AGRICULTURAL ENGINEERING SERIES E. B. McCORMICK, CONSULTING EDITOR FORMERLY DKAN OF ENGINEERING DIVISION KANSAS STATE AGRICULTURAL COLLEGE CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES PUBLISHERS OF BOOKS F O B^/ Coal Age ^ Electric Railway Journal Electrical World v Engineering News -Record Railway Age Gazette v American Machinist Electrical Merchandising v The Contractor Engineering 8 Mining Journal ^ Po we r Metallurgical & Chemical Engineering g. CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES BY ROY A. SEATON, M.S. PROFESSOR OF APPLIED MECHANICS AND MACHINE DESIGN AND SUPERINTENDENT OF CONSTRUCTION, KANSAS STATE AGRICULTURAL COLLEGE, ON LEAVE; CAPTAIN, ENGINEERING DIVISION, ORDNANCE DEPARTMENT, U. 8. ARMY; MEMBER OF AMERICAN SOCIETY OF MECHANICAL ENGINEERS, AMERICAN SOCIETY FOR TESTING MATERIALS, AND SOCIETY FOR THE PROMOTION OF ENGINEERING EDUCATION SECOND EDITION McGRAW-HILL BOOK COMPANY, INC. 239 WEST 39TH STREET, NEW YORK LONDON: HILL PUBLISHING CO., LTD. 6 & 8 BOUVERIE ST., E. C. 1918 . - COPYRIGHT, 1916, 1918, BY THE MCGRAW-HILL BOOK COMPANY, INC. PREFACE TO THE SECOND EDITION The reception accorded the first edition of this book has been very gratifying to the author. The principal change in this edition consists of the substitu- tion in Chapter II of the latest A. S. T. M. specifications for Portland Cement. A few typographical errors have been cor- rected and other minor changes made. 464293 PREFACE TO THE FIRST EDITION The extension of the use of concrete to farms and rural communi- ties has developed a need for a textbook treating the essential features of concrete construction in a thorough but simple manner. Most of the works now available either are written for engineers and are unsuited to the non-technical man, or are in the form of bul- letins dealing chiefly with the uses to which concrete is put in rural communities, without giving a systematic treatment of fundamental principles governing its use. The author has endeavored, in writing this book, to make it suitable for use as a text in a brief course in con- crete construction for agricultural or other students in colleges, when accompanied by laboratory exercises and field construction, and to make it valuable to others who have occasion to use concrete. Chapter II, on Cement Specifications and Tests, and Chapter VIII, on Strength of Reinforced Concrete, are necessarily somewhat techni- cal, but it is thought best to include this matter for the use of students. Persons who find the matter difficult or uninteresting may pass over these chapters. In the preparation of the book, various sources of information have been freely used, chief among which are Taylor and Thompson's "Concrete, Plain and Reinforced," and the bulletins published by the Universal Portland Cement Company, of Chicago, The Atlas Portland Cement Company of New York, the Association of Amer- ican Portland Cement Manufacturers of Philadelphia, and the Office of Public Roads and Rural Engineering, United States Department of Agriculture, Washington, D. C. The author wishes to express his gratitude to those who have given him permission to use their tables and figures and to acknowl- edge his indebtedness to Professors N. A. Crawford, P. J. Freeman, and M. R. Bowerman of the Kansas State Agricultural College for assistance in the preparation of the manuscript. MANHATTAN, KANSAS. R. A. SEATON February, 1916 vm CONTENTS PAGE PREFACE vii INTRODUCTION . 1 Definitions. Development of the use of concrete PART I MATERIALS CHAPTER I CEMENTS AND LIMES 7 Kinds of cements. Portland cement. Natural cement. Puz- zolan, or slag, cement. Lime. Hydrated lime. Choice of a cement. Care of cement. CHAPTER II CEMENT SPECIFICATIONS AND TESTS 13 For the small purchaser. For larger and more important work. Specifications of American Society for Testing Materials. Sound- ness test without special apparatus. CHAPTER III AGGREGATES 34 Aggregates. Selection of aggregates. Sand. Requirements of sand. Use of find sand. Uniformity coefficient. Impurities. Test for impurities. Organic impurities. Strength test for sand. Stone screenings. Broken stone. Size of stone. Gravel. Washing of gravel. Cinders. PART II PLAIN CONCRETE CHAPTER IV PROPORTIONS AND QUANTITIES OF MATERIALS . . . . 47 The problem in proportioning concrete. Voids. To find the per- centage of voids in coarse aggregate. To find the percentage of voids in fine aggregate. Proportions for maximum density. Fundamental laws of proportioning. Arbitrarily specified propor- tions. Other methods of proportioning. Trial mixtures. Mechanical analysis. Omission of coarse aggregate. Use of bank run gravel. Quantities of materials required. Fuller's Rule. Table of quantities. Problems. X CONTENTS CHAPTER V PAGE CONSTRUCTION OF FORMS 62 Materials used for forms. Use of wood for forms. Tying and bracing forms. Forms for straight walls. Forms for circular walls. Forms for overhead floors and roofs. Column forms. Time of removal of forms. CHAPTER VI MIXING AND HANDLING CONCRETE 71 Requirements of Good Mixing. Consistency. Methods of Mixing. Tools required for hand mixing. Measuring the mate- rials. Mixing the materials by hand. Machine mixing. Continuous mixers. Batch mixers. Transporting concrete. Depositing concrete. Curing of concrete. Bonding old and new concrete. Contraction and expansion joints. - Setting .of con- crete. Strength of Concrete. Effect of freezing on concrete. PART III REINFORCED CONCRETE .CHAPTER VII GENERAL PRINCIPLES 95 Necessity for reinforcing. Materials used for reinforcement. Grades of steel used. Specifications for reinforcing steel. Forms of reinforcing steel. Classes of structures in which reinforced con- crete is used. Hollow cylinders subjected to internal pressure. Placing the steel. Beams and slabs. Continuous beams. Depth of embedment of steel. Columns. Arches and hollow cyl- inders subjected to external pressure. Differences in materials and methods used for reinforced concrete. CHAPTER VIII STRENGTH OF REINFORCED CONCRETE Ill Stresses used in reinforced concrete. Hollow cylinders subjected to internal pressure. Columns. Beams and slabs. Rectangular beams. Floor and roof slabs. Continuous beams. T-beams. Arches and hollow cylinders subjected to external pressure. Problems. PART IV MISCELLANEOUS MATTERS CHAPTER IX CONCRETE SURFACE FINISHES. 125 Methods of treatment. Mortar face. Brushed surface. Scrubbed or etched surface. Tooled surface. Plastered surface. CHAPTER X STUCCO AND PLASTER WORK 131 Uses of stucco. Stucco surface finishes. Specifications for stucco CONTENTS xi CHAPTER XI PAGE WATERPROOFING AND COLORING CONCRETE 142 Necessity for waterproofing. Precautions to be observed in water- tight work. Methods of waterproofing. Use of hydrated lime. Use of water repellent compounds. Surface coatings. Bituminous shield. Coloring concrete. Colored aggregates. Mineral pigments. Painting concrete surfaces. CHAPTER XII CASTING IN MOLDS '. 150 Building blocks. Processes of manufacture. Block machines. Kinds of blocks. Waterproofing and coloring of blocks. Curing of blocks. Cost of concrete blocks. Specifications for concrete blocks. Concrete brick. Building trim. Drain tile. Rein- forced concrete pipe. Concrete fence posts. Corner and gate posts. Fastening fence to posts. Cost of posts. PART V TYPICAL APPLICATIONS OF CONCRETE CHAPTER XIII SIDEWALKS, FLOORS, AND ROADS 177 Sidewalks. Preparation of subgrade. Proportions. Forms. Mixing and placing the concrete. Surface finish. Curing. Cost of sidewalks. Use of concrete for floors. Cellar floors. Barn Floors. Feeding floors. Concrete roads. Construction of roads. Cost of Roads. CHAPTER XIV TANKS, CISTERNS, AND SILOS 188 Use of concrete for water tanks. Construction of tanks. Rein- forcing. Cisterns. Requirements of a good silo. Use of con- crete for silos. Size of silo required. Forms. Construction of the silo. Doors and chute. Reinforcing. Roofs. Cost of silos. CHAPTER XV SMALL HIGHWAY BRIDGES AND CULVERTS 206 Use of concrete for bridges and culverts. Types of bridges. Size of bridge. Foundations. Abutments. Flat slab bridges. Flat top box culverts. Circular top box culverts. INDEX 221 CONCEETE CONSTRUCTION FOR RUKAL COMMUNITIES INTRODUCTION Definitions. Concrete is an artificial stone made by cementing together particles of sand, gravel, broken stone, or other hard, inert substances, designated as the aggregate. Usually a cement is used which requires only the addition of water to make it harden, or set, and only concretes of this class will be considered in this book. The term cement is frequently applied to such objects as walks, building blocks, and silos, especially when they do not contain coarse particles of stone or gravel, but they should properly be called concrete walks, concrete building blocks, or concrete silos, and the name cement should be restricted to the material used to bind the aggregate together. Mortar is a name often given to concretes containing no coarse aggregate, or to that part of concrete which consists of cement, sand, and water. The name is almost universally ap- plied to mixtures especially before they have hardened used for filling the joints of masonry, and for plastering. Development of the Use of Concrete. Concrete was used for construction work by ancient peoples many centuries before Christ. The Egyptians and the Carthaginians are said to have used it to some extent, while the Romans employed it extensively in foundations, walls, and aqueducts. Their concrete was made by cementing together sand and broken stones with a mixture of volcanic ash and slaked lime. The cement resulting from this mixture was similar to the puzzolan, or slag, cement manufac- tured at the present time. Some of the structures built by the Romans are still in a good state of preservation after standing for two thousand years, thus testifying to the permanence of concrete construction. 1 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES During the Middle Ages the use of concrete was continued to some extent, but no great advance was made. In the eighteenth century it was discovered that the clay contained in certain limes conferred on them the property of setting, or hardening, under water, and the process of making natural cement was developed. In the last century the use of concrete was greatly stimulated by the invention of Portland cement, by the discovery of de- posits of rock suitable for making natural cement, and by the development of machinery which made it possible to produce these cements cheaply. 10 95 96 97 10 U 1900 01 02 03 04 OS 06 07 08 Year FIG. 1. Production of Portland and Natural Cements in the United States by Years. In the earlier part of the last century, most of the cement used in this country was imported. The first discovery of rock in the United States suitable for making natural cement was made in 1818. The first Portland cement produced in the United States was made by D. O. Saylor in the Lehigh Valley of Pennsylvania about 1875. The present century has seen a wonderful growth in the use of concrete in this country. The decreasing cost 'of cement and an increasing appreciation of the valuable properties of concrete have made its use grow enormously. Figure 1, showing the INTRODUCTION 3 production of Portland and natural cements in the United States, illustrates the phenomenal growth of the industry. New uses for this material have been discovered constantly, and its field has been widened until now it is used in all kinds of construc- tion work. In the cities, large buildings are made almost entirely of concrete and steel. Foundations, walls, columns, floors, and roofs are all made of concrete, most of which is reinforced by steel rods. Roads, pavements, bridges, dams, tunnels, sewers, and reservoirs are now built of concrete. Its use is not confined to the cities and to large construction work, however, but extends to smaller towns and to farms as well. The most progressive farmers use it for foundations, sidewalks, basement floors, feeding floors, root cellars, buildings, water tanks, silos, fence posts, and numerous other structures. Its value for such purposes is only beginning to be realized and it is reasonable to believe that its use on the farm will in the near future be greatly extended. The low cost of cement, the wide distribution of sand, stone, and gravel suitable for use in concrete, the ease with which it can be manufactured and molded to any desired form, and the permanence of concrete structures when properly made, to- gether with the fact that no skilled labor or expensive ma- chinery is required for its production, make concrete one of the cheapest and best of structural materials in use at the present time, and make it almost universally available. It is greatly to be desired that a knowledge of the valuable properties and the methods of use of this material be extended as widely as possible, in order that the tremendous benefits resulting from its use may be generally enjoyed. PART I MATERIALS CHAPTER I CEMENTS AND LIMES Kinds of Cements. The cements chiefly used in concrete con- struction may be classified as follows: (a) Portland cement, (b) Natural cement, (c) Puzzolan, or slag cement. Of these, Portland cement is by far the most widely used. The relative importance of the different kinds in the concrete industry of the United States is indicated by the fact that in 1910 there were produced in this country: 76,550,000 barrels of Portland cement, 1,100,000 barrels of natural cement, and 160,000 barrels of puzzolan cement. It is seen that the amount of Portland cement produced was about seventy times the amount of natural cement and almost five hundred times the amount of puzzolan cement. This being the case, this book will deal almost entirely with the use of Portland cement, the other kinds being treated only inci- dentally. Throughout the book, therefore, wherever the word cement is used, it should be understood to refer only to Port- land cement, unless it is explicitly stated otherwise. Pprtland Cement. Portland cement is defined by the Ameri- can Society for Testing Materials as "the product obtained by finely pulverizing clinker produced by calcining to incipient fusion, an intimate and properly proportioned mixture of argil- laceous and calcareous materials, with no additions subsequent to calcination excepting water and calcined or uncalcined gypsum." This is equivalent to saying that it is the product obtained when clay and limestone, or materials of similar chemical composition, are ground together in the proper proportion, are then heated 7 8 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES nearly 'jto : ihp melting, point, and the clinker thus produced is ground to a fine powder. The clause limiting additions to the material after burning is designed to prevent adulteration, but to permit of the addition of a small amount of gypsum or plaster of Paris to regulate the rate of setting of the cement. Usually 1 or 2 per cent of gypsum is required for this purpose. It is added to the clinker after the burning, but before the final grinding. As may be seen from the above definition, the name Port- land does not refer to cements made in any particular locality. It was given to the material by the inventor, because of the fancied resemblance of concrete made from it to the building stone quarried on the Isle of Portland, England. The essential materials for making Portland cement are cal- cium carbonate, the principal constituent of limestone; and aluminum silicate, the principal constituent of. clay. These materials are usually used in the proportion of about 3 parts of the former to 1 part of the latter. They occur naturally in many different forms and are very commonly found mixed together. In a few localities stone is found which contains them already mixed in approximately correct proportions. More commonly an excess of one of the materials is present, and the deficiency of the other must be made up by the addi- tion of some substance in which it occurs to excess. In the United States, the materials used to supply the calcium carbon- ate are chiefly limestone, chalk, or marl, while the aluminum silicate is supplied by clay, shale, blast-furnace slag, or cement rock. Suitable materials for the manufacture of Portland cement are widely distributed. More than half of the states in the United States have one or more plants for its manufacture, and it is lack of cheap fuel, of transportation facilities, or of a convenient market, rather than of suitable materials, that pre- vents its manufacture in other states. Table I gives the pro- duction of Portland cement in the United States, by states, in 1911. The color of Portland cement is usually a bluish gray, though the tint differs considerably in different brands. Certain mills CEMENT AND LIMES 9 make a special kind of Portland cement which is almost pure white in color, and which is much used for ornamental purposes. TABLE I PRODUCTION OF PORTLAND CEMENT IN THE UNITED STATES BY STATES IN 1911 States Producing plants Barrels Value Pennsylvania 25 Indiana. 