone screenings. The rods were cut from bar iron and were perfectly plain, without nuts or fox wedges. The results in- dicate that the force required is proportional to the area of contact. Comparing the different styles and sizes of plain rods, no difference in favor of one style or size can be determined; the apparent higher resistance per square inch offered by one- inch rods would probably disappear in a large number of tests. ADHESION TO STEEL RODS 285 TABLE 124 Resistance to Pulling of Iron Rods of Various Forms Imbedded in Mortar & Pouni s Pull. Ref. (S 03 H m o 12; Mortar, Bar No. Description of Rod. Perim- eter of Rod, Inches. Depth Im- bedded, Inches. Per In. Depth Im- bedded. Per Sq. In. Area in Con- tact. 1 3 2, 6, 7 Plain, |" diameter 1.57 8 to 10 700 447 2 3 " " 1" 3.14 " 1750 556 3 3 " ii u" " 3.93 " 2060 524 4 5 3 4 2,5,6,7 " \" square (( J// 41 2.00 4.00 u 1085 2250 543 562 6 3 2, 6, 7 " -l\" " 5.00 2170 434 7 3 4,5 ( Twisted 1" square, j ) 1 turn in 8" length £ 4.3 1 9+ 2595 608 8 3 1 1 I Twisted 1" square, ) } 2 turns in 8" length ( 4.3 1 9+ 2215 516 9 3 (< \ Twisted 1" square, { I 3 turns iu 8" length \ 4.3 1 9-9.5 2405 501 Notes: — Cement, Portland, Brand R. Sand, limestone screenings passing | inch slits, two parts by weight to one cement. Mortar one month old when tension was applied to rods. The rods given in lines seven to nine were made by twisting a piece of one inch square bar iron. The twisted portion was eight inches in length. Comparing the plain one inch square bolts with the twisted bolts, it appears that the former offered a resistance of 2,245 pounds per inch depth while the latter gave 2,405 pounds, an increase of less than eight per cent. 416. In the tests recorded in Table 125, the ordinary river sand used in construction was employed. The mortar was made neat and with two and four parts sand to one of cement. The depth the rods were imbedded varied from two inches to ten inches. The one-to-two mortar gave nearly as good results as neat cement, but the one-to-four mortar gave much lower results. The resistance seems to vary directly as the area of contact without reference to the depth imbedded, except as 1 In computing adhesion, or shear, or pounds pull per square inch of area in contact, perimeter considered circumference of a circle of diameter equal to the distance between opposite edges of rod after twisting. A core of mor- tar of this diameter, was torn from bar in pulling. To perceive effect twisting, compare pounds pull per inch depth imbedded. 286 CEMENT AND CONCRETE this enters in obtaining the said area. The results obtained in this table do not compare favorably with those obtained in Table 124, where limestone screenings were used. TABLE 125 Resistance to Pulling of Iron Rods Imbedded in Mortar. Variations in Depth Imbedded and in Richness of Mortar Adhesion, Pounds per Square Inch of Surface in Contact for Different Parts Depths Imbedded. Sand to < ) \ B Depths ) No. Imbed- ! ded,In ) 1.9-2.2 3.2 4 4.5-4.8 5.8-6 7.8-8 8.8 9.6-10 Re- sults. Mean. 340 346 313 228 340 5 313 2 272 294 270 202 255 247 275 15 264 4 74 119 117 100 142 10 111 Notes: — -Cement, Portland, Brand R. Sand, "Point aux Pins," river sand. Mortar one month old when rods pulled. Hods, round, 1 inch diameter. 417. Tables 126 and 127 are from similar tests made by Messrs. Peabody and Emerson. 1 The rods in Table 126 were of various shapes and included some having rivets through them. The \ inch by 1 inch bars gave lower adhesion per square inch than the square and round rods. When two rods are twisted together and imbedded in a small specimen, the tendency is to split the specimen. The bars containing rivets broke before the adhesion was overcome, although the depth imbedded Was but six inches. In Table 127 neat cement paste and concretes of several compositions are tried. These results are of interest as show- ing that concretes show as great adhesion to steel rods as do mortars. The very low result obtained with neat cement in this table is not explained and is in opposition to the results in Table 125. 1 Engineering News, March 10, 1904. ADHESION TO STEEL RODS 287 TABLE 126 Adhesion of Mortar to Steel Rods of Various Shapes, Imbedded about Six Inches No. OF Tests. Description of Rod. Perimeter of Rod, Inches. Pounds Pull. Per Inch Depth Imbedded. Per Square Inch Area in Contact. 4 3 4 4 4 4 4 Plain, \" square. Plain, \" square. Plain, \" round. Twisted," y square. 1" by 1 ". Two \" rods twisted together. i" by 1" with |" rivets through . 1.0 2.0 1.57 2.5 369 864 804 1259 744 369 432 512 293 Three specimens split. One rod broke at 8,000 lbs., or when adhesion was 1,250 lbs. per inch depth . One specimen split. Three rods broke at first rivet with 9,800 to 10,500 pounds, or when adhesion was 1,500 to 1,660 lbs. per inch depth. Notes: — Tests by Messrs. George A. Peabody and Samuel W. Emerson. Mortar composed of one part cement (Portland) to three parts sand. Specimens approximately 6 inch cubes. One rod imbedded in each, 6 to 6J inches. Rods pulled forty and eighty days after mortar was made. TABLE 127 Adhesion of Mortars and Concretes of Various Compositions to One Inch Square Steel Rods Imbedded about Ten Inches Cow position of Mortar OR Pounds > Pull. No. OF Concrete. Tests. Per Inch Depth Per Square Inch Area Cement. Sand. Stone. Gravel. Imbedded. in Contact. 4 1 1112 278 4 1 3 1644 411 4 1 3 6 1912 478 4 1 3 6 2062 516 4 1 2 4 2348 587 4 1 2 4 2187 547 Note: — Tests by Messrs. George A. Peabody and Samuel W. Emerson. CHAPTER XVI THE COMPRESSIVE STRENGTH AND MODULUS OF ELASTICITY OF MORTAR AND CONCRETE Art. 52. Compressive Strength of Mortar 418. The compressive strength of cement mortar is from five to ten times the tensile strength. As the result obtained in tests of either compression or tension depends upon the shape and size of the specimen, no definite value can be assigned to the ratio of compression to tension. Comparative tests have indicated in a general way that the cements giving the best results in tension show also the highest compressive strength; but with variations in treatment, different kinds and brands of cement do not give the same variations in the ratios of the two kinds of strength. Mortar is not usually employed alone in large masses. It more frequently forms the binding medium between fragments of other substances, such as brick and stone. The dependence of the strength of masonry upon the strength of the mortar increases with the roughness of the stone or brick, and the thickness of the bed joints. In fine ashlar masonry this depend- ence is comparatively small, in brickwork it is important, and in concrete any increase in the strength of the mortar increases the strength of the concrete in nearly the same ratio. Piers of brickwork may give a crushing resistance either greater or less than the strength of cubes made from mortar of the same composition as that used in building the piers. Thin beds of mortar between strong materials resist high com- pressive stresses, while in walls or piers built with weak blocks, the mortar is destroyed by the cracking of the blocks at a lower stress than the mortar would withstand in a cube pressed between steel plates. Since in brick and stone masonry the mortar forms but a small part of the structure, it is not econom- ical to use a poor quality of mortar with good brick and stone. 288 CEMENT MORTAR 289 419. Ratio of Compressive to Tensile Strength. — M. E. Candlot has made many experiments showing the effect of certain variations in the preparation of mortars upon the com- pressive and tensile strength. A few of the results of one series are presented in Table 128. The reduction from the metric system has been made, and a column added giving approximately the number of parts of sand to one of cement by weight, the accurate proportions appearing in the form of weight of cement to one cubic yard of sand. These results in- dicate that the ratio of the strength in compression to that in tension increases with the age of the mortar and also with its richness. TABLE 128 Resistance of Cement Mortars to Tension and Compression, with Varying Proportions of Normal Sand Specimens Hardened in Fresh Water [From Ciments et Chaux Rydrauliques, par M. E. Candlot.] Resistance in Pounds per Square Inch in Tension and , Z m oo O W % a J w * i r y O 2^ Compression. ? Z W oH H O -. w 2 v 7 days. 28 days. 1 year. 2 years. 3 years. g0 2 2 * n. IT. W [>. ft T. C. T. C. T. C. T. C. T. C. tf a^ 10.8 250 ■ 27 266 38 408 70 507 74 572 108 738 6.8 6.4 420 128 643 143 1164 212 1730 209 1630 219 1775 8.1 4.6 590 13!) 1040 234 1940 337 2980 284 2930 341 3080 9.0 3.5 760 233 1520 393 3080 435 4020 400 4400 462 4590 9.9 2.9 930 251 2110 462 3690 490 5580 490 5680 557 6060 10.9 2.5 1100 349 2630 551 5020 594 5820 557 6060 616 6480 10.5 2.0 1350 368 3360 550 5020 713 7750 805 7860 784 8710 11.1 1.6 1690 443 3310 561 5070 767 7670 907 8800 815 9180 11.3 From a study of the results of nearly three thousand tests made by Professor Tetmajer, the late Professor J. B. Johnson concluded that for mortars containing three parts sand to one cement the ratio of the compressive strength to the tensile strength is equal to 8.64 + 1.8 log. A, where A is the age of the mortar in months. It is shown above that the ratio in- creases with increasing proportions of sand. 420. Table 129 gives some results obtained at the Water- town Arsenal in tests of cement mortar cubes. 1 The mortars Prepared by Mr. George W. Rafter for the State Engineer of New York. 290 CEMENT AND CONCRETE TABLE 129 Compressive Strength of Cement Mortar. — Portland and Natural Tests of 12 Inch Cubes, Twenty Months Old, Made at Watertown Arsenal for State Engineer of New York Method of Storage of Cubes. Water 3 to 4 mo., then buried in sand. Covered with burlap; kept wet for several weeks, then exposed to weather. . . In cool cellar Fully exposed to weather . . . Means Grand mean . . Water 3 to 4 mo., then buried in sand Covered with burlap kept wet for several weeks, then exposed to weather. . , In cool cellar . Fully exposed weather . . to Means Cement. Kind. Nat. Nat. Nat. Nat. Port. Port. Port. Port. Brand. Buffalo Buffalo Buffalo Buffalo Empire Empire Empire Empire f Dry J Plastic ] Excess I Mean f Dry I Plastic | Excess L Mean f Dry J Plastic 1 Excess l Mean f Dry ! Plastic Excess Mean f Dry ■i Plastic [_ Excess f Dry I Plastic / Dry ^ Plastic f Dry 1 Plastic f Dry i Plastic J Dry 1 Plastic Crushing Strength, Lbs. per Square Inch, for Mortars Containing Parts Sand to One Cement by Volume: 3479 2795 2161 2812 3347 2476 2070 2631 2844 2514 2159 2504 3272 2667 1996 2645 3236 2613 2097 2649 2200 1783 1698 1894 2000 1294 1358 1551 2051 1256 1386 1564 1879 1356 1311 1513 2032 1421 1438 1630 3897 3642 3880 3672 3397 3313 4059 3589 3808 3554 1154 1000 776 977 961 692 738 797 987 883 678 849 1054 822 669 848 1039 849 '715 2494 2108 2492 2168 2132 2164 2450 2270 2392 2193 1782 1717 1489 1726 1614 1679 1715 1465 1650 1647 Mean 2278 1859 1545 1894 2103 1487 1389 1660 1961 1551 1408 1640 2068 1615 1325 1669 2102 1628 1417 1716 contained one, two and three volumes of sand to one of natural cement, and two to four parts sand to one volume of Portland. Result interpolated. 2,043 omitting interpolated result. CONCRETE 291 The proportions of water used were such as to give mortars of different consistency, "dry," like damp earth, " plastic," of the consistency usually employed by masons, and "excess," quak- ing like liver with slight tamping. The specimens were twelve inch cubes and four methods of storage were used, as indicated. Comparing the results with similar tests of tensile strength, it appears that the strength in compression decreases more rapidly as sand is added than does the tensile strength. The same conclusion was drawn from Table 128. The strength of the Portland mortar with four parts sand is about equal to the strength of the natural with two parts. The dry mortar gives the highest strength with natural cement, but with Portland the "dry" and "plastic" give about the same result. Concerning the consistency, it has already been pointed out that the conditions of the actual employment of mortar are such as to favor, in general, the use of a wetter mixture than that which gives the best results in laboratory tests of mortars. As to storage, the specimens kept in water for three or four months after made, give the highest results with natural ce- ment. There seems to be no choice between the other three methods of storage. Art. 53. Compressive Strength of Concrete with Various Proportions of Ingredients 421. With the increasing use of concrete in arch bridges, in foundation piers and in columns of buildings, and especially in connection with steel in beams, etc., the compressive strength of the material becomes of the greatest importance. Moreover, the composition of concrete may vary so much, the range of available aggregates is so wide, and the methods of manipula- tion are so diverse, that many tests must be studied before one can judge of the probable strength of a given mixture. For any very extended work, it may be found economical to make a series of tests using the materials available, and combining them as nearly as possible in the manner proposed in actual construction. This practice has been followed in several important works, and the data thus accumulated have added much to our hitherto somewhat vague notions of the probable strength of different mixtures under varying condi- 292 CEMENT AND CONCRETE tions of use. It is possible here to abstract but a few of the more reliable and complete tests of this kind, selecting those which indicate the value of certain special kinds of aggregate or the effect of certain variations in manipulation. 422. In connection with the design of the Boston Elevated R. R., Mr. George A. Kimball, Chief Engineer, prepared a series of concrete cubes of mixtures usually employed in practice, and with the materials available for the work in hand, and these cubes were tested at the Watertown Arsenal in 1899. A por tion of the results of these experiments are given in Table 130, where the details concerning character of the materials and the preparation of the specimens are shown. As each result in the table is the mean of at least twenty specimens, the ir- regularities frequently appearing in compressive tests have been largely eliminated, and the results are worthy of much confidence. TABLE 130 Compressive Strength of Concrete Tests of 12 Inch Concrete Cubes for Boston Elevated Railroad. Composition of Concrete by Volume. Crushing Strength, Pounds per Square Inch, at Age, Cement. Sand. Stone. 7 days. 1 month. 3 months. 6 months. 1 1 1 2 3 4 6 12 1525 1232 58.3 2440 2063 1042 2944 2432 1066 3904 2969 1313 Notes: — Materials: — Cement, mean results with four brands Portland, two Ameri- can, two German. Sand, coarse, clean, sharp, voids 33 per cent, loose. Broken stone, conglomerate passing 2h inch ring, voids 49J per cent, loose. Mixing: — Sand and cement turned twice, mortar and stone turned twice. Storage : — Cubes removed from molds three or four days after made and buried in wet ground until about a week before testing. Each result, mean of twenty or more tests. Tests made at Watertown Arsenal, for George A. Kimball, Chief Engineer, Boston Elevated R.R. "Tests of Metals," 1899. At the time of making these tests some cubes were .crushed with a die having a smaller area than the face of the cube. CONCRETE 293 With a die 8 by 8J inches on one compression face, the area of the die being thus about .46 of the area of the cube face, the strength per square inch under the die was about twenty-five per cent, higher than when the entire face of the cube was pressed. This is in line with the behavior of all brittle sub- stances under compression, as shown by Professor Bauschinger in testing sandstone specimens. 423. Tables 131 and 132 give a summary of a part of a very valuable series of tests of concrete cubes prepared by Mr. George W. Rafter and tested at the Watertown Arsenal for the State Engineer of New York. 1 The results summarized in Table 131 are those obtained with four brands of Portland cement made in the State of New York, namely, Wayland, Genessee, Empire and Ironclad. Tests were also made with a sand-cement, and with one brand of natural, but these results are not included in the table. The aggregate was sandstone of the Portage group, broken by hand to pass a two inch ring. The mortars used in making the cubes were of three degrees of consistency: (a) In the dry est blocks the mortar was only a little more moist than damp earth, and much ramming was required to flush water to the surface. (6) In another set the mortar was about the consistency of ordinary mason's mortar, (c) In the third set, the mortar was wet enough to quake like liver under moderate ramming. 424. The mortar was composed of one volume of loose ce- ment to two, three or four volumes of loose sand. Other pro- portions were also employed, but in this table only those re- sults are included in which the series of tests was complete as to variations in consistency and storage. The voids in the stone were about forty-three per cent, when the measure was slightly shaken, and thirty-seven and a half per cent, when rammed without mortar. The amount of mortar used was made either thirty-three per cent, or forty per cent, of the volume of the loose stone. Four methods of storage were used as follows: 1st, blocks immersed in water as soon as they were removed from the molds, and after three or four months they were buried in sand; 1 Report of State Engineer of New York, 1897. 294 CEMENT AND CONCRETE TABLE 131 Compressive Strength of Concrete Mean Eesdlts with Four Brands Portland Cement,, Illustrating Effects of Proportions, Consistency, and Methods of Storage. Tests of Concrete Cubes, about Twenty Months Old, Made for State Engineer of New York ■j- z < m O a CO CO < z o K O Cm O Ph . . . O .« o • i— i . . . "* 1 . . -CM .00 _ , .00 ] °° I . . . . 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CO riririH © CM •-*©-* K © Tfl © & I-H © © ^ CM CM i-h * ICO t~ CM I- © CM ICC ft © CM © g ©©00 Ph ^ CO CO CM -# i-H © „ » C h r- © O i-h CM CO CO CO CO t^Hcoo t© Ci -* Ci lO iO CD O CM CM CM CM CO © CO © iO CM 00 © © i© -# © CO CM CM CM to © © t- i-H © O CM iO © ^* LO CM CM CM CM CM • t- ■* i-H £ © o to .^ OXN V2 CO CM CM J > 1 n z H H O H Z o o n z < m 03 E-> « < Ph H m c o J o > (h O Z H ri < o o fcjg P-l O 02 1 2 6 1 39.5 2700 800 4 1 2 6 29.3 2820 763 5 1 2 3 o 35.5 2750 841 6 1 2 4 2 36.7 2840 915 Notes: — Materials: Cement, Portland, "Atlas" (American), 104 lbs. per cu. ft.; Natural, "Round Top," 70 lbs. per cu. ft. Sand, 15 per cent, retained on No. 8 mesh, 75 per cent, between 8 and 40 mesh, 10 per cent, passing 40 mesh. Sand was used damp, and weighed in that condition 90 lbs. per cu. ft. Stone, Bluestone, "Average," 93 percent, between \ inch and 2 inches. "Coarse," 89 per cent, between 1^ inches and 2£ inches. Gravel, "Average," 90 per cent, between \ inch and 1J inches. "Small," 90 per cent, between $ inch and f inch. Granolithic, 92 per cent, between ^ inch and \ inch. Mixing, thorough by experienced man. Tamping, light, in 4 inch layers, just sufficient to bring mortar to surface. Storage, cubes thoroughly wet twice a day. Age of specimens when broken, one year. The concretes all contained two parts sand and six parts aggregate to one cement, but the character of the aggregate varied as shown. The natural cement concrete gave from one- quarter to one-third the strength of the Portland concrete. The best result seems to be given by the average size broken stone, which was in reality a mixture of various sizes, ninety- 1 Mixture of one part granolithic size to one of concrete stone. CONCRETE 301 three per cent, of it being retained on a one-third inch mesh and passing a two-inch mesh. The mortar was probably in- sufficient to fill the voids in the stone for the first three cubes in the table, and under these conditions the gravel, with its smaller percentage of voids, makes a good showing. This illus- trates what we have already said, that the relative value of broken stone and gravel for aggregate depends upon the pro- portion of mortar used. TABLE 136 Compressive Strength of Concrete. Portland Cement Tests of Six-inch Cures of Various Mixtures u S K H h a £3 Pakts by Volume to One Cement. Crushing Strength, Pounds per Square Inch, at Age of, Sand. Gravel. Broken Stone. 7 days. 30 days. 90 days. 1 2 3 4 5 6 7 8 9 10 1 2 2 8 3 3 1 2 2 3 5 2i 3 o • 21 3" 4 5 1\ 4 3412 1077 1430 420 640 566 739 792 767 714 5318 1908 2215 2117 2 1199 1385 2033 . 1482 1345 1028 6140 2517 2903 1324 1290 1609 1783 2014 1409 1818 Means, Actual . Means, Pfvr Cent 1056 46 2003 88 2284 100 Note : — Results of Messrs. Ketchum and Honens. 431. The results in Table 136 were obtained by Messrs. R. B. Ketchum and F. W. Honens at the laboratory of the Uni- versity of Illinois, 3 and illustrate the rate of gain in strength of several mixtures. The cement used was Saylor's Portland, fine and of good quality. The sand and gravel were composed principally of silica, with 10 to 30 per cent, of limestone. About 60 per cent, of the sand passed a "number thirty" sieve. The unscreened gravel had about 42 per cent, caught on a " num- ber five" sieve and eighteen per cent, of it passed a "number 1 Unscreened. 2 Result irregular. 3 Technograph, 1897-98. 302 CEMENT AND CONCRETE thirty." Except in one mixture, however, the gravel and broken stone were screened, and only that portion passing a two-inch ring and retained on a "number five" sieve was used. The stone was a magnesian limestone. The concrete was mixed dry, so that considerable tamping was required to bring water to the surface. The cubes were first kept under a damp cloth for one day, immersed six days, and then stored in air in a room until broken. In crushing, "the direction of the force applied was parallel to the tamped surface." 432. Each result in the table is the mean of six specimens. Comparing number 2 with number 9 indicates that the strength obtained with one part cement to three parts unscreened gravel is much higher than with mortar of one part cement to three parts sand. Comparing 9 and 10 indicates that seven parts gravel and stone may be mixed with one-to-three mortar and give higher strength than the mortar alone. A comparison of 6, 7, and 8 shows that in case there is sufficient mortar to fill the voids in the aggregate, angular fragments give a somewhat higher strength than rounded ones, but that a mixture of broken st (mo and gravel is better than either alone. One of the most important points brought out by the tests is that the strength at seven days is 46 per cent., and at thirty days is 88 per cent., of the strength attained at three months. Art. 55. Cinder Concrete, etc. 433. For such purposes as floors for buildings, cinders are used in concrete to a considerable extent on account of their light weight. Cinder concrete weighs only from two-thirds to three-fourths as much as broken stone or gravel concrete. The strength, however, is correspondingly less, and whether for a given strength a floor may be made lighter by the use of cinders will depend upon the conditions of use and the charac- ter of the reinforcement. Table 137 gives the results of the tests of eight-inch cylinders, fifteen inches high, made by Mr. George Hill. 1 In these cylin- ders, cinders, broken stone, and gravel were used as aggregates. The character of the materials is shown in the foot-note of the 1 Trans. Am. Soc. C. E., Vol. xxxix, p. 632, CINDER CONCRETE 303 table. As the specimens were but one month old when tested, the results are low, but since in the construction of floor arches the centers are usually removed in less than one month, the strength developed in a short time has a special interest. TABLE 137 Compressive Strength of Concrete about One Month Old Tests of Cylinders, Eight Inches Diameter, Fifteen Inches High Aggregate. Proportions b^ Volume. AGS, Compressive Lbs. pee Strength, Sq In. American Slag Cement. Cement. Sand. Aggregate. Days. Portland Cement. Cinders. 1 3 6 33 246 u 1 3 6 18 292 u 1 2 5 oq 305 (C 1 2 5 DO 404 If 1 2 5 32 490 U 1 2.4 6 32 590 u 1 1.7 4.2 30 342 (1 1 1.6 4 30 330 l< 1 1.6 4 31 7b'5 u 1 1.6 4 31 765 Stone. 1 3 6 30 398 " 1 2.4 4.1 30 503 " 1 2.4 4 oq 645 " 1 2.4 4 30 730 Gravel. 1 3 6 30 917 618 4 t 1 2.4 4.8 30 650 U 1 2 7 25 880 u 1 1.6 6.5 31 730 Stone and gravel, ) graded . . . . ) 1 2 10 30 625 Notes: — Cement, American Portland, tensile strength 624 lbs. per sq. in., neat, seven days. Slag cement, a little less than 400 lbs. per sq. in., neat, seven days. Sand, clean, sharp, bank sand of mixed sizes, from moderately fine up to some pebbles size of bean. Cinders, ordinary steam, dust to \ inch size. tStone, broken trap, nearly uniform size passing 1^ inch ring. Gravel, clean, washed, \ in. to H in. Abstract of tests by Mr. George Hill, M. Am. Soc. C. E., Vol. xxxix, p. 632. It is evident that cinder concrete should not be loaded very heavily within a month after made. The gravel gives a better result than broken stone. 304 CEMENT AND CONCRETE 434. In Table 138 are given the results of some tests of twelve-inch cubes of cinder concrete made at the Watertown Arsenal for the Eastern Expanded Metal Companies. Steam cinders were used, practically as they came from the furnace, only the larger clinkers being broken. Two proportions were used and the specimens were broken at one month and three months. It is seen that the one-one-three mixture is about twice as strong as the one-two-five with all brands. The varia- tions between the several brands are also very great. TABLE 138 Crushing Strength of Cinder Concrete. Portland Cement Tests of Twelve-inch Cubes at Watertown Arsenal Brand of Cement. Strength, Pounds per Square Inch. Mixture A, 1-1-3. Mixture B, 1-2-5. Age of Specimens. Age of Specimens. 1 month. 3 months. 1 month. 3 months. A B C D 2329 1602 1438 1032 2834 2414 1890 1393 940 696 744 471 1600 1223 880 685 Notes: — Concretes mixed rather dry, 10 to 12 J pounds of water per cubic foot of concrete. Mixture "A," one part cement, one part sand, three parts cinders. Mixture "B," one part cement, two parts sand, five parts cinders. Weight of concrete, 104 to 116 pounds per cubic foot. Tests for Eastern Expanded Metal Companies. Data from "Tests of Metals," 1898. 435. Table 139 gives the results of other tests in the same series, using a single brand of cement and five mixtures, the richest containing three parts cinders and one part sand to one volume cement, and the poorest six parts cinders and three parts sand to one cement. The weight per cubic foot of the several concretes is also given. Tests of cinder concrete prisms made by the late Prof. J. B. Johnson at Washington University 1 indicated that the mixture "Materials of Construction," p. 628. CONCRETE WITH CLAY 505 containing one part sand and three parts cinders to one volume cement gave the highest strength, or about twelve hundred pounds per square inch, at one month. The same mixture gave the highest values for the ratios of strength to cost, and of strength to weight per cubic foot. TABLE 139 Crushing Strength of Cinder Concrete. Various Proportions with Germania Portland Cement Tests of Twelve-inch Cubes at Watertown Arsenal Proportions in Concrete. Weight per Cu. Ft. at 98 to 102 Days, Pounds. Crushing Strength, Pounds per Square Inch, at Age, Cement. Sand. Cinders. 29 to 39 days. 98 to 102 days. 1 1 1 1 1 1 2 2 2 3 3 4 5 6 110.4 112.8 107.9 105.3 103.5 1466 1098 904 769 529 2001 1634 1325 1084 788 Note: 1898. -Tests for Eastern Expanded Metal Companies, "Tests of Metals," 436. Clay in Concrete. — The effect of clay on the tensile strength of mortars has already been shown (Art. 49). Aggre- gates available for concrete frequently contain a certain amount of clay, and the question arises whether such aggregate must be washed, or whether certain small percentages may be per- mitted in the concrete, using, perhaps, a trifle richer mortar. The results in Table 140 were made to determine the effect of clay on the crushing strength of concrete. 1 The test specimens were six-inch cubes, and were broken when one week to twelve weeks old in an Olsen machine. The proportions were two parts sand and six parts gravel by weight to one of Portland cement, or two parts sand and four parts gravel by weight to one of natural cement. The clay is ap- parently expressed as the per cent, of total aggregates. It is seen that while six or twelve per cent, clay retards the harden- ing of both Portland and natural cement concrete, the strength of the Portland concrete after four weeks is increased by six per 1 Tests by Messrs. J. J Richey and B. H. Prater, Technograph, 1902-03. 306 CEMENT AND CONCRETE cent, clay, while at the same age the strength of the natural cement concrete is not greatly affected. The ramming of con- crete is facilitated by the presence of a small amount of clay, but larger amounts may render the mass sticky and difficult to ram. TABLE 140 Effect of Clay on Crushing Strength of Concrete Six-inch Cubes Cement. Proportions by Weight, No. Parts to One Cement. Age of Cubes When Broken. Crushing Strength, Pounds per Sq. In.; Clay as Per Cent, of Concrete, Sand. Gravel. 6 12 Port. 2 6 1 week 1030 1001 692 t i 2 6 4 weeks 1398 1525 1287 " 2 6 12 " 2110 2760 1865 Nat. 2 4 1 week 208 131 81 (i 2 4 4 weeks 428 3(54 283 2 4 12 " 780 722 480 Art. 56. The Modulus of Elasticity of Cement Mortar and Concrete 437. With the increasing use of concrete and steel in com- bination, the modulus of elasticity of cement mortar and con- crete assumes a new importance, since the ratio of the stresses in the two materials depends upon the relative moduli of elas- ticity. Some of the earlier determinations of the modulus of mortar gave very high values. This may have been due to the use of richer mixtures, and the exercise of greater care in the manipulation, than are employed in actual construction, and also to the fact that the determinations were based upon the deformations resulting from the application of very limited loads. It is now considered that the ratio of stress to strain is not constant, even for moderate loads, but that the modulus of elasticity decreases with increasing stress, and this fact is brought out in the following tables. The tests cited bring out a wide range of values for concretes and mortars made from a variety of sand and aggregate and of various compositions and ages. 438. Modulus of Elasticity of Natural and Portland Cement Mortars. — Table 141 gives the modulus of elasticity of mortars as determined by tests of twelve-inch cubes at the Watertown MODULUS OF ELASTICITY 307 y H C) <* H W J +J pq CI < '(-< H t/3 a s a h L~ 3 iM 1^ O i.O Tf ccmo O O l— OS -# O -ti o ©ocoo CO CM i-l CM i-i o o -^ [^ uO O CM ■^ CM CM CO CM O O O lo i— © o TjlCCCC L^ t-OMt- CO CO CM CO © o O CO O kffl M Ph ft- of a-> 11 £ be t3 a ro 0) m ~ 0) O eu '. h 308 CEMENT AND CONCRETE Arsenal. 1 These specimens were a portion of those prepared by Mr. Rafter, the compressive strength being given in Table 129. As each value is the result of but one determination, the results are not as regular as might be desired. In general the strength and the modulus decrease together as the amount of water used in mixing is increased. The modulus also decreases with the strength as the proportion of sand increases. 439. Modulus of Concretes One Month to Six Months Old. — In the compressive tests of twelve-inch concrete cubes made for Mr. George A. Kimball and abstracted in Table 130, many of the specimens were also gaged for compression under load to determine the modulus of elasticity, and a part of the results are presented in Table 142. TABLE 142 Modulus of Elasticity of Concrete Tests Made on Twelve-inch Cubes of Portland Cement Concrete at Watertown Arsenal for Boston Elevated Railroad Age of Cubes When Crushed. Concrete 1-2-4. Concrete 1-3-6. Concrete 1-6-12. Modulus of Elasticity in Thousands, between Loads, in Pounds per Square Inch, of 100-GOO 100-1000 1000-200) 100-600 lu0-1000 1000-2000 100-G00 100-1000 7 days 1 mouth 3 months 6 months 2592c 2062c 3670 3646 2053c 2444c 3170 3567 1351a 1462c 2157 2581 1869c 2438 2976 3608 15296 2135 2656 3503 1219a 1805 1868 1376 1642 1820 1363 1522 are means of five or more tests of one brand. are means of five or more tests on each of are means of five or more tests on each of Notes: — Results marked " a : Results marked "b two brands. Results marked "c three brands. Results not marked are means of five or more tests on each of four brands, two American, two German. For compressive strengths of similar cubes, see Table 130. It is seen that the modulus increases with the age and rich- ness of the specimens, and decreases as the load increases. For one-two-four concrete the modulus at one month, for loads between a hundred and a thousand pounds, is about two and 1 "Tests of Metals," 1S99. MODULUS OF ELASTICITY 309 one-half million, and for six months, three and a half million. The corresponding values for the one-three-six concrete are two million and three and one-half million. When the ultimate strength is approached, the modulus of elasticity decreases rapidly, and between loads of one thousand and two thousand pounds per square inch, the richest concrete gives only about one and one-half and two and one-half million at one month and six months, respectively. 440. Modulus of Concrete Dependent on Richness of Mortar. — The results in Table 143 are abstracted from the extensive tests made at the Watertown Arsenal for the State Engineer of New York. Although several brands were tested, the results in the table are from one brand only, namely, "Wayland" Portland. These cubes were all stored in the same manner, namely, in water three to four months, and then buried in damp sand until broken at the age of twenty months. The mean ultimate strengths of similar cubes stored according to four methods are given in Table 132. Since in all of these mixtures the quantity of mortar was a given percentage, either thirty-three or forty, of the volume of aggregate, the effect of the richness of the mortar may be studied. While the proportional strengths of the concretes made with mortars containing from one to five parts sand are 100, 77, 52, 44, and 38, the corresponding proportional moduli of elasticity are 100, 92, 77, 60, and 55, the modulus decreasing less rapidly than the strength, with the addition of sand. 441. Gravel and Trap Aggregates. — Table 144 gives the re- sults of the determinations of the modulus of elasticity of con- crete specimens made and tested at the Watertown Arsenal, 1 the strength of which was given in Table 134. As these are all rich concretes, the moduli and the strengths are high. The values of the modulus for the gravel concretes are about 70 per cent, of those for the trap, but the strengths of the gravel concretes are in general about 80 per cent, of those obtained with concretes having trap aggregate. In a general way, how- ever, the modulus and strength vary together. 442. Modulus of Cinder Concrete. — The modulus of elas- ticity of cinder concrete prepared for the Eastern Expanded "Tests of Metals," 1898. 310 CEMENT AND CONCRETE w K O a h d) o a « 0} w g 5 fc 5 w O H a « ns p {* „ H co • W O « 01 w as m to £ .3 T O a 9 CO O O CO ■* 00 "* t-»o I- H Tf( l~ CD O CO OS t^ S HHH HHH T-H O CO • -*l • CO CD o ~r . Vffl Tfl CO O O rH ■ O IO A e (3 1—1 r— 1 l-H . T-H '"' rH * ■ O -* •* CO -* © O CO 00 QO CO iffl T-H rH CM CO •* ■73 tH • r " 1 rH rH rH 1-1 r~ t~- CO t-h rN OS a cr> >o co rH L- CO 00 O CM O CO "# U3 -H Ph HriH rH rH rH 1-1 1ONCM O O •* CO -* CO oco CO 1 OO-Ji 00 O CO CO CO cj O l-l CMHH rH rH rH 1-1 OCOH CO CO t~ CM 05' CO O CO t- CO 00 CO CO 03 10 ■* O t- CO OS (NOH CM T-H T-H 0> os • t-h cq • CO CM . T-H CO . rH ■* T-H U0 CO rH e3 O A H 000 ia co co t^ iO IO t— ■* in OS CS CM CO GO 1 rH OS OS 1 d H(NH rH CM rH I-H S*. CO CO CO CO CO O CO 00 X— CO CO t- O O t- O O CM lO CO CM CM CM CM CM CM CM cS H CD t- t- t^ OS rH O CI CO CO CO CO rH rH CM 8 rH © © O CM rH rH T-H 3 ■d o WHO >o CO -* CO O IO O "* L^ 3 2 rH I— 00 CM "O O O (MHH CM CM CM CM 8 CO t- O 00 t» CO ,_, CO IT- CD CO t-OI> OS CO (M CO c~ I- T-H CM ■* CM t-H rH CM •* CM CM K ^ . • rH So^ o 1 ® ■g . . +3 u • ■ 2 H « c3 cS Pm h z '5 o H 0! M all o s « Ph W q .£ eg c3 O CS rj .rt co cS c« 0) ^ 3 H o < z if) H O CO = >*H W U O J !>1 § Ml 4J d (5 o a i-l w is H a h it Set Leng *.D OF >S PER NCH. O O T-H ^H t-H CM CO *# © t- CO L© C O O O O © © © CM © i© i— l CO O © Q © © © © © © © © T-H © © ©■ © © © © © O © © © © © ©' ©' ©' ©' © © © © ©' ©' © © © Z M . o o Q fc. o Z O CI i, « tNC7. O S u •tHCO c£ o o H« o Z ■< ►v£ o o & o O Oh © -H lO CM LO CO © t- >o Ol • • O H I- >0 1- © 00 >o © -ti H & iO 53 o i * Z J H j h S i « S K o »m - S. • " o io ■ . IN CM ■ t~- lO . IT- 1— 1 _ CO -* H . . 55 o _ CM 00 ' o -& ■ l~ CM _ i-l CM 33 t- CM 00 o • . T-H CM O CO a co ea Ph o> 0) H 3 LO » O LO 02 00 -# CM CO O # CO rH ' CO - " a EH •*oooo CO CO O CM rh o t- r~ O .00 . a a Q ta S 25 o o LO ^H -* . -* SO -+i CO "* h* • O -hh o • -t< CO 1-H . Ci 00 •* i-H CM • ea UOI^O • i-H 71 CO ' t- o lo CO I- i-H • CM ■■# LC5 t 3 00 00 CO . 00 Ci CO ia *c i- ffli*iO . CO i—i CO i—l CM 71 . •sn: iwirjad ao aoy S 7 da. 28 da. 3 mo. lyr. c3 c3 § ^ - 1>XMH CM ■iNawaQ PhPhPh?h fr^^X ft a i— i CM CO ■<# lO CO t- 00 C (S.S U ;ig CEMENT AND CONCRETE sized specimens either in compression or bending. In the prep- aration of such large specimens the conditions of actual con- struction may be closely reproduced, and the results, although likely to be quite irregular, as the strength of concrete in struc- tures is not uniform throughout, are nevertheless very valu- able On account of the expense connected with such tests, the number of specimens is usually so limited that the natural irregularities in strength mask the true conclusions. TABLE 147 Comparative Tensile, Transverse and Compressive Tests, of Varying Consistency of Mortar Effect Transverse and Water Mean Strength, Pounds per Compressive as Per Square Inch. Strength as Per Cement. Cent, of Cent, of Tensile. Ref. Dry Ingre- dients. Tensile. Trans. Comp. Trans. Com p. 1 P 9 516 837 1731 162 335 2 P 12 533 1187 2173 185 408 3 P 15 467 850 2498 180 533 4 P 18 461 966 2823 209 612 5 P 21 430 1022 2487 239 578 6 N 12 272 447 2270 164 835 7 N 14 325 516 2141 158 659 8 N 16 319 519 1481 163 464 9 N 20 304 509 1512 167 497 10 N 24 315 462 1317 147 418 Notes : — Cement, P = Portland, Brand R ; N = Natural, Brand In ; Sand, "Point aux Pins," pass No. 10 sieve. Age of specimens, one year. Two parts sand to one cement by weight. In Tables 148 to 156 are given some of the results obtained in testing over two hundred concrete bars at St. Marys Falls Canal. The molds for making the concrete bars were ten inches square by four and one-half feet long inside. The con- crete was rammed into the mold with a light wooden rammer. The bars were, in general, covered with moist earth soon after completed, to await the time of breaking. To break them they were supported on knife edges placed four feet apart, and the load was applied at mid-span through an iron bolt laid across the bar. In the earlier tests a direct load was imposed by means of a platform which was gradually loaded with one-man stone, but in the later tests the load was applied by means of hydraulic CONCRETE 317 jacks, an hydraulic gage being used to measure the force. In many cases the half bars were again broken at a later date with a twenty-inch span, as shown in the tables. 447. Variations in Richness of Mortar. — In Table 148 sev- eral concretes made with mortars having different proportions of sand are compared, and the results of briquet tests on similar mortars are also given. Although the briquets were not broken at the same age as the bars, the tests on the latter at the differ- ent ages show that they were not gaining strength rapidly, and the results may therefore be compared without serious error. TABLE 148 Transverse Tests of Concrete. Variations in Richness of Mortar £h . , 2 i. o o Modulus of Rupture. No. Bars. Date Made. Cem- ent. « fc e « W K *^ a n in !> H « B ° - g*H wg Four Foot Span. Twenty Inch Span. C 2 'f cos 5 <:£ o a ro No. Tests. Age. Mean. No. Tests. Age. Mean. Mo. Da. Yr. Mo. Yr. Mo. 76-77 11-2 Port. 717 2 1 7 593 4 2 9 600 78-79 t 4 " 1 790 2 u 689 4 " 698 80-81 " " 2 595 2 " 538 4 " 577 82-83 " u 3 432 2 " 489 3 " 415 84-85 11-3 " 4 335 2 " 379 4 " 385 86-87 " " 5 252 2 " 284 4 " 316 88-89 it " 6 218 2 u 262 4 t; 279 90-91 11-4 Nat. 1 483 2 u 420 4 u 450 92-93 " " 2 396 2 " 332 4 I £ 387 94-9") " " q 330 2 u 240 4 " 224 96-97 " " 4 237 2 ; i. 186 4 205 Notes: — Portland, Brand R,, Sample 82 M. Natural, Brand Gn, Sample 83 T. Sand, from " Point aux Pins" (river sand). Stone, Potsdam sandstone, retained on f inch square mesh, and no pieces larger than 3 inches in one dimension. Amount mortar used in each case equal to voids in stone measured loose, except in case 1-2 natural, when mortar exceeded voids by seven per cent. The fracture showed concrete very compact in nearly all cases. The results obtained with natural cement show that the tensile strength of the mortar in pounds per square inch was greater than the modulus of rupture obtained for the concrete. 318 CEMENT AND CONCRETE This is also the case with rich mortars of Portland cement, but for Portland mortars containing more than three parts sand to one of cement the concrete gives the higher result. The strength of the concrete with one-to-four mortar is fifty-five per cent, of the strength with one-to-one mortar for Portland, and forty-five per cent, for natural. The decrease in strength due to larger proportions of sand in the mortar is usually greater than the decrease in cost. TABLE 149 Transverse Tests of Concrete. Variations in Quantity of Mortar No. Bar. Date Made. < K £ HO 9 <; a Am't Rammed Concrete Made as Per Cent, of Loose Stone. Modulus of Rupture. Four Foot Span. Twenty Inch Span. No. Tests. Age. Mean. No. Tests. Age. Mean. 42 37-40 38-41 39-43 Mo. Da. 7 3 7 1 7 1 7 1,3 31 38 47 60 88 92 104 112 1 2 2 2 1 yr. u 247 284 350 346 2 3 4 4 Yr. Mo. 363 447 596 589 Notes: — Cement, Portland Brand R, Sample 64 T. Sand, " Point aux Pins," three parts by weight dry to one cement. Stone, Drummond Island limestone, passing 1 inch slits and retained on | inch slits. 448. Variations in Amount of Mortar Used. — Bars 37 to 43, Table 149, were all made with the same kind and quality of stone and the same quality of mortar, three parts sand to one cement by weight, but the amount of mortar varied; thus, in bars 41 and 38 sufficient mortar was used to fill the voids in the stone, while the bars above were deficient in mortar, and those below contained an excess. It is seen that the highest result is given by the bars in which the mortar was just suf- ficient to fill the voids in the stone, though the bars containing an excess of mortar gave practically the same result, while a deficiency of mortar resulted in decreased strength. 449. Variations in the Amount of Sand for Fixed Quantities of Cement and Stone. — In Table 150, bars 68 to 75 were all made with the same kind and quantity of cement and stone, but the amount of sand, and consequently the quantity and CONCRETE 319 quality of the mortar, varied. The highest strength is given by the concrete in which the weight of the sand was three times the weight of the cement; this quantity of sand gave sufficient mortar to fill the voids in the stone. The richer mortars, though stronger, were deficient in quantity, while four parts sand made an excess of mortar having a lower strength. TABLE 150 Transverse Tests of Concrete. Variations in Quantity of Sand for Fixed Quantities of Cement and Stone < pq 6 z P o Oh Eh" z a S a O q" . Z « * H £ ps m P o o o zX 2 c° s* o 55 W h 55 O O o 55 m K H CO s ►J z H 4 Foot Span, 13 Months 20 In. Span, 2 Years. Cem- ent, Lbs. Sand, Lbs. No. Tests. Mean. No. Tests. Mean. 138-139 Port. 120 237 0.61 7.31 2 354 2 289 a 509 13(5-137 it 120 237 0.83 7.12 2 450 3 482 b 404 140-141 u 120 237 1.03 7.00 2 450 4 442 c 415 142-143 it 120 240 1.16 7.12 2 385 4 417 d 400 146-147 Nat. 115 230 0.83 7.64 2 180 4 156 a 267 144-145 1 1 115 230 1.03 7.31 2 223 4 282 b 187 148-149 (i 115 230 1.16 7.12 2 234 4 256 c 145 150-151 ti 115 230 1.35 7.12 2 202 4 177 d 127 152-153 " 115 230 1.51 7.12 2 155 3 170 e 116 Notes: — Portland cement, Brand R, Sample M. Natural cement, Brand Gn, Sample 88 T. Sand, " Point aux Pins" (river sand). Stone, Potsdam sandstone, 7 cubic feet to each batch. Results in last column give tensile strength at one year of briquets made from similar mortar. Consistency : — a, very dry ; no moisture shown on ramming. b, slight moisture appeared at surface after continued ramming. c, quaked somewhat. d, quaked and water rose to surface in ramming. e, too wet to ram. TABLE 152 Transverse Tests of Concrete Bars. Value of Thorough Mixing No. Bar. Mixing of Concrete. Modulus of Rupture. Four Foot Span. Twenty Inch Span. No. Tests. Age. Mean. No. Tests. Age. Mean. 182-186 183-187 184-188 185-189 Turned once and back " twice " " " 3 times " " u 4 u (( t( 2 2 2 2 1 yr. ii 290 294 306 328 4 4 4 4 21$ mo. ii u 373 353 444 474 Notes: —Cement, Portland, Brand X, 200 lbs. Sand, "Point aux Pins," 600 lbs. Stone, Potsdam sandstone, 15 cubic feet. CONCRETE 321 concrete. Comparing the concrete turned once or twice, and back, with that turned three or four times, and back, it is seen that the mean strength of twelve tests with the former is 328 pounds per square inch, while the mean strength of the same number of tests with the more thoroughly mixed concrete is 388 pounds per square inch, an increase of eighteen per cent. TABLE 153 Transverse Tests of Concrete. Variation in Size of Aggregate No. Bar. g •i m Z H H O Stone. □ ggo O ffl « a B.f? Eh Amount Rammed Con- crete Made, Cubic Feet. Modulus of Rupture. Lbs. per Sq. In. Kind. Sizes. Per Cent. Voids IN- COM- PACT. One Bar, 4 Ft. Span, Age 1 Yr. Half Bar, 20 In. Span, Age, 21 Mo. 202 19!) 201 200 198 196 195 197 194 XR6 u u (( XM3 U U a a a «! ■1 d d d V IV 2 y 3 v ' 3 *- M \ each, V, F, & M \ each, K, V, F, & M V F M \ each, V, F, & M 45 43 44 J 40 }ao 32 33 34 I 30 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.75 3.86 3.75 259 259 216 245 288 216 186 ■ 131 207 367 347 269 292 390 311 302 208 302 Notes: — All mortar, three parts sand to one part Portland cement by weight. Quantity of mortar about one-third volume of compact stone. Stone : — ■ a = Potsdam sandstone ; d = gravel. Size : — K = T ' (T inch to \ inch. v = i u j " F = \ " 1 " M = 1 " 2 " 452. Variations in Size of Stone and Volume of Voids. — The bars given in Table 153 were all made with mortar composed of three parts sand to one of Portland cement by weight. The stone for these bars was sorted into different sizes, and these were recombined in the proportions indicated in the table. The sizes are denoted as follows: that passing one-half inch mesh and retained on one-quarter inch mesh, is called V; one- half inch to one inch is called F; one inch to two inches, M; two inches to three inches, C; and coarse sand, one-tenth inch to one-quarter inch, is called K. 322 CEMENT AND CONCRETE The first five bars were made with broken sandstone, and it is seen that the coarsest stone, size one inch to two inches, gave the lowest result. The size V, one-quarter inch to one- half inch, although containing no smaller percentage of voids, gave a much higher strength. The highest result was given by the bar made with a mixture of four sizes, the voids in this mixture being only thirty-six per cent. The bars containing gravel as aggregate indicate that the strength decreases as the size of stone and volume of voids increase, but a mixture of three sizes gives nearly as good a result as the fine gravel alone. Comparing the results with similar sizes of the two kinds of aggregate, it appears that the broken sandstone gives somewhat better results than gravel, notwithstanding that the proportion of voids in the former exceeds that in the latter. 453. Sandstone and Bowlder Stone Compared. — The results given in Table 154 are from a series of tests made for the Mich- igan Lake Superior Power Company by Mr. H. von Schon, Chief Engineer, 1 and show the strength of concretes made with two kinds of aggregate available at Sault Ste. Marie. Two samples of Portland cement, one made from marl and one from limestone, a slag cement, and a natural cement, are used in these tests. The two samples of Portland cement give nearly the same result, the slag less than half the strength, and the natural quite weak. The ratio of the strength obtained with crushed bowlders to that made with sandstone is about 1.6 with Portland, and the superiority of the former is shown with all cements. 454. Various Kinds of Aggregate. — Table 155 gives the re- sults obtained at St. Marys Falls Canal in using various kinds of stone. In bars 25 to 30, three kinds of stone are compared. The superiority of the Kelleys Island Limestone "shavings" from the stone planers is evident. The shape of the pieces may have had a considerable influence on this result, the planer shavings being flat, or lenticular in form. Bar 34 was made with a hard limestone from Drummoncl Island, 33 with gravel, and 31 and 32 with gravel and stone mixed in equal propor- 1 Tests reported by H. von Schon in Trans. A. S. C. E., Vol. xlii, p. 135. CONCRETE 323 TABLE 154 Transverse Strength of Concrete with Crushed Sandstone and Bowlders Aggregate. Mixture No. Modulus of Rupture, Pounds per Square Inch. Portland. (Marl.) Portland. (Bock.) Slag. Natural. Sandstone . . . it << 1 2 3 4 5 328 283 220 178 106 312 265 178 173 186 122 161 118 74 131 43 34 40 35 223 223 121 38 Bowlder Stone . i, u U U 11 u 1 2 3 4 5 407 377 332 327 333 397 395 374 351 325 145 167 176 146 123 36 67 55 52 60 Mean, Bowlder Stone .... 355 368 151 54 Ratio of ( Bowlder Stone j Moduli | Sandstone J 1.59 1.65 . 1.25 1.42 Notes: — Cross breaking tests of 6 in. by in. by 24 in. bars made for Michigan, Lake Superior Power Co. Materials: — Cement, representative brands of each of four classes. Sand, river sand, " Point aux Pins," mostly quartz, 96J lbs. per cubic foot. Voids, 41.7 per cent. Fineness, 96 per cent, passing No. 20 sieve, 39 per cent, passing No. 40 sieve. Stone, Sandstone, broken Potsdam 1 to 1^ inch size. Bowlder stone, broken gneiss and granite bowlders, 1 to 1| inch size. Proportion in mortar, 1 part cement to 2.4 parts sand by volume. Mixing: — Consistency, plastic; cement and sand mixed dry, then wet and mixed; mortar added to wet aggregate and concrete mixed by hand. Storage : — Bars stored in shed, protected from rain, fully exposed to air. Age of specimens when broken, sixty days. Mixture: — 1. Mortar 15 per cent, in excess of quantity required to fill voids. 2. Mortar 10 per cent, in excess of quantity required to fill voids. 3. Mortar 5 per cent, in excess of quantity required to fill voids. 4. Mortar just sufficient to fill Avoids in stone. 5. Mortar 15 per cent, in excess of amount required to fill voids in stone, but this 15 per cent, excess made with lime instead of cement. 324 CEMENT AND CONCRETE U a w w K P H B. P K P J P a c CO P, J3 C c o> is H a " O CO 00 CC t— ""* CO CO •"* "■# CO CO oj d O 3 j_; -l,-li-HrHi- H|NH|C1 z H a w 6 i 03 ^- = 2 - = W- - 5 So- 3 « « g +3 -+J 3 p^ t- OS i.o co t- co os o -+ co im --I "7 I 1 co fi d ^ 5S ■-fig rt|X fi .„■■"' CO .n ' — CO fi 12 fi o fi a fi fi cS -O .2 co £ co I I 1-1 I Ph Q £ O PQ e ~o « "e « CONCRETE 325 tions The gravel and hard limestone gave about the same result, but it is seen that the mixture gave a higher strength. Bars 154 to 159 were made to test the value of broken brick for use in concrete. It is seen that the strength obtained with brick is considerably lower than that obtained with the soft limestone. Had a poorer mortar been used, the brick would doubtless have given a better comparative result, since with the one-to-two mortar, the brick are not strong enough in them- selves to utilize the full adhesive strength of the mortar. TABLE 156 Transverse Tests of Concrete. Use of Screenings with Broken Stone Sand and Stone. Stone to 80 Lbs. Cement. Modulus of Rupture. No. Bar. O H H p O > Four Foot Span. Age, 11 Mos. Twenty Inch Span. Age, 2 Yrs. u § A St§ G Pm 3 u £« -A CO OP CO o a? No. Tests. Mean. No. Tests. Mean. 114-115 a 40 240 7.0 3.43 3.05 2 233 4 237 124-125 b 48.4 243 7.0 3.39 3.05 2 196 4 210 112-113 c 44 240 7.0 3.08 2 194 3 236 116-117 d 44 138 7.8 3.43 2.16 2 227 3 311 118-119 e 40.5 243 7.0 2.83 3.10 2 201 4 219 120-121 f 38.8 243 7.0 2.72 3.05 2 122 4 164 122 9 243 7.0 3.05 1 130 2 141 Notes: — Cement : Natural, Brand Gn, Sample 92 T, 80 lbs. Stone: — a = Drummond Island limestone, screened. b = 10 parts screenings to 100 parts stone. c = 17 parts screenings to 100 parts stone. d = 17 parts screenings to 100 parts stone. Screenings replacing equal amount sand. e = 50 parts screenings to 100 parts stone. / = 100 parts screenings to 100 parts stone. g = Screenings only, no broken stone. 455. Use of Screenings with Broken Stone. — Table 156 gives the results of a number of tests made to show the effect of mixing screenings with the broken stone. A smaller amount of mortar is required to fill the voids in a given volume of stone and screenings mixed than is required for the same volume of 326 CEMENT AND CONCRETE stone. It is seen that, with natural cement, when the same volume of mortar is used in the two cases, the presence of screenings to the amount of one-third of the total aggregate does not make a material change in the strength of the result- ing concrete, but when the screenings are allowed to take the place of a part of the sand in the mortar, as in bars 116 and 117, a much stronger concrete results. Natural cement mixed with sand and screenings alone, bar 122, does not make a strong concrete, but Portland cement with screenings without sand was found to give excellent results. 456. Deposition in Running Water. — A few tests were made to" show the effect of depositing concrete in rapidly running water. The molds were placed in the stream and weighted down in twelve inches of water. The concrete for two bars was deposited as soon as mixed, that for two other bars was allowed to stand in the air three hours before deposition, until it should have acquired an initial set, and two bars were made after the mortar had been allowed to stand five hours and twenty minutes before deposition; by this time the mortar had set quite hard. No attempt was made to ram the concrete, which was deposited by lowering it carefully into the water with shovels, the molds being filled as rapidly as possible. A very large amount of the cement was washed out by the current in all cases. After a few months the bars were removed from the stream and covered with earth as usual. The tests at eleven months did not appear to show any advantage in allowing the mortar to stand some time before deposition, but the tests at two years showed a distinct advantage in this treatment. 457. Use of Concrete in Freezing Weather. — Table 157 gives the results obtained with Portland cement concrete made in the open air during cold weather. The conditions as to tem- peratures and the character of the materials are fully given in the table. The experiments are too limited to permit of draw- ing definite conclusions, but the following points are indicated by the results obtained. The use of warm water, 100° to 156° Fahr., in freezing weather appears to give somewhat better results than cold water. Salt should not be used unless the temperature is below the freezing point, but in very cold weather the use of enough salt in the water to lower its freezing point below the temperature of the air seems to hasten the harden- CONCRETE 327 TABLE 157 Transverse Tests of Concrete Bars. Use of Concrete in Low- Temperatures r'. « e 8«* to to -a-y to to to to S — „— —"— —" ~ a S CO ■* GN CO _ CO t~ © 1^ o ~ t^ o r o CO (M ■■# LO O t^ GM 55 OQ PH ON GN CO Tft CO CO ■* ia UO iO ■* ■* -f GN CO ■* rt* Q a< p Stf CM iO r^ CO co GM CO «D r~ iO o -1- GN GM r~ GM CO » ■>* 1 GM GM GN GM GM GM co CO CO -ttl -T CO CO CO GM CO CO GM H X O to ©CO © to o to © to o cc c - c © ©© : CS ?! CVCN 00=iH l il 1—1 1—1 • he ■ o o — _ ■"* O -# lO t~ O GM 1—1 p a C« to ■* ~ IG -!- ^# -r CO CO CO -* co ■<* &>-> I— 1 1-1 K . &•& ~ 0) OS co GN co G<) © t~ !>■ *~ o CO "* -^ on rH rH GN GM GN CO CO CO 1—1 GN GN- H o -T ■<# c -* CO -|H on © ^ CO ©' GO © 00 © - Ir- © CO O - ^ o CO © CO © 2 2 - IPO « 2 gg o o • *» O 3 GM - IO O O CO g OD^o -3 ^ o © s 3 QQH I— "# CM ri o CM K 0) 00 »o ~ - Ph CO :r 1 — 1 T— ( U H h "# t -# -1" -* ^ iO ^ ^ ^ _ jX CI •* "" © " - •JJ "" © "* o 35 © B „ - CO - ,. GN - © - CO - IO - © -■ - - H 1-1 1-1 1-1 H a *, - - GM - - ,_, - CM - ri - ,_, - co ^ - - P 1 ' T_i 7-1 Remarks: — a, Concrete frozen after 10 to 20 min.; b, frozen in 45 min.; c, began to freeze in 15 min.; d, frozen hard following morning after molding; e, concrete still soft 9 a.m. morning after molding; /, bar defective. Notes: — Cement, Portland, Br. R. Sand, "Point aux Pins." Stone, Potsdam sandstone. 328 CEMENT AND CONCRETE ing as well as to increase the ultimate strength, mortars in freezing weather, see Art. 50.) (For tests of Art. 58. Resistance to Shear and Abrasion 458. Shearing Strength. — The shearing strength of mortars and concretes is of importance not only because of its intimate relation to the compressive strength, but because of the shear- ing stresses to which these materials are subjected in structures reinforced with steel. But few tests of shearing strength have been made, however, partly because of the lack of appreciation of their value, and partly because it is difficult to subject a specimen to a purely shearing stress. It is frequently stated that the shearing strength is somewhat in excess of the tensile strength, perhaps as much as twenty per cent. Table 158 gives the results of a series of tests made by Prof. Bauschinger in 1878. 1 The values in shear are very closely twenty per cent, in excess of the tensile strengths of similar mortars tested at the same time. TABLE 158 Shearing Strength of Portland Cement Mortar Cubes Hardened in Air Cement. 5 z ■ JO fr< rS ° B S H § ggH P ■"< Shearing Strength, Pounds per Square Inch. Tensile Strength of Similar Mortars at Eight Weeks. Age of Mortar. 1 week. 2 weeks. 4 weeks. 8 weeks. Quick setting Port- C laud, mean results ■] of four brands. ( Slow setting Portland, ( mean results of four j brands. ( None o •J 5 None o 5 225 108 07 301 124 78 270 128 94 323 164 122 257 154 112 341 199 138 259 196 168 377 237 199 210 169 139 256 181 169 Note : — ■ Cement, each result mean of four brands. Sand, medium grain, clean. Mortars hardened in dry air. Tests by Professor Bauschinger, 1878. 459. A distinction should be drawn between the resistance offered by a thin mortar bed to the sliding of one stone or brick on another and to shear of the mortar itself. The former re- 1 Quoted by Mr. Emil Knichling in a Report on Cement Mortars. SHEAR AND ABRASION 329 sistance involves the adhesion, of the mortar to the surface of the brick or stone, and the values for this resistance are usually much less than the shearing strength, and not greatly in excess of the adhesive strength. The one is of importance in the de- termination of the stability of masonry dams, retaining walls, etc., but the latter is the resistance in question in the design of monolithic concrete structures. 460. Resistance to Abrasion. — The resistance of cement mortar to abrasion depends on the quality of the sand as well as the cement. The abraiding surface wears away the cement or pulls the particles of sand out of their beds in the cement matrix. If the adhesion to the sand grains is strong, the sand particles receive the wear and withstand it until nearly worn away. With hard sand particles, therefore, the resistance to abrasion should increase as the proportion of sand increases, until the volume of the cement matrix becomes relatively too small to thoroughly bind the sand grains together. This limit is reached, however, when the mortar contains not more than two parts sand. With soft sand grains, the neat cement will usually give the highest resistance to abrasion, at least in the case of Portland. It has been found that specimens hardened in the air are brittle and wear more rapidly than those hardened in water. 461. Table 159 gives the results of several tests made to determine the relative wearing qualities of different mortars for such xises as sidewalk construction. The specimens were two- inch cubes, hardened in water and dried for a few hours just before grinding. An emery plate, set horizontally, was used in most of the tests. The results in any given line of the table are comparable, but, owing to changes in the grinding plate and in the methods used, the results in different lines are not all intercomparable. It is seen that when soft sand is used, such as limestone screenings, the greatest resistance to abrasion is offered by the neat cement mortar, and the resistance de- creases constantly as the amount of sand is increased. When hard sand, such as the siliceous river sand, from Point aux Pins ("P.P." in the table) is employed, the greatest resistance is offered by mortars containing about equal parts of sand and cement. A comparison of lines 5 and 10 indicates that rich natural cement mortars lose about twice as much as similar mortars of Portland, but natural cement mortars containing 330 CEMENT AND CONCRETE o a H u J o n En 05 CO CO CO 10 CM CD CO ■* 0? CO to *0 10 t- CO 1—1 CO t- CM CM £2 CO CM T}H i-H Ho CM 00 CO *o 05 1— ( CM t- CM CM CO CO CD 1-H O CO 00 . O CM '3 CM ^- 1-1 iO ■* ■* 1—1 CO 00 1-1 CM CO CO 00 C6 ■* T— 1 CM t- 1—1 05 LO r~ lO t~ TjH iO fc H CM Cn ■<* 02 *# CO i-( CM kO 00 CO CO 8 O a CM T— 1 ^ CM CO ■* CO CM CO CM 05 l-H O 10 i-l rt a> co 1 — 1 O Hn O t~ O CO i-H co C5 CO -* I-H O CO US CO i-H -* i-H CM ■ r+* CM 05 O O 1-1 T*1 CO CM O t~ B < •d CO O CM CO I-H © -* t^ O rH K & a « M & 05 00 05 CO CM 05 •* CO O CM T(H CO CM O CM VO O O CO CO 00 i-H Ph ■* CO TH •* X ■* t^ 10 Tj< 05 CO i-H CM ■* CM -* tO 1-H aaM O O .45 CM "* t- co 00 t~ LC5 CM t~ CD h] lO r-f -- 1 CM rHO ^ CM lO CO •Jst oixhoceohj Hova CM CM CM fN CM ■* ■>* Tt< 1-H ^ 50 ssawiaads so 'ox cS 6 0§?H *^* T3 ^ - - as - ^a ^ CO rt CM CO CM CO •sqrj'iiauipacls no ■uj 't>S - I9 d a.mssajj -si 00 •* "* t(H CO CO co 00 ■* CO O O O •8H3U -A8H "OJS cs 00 co CO CO SO co CO CO 00 CD s 'a^d no pasn a co - - CO CM O CM CM CD s ° O CM # z^j-Btii*) paqsruQ d a 3 O fi >j "C3