5 California 8 Kansas 12 Illinois . . .' 5 New Jersey 3 Missouri 4 Michigan 11 New York 7 Iowa 3 Ohio 5 Washington 3 Utah 3 Texas 4 Oklahoma 2 Tennessee 2 West Virginia 1 Kentucky 1 Virginia Maryland . . 2 Colorado 2 Montana 1 Alabama 2 Georgia 2 Total 115 26,864,679 7,407,830 6,317,701 4,871,903 4,582,341 4,411,890 4,114,859 3,686,716 3,314,217 1,952,590 1,451,852 960,573 662,849 2,438,493 1,981,341 1,487,753 1,162,081 858,969 $19,258,253 5,937,241 8,737,150 3,725,108 3,583,301 3,259,528 3,349,312 3,024,676 2,669,194 1,881,253 1,228,680 1,496,807 827,523 2,541,449 1,590,438 1,084,315 1,272,317 782,272 78,528,637 $66,248,817 Natural Cement. - Natural cement is defined by the American Society for Testing Materials as "the finely pulverized product resulting from the calcination l of an argillaceous 2 limestone at a temperature only sufficient to drive off the carbonic acid gas." It therefore differs from Portland cement in the follow- ing respects: (1) only one material is used in its manufacture, a telayey limestone of more or less varying composition, instead of a definitely proportioned mixture of clay and limestone; (2) this rock is not pulverized before being burned; (3) the tem- perature of burning is very much lower. The resulting prod- 1 That is, burning. 2 That is, clayey. 10 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES uct is much inferior to Portland cement in cementing power. On account of the simpler process of manufacture, natural cement can be sold more cheaply than Portland cement. It can often be distinguished from the latter by its lighter color, but this is not a positive indication, as some natural cements are dark, while some Portland cements are light in color. Natural cement was formerly of considerable importance, but its use is decreasing and its place is being taken by Port- land cement, as may be seen by reference to Fig. 1. While the latter costs more per barrel than the former, much less of it is required to produce a concrete of the same strength. Its use is therefore more economical, except perhaps in certain locali- ties or for certain special uses. Puzzolan, or Slag Cement. Puzzolan, or slag cement, is formed by mixing granulated blast-furnace slag of special com- position with slaked lime, and grinding the mixture to a very fine powder. In Europe volcanic ash is sometimes used instead of the blast-furnace slag. This cement differs from Portland cement made from the same materials, in that it is not burned after its constituents are mixed, and in that its properties are considerably different. While it may give about the same strength to mortars and con- cretes as Portland cement, concrete made with it is more likely to disintegrate or crumble to pieces, especially in dry air, than that made with Portland cement. Lime. When a pure or nearly pure limestone is heated sufficiently, the carbon dioxide is driven off, and calcium oxide remains as a white, hard stone. This forms the quicklime of commerce. It has a strong affinity for water and, when brought in contact with the latter, it will absorb a large quantity. At the same time, the lime swells in volume to two or three times its original bulk, the pieces break up and form a fine powder, or, if enough water is added, a smooth paste, and a large amount of heat is given off. This process is known as slaking, and the product is called slaked lime, lime putty, or lime paste. When lime paste is exposed to the air for some time, it hardens. This action is due to three distinct causes: (1) drying of the paste; (2) crystallization of the dissolved lime; and (3) CEMENT AND LIMES 11 absorption of carbon dioxide from the air, which changes the mass to calcium carbonate, similar to the original limestone. The last mentioned action goes on but slowly, and continues for a very long time. It takes place first and chiefly near the surface, and the interior of large masses never is completely changed. Hydrated Lime. -**- Hydrated lime is lime which has been slaked at the factory by the addition of just enough water to produce a fine, impalpable powder. It is being marketed in large quan- tities. Its advantages are that it is in powder form and is com- pletely slaked and ready for use. It is much more conveniently used in concrete work than quicklime, as its powdered form makes it easy for the workman to obtain proper proportions and to get the lime uniformly distributed throughout the mass. Choice of a Cement. Portland cement is well adapted for all kinds of concrete construction, and is usually to be chosen in preference to any other kind. It is recommended that it be used for all purposes by those not thoroughly familiar with concrete construction, and throughout this book all the propor- tions indicated, and all the designs given, are intended for use only with Portland cement. It is especially important that it be used in all places where high strength, water-tightness, or resistance to abrasion is required. Natural cement may be used for concrete where but little strength is required, and where it will not be subjected to wear or to the action of water. It should never be used in reinforced concrete work, nor in moist situations. Its use for any purpose is not recommended unless it is decidedly more economi- cal than Portland cement after one has taken into account the fact that, with the latter, a much leaner mixture may be used. Puzzolan cement may be used for concrete in moist or wet situations, but it is likely to crumble when exposed to dry air for any considerable length of time. It usually gives some- what weaker concretes than Portland cement, used in the same proportions, and this should be taken into account in consider- ing the relative economy of the two. The choice of the brand of cement to be used on small jobs must usually be based on the reputations which the different brands bear in the particular locality in question. An intelli- 12 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES gent choice between brands made by reputable manufacturers can usually be made only after tests which are much too elabo- rate to be performed by those unfamiliar with cement testing and unequipped with apparatus. These tests are described in the next chapter. Any brand which meets the standard speci- fications of the American Society for Testing Materials may be used with confidence, and all reputable manufacturers now guarantee their products to meet these specifications. Gener- ally it is sufficient for the small user to buy a brand which he knows has given satisfaction on other work in his locality and to see that the cement is not hard or lumpy. If desired, the soundness, or constancy of volume, test may be made as de- scribed at the end of the next chapter. Care of Cement. Cement is usually packed in cloth bags, each containing 94 pounds of cement. Occasionally it is packed in wooden barrels or in paper bags, or is shipped in bulk. The cloth bags are charged for when the cement is bought, but a full rebate is given for sacks returned in good condition, so that these make economical packages. The cloth bag is much more convenient to handle than the wooden barrel, and not so liable to breakage as the paper bag. Consequently it is replac- ing these forms of packages. Bulk shipments are not made at present to any considerable extent. It is important that cement be stored in a dry place. If it is piled directly on the earth, or on a floor laid on the earth, or if it is piled against an outside wall for any length of time, it will absorb moisture and will become hard and lumpy, even though it is kept from direct contact with water. A false floor should be made of boards laid on blocks, to separate the cement from the floor and to allow a circulation of air beneath it, and the sacks should be piled a few inches away from all outside walls. Much cement has been ruined by failure to observe these precautions. When well protected, cement can be stored for an indefinite length of time without injury. Cement which has become hard or lumpy should have the lumps screened out before it is used. The portion passing the screen may be used with confidence, but the lumps should be discarded. CHAPTER II CEMENT SPECIFICATIONS AND TESTS For the Small Purchaser. As was stated in the preceding chapter, the purchaser of small quantities of cement must usu- ally rely on the reputation of the brand used. All such cement should be purchased, however, with the understanding that it will meet the Standard Specifications for Cement of the Ameri- can Society for Testing Materials. These specifications have now been very generally adopted throughout this country, and all reputable manufacturers will guarantee their cement to meet them. In case of doubt as to whether a cement purchased is of good quality, samples 'should be taken as described later in this chapter, and sent to some engineering laboratory equipped to make the proper tests. Such laboratories may be found in prac- tically all large cities, and in the state colleges and universities. For Larger and More Important Work. All cement to be used for larger work should be sampled and tested according to the methods prescribed in the Standard Specifications of the American Society for Testing Materials, and should be required to meet the standards laid down in the Specifications. The specifications for Portland Cement are here given in full. STANDARD SPECIFICATIONS AND TESTS FOR PORTLAND CEMENT 1 SPECIFICATIONS 1. Definition. Portland cement is the product obtained by finely pulverizing clinker produced by calcining to incipient fusion, an intimate and properly proportioned mixture of argilla- ceous and calcareous materials, with no additions subsequent to calcination excepting water and calcined or uncalcined gypsum. 1 Copyrighted standard of the American Society for Testing Materials, Serial Designation C 9-17. 1916 Book of A. S. T. M. Standards, p. 429. 13 14 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES I. CHEMICAL PROPERTIES 2. Chemical Limits. The following ]imits shall not be exceeded : Loss on ignition, per cent 4.00 Insoluble residue, per cent 0.85 Sulfuric anhydride (SO 8 ), per cent 2.00 Magnesia (MgO), per cent 5.00 II. PHYSICAL PROPERTIES 3. Specific Gravity. The specific gravity of cement shall be not less than 3.10 (3.07 for white Portland cement). Should the test of cement as received fall below this requirement a second test may be made upon an ignited sample. The specific gravity test will not be made unless specifically ordered. 4. Fineness. The residue on a standard No. 200 sieve shall not exceed 22 per cent by weight. 6. Soundness. A pat of neat cement shall remain firm and hard, and show no signs of distortion, cracking, checking, or dis- integration in the steam test for soundness. 6. Time of Setting. The cement shall not develop initial set in less than 45 minutes when the Vicat needle is used or 60 min- utes when the Gillmore needle is used. Final set shall be attained within 10 hours. 7. Tensile Strength. The average tensile strength in pounds per square inch of not less than three standard mortar briquettes (see Section 51) composed of one part cement and three parts standard sand, by weight, shall be equal to or higher than the following: Age at Test, days Storage of Briquettes Tensile Strength, . Ib. per sq. in. 7 28 1 day in moist air, 6 days in water 1 day in moist air, 27 days in water 200 300 8. The average tensile strength of standard mortar at 28 days shall be higher than the strength at 7 days. CEMENT SPECIFICATIONS AND TESTS 15 III. PACKAGES, MARKING AND STORAGE 9. Packages and Marking. The cement shall be delivered in suitable bags or barrels with the brand and name of the manu- facturer plainly marked thereon, unless shipped in bulk. A bag shall contain 94 Ib. net. A barrel shall contain 376 Ib. net. 10. Storage. The cement shall be stored in such a manner as to permit easy access for proper inspection and identification of each shipment, and in a suitable weather-tight building which will protect the cement from dampness. IV. INSPECTION 11. Inspection. Every facility shall be provided the pur- chaser for careful sampling and inspection at either the mill or at the site of the work, as may be specified by the purchaser. At least 10 days from the time of sampling shall be allowed for the completion of the 7-day test, and at least 31 days shall be allowed for the completion of the 28-day test. The cement shall be tested in accordance with the methods hereinafter prescribed. The 28-day test shall be waived only when specifically ordered. V. REJECTION 12. Rejection. The cement may be rejected if it fails to meet any of the requirements of these specifications. 13. Cement shall not be rejected on account of failure to meet the fineness requirement if upon retest after drying at ' 100 C. for one hour it meets this requirement. 14. Cement failing to meet the test for soundness in steam may be accepted if it passes a retest using a new sample at any time within 28 days thereafter. 15. Packages varying more than 5 per cent from the specified weight may be rejected; and if the average weight of packages in any shipment, as shown by weighing 50 packages taken at random, is less than that specified, the entire shipment may be rejected. 16 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES TESTS VI. SAMPLING 16. Number of Samples. Tests may be made on individual or composite samples as may be ordered. Each test sample should weigh at least 8 Ib. 17. (a) Individual Sample. If sampled in cars one test sample shall be taken from each 50 bbl. or fraction thereof. If sampled in bins one sample shall be taken from each 100 bbl. (6) Composite Sample. If sampled in cars one sample shall be taken from one sack in each 40 sacks (or 1 bbl. in each 10 bbl.) and combined to form one test sample. If sampled in bins or warehouses one test sample shall represent not more than 200 bbl. 18. Method of Sampling. Cement may be sampled at the mill by any of the following methods that may be practicable, as ordered : (a) From the Conveyor Delivering to the Bin. At least 8 Ib. of cement shall be taken from approximately each 100 bbl. pass- ing over the conveyor. (6) From Filled Bins by Means of Proper Sampling Tubes. Tubes inserted vertically may be used for sampling cement to a maximum depth of 10 ft. Tubes inserted horizontally may be used where the construction of the bin permits. Samples shall be taken from points well distributed over the face of the bin. (c) From Filled Bins at Points of Discharge. Sufficient cement shall be drawn from the discharge openings to obtain samples representative of the cement contained in the bin, as determined by the appearance at the discharge openings of indi- cators placed on the surface of the cement directly above these openings before drawing of the cement is started. 