o CO 1— 00 05 O EXPANSION AND CONTRACTION 531 more than two parts by weight of sand do not give relatively as good results. Art. 59. The Expansion and Contraction of Cement Mor- tar, and the Resistance of Concrete to Fire 462. Change in Volume during Setting. — Cement mixtures shrink somewhat when hardened in air, while specimens stored in water expand a trifle during hardening. Although several experiments have been made on this subject the specimens used have been so small that the results obtained by various author- ities do not agree, and the effect of variations in the character of the mixtures has not been thoroughly investigated. The impor- tance of the question is found in the necessity of providing ex- pansion joints in long walls or sheets, and in the effect of such changes in volume in producing initial stresses in concrete or steel where these materials are used in combination. Certain general conclusions are well established and may be stated as follows: 1st. The shrinkage of mortar and concrete hardening in air is considerably greater than the expansion of similar specimens hardening in water; 2d. The amount of change in volume increases with the proportion of cement used in the mixture; 3d. The change in volume is continuous up to one year, but about one half of the change occurs in the first week, and it is very slow after 3 to 6 months. The following values of the change in linear dimensions are derived from the results of several experimenters, and show in a general way what changes are to be expected at the end of three months. 1 Variations in the character of the cement and the consistency of the mortar will affect the result. Composition: Parts Sand to One Portland Cement. Shrinkage of Mortars Hardened in Air. Expansion of Mortars Hardened in Water. Change in Linear Dimensions, One Unit in Neat cement .... One part sand .... Three parts sand . . . 300 to 800 600 to 1200 700 to 1200 500 to 2000 1200 to 3000 3000 to 5000 1 For more detailed results the reader is referred to the following authori- ties:— Dr. Tomei, Trans. A. S. C. E., Vol. xxx, p. 16. Mr. John Grant, Proc. Inst. C. E., Vol. lxii, p. 108. Prof. Bauschinger, Trans. A. S. C. E. Vol. xv, p. 722. 332 CEMENT AND CONCRETE 463. The Coefficient of Expansion of Cement and Concrete. — Concerning the coefficient of expansion of cement mortars of various compositions, we know but little. The result obtained by M. Bonniceau, giving the coefficient of neat Portland cement as about .000006 per degree Fahr., is frequently quoted. This is very nearly the value for iron and steel, and has formed a the- oretical basis for combining these materials. In the case of cement mortars and concretes, however, it is highly probable that the coefficient follows quite closely the behavior of the sand and stone used in the mixture, and is much less dependent upon the coefficient of the cement. This was indicated by the results of M. Bonniceau who obtained a value of about .000008 for con- crete. 464. A number of experiments to determine the coefficient of expansion of cement concretes were carried out under the direction of Prof. Win. D. Pence by students of Purdue Uni- versity. 1 As a mean of seven tests with one-two-four concrete of Bedford oolitic and Kankakee limestones combined with Portland cements of two well-known brands, the mean result for the coefficient was .0000055, the lowest result being .0000052, and the highest result .0000057. The coefficient of a bar cut from the Kankakee limestone was .0000056, the same result as obtained from the mean of three tests of concrete containing broken stone of this variety. The average result of four tests of gravel concrete composed of one part Portland cement, two parts sand and four parts screened gravel, or one part Portland cement to five parts un- screened gravel, gave .0000054 as the coefficient of expansion. These values differ from the coefficient of steel enough to indicate that in positions where the range in temperature is great, the resulting stresses in the concrete and steel may be considerable, and worthy of attention. 465. The Fire-Resisting Qualities of Concrete. — The value of concrete as a material to be used in the construc- tion of the walls and floors of buildings, is largely dependent on its fire-resisting qualities. That its use for such purposes is rapidly extending, is some evidence that these qualities are 1 Paper read before the Western Society of Engineers, Engineering News, Nov. 21, 1901. RESISTANCE TO EIRE 333 as satisfactory as in other classes of materials devoted to the same use. Under favorable circumstances, a fire in a building filled with combustible materials may reach a temperature of 2,000° to 2,300° Fahr. If a small specimen of cement mortar or concrete is subjected to a temperature approaching this intensity, the cement loses its water of crystallization and becomes friable. If cooled suddenly in water, the specimen cracks and disinte- grates. If cooled gradually, the outer edge of the specimen crumbles away. From such tests on small specimens some very erroneous conclusions have been drawn as to the value of con- crete as a fire-resisting material. Such conclusions have clone much to prejudice the public mind against concrete, and to re- tard its introduction in buildings designed to be fireproof. 466. Conductivity. — The great value of concrete as a fire resistant is due to its low conductivity of heat, and while the surface of a mass of concrete exposed to an intense flame for some time is ruined, and may be flaked off by the application of a strong stream of water from a fire hose, the depth to which the heat penetrates is very limited. Steel is said to lose ten per cent, of its strength at about 600° Fahr. and fifty per cent, at about 750° Fahr. The importance of protecting the steel framework of a building, not only from warping and complete destruction clue to flames, but from loss of strength from over- heating, is therefore evident. Among engineers and architects it is recognized that the term "fireproof construction" is only relative, although the lay mind is apt to give a definite and literal meaning to the term. It is well known that fireproofing tile, whether hard or porous, will fall to pieces if subjected to a temperature above that employed in its manufacture. The practical question then is, what type of construction will withstand long continued intense flame, and subsequent quenching with water, with the least injury to the strength of the structure. The results of fire tests that have been conducted in several places, and no- tably those made by the Department of Buildings of New York City, have shown that floor arches properly constructed of concrete-steel are equal to any style of floor with which they come in competition. The low conductivity of concrete is shown by the fact, 334 CEMENT AND CONCRETE stated by Mr. Howard Constable in connection with the dis- cussion of fire tests of concrete floor arches, 1 "that in some thirty-five cases where the temperature ranged from 1,500 to 2,400 degrees, the time of exposure being from one to six hours, the temperature of the upper flanges of six-inch to ten-inch beams might be approximately placed at not much above 200 degrees." He also says "in one case, where the beam was pro- tected by three inches of concrete, the fire was maintained for five hours, and the temperature went as high as 2,300 degrees, and there was no practical or permanent set produced in the beams." 467. Behavior in Conflagrations. — As to the behavior of concrete-steel arches in an actual fire, a board of experts was appointed by the insurance companies to investigate the causes and extent of damage to the fireproof buildings in the Pitts- burg, Pa., fire of May 3, 1897. This board stated in their report that they believed that in important structures of this class "the fireproofing should be in itself strong and able to resist severe shocks, and should if possible, be able to prevent the expansion of the steel work "; and continued, "There seems to be but one material that is now known that could be utilized to accomplish these results, and that is first-class concrete. The fire-resisting qualities of properly made concrete have been amply proven to be equal, if not better than fire clay tile, as shown by the tests carried on by the Building Department of the City of New York." 468. In a report on the Baltimore fire, Captain Sewall, 2 Corps of Engineers, U. S. A., says concrete "undergoes more or less molecular change in fire; subject to some spalling. Molecu- lar change very slow. Calcined material does not spall off badly, except at exposed square corners. Efficiency on the whole is high. Preferable to commercial hollow tiles for both floor arches or slabs and column and girder coverings. In form of reinforced concrete columns, beams, girders and floor slabs, at least as desirable as steel work protected with the best com- mercial hollow tiles. Stone concrete spalls worse than any 1 Trans. A. S. C. E., Vol. xxxix, p. 149. 2 Report to the Chief of Engineers, TJ. S. A., by Capt. John Stephen Sewall, Corps of Engineers. Published in Engineering News, March 24, 1904. RESISTANCE TO FIRE 335 other kind, because the pieces of stone contain air and moisture cavities, and the contents of these rupture the stone when hot. Gravel is stone that has had most of these cavities elimi- nated by splitting through them, during long ages of exposure to the weather. It is therefore better for fire-resisting concrete than stone. Broken bricks, broken slag, ashes and clinker all make good fire-resisting concrete. Cinders containing much partly burned coal are unsafe, because these particles actually burn out and weaken the concrete. Locomotive cinders kill the cement, besides being combustible. On the whole, cinder concrete is safe only when subjected to the most rigid and intelligent supervision; when made properly, of proper ma- terials, however, it is doubtful whether even brickwork is much superior to it in fire-resisting qualities, and nothing is superior to it in lightness, other things being equal." 469. Aggregate for Fireproof Work. — Since air is a poor conductor of heat, the more porous concretes are the better protectors against fire. On this account, as well as because of its lightness, cinder concrete is preferred for fireproofing. Care should be taken that cinders to be used in fireproofing concrete do not contain any appreciable amount of unburned coal; in concrete to be used next to steel members the cinders should also be practically free from iron rust. (See §473.) The strength of cinder concrete is much inferior to that made with the ordinary aggregates, and there should be no difficulty in making a porous concrete with the latter. In fact, in many other classes of construction it has been seen that great precautions must be taken to avoid porosity. By the use of insufficient mortar to fill the voids in the stone, voids may be left in the concrete, though at the expense of dimin- ishing somewhat the strength of the mixture. In adopting such an expedient one should not lose sight of the fact that in order to preserve the imbedded steel from corrosion, it must be fully covered with the mortar. 470. Broken bricks are excellent for fireproofing concrete. The bricks themselves are fire resistant, porous and light, while the adhesion of cement mortar to bricks is so great that unless a very weak mortar is used, the strength of the concrete is limited only by the strength of the brick employed. Sandstones, especially those with siliceous cementing ma- 336 CEMENT AND CONCRETE terial, are also well adapted for this purpose. Limestone, on account of the low temperature at which it is broken up, is not good, though as to just how far a limestone concrete would be disintegrated by the heat of an ordinary building fire has not, so far as the author knows, been fully investigated. It is known, however, that limestone masonry is calcined to a cer- tain depth in a conflagration. Granite in large pieces is cracked by only a moderate degree of heat, and spalls badly. Just how much danger there might be of a similar action in concrete aggregates of this material is not known, nor whether small pebbles or fine gravel would have this property in the same degree, though it is believed they would not, and this view has been confirmed by observa- tions of the Baltimore ruins. Before adopting a given aggregate for fireproof work, one should satisfy himself by actual test as to the suitability of the materials available, but such tests should be conducted upon concretes containing the proposed aggregates, rather than upon fragments of the materials not incorporated with mortar. Art. 60. The Preservation of Iron and Steel by Mortar and Concrete 471. The rusting of steel members in modern buildings and other engineering structures is one of the most serious menaces to their permanence. The introduction of concrete-steel con- struction has given rise to some discussion, especially among those unfamiliar with the properties of concrete, as to the effect of the concrete upon the steel. 472. Action of Corrosion. — The rusting of iron takes place only in the presence of moisture, air and carbon dioxide. In perfectly dry air, or in perfectly pure water, iron does not rust. Under the proper conditions, however, the iron, water and carbonic acid combine to form ferrous carbonate, which at once combines with oxygen from the air to form ferric oxide, the carbonic acid being liberated to act on a fresh portion of the metal. It is seen that only a very small amount of the carbon dioxide is necessary. If, however, the carbon dioxide or other acid filling the same role, is neutralized by the presence of. an alkaline substance, the foregoing reactions cannot take place. As cement is strongly alkaline, it thus furnishes an almost per- fect protection against rusting. EFFECT ON CORROSION OF METAL 337 473. Tests of Effect of Concrete on Corrosion of Metal. — To determine the cause of occasional rusting of steel surrounded by cinder concrete, and consequently the proper methods of applying cement mortar or concrete to steel, Prof. Chas. L. Norton, engineer in charge of the Insurance Engineering Ex- periment Station at Boston, made tests on several hundred bri- quets in which steel was imbedded in mortars and concretes of various compositions. 1 The briquets were subjected to air, steam and carbon dioxide, others to air and steam, to air and carbon dioxide and to the ordinarily dry air of a room. At the end of three weeks it was found that neat Portland cement had furnished a perfect protection in all cases. The corrosion of the steel in other specimens was always at a point where a void existed in the concrete, or where a badly rusted cinder had lain. In every case where the concrete or mortar had been mixed wet, and the surface of the steel had been thus coated with a thin layer of grout, no rust spots occurred. In the first tests made by Professor Norton the specimens were thoroughly cleaned before being imbedded in the concrete, but later tests indicated that in specimens that had begun to corrode before treatment, the rusting was arrested by the coat- ing of cement mortar or concrete. After from one to three months in tanks holding steam and carbon dioxide, specimens which had' been in all stages of corrosion before being im- bedded in the concrete had not suffered any sensible change in weight or size except when the concrete had been poorly applied. 474. The results of these experiments showed that the steel need not necessarily be freed from rust before being imbedded in the concrete; that the concrete to be applied next the steel should be mixed wet, or that the steel should be first coated with grout by dipping or brushing; and it appeared that the rust- ing sometimes found in cinder concrete is clue to the rust in the cinders rather than to the sulphur, and that if proper precau- tions are taken, cinder concrete is nearly as effective as stone concrete in preventing corrosion. Prof. Norton says, "In the matter of paints for steel there is a wide difference of opinion. I cannot believe that any of the paints of which I have 1 Report III of Insurance Engineering Exp. Station, Boston, Mass. 338 CEMENT AND CONCRETE any knowledge can compare with a wash or painting with cement." 475. Sulphur in Cinders. — The conclusions drawn by Booth, Garrett and Blair from a scries of tests made for the Roebling Construction Co., were that cinders from anthracite pea coal contained about two-tenths per cent, of sulphur which they considered sufficient to cause corrosion of unprotected iron- work, more or less rapidly, depending on the presence or absence of moisture; but they further concluded that a "full" concrete (one in which the voids in the cinders were entirely filled by mortar of cement and sand) would fully protect the steel. In a paper read before the Associated Expanded Metal Com- panies, Prof. S. B. Newberry has this to say concerning cinder concrete: x "The fear has sometimes been expressed that cinder concrete would prove injurious to iron, on account of the sulphur contained in the cinders. The amount of this sulphur is, how- ever, extremely small. Not finding any definite figures on this point, I determined the sulphur contained in an average sample of cinders from Pittsburg coal. The coal in its raw state con- tains rather a high percentage of sulphur, about fifteen per cent. The cinders proved to contain only 0.6 per cent, sulphur. This amount is quite insignificant, and even if all oxidized to sulphuric acid, it would at once be taken up and neutralized in concrete by the cement present, and could by no possibility attack the iron." 476. Precautions. — While so far as the corrosion of steel is concerned, the above experiments by Prof. Norton show that the rusting is corrected by the concrete, yet it is quite possible that the adhesion of cement to steel may be impaired by a coating of rust. The cleaning of the steel may be accomplished by first brushing with wire brushes to remove all scales, followed by treatment with hot dilute sulphuric acid, and finally apply- ing an alkaline wash such as hot milk of lime to neutralize all traces of the acid. Oxalic acid may be used in place of the sulphuric, and the application of the milk of lime dispensed with, since the acid oxidizes. The crystals of oxalic acid as purchased commercially should be mixed with about seven parts hot water and the solution applied with a brush or sponge. When the 1 Engineering News, Apr. 24, 1902. PRESERVATION OF STEEL 339 adhesion of the mortar or concrete to the steel is of any impor- tance, as it is in all concrete-steel construction where the stresses are divided between the steel and concrete, any of the ordinary oil paints will not only be quite unnecessary, but may be a very serious detriment to the construction. The experiments quoted indicate the importance of having the steel covered with an unbroken coating of cement or cement mortar. To insure this the steel must either be coated with a layer, preferably of neat Portland, by dipping or brushing, or the mortar placed next the steel must be wet enough to insure intimate contact throughout. It may be added also, that the addition of a small amount of thoroughly slaked lime to Port- land cement mortar or concrete will not only render the mate- rial more alkaline, but will make the mortar more plastic, and thus insure a better coating of the steel. Such small additions have no deleterious effect on the mortar. 477. Practical instances of the preservation of iron by con- crete are not wanting. The writer has stored in water, briquets with small iron plates imbedded in Portland cement mortar, and at the end of six months the plates were found moist, but entirely free from corrosion except where they projected beyond the mortar. A concrete-steel water main built on the Monier system at Grenoble, France, was taken out and examined after fifteen years service in damp ground. The metal imbedded in the mortar showed no signs of corrosion, and the mortar could only be detached from it by hammering. Mr. W. G. Triest 1 relates that in breaking up cast-iron, con- crete-filled pillars, a wrench was found that had been buried in the concrete for twenty-two years. The wrench had main- tained its metallic surface in the concrete, while a part of it that had been imbedded in coal ashes had corroded badly. Similar instances showing the action of concrete on steel and iron might be multiplied, but it is sufficient to state that the preservation of iron or steel properly imbedded in Port- land cement mortar or concrete is now seldom questioned, and the use of cement paint, in place of the ordinary oil paints, as a steel preservative, has been adopted in many places. 1 Trans. A. S. C. E., April, 1894. 340 CEMENT AND CONCRETE Art. 61. Porosity and Permeability; Efflorescence; Pointing; Use ii: Sea Water 478. The porosity and permeability of mortars have been thoroughly investigated by M. Paul Alexandre, who has pub- lished his results in " Recherches Experimentales Sur Les Mortiers Hydrauliques." l The results and conclusion in the following notes on the subject are largely a resume of the systematic investigation made by M. Alexandre. The two qualities, porosity and permeability, should not be confused, nor should it be thought that a porous mortar is always very permeable, or that a permeable mortar must of necessity be very porous. Porosity is measured by the amount of water which will be absorbed by a specimen after drying, while permeability is measured by the amount of water which will pass through a specimen in a given time under certain de- fined conditions of thickness, water pressure and area of face. 479. Porosity. — The porosity of mortars is due to, and in fact is measured by, the volume of the voids contained. These voids may be divided into three classes, according to the causes to which they may be attributed, as follows: 1st, apparent voids, due to the mortar not being properly compacted; 2d, latent voids clue to the imprisonment of air in the mortar when made; and 3d, voids resulting from the evaporation, during har- dening, of a portion of the water used in gaging. 480. Apparent voids may occur as the result of using insuf- ficient cement to fill the voids in the sand, or, in the case of concretes, insufficient mortar to fill the voids in the aggregate. They may also be due to improper manipulation as to tamping, or improper mixing, giving an excess of matrix in one place and a deficiency in another. It was found by experiment that mortars made with coarse sand had the largest volume of ap- parent voids. It has been shown elsewhere that if dry sand be moistened and agitated, the bulk of the sand is increased. This is caused partially by the imprisonment of air bubbles in the mass, and if a measure of sand so treated is filled with water, the bubbles will rise to the surface on jarring the vessel. Latent voids in mortar are due to a similar action, and hardened mortars con- 1 Ex trait des Annates des Ponts el Chaussees, September, 1890. POROSITY AND PERMEABILITY 341 taining such voids refuse to absorb water to replace the air bubbles, at least for a long time. 481. A portion of the water used in mixing mortar enters into chemical combination with the cement, another portion is absorbed by the sand grains, and a third portion goes to moisten the sand. The quantity absorbed by the grains depends upon the character of the sand, and the amount required to moisten the sand depends upon the superficial area of the grains in a given volume, being greatest for fine sands and least for coarse ones. At least one fourth of the water ordinarily used in mixing neat cement is given off later, if the hardened mor- tar is allowed to remain in dry air. The water required to moisten the sand, and at least a part of that absorbed by the sand grains, also dries out, leaving voids of the third class mentioned. The apparent voids may be reduced to a very small per- centage by care in the proportions and preparation of the mortar. The latent voids may amount to six or seven per cent, of the total volume. The evaporation of water may leave from six to eighteen per cent, of voids in the mass. 482. The conclusions drawn from M. Alexandre's experi- ments are briefly as follows: The porosity varies between wide limits according to the fineness of the sand and the richness of the mortar. It may be as low as thirteen per cent, and may exceed thirty-one per cent. With sand of the same degree of fineness, the porosity di- minishes as the proportion of cement in the mortar increases. With the same quantity of cement per volume of sand, the porosity increases with the fineness of the sand. This is es- pecially marked in rich mortars, where the increase in porosity may reach 50 to 100 per cent., while in lean mortars the use of a fine sand may not increase the percentage of voids more than 20 per cent. The least porous mortars are those rich in cement and made with coarse sand. Mortars made with fine sand are relatively very porous, even when made rich with cement. Mortars gaged dry are more porous than those of ordinary consistency, and mortars gaged wet are also likely to be more porous, unless the manipulation is such as to allow the excess water to rise to the surface of the mortar. 342 CEMENT AND CONCRETE 483. Permeability — The degree of permeability of mortars is a more important property than the porosity, since not only does it affect the suitability of the mortar for certain uses, but the life of the structure may depend upon the difficulty with which water may percolate the mass. The permeability of mortar decreases as the proportion of cement is augmented, and in the case of concretes the per- meability diminishes as the percentage of mortar increases, at least to the point where the latter is in excess of the voids in the stone. From experiments made at the Thayer School of Civil En- gineering, Messrs. J. B. Mclntyre and A. L. True found that a five-inch laj^er of concrete containing from 30 to 45 per cent, of one-to-one Portland cement mortar, and some of the speci- mens containing 40 to 45 per cent, of one-to-two mortar, were impermeable with pressures of 20 to 80 pounds per square inch, maintained for two hours. 484. Mortars made with fine sand are much less permeable than those made with coarse sand. This difference is so marked that a less permeable mortar is made with one barrel of cement per cubic yard of fine sand, passing a sieve having, say, fifty meshes per inch, than with two barrels of cement per cubic yard of very coarse sand in which the grains are, say, one- tenth inch in diameter. Mortars made with sands composed of a mixture of grains of various sizes are neither very porous nor easily permeated. Mortars mixed very dry or very wet have greater permeabil- ity than those of the ordinary consistency, and in the case of concretes, it would probably be found that a deficiency of water would result in a much more permeable mass than the use of what might be considered an excess. All of the above conclusions indicate that a mortar may be quite porous, and 3^et so long as the voids are very minute, the percolation of water through it will be slow. This is especially shown by the fact that mortars of coarse sand, not porous, are more permeable than the porous mortars of fine sand. 485. When water is permitted to percolate continuously through a mass of mortar, the interstices gradually become filled, and the permeability decreases in marked degree. M. Alex- andre found that a volume of water which passed a certain WATER-PROOFING 343 mass of mortar in twenty minutes at the beginning of the ex- periment, required five hours to percolate the mass at the end of a month. M. R. Feret has obtained similar results in making extensive experiments 1 on the subject of permeability, and considers that fine particles of cement or lime are carried along by the water, forming efflorescence at the surface and tending to stop the flow. 486. The Preparation of Water-Proof Mortar and Concrete. — To enumerate briefly the precautions necessary to attain water-tightness in mortars and concretes, it may be said that different brands of cement present different characteristics in this regard. Fine grinding is a prime requisite, and sand cement or silica cement, containing as it does very fine grains of sand intimately mixed with cement particles of extreme fine- ness, is admirably adapted to such uses. The sand should, if possible, be composed of a mixture of grains of various sizes, because such a mixture gives a mortar not only little permeable, but one that is not porous, and that has, besides, a good strength. The amount of cement in the mortar should be in excess of the voids in the sand, not less, in general, than three barrels of cement per cubic yard of sand. In concrete the volume of mortar should exceed the volume of voids in the aggregate, and to obtain this result without too great expense, the aggregate should be so selected as to have a minimum of voids. Gravel concrete properly proportioned may be made water-tight somewhat more easily than broken- stone concrete, but a mixture of gravel and broken stone will give good results not only in this regard, but in the matter of strength as well. 487. To make a compact mortar for use where the facilities for tamping are ordinarily good, the consistency should be neither very wet nor very dry. When the mortar is struck with the back of a shovel, moisture should glisten on the surface, but in a pile the mortar should appear but little moister than fresh earth. This is the consistency which, with a moderate amount of tamping, gives the least volume of mortar with given quantities of dry materials. In places difficult of access, 1 "La Capacite des Mortier Hydrantiques," Annates des Fonts et ChanssSes, July. 1892. 344 CEMENT AND CONCRETE or in the preparation of concrete, better results will be obtained with a mortar somewhat wetter than the above, since large voids will be less likely to occur in the more plastic mass. In fact, unless the supervision is very close, it is advisable to use a rather wet mixture in preparing concrete where water-tight- ness is desired. 488. Washes. — The application of certain washes to the surfaces of walls intended to be water-proof, and the introduc- tion of foreign materials into the mortar or concrete to make it less permeable, have been practiced to some extent. Alter- nate coatings of soap and alum solutions are applied with a brush, not only to concrete, but to brick and stone masonry surfaces. These penetrate the pores of the masonry, forming insoluble compounds which prevent percolation. Washes of grout, composed of cement, or of cement and slaked lime, are used for a similar purpose. "Sylvester's Process for Repelling Moisture from External Walls" consists in applying first a solution of three quarters of a pound of soap to one gallon of water, followed, after twenty- four hours, by the application of a solution containing two ounces of alum per gallon of water. Both solutions are applied with a brush, the soap solution boiling hot, and the alum so- lution at 60° to 70° Fahr. The applications are alternated, with twenty-four hours intervening each time. Experiments at the Croton Reservoir l indicated that four coats of each wash were required to render brickwork impervious to a head of forty feet of water and the cost of the four double applications was about ten cents a square foot. In Reservoir Number Two of the Pennsylvania Water Co., two washes of each solution were used on the walls at a cost for materials and labor of twenty-three cents per hundred square feet, and the results were said to be good. A modified recipe for such a wash in which but one solution is made is given as follows: 2 A stock solution is prepared of one pound lye, five pounds powdered alum, dissolved in two quarts water. One pint of the stock is used to a pail of water in which ten pounds Portland cement has been well mixed. 1 Trans. A. S. C. E., Vol. i, p. 203. 2 J. II. G. WOll', Engineering News, June 30, 1901. WATER-PROOFING 345 489. In a few cases the use of alum and soap solutions in the body of the mortar has been tried with apparently success- ful results. Mr. Edward Cunningham, 1 in making experiments on water-proof concrete vessels, used powdered alum equal to one per cent, of the combined weight of the sand and cement, mixing this with the dry ingredients. To the water used in mixing, one per cent, of yellow soap was added. The results were said to be very satisfactory. In the above proportions, however, the amount of alum is made to depend upon the amount of cement and sand used, while the soap added depends upon the amount of water, whereas the soap should bear a de- finite ratio to the alum. In experiments with mortar composed of one part cement to two and one-half parts of bituminous ash, Prof. W. K. Hatt 2 found that the alum and soap mixed with the mortar at the time of gaging increased the strength and hardness of the ash mortar about fifty per cent., and diminished the absorption by the same percentage. One half of the water used for gaging was a five per cent, solution of ground alum, the other half being a seven per cent, solution of soap. The alum solution was used first and the gaging completed with the soap solution. Mr. W. C. Hawley 3 employed a stock solution of two pounds caustic potash, five pounds powdered alum, and ten quarts water, and used in the finishing coat three quarts of this solu- tion in each batch of mortar containing two bags of cement. The mortar was mixed with two volumes of sand to one of ce- ment and covered forty-eight square feet to a depth of about one-half inch. The extra cost for materials and preparing so- lution was only about nine and a half cents per hundred square feet. With less than two parts sand to one cement, it was found the finishing mortar checked in setting. It was also found that any organic matter in the sand was softened by the potash, and an excess of potash caused checking, although an excess of alum had no deleterious effect. 490. Use of Lime, etc. — The introduction of slaked lime in mortars designed to be water-proof is suggested by the fact 1 Trans. A. S. C. E., Vol. li, p. 128. 2 Trans. A. S. C. E., Vol. li, p. 129. 3 Journal New England Water- Works Association, 1904. 346 CEMENT AND CONCRETE that the permeability of mortar diminishes if water is allowed to percolate it for some time, the theory being that fine par- ticles of cement and lime are dislodged by the passage of the water to form a deposit at or near the surface, and check the flow. This suggestion, however, needs experimental confirma- tion, since it seems quite possible that the introduction of a substance containing such a large proportion of water as does slaked lime, may increase the percentage of voids in the mor- tar, if not the permeability. The use of pulverized clay and pozzolanic materials for a similar purpose has been suggested. It has already been shown that moderate doses of clay have no deleterious effect on the strength of mortars for ordinary exposures. The action of the pozzolanic substances has been found by Dr. Michaelis and M. Feret to be not mechanical alone, but chemical, and the effect on the strength of the resulting mortar depends upon the exposure to which it is subjected, such admixtures being dele- terious for mortars hardened in air. 491. Efflorescence. — The white deposit sometimes formed at the surface of brick and masonry walls is usually due to the filtration of water through the mortar, dissolving out salts of potash, soda, etc., and depositing these salts on the surface by evaporation or by the formation of sodic carbonate. The ab- sorption of water from the atmosphere may also account for this deposit in some degree, especially near the sea. The same term is applied to a more harmful deposit, sulphate of calcium, which may be supplied by the filtrating water or may come from the cement, either from the addition of gypsum or from the fuel used in burning. The crystallization of this salt in the pores of the masonry at the surface may cause disintegration. On the other hand efflorescence may be quite harmless, as when it is formed by washing out from the mortar an excess of hydrate of lime. A portion of the latter may then be changed to carbonate of lime near the surface of the wall and actually stop up the pores or voids, and prevent further filtration. 492. The discoloration of brickwork and fine masonry by efflorescence is sometimes serious. To ameliorate these condi- tions, the use of water-proof mortars, and careful pointing of the work, are precautions to be recommended. General Gill- more, in "Limes, Hydraulic Cements and Mortars," suggests EFFLORESCENCE 347 the use of about ten pounds of animal fat to one hundred pounds of lime and three hundred pounds of cement; the object of the fat being to saponify the alkaline substance, the lime in form of paste serving only as a vehicle for the fat. A more practical method, however, would seem to be the application of soap and alum washes on the surface, or the use of soap and alum in the preparation of the mortar to be used near the face of the ^wall, and especially for pointing. The remedy to be adopted, however, will depend upon the cause of the efflorescence. 493. Pointing Mortar. — Pointing serves the double purpose of making the joint practically water-tight at the edges, and giv- ing a finish to the face of the wall. If the edge of the joint is not well filled, moisture collects there either from the face or from seepage through the wall. Subsequent freezing or the crystallization of certain salts may spall the stones or loosen them from their bed. In laying cut-stone masonry, the joints should be raked out for about two inches back from the face to be pointed. Pointing mortar should be prepared from fine sand and the best Portland cement. The proportion of sand should not exceed two parts by weight to one cement, and in the highest class work, equal parts of cement and sand are sometimes used. No advantage is gained, however, by using a mortar richer in cement than the one last mentioned. The use of fine sand and rich mortars are specified not only because such mor- tars are practically water-tight, but because they take a fine finish. 494. The tools required for pointing are a bent iron to rake out the joints (though this should be partially done while the mortar is green), a mortar board and small trowel, a calking iron and wooden mallet, a brush for moistening the joint, and one or more beading tools. After raking out the joint it is moistened by the brush, and the mortar, which is mixed quite dry, is filled in with the trowel. When enough mortar is in place to fill half the depth of the joint, it is tamped with the calking iron and mallet much as a ship's seam is calked with oakum. The joint is then filled to the face, and again tamped. The bead is then formed by running the beading iron back and forth over the joint. This beading iron is of steel with the handle parallel to, but some two or three inches out from, the 348 CEMENT AND CONCRETE line of the blade forming the bead. The blade is three to five inches long and "hollow ground" or finished with a smooth concave surface. Only such a length of joint is pointed at one operation as may be quickly carried to completion. The wall must be kept moist for some time after the pointing is done, and it should be protected from the direct rays of the sun, as fine cracks are very likely to appear in this rich, finely finished mortar. If possible, pointing should be done in moderate weather and must be entirely suspended in temperatures ap- proaching the freezing point. 495. Cements in Sea Water. — The theory of the action of sea water upon cements is not fully understood. It is known that some cement structures exposed to the worst conditions have given most satisfactory results, while others have failed in greater or less degree. It may be said at once, however, that many of the most eminent and conservative engineers consider that the failures that have occurred in the use of Port- land cement in sea water are due to improper specifications, proportions and manipulation, rather than to any defect in Portland cements as a class. 496. It is thought the following represents, in the main, the most generally accepted theory of the chemical action. In the setting of cements that are rich in lime, the whole of the lime is not engaged in stable compounds, and when placed in the sea the sulphate of magnesia of the sea water is able to com- bine with the lime, forming calcic sulphate, the magnesia being precipitated. The discovery of magnesia in decomposed mor- tars led, at first, to the supposition that the cause of failure was the presence of magnesia in the cement when used. If the water level about the structure changes frequently, as is usual, or if the wall is at times subjected to a greater head on one side than on the other as in tide docks, the percolation of water through the wall is stimulated, and the sulphate of lime may then be washed out if the mortar is quite pervious, and more will be formed from a fresh supply of sea water attacking the lime of the cement, until the latter is destroyed. If, how- ever, the sulphate of lime is not washed out, it may crystallize and thus cause swelling of the mortar. 497. It would appear from the above that for successful use in sea water the hydraulic index of the cement should be ACTION OF SEA WATER 349 high; that is, that the lime should be comparatively low in or- der that the lime compounds may be more stable. For this reason it is not impossible that some of our natural cements, which are so much more nearly uniform than the Roman ce- ments of Europe that have been condemned for this reason, may give fairly good results in sea water. The fact that the mortars of natural cement are more permeable than those of Portland, is, however, a serious defect. Following a similar reasoning, Dr. Win. Michaelis has ad- vanced the theory that if trass, or other pozzolanas of proper composition, be mixed with Portland cement subsequent to the burning, the hydrate of lime which separates from the ce- ment in hardening will at once combine with the pozzolanas, forming a stable compound. This view, however, has been vigorously opposed by the Society of German Portland Cement Manufacturers, as well as by many engineers, especially of France, and the discussion is not yet at an end. M. Candlot * says that, from the experiments of various en- gineers, "we have arrived at this conclusion, that the only remedy to adopt against decomposition is to prevent the sea water from penetrating the mortar. We are led thus to dis- miss the chemical reactions of sea water on mortars and to consider their action from a purely physical standpoint." 498. To resist the attacks of sea water the mortar should not only be impervious, but also as little porous as possible. The cement should be finely ground and should not contain free lime. The content of magnesia and of sulphuric anhydride should be as low as possible, the latter not exceeding one and five-tenths per cent. The proportion of lime should not be too high, and above all, special pains should be taken with the manufacture to insure proper comminution and mixing of the raw materials, and uniform burning. The addition of sulphate of lime to regulate the setting is believed to be injurious for cements to be used in sea water; even two or three per cent. is said to cause rapid disintegration, and in the specifications for recent extensive works in dock construction, the addition of gypsum or other foreign matter was entirely prohibited. 1 " Le Ciment,'" September, 1896, quoted by F. H. Lewis, M. Am. Soc. C. E. Trans. A. S. C. E., Vol. xxxvii, p. 523. 350 CEMENT AND CONCRETE Although slag cements have given good results in the sea for a short time, it is considered that they will not, in general, resist the action of sea water for long periods. 499. Sand or aggregate containing argillaceous or soft cal- careous matter should be avoided for works in the sea. Two instances of failure of sea walls in which shells were used as the aggregate are mentioned by Col. Win. M. Black, 1 and although the failures are not definitely traced to the calcareous matter in the concrete, the fact that experiments have shown that cal- careous sands do not withstand the action of sea water, makes it probable that this was an important cause of the failure. Fine sands that give porous mortars, though not easily per- meable, are to be strictly avoided. Coarse sands giving per- meable, though not porous, mortars are better, but still leave much to be desired as to immunity from decomposition. The best sands are those containing various grades of sizes of par- ticles from coarse to fine, as mortars made with such sands are not only compact, but practically impermeable. 500. Since the mortar and concrete should be made as com- pact as possible, the precautions mentioned under the head of water-proof mortar and concrete should be taken in the prep- aration of mortars and concretes for use in the sea. That is, the proportion of cement should exceed the voids in the sand and the mortar should exceed the voids in the aggregate. M. Alexandre has found that the mortars mixed to the or- dinary consistency are attacked least by sea water. When specimens are merely immersed in the water, those mixed dry suffer the most, but some tests indicate that if mortars are submitted to the filtration of water soon after made, those mixed wet are most easily decomposed. As to whether fresh or salt water should be employed in mixing mortars to be used in sea water, although Mr. Eliot C. Clarke, M. Paul Alexandre and many others have investigated this subject, the conclusions are not definite and it is probable that either may be used as convenient. 1 Trans. A. S. C. E., Vol. xxx, p. 601. PART IV USE OF MOKTAR AND CONCRETE CHAPTER XVIII CONCRETE : DEPOSITION 501. Concrete may be molded into blocks which are allowed to set and then are transported to the structure and laid as blocks of stone. This is the block system of construction. The adaptability of concrete to being built in place, however, is one of its chief merits, and consequently the monolithic method of construction is far more common Since it has been found that expansion and contraction, due to changes in temperature, affect concrete walls as they do any other walls of masonry, it has become customary to mold the concrete in sections, usu- ally alternate sections of equal size and shape being built first, and the omitted sections built in later. This method of con- structing a long wall is also called monolithic, since the blocks are of large size and are built in place. 502. When concrete is deposited either in air or in water, molds must be provided to keep the mass in the desired shape until it has lost its plasticity and acquired sufficient strength to stand alone. In foundations, the earth at the sides of the excavation may supply the place of a mold, and sometimes the mold forms a part of the permanent structure, as in the case of masonry piers with concrete hearting, and in steel cylin- der piers filled with concrete. Art. 62. Timber Forms or Molds 503. The construction, placing and removal of forms fre- quently represent a considerable percentage, from five to thirty per cent., of the total cost of the concrete, and it is therefore evident that an improper design may result in a considerable waste of money, as well as in marring the appearance of the 351 352 CEMENT AND CONCRETE work. The character of the form will of course depend on the character of the work; in the construction of a large number of small blocks of the same shape, where one mold may be used over and over, the thickness of the pieces should not be stinted, and the ease of knocking down the mold should be carefully considered. When a form can be used but once, the size of pieces should be no larger than necessary to give the requisite stiffness, and the ease of 'first construction is a main considera- tion. Forms should be left in place forty-eight hours to allow the concrete to set, and in the case of arches and beams a much longer time is necessary, so that the concrete may assume considerable strength before it is called upon to support its weight. 504. Sheathing. — Forms for massive walls of monolithic construction usually have vertical posts, with iron ties across, or braced by battered posts outside. The sheathing planks are then placed horizontal, in a few cases horizontal wales have been placed within the posts and vertical sheathing laid against the wales. The strength of the sheathing must be sufficient to stand the pressure transmitted to it through the concrete when the rammer is used close to the face of the mold. The concrete is seldom built up fast enough to bring upon the sheathing a great head of fluid pressure, but the ramming brings a heavy local pressure upon it. If supported at intervals of four feet, two- inch lumber dressed to one and three-quarters inches thick is usually sufficient; for spans of more than 5 feet, 2 f inch lum- ber is required to make a perfect face. Boards seven-eighths inch thick are suitable only when supports are not more than about 2 feet apart. In placing concrete in molds under water there is more danger of bursting the mold by the weight of semi-fluid concrete, and if the work is to be built up rapidly, this must be guarded against by sufficient bracing. 505. For exposed faces, the duty to be performed by the lagging includes leaving as smooth a finish as possible on the concrete after the removal of the forms. If green lumber is employed, the boards may shrink before use, leaving openings between the sheathing that will show plainly on the face of the work. A slight tendency of this kind may be checked by keeping the boards well wet with a hose until the concrete is TIMBER FORMS 353 placed. On the other hand, thoroughly seasoned lumber will swell when the concrete is placed; to obviate this difficulty the lower edge of each sheathing plank may be beveled on the outer edge; the thin edge on the inside will then crush when the planks swell. The use of tongue and grooved lagging has been tried, but is not usually satisfactory, as there is no opportunity to expand, and the planks are particularly hard to place a second time. To give a good face in work under water, however, tongue and groove sheathing will assist in preventing washing of the cement. Yellow pine lumber is found to be excellent for sheathing; on account of the large amount of pitch contained, it absorbs water slowly and holds its shape. For a similar reason, fir timber would be suitable. In order that the face of the mold shall be perfectly smooth, it is necessary to size and dress the plank on at least one side and two edges. As it is almost impossible to avoid having some line of de- marcation shown in the concrete at the joints of the sheathing planks, care should be taken that the lagging is of uniform width throughout, and laid horizontal so that consecutive sec- tions show the joint continuous. The sheathing may be placed for the entire form before concreting is commenced, or the plank may be raised on the posts as the work advances. The former method will usually give the neater appearance, but is too expensive for high walls. 506. Lining. — The appearance of the finished concrete is much improved, and the labor of preparing the forms probably not increased, since less care may be taken in surfacing, by lining the mold with thin sheet iron. Iron of number twenty gauge (.035 inch thick, 1.42 pounds per square foot) has been used for this purpose, but where the same lining is used several times, a heavier iron is preferable. The joining of one sheet of lining to another may present greater difficulties than the join- ing of planks, but joints will occur less frequently. In the construction of the Marquette Breakwater, Mr. Clarence Coleman, Asst. Engineer, used sheet steel one eighth of an inch thick for lining molds for building monolithic blocks. Concerning the use of the steel, Mr. Coleman says: 1 "Very 1 Report Chief of Engineers, U. S. A., 1898, p. 2254. 354 CEMENT AND CONCRETE smooth surfaces were produced on the slopes of the concrete and the work of the molders was greatly facilitated on account of the comparative ease with which the concrete was compacted under the slope pieces of the molds. The steel effectually pre- vented the aggressive friction of the sharp particles of broken stone on the wooden surfaces of the molds, thus increasing the life of the molds and decreasing the cost of molding the con- crete." 507. Oiling the Forms. — Oiling or greasing the face of the mold, in order that the latter may be removed without detach- ing particles from the concrete face, is usually advisable. Soap, crude oil, linseed oil, bacon fat, are some of the materials that have been used for this purpose; the first mentione:! probably gives the best results, and if not applied too freely will have no injurious effect upon either the finish or strength of the work. Applying shellac to the molds improves the appearance of the concrete surface. When the forms are lined with steel, the adhesion of the concrete to the lining is more difficult to over- come. In this case the ordinary oils are not entirely success- ful, but fat salt pork has been found to give satisfactory results. 508. Joints and Corners. — If desired, triangular strips may be nailed to the inside of the forms in such a way as to block off the face to represent stone masonry, and in this way the marks of joints between planks or between strips of lining may be avoided. Square corners should not be allowed on exterior angles, as it is difficult to so tamp the concrete as to make the corner perfect, and they are so likely to be chipped off. Tri- angular strips or moldings should be tacked along the corners of the mold as a fillet to cut off the corner by a plane making equal angles with the adjacent faces. This plane may be from one inch to two inches wide. To form water drips on projecting ledges, such as door caps and sills, abutment copings, etc., a small half-round should be nailed to the upper surface of the mold a short distance back from the projecting face. This leaves a ridge at the edge of the under side of projection so that the water must drip from the edge, and not follow back to the main wall face. 509. POSTS AND BRACES. — The sizes of posts and braces must be such as to make a practically unyielding support to TIMBER FORMS 355 the sheathing. With one and three-quarters inch lagging, posts may be four feet apart; if five feet four inches apart (three to each sixteen foot length), some yielding of the sheathing may be expected if it is less than two and three quarter inches. If sheathing is four inches thick, the distance between posts may be six or seven feet. Fir, yellow pine, and Norway pine are suitable for posts. Three-inch by eight-inch is an ordinary size, and a post of these dimensions should be supported, either by ties or braces, at intervals of four to six feet. Where the posts are four inches by ten inches, supports may be six to eight feet centers, while with six-inch by twelve-inch posts, the distance between cen- ters of supports may be eight to ten feet. Posts should be sized and dressed on the side which is to receive the sheathing, in order that the alignment may be perfect. 510. Methods of Bracing. — The general plan of the mold may vary according to conditions, the following methods hav- ing been employed on heavy, work to support the vertical posts : 1st, With outside inclined braces, leaving the interior of the mold unobstructed. 2d, Tie rods across the interior of the mold connecting opposite posts at frequent intervals. 3d, Each post trussed vertically and tied across at top and bottom only. 4th, Horizontal trussed wales outside of posts, spaced four to five feet apart in the vertical and tied across at the ends. 511. Inclined Braces. — The sizes of inclined braces depend on their lengths, the inclination to the vertical, and the amount of shoring used. An approximate rule for the size of braces under usual conditions and using ordinary dimension stuff, not boards, is that the number of square inches area of cross- section of brace should equal length of span in feet. If thin planks are employed, they should be in pairs, one on either side of the vertical post, and made to act together by cross- pieces nailed to the two planks. The aim should be to make the whole form practically un- yielding under the action of the tampers, as it has been found that this action is usually more severe than the mere pressure of the concrete in a semi-liquid condition. The sizes of pieces cannot, therefore, be accurately computed, but the above sizes are derived from the general result of experience as to what has proved satisfactory. 356 CEMENT AND CONCRETE <»■ The advantage of the form of construction just described is that the interior of the mold is left entirely unobstructed. On high walls, however, the amount of timber required for braces is excessive, and the braces may be almost as objectionable as tie rods, since the former prevent the laying of tracks along the side of the form. 512. Tie Rods. — When the vertical posts are supported by tie rods across the mold and the wall is thin, it may be possible in removing the mold to withdraw the bolts or rods if they have been thoroughly greased or wrapped with stiff paper be- fore the concrete is placed. If it is designed to leave the rods in the concrete, they should be provided with sleave nuts near the end, which, when unscrewed, will leave the end of the rod within the concrete mass not less than two inches from the face. The hole left by the nut should be carefully filled with mortar after the mold is removed. With vertical posts four feet apart, ,this method of support is objectionable, as it leaves a network of ties within the forms interfering seriously with the operation of a skip and with the ramming. It is not necessary, however, to place all of the tie rods to the top of the mold before beginning the concreting, as it is sufficient to keep one or two rods in place above the plane where tamping is being done. A modification of this method is to use wires of large diam- eter with an eye at the end just inside the finished face of the concrete. A short bolt, with hook at one end and threaded at the other, passes through the post, hooks into the eye of the wire, and is tightened by a nut on the threaded end outside the post. After removing the nut, the rod is unhooked and the hole in the face filled with mortar, the wire remaining in the concrete. 513. Trussed Posts. — The third method of support, where the posts are trussed and provided with heavy tie rods at the top, and held at the bottom either by tie rods or some other means, seems to have fewer objections than the methods just described. Less timber will usually be required to build this form than for that where inclined braces are used, and the obstruction to operations will usually be less than with either of the other styles. This mold is also very readily taken down, though the posts are heavier and more difficult to handle. TIMBER FORMS 357 To secure the bottoms of the posts, they may be set in the ground, or rest against sills braced to some other portion of the structure, or to piles. A suitable support may also be obtained by dumping a mass of concrete around the bottom of each post and allowing it to set. Forms erected on rock may have the posts rest against blocks bolted to the rock. 514. Trussed Wales. — The fourth method of supporting the posts is particularly applicable where the work is divided into blocks of moderate size in horizontal cross-section, say twenty feet square. In longer lengths the horizontal trussed wales become rather heavy for convenient handling. Within these limits, however, this is an excellent form. In the construction of Lock No. 2 between Minneapolis and St. Paul, 1 a form of this kind was used for blocks about twelve by fifteen feet at the bottom. The sheathing was one and three-quarters inches, lined with No. 20 galvanized iron. Verticals were four by twelve, spaced about two feet centers. The trussed wales were twelve by twelve inch, trussed with one and one-quarter inch rod, the king-post being of twelve by twelve inch about two and one-half feet long, making depth of truss three and one- half feet. The ends of opposite wales were connected by one and one-quarter inch rod passing outside of the sheathing. Each pair of the longitudinal wales was just above the corre- sponding pair of transverse wales, so that they did not inter- fere at the corners. The mold was twenty-nine feet in height. In describing this mold, Mr. Powell says: "One complete form weighs twenty-eight tons; each piece about seven tons. Each piece is moved separately by the cable-way in forty-five to sixty minutes. The operation of removing one complete form requires from three to four hours time. After being moved, a small crew of men occupy nearly a day in plumbing and bolting together the form." "The boxes containing 1.7 cubic yards of concrete are landed on top of the form by cable- way and tipped from that position. Although the jar and strain is severe, the forms have shown no ill effects therefrom, remaining; tight and secure." 1 Major Frederic V. Abbot in charge. Mr. A. O. Powell, Asst Engr., Report. Chief of Engineers, 1900, p. 2778. 358 CEMENT AND CONCRETE Art. 63. Deposition of Concrete in Air. 515. Transporting to Place of Deposition. — In depositing the concrete in place, care must be taken not to undo the work of mixing. If the concrete is allowed to fall freely a distance of several feet or to slide down an inclined plane, the stones will be likely to separate from the mass, and the result will be a layer of broken stone followed by a layer of mortar. If the concrete is deposited in a pile, the stone will roll down the out- side of the cone. This action is especially bad in concrete that is mixed rather dry. The author has seen a pavement founda- tion in which the limits of each wheelbarrow load of concrete could be distinguished, the foundation presenting the appear- ance of the cross-section of a honeycomb, made up of irregular hexagons outlined by broken stone having a deficiency of mortar. In all such cases, if the action cannot be avoided by some other method of dumping, then care must be taken to remix the concrete. There is one method by which the concrete may be deposited by gravity without separation of the materials. This consists in allowing the material to slide down a tube, but the tube must be kept continually full, the concrete being allowed to run out at the bottom only as fast as it is filled in at the top. This method is only applicable where the mixing is continuous, as in the case of machine mixers. 516. Sometimes it will be found possible to mix the con- crete so near to the place of deposition that it may be shoveled directly into place. In mixing by hand this is practicable, as the mixing platforms may usually be easily moved, and this method of deposition is carried out even in street work where the concrete is in thin layers and hence requires much moving of platforms. Where a machine mixer is used that is so mounted as to be portable, the concrete may be delivered in place by a belt conveyor. Such an arrangement for the building of walls and for foundations of pavements, has already been described in Chapter XIV. The conditions are usually such, however, as to preclude the possibility of mixing the concrete so close to the work that it may be shoveled into place or handled economically on a con- PLACING IN AIR 359 veyor of the style mentioned. The next cheapest method is to use a derrick to handle skips or bottom-dump buckets, pro- vided the work is sufficiently concentrated to have one posi- tion of the derrick serve to place a large quantity of con- crete. The skips should hold about a cubic yard, and if a batch mixer is used, the skip should hold a batch, whatever that may be. 517. If the concrete is mixed on the same level and within less than two hundred feet of the work, wheelbarrows may be used, but for greater distances, carts, or, what is usually cheaper, cars running on a track, should be employed. For large masses of concrete a cableway may be employed to advantage, provided there is sufficient use for it to repay the high original cost of plant. The selection to be made from among these common methods is dependent on economy as in handling other material, the only requirements being that the concrete shall be conveyed to place quickly, and that the ma- terials shall not be allowed to separate as a result of any of the manipulation. In laying large quantities of concrete, the dif- ference between success and failure from a financial standpoint may easily rest in the proper transportation of the materials to and from the mixer. 518. Ramming. — The concrete should be deposited in hori- zontal layers about six inches thick, leveled with a shovel and thoroughly rammed. The length of time ramming should be continued and the vigor with which it should be done depend largely on the degree of plasticity of the concrete. If the con- crete is made of such a consistency that when struck a smart blow with the back of a shovel a film of moisture will just show on the surface, it should have vigorous ramming to insure a compact mass. A flushing of water to the surface will then indicate when to cease tamping. With a little more water there is less danger of the larger stones "bridging" and leaving large voids in the mass, and less work will be required to flush water to the surface. With such a consistency, cutting the mass with a spade before start- ing, the ramming may assist in expelling air bubbles and pre- venting voids. With still wetter mixtures ramming becomes difficult, as the concrete will soon begin to quake, after which the ramming; should not be long continued as the mass is then 360 CEMENT AND CONCRETE semi-fluid, and the stones may gradually work themselves to the bottom of the layer, forcing the mortar to the top. 519. Rammers are frequently made of wood, but those of iron are believed to be better. The weight of a rammer is limited by the capacity for work of the man who wields it. They are usually made to weigh from twenty to forty pounds. If a man lifts and drops a forty-pound rammer with forty square- inch face twenty times a minute, he is doing less good to the concrete than if he dropped a twenty-pound rammer with twenty square-inch face forty times a minute. If the face of the ram- mer exceeds thirty-six square inches, the result is apt to be a mere patting of the surface of the concrete, unless the rammer is so heavy as to require two men to operate it. Iron rammers with face, say, three by .six inches, and weighing twenty to thirty pounds, are believed to be the most efficient. Still thinner rammers than this may be necessary in work involving such detail as for filling in between iron beams, and are desirable for tamping near the face of the mold. 520. Rubble Concrete. — In massive work the embedding of stones of "one-man size," or larger, in the concrete is a practice that has long been in vogue. The objection is sometimes made that this interferes with the homogeneity of the wall and that variations in expansion may result in injury to the work. It is thought, however, that in large masses this danger is more theoretical than real, and the author sees no objection to this form of construction for many purposes if properly carried out, and it is frequently permitted in important works. Thin walls, the arch rings of bridges, shallow foundations, etc., should not of course be built in this way, because the stresses to which such structures are subjected should be met by a uniform resistance, to avoid the effects of eccentric or irregular loading. In such structures as dams, lock walls, breakwaters, retaining walls, and in many cases bridge piers and abutments, the work may be considerably cheapened without sacrificing the fitness of the structure. The stones thus embedded should be perfectly sound and should not lie nearer one to another than six inches, nor should they lie nearer than this to the face of the wall. The concrete should be mixed rather wet, and much care taken that each stone is completely surrounded by a compact mass of concrete. The stones should be settled into the concrete al- PLACING IN AIR 361 ready laid far enough to assure their having a full bed. Stones used in this manner are sometimes called "plums." 521. Another class of rubble concrete differing from the above more in degree than in kind, is formed by placing large stones in the work, and filling the joints between them with a rather wet concrete in which spalls may be rammed if desired. The difficulty of obtaining a compact wall by this method is perhaps a little greater than when smaller stone are used, but in either case if really water-tight work is desired, the inspec- tion must be thorough. The saving in cost by the use of rubble concrete depends upon the local conditions, but under ordinary circumstances when broken stone is employed, the cost of crushing the stone and the cost of cement, for a volume of concrete equal to the volume of the stone imbedded, are practically saved. 522. Joints in Concrete. — In the construction of large masses of concrete in place, joints cannot be avoided; that is, it is not possible to make the entire mass monolithic, as force enough could not be employed to carry up the entire struc- ture at once. Even if this were possible, it would not be de- sirable, since the changes in length of the wall due to changes in temperature would probably result in cracks which would be irregular in outline and mar the appearance of the wall, if they had no more serious effect. When the concrete is subjected to vertical forces only, as in foundations for buildings, horizontal joints are less objec- tionable than vertical joints. But in the construction of con- crete lock walls, dams, and breakwaters, vertical joints are de- sirable to confine the cracks to predetermined planes. In the building of such structures, therefore, the method has been adopted of dividing the work into sections of such horizontal dimensions as may be thought best, and completing each sec- tion as a monolith. This will sometimes require the contin- uous prosecution of work for twenty-four or forty-eight hours. Whether this method, involving work at night, which is always more expensive and usually less thorough, is justified by the end sought, depends upon the character of the structure. 523. If this method is not adopted,, and a horizontal plane of weakness is a serious defect, special means should be pro- vided for avoiding this plane of weakness. Such provision may 362 CEMENT AND CONCRETE be made by iron dowels set in the concrete at the end of the days work and projecting above the surface to be covered by the concrete placed the next day; steps or hollows, or grooves parallel to the length of the wall, may be left to be filled by the next layer. Large stones weighing a hundred pounds or more are frequently imbedded one half their depth in the last layer of a days work to form a bond with the following layer. In any case special care should be taken to thoroughly wash and clean the surface of hardened concrete before continuing the work, using preferably wire brooms for this purpose and removing any stones at the surface that appear to be loose. A thin layer of rich cement mortar should then be laid upon it, into which the first layer of fresh concrete is well rammed. If the appearance of the finished face is of importance, special care must also be exercised in joining at this point. Before leaving a layer which is to be allowed to harden before contin- uing the work, the line limiting the height of the concrete at the face should be made perfectly horizontal, for a slight crack, or at least a noticeable line, may be expected at this point, and if not straight it will be the more unsightly. 524. If for any reason a layer of concrete cannot be carried over the whole area of the wall or foundation, it should neve r be allowed to taper off to a wedge, but a plank equal in width to the thickness of the layer should be set on edge, firmly se- cured, and the concrete tamped against it. In the construction of arches, culverts and sewers, such stop planks may well be set normal to the surface of the intrados instead of vertical. In case more than one layer is left incomplete, they should be stepped back, making an offset for each layer of at least one or two feet. The concrete should never be built up on a smooth batter if new concrete is to be joined to it later. 525. Keeping Concrete Moist. — All concrete should be kept moist from the time it is in the wall until it has become well hardened. Surfaces exposed to the air should therefore be sprinkled frequently for at least several days after placing. An excellent practice is to cover the surface with burlaps which may be kept saturated, as this not only furnishes the necessary moisture, but protects the work from the direct rays of the sun. The interior of a large mass will probably take care of SURFACE FINISH 363 itself in this regard, but the precaution has sometimes been taken of leaving vertical holes or wells in the mass, which are kept filled with water for some weeks and are then filled with concrete. 520, FINISH. — Some of the precautions that must be taken to secure a good finish to the face of concrete work have al- ready been mentioned in considering the forms and the meth- ods of deposition. These are usually supplemented, however, by certain special means when the appearance is of much im- portance. We must say first, that the application of a plaster of ce- ment mortar to a finished and set concrete face will almost never be permanent. It is seldom that it will adhere with suf- ficient strength to prevent scaling due to differences in expan- sion of the materials of different composition and age. If plaster must be used on the face of a wall, it should be applied before the concrete has set, but it is safer to avoid plastering. It is of course advisable to fill with rich mortar any voids that may appear in the face of the work, but such places should be few. If the molds are removed while the concrete is still moist, the face may be coated with a thin grout and then immediately scraped off with the edge of a trowel. This results in filling the small voids in the face of the work, but does not leave a coat of plaster on the surface to scale off. 527. A good finish may be obtained when the molds are smooth if the workmen will force the blade of a spade or shovel between the fresh concrete and the mold, and pull the handle away from the mold. This has the effect of forcing the large stone back from the face and allowing the mortar to flow in. A layer of mortar is thus left next the mold with no marked line of junction between mortar and concrete, as may be the case in using a mortar facing. A similar effect may be produced by throwing the concrete against the face of the mold with such force that the larger pieces of aggregate rebound. In very finely finished work this may mar the surface of the sheathing, but ordinarily this method is effective. 528. When a special layer of mortar is used for facing, there is more clanger, perhaps, of making the layer too thick than too thin. As to the richness of the mortar, two parts sand by 364 CEMENT AND CONCRETE measure to one volume packed cement is usually sufficient, though a more glossy finish may be made if desired, by using equal parts of cement and sand. It is better to avoid too great a variation between the richness of the mortar used for facing and that used in the body of the concrete. ( )ne of the best ways of applying such a layer is to prepare a sheet of steel of width equal to the thickness of one layer of con- crete, usually six to eight inches, with two handles on the upper edge to facilitate moving it. At the ends of the sheet, on the side next the mold, rivet short pieces of H in. by \\ in. or 2 in. by 2 in. angle iron. This sheet of iron with the projecting legs of the angles against the face of the molds, forms, with the latter, a space one and one-half or two inches thick, which is to be entirely filled with the finishing mortar made rather moist and tamped lightly with edge rammers. The concrete is filled in behind the iron, after which the latter is withdrawn by means of the handles, and the whole mass is thoroughly rammed. The end sought is that the finishing mortar shall have some approximately definite thickness, and that the stones of the concrete shall be tamped into the finishing mortar, but not through it, and thus destroy any sharp line of demarcation between mortar and concrete, and ensure a perfect bonding of the two. It is evident that this can only be accomplished by placing the mortar and concrete at the same time. 529. One other cautionary remark concerning the use of fin- ishing mortar. With the present state of our knowledge con- cerning the rates of expansion of mortars and concretes of dif- ferent composition, it is not considered wise to use too many combinations in the same structure. To illustrate, a pavement or surfacing of a large concrete structure was once built in layers as follows: first, thick natural cement grout was placed on the concrete foundation; second, natural cement concrete; third, Portland cement concrete; fourth, a richer Portland cement concrete; fifth, Portland granolithic; sixth, rich Portland mortar; and seventh, floated with dry Portland cement and sand. We cannot be absolutely sure that this is bad practice, but it would seem that this structure might have served its purpose with fewer varieties of material, and it is usually considered very doubtful whether Portland cement mixtures will always ad- here well to mixtures of natural cement, although the author SURFACE FINISH 365 knows of instances where they have been used in juxtaposition apparently with good results. 530. Granolithic is a facing or surfacing mortar composed of crushed granite and cement. The granite is usually specified to contain no particles larger than § inch to one inch, and about one and one-half to two and one-half parts are used to one vol- ume of cement. This is more frequently used for foot walks and other places where resistance to wear is required, but may also be used to surface walls, to line reservoirs, etc. It will be mentioned again in connection with cement sidewalk construc- tion. 531. Exposed concrete surfaces frequently present a patchy appearance. This may be the result of lack of care in placing the concrete next the mold, or it may be due to variations in the purity of the sand or in the amount of water used in mixing. On mortar-faced work this lack of uniformity is less noticeable. The use of slag sand, or of a little fine pozzolanic material, may be advantageous, and a small amount of lampblack in the facing- mortar also tends towards uniformity in appearance. A very pleasing finish may be given by applying to the set concrete a thin wash of cement and plaster of Paris, though the permanence of such a wash may be open to question. The sheathing should be removed as early as it is perfectly safe to do so, and the concrete surface cleaned from any oil or grease that may have come from the mold planks. The wash, which should be very thin, may be applied with a whitewash brush. A mixture of equal parts Portland cement and plaster of Paris gives a very light gray finish, and one part plaster to three parts cement gives a trifle darker shade. 532. A rubbed finish of excellent appearance may be given by removing the sheathing before the concrete has set very hard, say after twenty-four to forty-eight hours, and rubbing the surface with white brick or with a wooden float. If there are small voids in the surface, it may be covered with a thin grout of equal parts of cement and sand and then rubbed hard with a circular motion. The grout should not leave a scale on the work, the object being only to fill surface imperfec- tions. If the mold boards are removed at just the proper time, a good finish may be given by rubbing with a wooden float, with- 366 CEMENT AND CONCRETE out the coating of thin grout. A .somewhat similar effect is produced by brushing the surface with brooms or stiff brushes. 533. " Pebble-dash." — What is called a pebble-dash finish was used in the construction of a bridge in the National Park at Washington, D. C. 1 Eighteen inches of the concrete next the face was made of one part cement, two parts sand, and five parts of gravel and rounded stone from one and one-half to two inches in their smallest diameter. After the removal of the forms the cement and sand were b rushed from around the face of the gravel next the surface exposed to view. It was found by experiment that the brushing should be done when the concrete was about twenty-four hours old. At twelve hours the gravel was displaced by the brushing, and after thirty-six hours the mortar had be- come so hard as to be removed from the surface of the stones with difficulty. The forms were therefore designed so that sections of the lagging could be removed as desired. The cost of the brushing was said to be about sixty cents per square yard. A somewhat similar method is employed in giving to con- crete the appearance of cut stone. The materials used in the surfacing mortar are Portland cement and crushed rock, the character of the rock depending upon the color and texture de- sired in the finish. The molds having been removed after the proper time has elapsed, the mortar covering the face of the particles of crushed rock is removed by brushing or by washing the surface with a weak acid solution, followed by clean water, and finally by an alkaline solution to prevent any further action of traces of the acid which might be left on the face. This last method is said to be patented, "the patent covering the obtain- ing of a natural stone finish for concrete by mechanical, chemi- cal or other means." 2 It is hoped that such a blanket patent is somewhat less formidable than it appears. If the sheathing planks of the molds can be removed about twenty-four hours after the concrete is placed, the same effect may be produced without the use of acid. By using plenty of water the cement and finer portions of crusher dust in the face 'Capt. Lansing H. Beach, Corps of Engrs., U. S. A., in charge. Work described by Mr. W. I. Douglas, Engr. of Bridges, D. C, Engineering News, Jan. 22, 1903. 2 Engineering News, May 21, 1903. SURFACE FINISH 367 may be washed out with a stiff corn broom, leaving the facets of the crushed rock exposed. 534, Pointing and Bush-hammering. — If the molds have been left in place until the concrete is set hard and it is found that the face of the concrete is not what is desired, it may still be improved although it may not be plastered. With this ob- ject the face is sometimes tooled with the stone cutter's point to give the appearance of rough pointed or rock face masonry. Grooves may be cut to block off the work into rectangles of the proper size, then a draft of one to two inches may be left along all of these artificial joints, and within the draft line the rough pointing may be done. A cheaper method, however, is to bush-hammer the entire face, and this tends to mask any lack of uniformity in color or smoothness. Bush-hammering may be clone by ordinary labor- ers at a small cost, as one man can go over from fifty to one hundred square feet in ten hours, making the cost from If cents to 3^ cents per square foot, with labor $1.75 per day. Where it is decided beforehand to bush-hammer the work, less pains need be taken in dressing the lagging of the forms. 535. Colors for Concrete Finish. — The addition of coloring matter to cement and concrete is not at present widely prac- ticed, and consequently experience has not been sufficient to in- dicate just what colors may be used without detriment to the work. Lampblack has been most commonly employed, giving different shades of gray according to the amount used. In any large work where the use of coloring matter is desirable and there is not time to institute thorough tests, the advice of a cement chemist should be sought. The dry mineral colors, mixed in proportions of two to ten per cent, of the cement, give shades approaching the color used. Bright colors are diffi- cult to obtain and would not be in keeping with a masonry structure except in architecture. When mixed with an American Portland cement mortar, containing one part cement to two parts by weight of a yellow river sand, the particles of which are largely quartz, the colors indicated in the following table are obtained. With no coloring matter added, the mortar was a light green- ish slate when dry. Ultra marine green, in amounts up to 8 per cent, of the cement, had no apparent effect on the color of 368 CEMENT AND CONCRETE this mortar. Variations in character of cement and sand will affect the result obtained in using coloring matter. The colors indicated below are for dry mortars ; when the mortar is wet the shades are usually darker. None of the materials mentioned in the table seems to affect the early hardening of the mortar, though very much larger proportions might prove injurious. With red lead, however, even one per cent, is detrimental, and larger proportions are quite inadmissible. COLORED MORTARS. Colors Given to Portland Cement Mortars Containing Two Parts River Sand to One Cement. .So Weight of Dry Coloring Matter to 100 Pounds of Cement. 3^ Dry o? Material Used. >* \ Pound. l Pound. 2 Pounds. 4 Pounds. 6s Dark Blue Lamp Black Light Slate . Light Gray Blue Gray . Slate 15 Prussian Light Green Light Blue Bright Blue Blue . . Slate . . Slate . . . Blue Slate . Slate 50 Ultra Marine Light Blue Bright Blue Blue Slate . . . Blue Slate Slate . 20 Yellow Ochre . Light Green . Light Buff 3 Burnt Light Pinkish Dull Laven- Umber Slate . . Pinkish Slate . der Pink Chocolate . 10 Venetian Slate, Pink Bright Pinkish Light Dull Red . . Tinge . . Slate . . . Pink . . Dull Pink 2i Chattanooga Lie;ht Pinkish Light Terra Light Brick Iron Ore . Slate . . Dull Pink . . Cotta . . Red . . Li°ht Brick 2 Red Iron Ore Pinkish Slate Dull Pink . . Terra Cotta Red . . 91 536. In some cases it may be sufficient to color the surface of the work by painting. Ordinary oil paints are sometimes applied after washing the surface of the wall with very dilute sulphuric acid, one part acid to 100 parts water, but the per- manence of such a finish seems very questionable. The method of obtaining a gray finish by painting with a thin grout of cement and plaster of Paris has already been de- scribed (§ 531). Similar methods may be used with the dry mineral colors, and, while their permanency cannot be vouched for, it seems a more reasonable procedure than to paint a con- PLACING UNDER WATER 369 crete surface with oil paints. One pound red iron ore to ten pounds cement mixed dry, and then made into a very thin grout and applied to a well cleaned concrete surface with a white- wash brush, gives a pleasing brick-red color; and a rich dark red is given by one pound red iron ore to three pounds cement. The earlier this is applied after the concrete has set, the more likely is it to remain permanent. Art. 64. Placing Concrete under Water 537. In building a concrete structure under water where the site cannot be coffered, it must be expected that the expense of the work will be increased, and the quality of concrete poorer. The methods employed for subaqueous construction are: 1st, the lajdng of freshly mixed concrete in roughly prepared forms; 2d, placing the fresh concrete in bags of burlap or canvas which are deposited while the concrete is still soft; and 3d, molding in air concrete blocks which are placed in the work when well set. In the first method some cement will certainly be washed out of the concrete, the extent of this loss depending upon the condition of the water in which the work is done {i.e., its depth and the amount of current and wave action) and the care with which the concrete is lowered to place. Tamping cannot be done with this method, and any movement of the concrete to level it will cause further loss of cement. In the second method the loss of cement will be much less, but the adhesion between the different masses will be slight. In the third method there is no loss of cement and the concrete can be well rammed ; but if small blocks are used, there may be difficulty in so placing them under water as to make a solid structure, while if large blocks are used, special hoisting ma- chinery is required to handle them. 538. DEPOSITING IN PLACE.— The first method mentioned above, depositing fresh concrete in place, is usually the cheap- est and most expeditious method, though it is not likely to sive the best results. When concrete is lowered through water, there is a tendency for the cement to separate from the sand and stone. This tendency seems to be exhibited in a more marked degree with some cements than with others. In con- nection with the construction of the concrete foundations of 370 CEMENT AND CONCRETE the Charlestown bridge, a test was devised for determining the relative values of the different lots of cement for depositing in water. 1 Concrete was laid, through a small chute, in a cement barrel placed in a hogshead filled with salt water. It was found that while some specimens would retain their form after twenty- four hours when the barrel was removed, others showed but little cohesion after twenty-four to forty-eight hours. In the former, the cement and gravel remained well distributed through- out the mass, but in the latter much of the cement had sepa- rated from the gravel, and settled in the bottom of the barrel, where it remained in an inert state, while the central portion of the concrete, robbed of its cement, had many voids. As a result of this test, some lots of cement were not accepted for use. The finest portion of the cement is very liable to separate from the remainder as the concrete passes through the water, and if subjected to the action of waves or a current, much of the cement will be washed away. In exposed situations it is especially necessary to inclose the site of the work with sheet pil- ing or cribs, or a wall constructed by the bag or block method. When the water level outside the form is constantly changing, the flow of water through' the joints in the sheathing is especi- ally effective in washing out the cement, and in such conditions the sheathing should be made as nearly water-tight as possible. To this end tongue and groove lagging may be used, or the face of the mold may be covered with tarred felt, or canvas, tacked in place. 539. Laitance is the term applied to the whitish spongy material that is washed out of concrete when it is deposited in water. Before settling on the surface of the concrete, which it does slowly, it gives to the water a milky appearance, hence the name. In fresh water this semi-fluid mass is composed of the finest flocculent matter in, the cement, containing generally hydrate of lime. It remains in a semi-fluid condition for a long time and acquires very little hardness at the best. In sea water the laitance is more abundant and is made up of silica, lime and magnesia, with carbonic acid and alumina, its exact 1 Report of Mr. William Jackson, Chief Engineer. Third Annual Report Boston Transit Commission. PLACING UNDER WATER 371 composition depending upon the character of the cement. This interferes seriously with the bonding of the layers of concrete, and when it has settled it should be cleaned from the surface before another layer is placed. 540. The Tremie. — A method frequently employed to pre- vent, as much as possible, the loss of cement, is to make use of a large tube of wood or sheet iron, made in sections so that its length is adjustable, and provided with a hopper at the upper end. Such a tube is called a tremie. The hopper is always above water, and the lower end of the tube, which may also terminate in a hopper, rests upon the bottom of the founda- tion. The tremie is first filled with concrete, a box placed over the lower end serving to prevent the escape of the concrete while the tube is being lowered until the end rests upon the bottom. The tube is then lifted from the bottom sufficiently to allow the concrete to escape as fast as fresh concrete is added at the top. The surface of the concrete in the tube should be kept continuously above the water surface. The tremie may be held in position by a crane, or it may be so supported as to al- low of two motions at right angles to each other. Such an arrangement was used in building the piers for the Boucicault Bridge, the tube traveling along a platform, which in turn could move on a track at right angles to the first motion. In using a tremie the thickness of a layer may be regulated at will. In the construction of the Charlestown Bridge * a tube was used fourteen inches in diameter at the bottom, and about eleven inches in diameter at the neck, above which was a hopper to receive the concrete. When the attempt was made to place too thick a layer at one operation, it was found that the charge was likely to be lost, and the best results were believed to be obtained with layers two feet to two and one-half feet thick. Some experiments were made with a plug designed to keep the water from flowing up through the concrete when the tube was being refilled after a loss of the charge. This plug was made with a central core of wood and sides of canvas expanded by steel ribs. It worked fairly well, but its use was not con- tinued. 1 Third Annual Report, Boston Transit Commission. 372 CEMENT AND CONCRETE 541. This principle was employed by Mr. Daniel W. Mead in placing concrete in a small shaft in ninety feet of water. 1 An eight inch, wrought iron pipe was screwed together in sec- tions, and provided with a hopper at the upper end and a wooden plug at the lower end. After lowering the pipe into the shaft, the pipe was filled with concrete and it was expected that its weight would force out the plug at the bottom when the pipe was raised. On the first attempt, however, the plug failed to drop out, and on raising the pipe the cause was apparent. The plug had evidently leaked, and as the first concrete was dropped into the pipe it had separated, the broken stone being at the bottom, the sand next, and the cement above had so plugged the pipe as to support the weight of the concrete. The second attempt, when a tighter wooden plug was used and a small pipe placed inside the larger one to assist in loosening the plug if necessary, was successful. 542. The Skip. — Since in submerged work the concrete should be deposited in as large masses as possible, the use of a large skip will probably give better results than the tremie. A box form may be used with hinged lids at the top to permit filling, and two hinged doors at the bottom which may be opened from the surface by a tripping rope when the box has reached the place for depositing the concrete. A convenient form of skip is made in two halves, each half having a cross-section either of a right angled triangle or a quadrant of a circle. The two boxes are hinged at their upper inside corners and the pieces through which the hinge rod passes are prolonged upward, the lowering cables being at- tached to their ends. Two opening cables are fastened to the outer corners of the boxes. Two sheets of iron may be used as covers to the boxes, being attached to the hinge rod that serves for the two halves of the skip. It is seen that the skip will work on the principle of a pair of ice tongs. While being filled with concrete the box is sup- ported by the lowering cables, and the hinged lids are kept up by some simple contrivance. When full, the lids are closed and the skip lowered till it rests on the bottom; the skip being then hoisted slowly by means of the opening cables, the con- 1 Trans. Assn. of Civil Engineers of Cornell University, 1898. PLACING UNDER WATER 373 crete is gently deposited in place. Such skips are supplied by the makers of concrete machinery. 543. In depositing concrete by means of skips it is well to have the latter of large size, holding not less than a cubic yard, and preferably two cubic yards or more. The larger quantity will compact itself better on account of the greater weight, and the surface which is subjected to wash will have a lesser area in proportion to the volume of the mass. The skip should be completely filled with concrete and tightly closed while it is being lowered. It is important also that the skips be lowered slowly, in order that the inclosed air may be replaced by water without commotion. 544. The Bag. — Mr. Win. Shield l devised a bag for de- positing concrete under water which is said to work very satis- factorily. The top of the bag is closed, and has a three-quarter inch wrought iron bar fastened across the end with a loop to receive the hook of the lowering line. The mouth of the bag is slightly larger than the upper end, to facilitate the discharge of the concrete. The bag is inverted to be filled, and the mouth is then secured by a turn of a line provided with loops through which a small tapering pin is passed. This pin is attached to a tripping line, and when the bag has reached the place of deposition, a pull of the tripping line releases the pin; when the bag is gently lifted, the concrete is deposited in place with such slight commotion that but little cement is said to be lost. 545. Other Methods of Depositing in Situ. — For deposition under water the materials for concrete are sometimes mixed dry, but this is not good practice. The mere soaking of water into cement does not form a compact mortar; the moistened materials need to be thoroughly mixed and, if possible, rubbed together in order to obtain perfect adhesion. Then, too, if the dry materials are lowered to place and water is suddenly al- lowed access to the mass, much of the cement will be washed away in the disturbance caused by the sudden inrush of water. M. Paul Alexandre 2 found by experimenting on mortars of "dry" (stiff), "wet" and "ordinary consistency," that mortars 1 "Subaqueous Foundations," London Engineering, 1892. Abstract in Engineering News, Vol. xxviii, p. 379. 2 " Recherches Experimentales sur les Mortiers Hydrauliques," par M. Paul Alexandre, pp. 93-96. 374 CEMENT AND CONCRETE mixed "dry" suffered the greatest decrease in strength by im- mersion in running water. Mortars mixed "wet" suffered the least loss, though their resistance was less than those mixed to the ordinary consistency, since when not subject to the current of water, the wet mortars gave much lower results than those of ordinary consistency. 546. In order to avoid the washing out of the cement, the concrete is sometimes allowed to partially set before deposition. Mr. Robert W. Kinipple has used this method and advocates its adoption. 1 In employing this method, the concrete, which should be deposited when of the consistency of stiff clay, re- quires careful watching that it does not set so hard as not to reunite after being broken up. Under ordinary supervision, this will probably not prove as successful as some of the other devices, but it may be found valuable under certain circum- stances. The writer made a few experiments with this method on a small scale in swiftly running shallow water. Much of the cement appeared to be washed out by the current, but the results were somewhat better than were obtained when the concrete was deposited fresh. (See §456.) M. Paul Alexandre made some short time experiments on this point, which indi- cated that but little advantage was gained in allowing the cement to partially set before deposition. 547. Depositing Concrete in Bags. — The second method of depositing concrete under water, namely, by placing the freshly mixed concrete in coarse sacks and immediately lowering them to place, is very convenient under certain conditions. This method is of especial value in leveling a foundation to receive concrete blocks, or to form a base for concrete deposited in situ. Small bags of concrete have been successfully used in filling the spaces between pile heads which were to support an open caisson. In such a case the bags should be lowered to a diver who places and rams them. If the bags be properly leveled and the earth firm, a part of the load is thus transmitted to the material be- tween the pile heads, while if the earth be very unstable, the bag construction compels the piles to act together, giving lateral stiffness to the foundation and tending to prevent over turning. 1 "Concrete Work under Water," Pro c. Inst. C. E., Vol. lxxxvii. See also "Notes on Concrete," by John Newman, pp. 116 and 117. PLACING UNDER WATER 375 548. The bag method was successfully used in replacing with concrete the timber superstructure of the breakwater at Marquette, Mich. 1 The main portion of the breakwater was built of monolithic blocks on the rock-filled timber substruc- ture. After removing a portion of the rubble filling, a bed was made for the monolithic blocks by laying concrete in place two feet thick, extending from one foot below to one foot above low water datum. This method was afterward replaced by the use of concrete in bags, which made it safe to remove a lesser amount of the rock filling of the crib at the center, and thus decreased the expense of the work. The bags were of eight ounce burlap made 6 feet long and 6 feet 8 inches in cir- cumference, and held about one ton of concrete. They were filled while lying on a skip specially constructed, so that when the skip was in place it could be tripped and the bag placed in its exact position in the work. 549. In connection with this work a practical indication of the character of the concrete deposited in this manner was given by some small bags of concrete that were laid to protect, during the winter storms, a portion of the crib filling. Mr. Coleman says of this, 2 "Only one layer of these sacks, laid slightly overlapping from the lake side of the crib, was used. The sacks were so lightly filled that when laid as described, the average thickness of the concrete covering was not more than six inches. The crib was storm swept many times without displacing a single sack, and when they were removed in the following spring to facilitate the work, they came away, when pulled up with the floating derrick, a dozen or more at a time, so firmly were they cemented together, and in many cases large rubble stones were lifted up along with them, because of the adhesion of the cement to their surfaces." 550. The Cost of the concrete in bags was as follows: — Materials, cement, sand, stone, burlaps, etc $5,281 Mixing concrete and filling bags .912 Transportation 157 Depositing 408 Total cost per cubic yard $6,758 Or, cost in bags, exclusive of materials .... 1.477 Major Clinton B. Sears, Corps of Engineers, in charge; Mr. Clarence Coleman, Asst. Engineer. 