19. Treatment of Sample. Samples preferably shall be shipped and stored in air-tight containers. Samples shall be passed through a sieve having 20 meshes per linear inch in order to thoroughly mix the sample, break up lumps and remove foreign materials. CEMENT SPECIFICATIONS AND TESTS 17 VII. CHEMICAL ANALYSIS Loss ON IGNITION 20. Method. One gram of cement shall be heated in a weighed covered platinum crucible, of 20 to 25-cc. capacity, as follows, using either method (a) or (6) as ordered: (a) The crucible shall be placed in a hole in an asbestos board, clamped horizontally so that about three-fifths of the crucible projects below, and blasted at a full red heat for 15 minutes with an inclined flame; the loss in weight shall be checked by a second blasting for 5 minutes. Care shall be taken to wipe off particles of asbestos that may adhere to the crucible when withdrawn from the hole in the board. Greater neatness and shortening of the time of heating are secured by making a hole to fit the crucible in a circular disk of sheet platinum and placing this disk over a somewhat larger hole in an asbestos board. (6) The crucible shall be placed in a muffle at any temperature between 900 and 1000 C. for 15 minutes and the loss in weight shall be checked by a second heating for 5 minutes. 21. Permissible Variation. A permissible variation of 0.25 will be allowed, and all results in excess of the specified limit but within this permissible variation shall be reported as 4 per cent. INSOLUBLE RESIDUE 22. Method. To a 1-g. sample of cement shall be added 10 cc. of water and 5 cc. of concentrated hydrochloric acid; the liquid shall be warmed until effervescence ceases. The solution shall be diluted to 50 cc. and digested on a steam bath or hot plate until it is evident that decomposition of the cement is complete. The residue shall be filtered, washed with cold water, and the filter paper and contents digested in about 30 cc. of a 5-per cent solution of sodium carbonate, the liquid being held at a tempera- ture just short of boiling for 15 minutes. The remaining residue shall be filtered, washed with cold water, then with a few drops of hot hydrochloric acid, 1:9, and finally with hot water, and then ignited at a red heat and weighed as the insoluble residue. 23. Permissible Variation. A permissible variation of 0.15 will be allowed, and all results in excess of the specified limit but within this permissible variation shall be reported as 0.85 per cent. 18 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES SULPHURIC ANHYDRIDE 24. Method. One gram of the cement shall be dissolved in 5 cc. of concentrated hydrochloric acid diluted with 5 cc. of water, with gentle warming; when solution is complete 40 cc. of water shall be added, the solution filtered, and the residue washed thoroughly with water. The solution shall be diluted to 250 cc., heated to boiling and 10 cc. of a hot 10-per cent solution of barium chloride shall be added slowly, drop by drop, from a pipette and the boiling continued until the precipitate is well formed. The solution shall be digested on the steam bath until the precipitate has settled. The precipitate shall be filtered, washed, and the paper and contents placed in a weighed platinum crucible and the paper slowly charred and consumed without flaming. The barium sulfate shall then be ignited and weighed. The weight obtained multiplied by 34.3 gives the percentage of sulfuric anhydride. The acid filtrate obtained in the determination of the insoluble residue may be used for the estimation of sulfuric anhydride instead of using a separate sample. 25. Permissible Variation. A permissible variation of 0.10 will be allowed, and all results in excess of the specified limit but within this permissible variation shall be reported as 2.00 per cent. MAGNESIA 26. Method. To 0.5 g. of the cement in an evaporating dish shall be added 10 cc. of water to prevent lumping and then 10 cc. of concentrated hydrochloric acid. The liquid shall be gently heated and agitated until attack is complete. The solution shall then be evaporated to complete dry ness on a steam or water bath. To hasten dehydration the residue may be heated to 150 or even 200 C. for one-half to one hour. The residue shall be treated with 10 cc. of concentrated hydrochloric acid diluted with an equal amount of water. The dish shall be covered and the solu- tion digested for ten minutes on a steam bath or water bath. The diluted solution shall be filtered and the separated silica washed thoroughly with water. 1 Five cubic centimeters of concentrated 1 Since this procedure does not involve the determination of silicia, a second evaporation is unnecessary. CEMENT SPECIFICATIONS AND TESTS 19 hydrochloric acid and sufficient bromine water to precipitate any .manganese which may be present shall be added to the filtrate (about 250 cc.). This shall be made alkaline with ammonium hydroxide, boiled until there is but a faint odor of ammonia, and the precipitate, iron and aluminum hydroxides, after settling, shall be washed with hot water, once by decantation and slightly on the filter. Setting aside the filtrate, the precipitate shall be transferred by a jet of hot water to the precipitating vessel and dissolved in 10 cc. of hot hydrochloric acid. The paper shall be extracted with acid, the solution and washings being added to the main solution. The aluminum and iron shall then be repre- cipitated at boiling heat by ammonium hydroxide and bromine water in a volume of about 100 cc., and the second precipitate shall be collected and washed on the filter used in the first in- stance if this is still intact. To the combined filtrates from the hydroxides of iron and aluminum, reduced in volume if need be, 1 cc. of ammonium hydroxide shall be added, the solution brought to boiling, 25 cc. of a saturated solution of boiling ammonium oxalate added, and the boiling continued until the precipitated calcium oxalate has assumed a well-defined granular form. The precipitate after one hour shall be filtered and washed, then with the filter shall be placed wet in a platinum crucible, and the paper burned off over a small flame of a Bunsen burner; after ignition it shall be redissolved in hydrochloric acid and the solu- tion diluted to 100 cc. -Ammonia shall be added in slight excess, and the liquid boiled. The lime shall then be reprecipitated by ammonium oxalate, allowed to stand until settled, filtered, and washed. The combined filtrates from the calcium precipitates shall be acidified with hydrochloric acid, concentrated on the steam bath to about 150 cc., and made slightly alkaline with ammonium hydroxide, boiled and filtered (to remove a little aluminum and iron and perhaps calcium). When cool, 10 cc. of saturated solution of sodium-ammonium-hydrogen phosphate shall be added with constant stirring. When the crystallin am- monium-magnesium orthophosphate has formed, ammonia shall be added in moderate excess. The solution shall be set aside for several hours in a cool place, filtered and washed with water containing 2.5 per cent of NH 3 . The precipitate shall be dissolved 20 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES in a small quantity of hot hydrochloric acid, the solution diluted to about 100 cc., 1 cc. of a saturated solution of sodium-ammo- nium-hydrogen phosphate added, and ammonia drop by drop, with constant stirring, until the precipitate is again formed as described and the ammonia is in moderate excess. The precipi- tate shall then be allowed to stand about two hours, filtered and washed as before. The paper and contents shall be placed in a weighed platinum crucible, the paper slowly charred, and the resulting carbon carefully burned off. The precipitate shall then be ignited to constant weight over a Meker burner, or a blast not strong enough to soften or melt the pyrophosphate. The weight of magnesium pyrophosphate obtained multiplied by 72.5 gives the percentage of magnesia. The precipitate so obtained always contains some calcium and usually small quantities of iron, aluminum, and manganese as phosphates. 27. Permissible Variation. A permissible variation of 0.4 will be allowed, and all results in excess of the specified limit but within this permissible variation shall be reported as 5.00 per cent. VIII. DETERMINATION OF SPECIFIC GRAVITY *28. Apparatus. The determination of specific gravity shall be made with a standardized Le Chatelier apparatus which con- forms to the requirements illustrated in Fig. 2. This apparatus is standardized by the United States Bureau of Standards. Kero- sene free from water, or benzine not lighter than 62 Baume, shall be used in making this determination. 29. Method. The flask shall be filled with either of these liquids to a point on the stem between zero and one cubic centi- meter, and 64 g. of cement, of the same temperature as the liquid, shall be slowly introduced, taking care that the cement does not adhere to the inside of the flask above the liquid and to free the cement from air by rolling the flask in an inclined position. After all the cement is introduced, the level of the liquid will rise to some division of the graduated neck;, the difference between readings is the volume displaced by 64 g. of the cement. The specific gravity shall then be obtained from the formula Weight of cement (g.) Specific gravity = Displaced volume (cc.) CEMENT SPECIFICATIONS AND TESTS 21 j<- 5cm : . ( /.13cm- -^ Ground Glass Have two Q/cc Graduations extend - above I and below O Mark --'-. Capacity of Bulk approx. ZSOcc- --- >| v )L- 9cm FIG. 2. Le Chatelier Apparatus 22 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES 30. The flask, during the operation, shall be kept immersed in water, in order to avoid variations in the temperature of the liquid in the flask, which shall not exceed 0.5 C. The results of repeated tests should agree within 0.01. 31. The determination of specific gravity shall be made on the cement as received; if it falls below 3.10, a second determina- tion shall be made after igniting the sample as described in Sec- tion 20. IX. DETERMINATION OF FINENESS 32. Apparatus. Wire cloth for standard sieves for cement shall be woven (not twilled) from brass, bronze, or other suitable wire, and mounted without distortion on frames not less than 1J in. below the top of the frame. The sieve frames shall be cir- cular, approximately 8 in. in diameter, and may be provided with a pan and cover. 33. A standard No. 200 sieve is one having nominally an 0.0029-in. opening and 200 wires per inch standardized by the U. S. Bureau of Standards, and conforming to the following requirements: The No. 200 sieve should have 200 wires per inch, and the number of wires in any whole inch shall not be outside the limits of 192 to 208. No opening between adjacent parallel wires shall be more than 0.0050 in. in width. The diameter of the wire should be 0.0021 in. and the average diameter shall not be outside the limits 0.0019 to 0.0023 in. The value of the sieve as determined by sieving tests made in conformity with the standard specifica- tion for these tests on a standardized cement which gives a resi- due of 25 to 20 per cent on the No. 200 sieve, or on other similarly graded material, shall not show a variation of more than- 1.5 per cent above or below the standards maintained at the Bureau of Standards. 34. Method. The test shall be made with 50 g. of cement. The sieve shall be thoroughly clean and dry. The cement shall be placed on the No. 200 sieve, with pan and cover attached, if desired, and shall be held in one hand in a slightly inclined posi- tion so that the sample will be well distributed over the sieve, at the same time gently striking the side about 150 times per CEMENT SPECIFICATIONS AND TESTS 23 minute against the palm of the other hand on the up stroke. The sieve shall be turned every 25 strokes about one-sixth of a revolution in the same direction. The operation shall continue until riot more than 0.05 g. passes through in one minute of con- tinuous sieving. The fineness shall be determined from the weight of the residue on the sieve expressed as a percentage of the weight of the original sample. 35. Mechanical sieving devices may be used, but the cement shall not be rejected if it meets the fineness requirement when tested by the hand method described in Section 34. 36. Permissible Variation. A permissible variation of 1 will be allowed, and all results in excess of the specified limit but within this permissible variation shall be reported as 22 per cent. X. MIXING CEMENT PASTES AND MORTARS 37. Method. The quantity of dry material to be mixed at one time shall not exceed 1000 g. nor be less than 500 g. The proportions of cement or cement and sand shall be stated by weight in grams of the dry materials; the quantity of water shall be expressed in cubic centimeters (1 cc. of water = 1 g.). The dry materials shall be weighed, placed upon a non-absorbent surface, thoroughly mixed dry if sand is used, and a crater formed in the center, into which the proper percentage of clean water shall be poured; the material on the outer edge shall be turned into the crater by the aid of a trowel. After an interval of J minute for the absorption of the water the operation shall be completed by continuous, vigorous mixing, squeezing and knead- ing with the hands for at least one minute. 1 During the opera- tion of mixing, the hands should be protected by rubber gloves. 38. The temperature of the room and the mixing water shall be maintained as nearly as practicable at 21 C. (70 F.). 1 In order to secure uniformity in the results of tests for the time of setting and tensile strength the manner of mixing above described should be care- fully followed. At least one minute is necessary to obtain the desired plas- ticity which is not appreciably affected by continuing the mixing for several minutes. The exact time necessary is dependent upon the personal equation of the operator. The error in mixing should be on the side of overmixing. 24 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES XI. NORMAL CONSISTENCY 39. Apparatus. The Vicat apparatus consists of a frame A (Fig. 3) bearing a movable rod B, weighing 300 g., one end C being 1 cm. in diameter for a distance of 6 cm., the other having a removable needle D, 1 mm. in diameter, 6 cm. long. The rod FIG. 3. Vicat Apparatus is reversible, and can be held in any desired position by a screw E, and has midway between the ends a mark F which moves under a scale (graduated to millimeters) attached to the frame A. The .paste is held in a conical, hard-rubber ring G, 7 cm. in diam- eter at the base, 4 cm. high, resting on a glass plate H about 10 cm. square. CEMENT SPECIFICATIONS AND TESTS 25 40. Method. In making the determination, 500 g. of cement, -with a measured quantity of water, shall be kneaded into a paste, as described in Section 37, and quickly formed into a ball with the hands, completing the operation by tossing it six times from one hand to the other, maintained about 6 in. apart; the ball resting in the palm of one hand shall be pressed into the larger end of the rubber ring held in the other hand, completely filling the ring with paste; the excess at the larger end shall then be removed by a single movement of the palm of the hand; the ring shall then be placed on its larger end on a glass plate and the excess paste at the smaller end sliced off at the top of the ring by a single oblique stroke of a trowel held at a slight angle with the top of the ring. During these operations care shall be taken not to compress the paste. The paste confined in the ring, resting on the plate, shall be placed under the rod, the larger end of which shall be brought in contact with the surface of the paste; the scale shall be then read, and the rod quickly released. The paste shall be of normal consistency when the rod settles to a point 10 mm. below the original surface in J minute after being released. The apparatus shall be free from all vibrations during the test. Trial pastes shall be made with varying percentages of water until the normal consistency is obtained. The amount of water required shall be expressed in percentage by weight of the dry cement. 