2 Report Chief of Engineers, U. S. A., 1897, p. 2620. 376 CEMENT AND CONCRETE The cost of the first plan, placing a two foot layer of con- crete in situ, where different methods of handling were era- ployed, was, for labor: — Loading scow with materials $0,411 Mixing concrete - .846 Depositing 524 Cost in situ, exclusive of materials $1,781 551. When concrete bags are used in forming a foundation, the lower layers should usually cover a considerably greater area than that required for the top. Especially is this true if building upon insecure earth. This increased area at the bot- tom may be obtained by building the sides on a batter, or by the use of footing courses. If the latter are used, they should be so designed that in any case the projection beyond the course next above is not greater than the thickness of the layer. Before filling the concrete into the bags it should be thor- oughly mixed, as for deposition in the ordinary manner. The practice of using dry concrete for this purpose is reprehensible for the same reason as has been given in § 545. It has also been found that if the concrete is mixed and filled into the bags in a dry state, a layer of concrete on the outside may cake before the water has had time to reach the interior portion. The bags should be filled about three-fourths full, leaving the mass free to adjust itself to inequalities in the rock, or to the irreg- ular surface of the previously deposited layer. When strength and compactness are desired, the bags should be placed by a diver and gently rammed. In this way the mass may be well bonded by "breaking joints." 552. Large Masses in Sacks. — Very large bags of concrete are sometimes employed, as in the construction of a breakwater at New Haven, England. 1 "The top of the breakwater has a width of thirty feet, is ten feet above high water, and is sur- mounted by a covered way and parapet running along the outer - side, both sides battering one in eight. The breakwater is unsheltered from the force of the Atlantic, is founded on the rough, natural sea bottom, and the foundation course has a 1 From London Engineering, quoted in Engineering News, Vol. xxvii, p. 551. PLACING UNDER WATER 377 width of fifty feet; the lower portion of the structure, from the bottom up to a level of two feet above low water, consists of one-hundred-ton sacks of concrete deposited while plastic. The canvas with which the concrete was enveloped was of jute, weighing about twenty-seven ounces per square yard. The sacks were dropped into place by a specially designed steam hopper barge. The 'sack-blocks' in the finished work became flattened to a thickness of about two feet six inches. With the exception of this sack work the breakwater is built of plastic concrete laid in situ." Similar sack-blocks of one hundred sixty tons have been employed in breakwater con- struction. It is evident that this method of depositing concrete in large sacks is peculiarly suited to forming a foundation on a soft bottom, since, if the bags are made to project well beyond the sides of the molded concrete to be deposited above, they act in the double capacity of a mattress to prevent scour, and a foundation for the upper part of the structure. 553. Other uses for Bags of Concrete. — In the construc- tion of the Merchants' Bridge at St. Louis, bags of concrete were used to check the scour which occurred beneath the up- stream cutting edge of one of the caissons while it was being grounded. The bags were thrown into the river at such a dis- tance above the pier that they settled to the bottom at the point where the scour was taking place. Burlap bags were used at St. Marys Falls Canal for laying concrete under water next the face of the form to prevent washing .of the cement in building concrete superstructure for canal walls. As the bags were placed by hand they were made to hold only about two cubic feet of concrete. 554. Paper Sacks. — Paper sacks are sometimes employed instead of jute bags. Dr. Martin Murphy l describes the meth- ods employed in filling steel cylinders for the substructure of the Avon Bridge as follows: "Bags made of rough brown paper well stiffened with glucose, were employed and slipped into the water over the required place of deposition. Each bag held about one cubic foot of concrete; smaller ones were used be- " Bridge Substructure and Foundations in Nova Scotia," by Martin Murphy. Trans. A. S. C. E., Vol. xxix, p. 629. 378 CEMENT AND CONCRETE tween dowels. The bags wera quickly made up and dropped one after another, so that the one following was deposited before the cement escaped from the former one. The paper was immediately destroyed by submersion, and the cement remained; it could not escape. The bags cost one dollar thirty- five cents per hundred, or thirty-five cents per cubic yard." The success of this method will depend upon the character of the sacks, for in some experiments on a small scale with sacks of stiff manila paper the author found that the bags were not destroyed, and that no adhesion took place between the separate sacks. 555. the Block system of concrete construction. — The advantage of the block system of construction lies in the fact that the individual blocks may be made with the greatest care, and as they are allowed to harden thoroughly before being put in place, the loss of cement incident to the other systems is avoided. There is, however, the difficulty of forming a joint between adjacent blocks. The joints are of great importance when small blocks are employed, since the latter may not have sufficient weight to escape being washed out of the work. Large blocks may make a very solid structure by being simply super- imposed, but special hoisting machinery will be required to place such blocks. Sometimes a large bed of mortar is laid in coarse sacking and carefully lowered and spread on the block last laid, the next block being placed upon it immediately. A very rich mortar should be used for this purpose. Usually, however, it is not attempted to place mortar in the horizontal joints in concrete block work laid under water, but it is considered that all vertical joints should be filled with rich Portland cement mortar when the work is to be exposed to wave action. If settlement is anticipated, and large blocks are used, no attempt should be made to break joints in the direction of the longer dimension of the work, but the blocks should bond in a direc- tion transverse to the wall. Concrete blocks may be advan- tageously employed to form the faces of a structure built under water or exposed to wave action, the concrete hearting or backing being built in situ. 556. For convenience in handling, a groove to receive a chain or cable should be left down two sides and across the PLACING UNDER WATER 379 bottom of the blocks to enable them to be placed close together and to facilitate the withdrawal of the hoisting chain. These grooves may afterward be filled with concrete; such recesses are sometimes molded for the sole purpose of filling them with fresh concrete when in place, and thus binding the work to- gether. The molds for forming the blocks should be carefully made in order that the finished blocks may have good bearings one upon another. If the corners are rounded, they are less likely to be chipped off in handling or by having an undue strain come upomthe corner when in, place. If any recesses are desired in the blocks, the pieces placed in the mold to form them should be trapezoidal in cross-section with the longer parallel face against the side of the mold. If such filling pieces are made rectangular, difficulty will be ex- perienced in removing them when the concrete has set. The molds should, of course, be so constructed as to be readily taken apart to be used again. The opposite sides may be kept from spreading by rods which pass through the mold, but such rods are an inconvenience in packing the concrete into the mold, and it is therefore better to truss the mold- outside. If such tie rods are used, they may be left imbedded in the con- crete, or removed with the mold, as desired. 557. Cost of Molding Blocks. — An illustration of the use of the block method is furnished in the United States breakwater at Marquette. 1 The general plan of this breakwater has already been briefly noted and two methods of laying a two foot layer of subaqueous concrete, as a foundation for monolithic blocks forming the superstructure proper, have been described. A third method was to mold footing blocks, seven feet by five feet in section and two feet high, which were afterward laid flush with the lake side of the substructure cribs and filled in behind with concrete laid in place. The footing blocks thus assured a good quality of concrete beneath the toe of the monolithic block on the lake side where it was most necessary to provide a good foundation, and also served as a protection behind which the remainder of the two foot layer could be placed with greater facility. Many of these blocks were built during the winter in a shed 1 Report Chief of Engineers, 1897, p. 2624. 380 CEMENT AND CONCRETE artificially heated, the materials being thawed out as required. The molds were of six by six inch and four by four inch pine, lined with two by eight inch plank dressed on one side. Strips of trapezoidal cross-section, nailed inside the mold, provided for two parallel grooves on the bottom and two sides of the block to receive hoisting chains. A dovetail at the back of the block was also formed by three wedge-shaped pieces placed against the back face of the mold. The cost per cubic yard of making forty blocks is as follows: — 1.42 bbls. Portland cement, at $2.75 $3.90 .45 cu. yd. sand, at $0.45 20 1.0 cu. yd. stone screenings passing §" sieve, at $1.10, 1.10 Cost materia's in concrete per cu. yd $5.20 Superintendence, labor and watchman ....... $2.21 Fuel 31 10 per cent, of cost of warehouse and molds 52 Total cost of making per cu. yd 3.04 Total cost per cu. yd. of blocks ready to place in work $8.24 * CHAPTER XIX CONCRETE-STEEL 558. The ratio between the compressive and tensile strengths of steel is nearly unity. The same thing is approximately true of wood and some other materials of construction. In cement and concrete, however, the conditions are quite different, the strength in compression being from five to ten times the strength in tension. Concrete cannot, therefore, be economically used to resist tension, and in structures requiring transverse strength concrete is at a great disadvantage. 559. The idea of supplementing the tensile strength of con- crete by the use of iron in combination with it, seems to have been suggested independently by a number of men. It is known that combination beams were tested by Mr. R. G. Hat- field as early as 1855. In 1875 Mr. W. E. Ward, 1 M. Am. Soc. Mech. Engrs., constructed a dwelling entirely of "beton," the floors, roofs, etc., being reinforced with light iron beams and rods. These early uses of the combination have some bearing upon the ability of patentees to cover in their blanket patents more than the peculiar form of the steel member which they advocate in their particular system. Art. 65. Monier System 560. A much more picturesque beginning of the concrete- steel industry is furnished in the story, quite true, that about 1876, a French gardener, Jean Monier, used a wire netting as the nucleus about which to construct his pots for flowers and shrubs, and seeing that the practice might be extended, he called to his aid engineers and capitalists who developed the Monier system of construction. This system consists of imbedding in the concrete two sets of parallel rods at right angles to each other, the rods of the two sets being tied together with wire at all intersections. 1 Proc. Am. Soc. Mech. Engrs., VoJ . iv, p. 388. 381 382 CEMENT AND CONCRETE The larger wires run in the direction of the greater tensile stresses and are usually spaced two to four inches apart. The rods at right angles to these main tension members are to assist in dis- tributing the stress to the main members and may be of smaller diameter. The iron rods in this system are designed primarily to resist the tension only, and the form of the bars is not such as will stiffen the structure while the concrete is fresh. In an arch, two systems of netting are used, one near the intrados and one near the extrados. fc 561. The main advantages which this system has over some of its competitors are the simple shapes required, that is, round rods, which may always be obtained without difficulty, and the fact that these may be so readily put together by ordinary workmen under supervision. This system is especially adapted to vertical walls, whether curved or straight, and found its first extensive use in the construction of tanks and reservoirs. It has been extended, however, to the construction of sewers, floors, roofs, and arch bridges. One of the practical disadvantages of the system is that the nets are somewhat difficult to handle and keep in position, and in thin sections it has not been found practical to imbed the nets in concrete containing broken stone of the ordinary size. The use of cement mortar, usually one part cement to three sand, has been found necessary in order to p;et a perfect con- nection between the wires and the body of the work. This, of course, increases the cost. Another objection has been urged against it, namely, that the transverse rods do not in general have any duty to perform, and are simply a waste of material so far as the final strength of the structure is con- cerned. While this may be so in certain forms of construction, it may be met by the statement that these cross-rods may be made as small as desired if they are to act merely as spacers for the main rods. In slabs, walls, etc., however, these cross- rods have a purpose, and in some other systems members are supplied to fulfill this necessary function. 562. Some very bold arches have been built on the Monier system, including three bridges in Switzerland having 128 foot span, 11 foot rise, and a thickness of but 6f inches at the crown and 10 inches at the abutments. PATENTED SYSTEMS 383 A Monier arch of 32.8 foot span, rise one-tenth of span, width 13.2 feet, in which the mortar at the crown was six inches thick and eight inches at the abutments, was tested in Austria in 1890. It held a fifty-three ton locomotive on half the arch, and finally failed at the abutments under a load of 1,700 pounds per square foot over half the span. 563. Pipes are now made by this system at yards and trans- ported to the place of use. It has also been used as a substi- tute for iron in cylinders for bridge piers. A novel use of this system consists in making a pipe covering for piles exposed to marine borers. The pipe, which is long enough to reach from above the water surface to below the bed of the waterway, is slipped over the head of the pile and settled a short distance into the mud or silt of the bottom with the aid of a water jet. A question, however, has been raised as to the action of con- crete and iron in combination in sea water on account of the possible setting up of galvanic action. Art. 66. Wunsch, Melan, and Thacher Systems 564. Wiinsch System. — This system, which was invented in 1884 by Robert Wunsch of Hungary, consists of two iron members of angle irons and plates imbedded in concrete, the lower member being arched and conforming to the outline of the soffit, while the upper one is horizontal and continuous. The two members are riveted together at the crown, and at the abutment are rigidly connected by a vertical member. The several systems of rib bracing thus constructed are connected laterally at the abutment by channel bars running transverse to the arch and riveted to the bottom of each vertical in the abutment. Assuming that the abutments are stable, it is evi- dent that we have here not simply an arch, but also some ele- ments of the cantilever. The spandrels being built up solid of concrete, there is no definite arch ring, and the quantity of material required, especially in long spans, is likely to be much greater than in other systems. On the other hand, the great depth at the springing permits the use of concrete only moder- ately rich in cement. 565. A bridge of this type, built at Neuhausel, Hungary, consists of six spans of about 56 feet each, rise 3.7 feet, thick- ness at crown 9.8 inches, and at springing line 54.3 inches. 384 CEMENT AND CONCRETE The total width of the arch was 19.7 feet and contained thir- teen systems of arch ribs. Concrete in the abutments below water was made mainly of Roman cement. Above water it was composed of one part Portland to eight or ten parts sand and gravel. Ten to twelve inches of the arch was built of strong Portland concrete rammed in layers at right angles to radial lines of the arch, special care being taken with that part below the bottom arched member. • An arch was usually com- pleted in one day, and the centers remained in place thirty to forty days, the greatest settlement on the removal of centers being two-thirds of an inch. This bridge contained 1,346 cubic yards of concrete and 88,180 pounds of iron, and cost, complete, $13,700. 566. Melan System. — This system, invented by an Austrian engineer, Joseph Melan, consists of arched ribs between abut- ments as in bridges, or between beams or girders as in floor construction, the space between the ribs being filled with con- crete. Steel I-beams curved to the proper form are usually employed for the reinforcement, though angle iron flanges with lattice connections have been used in some of the large bridges. The steel members extend into the piers or abutments and are there connected by angles or other shapes, and firmly imbedded in the concrete. 567. This system as adapted to bridge construction has probably met with greater favor among American engineers than any other form. Perhaps this is because of the stiffness of the form of iron beam used, and because by assuming a rather high fiber stress for steel the reinforcement may be de- signed to withstand the entire bending moment without exces- sive dimensions for the steel members. There is thus a feeling of security in its use that is not felt in the same degree with other systems. The arch dimensions are determined by com- puting the forces and required thickness of arch ring after assuming certain safe working stresses for the steel and con- crete; but if desired, the size of steel members may then be increased slightly where necessary to such dimensions that with unit stresses of, say, one-half the elastic limit, the entire bending moment shall be taken by the steel. Some of the largest bridges built after this system in the United States are the five-span bridge at Topeka, Kan., and the three-span bridge at Paterson, N. J. PATENTED SYSTEMS 385 568. Thacher System. — A modification of the Melan system is that invented and patented by Mr. Edwin Thacher. Steel bars are used in pairs and imbedded in the concrete near the intrados and extrados of the arch and extending well into the abutments. The bars of each pair may be connected by bolts or stirrups, though Mr. Thacher's original idea seems to have been to have no connection between two bars of a pair ex- cept through the concrete. The bars are provided with pro- jections which may be in the form of rivet heads, lugs, or bolts, to increase the resistance of the bars to slipping in the concrete. 569. Mr. Thacher has more recently designed a special form of rolled bar having projections that serve the same purpose as the rivet heads mentioned above. Several bridges have been built on this system, one of the most notable of these being the Goat Island bridge at Niagara Falls, one span of which is 110 feet in length. 570. In the construction of arch bridges many of the other systems are simply modifications of the Melan. The shapes of the steel members may have different forms, and the connec- tions between the pairs of bars forming the arch ribs may vary to suit the idea of the inventors. But though these systems lose their identity in long-span arches, their distinctive features are more apparent in the construction of floors, roofs, columns, etc. Art. 67. Other Systems of Concrete-Steel 571. The Hennebique System. — The rods are here arranged in pairs, one above the other, in a vertical plane. In girders, the bar in the tension side is straight, while the other one of the pair is horizontal for a short distance along the center of the span, the ends being inclined upward near the ends of the beam. The two bars are connected by bent straps or U-bars so that the steel reinforcement may be compared to a queen post truss within the concrete. This system has been used in the construction of bridges, both arch and girder, floors, roofs, stairways, etc., but it is in beams and girders that its distin- guishing characteristics are best displayed. 572. A beautiful arch on this sjrstem is the bridge over the river Vienne at Chatellerault, France, consisting of three spans, 386 CEMENT AND CONCRETE the central one of which is 164 feet long, with rise of 15 feet, 8 inches. Four arch ribs 20 inches deep support the roadway, 25 feet wide, by posts forming a skeleton spandrel. 573. Kahn System. — In this system, which is somewhat similar to the Hennebique, the distinguishing feature is the care taken to provide against shear, or against that combination of tension and shear which tends to cause failure in a beam by cracks that extend diagonally upward toward the center of span from near the points of support. The steel plates forming the tension members are sheared longitudinally at intervals, and short ends are bent up at an angle of forty-five degrees with the horizontal. These ends, which may be compared to the tension diagonals of a truss, are thus a part of the maki steel member, and the stress is transferred directly to the latter without dependence on. the concrete. The advantages are the great resistance offered by the bar to being pulled out of the concrete and the thorough manner in. which all tension stresses may be provided against. The main disadvantages would seem to be the necessity of detailed shop work for each size of girder, the inconvenience of shipping the steel in its complete form and the difficulty of thoroughly tamping the concrete around the diagonals. 574. The Ransome System. — One of the earliest patents to be issued in this country for a method of using concrete and iron in combination was that issued to Mr. E. L. Ransome in 1884. The valuable and distinctive feature of this system is the use of a square bar that has been twisted cold. This twist- ing not only insures a good bond between the concrete and iron, but actually somewhat increases the strength of the bar. In building beams with twisted bars as tension members, the latter are given a slight inclination from the center upward toward the ends. For use in buildings, as in floors and columns, and for covers to area ways, and similar uses, this system is largely employed. 575. Roebling System. — As its name implies, wire forms the main feature of this system, and in a general way it resem- bles the Monier. Its application thus far is found principally in floor construction, two distinct methods being used. In the arched floor a wire netting, stiffened by round steel rods woven through it is sprung between the lower flanges of the main STRENGTH 387 I-beams of the floor. This netting, further stiffened and held in place by iron rods running parallel to the axis of the arch, forms a permanent center for the placing of the concrete, which fills all of the space to the level of the top of the I-beams. A level ceiling below is obtained by a similar netting laid flat against the under side of the I-beam and fastened thereto. This acts as a wire lath to receive a coat of plaster. If the level ceiling is not necessary, the plaster may be applied to the under side of the arch netting, in which case the lower flange of the I- beam should be encased in concrete to protect it from corrosion and fire. 576. For lighter loads, flat bars are placed at suitable in- tervals above and below the I-beams and clamped to the flanges. To these bars the wire netting is attached, a thin layer of con- crete laid on the upper wire incasing the bars, and plaster ap- plied to the lower netting forming the ceiling. Cinder concrete is usually employed with this system. 577. Expanded Metal. — The use of whac is commonly known as expanded metal lath has been extended to concrete-steel construction. As in the Monier and Roebling systems, the strength and stiffness of the structure are increased by the use of steel rods in connection with the expanded metal, the chief duty of the latter, where great strength is required, being that of a distributing member. Expanded metal is made from sheet steel by shearing short slits parallel to the grain, and extending the sheet at right angles to the slits, resulting in a network of diamond shaped openings. The metal used is of all weights up to one-quarter inch thick with meshes six inches long. 578. The steel bars used in connection with expanded metal by the St. Louis Expanded Metal Fireproofmg Co. are square, with frequent corrugations surrounding the bar. These- corru- gations serve only to prevent the slipping of the bars in the concrete without adding to the strength. The applications of this system include conduits, sewers, and walls of buildings, as well as floors and roofs. Art. 68. The Strength of Combinations of Concrete and Steel 579. While we have in this country been somewhat slow in acknowledging the worth of concrete-steel construction, there 388 CEMENT AND CONCRETE is now a strong interest displayed in the subject; many experi- ments are being made in our educational and commercial labo- ratories and the theory of the action of concrete and steel in combination is being rapidly developed. It is natural that in the investigation of a form of construction permitting so many variations in methods of preparation, that the opinions now advanced, based on insufficient data, should be more or less conflicting. 580. Experiments. — The experiments of M. A. Considere, made in France between 1898 and 1901, which have been made more available to us through the translation and collection of his articles on the subject by Mr. Moisseiff, 1 are exceedingly valuable. The effect of the quality of the steel and the con- crete, of repeated loads, of changes in volume in hardening, and many other points are carefully analyzed by experiment and theory. One of the most important deductions drawn by M. Con- sidere is that fibers of concrete within what may be called the sphere of influence of a reinforcing rod of iron or steel, is capable of enduring very much greater elongations without visible frac- ture than similar concrete without reinforcement. The expla- nation advanced for this is that the steel so distributes the stress throughout the length of the concrete in tension that the development of insipient fractures or excessive elongations at the weaker sections of the concrete is prevented until each section has taken its maximum load. The conclusion to which this theory leads is that the resistance of the concrete through- out the area of influence of the steel reinforcement, is main- tained far beyond that degree of deformation which, in concrete not reinforced, would cause its rupture. 581. Neglect of Tensile Strength. — Notwithstanding these conclusions, it is believed that it is sufficient in most cases of design to neglect the tensile strength of the concrete in concrete- steel combinations. This course may be defended by the fol- lowing considerations. The tensile strength of concrete is, at best, not usually above two hundred to four hundred pounds per square inch. If the stress on the extreme fibers of a beam 1 "Reinforced Concrete," by Armand Considere, McGraw Publishing Co., New York. STRENGTH 389 is three hundred pounds, and we consider that this stress de- creases uniformly toward the neutral axis, the mean stress is but one hundred fifty pounds per square inch. Again, if we disregard M. Considered conclusions, we find that since the modulus of elasticity of steel is, say, fifteen times that of con- crete, the former is only stressed to forty-five hundred pounds per square inch when the imbedding concrete has reached its ultimate strength. 582. The resistance of concrete to tension may easily be destroyed or impaired by accident, especially when fresh. The properties of concrete vary so much with the materials, the proportions, and the manipulation, and the investigation of the behavior of concrete and steel under stress is as yet so in- complete, as to make refinements in theoretical treatment not only unwarranted but really undesirable for practical purposes, since they lead to the appearance of greater accuracy than is in reality attainable. It is true that by the judicious selection of values for the constant appearing in formulas for the strength of concrete- steel beams, the results of such formulas sometimes show a re- markable agreement with the results of that series of actual tests for which the constants have been selected; but one has only to recall his experience in other lines, hydraulics for in- stance, to realize the importance of the almighty constant. The opinion sometimes advanced, that the strength of a given concrete-steel beam may be as accurately derived by formula as can the strength of a steel beam, the writer does not believe to be tenable, at least in the present state of our knowledge concerning the behavior of concrete. 583. To neglect the tensile strength of the concrete will result in a slight increase in the required area of steel reinforce- ment, and, in so far as the tensile strength of the concrete may be developed, will tend to make the compression side of the beam weaker than the tension side. The only objection to this is that the failure of the beam, though at a higher load, may be more sudden. This possibility, however, seems less serious than the error of depending on the tensile strength of the concrete only to find it lacking at the critical mo- ment. Since the aim here is to develop a formula that may be used 390 CEMENT AND CONCRETE with safety in the design of structures, and since to neglect the tensile strength of the concrete is to add an unknown, though probably small, factor of safety, the tensile strength will not be considered in the following analysis. Art. 69. Concrete-Steel Beams with Single Reinforcement 584. Definitions. — In this discussion the word strain has its technical meaning, the relative change in length of a piece under stress. It is usually expressed as the ratio of the elonga- tion (or shortening if in compression) to the original length of the piece. But for our purpose it is the ratio of the increment of change in length, occasioned by a given increment of stress, to the length of the piece before the increment of stress was applied. These two expressions for strain are usually consid- ered equivalent, since, according to Hooke's law, the ratio be- tween stresses and corresponding strains, for a given material, is constant within the elastic limit. But in dealing with con- crete it is found that, even before the stresses become excessive, Hooke's law does not hold true. Bearing in mind, then, the meaning of the word strain, we represent as usual the ratio of stress to strain by E, the modulus of elasticity, or „ stress E = ——r~- strain Let E a = modulus of elasticity of steel. E c = modulus of elasticity of concrete in compression. /„ = tension in steel, fbs. per sq. in. - f = compression in concrete, lbs. per sq. in. a = thickness of steel considered as a flat plate, or the area of imbedded steel bars per inch of width of beam z. y x = distance the extreme fiber of concrete in compression is from the neutral axis. y, 2 — distance the center of the steel reinforcement in the tension side of the beam is from the neutral axis. i = depth of concrete below reinforcement. d =yi + y2 and h = d + i. Xi = unit compression of extreme fibers of concrete in compression. Xo = unit elongation of steel in tension. E f Represent ^ by R, and j by r. E c U SINGLE REINFORCEMENT 591 585. Formulas for Constant Modulus of Elasticity. — The cross-section of the beam, the graphical representations of the strains and of the stresses are shown in the following diagrams: 1 i z. - - -a i f 1 * 1 it 1 A M GL t- A 1 I d 1 \ 1 -*-- ..- _*-*- \- L - eA a \) k £■>! rt c A ~J* ) i 1 y " i i as \ ■ Fig. 10. CROSS-SECTION. Fig. 11. STRAINS. Fig. 12. STRESSES. Fig. 13. STRESSES. Figure 12 shows the conditions when the stresses are so small that the modulus of elasticity of the concrete may be considered constant, and this case will be first considered. In the strain diagram, \ represents the deformation of the extreme fiber of concrete in the compression side of the beam, and X, the deformation of the steel. Since a section plane be- fore bending is considered to be plane after bending, the steel is considered not to slip in the concrete, and NN is the neutral axis, u A., = ^: but E s = and E c — u Xi ~lt or and 2/2 = 2/2 /. and x i = -ft' fs Ec r E J h = R m (Eq. 1.) In the stress diagram the triangle NAB represents the total compressive stress on the concrete for unit width of beam, and is equivalent to a single force £|1 applied at the center of gravity of the triangle. The total compressive stress for section of width z is The total tension in the steel is T = zaf s 392 CEMENT AND CONCRETE As we disregard the tensile strength of the concrete, and as the total normal compression and total normal tension on a section must be equal, as they are the two forces of a couple, we have P = T, or zff c = zaf s , whence a = f f = f' CEq- ?■) 2 The point of application of the force P is - y 1 above the neu- o tral axis, while the point of application of T is y 2 below the neutral axis; the arm of the couple is therefore [^ y 1 4- y 2 ), and the moment of resistance is equal to either force into this arm, substituting the value of y 2 given in (1) and reducing, M = zf c (| + I t |) y* = */, (I + 1^ yi ». (Eq. 3.) 586. Formulas for Varying Modulus of Elasticity. — The fore- going formulas are based on the supposition that the compres- sive stress in the extreme fiber of the concrete has not passed the point beyond which equal increments of stress no longer produce equal increments of strain or deformation. They are based, in other words, on the common theory of flexure, except so far as we have departed from the application of this theory in neglecting the tensile strength of the concrete. It is well known that even for steel and wooden beams this common theory does not, and is not meant to, apply outside the elastic limit. In the case of concrete, however, it has been found that, even for quite moderate stresses, the modulus of elasticity is not constant (Art. 56), but that after a certain stress is reached the modulus decreases with increasing stress. The effect of this upon the internal forces may be illustrated by the curve N B in Fig. 13. The extreme fiber is supposed to be subjected to the stress f c ; the fibers nearer the neutral axis have a smaller stress per square inch, and the modulus of elasticity for this smaller stress is greater; but in order that a section that is SINGLE REINFORCEMENT 393 plane before flexure shall be plane after flexure, the strain must be proportional to the distance from the neutral axis. It fol- lows, then, that the stresses in the inner fibers do not decrease accord' ng to the ordinates of the triangle, but are greater than indicated by such ordinates. The exact form of the curve B N is not known, but the examination of a number of de- formation curves has indicated that it is parabolic, and for the purpose of this discussion it may be considered a parabola with axis A B without serious error, although it is known the axis does not coincide with A B for stresses below the elastic limit of the concrete. 587. While the formulas derived in §585 may represent, then, the conditions existing in a beam subjected to very moderate stresses, it appears that beyond the limit of stress at which the modulus of elasticity of concrete becomes variable, they should be so modified as to take into account this variable modulus. Then if A B in Fig. 13 now represents / c and M S = f s , we have as before, 5/3 = fl f> = s ^ (Eq - 4 ° The total stress on the concrete above the neutral axis is 2 now represented by the area within the parabola, or - f c y 1} and o the total compression on section of width z is o and the total tension T' = mf s . As these are the two forces of a couple 2 / 2 v whence a = - '■£ y i = - — - (Eq . 5.) 3 .f t 3 r The point of application of P' is on a line through the center 5 of gravity of the parabola, or - y 1 from the neutral axis, while o the point of application of T' is at distance y 2 below the neutral 394 CEMENT AND CONCRETE 5 axis; the arm of the couple is, therefore, - y x + y 2 , and the 8 moment of resistance M = ^zy 1 f c Uy l +ijJ 5 2 Substitute value of y 2 given in (4) M = j^ zfcy? + 3 zfcVi j -^ y x / 5 9 r \ In applying these formulas, it must be remembered that (1), (2), and (3) are applicable where the stresses are below the point at which the modulus of elasticity of the concrete begins to diminish, while (4), (5), and (6) illustrate the con- ditions for stresses above that limit. 588. Example. — Design a beam of 10 foot span to carry a load due to 20 feet head of water. Load per square foot = 20 X 62. 5# = 1250#. Total load per foot width of beam = 125,000# = W x . First, using Eqs. 1,2, and 3. WL M = — = 187,500 inch-lbs. on beam 1 ft. wide, (z = 12). 8 Assume /, = 12,500, f = 500, r-'*j- 25; -p E s = 28,000,000, E c = 2,000,000, R = ~ = 14. From (3) M = 187,500 = 12 X 500 (± + ^j^j .Vf- y x * = 25.5, y x = 5.05 inches. From (2) Vi 5.05 1A1 . , a = 7T~ = K FTF = -101 mcn > 2 r 2 X 25 az = .101 X 12 = 1.21 sq. in. of steel for beam 12 in. wide. SINGLE REINFORCEMENT 395 From (1) r 25 y« = „ V, = 77 X 5.05 = 9.02 inches. ■ u R Jl 14 If i = thickness of concrete below center of steel bars = 2 inches, h = total depth beam = 5.05 + 9.02 + 2.00 = 16.07 inches. Second* using Eqs. 4, 5, and 6. Assume /. = 50,000, U = 2,000, r = f = 25; /c # 8 = 28,000,000, # e = 1,400,000, R = 'j = 20. As the stresses per square inch given above are approxi- mately the breaking strengths of the materials, we must supply a factor of safety, say 4; i.e., design the beam to withstand four times the required bending moment before the stresses assumed above are attained. 1 From (Eq. 6) M = 4 M = 4 X 187,500 = 12 X 2000 (~ + | X ^\ y* ; , . 187,500 x 4 x 12 whence «/ = — = 25, Ul 12 x 2000 X 15 or, y x =5. From (Eq. 5) a = - — = .133 inch, and az = 1.6 square inches of steel for 12-inch width of beam. 1 The method of using the breaking strengths of the materials, and com- puting the ultimate resistance equal to a certain number of times the desired strength, is considered inferior to that of assuming safe working stresses and computing directly the safe load. These safe working stresses should be fixed with reference to the elastic limit of the materials, rather than with reference to ultimate strength. The use here of the term factor of safety is for the momentary purpose of emphasizing the fact that the conditions assumed in deriving equation (6) are such as are supposed to exist under comparatively high stresses; but the formulas may evidently be applied to the safe working stresses the same as equations (1), (2) and (3), and in the present example the same size beam will result by eliminating "factor of safety" and using working stresses equal to one-fourth the values of the stresses assumed, 396 CEMENT AND CONCRETE From (Eq. 4) 2/2 = j^Vx - ^Vx = 1-25 X 5" = 6.25 inches. Xf i = 2 inches as before, h = total depth beam = 5.00 + 6.25 + 2.00 = 13.25 inches. It is seen that equations 4, 5 and 6 give, for the assumption made, a lesser depth of beam with more reinforcement than is given by equations 1, 2 and 3 with the corresponding as- sumptions as to stresses and moduli. 589. An inspection of the equations shows that to increase the amount of steel reinforcement in the tension side of the beam tends to move the neutral axis nearer to the tension side, and bring a greater area of cross-section of concrete into compression. If we arbitrarily decrease the depth of the beam which must withstand the same bending moment, it will in- crease the required area of reinforcement, and if carried too far will eventually raise f c beyond a safe value. On the other hand, if we take the beam as designed in accordance with equa- tions 1, 2 and 3 and subject it to a greater bending moment than that for which it is designed, then so long as R remains constant, r also remains constant, that is, the steel and con- crete are equally overstressed; but since R increases with the load, r will also increase, that is, the increment of stress in steel will be relatively greater than that in concrete. 590. Excessive Reinforcement. — In the solution of the above example if we introduce the requirement that the total depth of the beam shall be but 12 inches, while the quality of the con- crete is not improved, we may assume, as before, E s = 28,000,000 and E c = 1,400,000. Let us introduce the same factor of safety, 4, by using j c = £-° ? °-fi- = 500 pounds instead of designing the beam for four times the required bending moment; as we have seen, this does not affect the result. Since the depth of the beam is fixed, / s and r cannot be as- sumed, but must be found, together with a. We have d = y x + y 2 = 12 — 2 = 10 inches, and y 2 = 10 — y v From (6 a) M = j% X 12 X 500 y* + f X 12 X 500 y t (10 - y x ) = 187,500. EXCESSIVE REINFORCEMENT 397 Solving, we have y x = 6 inches nearly, and t/ 2 = 10*- 6 = 4 inches. From (4) vrm Substituting values of y 2 ,yi,f c , E c &nd E s , we have f g = 6,667 lbs. per sq. in. 2f r 2 5000 . Qn • From (5) a = Jjy x = 3 X ^ X 6 = .30 in. anc l az = 3.6 sq. in. of metal to each foot width of beam. This is more than double the amount of reinforcement required for a 13.25 inch beam, while the steel is stressed only 6,667 lbs. per square inch. It may be asked why not use a smaller area of metal, say 2 sq. in., stressed to 12,000 lbs. per square inch, giving the same total tension; but a moment's consideration shows that in order that the metal should assume this higher stress, its elongation must increase proportionally, involving a corresponding in- crease of strain in the concrete in compression with an accom- panying increase in stress beyond the assumed safe limit of 500 lbs. per sq. in. 591. To pursue this subject of excessive reinforcement a little further, let us examine some tests of concrete-steel beams made by Prof. Gaetano Lanza and reported in Trans. Am. Soc. C. E. for June, 1903. In these beams the width 2 = 8 inches, h = 12 and d = 10 inches nearly. The span was 11 feet. Proportions in concrete by volume 1 part Portland cement, 3 parts sand, 4 parts broken trap that would pass 1 inch ring, and 2 parts of the same rock that would pass \ inch ring. Both plain and twisted square steel bars were used as reinforcement, the plain bars having a tensile strength of about sixty thousand pounds per square inch and the twisted steel about eighty thousand pounds per square inch. If we assume the ultimate strength of the concrete to be 2,000 pounds per square inch, the modulus of elasticity at this high stress to be 1,400,000 and the modulus of the steel to be 28,000,000, we have, _ 28,000,000 = 2Q 1,400,000 " ' 80,000 = 40, 2,000 r -p?/i = 2y v 398 CEMENT AND CONCRETE and for twisted bars, r = From Eq. (4) y 2 = .'. Sy 1 = 10 inches, y t = —inches. o From Eq. (5) a = — ^ =^x-jX- = — = .055, and az =.444. That is, .444 sq. in. of twisted steel reinforcement is required in the beam 8 inches wide in order that the stresses in concrete and steel shall simultaneously reach the values of 2,000 and 80,000 lbs. per square inch, respectively. 5 . 2 40\ 100 20/ 9 From (6) I = 8x 2000 X (^ + | X = 311,100 inch-pounds. One beam having .56 sq. in. reinforcement, or an area very close to the theoretical amount called for above, broke under a bending moment of 470,000 inch-lbs. Eight other beams hav- ing a greater area of reinforcement gave moments of 355,000 to 443,000 inch-lbs., and the average of the nine bars was 403,000, or 30 per cent, greater than the value derived by formula. 592. Included in the series of tests were three beams, in each of which were placed two 1^ inch twisted rods. As we have seen, the correct amount of steel to develop the full strength of both steel and concrete is about .444 sq. in.; the three bars mentioned had 3.12 sq. inches of steel, or a large excess of reinforcement. To determine the theoretical moment of re- sistance of these beams, assume as before: E, = 28,000,000, E c = 1,400,000, fc = 2,000. f W f From (4) y % = £ g& = ^Ift, (*) 1.25 2 X2 qQ a = = .39. STRENGTH OF BEAMS 399 2 2 000 From (5) a = .39 = - -^- y u (b) ?/ 2 = 10 — 2/i. (c) Solving (a), (6) and (c), we obtain / s = 22,000, iji = 6.45 inches, and y 2 = 3.55 inches; /• whence r =-j — 11, 7 C and from (6), M = 8 x 2000 (^ + 1 x ^) ( 6 - 45 ) 2 = 522,000 inch-pounds. These three beams developed the following moments of re- sistance: 553,550, 663,700 and 783,500, mean 667,000 inch-lbs., or 28 per cent, greater than that derived by formula. None of them failed, however, by crushing of the concrete at the top of the beam, but by longitudinal shearing "at or a little above" the reinforcing rods. 593. It appears, then, that by increasing the area of steel reinforcement over 600 per cent., or from .44 sq. in. to 3.12 sq. ins., the strength of the beams was increased about 68 per cent, by theory, or 66 per cent, according to the few tests cited. The cost of the beam, however, was increased about one hun- dred per cent. This method of increasing the moment of resistance of a beam is not economical; it is better to improve the quality of the concrete. It may, however, be necessary at times to use excessive reinforcement on account of restrictions on the size of beam, but one may easily carry this so far that he passes outside the true theory of concrete-steel construction, and it becomes a question of the steel being sufficient to carry the entire load. In such cases double reinforcement may be adopted. 594. Tables of Strength. — In Table 160, equations (5) and (6) have been reduced to simpler forms by the introduc- tion of values of E s and /.,. Selecting in the table the division corresponding to the modulus of elasticity of the concrete which is to be used, and the line opposite the assumed stress in the concrete, M = quantity in column a times the square of the depth of beam, d; and the area of steel in a beam of 12 inch width, i.e. 12 a, equals quantity in column b times the depth 400 CEMENT AND CONCRETE m aj -— r— I — ~< > -r s-l — — 02 o3 CO pq o — 0) ^ H O r Z 4-= — r r*v efl ^ ~ a ~ sa 02 2 "o o u en ■* ID 02 d * fl LO X N « H ^1 O ^ CO SD OS <>) Sq. Ins. Area Equals d Times. -0 O (M -*f I - © •* © O © © i-H I-H 3.3 ® go-H 3 -*' © ■*' © i-i OS H M uo CO © a o_ "3 0Q © CO © t- t— 1 © © CN TJH. t- i-l IO Sq. Ins. Area Equals d Times. «3 N N lO fM M O © 'M iO © CO 00 OOOOHH C.2 S e 5.9 20.9 42.2 68.1 97.1 129.0 O O O O O O c-f '3 02 S £0 - 1-1 00 iO 00 I- 00 HWt-HOH 1-1 i-h CM Sq. Ins. Area Equals d Times. rfi CO © © (N © (M l-i Ttl OS ■* © © © © © ^h cm m r; m a) ® cS S CH e fl 1 P-> 02 -° ©^ CD ° --^ O o Esq 03 3 STRENGTH OF BEAMS 401 of beam, d. Column c gives the area of cross-section of steel expressed as the per cent, of the area of section above the center of steel reinforcement. 595. For example, suppose we wish to know the strength of a beam ten inches deep (d = h — i = 10 in.) and the amount of steel required to develop a stress in the concrete of 400 lbs. per square inch when the stress in steel is 10,000 lbs. per sq. in., and the modulus of elasticity of the concrete is assumed at 3,000,003. In column a under 3,000,000 modulus, and opposite 400 lbs. stress, we find 68.1 ; then the moment of resistance of a beam one inch wide is 68.1 inch-lbs. X 10 X 10 = 6,810 inch- lbs., and the resistance of a beam 12 inches wide is 6,810 foot- lbs. The area of steel required in 12 inches width of beam is .092 d or 0.92 sq. in. This beam is reinforced with .77 of one per cent, steel. Similar tables may be prepared for other values of ,?, and / s if desired. 596. In Table 161 the equations have been completely solved for certain typical values of E c and /,., assuming the values for E s and / s of thirty million and ten thousand respec- tively, as in Table 160. Having computed the bending mo- ment, and fixed upon the probable safe working stress and modulus of elasticity of the concrete which it is proposed to use, it is only necessary to take from the table the required depth of beam and the amount of steel reinforcement required. For example, a girder 10 feet long supported at the ends carries two loads of 5,000 pounds, each load being 2.5 feet from a support. If the width of girder is 15 inches, working stress of concrete 300 lbs. per sq. in. and modulus of elasticity of concrete 1,500,000, what is the required depth of girder and area of steel in tension side? The maximum bending moment (neglecting weight of beam) is 12,500 ft, -lbs. throughout the central five feet. The required 12 moment of resistance for twelve inches in width is — of 12,500 = 10,000 ft.-lbs. Looking in the table for this bending moment under 300 lbs. stress and 1,500,000 modulus, we find it is be- tween d = 12 and d = 14, or at about d = 13 inches. If we allow 2 inches below center of steel reinforcement, we have total depth of beam, h = 13 -f- 2 = 15 inches. In the same 402 CEMENT AND CONCRETE Jo i ~ £ C = d ~ "■' , | !-C ". a; 4> UJ W) tt-H — O - .3 3 i ^ P CJ o -^ ■P O GO o oj CO . £ II o & HNW^OOMlOSOfl^OOOHf^OfflO h- 1 pHrHHHHCNINIMCOCOCQ j (J iOO»OOtOO»00>OOOOOOiOOiOO>00 o o o CO +3 CO cr CO i— < co ■* co t- ~. c cm cc >-© 30 i— i -* i"- i— i o o o ca> -* ^HHHHIM(N(MM03'*'*^i3 M o o W -t-3 no OOOOiOCCt-OOOOOOOOOOOOOOO H O M W M O O H o: N W O M H 3 1-- O O O .' O K H-*Oit>t-OicofflN*ej"ir^Oi>'*OMCsco H (M M "C - XOLOHN'Ot-lNCSt-NO p o HH!MINO:-*CDt-OHTf CO K a § h ^H T-H . - - N-*OQOOlN'*!OCOONOO«ONOO'*OeO(M 'V o< H N IO ■* CO N X OJ O -0 HH(NCSWOOt-OH 8 (=1 1-1 _• ■*aMt-HU30'#»(MHcSt>50COH'*®aSH a o o o o co do CO H'M^L.Ot^*CH«-*^C. ^LOffiTttOONCDH HHrtHrtHIMNINnM^^iO ^ h o +s •Xi O O H ■* »3 Tf( r- ICcOOOOOOOOOOOOO a ,Q CS C5 C4 00 N C O CO "M CO CO O -t" CC CO -CM t- CO C O 1^ M CO O -t O X W C C. "M -t CO C O O H H 0C M H CM OJ •* •£ » C ■+ ~. 'I CM C-T t^ CM CI t- 00 O I— IH(MM-*U3t>OOOCM S ta 1-1 rH ^ g H H H CM CM CO CO CO -t -t< lO CO N N a O H O) CO uO M i-l a o o d o CO O 1 CO HCMK)'*'.'5cOt-C»CftOCM-tcOOOHia»H'^t- H H H H rt CN CM CM CO CO CO ^ COMSMiOa^O-OOOOOOOOOOOO o N O M H ~. CM H 'C! Tt 00 H C _ C CO 'O CO: CM UO -rf o l-jl CO O CM 00 N l^ CC H iO G CO •* O -* O (M CM uO CM (M rHrHCNOO^CCt-O-tlOlTtlCOeiCJlOOOCNOO HHHCMCO'ltiOOOOCB S h jj OMNO'O'+MCMHOMO-f'NOCCDMCt-'* o d o o CO CO CO H CO OH!M(»-CS!N HHHHriCMCMCMCMCO M o ■M a-' acoH^iaoia^wooooooooooo lO O uO 00 CO H "M O CO CO CO O CC -f CO Cc i— l _. -t i-O 05 tf ►7 CM O O CO CO H H CM ^ CO C i.C C. •+ CM _ H CO W H H CM CO ■* l-O CO CS CM CO C X N N CO O CO o i-Hi-HCMCMCO^LC;ir~00 CO K s h fl - OCOOOCMOiOHX^t-OMi-OiOifcOCMHH n d o o d CO co 6< CO O H rt CM CO CO i< lO O CO t- C5 O H CO i-O N C H CO rHrHr-Hi-li-Hi-HCMCNJ +-> COHCJCOCSIM-WTfOOOOOOOOOOO o w •*CCM CCCSr- 1 CO iO CO N t- id H O -t X N CO t)( cm o hj! H'*t-HI>MOOOI'COM'N^O'*t^Osa» CM H HH(NCOCOt1IOO«>OhNt|I(NHH HHCMCMCO-^iOcD H h q i-OC3i"*00C0t^CMt COO-fCOCMCOOTtlNHO CO K a • P, X . o S 2! d o o d CO CO CO H CO O C H H CM CM CO CO ^ i< O CO t- X CC H CM CO LO CO ^ ,_, ^H ,_, ^H w - •f"^cocMXHcsioiocoooooooco B CO CO O ■* LC CM I-- X CO H H X CO 'C ^ ■* co a -t o J hJ H CO C X CM CO >h L )< a CO N C O CO X CO H 'CM HHCMCMCO'ClCOXHOO^Ot 1< i-H i— 1 ^h CM CO 00 ■* CI H te 6 0) 1" o o "J -M H i 1 t n i l h i i + ■ Area m Sl-et /, a a i AreaStee/, A' Area Steel A-ha Fig. 14. Fig. 15. Fig. 16. CROSS-SECTION CROSS-SECTION STRAIN DIAGRAM. (Single Reinforcement.) (Double Reinforcement.) Let Fig. 14 represent the cross-section of a beam reinforced on the tension side with sufficient steel, area a, to develop the proper working stresses in the materials, and let the position of the neutral axis be N N. If at distance x from the neutral axis we add an area of steel A' in the compression side, the position of the neutral axis would be changed for similar load- ing; but if at the same time we place in the tension side an ad- A T ditional area of steel A such that —f = — a y 2 1 the position of the neutral axis will be unchanged. Let // = stress in steel in compression; then since the steel must suffer the same deforma- Multiplying the last tion as the surrounding concrete ji ~ ~ Is & two equations, we have, / s A = j\ A' , that is, we have added equal forces to the two sides of the beam, and have increased the moment of resistance by f s A (x + ?/ 2 ) inch-pounds. 599. To illustrate the application of this principle we may take the beam considered in § 591, in which 2 = 8, R — 20, .444 _ „ „ 20 8 .055, / s = 80,000, y. 3 inches, y 1 = — in., and M = 311,100 inch-pounds. r = 40, az — .444, a 10 "3~ When the area of reinforcement in the tension side of this beam was increased to az = 3.12 sq. in. or a = .39, the theo- retical bending moment was increased to 522,000 inch-pounds (§ 592). What will be the result of a similar increase in steel distributed between the two sides of the beam? A'z = 1.92 Az = .76 az = .44 DOUBLE REINFORCEMENT 405 Let k — distance from top of beam to center of reinforce- ment on compression side = 2 inches, 10 4 then x = y, — 2" = —. — 2 = - inches, ^ = ^=^-^ = 0.4 or A = 0.4 A'. A y.i 3 6 A + A' = .39 - .055 = .335 1.4 A' = .335 A' =.24 A = .095 whence a = .055 A + a = .150 Total steel, 3.12 sq. inches. // = t^L _ .4 ; = 32,000. Added moment of resistance equals ^2 (* 4- y 2 ) / s = .095 x 8 x ^x 80,000 = 4S6.400 in.-lbs. And total moment of resistance equals 311,100 + 486,400 = 797,500 inch-pounds. ' None of the bars in the series mentioned in §591 had as large an area of reinforcement as 1.92 sq. in. on the compression side. It is noticed, first, that the double reinforcement gives bet- ter results than such excessive reinforcement on the tension side; second, that the stress in steel on the compression side is less per square inch than that in tension; and third, that in case a large addition of steel is made, this results in a greater area of steel in compression than the total area of steel in ten- sion. In practice the area of steel in compression is usually made equal to, or less than, the area in tension, but beams with double reinforcement are seldom accurately designed. Art. 71. Shear in Concrete-Steel Beams 600. There are several methods of failure of concrete-steel beams other than those considered above, direct tension in the steel or direct compression in the concrete due to the bending mo- ment. These other methods of failure are popularly called failures in shear, although some of them cannot properly be so classed. 