41. The consistency of standard mortar shall depend on the amount of water required to produce a paste of normal consist- ency from the same sample of cement. Having determined the normal consistency of the sample, the consistency of standard mortar made from the same sample shall be as indicated in Table II the values being in percentage of the combined dry weights of the cement and standard sand. 26 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES TABLE II PERCENTAGE OF WATER FOR STANDARD MORTARS Percentage of Percentage of Percentage of Percentage of Water for Neat Water for One Water for Neat Water for One Cement Paste of Cement, Three Cement Paste of Cement, Three Normal Standard Normal Standard Consistency Ottawa Sand Consistency Ottawa Sand 15 9.0 23 10.3 16 9.2 24 10.5 17 9.3 25 10.7 18 9.5 25 10.8 19 9.7 27 11.0 20 9.8 28 11.2 21 10.0 29 11.3 22 10.2 30 11.5 XII. DETERMINATION OF SOUNDNESS 1 42. Apparatus. A steam apparatus, which can be main- tained at a temperature between 98 and 100 C., or one similar to that shown in Fig. 4, is recommended. The capacity of this apparatus may be increased by using a rack for holding the pats in a vertical or inclined position. 43. Method. A pat from cement paste of normal consistency about 3 in. in diameter, J in. thick at the center, and tapering to a thin edge, shall be made on clean glass plates about 4 in. square, and stored in moist air for 24 hours. In molding the pat, the cement paste shall first be flattened on the glass and the pat then formed by drawing the trowel from the outer edge toward the center. 44. The pat shall then be placed in an atmosphere of steam at a temperature between 98 and 100 C. upon a suitable sup- port 1 in. above boiling water for 5 hours. 1 Unsoundness is usually manifested by change in volume which causes distortion, cracking, checking, or disintegration. Pats improperly made or exposed to drying may develop what are known as shrinkage cracks within the first 24 hours and are not an indication of un- soundness. These conditions are illustrated in Fig. 5. The failure of the pats to remain on the glass or the cracking of the glass to which the pats are attached does not necessarily indicate unsoundness. CEMENT SPECIFICATIONS AND TESTS 27 45. Should the pat leave the plate, distortion may be detected best with a straight edge applied to the surface which was in con- tact with the plate. XIII. DETERMINATION OF TIME OF SETTING 46. The following are alternate methods, either of which may be used as ordered: 47. Vicat Apparatus. The time of setting shall be determined with the Vicat apparatus described in Section 39. (See Fig. 3.) 48. Vicat Method. A paste of normal consistency shall be molded in the hard-rubber ring G as described in Section 40, and placed under the rod B, the smaller end of which shall then be carefully brought in contact with the surface of the paste, and the rod quickly released. The initial set shall be said to have occurred when the needle ceases to pass a point 5 mm. above the glass plate in J minute after being released; and the final set, when the needle does not sink visibly into the paste. The test pieces shall be kept in moist air during the test. This may be accomplished by placing them on a rack over water contained in a pan and covered by a damp cloth, kept from contact with them by means of a wire screen; or they may be stored in a moist closet. Care shall be taken to keep the needle clean, as the collec- tion of cement on the sides of the needle retards the penetration, while cement on the point may increase the penetration. The time of setting is affected not only by the percentage and tem- perature of the water used and the amount of kneading the paste receives, but by the temperature and humidity of the air, and its determination is, therefore, only approximate. 49. Gillmore Needles. The time of setting shall be deter- mined by the Gillmore needles. The Gillmore needles should preferably be mounted as shown in Fig. 6 (6). 50. Gillmore Method. The time of setting shall be deter- mined as follows: A pat of neat cement paste about 3 in. in diam- eter and J in. in thickness with a flat top (Fig. 6 (a) ), mixed to a normal consistency, shall be kept in moist air at a temperature maintained as nearly as practicable at 21 C. (70 F.). The cement shall be considered to have acquired its initial set when the pat will bear, without appreciable indentation, the Gillmore 28 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES 1/003 ipui^ CEMENT SPECIFICATIONS AND TESTS 29 30 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES needle fV in. in diameter, loaded to weigh J Ib. The final set has been acquired when the pat will bear without appreciable indentation, the Gillmore needle ^ in. in diameter, loaded to weigh 1 Ib. In making the test, the needles shall be held in a vertical position, and applied lightly to the surface of the pat. (a) Pat with top surface flattened for determining time of setting by Gilmore method. (b) Gillmore Needles. FIG. 6 XIV. TENSION TESTS 51. Form of Test Piece. The form of test piece shown in Fig. 7 shall be used. The molds shall be made of non-corroding metal and have sufficient material in the sides to prevent spread- ing during molding. Gang molds when used shall be of the type shown in Fig. 8. Molds shall be wiped with an oily cloth before using. CEMENT SPECIFICATIONS AND TESTS 31 52. Standard Sand. The sand to be used shall be natural sand from Ottawa, 111., screened to pass a No. 20 sieve and retained on a No. 30 sieve. This sand may be obtained from the Ottawa Silica Co., at a cost of two cents per pound, f. o. b. cars, Ottawa, 111. FIG. 7. Details for Briquette 53. This sand having passed the No. 20 sieve shall be con- sidered standard when not more than 5 g. pass the No. 30 sieve after one minute continuous sieving of a 500-g. sample. 54. The sieves shall conform to the following specifications: The No. 20 sieve shall have between 19.5 and 20.5 wires per whole inch of the warp wires and between 19 and 21 wires per 31a CONCRETE CONSTRUCTION FOR RURAL COMMUNUTIES whole inch of the shoot wires. The diameter of the wire should be 0.0165 in. and the average diameter shall not be outside the limits of 0.0160 and 0.0170 in. The No. 30 sieve shall have between 29.5 and 30.5 wires per whole inch of the warp wires and between 28.5 and 31.5 wires per whole inch of the shoot wires. The diameter of the wire should be 0.0110 in. and the average diameter shall not be out- side the limits 6.0105 to 0.0115 in. 55. Molding. Immediately after mixing, the standard mor- tar shall be placed in the molds, pressed in firmly with the thumbs, and smoothed off with a trowel without ramming. Additional mortar shall be heaped above the mold and smoothed off with a FIG. 8. Gang Mold trowel; the trowel shall be drawn over the mold in such a manner as to exert a moderate pressure on the material. The mold shall then be turned over and the operation of heaping, thumbing, and smoothing off repeated. 56. Testing. Tests shall be made with any standard ma- chine. The briquettes shall be tested as soon as they are removed from the water. The bearing surfaces of the clips and briquettes shall be free from grains of sand or dirt. The briquettes shall be carefully centered and the load applied continuously at the rate of 600 Ib. per minute. 57. Testing machines should be frequently calibrated in order to determine their accuracy. 58. Faulty Briquettes. Briquettes that are manifestly faulty, or which give strengths differing more than 15 per cent from the average value of all test pieces made from the same sample and broken at the same period, shall not be considered in determining the tensile strength. CEMENT SPECIFICATIONS AND TESTS 316 XV. STORAGE OF TEST PIECES 59. Apparatus. The moist closet may consist of a soap- stone, slate, or concrete box, or a wooden box lined with metal. If a wooden box is used, the interior should be covered with felt or broad wicking kept wet. The bottom of the moist closet should be covered with water. The interior of the closet should be provided with non-absorbent shelves on which to place the test pieces, the shelves being so arranged that they may be with- drawn readily. GO, Methods. Unless otherwise specified all test pieces, immediately after molding, shall be placed in the moist closet for from 20 to 24 hours. 61. The briquettes shall be kept in molds on glass plates in the moist closet for at least 20 hours. After 24 hours in moist air the briquettes shall be immersed in clean water in storage tanks of non-corroding material. 62. The air and water shall be maintained as nearly as prac- ticable at a temperature of 21 C. (70 F.). 32 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Soundness Test without Special Apparatus. Of the above tests, the one relating to constancy of volume or soundness is probably the most important. Fortunately this test can easily be made by anyone who is careful, without any special appara- tus or training. The cement should be sampled as previously described in this chapter. This sample should then be thoroughly mixed with water on some nonabsorbent surface, such as glass or porcelain, to the consistency of putty. What is known as the Boulogne method of determining the proper consistency requires that "the paste shall be firm but well bonded, shining and plastic, and shall satisfy the following conditions: "1. The consistency shall not change if it is worked 3 minutes longer than the original 5 minutes. "2. If dropped 20 inches from a trowel, it should leave the trowel clean, and fall without losing its shape or cracking. "3. Light pressure in the hand should bring water to the surface, and the paste should not stick to the hand. If a ball thus formed falls from a height of about 20 inches, it should retain its rounded form without showing cracks. "4. The proportion of water should be such that more or less will produce opposite effects from those just described for the proper consistency." 1 After repeated trials have given a paste which is just right, it should be made, on pieces of glass, into three or more pats, about 3 inches in diameter and one-half inch thick at the middle, tapering to a thin edge.. These pats should be kept in moist air for the first twenty-four hours. This may be done by sus- pending a damp cloth, such as a piece of burlap, over the pats so that it does not touch them but has its ends dipping into pans of water. After twenty-four hours one of the pats should be placed in dry air and one in water, and left for twenty-seven days, observations being taken at intervals. The third pat should be suspended over boiling water for five hours in a loosely closed vessel, such as a teakettle. The pat should be far enough above the water so that the latter will not strike it in boiling. 1 Taylor and Thompson, "Concrete, Plain and Reinforced," 2d edition, page 71. CEMENT SPECIFICATIONS AND TESTS 33 Any radial cracks formed near the edges, or any evidences of crumbling of the air or water pats, show the cement to beun- sound, and call for its rejection. Cracks which are sometimes found near the middle of the pat are usually evidence that the air in which the pats were kept during the first twenty-four hours was not moist enough, and these should not be confused with the evidences of unsoundness. Figures 10 and 11 show FIG. 10. Pat showing Shrinkage Cracks due to too Rapid Drying. These Cracks are Harmless. FIG. 11. Pat showing Expansion Cracks due to Unsoundness of Ce- ment. These warrant Rejection of the Cement. the difference between dangerous and harmless cracks. If the pat which is given the steaming test shows radial cracks at the edges, the cement is not necessarily unsound, but should not be used until the end of the twenty-eight-day tests of the air and water pats. If the latter pats are all right, the cement may be used, notwithstanding the failure of the pat under the accel- erated test. On the other hand, if the pat passes the steaming test, the cement may be used, without waiting for the twenty- eight-day tests. CHAPTER III AGGREGATES Aggregates. The name aggregates is given to those mate- rials, such as sand, gravel, broken stone, and cinders, which are bound together by the cement to form concrete. These are divided into two classes: (1) fine aggregates, such as sand and stone screenings; and (2) coarse aggregates, such as gravel and broken stone. For concrete, it is customary to use a mixture of coarse and fine aggregates in about the proportion of two parts of the former to one part of the latter, though sometimes the coarse aggregate is omitted altogether, or a mixture of coarse and fine material may be used as it comes from the gravel bank. The subject of proportions will be discussed fully in the next chapter. Selection of Aggregates. The quality of the aggregates used has a very important effect on the quality of the concrete pro- duced, and a careful study of them will be well worth while. Cements are manufactured under careful supervision, samples of the product being taken at regular intervals and carefully tested by the manufacturers. The aggregates, on the other hand, are usually local materials, often being taken from the site of the work, and not subjected to expert inspection or to tests of their quality. Consequently, much more poor concrete results from using poor aggregates than from using poor cement. Fur- ther, certain aggregates will produce a concrete, of given quality with a much smaller proportion of cement than will other ag- gregates. Tests show that a very fine sand may require several times as much cement as a coarse sand requires to produce the same strength of concrete. As cement is the most expensive element in concrete, it will, therefore, often be uneconomical to use the cheapest sand or gravel, or the one which can be had with the least hauling. The quality of the aggregates should be carefully studied, and only those should be used which will 34 AGGREGATES 35 produce a sound concrete of a strength sufficient for the purpose, at the lowest total cost, all things considered. Sand. Sand is most commonly used for the fine aggregate in concrete. As a matter of convenience the term may be re- stricted to particles which will pass through a J-inch mesh screen, the coarser particles being considered as gravel. It may come either from a natural sand-bank, or from a river, a lake, or a sea. Sands from each of these sources are extensively used in concrete construction. Requirements of Sand. The principal requirements of sand for concrete are that it be coarse and dean. It is often speci- fied that the sand shall be sharp. Careful tests show that this is entirely unnecessary, sands with rounded grains being in every respect as suitable for the purpose as those with angular grains, usually designated as sharp sands. The terms coarse and clean are rather indefinite, as sand which would be regarded as coarse in some parts of the country would be called fine in others, and while a considerable amount of some impurities in sand is harmless, a minute quantity of others will destroy its usefulness for concrete. It may be said in general that the coarser the sand is, the stronger concrete it will make, other things being equal, and that sand containing a mixture of coarse and fine particles, with the coarse grains predominating, is ideal. Probably most coarse sands have at least enough fine sand in them as they come from the bank or bed. The fine grains will be hidden between the coarser ones, and will escape notice, so that a sand which, on superficial examination, appears to be made up almost wholly of coarse grains will, upon screen- ing, show a considerable percentage of fine particles. Use of Fine Sand. The relative coarseness of sand is best determined by screening a sample over a set of screens of dif- ferent meshes l and noting the percentage of the sample passing each screen. As an approximate guide to the meaning usually given to the terms coarse, medium, and fine, particles which would be retained on a 15-mesh screen may be considered as coarse,, those passing a 15-mesh screen and retained on a 40- 1 By the mesh of a screen is meant the number of meshes or openings in one linear inch, measured at right angles to the wires. Ordinary window screen is about twelve mesh. 36 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES mesh screen as medium, and those passing a 40-mesh screen as fine. A sand containing a large percentage of fine particles will usually give a concrete of low strength, and one which is readily penetrated by water, unless a large percentage of cement is used. One reason for this is that when the particles are fine, the total surface area of the particles in a given volume- is con- siderably greater than when the particles are coarse, and hence more cement is required to coat them as perfectly. It is also more difficult to get the cement paste well distributed through the sand when the particles of the latter are fine than when they are coarse. TABLE III 1 EFFECT OF FINENESS OF SAND ON STRENGTH OF 1 : 3 MORTAR Sand Per cent passing sieves of given mesh Strength Ibs. per sq. in. Age 28 days No. 10 20 30 40 50 80 100 Tensile Compressive 1 73 46 30 15 7 2 1 654 5600 2 84 69 53 32 14 5. 