601. We have seen that the shearing stress of concrete is usually considered to be somewhat in excess of the tensile 406 CEMENT AND CONCRETE strength (§458) and that the latter is one-fifth to one-tenth the compressive strength. With a beam having only a normal amount of reinforcement, then, there is little danger to be feared from simple vertical shear, and as a matter of fact, tests have not developed instances of such weakness. In compara- tively short spans, however, failures have occurred near the quarter points, in cracks starting at the under side of the beam and extending upward in a direction inclined toward the center. This method of failure has the appearance of being due to a combination of shear and tension in the lower section of the beam, since the cracks are approximately at right angles to the theoretical "lines of direct tension." Such failures, however, are almost always accompanied by a slipping of the steel bar in the concrete, and may frequently be prevented by taking proper precautions against such slipping. 602. A more frequent cause of failure is a longitudinal shear in the plane near the steel reinforcement and on that side of it lying nearer the concave side of the beam. It is evident that a failure caused by slipping of the bar in the beam, although caused primarily by shearing forces, is really a failure in adhesion, yet the two forms of weakness are so closely connected that it is simpler to consider them together. 603. Comparison with Plate Girder. — In a steel plate girder the lower flange is considered to carry the tension, the upper flange the compression; the web connects the two flanges, caus- ing them to act together as one beam, and we may think of the web as preventing the ends of the compression flange sliding beyond the ends of the tension flange. When the web is not able to accomplish this without buckling, it is stiffened by vertical angles. In a concrete steel beam we have considered the entire tension to be carried by the steel reinforcement, and the entire compression to be carried by the concrete on the other side of the neutral axis. The connecting web is also concrete. This web is thick and not liable to buckle, but it may shear in a lon- gitudinal plane as a wooden beam may do when short and deep. All of the tension in the steel reinforcement must be trans- mitted through the surrounding concrete. If there are no pro- jections on the steel bar, the adhesion of the concrete to it may, under certain circumstances, be not strong enough to SHEAR IN BEAMS 407 safely carry this stress; and if the adhesion is sufficient, then the shearing strength of the concrete may be too low to transmit the stress to contiguous fibers or layers. 604. Illustration. — Let us consider a concrete-steel beam twelve inches wide, twelve inches deep and of ten foot span, supported at the ends; reinforcement, one square inch of metal properly distributed in a plane two inches above the bottom of the beam. Let us suppose this beam carries a uniform load of 600 pounds per foot, giving a maximum bending moment of 90,000 inch-lbs., and a stress in steel of 10,000 pounds at the center. The ends of the steel bars are of course without stress. Since the bending moment at any section of such a beam is proportional to the product of the segments into which the section divides the span, the bending moment one foot from the ends will be =-^-= X 90,000 = 32,400 inch-pounds. 5x5 F Let us consider the neutral axis in the same position at the end of the beams as near the center. (This is not strictly true, because of the lighter stress near the ends of the beam, but the error made b}' such an assumption will be unimportant for our present purpose.) Then the tension in the steel will have the same proportion, or, tension in steel one foot from the end = <|V X 10,000 = 3,600 pounds. The stress in steel, then, which is zero at the end, has in- creased to 3,600 lbs. in one foot of length. To provide against poor contact near the end, consider two-thirds of this length, or eight inches, to be operative. If the reinforcement consists of four one-half-inch square bars, the necessary adhesion per square inch is — — — — = 57 lbs. per sq. in.; but if only one bar o X o is used one inch square, the required adhesion is 114 lbs. per sq. in. The latter would not be good practice, not only be- cause of high adhesion required, but because the steel is not properly distributed. Where the stress in adhesion is greater than can be safely relied upon for plain rods, it is necessary to use some kind of deformed bar, or to anchor the bar securely at the end. This may be done by passing the end of the tension bar around a 408 CEMENT AND CONCRETE rod transverse to the beam near the end. Care should be taken that the safe value of adhesion is not assumed too high. 605. Value of Shear. — The same total stress of 3,600 lbs. must be transferred through the concrete immediately above the bar. If the reinforcement is so distributed that the entire width of the beam has practically the same stress, and we consider, as before, that two-thirds of the length of the end . . . n , 3,600 _„ _ .. loot is operative, we have mean shear = — = 37.5 lbs. 1 12 X 8 per sq. in. The value of stress in shear should not exceed one- tenth the safe value in compression, and there is a general ten- dency to use not more than one-twentieth. If the same form of beam had a span of but five feet with same bending moment, the value of the shearing stress by this method becomes 75 lbs. per sq. in., and it will be necessary to provide against this stress coining upon the concrete. Another approximate method is the ordinary one for rect- angular beams, viz. to consider the shear in horizontal plane just above the steel reinforcement to be § times the total shear at any section, divided by the area of vertical section of the beam. 606. Provision is sometimes made for relieving the concrete of all shearing stresses. In this case the beam is divided into imaginary panels of length equal, say, to the depth of the beam, and the diagram of maximum shear is drawn. The shear in each imaginary panel is then provided for by a vertical or inclined bar of the proper dimensions. Or, what is usually better, the shear bars are all of one size and the proper num- ber of them are distributed throughout each panel length; the spacing of the shear bars thus becomes wider near the center of the beam. 607. Resistance to Shear. — When provision against shear is made by using small steel rods placed either vertical or in- clined downward toward the center of the beam, as mentioned above, these rods may well be made in the form of inverted U-shaped stirrups, with their ends securely fastened to the reinforcing metal in tension. In many cases all the provision necessary is given by the use of two longitudinal bars, parallel and close together near the cen- ter of the span, but one of them leading to a plane near the top of SHEAR IN BEAMS 409 the beam at the supports. This system is very conveniently applied in concrete slabs supported by I-beams, one bar of the pair being hooked over the upper flanges of the I-beam and sagging toward the center. The Hennebique system (§571) is a combination of the inclined bar and U-shaped stirrups. 608. A modification of the single inclined bar is the Cum- mings system, wherein there are several pairs of bars of vary- ing lengths; these are all horizontal and near the bottom along the center of the beam; a short distance from the center the shortest pair turns up at an angle of about forty-five degrees ; a littVe farther toward the end a second pair of bars is turned up, and so on, leaving a single pair to go through straight to the support. Another, and more radical modification, is the Kahn system (§573), in which the bar is square with wings of metal on oppo- site corners which are sheared and bent up at angles of forty- five degrees, so that the outline of the steel work in a beam resembles the tension members of a Pratt truss. CHAPTER XX SPECIAL USES OF CONCRETE: BUILDINGS, WALKS, FLOORS AND PAVEMENTS Art. 72. Buildings - 609. While the use of concrete and steel for the walls and floors of buildings is about fifty years old, yet it is only in com- paratively recent years that its value has become generally known. It is now applied to all classes of structures, ware- houses, factories, residences, station and office buildings, and it is anticipated that in the next twenty years concrete-steel will be as familiar in architecture as steel skeleton, stone, and brick are now. ,- 610. It happens that at present the concrete-steel building- industry is largely in the hands of companies who are exploiting some particular form of steel rods or bars applied according to some one of the many "systems" of reinforcement. This con- dition has both good and bad features. A reputable concern of this kind will have in their employ engineers who should sat- isfy themselves that each design is a safe one, for the failure of a building will cast disrepute on their particular system. It is this fact that leads the companies to keep the construction entirely, and the design largely, in their own hands. Another advantage is that these concerns are able to perfect methods of construction by experience, and to lessen the expense of one structure by making use of the concrete plant and the molds that have been used on another. ,- 611. In making plans for a building, the owner is usually represented in the first instance by an architect whose business it is to dictate the design. If concrete-steel is considered, the architect may call an engineer in consultation and they may together harmonize the features of utility and appearance with economy and strength, but in letting the contract it is found that the competition is limited to one or two companies using the particular system which the engineer considers the best adapted to the particular conditions in question. 410 BUILDINGS 411 On the other hand, the architect will hesitate to go to the con- struction company for assistance, since he must first select the system he shall use, a question upon which his ideas may be neither clear nor well grounded, and he is then having the prospective contractor assist in the design. Under these circumstances the architect will usually consider concrete-steel construction as something he wishes to avoid if possible. But this condition will correct itself in time, for owners will demand a considera- tion of this form of construction, engineers will become fa- miliar with its use and will be employed to design the engineer- ing features, while reliable contractors in every city will obtain permission to build in accordance with any "system" under the supervision of a competent engineer. 612. Roof. — While a pitch roof is sometimes built of con- crete-steel, this form of construction is particularly adapted to so called flat roofs. The roof is constructed much the same as a floor slab (Art. 65-67), except that expansion joints are some- times provided, and the roof is covered with tar and gravel, or some of the patent roofings ordinarily used. While the roof loads are usually light, permitting a greater span of slab between beams than for floor construction, it will seldom be economical to introduce these longer spans because of the changes necessary in the molds. In most buildings it is neces- sary to provide against condensation, and for this purpose a flat ceiling may be suspended at the level of the under side of the beams giving an air space. 613. Floor System. — The floors may be constructed in con- formity with the principles stated in Chapter XIX. The strength of short span arches, such as are used for floors, where the haunches are built up level with the top of crown of arch, is a matter of experiment and cannot be accurately determined theoretically. Empirical formulas may be derived for a certain system based on a sufficient number of tests. The principles underlying the strength of slabs may be con- sidered the same as those applying to beams (Art. 69), although if the length of slabs is not much greater than the span, they are not strictly applicable, but will err on the safe side. 614. A decision must first be made as to the size of bays into which the floor space is to be divided. This will of course de- pend on the use of the building, the engineering features con- 412 CEMENT AND CONCRETE forming to requirements of utility. If the bays are not square, the girders should usually take the shorter span between columns. This length is then divided into the number of slab spans that will give maximum economy. The shorter these spans the less the amount of material required in slabs and the greater the number and cost of floor beams. Computations should be made, therefore, for two or three arrangements to determine this point. As this distribution for maximum economy will vary with the loads to be provided for, it is well, if the floors are not all to carry the same load, to take for this computation a load intermediate between the heaviest and lightest, and use if possible the same arrangement of spans throughout the building. The strength of slabs for given bending moments may be taken directly from Table 161, after deciding upon the working- stress to be allowed in the concrete and the probable modulus of elasticity. The beams and girders, if single reinforcement is used, are taken from the same table or computed by the methods of Art. 69. 615. In some instances it may be found economical to use concrete-steel slabs for floors supported by concrete protected steel beams and girders. One advantage of this system is that the forms for building the protecting concrete and for the floor slabs may be hung from the steel girders and beams. For this method of construction the enveloping concrete should not be less than one and one-half inches thick over the edges of flanges, and wire fabric or metal lath wrapped about lower flanges of beams will insure the concrete remaining in place. This is not properly concrete-steel construction, but simply concrete protected steel, and except in case the concrete ex- tends well above the steel, forming an independent compres- sion flange, no added strength should be computed for the con- crete covering. 616. Columns. — In the foundations of buildings of moder- ate height the supporting columns may be built entirely of concrete. Since, however, the pressure on the concrete, even when it is constructed with the greatest care, should not ex- ceed two hundred to three hundred pounds per square inch, the required area of cross-section in the lower stories is usually so great as to preclude the use of columns built entirely of concrete. BUILDINGS 413 617. Concrete Filling and Covering. — A steel column of any of the ordinary styles, built up of steel shapes may be used, and protected from corrosion and fire by filling and cover- ing with concrete. This not only serves as a protection against rust, but materially increases the stiffness and permits the use of a somewhat higher working stress in the steel. The concrete filling should be mixed quite wet in order that it shall work into all angles. The edges of the metal should not approach nearer than one and one-half inches to the exterior of the con- crete, and flat surfaces of metal should have a covering of at least two and one-half inches. Where it is necessary to cover large, flat surfaces, they should be first covered with expanded metal or wire fabric, locked on by twisting around the edges of the plate or channel. 618. Columns of Concrete Steel. — Concrete-steel col- umns differ from the above in that the main dependence is placed on the concrete rather than on the steel. For such columns longitudinal reinforcement has generally been employed. Steel bars extending from end to end of the column are dis- tributed throughout the cross-section, and are tied together at intervals of four to twelve inches by smaller bars forming loops to hold them in place. The splicing of the bars is effected by placing a small tube over the upper end of the lower bar and projecting above it, and then setting the lower end of the upper bar within the tube resting on the lower bar. Where this is done it is essential that the two ends be planed perfectly square, and it is much better to avoid splices in a column between lateral supports. In a building the reinforcing rods project up through the floor above and are spliced into the bars of the columns in the next story. 619. Strength of Columns. — When a column reinforced with longitudinal bars is subjected to pressure, the concrete and steel must shorten together. The relative stresses in the two materials will then be proportional to their moduli of elasticity. From this follows the formula, P=f c (C +RS) where P = total pressure on column, f c = stress in concrete, C and S = areas of concrete and steel respectively, 414 CEMENT AND CONCRETE E . . and R = jt, or ratio of the modulus of elasticity of the steel to that of the concrete. In a series of tests of twenty-one columns made by Prof. Gaetano Lanza, 1 but three failed under a lower stress than that computed by the above formula. The columns were eight to ten inches square, six to seventeen feet long and reinforced with either one or four bars, the latter being from f inch to l\ inches square. The lowest breaking load was fifty tons on an 8 by 8 inch column with one bar one inch square, and the strongest column, 10 by 10 inches, with four f inch longitudinal bars, was not crushed with a load of one hundred fifty tons, the limit of the testing machine. The lowest result was twenty per cent, less than that given by the formula, and the greatest excess strength over the theoretical was fifty per cent. 620. While longitudinal reinforcement undoubtedly strength- ens a long column against flexure, as well as adds to the resist- ance to crushing, yet the added strength is gained at the ex- pense of considerable additional cost. Suppose we have a ten inch square column, twelve feet long, made of concrete with a breaking load of 1,800 lbs. per square inch, or 180,000 lbs. total breaking load. Suppose eight f inch square bars to be built into this column as longitudinal reinforcement, and that the modulus of elasticity of the steel is ten times that of the concrete. Then the strength of the reinforced column would be, by the formula above, P = 1,800 (95.5 + (10 X 4.5)) = 252,900. The longitudinal reinforcement has thus resulted in an increase of strength of 40 per cent., while by the addition of 180 pounds of metal, the cost of the column has risen from about $3.00 to say $8.50, an increase of about 180 per cent, without counting the cost of lateral ties, and the additional trouble in building a reinforced column. 621. Hooped Concrete. — In extended experiments on what he has called "hooped concrete," M. Considere 2 has shown that 1 Trans. A. S. C. E., Vol. 1, p. 487. 2 Comptes Rendus de I'Academie des Sciences, 1898-1902. Translation, "Reinforced Concrete," by Armand Considere, translated by Leon S. Mois- seiff, McGraw Publishing Co., New York. BUILDINGS 415 reinforcement is much more important and beneficial in a transverse or circumferential direction than if longitudinal. This may be accounted for by the fact that the natural' method of failure of concrete prisms, is by splitting along planes parallel to the direction of pressure, and the ordinary method of failure by shear along inclined surfaces is induced by the friction of the plates transmitting the pressure to the prism. It was also shown that while concrete reinforced by longitudinal bars with the ordinary amount of lateral ties breaks suddenly, hooped concrete fails gradually under a much heavier load. 622. M. Considere concluded from his experiments that the circumferential ties should not be farther apart than one- seventh to one-tenth the diameter of the column, even when longitudinals were used to assist in completing the network, and that the results were more successful the nearer together the hoops or ties were placed. He found that spirals were better than individual single ties and that longitudinals were of value chiefly in assisting to confine the concrete, transmitting the bursting pressure at a given plane to the contiguous spirals above and below. 623. M. Considere says 1 that the "compressive resistance of a hooped member exceeds the sum of the following three ele- ments: — "1. Compressive resistance of the concrete without rein- forcing. "2. Compressive resistance of the longitudinal rods stressed to their elastic limit. "3. Compressive resistance which could have been produced by imaginary longitudinals at the elastic limit of the hooping metal, the volume of the imaginary longitudinals being taken as 2.4 times that of the hooping." To subject hooped concrete to a practical test, M. Considere constructed, in 1903, a truss bridge of sixty-five foot span with parabolic top chord of seven and one-half feet rise, 2 the com- pression members being of hooped concrete, and the tension members of concrete-steel with longitudinal reinforcement, or concrete protected steel. A central panel of the truss was con- " Reinforced Concrete," p. 159. Engineering News, May 5, 1904. 416 CEMENT AND CONCRETE strueted with a reduced section of top chord about eight inches diameter reinforced by eight longitudinal bars .43 inch in diameter and a helix 6J- inches in diameter of .43 inch metal coiled to a pitch of about one inch. This reduced top chord section showed signs of failure when the computed stress reached about 5,000 pounds per square inch. 624. FORMS FOR BUILDINGS. — One of the most serious prob- lems in the construction of concrete-steel buildings is the de- signing of the forms. They must be as light as is consistent with strength to facilitate handling. They should be of simple construction so that they may be set up and removed without too much supervision, and they should be so assembled with bolts and screws that they may be used repeatedly. In erect- ing a large building sufficient forms are usually provided to set up one floor complete, including columns, beams, girders and floor slabs. After placing the reinforcement, the concrete is filled in as rapidly as possible, making the slabs, girders and columns practically monolithic. The forms for the girders usually rest upon the column molds and are supported at intermediate points by posts rest- ing on the completed floor below. While column molds are sometimes filled from the top, better work is assured by having one side of the mold built up as the concrete is filled in from the side. The mold to receive the concrete forming the floor slab is either a part of, or is supported by, the pieces forming the sides of the girders and beams. Provision is sometimes made for leaving supports at intervals under the completed beams and girders after removing the forms from the sides of the beams and the bottom of floor slabs. This is done by making the bottom piece of the girder mold separate, and attaching the side pieces to it by screws which may be removed without dis- turbing the bottom. The caps of the supporting posts are then made long enough to permit the lower edges of the side pieces to rest directly on them. This method was adopted in build- ing the Central Felt and Paper Company's factory at Long Island City. 1 1 Wight-Easton-Townsend Company, Contractors, Engineering Record, Jan. 16, 1904. BUILDINGS 417 625. In the same building the walls were built with molds three feet high and sixteen feet long, placed in pairs on oppo- site sides of the wall. When one section was completed, the molds were "lifted until the lower edges were two inches below the top of the concrete. In the new position they were sup- ported by horizontal bolts through their lower edges, across the top of the concrete; the upper edges were tied together by transverse wooden strips nailed to them about three feet apart, and they were braced to the false work supporting the roof and column molds." "The bolts passed through sleeves which were left permanently embedded in the walls. At first, iron pipes were used for this purpose, but afterwards it was dis- covered that pasteboard tubes were equally efficient and much easier to trim and point after the molds were removed." 626. An excellent system of molds was used in the con" struction of the Kelley and Jones Company's factory at Greens- burg, Pa. 1 The floor molds were especially convenient, being made collapsible by a hinge joint at the top along the longi- tudinal center line. These floor molds were in reality cores between adjacent floor beams; when in place the top surface was horizontal, to form the under side of the floor slab, and the vertical side pieces formed the sides of the floor beams. When the concrete had set sufficiently, the lower edges of the form were made to approach each other, thus coming away from the concrete gradually. A special light wooden framework or tower, with a working platform six feet below the floor, and a rope sling to receive and lower the floor mold, permitted of removing the molds rapidly and without injury. A special truck was also used for moving the floor molds about the building. 627. A convenient adjunct for the construction of concrete wall forms consists of a short section of I-beam having a width between flanges equal to the thickness of the plank to be used. These plank holders are laid in pairs, with web horizontal, one on either side of the wall, and connected by a bolt passing through them and through the wall. 2 Two rows of planks on edge are first placed around the building so as to inclose the pro- 1 Mr. E. L. Ransome, Architect and Engineer, Engineering Record, Feb. 6 and 13, 1904. 2 Patented by Thomas G. Farrell, Washington, N. J. 418 CEMENT AND CONCRETE posed wall. At the upper side of each junction between two planks in the same horizontal row is placed one of these plank holders. Another horizontal row of planks may now be placed, with the iron plank holders at the joints as before. As the wall is built up, the lower planks and holders may be removed and placed on top, and thus few forms are required. Tees and L-forms are provided for partition walls and corners. When an air space is desired in a wall a special terra cotta tile or building block may be built into the wall, but this is quite expensive, and an interior collapsible form may be made of timber by the use of two planks held apart by a wooden brace which may be knocked out. Special means of handling the interior plank should" be provided, and the building of a high wall cannot be continuous with this method. 628. New York Building Regulations. — While city building regulations are not always criteria of good practice, yet the Regulations of the Bureau of Buildings of the Borough of Manhattan concerning the use of concrete-steel construction are exceptional. Emanating from a bureau that has been dis- tinctly hostile to concrete-steel, they are naturally conservative, but are, on the whole, excellent, and work conscientiously done in accordance with them will not bring discredit on concrete construction. It is specified that the cement shall be only high grade Portland standing certain tests, that the sand shall be clean and sharp, aggregate, broken trap, or gravel of a size that will pass a three-quarter inch ring, and that the proportions used shall be one cement, two sand and four of stone or gravel, or that the concrete shall have a crushing strength of two thou- sand pounds per square inch in twenty-eight days. Only the best quality of concrete is thus permitted. 629. The Regulations concerning the design are then stated as follows : — "Concrete-steel shall be so designed that the stresses in the concrete and the steel shall not exceed the following limits: — Extreme fiber stress on concrete in compression, 500 lbs Shearing stress in concrete 50 Concrete in direct compression 350 Tensile stress in steel 16,000 Shearing stress in steel 10,000 per sq. in. BUILDINGS 419 "The adhesion of concrete to steel shall be assumed to be not greater than the shearing strength of the concrete. "The ratio of the moduli of elasticity of concrete and steel shall be taken as one to twelve. "The following assumption shall guide in the determination of the bending-moments due to the external forces: Beams and girders shall be considered as simply supported at the ends, no allowance being made for the continuous construction over supports. Floor plates when constructed continuous and when provided with reinforcement at top of plate over the supports, may be treated as continuous beams, the bending-moment for- W L uniformly distributed loads being taken at not less than -— — ; W L the bending-moment may be taken as — — in the case of square floor plates which are reinforced in both directions and sup- ported on all sides. The floor plate to the extent of not more than ten times the width of any beam or girder may be taken as part of that beam or girder in computing its moment of resistance. "The moment of resistance of any concrete-steel construc- tion under transverse loads shall be determined by formulas based on the following assumptions: — "(a) The bond between the concrete and steel is sufficient to make the two materials act together as a homogeneous solid. "(6) The strain in any fiber is directly proportionate to the distance of that fiber from the neutral axis. "(c) The modulus of elasticity of the concrete remains con- stant within the limits of the working stresses fixed in these Regulations. "From these assumptions it follows that the stress in any fiber is directly proportionate to the distance of that fiber from the neutral axis. "The tensile strength of the concrete shall not be considered. "When the shearing stresses developed in any part of a construction exceed the safe working strength of concrete, as fixed in these Regulations, a sufficient amount of steel shall be introduced in such a position that the deficiency in the resist- ance to shear is overcome. "When the safe limit of adhesion between the concrete and 420 CEMENT AND CONCRETE steel is exceeded, some provision must be made for transmitting the strength of the steel to the concrete. "Concrete-steel may be used for columns in which the ratio of length to least side or diameter does not exceed twelve. The reinforcing rods must be tied together at intervals of not more than the least side or diameter of the column. "The contractor must be prepared to make load tests on any portion of a concrete-steel construction, within a reasonable time after erection, as often as may be required by the Super- intendent of Buildings. The tests must show that the con- struction will sustain a load of three times that for which it is designed, without any sign of failure." Art. 73. Concrete Walks 630. One of the most important uses of concrete is in the construction of street and park walks. It has not only driven stone flagging almost out of use, but it is being employed to a large extent in towns and villages where board walks have formerly been used almost exclusively. A concrete walk is made up of a sub-base or foundation, a base, and a wearing surface. 631. Foundation. — As in other structures, one of the most important essentials for success lies in the preparation of the foundation, and the care that must be bestowed on it will depend upon the character of the soil and the climate. In the higher latitudes of the United States, frost may soon destroy a walk the foundation of which is not well drained. The excavation should be made to the sub-grade previously determined upon, any objectionable material such as loam or organic matter being removed, and the bottom of the excava- tion smoothed and well rammed. Upon this is laid the sub- base, its thickness varying from nothing to twelve inches. In a sandy soil with good natural drainage and little danger from frost, and where light traffic is expected, it may be unnecessary to provide any special sub-base, since the soil itself furnishes a good foundation for the concrete, but in clay soil in northern climates, twelve inches of sub-base may be required. The best material for this sub-base is broken stone varying in size from one-half inch to two and one-half inches. Usually broken stone is considered too expensive, and gravel, coarse sand, CONCRETE WALKS 421 cinders, or broken brick is employed. A layer four inches thick is usually sufficient for good materials, but six to twelve inches of cinders are sometimes required. It should be well rammed to a level surface, and when completed should be firm but porous. The most important point is that this course shall have good drainage, otherwise it may be a menace to the walk. If it is more porous than the retaining soil, it will naturally drain this soil, and if the water is not able to escape into the sewer or elsewhere, it may be frozen and heave the walk. An ex- cellent plan sometimes adopted is to lay at intervals of twenty to twenty-five feet, a blind stone drain from the walk founda- tion to the foundation of the curb. In exceptional cases it may be necessary to lay a tile drain in the sub-base to lead the water away from the walk. 632. Base. — The base is the body of the walk giving stiff- ness to the structure. Its functions are to furnish a solid foundation for the wearing surface and to give transverse strength to the walk, transmitting the pressure uniformly to the sub-base. The base is of concrete, which need not be very rich for ordinary traffic. A proportion of one part packed Portland cement to two and one-half volumes of dry sand and six volumes broken stone is excellent, and proportions of one, three and seven parts cement, sand and stone, respectively, will usually be found sufficient, though the richer the concrete in the base the better will the top dressing adhere to it. The broken stone for this concrete should be of a size not exceeding one and one-half inches in any dimension, some cities requiring three-quarters inch or less. Crushed granite and trap are excellent, though limestone or any other moderately hard rock may be used that is suited to making concrete for ordinary purposes. If of a hard rock, the screenings may well be left in the broken stone, and when this is done, the dose of sand should be diminished. (See Art. 37.) The thickness of the layer of concrete should not be less than three inches. Four inches is much better and is recom- mended for general use in sidewalks, while in exceptional cases six inches is required. The top of the concrete base should be finished to a plane parallel to the proposed surface of the walk and at a distance below it equal to the proposed thickness of the top dressing. 422 CEMENT AND CONCRETE 633. Wearing Surface. — The preparation and application of the wearing surface require much care if satisfactory results are to be obtained. The most evident service of this layer is to withstand wear, and it should therefore be made of rich Portland cement mortar. With a sand consisting principally of quartz particles, it is found that a mortar composed of equal parts cement and sand gives about the best results in tests of abrasion. If the mortar is used richer than this, it is likely to check or crackle in setting, marring the appearance of the walk. Mortar containing two parts cement to three parts sand gives nearly as good results, and two parts sand or fine crushed granite to one of Portland cement is usually satisfactory. The sand for the mortar should be quartz if possible, or crushed granite or trap. It should be screened through a quarter inch mesh, and there should not be a large proportion of very fine particles. The thickness of the layer of top dressing is usually about one inch, and this is probably the maximum thickness ever required. One-half inch of top dressing is believed to be suf- ficient when the wear is not excessive, provided the base has been carefully leveled. 634. The Construction of the Walk. — If the walk has not a considerable longitudinal slope, it should be given a transverse slope of about a quarter inch to the foot to provide for draining the surface. Stakes for grade and line having been given, a maitre cord is stretched along the line stakes to mark the sides of the exca- vation. After the material has been excavated to the proper sub-grade and all soft material in the bottom removed, the bottom of the trench is well rammed. If tile drain is necessary, it is laid with open joints on this foundation. The material to form the sub-base is now wheeled in and rammed to the proper thickness, water being used freely if it facilitates the packing. The top of the sub-base is brought to a level plane at the proper distance below the grade stakes. The molds for the walk are now to be laid. These are made of two by four or two by six inch scantling, sized and dressed on at least one side and one edge. Stakes are first securely driven, about five or six feet apart, with their faces two inches back from the side lines of the proposed walk, and their tops CONCRETE WALKS 423 at grade. Against these stakes the scantlings are placed on edge with dressed side toward the walk, and smooth edge level with the grade stakes. These molds are held in place by nail- ing through the supporting stakes into the scantling, and if these nails are not driven "home," they may easily be pulled to release the mold when the work is completed. On the upper edges of the mold are then marked off the sizes of blocks de- sired, being careful that the marks defining a joint are exactly opposite each other on the two scantlings. 635. The concrete materials having been previously deliv- ered near the work, the concrete is mixed, either by hand or machine, according to the methods already given, and rammed in place after the sub-base has been well wet down to receive the concrete. The concrete should be just short of quaking, and in ramming care must be taken not to disturb the molds. For tamping next the molds, the makers of cement working tools offer a light rammer with square face at one end and blunt, chisel shaped tamper at the other. The surface of the base is brought to a plane parallel to the proposed finished sur- face of the walk, and at a distance below it equal to the thick- ness of the top dressing. A straight edge, long enough to span the walk and notched out at the ends so that when placed on the molds the straight edge will define the correct grade of the base, is a convenience here. 636. The concrete is now cut into blocks exactly corre- sponding to the proposed blocks in the top dressing. For this purpose a straight edge is laid across the walk in line with marks previously made on the molds to define the joints, and with a spade or special tool the concrete base is cut entirely through to the sub-base. This division is necessary to allow for expansion and contraction, and prevent cracks in the top dressing elsewhere than at the joints. This joint in the base should then be filled with clean sand. If preferred, these joints in the base may be made by placing thin steel strips across the molds to be removed after the concrete for the next block is in place. The end block made from a given batch of concrete should be limited by a cross mold set exactly on line of a proposed joint. When the base is continued, this cross mold is removed. A part of a block should never be molded and then built on after having stood long enough to begin to set. Any concrete 424 CEMENT AND CONCRETE left over from finishing a block should either be mixed in with the next batch, if this is to follow in a very short time, or it should be wasted. A disregard of this rule will probably result in a crack in the top dressing above the line of division between adjacent batches. 637. When a block of base is finished, the top dressing or wearing surface should be applied immediately. The lack of adhesion between the base and wearing surface is one of the most frequent causes of failure in cement walks. The mortar should not merely be laid on in a thick layer and then struck off to grade, but it should be worked and beaten into close con- tact with the concrete at every point. The mortar should be tamped with a light rammer and beaten with a wooden batten, and to accomplish this properly the mortar must not be very wet. The surface is then to be struck off with a straight edge bearing on the top of the mold planks. Some hollows or rough places will remain, and the straight edge should be run over a second or perhaps a third time, a small amount of rather moist mortar, made from thoroughly screened sand, having been first applied to such places. When the surface film of water is being absorbed, the surface is worked with a wooden float. The exact time when the work should be floated will soon be known by experience. After the floating is completed, the trowel may be used to give a smoother surface, but this makes the walk so slippery that it is not usually desirable. 638. If the top dressing is worked too long, the cement is brought to the surface, robbing the next lower layer of its ce- ment and resulting in scaling. The top dressing is now cut entirely through on exact line above the joints in the base. This may be done by a trowel working against a straight edge, but special tools are made for cutting through the mortar and rounding the edges of the joint at one operation. A quarter- round tool is also run along the edges of the mold to give a neat finish. When desired, an imprint roller run over the walk gives it the appearance of having been bushhammered. It is important that the top dressing be applied before the concrete has begun to set, and it must" not be applied to a por- tion of a block and then some time allowed to elapse before applying the remainder. The edge of the top dressing must CONCRETE WALKS 425 be cut off squarely at the end of the block. If desired, the wearing surface may be colored by the use of lamp black in the mortar, giving a uniform gray color to the walk. (§ 535.) 639. When the walk is completed, it should be fenced off so that animals may not walk over it while still fresh, and it should be protected from a hot sun. The surface should be kept moist, and this may be done after the first twenty-four hours by spreading a layer of damp sand over the walk and wetting the sand with a rose nozzle as often as may be needed. The walk may be opened to light travel after about four days, but it is better to remain covered with the damp sand for a week. 640. Cost of Concrete Walk. — The cost of concrete walks varies from ten cents to twenty-five cents per square foot. A fair price for a walk of average quality where there are no special difficulties is twelve to eighteen cents per square foot. As an instance of a walk built with special care, the one constructed about the top of the bank of the Forbes Hill Reser- voir may be mentioned. 1 , The sub-base of this walk was of stone and twelve inches thick, the layer of concrete- was five inches thick at the center of the walk and four inches at the sides. The top was of granolithic finish one inch in thickness. The walk was laid in separate blocks about six feet square. The average gang employed on the concrete consisted of six men and one team, while the finishing was clone by two masons and one tender. The amount laid per day was about forty square yards. The cost per square yard was as follows: — £ cu. yd. stone in foundation or sub-base, at $.40 per cu. yd. . $0,133 Labor, placing stone at $1.50 per day 502 Total cost stone foundation per sq. yd. of walk . . . $0,635 .158 bbl. cement, at $1.53 per bbl. . . $0,242 .065 cu. yd. sand, at $1.02 per cu. yd 063 .109 cu. yd. stone, at $1.57 per cu. yd 170 Labor, mixing and placing concrete 450 Total cost concrete base per sq. yd $0,928 .11 bbl. cement, at $1.53 pei bbl $0,108 .022 cu. yd. sand, at $1.02 per cu. yd 022 Lamp black 008 Labor, preparing and finishing surface 149 Total cost top dressing or wearing surface $0,347 rp . i . n , ( Materials $0.81 .... Total cost walk per sq. yd. ) Labor _ UQ _ $1 910 1 C. M. Saville, M. Am. Soc. C. E., Engineering News, March 13, 1902. 426 CEMENT AND CONCRETE 641. The following is given as an estimate of cost of items in a walk built with six inch cinder sub-base, four inch concrete base and one inch top dressing . Cost per sq. yd. of Walk Materials Labor Preparation of foundation, excavation and ramming . . . $0.20 Sub-base, 6 in. cinders \ cu. yd., at $0.40 cu. yd $0.07 Placing and ramming cinders 0.04 I cu. yd. concrete, at $3.00 per cu. yd. for materials alone . 0.33 I cu. yd. concrete, placing, at $1.80 per cu. yd 0.20 Top dressing ^ cu. yd. mortar, at $9.00 per cu. yd. . . . 0.25 Placing top dressing and finishing walk 0.25 Superintendence and molds 0.10 Totals $0.65 $0.79 Total cost per sq. yd., $1.44, or 10 cents per sq. ft. 642. As an example of a low priced walk, the concrete walks in San Francisco * are but three inches thick, two and one-half inches of concrete composed of one part Portland cement, two parts beach gravel, and six parts of crushed rock of size not exceeding one inch; the top dressing being one-half inch thick of equal parts Portland cement and beach gravel. With ce- ment $2.50 per bbl., crushed rock and gravel from $1.40 to $1.75 per cu. yd., and wages twenty cents an hour for laborers and forty cents for finishers, this walk is constructed at from nine to ten cents per square foot. It is stated that a gang of three or four men will lay 150 to 175 square feet per day. Art. 74. Floors of Basements, Stables and Factories 643. The principles governing the laying of walks apply also in a general way to the construction of floors that rest directly on the ground. For residences, basement floors may be laid with three inch base of concrete and one-half inch wearing surface. The thick- ness of sub-base will depend upon the character of the soil. Where natural conditions do not assure good drainage of the foundation, this should always be provided for by either a blind stone or tile drain laid around the outer edge of the building and leading to the sewer or other outlet. The finished surface of the floor should always have a slight slope toward the center 1 Engineering News, March 4, 1897. FLOORS 427 or one corner of the basement, and a trapped sewer connection set at this lowest point in such a way that it is accessible for repairs and cleaning. 644. Wet Basements. — Where much ground water is en- countered, and especially where a basement is subjected to a head of water from without, special precautions must be taken in building the floor. The concrete must be made thick enough so that its weight and the arch action set up, shall be able to withstand the upward pressure of the water. In building such a floor it is necessary to keep a sump hole, preferably in the center, towards which the construction proceeds from the sides. A pipe placed in the sump hole permits pumping until the con- crete is laid about the pipe, when the latter may be filled with rich cement mortar. In such cases the side walls of the base- ment should be plastered with Portland cement mortar on the outside and special care taken in joining the floor to the wall. 645. Size of Blocks. — As the changes in temperature in a building are usually much less than in open air, the blocks of concrete may be of much larger size, say ten feet square, and many basement floors are laid without any joints, though sooner or later they will probably crack if so laid. In factories for certain purposes, however, the floors may be subjected to greater changes in temperature than walks laid in the open air. In such cases the blocks should not be more than three or four feet on a side, and the joints may well be filled with asphalt, especially if water-tightness is desired. 646. Stable floors may be made of six inch cobble or broken stone sub-base, six inches of concrete made with mortar con- taining three parts sand to one cement, and one inch of top dressing containing three parts sand (mixed sizes) or crushed granite to two parts cement. Factories having heavy machinery with much vibration re- quire strong floors. Such a floor may be made of six inches of cobble stone sub-base covered by six inches of a lean concrete made with one-to-four mortar, and above this, three to five inches of rich concrete made with mortar containing two and one-half parts sand to one cement, and one inch of top dressing, equal parts cement and sand or cement and crushed granite. 647. Example and Cost. — In the construction of the new printing building for the Government Printing Office at Wash- 428 CEMENT AND CONCRETE ington, the basement floor is nine inches thick, made as fol- lows: * — 1. Concrete sub-base, six inches thick of one part natural cement, two parts sand and four and one-half parts broken brick. 2. Concrete base, two and one-half inches thick of Portland cement one part, sand two parts and fine broken gneiss four parts. 3. Top dressing, one-half inch in thickness, of two parts sand to one part Portland cement. The cost of this floor was about $1.50 per square yard, or about seventeen cents per square foot. Art. 75. Concrete in Pavements and Driveways 648. PAVEMENT FOUNDATIONS. — The principal use of con- crete in connection with city pavements has been as a founda- tion, the wearing surface being of some other material, as brick, asphalt, cedar blocks, etc. Concrete for pavement foundations should not be less than six inches in thickness, and a greater thickness will be required where the ground is insecure. The excavation having been made to the required sub-grade, and all loose soil removed and the places refilled with broken stone, the earth is thoroughly rolled to a smooth surface parallel to the surface of the proposed pavement. Drainage for the foundation should be provided where necessary by broken stone or tile drains beneath the curb. Before beginning the placing of concrete, stakes may be driven in the foundation, with their tops at grade, at intervals of five to ten feet over the entire pavement, to assist in securing the proper grade of concrete surface. 649. The stone for the concrete should be broken so that no piece is larger than two and one-half inches in its greatest di- mension. If the stone is of good quality, it need not be screened except to remove the finest dust, if this is present in consider- able quantities. Sufficient mortar should be used to fill the voids in the stone, this mortar being composed of about two parts sand to one of natural cement, or better, two and one- half or three parts sand to one of Portland cement. This con- Report of Capt. John S, Sewall, Report Chief of Engineers, 1896. PAVEMENTS 429 crete is thoroughly rammed in place, care being "taken that adjacent batches as laid in the street mingle with each other so as to show no line of demarcation. In stopping work for the night, the concrete should cut off sharply on a straight line parallel to the direction of the proposed joints in the wear- ing surface. Joints extending across the street should be left at intervals of thirty to forty feet to allow for expansion and contraction. 650. The concrete is finished to a surface parallel with the proposed street surface, a templet being employed to secure this. The concrete should be kept damp for a few days, and . no traffic allowed upon it until the wearing surface is laid. If the wearing surface is of 'brick or wooden blocks, a layer of sand about one inch thick is first spread over the concrete. The advantages of a concrete foundation for street pave- ments are its strength and durability and water-tightness. 651. CONCRETE PAVEMENT. — Concrete has not been a popu- lar material for a street surface except for short driveways and in courts where both vehicles and pedestrians must.be accom- modated. One reason for this is that concrete is slippery, and another, that owing probably to carelessness or ignorance, the wearing qualities have not been good. The first objection may be largely removed by cutting the surface into blocks, four by eight inches, by deep grooves, or by the use of a deep imprint roller on the wearing surface. As to wearing qualities, there seems to be no good reason why a concrete cannot be made tough enough to withstand heavy traffic. It will of course be necessary to divide the work into blocks of twenty to twenty- five square feet, with expansion joints of sand, asphalt, or tarred paper between. A third objection is the glare of the surface in summer. A partial remedy for this may be had by placing some coloring matter, such as lamp black, in the top dressing. 652. The sub-base may consist of a six inch layer of broken stone, or twelve inches of cinders, well drained and thoroughly compacted by rolling. For exceptionally heavy wear it may be advisable to use a five inch layer of lean concrete for the sub- base, after rolling the bottom of the excavation and providing drainage. Upon the sub-base should be laid a base, composed of four 430 CEMENT AND CONCRETE inches of concrete made with first class stone, such as granite, trap or hard limestone crushed to pass a ring one and one-half inches in diameter, and containing enough mortar, one part Portland cement to two or three parts sancl, to fill the voids in the stone. The top dressing, a layer of granolithic one and one- half or two inches thick, should then be immediately applied. This mortar should be made with one or two parts granite, trap, or other hard rock crushed to pass a five-eighths inch screen, to one part Portland cement. These two layers are placed in much the same manner as that described for laying concrete sidewalks, but the joints in base and top dressing should run at angles of forty-five degrees with the curb to prevent ruts following the lines of the joints. A roller making deep imprints is then run over the finished surface to furnish a foothold for horses, or, for this purpose a special roller may be used to mark the top dressing into blocks approximately four by eight inches, with deep (one-half inch) grooves. When completed, the pavement should be kept moist, pref- erably by a layer of damp sand, and no traffic should be al- lowed upon it for at least a week or ten days. 653. Concrete pavement laid in Belief ontaine, Ohio, was found to be in good condition after ten years' service; 1 the only serious defect apparent being that, since the blocks were marked off parallel to the curb, ruts have sometimes formed along these joints. This pavement was made with four inches base concrete, laid directly on sub-grade where foundation is gravel, sand or porous soil; or if soil is impervious, the base was laid on four inches of broken stone or cinders. The top layer was two inches thick, equal parts cement and sand or pea granite. Sub-drains of three inch tile were laid inside each curb line, and the curb is formed as part of the outer blocks. Both the base and top dressing were cut through in squares, five feet on a side. The cost of the pavement is said to have been $2.15 per square yard, and very few repairs have been found necessary. In Germany a cement macadam, made with six inch sub- 1 Municipal Engineering, December, 1900, and Engineering News, Jan. 7, 1904. CURBS AND GUTTERS 431 base of broken stone or gravel, with a wearing surface of hard macadam stone mixed with cement, has been successfully used. Art. 76. Curbs and Gutters 654. The use of concrete for curbs and gutters is rapidly- increasing. Curbing is sometimes molded and afterward put in place like stone curbing, but the greatest advantages in the use of concrete for this purpose are only attained by molding in place the curb and gutter as one structure. The Parkhurst combined curb and gutter is a patented form that has proved very satisfactory. This form has a projection of about one inch at the back and another along the bottom just below the curb, this feature being patented. A combined curb and gutter may consist of a curb four to six inches wide at the top, and five to seven inches at the bot- tom, and have a face of six to seven inches above the gutter. The upper face corner of the curb and the angle between curb and gutter should be rounded with a radius of one and one- half to two inches. The gutter is sixteen to twenty inches wide, and from six to nine inches thick, with top surface con- forming to the grade of the street. 655. The sub-base should consist of a layer of broken stone six inches thick, or six to twelve inches of cinders thoroughly rammed. The preparation of the foundation should be similar to that required for a pavement, care being taken that the sub-base be thoroughly drained, tile being used if necessary. Forms to receive the concrete are held in place by stakes, the molds being carefully set to grade. The sub-base may now be covered by a layer of four to six inches of Portland concrete of only moderate richness, as one to three to six, and the concrete to form the curb and gutter placed upon it before it has set, or a six inch layer to form the gutter may be placed directly on the sub-base. 656. Concrete to form the curb and gutter should be of good quality, not more than two and one-half parts sand to one part Portland cement being used for the mortar, and sufficient mor- tar used to entirely fill the voids in the stone. The broken stone for this concrete should be rather fine, with few, if any, pieces larger than one inch in greatest dimension. The ex- posed faces receive a top-dressing, or wearing surface, of one- 432 CEMENT AND CONCRETE half inch to one inch of granolithic containing not more than one and. one-half parts of trap or granite, pea size, to one part Portland cement. This coating is applied as soon as possible after the concrete is placed, as in sidewalk work. The surface is troweled or floated, but a smooth, glossy finish is avoided. The curb and gutter may well be laid in alternate blocks about six feet long, but a somewhat neater appearance is se- cured by making the work continuous, and cutting it entirely through at intervals of six feet to provide for slight movement. As the molds may be used repeatedly, they should be sub- stantially made. Special forms are of course required at corners, catch basins, etc. As in other concrete construction, the work should be protected from injury and kept moist for at least a week. 657. On business streets it is desirable to build the sidewalk close to the curb, with only a joint between, the grade of the walk conforming to the curb and sloping up toward the build- ing line one-quarter inch to the foot. On residence streets the walk should be separated from the curb by a park strip, the walk being high enough to give drainage toward the curb. Steel facing is sometimes used for curbs subjected to excep- tional wear, as in front of shipping warehouses and freight sheds. Where these are applied, they should cover the top and the upper part of the face of the curb and must be well anchored, by bolts or special webs, to a substantial mass of concrete, otherwise they will work loose and defeat the object for which they are used. 658. Cost of Concrete Curb and Gutter. — At Champaign, 111., 1 a curb was built seven inches high and five inches thick, the gutter, six inches thick, extending nineteen inches into the roadway from the face of the curb. The foundation consisted of six inches of gravel or cinders well rammed. The concrete was composed of one part Portland cement to five parts fine gravel, and the finishing coat, one inch thick, was of one part Portland cement to one part clean, sharp, coarse sand. The cost per foot was thirty-nine cents, including all excavation, A similar curb at Urbana, 111., was 4^ inches thick at the top, 5 inches at the base and 7\ inches high; the gutter being W. H. Tarrant, Engineer, Proc. 111. Soc. Engr. and Surveyors, 1899. STREET RAILWAY FOUNDATIONS 433 5 inches thick and extending 18 inches into the roadway. The foundation was composed of eight inches of cinders or gravel. The concrete was of one part Portland cement to five parts clean gravel, and the finishing coat was one inch thick, com- posed of one part Portland cement to two parts sharp sand. The price per linear foot, including the excavation, removal of old curbing, and refilling, was forty-six cents. At South Bend, cement curb alone, 6 inches wide at top, 7 inches at bottom and 16 inches depth, with the upper half composed entirely of one to two Portland cement mortar, has been constructed for eighteen cents per linear foot. Art. 77. Street Railway Foundations 659. The heavy motor cars used on city and urban electric railways subject the track to very severe service. As the head of the rail must be practically flush with the pavement on city streets, cross-ties, when used, are so far beneath the surface that they decay rapidly and their renewal entails the tearing up of the pavement. As there is not the same necessity for a cross-tie on street tracks as on railroads, since the rails are held to gage by the pavement, these objections to the cross-tie have led to the adoption of a concrete girder under each rail. The rails and ties (if ties are used) should not only rest upon the concrete, but should be imbedded in it. Track in which the rails rested upon concrete, but were not imbedded in it, has been found to yield laterally and get out of alinement, while on the other hand, if the ties rest upon earth or gravel and are filled between with concrete, the track is likely to settle, break- ing the bond of the concrete. 660. The method of placing concrete beams for street rail- way tracks in Minneapolis was as follows: l The rails were first spiked to cross-ties at intervals of six to eight feet, and the rail joints cast-welded. In laying the street pavement foundation of natural cement concrete, a rough groove, fifteen inches wide at the bottom and eighteen to twenty inches at the top, was left under each rail. This groove was immediately filled be- tween ties with concrete made of one part Portland cement, 1 F. W. Cappelen, M. Am. Soc. C. E., Engineering News, Oct. 14, 1897; Municipal Engineering, November, 1896. 434 CEMENT AND CONCRETE two and one-half parts sand, and four and one-half parts broken stone. The rails were tied together every ten feet with wrought iron tie bars, three-eighths inch by two inches, set on edge. These tie bars were rounded at the ends, threaded and attached to the web of the rail by two nuts, one on either side of the web. The rails were then spiked to the concrete beam, the temporary wooden ties removed, and the spaces left by them filled with concrete, completing the beam. As the concrete beam was eight inches thick and the rail five inches, the sub-grade was thirteen inches below the top of the rail. On the gage side of the rail were placed toothing blocks of granite, 3| by 9 inches by 4^ inches deep, held away from the rail I j inches by temporary wooden strips. After removing these strips, cement grout was poured into the groove to fill 2\ inches over the base of the rail, the remaining 2\ inches to the top of the rail being filled by asphaltic cement which re- mained soft enough to permit a flange groove to be made by the first car over the track. The asphalt wearing surface was laid against the rail on the outer side. Mr. Cappelen, in describing this construction, says that a rail six inches high with six-inch base should be used, with granite toothing blocks, six by nine inches by five and one-half inches deep. The cost per foot of rail for the concrete beam construction only, was twenty-six to twenty-seven cents, and for the filler, five cents per foot. The cost per mile of double track, exclusive of rails and pavement, was about $8,670.00. Somewhat similar methods have been employed in Toronto and Montreal, Canada, Indianapolis, Ind., and Scranton, Pa., Denver, Detroit and Cincinnati. 661. At Scranton, Pa., 1 the rails were laid directly on the six-inch concrete base of the pavement. This thickness was increased to twelve inches under the joints (which were rein- forced by an inverted rail four feet long) and under steel cross- ties spaced ten feet centers and formed of old girder rails in- verted and riveted through the flanges at the intersection. Flat steel tie bars, threaded at the ends, spaced ten feet centers, were also used here as at Minneapolis. 1 Description of the systems employed in several cities are given in En- gineering News, Dec. 26, 1901, STREET RAILWAY FOUNDATIONS 435 The concrete mixing plant was mounted on a car running on the track; the materials were delivered to the machine by hand measuring boxes, and the Drake mixer deposited the con- crete directly into the trench. The total cost per foot of track is given as $2.65, $1.17 of which was for grading, rolling, con- creting and brick paving at $1.97 per square yard, and for extra concrete at joints and ties at $0.72 per square yard. 662. At Toronto, Canada, the six-inch concrete base of the pavement is increased to eight inches in thickness for twenty inches width under each rail, and the base of the latter is im- bedded one inch in the concrete. A 6^-inch grooved girder rail is used, with mortar rammed between the web and the adjacent paving blocks. 663. At Cincinnati the bottom of the concrete stringer is nine inches below the base of the nine-inch grooved girder rail, and the concrete is built up from three to six inches on the web, according to the thickness of the wearing surface of the pave- ment. The space between the upper part of the web and the adjacent paving is then filled with cement mortar, . thus sup- porting the head of the rail as well as protecting the web from corrosion. CHAPTER XXI SPECIAL USES OF CONCRETE (CONTINUED). SEWERS, SUB- WAYS, AND RESERVOIRS. Art. 78. Sewers 664. There seems to be no very good reason why concrete is not more generally employed in the construction of all large sewers. With sizes less than two or two and one-half feet in diameter the difficulty of removing the centers prohibits the use of concrete in the ordinary Avay, and although some appliances have been devised for building these small sewers as monoliths by a mold that advances as fast as the concrete is tamped in place, they have not proved popular. The difficulty of obtain- ing a perfect grade, and the undesirable feature of leaving the green concrete unsupported, are probably reasons sufficient for this lack of popularity. For the larger size sewers concrete has several advantages over brick. First may be mentioned the very smooth finish that may be obtained on the invert, appreciably increasing the velocity of flow over that usually obtained with brick inverts. Cheaper labor may be employed in concrete work with less danger of annoyances from strikes. The cost is from one- third to one-half less than for brick. 665. Methods of Construction. — The City of Washing- ton was one of the first^to use concrete extensively in sewer construction 1 . For sizes up to twenty-four inches internal diam- eter the concrete is used only as a foundation and bedding for the ordinary sewer pipe. For a twenty-four inch sewer the pipe rests in a bed of concrete twenty-seven inches wide at the bottom, flaring to forty inches wide at the level of the center of the pipe, and then carried up with plumb sides for six inches, and finally finished by planes tangent to the upper curve of the pipe. At the joints there are bands of concrete 1 Described by Capt. Lansing H. Beach, Corps of Engrs., U. S. A. Report Operations District of Columbia, 1895. 436 SEWERS 437 extending over the top, so that at these places the pipe is en- tirely inclosed. Similar forms are used for the smaller sizes with corresponding decreased dimensions. For all sewers be- tween ten inches and twenty-four inches the sub-grade is six inches below the exterior of the pipe, and in all cases the band about the joint is four inches thick at the top. 666. The method of laying these sewers is as follows: The trenches are 2\ to 3 feet in width, with " headers" about 2 feet wide, left at intervals of 10 to 16 feet, which are tunneled through. The grade and line pegs are placed in the headers at the ground surface, and a cord is stretched on the sewer line over at least four stakes, at a convenient height above the grade, and thus parallel to the bottom of the sewer. When the trench is to the required grade, a six inch layer of concrete, made with one barrel natural cement, two barrels sand and four barrels gravel, is placed. This concrete is rammed with iron rammers weighing sixteen pounds, and having eighteen square inches ramming surface. The pipe is then laid upon this bed and each section is tested for line and grade, For the former, a plumb bob is used with its cord held against the grade cord already mentioned, and for testing the grade a grad- uated pole is used, with a projection at the bottom which sets on the interior of the pipe, just within the open end. Concrete is then lowered in buckets, deposited on top of the pipe and allowed to fall down on the sides so as not to disturb the alinement. When enough concrete to secure the pipe has been thus placed, it is rammed and the concreting continued until the required form is obtained, as already de- scribed. The concrete in the bands carried over the joints is not rammed but is beaten with wooden paddles and heavy trowels to compact it and bring it to the desired form, four inches thick and four inches wide at the top, and flaring to twelve inches wide (in the direction of the sewer) at the top of the pipe. 667. Cost. — The quantities of concrete materials required to lay one hundred linear feet of pipe sewers as described above are given as follows : — Size of sewer 8 inch 12 inch 18 inch 24 inch Cement, bbls G.76 10.58 14.77 19.14 Sand, cu. yd 2.07 3.23 4.52 5.85 Gravel, cu. yd 4.16 6.47 9.04 11.70 438 CEMENT AND CONCRETE With natural cement costing $0.79 per barrel in sacks, sand $0.47 per cu. yd., gravel $0.75 per cu. yd., and laborers $1.50 to $1.75 per day, foremen, masons and inspectors $4.00 per day, the average cost of laying pipe sewers in this manner was approximately as follows, exclusive of the cost of the pipe: 8-inch, $1.11; 12-inch, $1.14; 15-inch, $1.46; 18-inch, $1.60; 21-inch, $1.67; 24-inch, $2.32 per foot. 668. Sewers at Chicago. — In the construction of some 17,000 feet of sewers for the Chicago Transfer and Clearing Yards, 1 concrete was used for all sewers of thirty-six inches diameter and over. The excavation was mostly in blue clay and done by steam shovel to a depth of twenty feet, the re- mainder being removed by hand shovels and swing derrick. The material was such that in general the bottom of the trench could be trimmed to the form of the exterior of the sewer. The thickness of the ring of concrete was 8 inches for 36 and 42-inch sewers, 10 inches for 48-inch, and 12 inches for 84 and 90-inch sewers. The concrete was composed of one part "Steel Pozzolana" (slag) cement, three parts sand and five parts broken stone. The cement was of course very finely ground and showed high seven-day tests. The cost was $1.30 per barrel delivered. The sand was the Chicago "torpedo" sand, coarse and of good quality, and cost about ninety cents per cubic yard delivered. The stone was a limestone from Summit, 111., crushed in two sizes, namely, 1 to 2\ inches and \ to \\ inches. These two sizes of stone were mixed in proportions one part of the coarser to two of the finer. The cost of stone was about $0.80 per cubic yard delivered. The concrete was mixed by a rotary mixer of the continuous type provided with radial blades. The mixer was mounted on a flat car, with engine and upright boiler. Three cars of stone, the mixer car, two cars of sand and one of cement made up the concrete train, which ran on a track laid close to the trench and was kept near the work by a small locomotive. The mixer was supplied by wheelbarrows running from the material cars on plank runways attached to the cars. The concrete was also transported in wheelbarrows from the mixer to the trench. 1 E. J. McCaustland, Trans. Assoc. C. E., Cornell University, 1902. SEWERS 439 669. The bottom of the trench being cut to form, the con- crete for the invert was laid directly on the sub-grade, tamped in layers carried up until the invert occupied about one hundred forty degrees of arc. The form of the inner face of the invert was maintained by template, grade stakes being set 12^ feet apart along the trench. The remainder of the sewer was laid on centers resting on the invert. The ribs for this centering were made in a complete circle, of three thicknesses of one by twelve inch boards nailed together and cut to a true circle. Ribs were placed four feet center to center, and covered with lagging two inches thick and three inches wide, planed to radial joints. The strips of lagging were held in place at each end of a section by a j\ by 2 inch iron band passing over all of the strips, and turned in at the ends, forming a hook in which rested the low r er lagging strip, the other strips being supported by this one. The lower part of each rib rested on the invert, the upper portion being cut to a diameter four inches less (that is, smaller by twice the thickness of the lagging). While the trench was near enough to the outside of the sewer ring not to measurably increase the amount of concrete over and above the desired thickness, the trench served as the outside form. Above this point, planks were inserted and braced to the sides of the trench. From the haunches to the crown the exterior was finished with a template. When completed, the exterior form planks were removed, and a light covering of earth placed on the surface to protect it from drying too rapidly. This was especially necessary in this case on account of the kind of cement used. The centers were removed usually after forty-eight hours, by swinging the ribs about the vertical diameter and removing the lagging. As soon as the centering was removed, the inner surface was plas- ered with a mortar composed of three parts lake sand to one part cement. 670. Cost. — The company furnished the materials used in the sewer ring and manholes, and delivered it on the work, while the contractor furnished all tools and labor to dig the trenches, complete sewer and manholes, and do the back filling. The contract prices per foot are given by Mr. McCaustland, the resident engineer, as follows: — 440 CEMENT AND CONCRETE 36-inch sewer in trench avi 42 48 84 90 lg 11 feet deep, 3,340 feet, at $2.30. 14 " 2,660 " 3.00. 17 " 4,540 " 3.57. 22 " 1,000 " 5.91. 24 " 5,400 " 6.68. From the data given we have computed the approximate quantities of concrete per foot of sewer, and assuming the cost of the materials for a cubic yard at $3.00, we obtain the follow- ing approximate costs: — Size Sewer. Depth Trench. Mati Approximate Cubic Yards Concrete. RIALS. Approximate Cost Concrete. Construction Contract Price per Foot. Estimated Total Cost per Foot. 36 in. 42 " 48 " 81" 90 " 11 feet. 14 « 17 " 22 " 24 " .285 .325 .47 .93 .99 $ .85 .97 1.41 2.79 2.97 $2.30 3.00 3.57 5.91 6.68 $3.15 3.97 4.98 8.70 9.65 671. Special Molds for Small Sewers. — In the construction of a thirty inch sewer at Medford, Mass./ Mr. William Gavin Taylor made use of a very convenient form. The lower 240 degrees of the sewer was of concrete, the upper 120 degrees being of brick. To construct the concrete portion as a mono- lith, the forms were constructed in lengths of ten feet, separat- ing on a vertical line into two halves. The two halves were connected by clamps, and held at the proper distance apart by dog irons in the end ribs of each form. After smearing the forms as usual, the concrete was deposited and rammed. When it had partially set, the dog irons were removed and turn-buckles used to slowly pull the two halves together. This method pre- vented the green concrete being broken, although the concrete extended up on the sides thirty degrees above the horizontal diameter. 672. The centers used for the brick arch were also ingen- iously arranged, and since they might have been used for a con- crete arch they may be described here. These centers were also in ten foot lengths. The ribs, of two inch plank, were 1 Abstract from Armnal Report of City Engineer, Engineering Record, NOv. 7, 1903. SEWERS 441 spaced two feet centers, with lagging f inch thick by 1^ inch wide, with one bevel edge to make a tight upper surface. The rear end of each center was supported by wedges securely fastened to the outer end of the preceding section, the forward end being supported by a screw jack. After turning the arch, these centers were removed by the aid of a special truck the axles of which were bent at such an angle as to make the cast iron wheels fit the concrete invert. The axle of a roller was first fastened to the outer rib of the center to be removed ; the truck was then run back a foot or so under the center and the screw jack supporting the forward end of the center released. This allowed the forward end to drop a short distance, the roller resting on the running board of the truck. The latter was then pulled into the sewer far enough to let the roller run off the end of the truck and lock itself. The truck being then pulled out of the sewer toward the finished end, drew the center away from the wedges sup- porting the rear end, allowing the form to drop on the truck and be wheeled out of the sewer. By this method the centers were successfully removed without injuring the concrete. 673. Cost. — From data given, the cost of this sewer — ■ about sixteen hundred feet in length — is approximately as follows, labor costing twenty-five cents an hour: — 1.25 cu. yds. excavation and back fill, at $.59 $0.74 .15 cu. yd. concrete, at $6.70 1.00 .037 cu. yd. brick masonry, at $12.05 44 Cost of linear foot, exclusive of manholes, estimated at . . $2.18 The total cost per linear foot is given as $2.39 674. New York Sewers. — In connection with the construc- tion of the New York Rapid Transit Railway, some of the sewers were built of concrete. This work was done with ex- ceptional care, and on a large scale, and it was found that the concrete sewers cost one-third less than similar sewers of brick. The method of construction of one section may be described as follows : 1 The forms for the invert of the straight lengths of sewer were twelve feet in length, consisting of a strong frame- work covered with closely matched lagging, planed smooth and 1 Engineering News, March G, 1902. 442 CEMENT AND CONCRETE greased with machine oil. After the trench was prepared, con- crete was placed and rammed until the top of the concrete was within about one-half inch of the flow line of the invert. To accomplish this, a straight edge was used, bearing on the fin- ished invert in the rear and a template secured to the trench timbering just ahead of the section under construction. The invert centers were then placed, resting on the finished invert at the rear and on a solid foundation accurately set to grade at the forward end. Mortar composed of equal parts Portland cement and sand was then tamped between the invert form and the bottom concrete already laid. When the flow line had been thus accurately formed, the center was braced and vertical planking set to form the outside of the walls. The concrete was then rammed in place. Joists of two inch by four inch scantling laid along the center of the top of each side wall of the invert section, formed, when removed, a mortise into which the fresh concrete of the arch section was rammed to form a bond. Similar mortises were also made in the forward end of each section as built. After twenty-four hours or more the forms were removed, and a thin cement wash was applied to the interior, sufficient only to fill any slight imperfections in the surface. The arch centers, similar in construction to the forms for the invert, were put in place and plastered with one inch of rich Portland mortar. Concrete was then placed sufficient to make the arch eight inches thick, the outside of the walls being formed by inclined boards braced to the trench, and the top of the extrados was formed by hand. 675. Steel Forms. — Two novel types of centering have been devised, in which the surface next the concrete is of steel. In one of these 1 the forms are in sections about three feet long. Two of the pieces of steel are of a width suitable to reach from the bottom of the sewer to just above the spring line of the arch, while a third piece forms the arch center. The strips are bent at an acute angle at the sides, thus projecting into the sewer along an element of the surface where the plates join ; the two sides of adjacent plates, which flare away from each other, are then connected by a continuous U-shaped clip of steel slipped 1 Engineering Record, Jan. 9, 1904. SUBWAYS AND TUNNELS 443 on from the end of a three foot section, and the intervening space in the clip filled with clay or melted paraffin. The form is assembled outside the trench, and after the paraffin is in place, the center may be handled. When the sewer is completed, the paraffin is melted by a suitable heater, or the clay is washed out, and the form may be collapsed and removed. 676. In the other form * the steel plates are in continuous strips about six inches wide and are applied by setting up the wooden form on an improvised axis, revolving the form and wrapping the steel sheet about it as it is revolved. The wooden form is in two parts, upper and lower, firmly connected while in use, but the two parts may be made to approach each other by driving out the wedges between them. After the winding, the center, with its sheet steel jacket, is lowered into the trench. When the concrete is completed, the form is collapsed and removed, leaving the spiral of steel in place to support the con- crete until the latter is well set. The steel is then removed by simply pulling on one end. As it comes away from the concrete it is wound into a coil, and is then ready to be rewound on the wooden form. Both of the above styles have been patented. Art. 79. Concrete Subavays and Tunnel Lining 677. The advantages of concrete in subway construction and in tunnel lining are now well established. In subways built in open cut, the side walls and invert are of concrete built in place, while the roof is frequently made with I-beams with concrete arches turned between them. The I-beams are sup- ported directly on the side walls, which are usually made mono- lithic with the invert. 678. Special precautions have to be taken to exclude water from a subway, and for this purpose tarred felt and Portland cement plaster are employed. The specifications for the New York Rapid Transit Subway 2 were carefully framed to secure a waterproof construction. On the sub-grade was placed a layer of concrete, smooth and level on top. This was covered by alternate layers of hot asphalt and felt, from two to six layers of each being used as deemed Engineering News, Feb. 18, 1904. Abstracted in Engineering News, Feb. 13, 1903. 444 CEMENT AND CONCRETE necessary for the conditions encountered. The remainder of the concrete forming the floor was then laid upon the top layer of asphalt. In dry, open soil the felt was not required, and in dry rock excavations above water level both the asphalt and felt were omitted. Similar provisions were made for water- proofing the side walls and roof, resulting in a complete layer of asphalt and felt imbedded in concrete about the entire tun- nel, the waterproofing being protected both inside and out by concrete. 679. In the construction of the Boston Subway 1 the por- tion built in open cut was made as follows: The work was di- vided into sections of convenient length, about twelve feet, so that work on a section could be carried on continuously until completed. Upon the prepared grade were laid three thick- nesses of tarred felt with six-inch lap joints, well pitched be- tween the layers, and the top of the upper layer thoroughly covered with the pitch. When the latter had hardened, the invert was laid over the entire width of the section. At each side a back wall six inches thick was built up to a convenient height and braced. The forms were then removed and the face of this back wall was plastered with rich Portland cement mortar. The main side walls were then built up be- tween this layer of plaster and the forms defining the interior face of the wall. This portion of the subway had an arch roof, two feet thick at the crown, which was laid on wooden centers. The exterior of the roof was plastered like the side walls, and then covered with four inches of concrete to protect the plaster from injury. The centers were removed after from ten to thirty days; the span of the arch was about twenty- three feet. 680. Tunnel Lining in Firm Earth. — In building tunnels in earth that is sufficiently firm not to require extensive timber- ing, concrete is well adapted for lining. An instance of this is furnished by the extensive system of tunnels constructed for telephone and telegraph service under the streets of Chicago. 2 The trunk conduits for this system are about thirteen by four- 1 Annual Report Boston Transit Commission, 1900; also described in Engineering News, April 4, 1901. 2 Mr. George W. Jackson, Engineer, Proc. W. Soc. Engrs., 1902; also in Engineering News, Feb, 19, 1903. SUBWAYS AND TUNNELS 445 teen feet inside, and the laterals about six by seven feet, all of the five center horseshoe form. The excavation was in hard clay which stood up well. Shafts were located in basements of buildings rented for the purpose, and in these basements were placed the compressed air plants, material bins, concrete mixers, etc. The large air locks, some of which would hold ten small construction cars, were placed at the bottoms of the shafts. Work was done in three shifts, working eight hours each. The two night shifts could excavate about twenty-one feet of lateral tunnel in the sixteen hours, and the day shift placed the lining. 681. The concrete was in general composed of five parts of broken stone and screenings, or of mixed gravel and sand, to one part Portland cement. For intersections but four parts aggregate were used. This should make a very strong concrete. The centers for the smaller conduits were made of three-inch channels, each rib being in five parts bent to the proper form and connected by flange plates bolted to the inside of the chan- nels at the ends. These ribs were placed three feet apart, and two-inch plank used for lagging. The ribs for the trunk sewers were of similar construction, but with heavier channels braced with angles. Steel lagging was used, made of plates about twelve by thirty-six inches, stiffened by H inch angles on four edges. There were also provided bulkheads or steel end plates of voussoir shape, twelve inches along the intrados and twenty inches high, for the pur- pose of retaining the end of each section of lining and permit thorough tamping. These bulkhead sheets, or "end flights," were also stiffened along three edges, and could be attached to the webs of the channel ribs by short bolts. The concrete was mixed at the shaft head and conveyed to the work in cars twenty inches wide and four feet long, running on a fourteen-inch gage track. The floor of the tunnel was first laid in the excavation, the steel ribs then put in place on the floor, and the lagging placed at the bottom and built up the sides just ahead of the concrete. When near the crown, short pieces of lagging three feet in length covering but two ribs were used, and the concrete rammed in from the end of these short sections until they were complete, and then another row of short pieces placed and the operations repeated. 446 CEMENT AND CONCRETE The concrete floor of laterals was designed to be thirteen inches, and the sides and arch ten inches thick, but in all cases the entire space between the lagging and the sides of the ex- cavation was filled with concrete. 682. In such work as this only the best materials should be used, and, as early strength is desired, the use of Portland cement is general in order that the centers may be removed within a reasonable period. The ends of the sections into which the work is divided should, if possible, be brought up square, the bulkhead sheets described above being an ingenious and effective method of providing for this. Where it is not practicable to finish with a square end over the entire area of section, then the work on the sides should be stepped back from the bottom toward the crown, each step being bounded by planes corresponding to coursing and heading joints in a masonry arch. 683. Tunnel Lining in Soft Ground. — For tunnels in soft ground requiring the use of a shield, some difficulties in using a concrete lining are apparent. The principal one of these lies in the fact that the fresh concrete is not capable of taking the thrust of the jacks used in forcing the shield ahead. Attempts have been made to overcome this difficulty by so constructing the centers that the jacks may bear against them instead of on the fresh concrete. Another difficulty is that in materials requiring almost con- tinuous support, the temporary timbering is in the way of the centering for the concrete construction; and still another is the difficulty of properly tamping the arch at the crown where the tail of the shield confines the working space. Concrete blocks were tried in the construction of sewers in Melbourne, but without entire success. Such blocks were successfully em- ployed in the underground road system of Paris, though at- tempts to use fresh concrete in shield tunneling for this work proved a failure. 684. East Boston Tunnel. — In the construction of the East Boston Tunnel Extension of the Boston Subway, however, a monolithic concrete lining has been successfully built, the tunnel being excavated by shield. This tunnel is about twenty by twenty-four feet for double track electric line. The arch ring and the walls are thirty- SUBWAYS AND TUNNELS 447 three inches in thickness, while the invert is twenty-four inches. Two side drifts, eight feet square, were first driven a certain dis- tance and timbered. The bottoms of these drifts were then excavated, and the side foundations of concrete were placed in lengths of sixteen to twenty feet. When the foundations had set, the interior forms for the side walls were placed upon them, supporting the caps, the exterior plumb posts removed, and the concrete side walls, three feet thick, built up to within sixteen inches of the springing line of the arch. This work was kept about one hundred feet in advance of the shield. The shield, provided with live rollers, rested upon these side walls, the rollers running in a flanged plate placed on top of the walls. The shield was forced ahead thirty inches at a time, and sections of the arch thirty inches in length were turned directly behind the shield. 685. The centers of the arch were of curved, ten inch steel channels spaced thirty inches apart, and the lagging, four inches thick, was placed from the bottom toward the key as the concrete was built up. Each section of arch is keyed with concrete pressed through two holes in the rear girder at the top of the shield, special rammers being used to tamp the con- crete into the space at the crown of the arch, the concrete being directed into place by curved sheet-iron troughs. In each section of arch sixteen cast iron bars, three and one- quarter inches in diameter and thirty inches long, are built into the concrete in position to receive the thrust of the shield jacks. Wooden bulkheads on the jack plungers serve to con- fine the fresh concrete, but the reaction is taken on the cast iron bars which, being butted end to end in successive sections of the arch, carry the stress back to concrete that is able to sustain it. As the shield advanced, the space left over the completed arch by the tailpiece of the shield was filled with grout under pressure. The centers remained in place thirty days. The invert was excavated and laid in ten-foot sections about twenty-five feet in the rear of the shield. The concrete was mixed at the bottom of the shaft and passed through the air lock on cars. The concrete cars ran on a higher level than the muck cars, in order not to interfere with the excavation. 686. Lining Tunnels in Rock. — If the rock through which a tunnel is driven is seamy and insecure, concrete is in most 448 CEMENT AND CONCRETE cases the cheapest and best lining. The cost of the lining is, of course, less if it can be built in connection with the excava- tion, but it is frequently difficult to foresee how a given rock will stand exposure to the air and water, and it becomes an exceedingly nice question to determine at the time of building a tunnel whether lining is required. In many cases this ques- tion is settled in the affirmative by other considerations than the character of the rock, as the resistance to flow, in water- works and sewers, or the ease of ventilation and the necessity of a good appearance, as in street or steam railway tunnels. 687. New York Subway. — In the construction of portions of the rapid transit subways of New York, a traveling center which served also to support a working platform was carried on six wheels running on a track laid on the footing courses of the side walls. This center carried at the side, sections of lag- ging curved to the required form of the side walls. This lagging was adjusted in place, and braced from the platform or center by means of wedges. Directly behind this traveling center was a similar platform carrying a derrick; and behind this, the traveling center carrying the lagging for the roof. This third platform was jacked up to place the roof lagging at the correct elevation, and firmly supported by wedges. The concrete was brought in skips on cars that ran on the floor level and stopped beneath the derrick platform. The derrick hoisted the skips through a hole in the platform and placed them on cars on either the side wall or the roof platform, so that the concrete was delivered either to the side wall forms in advance, or the roof forms in the rear as required. The concrete was rammed in a direction transverse to the tunnel axis until the roof was completed, except for a space about five feet wide at the crown. The arch was then keyed by tamping the concrete in from the end of the form. The two platforms carrying the forms were each forty feet long, and the derrick platform was eighteen feet. 688. The excavated rock was crushed for the concrete on a working platform erected over and around the shaft head. Cars delivered the excavated material at the shaft in steel skips, which were hoisted to the working platform, set on push cars and dumped into bins, from which stone was delivered to the crusher; these cars then passed under the crushed stone bins, SUBWAYS AND TUNNELS 449 were loaded with broken stone, run back to the shaft head, and the broken stone dumped into bins mounted over the mixer. The skips were then lowered into the shafts by the derricks, to be run to the headings and reloaded. The stone and sand were fed to a measuring box by means of a hopper, the measuring box discharging directly into a cubical mixer, which was high enough above the tunnel floor to dump directly into skips on the cars. 689. Cascade Tunnel. — In the construction of the Cascade Tunnel of the Great Northern Railway a somewhat different arrangement was used. 1 The working platform in the tunnel was erected five hundred feet in length, and cars hauled by cable up an incline to the platform. The side walls were built in alternating sections, eight to ten feet in length, the support of the arch timbering being thus gradually transferred from the plumb posts to the concrete of the side walls. Arch sections were built in twelve foot lengths, the centers being made of four by sixteen inch plank without radials, so as to leave a clear way for concrete cars on the working platform. The latter were high enough to allow the material cars to run be- neath them. 690. Concrete vs. Brick. — There are frequent instances in engineering construction where brick masonry might well have been replaced by concrete, and the use of brick for tunnel lining is still adhered to in many cases. This is partly because some- what less elaborate centers can be used for brick arches, and the centers may be struck somewhat earlier, and partly be- cause of extreme conservatism on the part of the designer, although without doubt there are cases where the use of brick is entirely warranted. An interesting instance of the greater adaptability of con- crete under unforeseen conditions, however, is presented by the Third Street concrete and brick lined tunnel at Los Angeles, Cal. 2 This tunnel was excavated mostly through an argilla- ceous sandstone. The side walls were of concrete up to the haunches, the upper part of the arch being of six courses of brick. A streak of yellow clay was encountered, and it "was 1 Mr. John F. Stevens, M. Am. Soc. C. E., Engineering News, Jan. 10, 1901. 2 J. H. Quinton, M. Am. Soc. C. E., Engineering News, July 18, 1901. 450 CEMENT AND CONCRETE soon demonstrated that the six ring brick arch, which occupies the central portion of the roof, was not strong enough to hold up the immense weight above it, and the temporary timbering was crushed and broken in a most alarming way." The strength of the arch was increased by using nine rows of brick instead of six until the clay seam was passed. In such portions of the six ring arch as had cracked, it was found that the inner ring of brickwork had separated from the second ring, and in places the second ring had separated from the third. The concrete walls had shown no evidence of weakness. To repair the brickwork, steel concrete beams or arches were inserted in the brickwork at intervals of four feet, and extend- ing from one concrete wall to the other. These beams were twelve inches wide and eight to twelve inches deep, made of rich concrete, and had imbedded in each beam two pieces of three inch by three-quarter inch steel. The steel ribs were set in recesses cut out of the brickwork, and rested at the ends upon the concrete of the side walls. Substantial centers were used for building the concrete beams, and when the latter had set, the defective brickwork between adjacent beams was cut out and replaced by rich concrete. 691. Aspen Tunnel. — Another illustration of the adaptabil- ity of concrete when unexpected difficulties arise, is furnished by the construction of the Aspen Tunnel on the Union Pacific Railroad. 1 The original design provided for sets of timbers to support the excavation, spaced about three feet, center to center, but for nine hundred feet of the tunnel such pressures were encountered that in places a solid wall of twelve by twelve inch timber was forced in. For a portion of this section the lining was built of a combination of concrete with steel ribs. The latter were 12-inch, 55-pound I-beams spaced from twelve to twenty- four inches, center to center, curved to conform to the interior of the tunnel. The concrete was built up around and between the beams, the inner flange being covered by from four to seven inches, and the total thickness of the walls two to three feet. 692. The Perkasie Tunnel of the Philadelphia and Reading Railroad was constructed through a firm rock, which, however, was intersected by several strata of seamy rock. As trouble 1 W. P. Hardesty, Engineering News, March 6, 1902. SUBWAYS AND TUNNELS 451 was experienced from rock falling from these strata, it was decided to line the tunnel at such places. This lining had a minimum thickness of eighteen inches at the crown and twelve inches at the sides. Traffic through the tunnel was not ob- structed during the work of placing the lining, In laying about five hundred cubic yards of concrete, the cost was about ten dollars and eighty cents per cubic yard, exclusive of cost of centering and dry filling. 1 693, "Water Works Tunnel. — The lining of portions of the Beacon Street Tunnel of the Sudbury River Aqueduct was undertaken some fourteen years after its excavation, and at a time when it was necessary to use the tunnel intermittently to supply water to the city of Boston. The methods employed are described by Mr. Desmond FitzGerald in Transactions American Soc. C. E. for March, 1S94. A substantial track of 2 feet 1^ inch gage was laid from a manhole furnishing access to the sewer to the portion of the tunnel to be lined. The rails, weighing thirty-six pounds to the yard, were supported on small but substantial trestles, built of three by four inch spruce joists, and placed eight feet between centers. Every third trestle was braced from the sides and roof of the tunnel to prevent the track being floated when the tunnel was in use. The trestles also carried five rows of planks for the workmen to walk on in pushing the cars. The track was elevated by these trestles, so the work was not seriously interfered with by a small amount of water in the tunnel. The track cost about eighty-seven cents a foot. Cars to run on these tracks to deliver materials and concrete had frames five feet by one foot nine inches, with twenty inch wheels, and cost about fifty-six dollars each. 694. Centers. — The centers were in three parts, two for side walls and one for roof. The ribs were of three thicknesses of two by ten spruce plank, without interior bracing for the roof section. The side sections had each an inclined brace. Wedges were inserted between the tops of the side sections and the bottoms of the roof ribs to hold the latter in place. The lagging was two by four inch spruce, in eight foot lengths, with beveled edges and planed both sides. The centers were 1 P. D. Ford, M. Am. Soc. C. E., Trans. A. S. C. E., March, 1894. 452 CEMENT AND CONCRETE spaced four feet apart, and seventy-five full centers were built; these, with the lagging, contained 14,000 feet B. M. of lumber, and cost $1,460.55, or $104.30 per thousand feet B. M. 695. Methods of Work. — Broken stone, sand and cement were stored in shanties over and around the manhole leading to the tunnel, and arrangements made by which the materials could be delivered through chutes down the manhole to the cars As it was found more convenient to work in winter, special provision was made for storing large quantities of ma- terial in the shanties. The sand was piled around an iron lined, wooden bulkhead, in the center of which was a large stove. The concrete was mixed within the tunnel as close to the work as possible, and in places where the cross-section had been sufficiently enlarged by falls of rock to permit easy work- ing. The materials, delivered to the material cars down the chutes already mentioned, were pushed to the mixing platforms and combined in the proportions of 18.56 cubic feet of crushed stone and 7.35 cubic feet of sand to one barrel of Portland cement, being approximately 1 to 2 to 5^. The above quanti- ties of materials made 20 to 21 cubic feet of concrete. When mixed, the concrete was shoveled into cars, conveyed to the work and then shoveled into place. The tamping was done principally with oak rammer five inches square, twelve inches long, with a short wooden handle in one end. In tamping the key of the arch, long-handled iron rammers were used. Much care was requisite here to prevent the aggregate separating from the mortar and lodging next the lagging, as it always has a tendency to do, thus resulting in voids in the face of the work when the lagging is removed. The concrete was built up on the sides in horizontal layers and stepped back by inserting bulkheads, so that the adjacent sections bonded together. 696. Cost. — The cost of this concrete lining, which was built under great disadvantages, amounted to $16.15 per cubic yard. This cost must be considered reasonable in view of the fact that the materials had to be transported an average dis- tance of more than one-half mile on small push cars, and the work in the tunnel was suspended for three days of each week to allow the tunnel to be used to maintain the water supply of the city. RESERVOIRS 453 Art. SO. Reservoirs: Linings and Roofs 697. Although the choice of the material with which to construct a reservoir may in some cases be varied by local conditions, it is found that under ordinary circumstances con- crete offers the greatest advantages for a minimum cost. For the side walls of small reservoirs, concrete furnishes the requi- site strength and water-tightness with a moderate thickness; earthen embankments and floors may be made practically im- pervious with concrete and mortar, combined with asphalt when considered necessary; while for the roofs, groined arches or beams and slab construction, with supporting piers, all of this material, make a neat, permanent, and altogether satis- factory covering, at a smaller expense than would be required for brick or stone masonry. 698. Details of Construction. — In the walls and floors, water-tightness is a prime consideration, and this is best at- tained by a layer of mortar on the inner surfaces or between two layers of concrete. As in floors, walks, etc., the necessity of providing for ex- pansion and contraction will depend upon the extremes of temperature to which the surface is to be subjected. In covered reservoirs which are to be almost constantly filled with water, or in very equable climates, the blocks may be large, say twenty feet square, while under more severe conditions the blocks may not contain more than twenty square feet. The joints between the blocks may well be wide enough to be filled with asphalt. This furnishes an elastic joint which is compressed as the blocks expand, and swells when the blocks again con- tract. 699. Reservoir Floors. — One of the principal difficulties ex- perienced in the construction of floors is from settlement of the foundation. The floor should, therefore, have strength enough to bridge any small irregularities in the foundation that may result from inequalities in settlement. For a similar reason, it is not well to make the blocks too large, as smaller blocks with compressible joints will more readily conform to an uneven surface without permanent injury. In order that the reser- voir shall not leak even if the foundation settles, the concrete and mortar may be covered with one or more layers of asphalt. 454 CEMENT AND CONCRETE In building the floor lining, alternate blocks are sometimes placed first in molds and the intermediate blocks built in later. In other cases the blocks are laid consecutively. The advan- tage of the former method seems to lie principally in the ease of construction, as access may be had to all sides of the block. 700. In hard clay soil not liable to settlement, four inches is sufficient thickness for the floor, the concrete to be covered before it has set with a half-inch layer of rich Portland mortar, troweled to a smooth surface. If the reservoir when empty will be subjected to hydrostatic pressure from without, the floor must be designed to resist this pressure. In this case, if seepage from without into the reservoir is objectionable, a layer of mortar may be placed over the first layer of concrete and protected by the concrete laid upon it. This outside pressure may be provided for in a covered reservoir by making the floor of inverted arches between piers, the weight of the floor, piers, roof, and earth filling over the roof, being made sufficient to balance the upward pressure on the floor. If there is no ob- jection to the water from without being led into the reservoir, a porous layer of broken stone or gravel beneath the floor may be connected with the interior of the reservoir through pipes provided with check valves, and the outside pressure be thus removed. Where it can be accomplished, it will usually be better to lead this ground water through a pipe to a sewer or a lower level rather than into the reservoir. 701. Walls. — The thickness of the wall is determined by methods similar to those used in designing a retaining wall or a dam according as the pressures are greater from the embank- ment without or the water pressure within. In the case of a covered reservoir, the thrust of the roof arches may convert any vertical section of the wall into a beam, the earth pressure from without being supported by the floor at the bottom and the roof at the top. Or in case there is no back pressure from earth filling, the thrust of the roof may be added to the inner water pressure. In circular covered reservoirs the arch thrust is usually taken by steel bands laid in the concrete and en- circling the reservoir near the top of the wall. In narrow reservoirs rectangular in plan, tie rods may be used, or the wall may be buttressed to take the roof thrust. Concrete side RESERVOIRS 455 walls are usually built vertical, or nearly so, on the inside, and with a batter on the outside. 702. Linings. — Linings of sloping earthen embankments are laid the same as the floors, and similar precautions are required. There is greater danger of settlement of embankments than of the floor foundation, and the blocks, therefore, may well be made smaller. Some difficulty may be experienced with laying hori- zontal asphalt joints on a sloping face, and some sliding of the lining may be expected under ordinary conditions, the asphalt joints being compressed. For this reason it would seem to be better to use asphalt in the inclined joints only, and a mastic in the horizontal joints. Another method which would probably prove satisfactory is to lay first a tier of blocks next the floor, and when these have set, apply a very thin coat of asphalt to the upper edges of these blocks, following with another tier, and so on. 703. ROOFS. — Where it is necessary to cover a reservoir, either to prevent the formation of ice, or the growth of alga?, or for other reasons, the groined arch is an excellent design for the roof on account of the small amount of concrete required, the clear head room given, and the ease of ventilation. The extending use of reinforced concrete will also probably enter this field to a greater extent in the future than it has here- tofore. The determination of the stresses in a groined arch roof is complicated not only by the peculiar form of the arch itself, but by the fact that the spandrels of the arches are filled with concrete over the piers to the level of the extrados at the crown. This evidently results in making of any given unit of the roof, having a pier as its center, a cantilever, and the arch action is interfered with. Unless, however, tension members of steel are laid in the concrete near its upper surface, it is not wise to count on the strength of the cantilever except to consider it a factor of ignorance on the safe side. If one wishes to depart from the ordinary and tried dimensions for groined arches in concrete, such departure had better be based on some special experi- ments and tests on full sized sections. Some of the dimensions that have been used are given in the examples cited below. 