3 622 4099 3 93 78 61 38 16 4 2 ' 527 3415 4 93 71 59 46 30 15 8 444 3159 5 92 84 76 62 38 12 5 361 3112 6 99 97 95 87 61 16 6 335 1898 Table III shows the effect of fineness of sand on the strength of mortar. Sand No. 6, with 87 per cent passing the No. 40 sieve, gives a mortar only about one-half as strong in tension, and one-third as strong in compression, as that from sand No. 1, of which only 15 per cent passes the No. 40 sieve. The other sands also show a decrease of strength with the fineness of the sand. If the sand contains too large a percentage of fine particles, it will sometimes pay to screen it over a 40- or 50-mesh screen, and reject the particles passing through. If this is not done, a large percentage of cement should be used to give the necessary strength. Uniformity Coefficient. It is desirable that the particles of a sand should vary in size from fine to coarse, with the coarser 1 Prepared from tests reported in Bulletin No. 331, United States Geologi- cal Survey. AGGREGATES 37 ones predominating, since the fine grains will help to fill up the spaces between the coarse grains and so reduce the amount of cement required for this purpose. The effect of a proper grada- tion of the sand grains on the strength of mortars is especially noticeable in the leaner mixtures. A convenient method of comparing the gradation of differ- ent sands is by means of their uniformity coefficients. To de- termine this coefficient, a sample of sand is screened over a set of screens and the percentage by weight of the sample passing each is noted. A plot is then made of this, as shown in Fig. 12. 100 ^T t : 4 --100 j a> 1 ^^ -- T vQ W 7C . f^- -*-- 'm rt ^ ^--* 1* > OJ 5 a s ^^* _j_ !> > "^ i >> '5 is f ^"^ jj 1 2 'i i-. " j) r iO fs ? > /* t J J 2 g ii' ,rj o 5 n' d ^> $~ ' / S A ^ OP t io s ^ or H ^ 25 *2^ J , UP o fl E^ ^ P< g 1 ^ t" Ji g I n cr | ~\ 1, --0 Diameter of Particles in Inches FIG. 12. Mechanical Analysis Curve for Blue River Sand. This is called a mechanical analysis curve. It is made by laying off to scale on the horizontal lines the diameters of the meshes of the sieves, and on the vertical lines the percentages of sand passing sieves of the given diameters of mesh. A smooth curve is then drawn through the points found, and this indi- cates approximately the percentage of particles smaller than any given size. To illustrate the plotting of the curve, the analysis from which Fig. 12 was plotted is shown below: Sieve No. Diameter of mesh Per cent of sample passing i" 4 0.234 10Q 10 0.081 63 16 0.046 42 20 0.036 31 30 0.022 10 50 0.013 2.5 100 0.0067 1.5 38 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES After the curve is plotted, the diameter of the mesh through which 60 per cent of the sample would pass and the diameter through which 10 per cent would pass are noted. The quotient of the former divided by the latter is known as the uniformity coefficient. This term may therefore be defined as the ratio of the diameter of the particles having 60 per cent smaller than themselves to the diameter of the particles having 10 per cent smaller than themselves. In the given curve, the diameter of mesh through which 60 per cent of the sample would pass is about 0.075 inch, while that through which 10 per cent would pass is about 0.022 inch. Hence the uniformity coefficient is 0.075 -r- 0.022 = 3.4. A high value of the uniformity coefficient indicates a good gradation of sizes from fins to coarse. However, there may be considerable variation in the coarseness of different sands hav- ing about the same uniformity coefficient, and hence this quantity alone cannot be regarded as a fair index of the quality of the sand. A high uniformity coefficient, together with a high value for the size of particles at the 60 per cent line, indicates a well-graded coarse sand, which if free from injurious impurities is well suited to concrete work. Impurities. The effect of impurities in the sand on the strength of concrete will differ greatly with the nature of the impurities and the richness of the mixture. It seems to be well established that a considerable amount of clay or other mineral impurities, in some cases 20 per cent or more, may be present in lean mortars, such as 1:3 or 1:4, without injury, and that a small amount of clay may even increase the strength. On the other hand, in richer mixtures such as 1:1 or 1:2, a much smaller percentage of clay will reduce the strength. The explanation seems to be that in the lean mixtures the clay helps to fill up the voids, or spaces between the grains, while in the richer mixtures there is more than enough cement paste to fill all the voids in the sand. It should not be- understood from this that it is desirable to add clay to any sand, but only that sands which contain a small amount of clay may be used for concrete, if clean sand is not available. A clean sand should always be chosen in pref- AGGREGATES 39 erence to a dirty one, and sands with more than 5 per cent of clay should not be used without first being tested. Dirty sands are sometimes washed in the manner described for washing gravel on page 43, but the results are much less satisfactory. On account of the small size of the grains, the particles of clay and silt are difficult to get rid of, and without special washing apparatus it is not usually worth while to try it. Clay which is present in the form of balls is injurious, as these balls have about the same effect as open pockets in the concrete. Most of them can be removed by screening the sand through a J-inch screen. Test for Impurities. A simple test to determine approxi- mately the amount of .clay and loam in the sand is to put about four inches of the latter into a quart fruit jar and then to fill the jar with water to about an inch from the top. The top should then be fastened on, and the jar shaken vigorously for several minutes, after which the contents should be allowed to settle. The sand will settle to the bottom, with the clay and loam above it. The dividing line can be seen by the differ- ence in color. If more than about one-fourth of an inch of clay and loam shows, the sand should not be used without being tested, and, if possible, a cleaner sand should be obtained. A more accurate test can be made by washing a known weight of sand through several waters to remove all the silt, evaporating the water and weighing the residue. Organic Impurities. Vegetable, or organic, impurities in the sand are extremely injurious even in very small quantities. An amount of vegetable loam so small as to escape notice, except on very careful examination, may render sand entirely unfit for concrete. It seems to form a coating .on the sand grains which prevents the adhesion of the cement and retards its setting. Sand suspected of containing any organic matter should there- fore be tested before being used in work of any importance. Those sands commonly designated as " quicksands" are en- tirely unsuited for concrete work, as they are very fine and are often coated with organic matter. Strength Test for Sand. A strength test of mortar made with a given sand is the final test as to whether it is suitable for use in 40 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES concrete, but such tests can be made only in laboratories suitably equipped. The Joint Committee on Concrete from the national engineering societies recommends the following requirements: "Fine aggregates should be of such quality that mortar composed of one part* Portland cement and three parts fine aggregate by weight when made into briquettes will show a tensile strength at least equal to the strength of 1 : 3 mortar of the same consistency made with the same cement and standard Ottawa sand. 1 If the aggregate be of poorer quality the proportion of cement should be increased hi the mortar to secure the desired strength. "If the strength developed by the aggregate in the 1:3 mortar is less than 70 per cent of the strength of the Ottawa-sand mortar, the material should be rejected. . To avoid the removal of any coating on the grains, which may affect the strength, bank sand should not be dried before being made into mortar, but should contain natural moisture. The percentage of moisture may be determined upon a separate sample for correcting weight. From 10 to 40 per cent more water may be required in mixing bank or artificial sands than for stand- ard Ottawa sand to produce the same consistency." It should be noted that this is given as the minimum requirement for sand. The Ottawa sand is adopted as a stand- ard, not because it is an unusually good sand, but because it is a definite sand, uniform in its properties, and is available any- where in the country. It is used only as a sort of yardstick by which other sands are measured. Any really good sand will give a strength at least equal to the Ottawa sand, and the better sands will show a considerably greater strength. Stone Screenings. Stone screenings are sometimes used instead of sand for the fine aggregate of concrete. A screen of about J-inch mesh should be used. The stone should be reasonably hard and the screenings should be free from im- purities and an excessive amount of dust. Screenings from any of the stones suitable for the coarse aggregate may be used. The discussion of sand will, for the most part, apply equally well to, stone screenings, and the same general properties are required; namely, cleanness and coarseness. Broken Stone. The coarse aggregate used in concrete work consists usually of broken stone, gravel, or cinders. Broken 1 See footnote on p. 16. AGGREGATES 41 stone for concrete should be clean, hard, and of a size suited to the character of the work in which it is used. Any sound stone, such as is used for building purposes, may be used. Trap, granite, conglomerate, hard limestone, and hard sandstone are all widely used and are well suited to the purpose. Soft lime- stones, soft sandstones, slate, and shale should not be used if it is possible to obtain hard stone. Broken hard-burned brick may be used, but softer grades of brick make weak concrete. A stone which breaks into cubical or other angular shapes is much to be preferred to one that gives thin, flat pieces. Size of Stone. The size of broken stone to be used de- pends on the character of the work. For thin walls or for rein- forced concrete, in which there is an intricate network of steel, stone, the largest particles of which will pass through a 1-inch or smaller ring, should be used. For heavier work the size may be increased to 2 or 2J inches. The general rule may be given that the diameter of the largest pieces of stone should not be greater than J to J the thickness of the concrete. As with sand, a mixture of coarse and fine particles will give ^U^\^^ 'screen stronger concrete than will a stone of uniform size. In unimportant work, the stone may be used as it comes from the crusher without screening, if the amount of fine material runs fairly uni- form and is not excessive. If this is done, the proportion of sand used should be cut down to make allowance for the fine material in the stone. It will often pay, if a dense FIG. 13. Details for Construction of In- f . clined Screen for Stone or Gravel, waterproof concrete is re- quired, or if the fine material is distributed unevenly, or if an ex- cessive amount of clay or loam is present, to screen the stone over a J-inch screen. The screenings may be mixed with, and used as a part of, the sand, if they are clean. Figure 13 gives details for J^Bolt 42 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES constructing a suitable screen. If large quantities of material are to be screened, or if the material is to be separated into several sizes, it will be desirable to have a revolving screen, such as is shown in Fig. 14. FIG. 14. Small Portable Revolving Screen. Any large flat pieces of stone which may pass through the crusher should be picked out by hand. Gravel. Gravel can frequently be obtained more cheaply or conveniently than broken stone. If it is clean and well graded, it will make as good concrete as the stone. Gravel, as it occurs in the bank or the creek bed, is usually mixed with a larger or smaller amount of sand than is suitable for concrete, and the different sizes are usually distributed in seams and pockets. It should therefore be screened over a J-inch screen, the part passing through being used as sand. If larger pebbles are present than should be used, 1 it may be necessary to screen these out with a coarse screen. Thin, flat pieces are unsuitable and should be thrown out. 1 See the discussion of the size of particles in connection with the subject of broken stone. AGGREGATES 43 Washing of Gravel. Dirty gravel can be washed by spread- ing it out in a thin layer on a mixing platform which has one edge elevated six inches or more and turning a hose on it, or throwing water on it from buckets. The remarks made about impurities in sand apply with equal force to impurities in gravel. Slope 5 Ft. in 12 Ft r Trough to Kun off Dirty Water to be Lined with Tar Paper FIG. 15. Trough for Washing and Screening Sand or Gravel. Figure 15 shows a method which can be used for screening and washing dirty gravel or sand. For sand, a screen of 30 meshes to the inch should be used and the cleats should be placed close together to support the weight of the sand. For gravel, use a J-inch screen. The sand or gravel is thrown on the upper end of the screen by one man, while another throws a stream of water on it from a hose. The water washes the sand or gravel down the screen, while the dirty water drains off in the trough provided for this purpose. Cinders. Cinders are sometimes used for concrete where little strength is required and where the concrete acts chiefly as a filler. They are entirely unsuitable for general concrete work. When used, they should be free from dust, ashes, and particles of unburned coal. PART II PLAIN CONCRETE CHAPTER IV PROPORTIONS AND QUANTITIES OF MATERIALS The Problem in Proportioning Concrete. Too little consid- eration is generally given to the subject of proportioning, es- pecially on small work. A little cement, sand, and stone, or cement and sand only, are often mixed together without any regard for the proper proportioning or accurate measuring of the materials, with the result either that the concrete is inade- quate for its purpose, or else that the cost for materials, espe- cially cement, is higher than is necessary. When one sees the abuse to which concrete is subjected in this respect, one ceases to wonder that it is not always satisfactory, and, instead, is surprised that it can stand up as well as it does. The proper proportions differ with the materials used, and with the purpose for which the concrete is being made. The three properties which are most often required are (1) strength, as in bridges and buildings; (2) resistance to abrasion or wear, as in concrete sidewalks and roads; and (3) impermeability, or water-tightness, as in water tanks and silos. The problem in proportioning concrete is to determine the amounts of cement, sand, and stone which must be used in order to obtain these properties at the least expense for materials and labor. Before one can understand the fundamental principles governing the proportioning of materials, one must consider the places occupied by the different materials in the concrete. Voids. The voids, or open spaces, between the particles form a considerable part of the volume of a material such as sand or broken stone. This is readily understood when one realizes that