704. Forms. — The preparation of forms or centers for groined arches is one of the most difficult and expensive details 456 CEMENT AND CONCRETE of the construction of such a roof. It will probably be best to have each section of the form cover the space, square in plan, between four piers. The ribs of the centering may well be built up of planks, nailed together and sawed to proper form. The lagging should be planed to size, and have radial joints to make a smooth and even top surface. Care is necessary to make a neat fit along the valley extending diagonally between piers, and a small fillet may well be fitted into this valley to avoid a sharp corner on the finished concrete, as well as to cover up possible imperfections in the joints. The forms should, of course, be designed to take the thrust of the adjacent com- pleted arches, and if sufficient forms are not built to cover the entire reservoir, and thus transmit the thrust to the walls, the piers at the border of the forms must be thoroughly braced to the opposite side walls or the piers will be toppled over and the roof wrecked. This accident occurred to one reservoir roof during construction, the pier braces having been removed without the knowledge of the engineer. 705. In laying the concrete, joints between the work done on consecutive days should cut the arches at right angles to their axes, and bulkheads should be used to make such a joint a vertical plane. The covering of each unit between four piers is made monolithic, and care is necessary to prevent the stones working to the bottom of the mass and thus becoming exposed when the forms are removed. This may be prevented by plas- tering the forms with mortar and placing the concrete upon it before the mortar has begun to set. 706. A roof consisting of a network of concrete-steel beams intersecting at right angles, supported by piers and covered by concrete-steel slabs, makes a very simple design. The forms are much easier to construct, and forms for only a limited area need be erected at one time. An excellent article on "Covered Reservoirs and Their Design," by Mr. Freeman C. Coffin, M. Am. Soc. C. E., is contained in the July, 1899, number of the Jour, of the Assn. of Engr. Soc. An article on the "Groined Arch/' by Mr. Leonard Metcalf, Assoc. M. Am. Soc. C. E., ap- pears in Trans. A. S. C. E. for June, 1900; and Mr. Frank L. Fuller presents an article on "Covered Reservoirs," in Jour. Assn. Engr. Soc. for Sept., 1899. 707. Examples of Concrete Reservoirs. Wellesley. — ■ The RESERVOIRS 457 reservoir at Wellesley, Mass./ a part of the water supply sys- tem, was designed by Mr. Freeman C. Coffin. It is eighty-two feet in diameter, walls fifteen feet high, four feet thick at bottom and two feet at top. The walls are of concrete and rubble masonry. In the construction of the walls, concrete was used containing three parts sand and five parts of stone to one of cement, one cubic yard of concrete containing about 1.2 barrels of cement. The bottom of the walls, which were de- signed to be built of concrete three feet four inches thick, were actually built of rubble four feet thick, as a large quantity. of bowlders was at hand. The excavation was in hard clay con- taining but little water, and the floor was made only four inches thick, of concrete of the same quality as that used in the walls. The floor and side walls were plastered with two coats, the first, one-half inch thick, of mortar containing two parts sand to one of Portland cement, and a coat about one-eighth inch thick, of neat Portland carefully rubbed and smoothed with trowels. Such a plaster coat should be applied before the con- crete has set. The two plaster coats cost twenty cents per square yard. 708. The piers to support the groined arch roof were two feet square, and built of brick. The span of the arches was 12 feet, rise 2.5 feet, and the concrete 0.5 foot thick at the crown. A channel iron ring or band was set in the concrete walls at the springing of the roof arches to take the thrust of the latter. The centers were placed over one-fourth of the area at a time, the piers being braced to take the thrust of the arches until the roof was completed. The concrete in the roof was composed of two and one-half parts sand and four and one- half parts broken stone to one part Portland cement. The centering cost twenty-two and one-half cents per square foot of area covered. The spandrels were filled in level with top of concrete at crown. On top of the concrete roof was placed six inches of clean gravel for drainage and to prevent the earth freezing to the concrete. This gravel was drained by four inch vitrified pipe discharging at the toe of the slope wall. 1 Engineering News, Sept. 30, 1897; Jour. Assn. Engr. Societies, July 1899; Trans. A. S. C. E., June, 1900. 458 CEMENT AND CONCRETE One foot of earth filling and one foot of loam were placed upon the gravel. 709. Astoria. — The reservoir for the Astoria City Water Works * was designed and built by Mr. Arthur L. Adams, M. Am. Soc. C. E. The reservoir has a capacity of six and one- fourth million gallons, walls twenty feet high. The excavation was in hard clay and sand mixed with clay, which in some places resembled a soft sandstone. The embankment was in general about five feet, the remainder of the depth being in excavation. The floor consisted of six inches of concrete, f inch cement mortar, one coat liquid asphalt and one coat harder asphalt. The slope lining was of six inches concrete, one coat asphalt, one layer of brick dipped in asphalt and laid flat, and a final finishing coat of asphalt. The concrete was composed of one barrel Portland cement, one-tenth cubic yard sand, five-tenths cubic yard gravel and nine-tenths cubic yard of crushed stone, these quantities of the ingredients making one cubic yard of concrete. Here we have an instance of the use of a mixture of broken stone and gravel, a practice which has already been commended as resulting in a small amount of voids. The concrete of the floor was laid in blocks twenty feet on a side, molds of two by six inch plank forming the outside edges of a block, and serving as a guide to the straight edge used in finishing, as in concrete walk construction. The finishing coat was of two parts fine sand to one of Portland cement and was applied, before the concrete base had begun to set, by two finishers with smoothing trowels. When the next block was to be laid, the plank were replaced by one-half inch weather boarding. When the concrete had thoroughly set, these boards were removed and the joints so formed were run full of asphalt, when the first layer of this material was spread. The concrete on the sides was also six inches thick and laitl in sheets ten feet wide, extending up and down the slopes, expansion joints being provided on the inclined joints only. The finishing coat of mortar was not used here, but all inequali- ties in surface were smoothed by using a little mortar from the next batch of concrete. 710. Each concrete gang was composed of twenty men and 1 Trans. A. S. C. E., December, 1,896. RESERVOIRS 459 one water boy. All concrete was mixed by hand on movable platform in half -yard batches. On the entire work 1.84 cubic yards of concrete per clay were mixed and placed per man employed, and on the floor alone this quantity was increased to 2.35 cubic yards, an excellent showing for this class of work. The cost of concrete per cubic yard, without profit, was as follows: — On Slopes: — Cement, at $2.45 per bbl $2.82 Other materials 1.94 Labor 1.07 Total per cubic yard for 600 yards $5.83 On Floor: —Cement, at $2.45 $2.64 Other materials 1.92 Labor 68 Total cost per cubic yard for 680 yards $5.24 The costs of the slope lining and floor complete, per square foot, are given as follows: — Slope: —6 inches concrete $0.1187 .649 inch asphalt 0100 Brick in asphalt 0889 .851 inch asphalt 0131 Chinking crevices .0030 Ironing 0036 Total cost per square foot of slope $0.2373 Bottom: — 6 inches concrete $0.1031 Cement mortar finish .0113 .537 inch coat asphalt 0077 .573 inch coat asphalt 0082 Total cost of bottom per square foot $0.1303 711. Forbes Hill. — The Forbes Hill reservoir 1 forms a part of the distribution system of the Metropolitan Water Works of Boston and was built under the direction of Mr. Dexter Brack- ett, M. Am. Soc. C. E. The reservoir is two hundred eighty by one hundred feet, partly in embankment. The soil under the embankment was first stripped to a depth of two and one-half 1 Described by Mr. C. M. Saville, M. Am. Soc. C. E., Division Engineer, before the N. E. Water Works Assn. Abstracted in Engineering News, March 13, 1902. 460 CEMENT AND CONCRETE feet at the toe, increasing to five feet stripping at the inner edge of the slope. The material was hard pan, and the embank- ments were built in four inch layers, rolled with four thousand pound rollers, so made as to leave a slightly corrugated surface. The bank was extended one foot inside of the finished line to assure a compact face, and afterward trimmed to grade. 712. The bottom of the reservoir was covered first with a layer of concrete about four and one-half inches thick, com- posed of one part natural cement, two parts sand, and five parts stone. The sand was of good quality; the stone came from the excavation and was washed before crushing. This layer of natural cement concrete was covered by a layer of Portland cement mortar one-half inch thick, made of two parts sand to one cement, and finished with a richer mortar, one part sand to four of cement. This half-inch layer was laid in strips four feet wide and finished like a cement sidewalk. Although this mortar coat was kept well moistened, some cracks developed which were filled with grout before applying the second layer of concrete. If no joints were used in the lower layer or base concrete, and joints in the coat of mortar were provided in one direction only, as appears to have been the case, the cracking should have been anticipated. At any rate, the value of the mortar coat be- tween the two concrete layers was greatly impaired by this cracking, and the experience points to the advisability of plac- ing the upper layer of concrete on the mortar before the latter has set, thus avoiding the expense of finishing the mortar layer. The upper layer of concrete was composed of one part Port- land cement, two and one-half parts sand and four parts broken stone, and was laid in blocks ten feet square. These blocks were laid alternately each way. The slope was first lined with Portland concrete of 1 to 2^ to 6;|, then one-half inch layer of mortar as for the bottom. The upper layer of concrete on slope was same as the upper layer of the bottom lining, but the blocks were eight by ten feet and finished with one inch of granolithic, in which stone dust and particles smaller than three-eighths inch were sub- stituted for the one and one-half inch stone of the concrete. 713. Cost. — The cost of lower layer of concrete on bottom, natural 1 to 2 to 5, was as follows: — RESERVOIRS 461 1.25 bbl. natural cement, at $1.08 $1,350 .34 cu. yd. sand, at $1.02 347 .86 cu. yd. stone, at $1.57 . 1.350 Materials in concrete $3,047 Forms, lumber, at $20.00 per M ... $0,090 Forms, labor 0.100 Total forms , .190 General expenses $0.08 Mixing and placing 1.17 1.250 Total cost per cu. yd $4,487 Cost of lower layer on slopes, Portland 1 to 2\ to 6^, was as follows: — 1.08 bb:s. Portland cement, at $1.53 . . . . . . $1,652 .37 cu. yd. sand, at $1.02 377 .96 cu. yd. stone, at $1.57 1.507 Materials in concrete . $3,536 Forms, lumber, at $20.00 per M $0,016 Forms, labor 0.121 Total forms .137 General expenses $0,177 Mixing and placing 1.213 1.390 Total cost per cubic yard $5,063 The cost of the upper layer on bottom and slopes, including the finish on slopes, Portland 1 to 2+ to 4, was as follows: — 1.37 bbls. Portland cement, at $1.53 $2.09 .47 cu. yd. sand, at $1.02 48 .745 cu. yd. stone, at $1.57 1.17 Materials in concrete $3.74 Forms, lumber, at $20.00 per M $0.25 Forms, labor 0.26 Total forms .51 General expenses $0.15 Mixing and placing 1.53 1.68 Total cost per cu. yd $5.93 462 CEMENT AND CONCRETE The cost of the half-inch plaster coat between the layers of concrete was twenty cents per square yard. 714. Rock ford. — A reservoir for the city of Rockford, 111., 1 was built almost entirely of concrete after plans prepared by the City Engineer, Mr. Chas. C. Stowell. The soil was a loose gravel, and after excavation was completed, parallel lines of drain tile were laid in trenches nine to ten feet centers and leading to a fifteen inch vertical sewer pipe carried to the sur- face of the street and capped. This sewer pipe served as a sump for a pump should it be found necessary at any time to repair the bottom. These trenches were filled with broken stone and the whole area of the foundation covered with three inches of clay. The concrete bottom was in two layers, eight inches and seven inches thick, respectively, and composed of two parts sand and five parts stone to one of Portland cement. The walls were of similar concrete for the bottom twelve feet, natural cement being used in the upper eight feet of the walls. The thickness at the bottom was 4^ feet, walls being straight on outside with one to ten batter on inside. The entire inner surface of floor and walls was plastered with one-half inch of Portland mortar, one to one. The cost of concrete in the work averaged $6.50 for Portland and $4.00 for natural, and the finishing coat cost seventy-five cents a square yard. 715. The roof was of concrete, expanded metal lath, and steel rods, the finished thickness being but two inches. This was supported by ribs of concrete, each twelve inches thick at the crown and having a seven-inch channel on the under side. The springing line of the ribs was eight feet below the top of the walls, giving a good depth at the skew back. Ribs were spaced about seven feet centers. The span of the roof was about fifty-five feet, and rise about eleven feet. The cost of roof was less than twenty-five cents a square foot. 716. Concord. — The groined arch roof of the Concord, Mass., 1 sewage storage well, designed by Mr. Leonard Metcalf, Assoc. M. Am. Soc. C. E., was fifty-seven feet diameter and contained about one hundred cubic yards of masonry. The cost of the roof per square foot of surface was as follows: — Described in Engineering News, Feb. 22, 1894. RESERVOIRS 463 Centering . $0.18 Concrete materials .15 Labor and supervision .05 Total $0.38 717. Albany. — The Albany filter plant roof/ designed by Mr. Allen Hazen, Assoc. M. Am. Soc. C. E., was also of the groined arch type, the arches having the same span and rise as the Wellesley reservoir. The cost of the roof per square foot of area was as follows: — .029 eu. yd. concrete, at $6.30 $0,182 Piers .054 Earth filling and seeding .014 Manholes, entrances, etc .027 Total cost per sq. ft $0,277 For a list of reservoirs and filter beds, in the roofs of which groined arches have been used, giving in tabular form the general dimensions, the proportions used in the concrete, and in several cases the cost of the roof per square foot of reservoir, the reader is referred to Engineering News of December 24 1903. 1 Trans. A. S. C. E., June, 1900. CHAPTER XXII SPECIAL USES OF CONCRETE (Continued) BRIDGES, DAMS, LOCKS, AND BREAKWATERS Art. 81. Bridge Piers and Abutments and Retaining Walls 718. The use of concrete in large bridge piers was at first confined to the hearting or backing of stone masonry shells. It was soon found, however, that in many cases the concrete was able to withstand the effects of frost and ice better than was the variety of stone available for building the masonry shell, and many important bridges are now supported by piers built entirely of concrete. As an example of this use may be mentioned the bridge "across the Red River l in Louisiana, which has six concrete piers of heights from forty-four to fifty-three feet. The pivot pier is twenty-seven feet in diameter, with vertical sides. The draw rest piers are seven feet wide under the coping, nineteen feet between shoulders and twenty-six feet long over all. The sides have a batter of one-half inch to the foot. The coping is of limestone. 719. In the construction of the Arkansas River Bridge 2 of the K. C. P. & G. R. R., ten piers and two abutments were built of concrete. The piers varied in height from twenty to sixty-five feet, some of them containing over six hundred cubic yards of concrete. The entire work was completed in eleven months, although many difficulties were met. The concrete was composed of one part Portland cement, two and one-half parts coarse, sharp sand, and five parts of clean, broken stone. The lagging for the forms was of two-inch yellow pine, sur- faced one side and sized to one and three-quarters inches. On 1 George H. Pegram, Consulting Engineer. Walter H. Gahagan, Engi- neer for Contractors. 2 Engineering News, Aug. 25, 1898. 464 BRIDGE PIERS 465 the straight part of the pier this lagging was laid horizontal and supported by four by six vertical posts set four-feet centers, posts on opposite sides of the pier being tied together by three- quarter inch bolts passing through one and one-half inch gas pipes spaced five feet vertically. The gas pipe was allowed to remain in the finished pier, the bolts being withdrawn. The lagging for the semicircular ends was of two by six with bevel joints, placed vertical, and supported at five-foot inter- vals by segmental ribs of double two by twelve planks. At the ends of the ribs were bolted short pieces of eight by eight inch angle irons with edge horizontal. These angle irons were, in turn, bolted to four by six verticals at the corners or shoul- ders of the pier. 720. The foundation piers of the Lonesome Valley Viaduct, 1 thirty-six piers and two abutments, are entirely of concrete. The piers are four feet square on top with batter of one inch to the foot, and are from five to sixteen feet in height. The total concrete laid was 926 cubic yards at a contract price of about $7.00 per cubic yard. The piers were finished On top with a steel plate, four feet square and one-half inch thick, taking the place of coping stones. Where rock foundations were not found, the lower portion of the pier was given an increased batter to secure such a cross-sectional area at the bottom that the unit pressure on the earth did not exceed one ton per square foot. The cost of the concrete for this work has already been given (§ 322). 721. Steel Cylinders. — Steel shells filled with concrete have been used to good advantage, especially for bridge approaches. Such shells are usually in pairs placed abreast, one under each truss of the bridge or viaduct. The two cylinders of a pair are usually connected by lateral bracing, and if desired in heavy work, this bracing may be inclosed in a concrete wall and thus protected from injury by running ice, etc. The thickness of metal in the shells need not be great, three-eighths of an inch usually being sufficient, though this depends on the height, the stresses, and the liability to injury. In soft ground requiring piles, most of the piles are sawn off below the limit of scour, or below the water line for land piers, but one or more may be 1 Gustave R. Tuska, Trans. A. S. C. E., September, 1895. 466 CEMENT AND CONCRETE allowed to project up into the cylinders and the concrete filled in around the heads, thus anchoring the pier. In foundations on rock if the cylinders require an anchorage, this may be pro- vided with bolts fox-wedged or cemented in the rock and pro- jecting into the cylinder. (For details of methods adopted in this class of construction, see "Bridge Substructure and Foun- dations in Nova Scotia," by Martin Murphy, Trans. A. S. C. E., September, 1893.) 722. Repair of Stone Piers. — Where masonry piers are being destroyed by the abrading or expansive action of ice, or by other causes, concrete is successfully used to arrest such action, the entire pier being incased in a layer, one to three feet thick, of Portland cement concrete of good quality. The piers of the Avon River bridge, 1 originally built of ashlar masonry, failed entirely to withstand the deteriorating in- fluences of an extreme range in tide coupled with the severe temperature of a Nova Scotia winter. Five of them were sub- sequently incased in concrete, as follows: A form was made of ten by ten inch spruce timber surrounding the ashlar masonry of the piers and forming a mold to receive the concrete and retain it in place until set. The thickness of the concrete casing was two and one-half feet at the bottom and one and one- third feet at the top, which was three feet above high water. The concrete was composed of one barrel Portland cement, one and one-half barrels clean sand, one barrel of clean gravel, and in it was placed by hand four parts of common field stone weighing from eight to twenty pounds each. This treatment was entirely successful in preventing further disintegration. 723. An efficient cutwater for bridge piers is made by placing old rails vertically on the upstream nose of the pier, anchoring them to the masonry and filling between with concrete, leaving only the wearing surface of the rail head exposed. 724. Pneumatic caissons are usually filled with concrete, the filling over the working chamber being carried up fast enough to keep the work above water as the caisson is sunk. The filling of the working chamber calls for special care in tamping under and around the longitudinal and cross-timber braces. A space of about three inches next the roof of the chamber is 1 Trans. A. S. C. E., December, 1893. RETAINING WALLS AND ABUTMENTS 467 filled with a rich concrete, containing no stone larger than one inch, mixed quite dry and solidly tamped with special edge rammers. 725. RETAINING WALLS AND ABUTMENTS. — Concrete is used very largely for constructing retaining walls and bridge abut- ments. The foundations of a retaining wall should be of ample width, and if the wall is not founded on rock, some settlement and outward movement may be expected if the conlmon for- mulas are used in computing the dimensions. If this movement is not equal throughout the wall, cracking is likely to take place, and to confine these cracks to prede- termined vertical planes, it is well to construct the wall, if a long one, with vertical joints at intervals of fifteen to thirty feet. Such a joint is made by building one section and fol- lowing with another, without special precautions to make a bond between. If there is an opportunity for water to accumulate, care should be taken to drain the earth back of the wall, either by drains leading around the ends, or by pipes passing through the wall. The latter may result in discoloration of the face. 726. Coping. — The face of a retaining wall or abutment is preferably given a batter, and a coping is provided to improve the appearance. The coping should have a slight inclination toward the back to prevent the discoloration of the face by dripping. It should be divided by vertical joints into blocks, not more than six to eight feet in length. The projection of the coping will depend upon the dimensions of the wall. Wing- walls are preferably built with a sloping top or coping, but this should be made monolithic with the wall by special molds (§729). 727. Rules for Use of Concrete in Abutments. — In the use of concrete for abutments and piers, the practice of the Illinois Central Railroad, as set forth in their specifications, can hardly be improved upon. The engineer of bridges and buildings on this road, Mr. H. W. Parkhurst, M. Am. Soc. C. E., is one of those engineers who early recognized the value of concrete in bridge work, and as the result of his extensive experience along this line, he is widely known as an able and conservative au- thority. These specifications are printed nearly in full in Engineering 468 CEMENT AND CONCRETE News of July 18, 1901, from which the following extracts are made : — 728. Natural and Portland Cement : Where used : — Natural cement concrete "may be used where foundations are entirely submerged below low-water mark, or where there is no risk of the same being exposed to the action of the weather by cutting away the surrounding earth. It, however, shall be used only where a firm and uniform foundation is found to exist after excavations are completed. In all cases where foundations are liable to be exposed to the action of the water, or where the material in the bottom of excavations is soft or of unequal firmness, Portland cement concrete must be em- ployed for foundation work. "The natural cement concrete shall usually be made in the proportions (by measure) of one part of approved cement to two parts of sand and five parts of crushed stone, all of char- acter as above specified. For Portland cement concrete foun- dations, one part of approved cement, three parts of sand and six parts of crushed stone may be used. Wherever in the judgment of the engineer or inspector in charge of the work, a stronger concrete is required than is above specified, the pro- portions of sand and crushed stone employed may be reduced, a natural cement concrete of 1, 2 and 4, and a Portland cement concrete of 1, 2 and 5 being substituted for those above speci- fied. " Portland Cement Concrete: — Concrete for the bodies of piers and abutments, for all wing-walls for same, and for the bench walls of arch culverts, shall generally be made in the pro- portions (by measure) of one part of cement, two and one-half parts of sand and six parts of crushed stone. Where special strength may be required for any of this work, concrete in the proportions of 1, 2 and 5 may be used; lout all such cases shall be submitted to the judgment of the engineer of bridges, before any change from the usual specification is to be allowed. "For arch rings of arch culverts and for parapet head walls and copings to same, Portland cement concrete, in proportions of 1, 2 and 5, shall generally be used. Concrete of these pro- portions shall also generally be used for parapet walls behind bridge seats of piers or abutments, and for the finished copings (if used) on wing-walls of concrete abutments, also for arch USE OF CONCRETE AND ABUTMENTS 469 work in combination with I-beams or in combination with iron- work for transverse loading. "Bridge seats of piers and abutments and copings of con- crete masonry which are to carry pedestals for girders or longer spans of ironwork, shall generally be made of crushed granite and Portland cement, in the proportion (by measure) of one part of approved cement, two parts of fine granite screenings, and three parts of coarser granite screenings, the larger of which shall not exceed three-quarters inch in greatest dimension." 729. After specifying the method of building molds, which is treated elsewhere (Art. 62), the specifications proceed: — "The planking forming the lining of the molds shall in- variably be fastened to the studding in perfectly horizontal lines ; the ends of these planks shall be neatly butted against each other, and the inner surface of the mold shall be as nearly as possible perfectly smooth, without crevices or offsets be- tween the sides or ends of adjacent planks. Where planks are used a second time, they shall be thoroughly cleaned, and, if necessary, the sides and ends shall be freshly jointed so as to make a perfectly smooth finish to the concrete. "The molds for projecting copings, bridge seats, parapet walls, and all finished work shall be constructed in a first-class workmanlike manner, and shall be thoroughly braced and tied together, dressed surfaces only being exposed to the contact of concrete, and these surfaces shall be soaped or oiled if necessary, so as to make a smoothly finished piece of work. The top surfaces of all bridge seats, parapets, etc., shall be made per- fectly level, unless otherwise provided in the plans, and shall be finished with long, straight edges, and all beveled surfaces or washes shall be constructed in a true and uniform manner. Special care shall be taken in the construction of the vertical angles of the masonry, and where I-beams or other ironwork are not used in the same, small wooden strips shall be set in the corners of the mold, so as to cut off the corners at an angle of 45°, leaving a beveled face about one and one-half to two inches wide, instead of a right-angled corner. "Where wing- walls are called for, which have slopes corre- sponding to the angle of repose of earth embankments, these slopes shall be finished in straight lines and surfaces, the mold for such wing-walls and slopes being constructed with its top 470 CEMENT AND CONCRETE at the proper slope, so that the concrete work on the slope may- be finished in short sections, say from three to four feet in length, and bonded into the concrete of the horizontal sections before the same shall be set, each short section of sloped sur- face being grooved with a cross-line separating it from adjacent sections. It will not be permitted to finish the top surface of such sloped wing-walls by plastering fresh concrete upon the top of concrete which has already set, but the finished work must be made each day as the horizontal layers are carried up, to accomplish which the mold must be constructed complete at the outset; or, if the wing-wall is very high, short sections of the mold, including the form for the slopes, must be completed as the horizontal planking is put in place." 730. This is followed by directions concerning foundation work; the following is given relative to building steel into the masonry : — "Iron rails to be furnished by the railroad company shall be laid and imbedded in such manner as may be specified in such foundation concrete as in the opinion of the engineer of bridges needs such strengthening, and no extra charge, except the actual cost of handling the same, shall be made by the contrac- tor for such work, but the volume of such iron shall be esti- mated as concrete. "Where I-beams are to be placed in the angles of concrete piers as a protection against ice, drift, etc., these shall be set up and securely held in position so that they will extend one foot or more into the foundation concrete. The planking of molds shall be fitted carefully to the projecting angles of these I-beams, and small fillets of wood shall be fitted in between the inner faces of the mold and the rounded edges of the I-beam flanges, so that no sharp projecting angle of concrete will be formed as the work is constructed. "These fillets may be made in short pieces and fastened neatly into the mold as the layers of concrete are carried up. Such I-beams will generally be furnished of sufficient length to extend at least six inches above the top of the battered masonry into the concrete coping, and special pains shall be taken to tamp the concrete thoroughly around the I-beams, and to finish the coping above and around the ends of the same, so as to make a compact and solid bearing against the ironwork. CONCRETE PILES 471 "Where anchor bolts for bridge-seat castings are required, they shall be set in place and held firmly as to position and elevation, by templets, securely fastened to the mold and framing. Such I-beams and anchor bolts shall be imbedded in the concrete work without additional expense beyond the price to be paid per yard for the several classes of concrete in which such iron is placed, the volume of iron being estimated as concrete. "After the work is finished, and thoroughly set, all molds shall be removed by the contractor. They shall generally be allowed to stand not less than forty-eight hours after the last concrete work shall have been done. In cold weather, molds shall be allowed to stand a longer period before being removed, depending upon the degree of cold. No molds shall be re- moved in freezing weather, nor until after the concrete shall have had at least forty-eight hours, with the thermometer at or above 40° Fahr., in which to set."' 731. After giving in detail the methods to be followed in placing and ramming concrete and the use of facing mortar, the following paragraph is especially applicable to the subject in hand: "Layers of concrete shall be kept truly horizontal, and if, for any reason, it is necessary to stop work for an indefinite period, it shall be the duty of the inspector and of the contractor to see that the top surface of the concrete is properly finished, so that nothing but a horizontal line shall show on the face of the concrete, as the joint between portions of the work con- structed before and after such period of delay. If for any reason it is impossible to complete an entire layer, the end of the layer shall be made square and true by the use of a temporary plank partition. No irregular, wavy or sloping lines shall be per- mitted to show on the face of the concrete work as the result of constructing different portions of the work at different periods, and none but horizontal or vertical lines shall be per- mitted in such cases." Art. 82. Concrete Piles 732. Piles may be made of concrete either with or without steel reinforcement. In the former case they are built in place, but where steel is used, the piles are usually driven after they 472 CEMENT AND CONCRETE have been prepared in suitable molds. Concrete is also em- ployed to protect from decay, or from the ravages of the teredo, wooden piles already in service. Concrete piles are much more durable than wooden piles, and may be used without reference to the water line. A sav- ing may thus be made under certain conditions, as the use of concrete piles may obviate the necessity of excavating to the water line and building up with masonry resting on a wooden pile foundation. As the diameter of the pile is not limited, a much greater load per pile may be provided for. There are of course many places where piles of concrete are not as suit- able as wooden piles; they are not as well adapted to with- stand certain kinds of hard usage, such as violent shocks, and they are much less flexible. 733. Building in Place. — In certain kinds of soil, such as stiff clay, a wooden pile, or dummy, of the proper length may be driven and withdrawn, the hole left being at once filled with concrete. The application of this crude method is very lim- ited, as it is seldom that the soil will stand until the hole is filled with concrete. For the building of piles without reinforcement, Mr. A. A. Raymond 1 has patented a system by which a thin steel shell or casing is driven to the desired depth and then filled with concrete in place. A shell is first slipped over a steel pile core made to fit it, and the shell and core are driven by a pile driver in the ordinary manner. The core is then slightly shrunken in diameter, by a simple device, and withdrawn, leaving the shell in the ground. The core is hoisted in the pile driver leaders, another shell is lowered into the one just driven and then slipped up on the core, after which the driver is shifted to the next location, and this shell is driven in the same manner as the first. The filling of the shells with concrete is done as soon as convenient. While the shape of the shells may be varied to suit conditions, the ordinary size is about twenty inches diameter at the top and six inches at the bottom, and such a shell twenty feet long weighs about seventy pounds. 734. The same company has a system of sinking shells in sand by the water jet. For this purpose the shells are in Raymond Concrete Pile Co., Chicago, 111. CONCRETE PILES 473 conical telescopic sections about eight feet in length. A two and one-half inch pipe with three-quarter inch nozzle is attached to the center of a cast iron point fixed to the inner section. Water forced through the pipe causes the shell to settle, and as the inner shell descends, its upper end engages with the lower end of the second section, so that when fully lowered the sec- tions form a continuous cone. The concrete is filled in simul- taneously with the sinking, imbedding the two and one-half inch pipe which remains permanently in the center of the con- crete pile. 735. Concrete-Steel Piles : Molding. — Piles of concrete-steel usually have three or more steel rods of about one square inch cross-section imbedded longitudinally in the pile, and connected by smaller rods or wires at intervals of six to ten inches. Molds are so made that they may readily be detached and used again. At least one side of the mold should also be in short sections that may be put in place as needed, in order to facil- itate placing the concrete. The molds should be set up verti- cally with the longitudinal steel rods in position. Enough con- crete is put in the molds to fill six to ten inches in length, when a set of transverse tie rods or wires is placed, then another layer of concrete, etc. The concrete, which is of Portland cement, should be mixed rather wet, as thorough tamping is difficult in the confined space. The piles should be provided with a cast iron shoe at the bottom, or a steel plate covering to protect the point. At the top, one of the main rods is bent over to form a ring to facilitate handling the piles. 736. Driving. — When the concrete has hardened suffi- ciently, say at the end of four to eight days, the mold should be removed, and the pile allowed to remain in its original posi- tion twenty to twenty-five days longer, sprinkling it occasionally. When thoroughly set, they may be driven with an ordinary pile driver, using a heavy hammer and short drop. A steam hammer is preferred, however, and a special cap must be used to prevent injury to the pile head. Such a special cap may well be made of cast steel, fitting over the head of the pile like a helmet. The space between the lower end of the cap and the side of the pile is calked with clay and rope yarn or other suitable material. Through a hole provided in the top of the helmet, the space between the pile and cap is then com- 474 CEMENT AND CONCRETE pletely filled with dry sand. Such a cushion cap effectually protects the pile head, distributing the pressure to the entire head. Caps in the form of a steel ring filled with sawdust sur- mounted by a wooden block, and also caps made of alternate layers of lead, wood and iron plates have been successfully used. Art. 83. Arches 737. The use of concrete in the construction of arch bridges is becoming so extended and diversified that it would require a volume by itself to adequately cover the subject, and such a treatment of it is well merited. All that can be attempted here is to describe briefly one or two examples of well propor- tioned arches, and to give a few hints on methods of design and construction. 738. DESIGN. — Concrete arches may be built with or with- out steel reinforcement, but for long spans concrete-steel is usually employed. The design of a concrete arch without steel is entirely similar to that of a stone masonry arch, except that planes of weakness corresponding to joints between voussoirs in a masonry arch, may be somewhat more arbitrarily arranged in the former. In fixed concrete-steel arches, the arch ring is continuous, and is capable of resisting a bending moment. The compu- tations are therefore somewhat more complicated, and until the action of concrete and steel in combination has been more carefully determined, it may be said, in the words of a promi- nent engineer, that "the development of the system of arches of concrete must necessarily be largely based upon empirical information coupled with sound judgment and work executed with great care." '■ Fortunately, the saving effected by this con- struction over a masonry arch is usually so great that it is possible to use a large factor of ignorance, and it is to be hoped that the use of concrete-steel for arches of long span will not be given a serious check by the failure, perhaps under unforeseen conditions, of some of the web-like structures that have been built of it. 739. Where the span and rise of the arch are not fixed by the local conditions, the comparative economy of different 1 L. L. Buck, Trans. A. S. C. E., April, 1S94. ARCHES 475 arrangements and the appearance of the completed structure must govern. Shortening the spans decreases the amount of concrete required in the arches, but increases the pier work, which is usually the most expensive part of the structure. These points having been decided, the form to be given the arch ring is next to be considered. While it is desirable that the neutral axis of the arch ring should nearly correspond with the line of pressures for a full load, there is still considerable choice allowed the designer as to the actual form to be given the intrados without serious changes in the amount of material required. As the semicircular arch can usually be adopted for very short spans only, the choice must lie between the seg- mental, the elliptical, and the polycentered arch approaching more or less closely the ellipse, the parabola, or the transformed catenary. The segmental arch, the parabola and the catenary do not give a pleasing effect at the junction of the arch ring and the abutment, and the curve is sometimes departed from near the springing to make the intrados tangent to the face of the abut- ment. The final choice will thus usually lie between a true ellipse and the basket-handled arch. Mr. Edwin A. Thacher, M. Am. Soc. C. E., designer of the Topeka bridge, considers that "arches with solid spandrel fill- ing should be flat at the center and sharper at the ends, ap- proaching an ellipse; while arches with open spandrel spaces should be sharp at the center and flatter at the ends approach- ing a parabola, or, which is better, sharp at the ends and center and flat at the haunches." : The form of the intrados having been fixed, the depth of key- stone for an arch without reinforcement is derived, tentatively, from the rules of either Rankin or Trautwine, to be corrected later if necessary. The form of the extrados is then so chosen as to give the required depth of arch ring to confine the line of pressure within the middle third. 740. Concrete-steel Arch. — The computation of a concrete- steel arch is, as already stated, more, involved. The graphical analysis is much the simplest method of deriving the bending moment, direct thrust and shear. The experience of Mr. Engineering News, Sept. 21, 1899. 476 CEMENT AND CONCRETE Thacher has led him to endeavor to have the line of pressure lie within the middle third of the arch ring, although this is not absolutely necessary in reinforced concrete. The same author- ity considers it good practice to design the steel work to be capable of taking the entire bending moment with a unit stress of about one-half the elastic limit of the steel. The thrusts, bending moments and shears at successive sec- tions of the arch ring having been determined, both for full and half span loads, by the graphical methods explained in Greene's "Arches" or Cain's "Elastic Arches," or by the analy- sis given in Howe's "Treatise on Arches," the dimensions of the arch ring and the steel reinforcement are to be derived by the aid of such formulas as are given by Mr. Thacher 1 involving the allowable unit stresses in steel and concrete, and their respective moduli of elasticity. 741. General Considerations. — In short spans, parallel spandrel walls with earth filling between, may be used, but for long spans the spandrels are usually open, that is, built of vertical piers or walls running parallel to the axis of the soffit, and arched over at the top to support the pavement or ballast. This treatment has the following advantages: only vertical forces are transmitted to the arch ring; decreased loads on arch and abutments; increased waterway in case of unusual floods; and better architectural effects. The beauty of the structure is an important consideration, inasmuch as the decision in favor of a concrete arch as against a steel bridge is usually affected quite as much by considera- tions of aesthetic effect as of cheapness and durability. In this connection it may be said that in concrete-steel construc- tion there may be little difference in the thickness of arch ring required at the crown and near the springing, but the appear- ance of the structure will usually be improved by accentuating a little, if necessary, the increased thickness at the springing, except in the case of the semicircular arch in which the eye is accustomed to a nearly uniform thickness of the voussoirs. The appearance is also frequently improved by molding the concrete at the crown to represent a keystone, projecting a little beyond the face of the rest of the arch ring. 1 Engineering Neivs, Sept. 21, 1899. ARCHES 477 The beauty of a concrete arch may easily be marred by faulty design, and some very ugly, as well as some very beau- tiful, arches have been erected. 742. Stone Facing. — The practice which has been followed to some extent of facing the spandrel walls with cut stone masonry, is considered questionable. The cost of ashlar facing is likely to be so great as to discourage the use of headers of sufficient length to give a good bond with the concrete, and it is next to impossible to make this equal to monolithic concrete construction. Again, since concrete is frequently used to pro- tect ashlar masonry that has started to disintegrate, it is rather a reversal of what has been found good practice to face con- crete with a thin layer of cut stone. No criticism is intended of the method of building a pier of large dimension stone with concrete hearting, as this is a different matter. But a thin parapet or spandrel wall faced with a mere shell of cut stone, however beautiful it may be when built, is likely to take on a somewhat dilapidated appearance after ten years' service, espe- cially if it is called upon to pass through one or two floods of "unusual violence. 743. Quality of Concrete. — As already intimated, the cost of a concrete arch, especially where reinforcement is used, is, under ordinary circumstances, considerably less than a masonry arch of equal appearance and strength. The only exceptions to this rule are where the facilities for obtaining stone suitable for masonry are exceptional, and where the work is far re- moved from cement-proclucing regions and from the coast. The ability to employ common labor for much of the construc- tion work in a concrete arch is an advantage only partially offset by the necessity of having somewhat more careful work done upon the arch centers and more careful supervision of construction. The concrete of the arch ring should be of the best quality, especially if steel reinforcement is not used. For this purpose, the stone, broken to a size not exceeding two inches in any dimension, should be mixed with a quantity of mortar a little more than sufficient to fill the voids, and composed of one part Portland cement to two parts sand. Interiors of piers and abutments may be made of a poorer mixture, such as one Portland cement to three of sand and six of broken stone, or 478 CEMENT AND CONCRETE even in some cases where abutments are massive, one to four to eight concrete may be employed. 744. Centers. — Substantial centers must be provided for concrete arches, and the lagging should be sized, dressed on the upper side, and laid with radial joints parallel to the arch axis. Two inch plank sized to one and three-quarters inches is usually employed for lagging, and the supporting ribs should be from three to four feet centers. For spans up to forty feet a braced wooden rib with one center support and two end supports is used, but for longer spans a trussed center with supports ten to eighteen feet apart is employed. The centers should be made rigid and the camber need be very slight, say from laVo to e^o °f the radius at the crown. Not less than twenty-eight days should be allowed to elapse after building the arch before striking the centers. 745. Construction. — A method that has been largely em- ployed in building the arch ring is to divide the arch into lon- gitudinal rings by planes at right angles to the arch axis. It is believed to be better practice, however, to build the arch as a series of voussoir courses beginning with the spring course, but not necessarily proceeding in order from the springing to the crown. The advantages of this method of building the arch, in transverse courses parallel to the axis of the intraclos, are that the planes of weakness may be made at right angles to the line of pressure; the unequal loading, and consequent settlement of the centers, has less tendency to crack the sec- tions or to separate one section from another. In cases of failure of concrete arches under excessive floods, the tendency of the arch to separate along a longitudinal joint forming a plane of weakness has been clearly shown. 746. The tendency of the center to rise at the crown as the arch ring is built up on the haunches is sometimes overcome by temporarily loading the crown. In constructing the ring in voussoir courses, the order of the work may be so arranged as to distribute the loading on the centers in any manner desired. Such an expedient was adopted in the construction of the Illinois Central R. R. arch across Big Muddy River, where the arch ring was divided into nineteen voussoirs. The two spring- ers were built first, then the fifth row of voussoirs towards the crown on each side, followed by the ninth row, the third and ARCHES 479 seventh. The intermediate blocks were then built in order toward the crown, the second, fourth, sixth and eighth, and finally the keystone. In this way the weight was well dis- tributed on the centers, and the load on the two sides of the crown was kept symmetrical. The monolithic blocks forming the voussoirs that were built in molds had recesses on either side, which were made by securing planks to the interior of the mold. When the intermediate blocks were built, the con- crete thus keyed into the blocks first made. The division of the work into voussoir courses will usually admit of such size molds or blocks that two, one on either side of the center, may be completed in a day. If it becomes ne- cessary to interrupt the laying of a block, however, a vertical bulkhead should be constructed in the mold, with key or dowel pins if desired, to assist in making a bond when the block is completed. 747. Finish and Drainage. — To provide a smooth face, a thin facing mortar of one part Portland to two parts sand is desirable, laid at the time of building the concrete in accord- ance with methods already described. A thicker layer of granolithic may be used on the soffit and will somewhat more effectually prevent the broken stone of the concrete settling on the lagging, which is always likely to occur to the detriment of the appearance of the finished work. The division between adjacent voussoirs should be clearly marked on the face, and additional joints may be indicated if desired, by lines in a plane approximately perpendicular to the line of pressure. Such lines are obtained by securing triangular strips on the inner face of the molds. When spandrel walls are used, these may be similarly marked on the face by horizontal and vertical joints. On long spans the spandrels should have expansion joints, and the coping and parapet, when of concrete, should also have vertical joints to provide for changes in length due to loading or thermal variations. The arches over the spandrels should be provided with a waterproof covering, either of Portland cement grout or an asphalt mixture to prevent the percolation of water to the arch ring. Pipe drains should be provided to carry the water to a point over the piers where it may be discharged. Care should be taken that such pipes have their outlets so located that the 480 CEMENT AND CONCRETE drip shall not disfigure the wall. Open spandrels may be drained by pipes built into the arch ring at suitable places. 748. Highway Arch without Reinforcement. — A good ex- ample of a highway bridge built of concrete without reinforce- ment is the monolithic arch spanning San Lea.ndro creek, be- tween Oakland and San Leandro, Cal. 1 This arch has a five centered, elliptical intrados, with span of 81i feet, rise of 26 feet and width of about 60 feet. At the crown the thickness of the arch ring is 3 feet, the radius of the intrados 61^ feet and of the extrados 88 feet. As the arch rests directly on a bed of clay containing some gravel, the footings are made 30 feet wide, and they extend 5 feet below the creek bed. The lagging for the forms was of 2 by 6 inch scantling laid transverse to the axis of the structure or parallel to the axis of the intrados. The ribs of the centering were built of two 1 by 12 inch boards and the braces of 4 by 6 inch timbers, converged to three short 12 by 12 inch timbers supported by wedges bearing on 12 by 12 inch longitudinals. 749. The concrete was composed of one barrel Portland cement, two barrels sand to seven barrels of broken stone of varying sizes. When the haunches had been built up about one-third the way, as flooding of the work was anticipated, an arch ring one foot thick was first completed, the remainder being placed as a second layer. There is a parapet wall three feet six inches high on either side of the bridge. The spandrel walls show a solid face, and are paneled to bring out the outlines of the extrados and parapet. The centers were struck ten clays after the completion of the second arch ring, and the settlement at the crown was about one and one-half inches. The forms con- tained 90,000 feet, B. M., of lumber, and 3,384 cubic yards of concrete were used. The cost of the bridge was $25,840.00, or less than $8.00 per cubic yard of concrete. The contractors were the E. B. and A. L. Stone Co. of Oakland, Cal., and the plans were prepared by the County Surveyor's office of Alameda County, Cal. 750. A Three Span Arch. — The three span arch spanning a 1 Described by Mr. William B. Barber, Engineering News, Aug. 27, 1903. ARCHES 481 mill pond on Anthony Kill, near Mechanicsville, N. Y., 1 is worthy of notice on account of some peculiarities in the center- ing and because of the location of the plant on a side hill, so that the concrete was delivered on the work with very little labor. Two of the arches were of 100 ft. span, with rise of 20 feet, and the remaining arch was of 50 ft. span. The width is but 17 feet, and the piers are founded on rock at a depth not exceeding 12 feet. For the centering, piles were first driven, six feet centers, in bents ten feet apart, and the bents capped with ten by twelve inch timbers. Stringers of the same dimensions were then laid longitudinally, and eight by ten inch posts were erected on the longitudinals and spaced three feet centers. These posts, which were cut to proper length, so that their tops conformed to the curve of the intrados, were then capped with eight by ten inch timbers parallel to the axis of the intrados, and the lagging- laid upon them transverse to this axis or parallel to the center line of the bridge. This lagging was of two thicknesses of one inch boards sprung into place and nailed, the upper layer being of dressed lumber to give a smooth surface to receive the con- crete. The concrete was of one part Portland cement, three parts sand, three parts gravel and three parts broken stone, except for the arch ring, in which but two and one-half parts each of gravel and stone were used. The concrete plant was so ar- ranged that the stone could be passed from the crusher to the mixer by gravity. The concrete was delivered on the arch in cars of three feet gage drawn by cable. From fifty to sixty cubic yards of concrete were placed in ten hours with but nine laborers. 