a large amount of water can be poured into a bucket already filled with these materials. In broken stone screened to a uniform size, the voids will be about 50 per cent of the total volume, and in screened gravel about 40 per cent. 47 48 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES In sand the voids form about 30 to 40 per cent of the volume. In cement, although the particles are very fine and the indi- vidual voids are small, the proportion of voids to the whole volume is high, being about 50 per cent when the cement is packed, and more when it is loose. When these materials are mixed together with water to make concrete, most of the sand goes into the voids of the stone, and the cement paste remaining after the particles of sand and stone are coated goes into the voids of the sand. The result is that, while the volume of concrete produced is not much greater than the volume of stone used, the concrete is much denser; i.e., has a smaller percentage of voids than any of the separate materials used. This is illustrated in Fig. 16. ; l It J L IT J L H 12- H K 12" H H 12'- *| Cement Sand Stone Concrete FIG. 16. Diagram Showing Relation of Materials to Each Other in Concrete. To Find the Percentage of Voids in Coarse Aggregate. The voids in the broken stone or gravel may be found approxi- mately by finding how much water can be poured into a certain volume of the material. Fill the measure level full with the materials to be tested, weigh it, and pour in water until the measure will hold no more. Weigh the measure again and take the difference in the two weights. This is the weight of water required to fill the voids. Now empty the measure and find the weight of water required to fill it. Divide the weight of water required to fill the voids by this weight, and the quotient multiplied by 100 will be the percentage of voids in the material tested. The result is likely to be a little lower than the true value, on account of air trapped between the particles, but the error will be small. More nearly accurate results may be ob- tained by introducing the water into the bottom of the vessel through a pipe than by pouring it in at the top. PROPORTIONS AND QUANTITIES OF MATERIALS 49 Problem: Weight of measure filled with stone = 123 Ib. Weight of measure filled with stone and water = 155 Ib. Weight of measure filled with water = 84 Ib. Weight of measure empty = 13 Ib. Find percentage of voids in the stone. Solution: 155 - 123 = 32 Ib. = Weight of water required to fill voids. 84 - 13 = 71 Ib. = Weight of water required to fill measure, f Y X 100 = 45 = Percentage of voids in stone. To Find the Percentage of Vojds in Fine Aggregate. The foregoing method will not give accurate results for sand because of the large amount of air entrapped. Sometimes a somewhat similar method is used, the sand being poured into the water instead of water into the sand. A better and simpler method is to use the specific gravity of the sand. By the term specific gravity is meant the ratio of the weight of a given volume of any material to the weight of an equal volume of water. With such substances as sand, consisting of a large number of individual particles, the volume meant in this definition is the actual volume occupied by the particles them- selves, not including the spaces between the particles. The spe- cific gravity of all sands used for concrete work is practically constant and is 2.65. This being known, it becomes a simple matter to find the percentage of voids in a given sand. All we need to do is to find the weight of a given volume of the sand and compare this with the weight of the same volume of solid material. To use this method, we may fill any convenient measure with sand in the condition in which we wish to find the per- centage of voids. This sand is then dried at a temperature of not less than 212 F. and is weighed. The weight of water re- quired to fill the vessel (with no sand in it) is now found. The latter, multiplied by 2.65, gives the weight of an equal volume of solid sand (no voids). Subtract from this the actual weight of sand found, divide the result by the weight of the solid sand, and multiply the quotient by 100. The result is the percentage of air and moisture voids in the original sample of sand. If the percentage of air voids alone is required, the 50 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES percentage of the volume occupied by the moisture may be found and subtracted from the total voids. Problem: The sand required to fill a given vessel weighs 122 oz., and the weight after drying is 117 oz., while the weight of water required to fill the same vessel is 67 oz. Find the percentage of voids. Solution. The weight of the vessel full of solid sand (no voids) is 67 x 2.65 = 177.6 oz. Hence, the difference between weight of vessel full of solid sand and weight of given sand required to fill the vessel is 177.6 - 117 = 60.6 oz. and there is X 100 = 34.1 per cent of air and water voids in the sample of sand. The weight of water in the sand was 122- 117 = 5 oz. hence there is x 100 = 7.5 per cent of water voids, and 34.1 - 7.5 = 26.6 per cent of air voids in the sand. Proportions for Maximum Density. Since most of the sand particles go into the voids of the broken stone, it is easily seen that a mixture of sand and stone in the proper proportions will contain a smaller percentage of voids, and hence will have a greater density than either of the materials separately. The greatest density will be obtained when the least amount of sand is used which will fill all the spaces between the pieces of stone in the mixture. It must be remembered, however, that some of the sand particles will get between the pieces of stone, hold- ing them apart slightly and thus increasing the percentage of stone voids over what it would be with no sand present. Hence the percentage of sand used must be a little greater than the per- centage of voids in the stone. Likewise, the densest concrete will be produced by using an amount of mortar just sufficient to fill the spaces between the stones or a little more than the percentage of voids in the broken stone. The usual allowance of excess is about 10 per cent by volume. Fundamental Laws of Proportioning. Experiments have shown that PROPORTIONS AND QUANTITIES OF MATERIALS 51 (1) With the same percentage of cement, the concrete will be strongest and most impermeable when the fine and coarse aggregates are so proportioned to each other as to give the greatest density; (2) With the same aggregates in the same proportions to each other, that concrete will be strongest and most imper- meable which contains the greatest percentage of cement. It is generally true that the concrete which is strongest and most impermeable will also best resist wear or abrasion, so that the above laws are fundamental for all classes of concrete work. It follows from these laws that we should use only enough sand to fill all the voids in the broken stone with mortar (mak- ing due allowance for the unavoidable separation of the particles of stone by the sand grains which get between them) and that we should use enough cement with this mixture of sand and stone to give the strength, impermeability, or resistance to abrasion that we desire. If the amount of sand used is either more or less than enough to fill the stone voids with mortar, the concrete will be less dense, and more cement will be required to give the same strength. This is one important reason why it is desirable to screen the fine material out of broken stone and gravel, and then remix the materials in the proper proportion of fine to coarse, or at least to reduce the amount of sand to allow for the fine material which may be present in the stone or gravel. The amount of cement which can be saved by screening the materials will usually more than pay for the labor of screening, unless the materials run very uniform. Arbitrarily Specified Proportions. As the percentage of voids in different lots of screened broken stone or gravel will not differ very greatly, it is customary in much construction work to specify definite ratios of the materials, without special refer- ence to the particular materials to be used in any given instance. If the materials are of good quality and of average character, this works out very well, and it is doubtful if the method can be improved upon for small work. The voids in broken stone with the dust screened out aver- age about 45 per cent. In gravel screened over a J-inch screen the voids run about 40 per cent. With allowance for the 52 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES separation of the coarse aggregate by the particles of sand, for the lack of perfect uniformity in the distribution of the coarse and the fine particles, and, on the other hand, with considera- tion of the fact that the cement and water will slightly increase the volume of the mortar over that of the sand, it will be approximately correct to use an amount of sand equal to one- half the volume of the stone or gravel. This ratio will be nearly correct, irrespective of the amount of cement used, except in rich mixtures. For ratios of cement to sand greater than 1:2, the ratio of sand to stone should be somewhat decreased. If the stone or gravel has not been well screened, the amount of sand may be decreased a little from the ratio given above. If, when the concrete is well spaded or tamped, more mortar flushes to the top than is sufficient to cover all the stones, then less sand may be used; but, if it is difficult to get any mortar to flush to the top, a little more sand should be used. The following proportions are much used in concrete work. A rich mixture of 1 part of cement, 1J parts of sand, and 3 parts of broken stone or gravel, usually known as a 1 : 1| : 3 mix, is used for columns of reinforced concrete buildings, for thin walls that must be water-tight, and wherever a very strong, dense concrete is required. The mixture is richer than is re- quired for most work and is to be considered exceptional. In most cases the concrete would be improved by reducing the amount of sand to 1: 1J: 3 or 1: 1: 3. A good mixture of 1 part of cement, 2 parts of fine aggregate, and 4 parts of coarse aggregate, known as a 1:2:4 mix, is used for reinforced concrete work of all kinds, for water tanks, for thin walls, and for any purpose where a good concrete is required, with considerable strength and impermeability. This and the following mixture are the ones best adapted for gen- eral concrete work. A medium mixture of 1 part of cement, 2J parts of sand, and 5 parts of broken stone is used for plain concrete 1 work of all kinds, for foundations, for walls, for floors, and for all other purposes for which a good concrete is required, but not so much strength nor impermeability is necessary as to require a 1:2:4 mix. 1 Concrete not reinforced by steel rods. PROPORTIONS AND QUANTITIES OF MATERIALS 53 A lean mixture of 1 part of cement, 3 parts of fine aggregate, and 6 parts of coarse aggregate, known as a 1:3:6 mix, is used in heavy masses where the loads are wholly compressive and of moderate intensity, and where the principal requirements are weight and stability combined with a moderate degree of strength, as in heavy walls, foundations, and bridge piers. Still leaner mixtures are sometimes used, but they are not recommended for general use. Other Methods of Proportioning. Of the more scientific methods of proportioning, but two will be mentioned: (1) Use of trial mixtures to determine proportions for maximum density. (2) Use of a mechanical analysis curve. On large jobs the saving of cement will more than repay the trouble and expense of determining the best proportions by one of these methods. Trial Mixtures. A strong, rigid vessel, such as a piece of steel pipe, 8 or 10 inches in diameter and 2 feet high, is re- quired for the application of this method. Weigh out the ma- terials in the proportions thought desirable, using such amounts as will make enough concrete to fill the cylinder nearly to the top. Mix the materials on a non-absorbent surface, such as a piece of sheet iron, and tamp the mixture into the cylinder, noting carefully the height to which it is filled. It is well to weigh the cylinder before and after filling it, as a check on the amount of materials used and the amount lost in handling. Now throw out this batch, clean the cylinder, and mix up another batch, using the same weights of cement and water and the same total weight of sand and stone as before, but making the ratio of sand to stone slightly different. Note whether this batch fills the cylinder to a higher or a lower point than the first batch. If it fills it higher, it is a poorer mixture, and if lower, it is a better mixture than the first. Con- tinue making trial batches until the proportion is found which gives the least height in the cylinder. This will be the best which can be obtained with the. given materials, using the given amount of cement, since it will make the densest concrete. Mechanical Analysis. This method of proportioning is recom- 54 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES mended and used by some of the best concrete engineers in the country. It requires special apparatus and is not generally applicable for small work, but is mentioned here for the light it throws on the proper grading of the size of particles for the best concrete. 1 Extensive tests have shown that the best prac- tical mixture is one in which the particles are so graded from fine to coarse that a mechanical analysis curve, similar to the one shown in Fig. 12, but made to include the cement and .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 1.3 1,4 1.5 Diameter of Particles in Inches FIG. 17. Maximum Density Curve with Stone Screened Through H-Inch Screen. broken stone as well as the sand, follows most closely a certain curve known as a maximum density curve. This maximum density curve is a combination of an ellipse and a straight line and can easily be plotted for any given materials. Figure 17 shows such a curve. As proof of the efficiency of this method, it is stated that water-tight concrete has repeatedly been secured with as lean a mixture as 1:3:7, whereas the ordinary mixture for water- tight concrete is about 1:2:4. The saving in cement is evident. Omission of Coarse Aggregate. It is often convenient, and sometimes desirable, to omit the coarse aggregate from con- crete. Sidewalks and floors are often made in this way, as are also thin walls, building blocks, brick, tile, and other objects. 