140,000 feet B. M. lumber was used in centers. The entire work consumed about 2,500 cubic yards of concrete. 751. Railroad Arch without Reinforcement. — The concrete bridge carrying the Illinois Central R. R. over the Big Muddy River furnishes an excellent example of a long span arch, built without reinforcement so far as the arch ring is concerned. The bridge is very fully described by Mr. H. W. Parkhurst, Engineer of Bridges and Buildings I. C. R. R., in Engineering News of Nov. 12, 1903. There are three spans, each 140 feet Described in Engineering News, Nov. 5, 1903. 482 CEMENT AND CONCRETE in the clear, with 30 feet rise above springing lines. The arch ring proper is five feet thick at the crown, but as the spandrels, which are built open over the haunches, have near the crown only a false opening on the face, the actual thickness of concrete at the crown is seven feet. The piers and abutments already in place for the three Pratt trusses formerly in use, were surrounded with new con- crete masonry, making the piers 21 ft. 6 in. wide at the top. As rock was found only at considerable depth, the piers rested on piles. To relieve the load on foundations as much as possible, as well as to avoid cracking, which would be likely to occur in heavy longitudinal spandrel walls from temperature strains, transverse spandrel arches were adopted. Since in case of de- railment of trains these spandrel arches would be subjected to shock, the concrete in this portion of the structure was rein- forced by a self-supporting skeleton structure built of steel rails. Longitudinal rails were laid horizontally, three feet center to center, connected at frequent intervals by one inch rods and held in place by vertical posts, which in turn rested upon transverse horizontal rails laid in recesses left in the arch rib. 752. Expansion joints were provided in the spandrel arches at the ends, two at each pier and one at each abutment, to al- low some movement due to changes in temperature. The ex- pansion joints were made by placing in the joint several thick- nesses of corrugated asbestos board protected by a ^-inch lead plate folded into the joint, forming a trough at the top. The lead plate lies flat on top of the concrete for five inches from the joint, and about two inches at each end of the plate is bent down at right angles and set into the concrete. An asphaltic composition is then laid over the lead plate, entirely covering it and filling the trough. The centering was erected on pile bents spaced about 14 feet centers, the calculated pressure on each pile being about eighteen to twenty tons. For the center span, five 60 foot deck plate girders resting on pile piers were used over the deep- est portion of the channel to provide for possible floods bring- ing large amounts of drift. 753. The arch ring was laid in voussoir courses as described in § 746. Face joints were made by securing triangular shaped ARCHES 483 pieces to inner face of the molds in lines approximately at right angles to the line of pressure. All exposed work was faced with a layer of about 1J inches of Portland cement mortar placed and rammed with the concrete. The surfaces were not given, in general, any further finish, no attempt being made to remove or conceal the usual marks left by the mold boards. Portland cement was used throughout, the quality of the concrete being varied by the amount of cement used to given quantities of the aggregates. In the centers of large masses the poorer mixtures were employed, while the richer concretes were used in those places subjected to the most trying conditions. In making the concrete the principle followed seems to have been to keep the mixer as near the work as practicable, moving the mixer and carrying materials to it, rather than to transport the mixed concrete from a certain fixed location of the mixing plant. Much of the concrete was handled in barrows, but derricks were also used in portions of the work. As traffic on the old bridge had to be maintained during the erection of the new structure, considerable extra handling of concrete was necessary and additional work was involved in ramming the con- crete in places difficult of access. The concrete was mixed rather wet, so that but little tamping was required to make it quake. 754. Cost. — The total amount of concrete was over 12,000 cubic yards, which was placed at an average cost of $5.43 per cubic yard. In cofferdams and centers 400,000 feet B. M. of timber was used, and about 300,000 pounds of steel was em- ployed in the skeleton structure of the spandrels. This steel cost 1.2 cents per pound, the punching, fitting and erecting costing but about 0.61 cent per pound. The total cost of the bridge is estimated to have been $125,000.00, or about the same as the estimated cost of a steel structure designed for the same duty. 755. The Melan Arch Bridge at Topeka, Kan., is one of the most important concrete-steel structures yet erected in the United States. It consists of one span of 125 feet, two of 110 feet each, and two of 97.5 feet each. The foundations for piers and abutments are piles in soft sand. The steel rein- forcement is in the form of a latticed member. The bridge is fully described in Engineering News of April 2, 1896, and En- gineering Record, April 16, 1898. 484 CEMENT AND CONCRETE 756. Concrete-Steel Viaduct. — A viaduct of ten concrete- steel arches, of about 65 foot span, carries a double track elec- tric line across West Canada Creek near Herkimer, N. Y. 1 The piers rest on piles driven into hard blue clay, the surface of which is 6 to 12 feet below the creek bed. The segmental arches have a rise of 12 to 14 feet, with thickness of 21 inches at the crown and 4 J feet at the springing; the radius of intrados is about 46 feet, and of extrados about 57 feet. The stresses were computed for full load and for live load on half span, Prof. Cain's graphical method being employed. The maximum stresses allowed were six hundred pounds per square inch com- pression in concrete and ten thousand pounds per square inch tension in steel. The stresses caused by a variation of fifty degrees in temperature were allowed for. The tensile strength of the concrete was disregarded. Thacher bars, 1J inches dia- meter, were used for the reinforcement, being placed eleven inch centers near both intrados and extrados. 757. Expansion joints were provided in spandrel walls by nailing to the sides of the forms for arch pilasters a narrow strip of timber, thus forming a groove into which the spandrel wall is tongued. These joints show some motion and allow some water to leak through. The concrete was mixed three parts sand and seven parts gravel to one volume packed cement for foundations and piers, and two and one-half parts sand and five of gravel to one ce- ment for the arch rings and spandrel walls. All concrete was mixed wet and by hand. The work was faced with mortar composed of one part cement to two and one-half parts sand, and after the removal of forms the face was brushed with thin mortar wash and rubbed with sandstone blocks, giving a uni- form color to the surface. Art. 84. Dams 758. Concrete vs. Rubble. — Concrete has been employed to some extent in most of the important masonry dams of recent construction, and has formed the main portion of some of the largest dams yet built. The relative value of concrete and uncoursed rubble masonry Engineering Neios, Feb. 27, 1904. DAMS 485 laid in Portland cement mortar is perhaps still an open ques- tion, though it is believed that the former will eventually be preferred by engineers who are familiar with both. Concrete will require in general a larger proportion of cement than does the masonry, so that in localities difficult of access, the ma- sonry may for this reason be cheaper. Usually, however, con- crete will be the cheaper, and less skilled labor will be required in the building. With the same amount of inspection, concrete of good materials properly proportioned will form at least as impervious a wall as will rubble. 759. Quality of Concrete. — The up-stream face of the dam should be made as nearly water-tight as possible, and therefore a rich concrete employed in which the mortar is in excess of the voids in the stone, and the mortar itself contains about two parts sand to one cement. The body of the wall, however, may be made of a poorer mixture, one to three to six usually being sufficient. Bowlders may also be imbedded in the mass to cheapen the concrete without any serious detriment. Such bowlders should, of course, be sound and clean, and- well wet before being placed. They should be kept well back from the face of the wall and should be separated one from another by at least six inches, to allow of thoroughly tamping the concrete between them. 760. Building in Sections. — In a wall of rubble the con- traction and expansion are taken care of by minute cracks between the stone and mortar which frequently are not notice- able. In a concrete wall, unless provision is made for this, these signs of movement may be concentrated in cracks at in- tervals of thirty to sixty feet; these are always unsightly, and may in exceptional cases be a serious defect. The remedy evidently lies in so building the dam that if these cracks appear, they shall be confined to predetermined planes where they will not do any serious harm. Such contraction cracks will be very much less likely to occur in a dam arched in plan than in a straight clam, since in the former a slight movement of the masonry up or clown stream changes the length of the wall and relieves the tension strains. 761. Joints. — The joints ia a concrete clam should not be unbroken planes for any great distance. That is, the concrete should be so placed that the joints between work of different 486 CEMENT AND CONCRETE days are not planes extending through the wall. The wall may well be kept higher on the down-stream side and step down toward the up-stream side. The vertical joints should also be broken by right-angled off-sets, but the wisdom of using a dove- tail joint in such work is very questionable. The joining of one day's work to another necessarily forms a plane of weak- ness, and therefore the work should be carefully planned to the end that these planes shall be, in direction and location, where they will not unnecessarily weaken the structure or render it pervious to water. 762. Examples: St. Croix Dam. — A dam at St. Croix, Wis., 1 was built of sandstone masonry of uncoursed rubble in one-to- three mortar, and faced with concrete of one Portland cement to three parts sand to four parts broken stone of 1 to 3 J inch size. The concrete was rammed in place between the stone- work and the concrete forms. The selection of the uncoursed rubble was probably made on account of the site being five miles from the railway and the consequent difficulty of getting cement. The clam was arched in plan, and in preparing the foundation, several grooves or trenches were cut in the rock in a longitudinal direction, to avoid, as usual, a through course at the bottom, and these trenches were also filled with concrete. Had the concrete for the facing contained five parts of broken stone having maximum size of 2 or 2\ inches, it would have been more nearly in conformity with the best practice. 763. Massena Dam. — In the construction of the dam at the forebay of the Massena Water Power Company, Massena, N.Y., 2 it was sought to take up the tension stresses due to contrac- tion by imbedding in a longitudinal direction in the concrete, T-rails two feet apart horizontally and four feet apart ver- tically. 764. Butte Dam. — The dam built in connection with the Butte, Montana, water system is 120 feet high, 350 feet long, 10 feet wide at the top and 83 feet wide at the 120 foot point. The bed rock was granite, which was first covered with four inches of concrete made with small sized stone. In the body of the dam, granite bowlders were thickly imbedded in the 1 Engineering News, June 13, 1901. 2 Engineering News, Feb. 21, 1901. DAMS 487 concrete, care being taken that each bowlder was entirely en- veloped in concrete and that there were no horizontal or nearly horizontal courses either of concrete or bowlders. 765. San Mateo Dam. — The San Mateo Dam of California, one of the highest dams in existence, is built entirely of concrete, 170 feet high. It is 126 feet thick at the base and is arched up- stream with a radius of 637 feet. The dam was constructed in blocks of 200 to 300 cubic yards each, of irregular heights, so as to bond the courses together and have no through joints. Con- crete, one, two to six, was delivered in small push cars on a high trestle over the dam, and was dropped through iron pipes 16 inches in diameter to the place of deposition. In some cases this drop was 120 feet, and it is stated that the concrete appeared not to be injured by this method of handling. 766. Barossa Dam. — The Barossa Dam in South Aus- tralia 1 is of a bold arch design. The arch has a radius of 200 feet, and the chord is 370 feet subtending an angle of 135 degrees, 20 minutes, and the length of the arc 472 feet. The height of the dam is 94 feet above the ground line, yet the greatest thick- ness above the foundation is only 34 feet, with a top width of only 4^ feet. Special care was taken in selecting the materials and fixing the proportions. The cement was aerated fourteen days before use. Test cubes of concrete two feet on a side were prepared with different proportions of materials and subjected to a hydrostatic pressure of two hundred feet before deciding upon the proportions to use in the concrete. As a result of these tests, the aggregate was made up of one part screenings ^ to h inch, two parts "nuts" \ to \\ inch, and four and one-half parts "metal" \\ to 2 inch. This mixture contained about 35 per cent, voids. The mortar was made of one part Portland cement to one and one-half parts sand, and was from seven and one-half to fifteen per cent, in excess of voids in aggregate. Plumbs were used in the dam to within fifteen feet of the top, and above this level iron tram rails were placed in string courses. The success accompanying the use of concrete in structures of this magnitude is sufficient evidence of its value and adapta- bility. 1 Mr. A. B. Moncrieff, Engineer in Chief, Engineering Neios, April 7, 1904. 488 CEMENT AND CONCRETE Art. 85. Locks 767. The use of concrete in the construction of canal locks is comparatively recent, but it has met with much favor, and its use is extending. The requirements for a lock wall are that it shall be reasonably water-tight, that its strength shall be sufficient to withstand the thrust of the gates and support the earth filling behind it (or in a river wall, the difference in water pressure on the two sides), and that it shall withstand the impact and abrading action of boats using the canal. In all of these respects concrete is believed to be the equal of a good class of stone masonry. At St. Marys Falls Canal, portions of the lock walls which have been injured by boats and re- paired with concrete have given entire satisfaction, although in such cases the concrete had to be patched on, and some- times in places difficult of access for work of this character. 768. Methods of Building. — The present accepted method of concrete lock construction is to build the walls in alternate sections, filling in the intermediate sections after the others have set. It is sometimes thought necessary to make the work on a section continuous from time of starting the con- creting to its completion. That the exterior appearance of the work may be somewhat better if such a course is followed, is true, but it is very questionable whether the attainment of this desirable result is worth the additional expense and the addi- tional liability of having poor work done under the cover of darkness when work at night is necessitated by such a rule. With proper precautions, such as making steps in the top surface of work left for the night, as already detailed elsewhere, and being careful that the limit of work on exposed faces is bounded by true horizontal and vertical lines, the plane of weakness occasioned by a horizontal joint extending only par- tially through the work cannot be a serious defect in a con- crete wall. 769. The molds, so far as the walls alone are concerned, are comparatively simple and have already been described under the head of forms (Art. 62). Cable passages, gate recesses, hollow quoins, culverts, etc., call for special carpentry work, sometimes of quite intricate character. While the efficiency of the machinery and the lock as a whole should not be sacrificed LOCKS 489 to obtain easy construction, yet sharp corners should always be avoided, and simplicity of outline should be the constant aim. Linings of hollow quoins (when steel quoins are consid- ered necessary), gate anchorages, cable sheaves and other parts built into the masonry, are in general placed with greater difficulty in concrete forms than in stone masonry. Aside from such special constructions, the walls may be built up much more rapidly of concrete than of stonework. As to the proportions to be used in concrete for locks there is no rule of thumb. As a guide, the stresses in each part of the structure should be determined as well as the knowledge of the forces will permit, but the proportions will depend on the question of water-tightness and freedom from deterioration quite as much as upon required strength. It may be said, however, that in a considerable portion of the cross-section of the walls, weight is the main consideration and the concrete need not be very rich. The concrete surrounding the culvert, however, should be of good quality, as the stresses which may be devel- oped here do not admit of close analysis. 770. The walls should be faced with mortar made of one and one-half or two parts sand, or, better, two parts of granite screenings one-half inch and smaller, to one part of the same kind of cement used in the body of the concrete. This facing need not be more than three inches thick, and if made of sand and cement, it will probably be better if not more than one inch thick, though this may depend on the materials and local con- ditions. In any case this facing should be laid with the con- crete by means of a removable steel plate similar to that de- scribed in § 528. The top of the wall should be finished with mortar or granolithic similar to a concrete walk or driveway. While the walls should in general have a vertical face, a slight batter is allowable at the top, starting at about upper pool level, to protect the concrete from being chipped by the impact of boats, and for a similar purpose the outer corner of the wall should be rounded with six to twelve inch radius. Special care must be taken in lining the culverts, particularly in silt-bearing streams, and in such places as a change is made in the direction of the flowing water. For high heads it may be necessary to line the culverts with cast iron for a portion of their length. Granite and hard burned bricks have also been 490 CEMENT AND CONCRETE used for this purpose, but in locks of moderate lift, granolithic lining will usually be found sufficiently resistant. AH necessary irons and bolts should be built into the masonry as the work progresses, as they will be much more secure than if set later in recesses left for them. 771. Cascades Lock. — The large lock in the canal at the Cascades of the Columbia was one of the first in the United States to be designed of concrete in this country. In this lock the walls, wells, copings and portions of culverts were faced with stone. The foundation rock was covered with eight inches of rich concrete, one part Portland cement, two parts sand to four parts gravel. Fourteen feet of the chamber walls and ten feet of gate, abutments or wide walls were of concrete, one to three to six, while balance of masonry was of one to four to eight concrete. The molds were of four by six posts four feet apart, and lagging of two-inch lumber, dressed to size for exposed faces. The work was carried up in horizontal layers, not more than two feet being placed in one day. The set concrete was picked and washed when fresh concrete was to be laid upon it so as to get as good a bond as possible. The inlet pipes to the turbines to operate the machinery were built in the lock walls, and as it was not desirable to place an iron pipe in this location, the pipe was molded of concrete and afterwards laid in the wall. The pipe was thirty-nine inches diameter, walls six inches thick and contained about 0.22 cubic yard of concrete per foot. It was made in three foot lengths in vertical molds, and the cost of about six hundred feet of it was at the rate of $3.56 per foot, or $16.19 per cubic yard. 772. Hennepin Canal. — In the locks for the Illinois and Mississippi Canal the walls are entirely of concrete, and were built in alternate sections about thirty feet long. Work on a given section once commenced was continued to completion without intermission. The top was finished without any plas- ter or wet coat, the excess concrete being simply cut off with a straight edge and rubbed smooth and hard with a float. Vertical wells one foot square were left in the walls at intervals, and these were kept filled with water for about three weeks after the completion of the section, and then filled with concrete. To avoid weak places due to single batches made from cement LOCKS 491 of poor quality which might have passed inspection, the ce- ment was mixed in lots of five to ten barrels before being used in the concrete. The quoins of these locks were of cast iron. The founda- tions and the spaces in rear of lock walls are cut off from upper pool by cross-walls, and are underdrained to the lower pool to prevent the action of water pressure due to the upper pool level tending to force up the foundation. Ten inch and twelve inch tile drains were used for this purpose. The proportions used in general were one part Portland cement, three to three and one-third parts gravel, and four parts broken stone, the concrete containing about one and four- tenths barrels* of cement per yard. The average cost of con- crete in quantities of two thousand to four thousand yards was from $8.50 to $9.15 per cubic yard, distributed approximately as follows: — Materials $5.00 to $6.00 Molds 82 to 1.42 Mixing and placing 1.64 to 1.82 Miscellaneous 12 to .47 773. Herr Island. — In the Herr Island Locks, Alleghany River, the failure of the cofferdam to exclude water from the lock pit on account of porosity of the river bed, led to the adop- tion of a concrete foundation, laid under water, of sufficient weight to balance the hydrostatic pressure. After this founda- tion was in place, the cofferdam was pumped out and the con- crete side walls built in the dry. The concrete was placed in one foot courses covering the entire area of the wall, the forms being made of one course of two by twelve inch plank set on edge and halved at the ends to form two inch lap splices. Iron rods one-quarter inch diam- eter were placed six feet eight inches apart to tie face and back plank together. A two by twelve inch cross-plank was placed on edge beside each tie rod, dividing the work into short sections. After completing the concreting to the top of the forms throughout, the cross-planks were removed and the space nlled with concrete, thus making a vertical joint. The forms for the next course were then put in place in a similar manner. The size of stone used as aggregate was first two inches in one dimension, but this size was afterward reduced to one and 492 CEMENT AND CONCRETE one-half inches, and finally to one inch, the smaller size stone being preferred. 774. Mississippi River. — The lock in the Mississippi River between Minneapolis and St. Paul was founded on a soft sand- stone rock having many water-bearing seams. The lock was surrounded on three sides by a cut-off wall. A trench two inches wide and ten feet deep was cut in the soft rock by jet- ting a series of holes in close juxtaposition and then breaking out the intervening wall with a drill and saw of special con- struction. In this trench was first laid a double thickness of three-quarter inch boards and the remaining space was grouted full. Sections of this wall afterward uncovered, showed the method to have been very effective. Similar methods of seal- ing open seams in rock by the use of grout under pressures have been used elsewhere. The forms for the construction of this lock were of excellent design 1 and have been described under the head of "forms" (§ 514). The walls were built in alternate blocks, twelve feet long. At the ends of the blocks are left vertical spaces five by seven inches, to be filled with mortar and other water-tight composition. The forms are lined with sheet iron, and to obtain a smooth face the concrete is thrown against the lining, the stones rebound, leaving only mortar on the face. The face is rammed with tampers of special form, wedge shaped, and measuring f inch by 5 inches on the lower edge. This is followed by a flat rammer. The finish is said to be excellent. 775. Sand-cement was used quite largely in the lock con- struction. It was prepared at the site of the work, of equal parts Portland cement and siliceous sand ground together in a tube mill. Proportions in the concrete were varied somewhat from time to time, though in general it was mixed one part silica cement, two and one-third parts sand and six and two-thirds parts of crushed stone without screening. Tests showed that about ten per cent, of this crusher product was fine enough to be consid- ered sand, and account of this fact was taken in fixing the pro- portions as above. The cost of the concrete, over 11,000 yards, was as follows : — 1 Mr. A. O. Powell, Asst. Engr., Report Chief of Engrs., 1900, p. 2778. BREA K WA TERS 493 Cement $2.76 Stone . .' $1.29 Breaking stone for crusher .38 Crushing stone .82 Total stone $2.49 Sand .52 Total materials $5.77 Forms 1.21 Mixing and placing concrete 1.44 Total cost per cubic yard concrete . $8.42 Art. 86. Breakwaters 776. The use of concrete in the construction of breakwaters in the United States was suggested as early as 1845. In recent years it has been employed quite extensively, especially for harbor improvements on the Great Lakes, where it has with- stood the rigorous winters, the severe storms, the attrition of ice, and the impact of boats, in a highly satisfactory manner. Its use has been confined largely to the construction of a super- structure on timber cribs, the concrete work being in the form of blocks set with derricks, or of monolithic blocks molded in place, or more frequently composed of a combination of these two forms. Since in breakwater construction weight is of prime impor- tance, it is not necessary, in general, to use an exceptionally strong concrete, as the increased expense had better be in- curred in increasing the cross-section. 777. Buffalo Breakwater. — In the construction of the ex- tensive breakwaters at Buffalo, 1 concrete has been used in large quantities and according to various plans. In 1887 the super- structure of some 750 feet of timber-crib breakwater was re- newed, mainly with natural cement concrete. 250 feet of this superstructure was built with a facing of Portland cement concrete, while 500 feet of it was faced with stone masonry. The concrete started two feet below mean lake level. The cross-section of the superstructure was about 350 square feet, and the cost of concrete, exclusive of materials, was about $2.36 per cubic yard. 1 Described by Mr. Emile Low, U. S. Asst. Engr. Trans. Am. Soc. C. E., December, 1903. 494 CEMENT AND CONCRETE During the following year concrete footing blocks were used on both the lake and harbor faces, since it was found that the cement was washed out of the concrete laid in place below water. The blocks contained about 3£ cubic yards and cost on the average a little more than $30.00 each, or $37.35 each in- cluding the setting, or at the rate of $11.29 per cubic yard. The molds or forms, which were used repeatedly, cost about $40.00 each. 778. Another style of concrete superstructure developed at Buffalo is that recommended by Major F. W. Symons. It consists of three longitudinal walls, connected at intervals by cross-walls, filled between with rubble stone and provided with heavy parapet and banquette decks. The longitudinal wall on the lake side is founded on heavy concrete blocks 5' feet high, 8 feet thick at the base and 7-2 feet long ; the two minor walls are formed by smaller blocks, 4 feet by 4.5 feet by 12 feet. The total width at base is 36 feet. The space between lake face blocks and center row is 14 feet, and between center row and harbor face blocks is about 5 feet. The cross-wall blocks are 7 by 6 by 4 feet under the parapet, and 4 by 3 by 4 feet under the banquette, all spaced 36 feet centers. All concrete blocks have their bases set two feet below mean lake level and have panels in their upper surfaces to provide a bond with the concrete laid in place. The lake wall above the concrete block is 8 to 4 feet thick, with batter on face, and the decks are 3 to 4 feet thick, built of concrete in place. The forms for the harbor face wall and cross- walls were of § inch matched pine, with vertical posts two to three feet centers tied through the wall with one-half inch tie rods. The concrete was composed of the following volumes: one part Portland cement, one part screened gravel (about f inch), two parts sand grit (nearly half of which was |- inch to \ inch gravel),, and four parts unscreened broken limestone (about 11 per cent. dust). The cost of the concrete in blocks Avas $10.00 per cubic yard, and that in place cost $9.40 per cubic yard. 779. Cleveland Breakwater. — Several forms of concrete su- perstructure have been employed in the work at the Cleve- land breakwater. One section on a thirty-two foot crib has three rows of concrete blocks, one each on lake and harbor BREAKWATERS 495 sides and one in center of the crib, extending three feet below mean lake level. The concrete in place is started at mean lake level and is composed of a base five feet thick, with vertical faces over the entire crib, and surmounted on the lake side by a parapet five feet high and about twelve feet wide. The stone filling of the cribs was covered with a cheap decking of wood before laying the concrete in place. 780. Marquette Breakwater. — In the construction of the superstructure of the breakwater at Marquette, Mich., the con- ditions were peculiar in that it was desirable to provide a pas- sageway within the superstructure through which the lighthouse on the outer end might be reached in stormy weather. This was accomplished by leaving near the harbor face a conduit, 6 feet 3 inches high and 2 feet 10 inches wide, the entire length of the structure. The old timber structure having been removed to about one foot below mean lake level, a foundation course two feet thick of Portland cement concrete was laid on a burlap carpet placed over the stone filling of the crib. Upon this the monolithic blocks were built in place, substantial molds being set up for alternate blocks ten feet apart. After these had set, the molds were removed and other molds set up to form the two faces of the intervening blocks, the ends of the blocks already com- pleted taking the place of end molds. The monolithic blocks were of natural cement concrete in proportions of 489 pounds of cement to one-half cubic yard of sand and one cubic yard of broken stone. About twenty per cent, of these monoliths was composed of rubble stone ranging in size from one-half to three cubic feet, care being taken that no rubble should be placed nearer than one foot to any outside surface. The standard block was twenty-three feet wide on the base, which was one foot above mean lake level. The lower five feet of the face had a 45° slope. There was then a nearly level berm, 7.5 feet wide, forming the banquette deck; from the back of this deck the face sloped at an angle of 45° to the parapet deck, which was 6 ft. 4 inches wide. The harbor side of the block was vertical, 9.4 feet high. Since the structure proved very stable and free from vibrations in heavy seas, the horizontal dimensions of the block were reduced as the shore was approached. 781. The method of placing the Portland cement concrete 496 CEMENT AND CONCRETE foundation was modified as described under the head of the block and bag systems of concrete constructions (Art. 64). The cost of the monolithic blocks of natural cement concrete was as follows : — 490 lbs. cement, $1.04 per bbl $1,815 .5 cu. yd. sand, $0.50 per cu. yd. ........ .25 1.0 cu. yd. stone, $1.58 " " "........ 1.58 Materials in one cubic yard concrete $3,645 80 per cent, concrete in the finished block, .80 of $3,645 $2.91 Loading materials .33 Mixing concrete .52 Depositing concrete .41 Handling rubble 09 Finishing blocks .09 Moving and setting forms .25 Timber waling, anchor bolts, etc .13 Total cost in place per cu. yd $4.73 Very interesting and detailed accounts of the construction of this breakwater, which was carried out with special care as to all details, were made by Mr. Clarence Coleman, Asst. Engr., and may be found in the reports of Major Clinton B. Sears, Reports Chief of Engineers, U. S. A., 1896 and 1897. INDEX Abrasion — Resistance to, 329. Tests of, 94. Abutments, 467. Accelerated Tests (see Soundness), 77. Acceptance of Cement, 153. Accuracy Obtainable in Tests, 137. Acid — Sulphuric, in Cement, 34. Use on Concrete Surface, 368. Adhesion — ■ Cement to Brick, 273, 27S. Glass, 273. Iron, 273. Steel Rods, 284. Stone, 272. Terra Cotta, 273. Effect of Character Surface, 276. Plaster Paris, 277. Regaging, 276. Richness Mortar, 274. Neat and Sand Mortars, 279. Portland to Natural, 270. Results of Tests, 270. Tests of Cement, 92. Adulteration, 4, 43. Age and Aeration of Cement — Effect on Time Setting, 68. Specific Gravity, 42. Strength, 235. Aggregate — Bowlder Stone, 322. Brick, 186, 324, 335. Cinders, 302, 309, 338. Clean, 188. Cost, 195. Crushing, 194. Fireproof Concrete, 335. Aggregate — Granite, 322. Gravel as, 192, 298, 303, 309. Material for, 186. Sand in, 202. Sandstone, 294, 322. Sea Water, 350. Size and Shape of Fragments, 188. Tests of, 298, 322. Trap, 298, 309. Voids in, 190. Weight of, 189. Air Hardened Mortars, 122, 232. 260. Alum and Soap Washes, 344. Alumina in Cement, 33. Aluminous Natural Cement, 25. Amount of Mortar in Concrete, 200. Effect on Compressive Strength, 293. Effect on Transverse Strength, 318. Analysis — Methods, 35. Materials, 11. Natural Cement, 8. Portland Cement, 6. Anchor Bolts, 284, 471. Arch — Big Muddy River, 481. Highway, 480. Mechanicsville, 480. Melan, 384, 483. Monier, 382. Plain Concrete, 480, 481. San Leandro, 480. Thacher, 385. Three Span, 480. 497 498 INDEX Arch — Topeka, Kan., 483. Wtinsch, 383. Arches — Centers, 478,' 482. Construction, 478. Cost, 483. Design, 474. Drainage, 479. Finish, 479. Viaduct, 484. Bag for Depositing Concrete, 369, 373, 377. Bag Method, 374. Bags of Concrete to Form Face, 377. to Prevent Scour, 377. Baker, Classification of Hydraulic Products, 3. Ball Mills for Grinding, 20. Barrels, Cement — Capacity, 172. Records, 146. Basement Floors, 426. Base of Concrete Walk, 421. Beams — Concrete-Steel, 390. for Street Railway Tracks, 433. Steel, Protected, 412. Strength, Experiments, 313, 403. Formulas for, 391,393. Tables of, 400, 402. Belt Conveyor for Concrete, 358. Blast Furnace Slag — Cement, 22, 23. Sand, 159. Block System, 351, 378. Blocks, Concrete, in Breakwaters, 379, 493. Blowing of Cement (see Soundness). Board, Mixing, for Concrete, 204. Bohme Hammer Apparatus, 114. Boiling Test, 77. Bolts, Adhesion of Mortar to, 284. Boston Elev. R.R. Tests Concrete, 292, 308. Boston Subway, 444, 446. Bowlder Stone as Aggregate, 322. Box Mixer (see Cubical). Braces for Forms, 354. Breaking Briquets, 123. Breaking Stone by Hand, 194. Breakwater, 493. Buffalo, 214, 493. Cleveland, 494. Concrete in, 379, 493. Marquette, 375, 379, 495. Brick — Adhesion of Cement to, 272. as Concrete Aggregate, 186, 324, 335. Dust with Cement, 258. Bridge — Abutments, 467. Piers, 464. Forms for, 464. Bridges (see Arches). Briquets — Area Breaking Section, 109. Breaking, 123. Form of, 108. Machine for Making, 1 14. Methods of Making, 113. Records, 147. Storing, 117. Broken Stone (see Aggregate). vs. Gravel, 192. Brushing Concrete Surface, 361, 366. Buffalo Breakwater, 214, 493. Buffalo, Concrete Mixing at, 214. Buhr Millstones, 20. Building Regulations, New York, 418. Buildings of Concrete, 410. Burlap Bags for Placing Concrete, 369, 377. Burning — Natural Cement, 26. Portland Cement, 16. Bushhammering Concrete, 367. Caisson Filling, 466. Calcium Chloride — Effect on Setting, 70. Test for Soundness, 81. INDEX 499 Calcium Sulphate — Effect on Strength, 249. Time Setting, 69. Canal Locks — Concrete for, 224, 357, 488. Forms for, 357. Capacity Cement Barrels, 172. Carbonic Acid, 34. Cars- — Concrete Plant on, 216. for Transporting Concrete, 213. Cascades Canal, Concrete for, 224. Centers (see also Forms) — for Arches, 478, 480. for Tunnel Lining, 451. Chamber Kilns, 17. Chemical Tests, 31. Chicago Drainage Canal, Concrete on, 224. Cinder Concrete — Strength, 302. Modulus Elasticity, 309. Cinders, Sulphur in, 338. Classification Hydraulic Products, 1. Clay- for Cement Manufacture, 10. in Concrete, 305. in Mortar, 253. Clip for Breaking Briquets, 124. Cock, 128. Form Suggested, 133. Gimbal, 130. Requirements for Perfect, 132. Russell, 129. Tests of, 131. Clip Breaks, 126. Cause, 126. Prevention, 127. Strength, 127. Coarse Cement and Fine Sand Com- pared, 57, 62. Coarse Particles (see Fineness) — Effect of, 52. on Time Setting, 69. Cock Clip, 128. Cockburn Concrete Mixer, 211. Coefficient Expansion, 332. Cohesion and Adhesion Compared, 275, 279. Cold, Effect on Cement, 260. Color for Concrete Finish, 367. of Cement, 36. of Concrete Surface, 365, 367. Columns, 412, 415. Concrete-Steel, 413. Steel, Filled and Covered, 413. Strength of, 413. Comparative Tests — Natural Cements, 138. Portland Cements, 138. Compression Tests, 89. Compressive Strength — Concrete, 291. Mortar, 288. Compressive and Tensile Strength Compared, 288, 313. Compressive and Transverse Strength Compared, 313. Composition, Chemical, 6, 8. Effect on Specific Gravity, 42. Concrete — Amount of Mortar in, 200. Compressive Strength of, 291. Construction, Rules for, 467. Cost, 218. Definition, 186, 200. Deposition in Water, 326, 369. Making, 200. Mixers, 207, 212. Mixing, Cost, 212. Mixing by Hand, 203. Mixing Plants, 212. Proportions in, 200. Thorough Mixing, 203. Concrete-Steel, 381. Conductivity of Concrete, 333. Considered Experiments, 388. Consistency Concrete, Effect on Strength, 293, 296, 319. Consistency Mortar, 176. Determination, 97. Effect on Adhesion, 274. Tensile Strength, 99, 232, 314. 500 INDEX Consistency Mortar — Effect on Time Setting, 71. Transverse and Compressive Strength, 314. Effect in Low Temperatures, 268. Constancy of Volume (see Soundness). Contraction Concrete in Setting, 331. Coosa River Concrete Plant, 212. Coping for Retaining Wall, 467. Corners of Concrete Forms, 354. Corrosion, Action of, 336. Cost — Aggregate, 195. Concrete, 218. Arch, 483. Curb and Gutter, 432. Floor, 427. Mixing, 212. Tunnel Lining, 452. Walk, 425, 426. Mortar, 182. Sand, 171. Sand Washing, 170. Cracks in Concrete, 361. Crushing Strength (see Compression). Cubes, Concrete, Tests of, 292. Cubical Concrete Mixer, 208, 212. Curb and Gutter, 431. Cut Stone Facing, 477. Finish, 366. Cylinder, Steel, Bridge Pier, 465. Dams, 484. Barossa, 4S7. Butte, 486. Concrete vs. Rubble, 484. - Massena, 486. San Mateo, 487. St. Croix, 486. Definitions, 1. Delivery of Cement, 144. Density, Apparent, 37. Deposition Concrete in Running Water, 326, 369. Deterioration of Cement, 235. Deval, Test for Soundness, 78, 81. Diary, Use of, 153. Dietsch Kiln, 17. Drake Concrete Mixer, 211, 216. Dromedary Concrete Mixer, 209. Efflorescence, 346. Estimates, Cost Concrete, 218. Mortar, 182. Excessive Reinforcement, 396. Expanded Metal, 387. Expansion — Coefficient of, 332. Concrete in Water, 331. Joints, 482, 484. Experiments — Columns, 413, Concrete-Steel, 388, 397, 403. Considered, 388. Hooped Concrete, 414. Face of Concrete (see also Finish) — Bushhammer, 367. Colors for, 365. Cut Stone, 366, 477. Efflorescence, 346. Lock Walls, 489. Mortar, 363. Pointed or Tooled, 367. Face Pressed in Compressive Tests, 292. Faija, Mortar Mixer, 107. Tests for Soundness, 78. Failure of Concrete in Sea Water, 348. Farrel's Wall Molds, 417. Filtration through Concrete, 340, 342. Fineness Cement — Effect on Specific Gravity, 52, 59. Strength, 54, 60. Time Setting, 52, 60,69. Weight, 59. Importance, 45. Specifications, 51. Tests, 45. Fineness of Sand, 97. in Freezing Weather, 268. Finish of Concrete Surface, 363. Colors, 367. INDEX 501 Finish of Concrete Surface — Mortar, 863. Pebble-dash, 366. Plaster Paris, 365. Rubbed, 365. Shovel, 363. Tooled or Pointed, 367. Fire, Resistance Concrete to, 332. Fireproof Buildings, 332. Fireproof Concrete, Aggregate for, 335. Flexure, Concrete-Steel Beams, 390. Tests Concrete, 314. Mortar, 90, 313. Floor, Systems of Concrete-Steel, 381, 411. Floors — Basement, 426. Buildings, 411. Reservoirs, 453. Forms, Concrete, 351. for Buildings, 416, 417. Bridge Piers, 464. Columns, 416. Lock Walls, 4S8, 492. Piles, 473. Reservoir Roofs, 455. Subways, 445, 448. Tunnel Lining, 447, 449, 451 . Oiling, 354. Time Left in Place, 352, 439, 471, 478. Formulas for Concrete-Steel Beams, 391, 393. Foundation — Concrete Walks, 420. Pavements, 428. PiLes, 471. Free Lime in Cement, 31 , 76, S3. Freezing Weather — Use of Cement Mortar in, 260. Use of Concrete in, 326. Gage of Wire for Sieves, 46, 47. Gaging Mortar — by Hand, 105. Effect of Thorough, 236. with Hoe and Box, 106. Gaging Concrete (see Mixing). German Normal Sand, 96. Gilmore Wires for Time Setting, 66. Gimbal Clip, 130. Glass, Adhesion of Cement to, 274. Granite as Aggregate, 322. Granolithic, Facing, 365. Top Dressing, 422. Granulometric Composition — Aggregate, 189. Sand, 163. Gravel as Aggregate, 186, 192, 298, 303, 309. vs. Broken Stone, 192. Gravity Concrete Mixer, 212. Griffin Mill, 21. Grinding Cement (see Fineness), 20. Grout, to Seal Cracks, 492. on Surface Concrete, 363, 365. Gutters and Curbs, 431. Gypsum (see Plaster Paris). Hammer, Bohme, 114. Heat, Effect on Concrete, 332. Heating Materials in Cold Weather, 267, 452. Hennebique System, 385, 409. History, Hydraulic Products, 1. Hoe and Box for Mortar Mixing, 106. Hoffman Kiln, 17. Hooped Concrete, 414. Hot Materials in Cold W r eather, 267. Hot Tests (see Soundness). House Walls, 417. Hydraulic Limes, 2. Immersion of Briquets, 119. Impervious Concrete, 340, 343. "Improved" Cement, Strength of, 244. Impurities in Sand, 168. Ingredients — in Cubic Yard Concrete, 218. Mortar, 179. Portland Cement, 5. Interpretation Tensile Tests, 137. 502 INDEX Iron — Adhesion Cement to, 274, 284. Corrosion in Concrete, 336. Iron Oxide, 33. Jig for Mortar Mixing, 107. Johnson Bar, 387. Joints — Expansion, 482, 484. in Concrete, 361. Blocks, 378. Dam, 485. Molds, 354. Walks, 423, 424. Kahn System, 386, 409. Kilns, Cement, 16. Output, 19. Lagging for Forms, 352. Tongue and Groove, 352. Laitance, 370. Lamp Black, in Concrete, 365, 367. Surface Finish, 368. Laying Fresh Concrete on Set Con- crete, 361. Le Chatelier, Apparatus for Specific Gravity Test, 40. Test for Soundness, 81. Time Setting, 66. Lime, Classification, 3. Hydraulic, 3. Lime in Cement, 31, 245. Lime Paste, Effect on Adhesion, 280. Lime, Slaked, with Cement, 245, 345. Limestone, Adhesion Cement to, 274, 277. Limestone, Crushed as Aggregate, 297, 322, 335. Limestone Dust with Cement, 160, 187, 258, 325. Lining of Forms, 353. Reservoirs, 455. Loam in Sand, 168. Lock — Cascades, 490. Hennepin Canal, 490. Lock — Herr Island, 491. Mississippi River, 492. Locks, 488. Culvert Lining, 489. Facing, 489. Methods Building, 488. Molds, 488, 490. Louisville and Portland Canal, Con- crete on, 223. Machine for Breaking Briquets, 123. Concrete Mixing, 207. Mortar Mixing, 107, 178. Maclay, Test for Soundness, 78. Magnesia in Cement, 32. Magnesian Natural Cements, 24. Manufacture Natural Cement, 24. Portland Cement, 10. Marking Briquets, 117. Materials — for Cubic Yard Concrete, 218. Mortar, 179. Natural Cement Manufac- ture, 24. Portland Cement Manufac- ture, 10. Melan System, 384. Arch, Topeka, 483. Microscopical Tests, 36. Mills — Ball, 20. Griffin, 21. Tube, 20. Mixing Concrete — by Hand, 204. Cost, 206. by Machine, 207. "Cost, 212. Necessity of Thorough, 303, 319. Mixing Mortar — for Tests, 105. Use, 177. Necessity of Thorough, 236. Mixing Natural and Portland Cement, 243. INDEX 503 Modulus of Elasticity — ■ Concrete, 308. Mortar, 306. Modulus of Rupture in Flexure — Concrete Prisms, 314. Mortar Prisms, 313. Moist Closet for Briquets, 119. Moistening Concrete, 362. Moisture, Effect on Volume Sand, 166. Molder's Record, 147. Molding — Bohme, Hammer, 114. Hand, 115. Jamieson Machine, 114. Machine, 114. Methods, 113. Molds — Briquet, Cleaning, 113. Forms of. 108. Kinds of, 112. Concrete (see Forms). Blocks, 378. Sewers, 439, 442. Walks, 422. Walls, 417. Monier Arch, Test, 382. Monier System, 381. Mortar — Amount in Concrete, 200. Cost, 182. Definition of, 155. Facing, 363. for Plastering Concrete, 363. Ingredients for Cubic Yard, 179. Mixing, 105, 177. Varying Richness, 227. Natural Cement — ■ Analysis, 8. Definitions, 8. Manufacture, 24. Natural Cement Concrete, Strength of, 300. Neat vs. Sand Tests, 95. Needle Test for Time Setting, 66. Numbering Briquets. 1.17. Oiling Forms or Molds, 354. Painting Concrete, 368. Pan Mixer — for Cement, 14. Concrete, 210. Paper Sacks for Concrete, 377. Pat Test (see Soundness). Pavement, Concrete, 429. Pavement Foundation, 428. Pebble-Dash Finish, 366. Permeability of Mortars, 340, 343. Piers, Bridge, 464. Forms for, 464. Piles, Concrete, 471. Protection by Concrete, 383. Pipe, Sewer, in Concrete, 436. Placing Concrete under Water, 326, 369. Placing Consecutive Layers Concrete, 361. Plant, Portland Cement, 14. Plants, Concrete, 212. ' Plaster Paris — Effect on Adhesion, 277. Strength, 249. Soundness, 250, 251. Time Setting, 69. Plastering Concrete Surface, 363. " Platform, Mixing, 204. Plums in Concrete, 361, 485, 495. Point, Dressing Surface Concrete, 367. Pointing Mortar, 347. Porosity of Mortars, 340. Portland and Natural Compared, 279, 282. Portland Cement — Composition, 5. Definition, 4. Manufacture, 10. Posts for Forms, 354, 356. Pot Cracker for Grinding, 26. Pozzolana Cement (see Slag Cement), 7. Pozzolana with Cement, 365. Preservation of Iron and Steel, 336. Proportions in Concrete — Theory of, 200. 504 INDEX Proportions in Concrete — ■ Effect on Strength, 295, 301, 317. Modulus of Elasticity, 309. Proportions in Mortar, 173. Effect on .Strength, 227. Puzzolana (see Pozzolana). (Qualities, Desirable, in Cement, 28. Kails Imbedded in Concrete, 170. Rammers for Concrete, 360, 492. Ramming Concrete, 359. Effect on Strength, 297. Ransome Bars, 284, 386. Concrete Mixer, 209. System, 386. Rate of Applying Tensile Stress, 133. Ratio Compressive to Tensile Strength, 289. Records of Tests, 146. Regaging Mortar, 237. Effect on Adhesion, 276. Regrinding Cement (see Fineness). Reinforced Concrete (see Concrete- Steel). Reinforcement, Double, 403. Excessive, 396. Longitudinal, 413. Single, 390. Repair of Stone Piers, 466. Reservoirs, 453. Examples, 456. Floor, 453. Lining, 455. Roof, 455. Walls, 454. Results of Tests, Treatment of, 135. Retaining Walls, 467. Retardation of Setting of Cement, 69. Richness of Concrete, Effect on Strength, 296, 317. Rods, Adhesion of Mortar to, 284. Tie, for Forms, 356. Roebling System, 386. Roman Cement, Definition, 2. Roof, Concrete, for Building, 411. for Reservoir, 455. Roscndale Cement (see Natural). Rubbed Finish for Concrete, 365. Rubble Concrete, 360. Rubble vs. Concrete, 484. Rules for Concrete Construction, 467. Russell Clip, 129. Rust, Prevention of, 336. Sacks of Concrete, 374, 377. Salt, Effect on Mortars, 263. Time Setting, 70. Use in Freezing Weather, 260, 326. Sampling, Method, 145. Per cent, of barrels, 144. Sand — Character, 154, 157. Cost, 171. Damp — Mortars Hardened in, 278. Volume of, 166. Detecting Impurities in, 168. Fineness, 97, 159. for Tests — Comparison of, 96. Fineness, 97. German Normal, 96. Natural, 96. for Use in Sea Water, 159. Heating in Winter, 452. Impurities in, 168. in Aggregate, 202. Quality, 170. Shape and Hardness Grains, 155, 159, 162. Slag, 159. Varying Amounts of, 227. Voids in, 162. Measuring, 164. vs. Neat Tests, 95. Washing, 169. Weight, 170. Sand-Cement — Manufacture, 21. Use in Locks, 492. INDEX 505 Sandstone — Adhesion of Cement to, 274. as Aggregate, 294, 322. Sawdust in Mortar, 359. Screenings in Broken Stone, 187, 325. Screw Concrete Mixer, 211. Sea Wall, Concrete in, 216. Sea Water — Cements in, 348. Concrete in, 34S. Storing Briquets in, 121. Section, Breaking, of Briquets 109. Setting, Process of, 65. Setting, Rate or Time of, 66. Approximate Method Determ n- ing, 67. Effect of Aeration, 68. Age, 68. Composition, 67. Consistency, 71. Fineness, 69. Gaging, 73. Gypsum, 69. Medium, 74. Plaster Paris, 69. Salt and Sugar, 70, 71. Temperature, 72, 73. Gilmore Wires, 66. in Air and Water, 74. Mortar and Neat Cement, 72. Requirements as to, 74. Variations in, 67. Vicat Needle, 66. Sewers — Cost, 437, 439. Forms, 439, 441. Steel, 442. Methods Construction, 436, 443. Pipe, in Concrete, 436. Shear — in Concrete-Steel Beams, 405. Strength in, 328. Tests of, 90. Sheathing for Forms, 352. Tongue and Groove, 352. Shoefer Kiln, 17. Short Time Tests, Interpretation, 137. Shrinkage in Setting, 331. Sidewalk, Concrete, 420. Base, 421. Construction, 422. Cost, 425. Drainage, 420, 422. Foundation, 421. Wearing Surface, 422. Sieves for Cement, 46, 51. Value cf Coarse, 63. Sifting (see also Fineness). Mechanical and Hand, 49. Time of, 50. Silica, 10. Silica Cement — Manufacture, 21. Use in Locks, 492. Skip for Placing Concrete, 372. Slaked Lime with Cement, 245, 280. Slag Cement — Definition, 7. Manufacture, 23. Slag Sand, 159. Smith Concrete Mixer, 210. 216. Soap and Alum Solutions, 344. Soundness, 76. Tests for — A. S. C. E., 76. Boiling, 77. Chloride Calcium, 81. Deval, 79. Discussion, 82. Faija, 78. German Normal, 77. Hot, for Natural, 87. Hot Water, 78. Kiln, 77. Le Chatelier, SI. Records of, 151. Warm Water, 78. Spandrels, Arch, 476. Special Test Records, 153. Specific Gravity Cement, 39. Effect Aeration, 42. Coarse Particles, 52. 06 INDEX Specifications for Concrete Work, 467. Specimens, Marking, 146. Steel Beams, Concrete Covered, 412. Steel Facing for Curbs, 432. Forms for Sewers, 442. Lining for Forms, 353. Shell for Bridge Piers, 465. Steel with Concrete, 387. Steinbriich Mortar Mixer, 107. Steps in Concrete Construction, 362. Stone, Broken (see Aggregate) — vs. Gravel, 192. Character Surface of, 276. Crushers, 194. Crushing, 195. Facing for Concrete, 477. Finish for Concrete, 367. Stop Planks, 362. Storage for Cement, 144. Storing Briquets, 117. before Immersion, 117. in Air, 122, 232, 246, 260. in Sand, 123, 27S. in Water, 119. Storing Concrete Cubes, Effect of Medium, 293. Street Railway Foundations, 433. Strength (see Tensile, Transverse, etc.). Compressive, of Concrete, 291. Mortar, 288. of Concrete-Steel, 390, 403. Tensile, of Mortar, 227. .Transverse, of Concrete, 313. Stringers for Street Rails, 433. Subways, Concrete, 443. Boston, 444, 446. Chicago Telephone, 444. New York, 443, 448. Sugar, Effect on Time Setting, 71. Sulphuric Acid, 34, 368. Summary of Tests, Record, 147. Surface Concrete (see Finish). Surface Stone, Effect on Adhesion, 276. Sylvester's Process, 344. Tamping Concrete, 359. Temperature Cement and Water — Effect on Tensile Strength, 103. Time Setting, 72. Temperature, Low — Use of Concrete in, 326. Mortar in, 260. Tensile and Compressive Strength Compared, 288, 313. Tensile Strength — Effect Sand, 227. Neglect of, in Concrete-Steel, 388. Tensile Tests Cohesion, 95. Terra Cotta, Adhesion of Cement to, 274. Dust with Cement, 200. Test Monier Arch, 382. Testing Machine, Tensile, 123. Testing, Uniform Methods, 30. Tests (see also Tensile, Transverse, etc.) - Abrasion, 94, 329. Adhesion, 92, 270. Chemical, 31. Cohesion, 95. Compression, 89, 288. Concrete, 291, 314. Fineness, 45. Sand, 96, 155. Shear, 90. Soundness, 76. Specific Gravity, 39. Tensile, 95. Time Setting, 65. Transverse, 90. Weight per Cubic Foot, 37. Tetmajer, Boiling Test, 77. Kiln Test, 77. Thacher System, 385. Theory of Concrete-Steel Beams, 387, 390, 403. of Proportions in Concrete, 200. Thermal Expansion Cement, 332. Tile, Pulverized, Use of, 260. Time Required to Sift, 49. Time Setting (see Setting, Rate of). Tooling Concrete Surface, 367. INDEX 507 Top Dressing, Concrete Walks, 422, 424. Topeka Bridge, 483. Transporting Concrete, 358. Transverse Strength — Comparison with Tensile, 313. Concrete, 314. Mortar, 313. Tests of Cement, 90. Tremie for Placing Concrete, 371. Trussed Posts, 356. Wales, 357. Tube Mill, 20. Tunnel Lining — Brick vs. Concrete, 449. Cost, 452. Forms for, 447. in Firm Earth, 444. in Rock, 447. in Soft Ground, 446. Tunnels — Aspen, 450. Cascades, 449. East Boston, 446. Perkasie, 450. Sudbury River Aqueduct, 451. Twisted Rods — Adhesion to, 284. Ransome, 386. Uniformity in Methods Testing, 30. Viaduct, Concrete-Steel, 484. Vicat Needle for Time Setting, 66. Voids in Aggregate, 190, 201. Voids in Sand. 162. Effect Moisture, 106. Voids in Sand — Effect Shape Grains, 162. Size Grains, 163. Volume, Proportions by, 173, 200. Changes in, During Setting, 331. . Wales, Trussed, 357. Walks of Concrete, 420. Wall Molds, Buildings, 417. Farrel's, 417. Warehouse for Cement, 144. Washes for Concrete Walls, 344. Washing Sand, 169. Water in Mortar and Concrete (see Consistency) . Water, Deposition Concrete in, 326, 369. Water of Immersion for Briquets, 119. Water, Stale, for Immersing, 121. Waterproof Construction in Sub- ways, 443. Waterproof Mortar and Concrete, 340, 343. Waterproof Work in Reservoirs, 453. Wearing Surface of Walks, 422. Wedge Rammers for Concrete, 492. Weight of Concrete, 299, 305. Weight per Cubic Foot Cement, 37. Wells in Concrete, 362, 490. Wheelbarrows for Conveying Con- crete, 359. White Finish for Concrete, 365. Wire in Sieves, 47. Wires for Testing Time of Setting, 66. Wiinsch System, 383. ■^ a \ y

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