1 For a full discussion of the method, the reader is referred to Taylor and Thompson's "Concrete, Plain and Reinforced," John Wiley and Sons, New York. PROPORTIONS AND QUANTITIES OF MATERIALS 55 When this is done, a little larger proportion of sand may be used than in the usual cement, sand, and stone concrete, but the number of parts of sand should' be made less than the number of parts of stone commonly used in such a mixture. For example, a mixture of 1:4 might be used in a place where concrete of about 1 : 2| : 5 quality is required. This concrete will usually be a little weaker and less dense than stone con- crete, and a mixture of 1 : 3J would be better. It will usually be cheaper to make concrete of a given strength with stone or gravel than with sand alone. Tests made in the laboratories of the Kansas State Agricultural College indicate that it may be economical to pay as much as $2.50 per yard for stone, rather than to use enough extra cement to get the same strength with cement and sand only. Use of Bank-run Gravel. Sand and gravel are sometimes used mixed as they come from the bank. This is seldom eco- nomical, as there will almost always be an excess of sand. Even if these materials are mixed in just the right proportions, it should be remembered that the sand has not increased the volume of the gravel to any great extent. Hence the volume of mixed sand and gravel used with a given amount of cement should be no greater than would be required of the gravel only, if the sand were to be added separately. Thus, a mixture of 1 part of cement to 5 parts of a well-graded bank-run gravel is in reality approximately equivalent to 1 part of cement, 2J parts of sand, and 5 parts of gravel, the sand grains being pres- ent in the voids of the gravel. With the excess of sand usually present, not more than 4 or 4J parts of the gravel can be used with 1 part of cement to get as good concrete as a 1:2^: 5. If the sand and gravel run very uniform and are not in the desired proportions, a small amount may be screened with a i-inch screen and the proportions of sand and gravel determined. An additional amount of screened gravel may then be added to the bank-run gravel to give the desired proportions. Suppose, for example, that when 3 cubic feet of a bank-run gravel is screened, it is found to contain 2 cubic feet of gravel and 1.8 cubic feet of sand, and it is desired that the concrete shall con- tain twice as much gravel as sand. Then to each 3 cubic fee* 56 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES of bank-run gravel it will be necessary to add 1.6 cubic feet of screened gravel, making a total of 3.6 cubic feet of gravel to cor- respond with the 1.8 cubic feet of sand. It will be necessary to use 0.9 cubic feet of cement with this for a 1: 2: 4 mix, since half as much cement as sand is required. As one bag of cement may be considered equal to a cubic foot, we must use for each bag of cement 3 -J- 0.9 = 3.3 cubic feet of the bank-run gravel, and i n t-^r =1.8 cubic feet of screened gravel. Quantities of Materials Required. In estimating the quan- tities of materials required for a job, one must not make the mistake of assuming that one cubic yard of cement, sand, and stone will make a cubic yard of concrete. As has been pre- viously pointed out, most of the cement and sand go to fill up the spaces between the pieces of stone, so that with ordinary mixtures the volume of the concrete formed is not much greater than the volume of the stone used. Neglecting to consider this point will cause a very serious error in the estimate of the quantities of materials required, and will result in a considerable increase in the cost of the structure over the estimated amount. A simple method of estimating the quantities of materials required, sufficiently accurate for most small jobs where about half as much sand as stone is used, is to figure on a volume of stone equal to the volume of concrete required, and then de- termine the quantities of sand and cement from the relative parts used in the mixture. Thus for 8 cubic yards of 1 : 2 J : 5 2 1 concrete, 8 cubic yards of stone would be allowed, -^ or J as o much sand as stone, or 4 cubic yards, and y as much cement as stone, which is f cubic yards, or 43^ cubic feet. As one bag of cement is taken as 1 cubic foot, this is equivalent to about 43 bags of cement. When sand or bank-run gravel alone is used, it is obvious that by the foregoing method we should allow as many cubic yards of sand, or of gravel, as there are cubic yards of concrete required, and a proportionate amount of cement. Thus for 8 cubic yards of 1:4 concrete there would be allowed 8 cubic yards of bank- run gravel and 2 cubic yards, or 54 cubic feet, or bags, of cement. PROPORTIONS AND QUANTITIES OF MATERIALS 57 This method will usually give a small excess amount of ma- terials because the volume of concrete is a little greater than that of the coarse aggregate. In rich mixtures the inaccuracy will be greater than in ordinary mixtures, on account of the larger amount of cement used over that required to fill the voids. When there is a considerable excess of sand, as in a 1:3:4 mix, this method will give large errors and should not be employed. Fuller's Rule. A more nearly accurate method of estimat- ing quantities, which takes into account the variation in the richness of the mixture, and the increase in the volume of the concrete over that of the coarse aggregates, is the following, adapted from Fuller's Rule : 1 To find the number of bags of cement required for each cubic yard of concrete, divide 42 by the sum of the parts of the materials, the number of parts of cement being expressed as 1. The amounts of the other materials can be then found from the ratio of their volume to that of the cement. The rule can be expressed algebraically as follows: Let c = number of parts of cement ( =1), s = number of parts of sand, g = number of parts of gravel, C = number of bags of cement required for each cubic yard of concrete, S = number of cubic yards of sand required for each cubic yard of concrete, G = number of cubic yards of gravel required for each cubic yard of concrete. Then C = S = G = c + s CXs 27 CXg 27 1 For the original form of this rule, see Taylor and Thompson's "Con- crete, Plain and Reinforced," 2d edition, p. 16. 58 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES By this rule, the quantities of materials required for 8 cubic yards of 1 : 2 J : 5 concrete are found as follows. For each yard of concrete there will be required 42 4.94 bags of cement, 1 + 2} + 5 4.94 X 2| 27 4.94 X 5 27 = 0.46 cubic yards of sand, and = 0.92 cubic yards of stone. For 8 cubic yards of concrete the amount of each material will be 8 times as great, or 39.5 bags of cement, 3.68 cubic yards of sand and 7.36 cubic yards of stone. These values are slightly less than those found by the more roughly approximate method. In applying any rule, one should bear in mind that the volume of materials required will vary somewhat with the amount of fine material in the stone or other coarse aggregate. If the stone is screened to a uniform size, about 5 per cent more of all materials will be required than is given by Fuller's Rule, while if the stone contains a considerable amount of small particles, the quantities given by the rule may be decreased by about 5 per cent. Tables of Quantities. A still more exact method of deter- mining the quantities of materials required is by means of tables which have been carefully worked out and checked by actual experience. This method has the additional value of requiring less computation. The table on the opposite page applies to ordinary materials and covers the usual range of proportions. To determine by this table the quantities of materials for 8 cubic yards of 1 : 2 J : 5 concrete, with screened stone, we find that for each cubic yard of concrete there will be required 5 bags of cement, 0.46 cubic yards of sand, and 0.92 cubic yards of stone; then for 8 cubic yards of concrete the quantities required will be 8 X 5 = 40 bags of cement, 8 X 0.46 = 3.68 cubic yards of sand, and 8 X 0.92 = 7.36 cubic yards of stone. These values are in close agreement with those value found by Fuller's Rule. PROPORTIONS AND QUANTITIES OF MATERIALS 59 TABLE IV QUANTITIES OF MATERIALS FOR ONE CUBIC YARD OF RAMMED CONCRETE Proportions by volume Quantities required for broken stone with dust screened out Quantities required for crusher-run stone or screened gravel Packed Loose Loose Cemem Sand Stone Cemen! Sand Stone cement sand stone bags cu. yd. cu. yd. bags cu. yd. cu. yd. 1 33.24 33.24 1 1 19.5 0.72 . 19.5 0.72 1 1 H 11.9 0.44 0.66 11.5 0.42 0.64 1 1 2 10.5 0.39 0.78 10.1 0.37 0.75 1 1 2| 9.4 0.35 0.87 9.0 0.33 0.83 1 if 15.5 0.86 15.5 0.86 1 it 2 9.2 0.51 0.68 8.9 0.49 0.66 1 li 3 7.6 0.42 0.85 7.3 0.41 0.81 1 H 4 6.6 0.36 0.97 6.2 0.35 0.92 1 2 12.8 0.95 12.8 0.95 1 2 3 7.0 0.52 0.77 6.7 0.50 6.74 1 2 4 6.0 0.45 0.89 5.8 0.43 0.85 1 2 5 5.3 0.39 0.98 5.0 0.37 0.93 1 2 11.0 1.01 11.0 1.01 1 2* 4 5.6 0.51 0.82 5.3 0.49 0.79 1 2 5 5.0 0.46 0.92 4.7 0.44 0.87 1 2i 6 4.5 0.41 1.00 4.2 0.39 0.94 1 3 9.6 1.06 9.6 1.06 1 3 4 5.2 0.58 0.77 5.0 0.56 0-74 1 3 5 4.6 0.52 0.86 4.4 0.49 0.82 1 3 6 4.2 0.47 0.94 4.0 0.45 0.90 1 3 7 3.9 0.43 1.00 3.7 0.41 0.95 1 4 7.6 1.13 7.6 1.13 1 4 7 3.5 0.52 0.91 3.3 0.49 0.86 1 4 8 3.2 0.48 0.96 3.1 0.46 0.91 1 4 9 3.0 0.45 1.01 2.9 0.43 0.96 1 5 6.3 1.17 6.3 1.17 1 5 10 2.6 0.49 0.98 2.5 0.47 0.93 1 6 5.4 1.20 5.4 1.20 1 7 4.7 1.22 4.7 1.22 1 8 4.2 1.24 4.2 1.24 NOTE. Variations in the fineness of the sand and the compacting of the concrete may affect the quantities by 10 per cent in either direction. 1 Adapted from Taylor and Thompson's forced," 2d edition p. 232. Concrete, Plain and Rein- 60 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES PROBLEMS 1. A certain measure weighs 8 pounds when empty. When filled level full of water, it weighs 56 pounds; when filled with gravel, 83 pounds. When water is poured into the gravel to fill the voids, the total weight is 102 pounds. Find the percentage of voids in the gravel. Ans. 39.6 %. 2. Find the percentage of air and water voids in a sand, if a certain vessel holds 38 pounds of the sand, and the same vessel holds 20.9 pounds of water. The sand in the vessel loses 1.2 pounds in weight when dried at 212 F. Ans. 33.6 % total voids. 5.7 % water voids. 27.9 % air voids. 3. In a test of certain materials by the method of trial mixtures, different batches filled the cylinder to within distances from the top as given in the table. What are the best proportions for the materials when one is using one part cement to a total of seven parts of sand and gravel? Mix Distance from top of cylinder to concrete l:2f:4i 1" If* 2" l:2J:4f 1: 2|: 4| 2" 1:2:5 ir Ans. l:2i:4f 4. If a certain bank-run gravel is screened over a |-inch screen, and each cubic foot is found to contain 0.7 cu. ft. of gravel and 0.6 cu. ft. of sand, how much of this gravel and how much screened gravel must be used with each sack of cement to make a 1 : 2 : 4 concrete? Ans. Use 0.5 cu. ft. of screened gravel for each cubic foot of bank-run gravel. For each sack of cement use 3 cu. ft. of bank-run gravel and If cu. ft. of screened gravel. 6. If a stone from a crusher is found to contain one cubic foot of screened stone and 0.12 cu. ft. of screenings for each cubic foot, how much sand and how much crusher-run stone must be used with each bag of cement to make a 1 : 2 : 5 concrete? Ans. 1.9 cu. ft. of sand and 5 cu. ft. of crusher-run stone. 6. Find the quantities of materials required for 12 cu. yd. of 1:2:5 con- crete by each of the three methods given in this chapter. Ans. By approximate method, 65 bags cement, 4.8 cu. yd. sand, and 12 cu. yd. stone. By Fuller's Rule, 63 bags cement, 4.7 cu. yd. sand, and 11.7 PROPORTIONS AND QUANTITIES OF MATERIALS 61 cu. yd. stone. By table, 63.6 bags cement, 4.7 cu. yd. sand, and 11.8 cu. yd. stone. , In the following problems, use the table of quantities. 7. Find the quantities of materials required for a sidewalk 160 feet long and 5 feet wide, if the base is made 4 inches thick of 1:3:4 gravel concrete, and the top is f inch thick of 1 : 2 mortar. Ans. For top coat, 23.7 bags cement, 1.8 cu. yd. sand. For base, 49.5 bags cement, 5.5 cu. yd. sand, and 7.3 cu. yd. gravel. Total, 73.2 bags cement, 7.3 cu. yd. sand, and 7.3 cu. yd. gravel. 8. Find the cost of materials for the walk of Problem 7 if cement costs 45 cents per bag; sand, $0.75 per load of one and one-half yards; and screened gravel, $1 per yard, all delivered on the site of the work. Ans. $43.89 or, allowing for uncertainty as to exact amounts of materials, say about $45. 9. Find the cost of the concrete materials for a circular water tank 8 feet inside diameter by 2 feet high, with sides 5 inches thick and floor 6 inches thick. All concrete is mixed 1:2:4, using screened gravel. Use the same prices as in Problem 8. 10. Find the cost of the materials for a concrete wall 4 feet high, 20 feet long, and 12 inches thick, using the materials of Problem 4, so proportioned as to get an actual 1:3:6 concrete. Use the prices of Proble.ni 8. 11. How much could be saved in the cost of a cubic yard of gravel concrete by using a 1:2:5 mixture instead of a 1: 2: 4 mixture? Would the cost be more or less than for a 1:2^:5 mixture, and how much? Use the prices of Problem 8. CHAPTER V CONSTRUCTION OF FORMS CONCRETE is a plastic material and will take the shape of any mold or container in which it is placed. Hence, all that is necessary in order to make concrete objects of any desired shape or form is to construct molds of the proper shape. For different objects, the forms vary in character from simple two by fours laid on edge, for sidewalks, to intricate molds in sand, clay, or plaster of Paris, for ornamental objects, or to the maze of walls, floors, columns, and braces in reinforced concrete buildings. According to the character of the objects being made or the subsequent treatment of the surface, the character of the workmanship required varies from the simple nailing of rough boards on studs to work approaching joinery in its requirement of accuracy and skill. Materials Used for Forms. Any material which will hold the concrete in place until it has set may be used for a form. Materials frequently used are earth, sand, cast iron, sheet steel, and wood. For foundation and other walls below ground, where the earth will stand, the concrete is often placed directly against the earth. If this is done, the excavation should be made ac- curately in the proper place and the walls should be cut true to surface. If the earth walls have caved out to any extent, it may be more economical to use wooden forms to save concrete. Moist sand is used for molds for ornamental work of concrete in much the same manner in which it is used for molten metal in the foundry. A wooden or other model is used as a pattern and a liquid mortar or grout is poured into the impression made in the sand. Cast-iron molds are often used for objects of small size which are to be made repeatedly, such as building blocks and concrete tile. Sheet steel is often used for forms on either small or large work, where they can be used repeatedly, 62 CONSTRUCTION OF FORMS 63 as in tunnels, culverts, fence posts, and silos. Wood is, however, the most commonly used forming material. The ease with which it can be worked into the desired form, and its cheap- ness, make it highly useful for this purpose. Use of Wood for Forms. The woods most used for forms are yellow pine, fir, and spruce. A wood should be chosen which will not warp or swell excessively when wet. Green lumber is better than well-seasoned material for this reason. For exposed faces of concrete walls, the lumber should be free from knotholes, irregularities, and slivers, and should be sur- faced on one side. The edges should be surfaced, or tongued and grooved. Six-inch tongued and grooved flooring or eight- inch shiplap makes excellent form lumber. Boards wider than 8 inches are difficult to draw together, if they are crooked, and are not recommended for general use. Either one-inch or two-inch lumber may be used for face boards, and the choice will depend on the character of the work. Where a considerable amount of lumber is required-, and it can be used only once or twice, it will be cheaper to use one-inch material, while if the forms can be used repeatedly, two-inch stuff will be better. When the face of the work is to be cov- ered, or when its appearance is unimportant, any rough lumber may be used if the cracks are battened to prevent leakage of the mortar. The face boards of forms should be nailed to the studs or joists as lightly as is consistent with keeping them in place until the concrete is poured. This is especially important when the form is to be torn apart after it has been used once. Six- penny nails should be used for one-inch forms. One nail to each intermediate stud and two nails to each end stud are usually sufficient. Heavily nailed forms cannot be torn apart without considerable injury to the lumber. It is important that no cracks or knotHoles be left in' the forms large enough to permit the thin grout to flow out. This grout contains the cement, and a small crack . will drain out enough of it to weaken the concrete near the crack and make it open and porous. It will help to preserve the forms and prevent particles of 64 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES concrete from sticking to them if they are covered with a coat of crude oil, linseed oil, or soap solution. When this is not done, they should be thoroughly wet down immediately before the concrete is poured. The forms should be thoroughly cleaned of all particles of concrete adhering to them before they are used a second time. If this is not done, the surface of the wall will be made rough and irregular. The .cleaning can easily be done with 'a wire brush or a stiff broom, assisted by a little scraping. Tying and Bracing Forms. The tying and bracing of forms is highly important. Those who have not investigated the matter cannot realize the tremendous pressure exerted by the concrete, tending to separate and displace the forms. The pres- sure, in pounds, exerted on each square foot of the forms by concrete in a semifluid state can be found approximately by multiplying the height of the concrete above the point in ques- tion by 150, since 150 pounds is about the weight of a cubic foot of concrete. For example, in a wall five feet high and of any thickness, the pressure tending to separate the forms at the bottom, while the concrete is semifluid, is about 5 X 150 = 750 pounds per square foot. The forms must therefore be tied and braced sufficiently to resist this pressure. Most carpenters and others who have not had actual experience in forming for concrete .fail to realize this, and many bulged walls are the result. Where forms are used on both sides of the wall, the ties should extend through the wall from one form to the other, to take up this pressure. The bracing to the ground then re- quired is only sufficient to hold the wall itself upright, and to support the shock and jar of placing the concrete. It is en- tirely unsatisfactory to try to resist the pressure of the concrete by means of stakes and diagonal braces running to the ground. If this is attempted for walls more than a foot or two in height, failure is almost sure to result. Forms for Straight Walls. The form for a straight wall is the most common type required and is one of the most easily constructed. It is usually made of face boards running hori- zontally, nailed to upright studding or posts. If a long wall is to be constructed, use 2"X 6" plank for the face boards, nailing CONSTRUCTION OF FORMS 65 Space these to 4"X 4" or 4"X 6" uprights spaced about three feet on centers, with twelve-penny nails. The forms may be built con- tinuous, or may be made as panels, each about twelve feet long and as wide as the wall is high. These panels can be made up flat on the ground and then raised and supported in place. The form for one side of the wall should be lined up and plumbed, and then thoroughly braced by diagonals running from the studs to stakes driven in the ground or to some adja- cent structure. The form for the opposite side of the wall should then be placed and secured to the one already set by ties and braces. Sticks about 1"X 2", and of a length exactly equal to the thickness of wall desired, should be cut for use as spacers between the forms. The ties may be made of No. 10 smooth wire passed around the studs on opposite sides of the walls in a loop and then twisted up by means of a rod or stick. To prevent drawing the forms too close together, a spacer should be placed near each tie when it is being twisted up. Most of these spacers can be removed immediately after the adjacent wire has been tightened, but a sufficient number should be left in place to hold the forms apart until the concrete is placed. These should be knocked out and removed just before they are covered by the con- crete. The method of tying and bracing is clearly shown in Fig. 18. Bolts are sometimes used instead of wires for the ties, but they are more expensive and are not so adaptable to different FIG. 18. Tying and Bracing of Straight Wall Forms. 66 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES situations. Cleats nailed to the studs may be used as ties at the top of the wall if desired. The number of ties required does not depend on the thick- ness of the walls, since the pressure of the concrete tending to force the forms apart is practically the same for a wall 6 inches thick as for one 3 feet or more thick. Usually ties should be placed about 3 feet or less apart on each stud vertically, but a little closer together near the bottom than near the top of the wall. Where there will be a greater depth of unset concrete at any one time than four to six feet, the spacing of the tie wires near the bottom must be less than the value just given. It should be kept in mind that, when the forms have once bulged, it is practically impossible to straighten them, so that ample tying or bracing must be provided before work is begun if the job is to be satisfactory. After the concrete becomes sufficiently hard, the tie wires may be cut, and the panels moved along the wall to be used again for forms. The wires may then be clipped close to the face of the concrete. More commonly the forms for small jobs can be used only once without being torn apart, and for such work one-inch face lumber should be used. The uprights may be 2"X 4" sticks and should be spaced about 18 inches apart on centers along the form. It will often be desirable to build the forms on the ground in sections as described for the two-inch panels, afterwards raising and bracing them. This is especially con- venient in constructing thin walls. The bracing and tying may be done as described above for the two-inch panels. Forms for Circular Walls. Concrete is often required in the form of cylindrical walls, as in water tanks, silos, and cis- terns. In such cases, the forms, if of wood, must be made with the face lumber running parallel to the axis of the cylin- der, this being held in place by circular ribs, cross braced to make them keep their shape. The face lumber, or lagging, as it is called, may usually be six inches wide, but, for small circles, four inches is better. If wooden forms are used on both sides of the wall, ties may extend through the wall, as in the case of straight forms. When the earth answers for the outer CONSTRUCTION OF FORMS 67 form, the inner one may be held against the pressure of the concrete by cross bracing to the opposite wall. Pencil Outer Form Inner Form FIG. 19. Laying Out Ribs for Circular Forms. A simple method of laying out the circular ribs is as follows: Fasten a string to a nail in the top of a stake driven firmly in the ground. Measure off a length of string equal to one- half the diameter of the circle desired and tie a knot through which a nail is forced. Keeping the string tight, draw a circle UJ Forms.in Place Vertical Section FIG. 20. Construction of Circular Forms. on the ground with the nail. Lay the boards around this circle as shown in Fig. 19, and nail them together securely. Then mark out the circle on them, and saw along the line. Figure 20 shows the form completed, with the concrete in it. 68 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Forms for Overhead Floors and Roofs. Overhead floors and roofs of reinforced concrete usually consist of floor panels or slabs of comparatively small thickness, resting on beams and girders, the latter being supported by the columns or walls. The beams project below the floor panels, the top of the floor being carried level over the beams. FIG. 21. Reinforced Concrete Floor Forms Under Construction. In the construction of forms for such a floor, the forms for the beams are usually made first. These forms are supported near the ends and at one or more intermediate points by up- right timbers resting on planks or other timbers laid flat on the floor below. The forms for the floor panels are made of 1"X 6" boards on two-inch joists. The ends of these joists are supported on the sides of the beam forms. Usually it will also be necessary to put one or more intermediate rows of supports under the floor panel joists. In putting the forms together, the problem of taking them down easily with as little damage as possible to the lumber CONSTRUCTION OF FORMS 69 and to the concrete should be kept constantly in mind. The boards should be nailed very lightly to the joists, and all pieces of face lumber should be cut just a little short, so that their ends will not project into the concrete. All supports should be cut an inch or two short, and wedges used to bring them up to the proper place. This makes it easy to level up the floor, and to remove the supports when the forms are being taken down. Figure 21 shows the forms for a reinforced concrete floor under construction. FIG. 22. Column Forms. Column Forms. Column forms may often be used re- peatedly if properly designed and handled. The sides may be built in separate panels, these being lightly tacked together and the whole 'being .securely clamped either by means of wooden frames nailed around the columns as shown in Fig. 22, or by means of bolts or some kind of patented clamp, of which there are several on the market. An opening should be left at the bottom of each column, through which chips, 70 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES sawdust, and dirt may be cleaned out. This can be closed up just before concreting is begun and after the base of the form is carefully cleaned out. The same precaution should be ob- served of cutting the forms a little short as in the case of the floor forms, to make their removal easy. Time of Removal of Forms. The setting and hardening of concrete is a gradual process, and its rate is dependent on several factors. (See " Setting of Concrete," in Chapter VI.) For this reason the time which must elapse before the forms may be removed will differ materially in different cases for the same kind of structure. Warm weather, rich mixtures, and a dry consistency all tend to make concrete set rapidly and hence make it possible to remove forms earlier than when these conditions are not present. Concrete not subjected to any load but its own weight, and in such a form that no tendency to bend is produced, will be set sufficiently to permit the forms to be removed when it is so hard that it cannot be indented by pressure with the thumb. If the concrete is loaded, or if there is a bending tendency, as in beams and floors, the forms must be left much longer. In reinforced concrete work the forms should usually remain in place for from one to four weeks. In cool weather they should be allowed to remain at least twice as long as in warm weather. It is often desirable to leave certain braces and supports in place under beams and slabs when the rest of the forms are removed. An experienced judgment is required to tell the earliest date at which many forms may be removed, and the novice should be careful to allow plenty of time. CHAPTER VI MIXING AND HANDLING CONCRETE Requirements of Good Mixing. A concrete is well mixed when (1) every particle of sand and stone is coated with cement paste, (2) the sand and stone are evenly distributed through the mass, and (3) the whole mass is of a uniform consistency. If a batch of concrete has light streaks through it, or has some stones uncoated, or if parts of it have a deficiency or excess of mortar or of water, then the mixing has not been sufficiently thorough, and an inferior grade of concrete is produced. Consistency. It is impossible to give definite rules regard- ing the amount of water to be used in mixing concrete, as this depends to a considerable extent on the brand of cement, the characteristics of the sand and stone, the proportions used, and the consistency desired in the concrete. The amount of water required should be determined by trial for the first batch mixed on any given job, and the same amount should be used for subsequent batches. All water used should be reasonably clean, and free from acid, alkaline or organic impurities. The proper consistency for concrete differs according to the purpose for which it is to be used. Experiment has -shown that the strongest concrete is obtained by the addition of just enough water so that, by hard tamping in the forms, a little water may be made to flush to the surface. The difference in strength is slight, however, and if the concrete does not receive sufficient tamping, such a concrete will be considerably weaker than a wetter mixture, for which less tamping is required. The three different consistencies most used are: (1) Wet mixture, which is thin and mushy. It will run off 71 72 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the shovel unless handled quickly, will spread out level in a wheelbarrow after being wheeled a very few feet, and will not support the weight of a man when placed in the forms. This mixture is suitable for thin walls and for reinforced work where there is an intricate network of steel around which the concrete must run. But little compacting is required, a little " joggling" with a spade being sufficient. Care must be taken not to use enough water to cause separation of the materials while the concrete is being handled. Cement and sand mixed to this consistency is much used under the name of grout. (2) Medium mixture, which is wet enough to be compacted by spading or light ramming, but not so wet that much free water will be brought to the surface by such compacting. It will quake freely like jelly and will support a man only after he has sunk into the surface some distance. This is the most suitable consistency for general concrete work. But little labor is required to work the concrete into place and compact it, yet it is not so wet that there is danger of the separation of the materials. (3) Dry mixture, which is of the consistency of damp earth. A handful of it squeezed together will retain its shape after the pressure is removed, but it is wet enough so that a thin film of water can be brought to the surface by hard ramming. In making dry mixtures, great care must be taken that enough water is used, as a little deficiency will produce a permanently weak concrete. Care must also be taken to see that this mix- ture is thoroughly rammed. Tests show that concrete made of dry mixtures may be ten to twenty times as strong when heavily rammed as when the concrete is lightly pushed into the molds. Figure 23 shows the results of such a test made at the Kansas State Agricultural College. This mixture is used when it is desired to remove the molds, or forms, almost immediately, as in building blocks, and also when the concrete must carry a considerable load in a few days. Concrete made of it and well rammed will be stronger than the wetter mixtures for several weeks after being mixed, but the latter will gradually catch up, and, unless the dry mix- tures are very well rammed, will eventually become stronger. MIXING AND HANDLING CONCRETE 73 Dry mixtures also give a more porous concrete and hence are unsuitable for structures that must be water-proof. On account of the difficulties in the use of dry mixtures, it is recommended that they be avoided wherever possible. FIG. 23. Effect of Tamping on the Strength of Concrete. These Cylinders were made of the Same Batch of Concrete. One was Thoroughly Tamped : the Other was not. Note the Difference in the Breaking Loads. Methods of Mixing. Two principal methods of mixing are in use, hand mixing and machine mixing. By either of these methods excellent concrete can be obtained, and the choice will usually be determined by the size of the job and the readi- ness with which a mixer can be obtained. For very small jobs hand mixing is often preferable on account of the labor re- quired to move the machine to the job, though in recent years small and readily portable mixers have been developed to such an extent that it is profitable to use them on much smaller jobs than formerly. On large jobs, machine mixers should always be used on account of the saving of labor. Tools Required for Hand Mixing. Very few tools are really required for mixing concrete by hand. A shovel, a bucket, and a place to do the mixing where the water will not wash away 74 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the cement will answer. It will be found convenient, however, to have the following list of tools, if much work is to be done: Shovels, No. 3 square pointed, one for each man, Wheelbarrows, preferably at least two, with sheet-steel bodies, Water barrel, Water buckets, two or more, Rammer. A 2" X 4" stick may be used, or better, a 4" X 4" stick 30 inches long with handles, Spade, to force stones back from the forms, Sand screen (see Fig. 13), Mixing platform, Measuring boxes. A suitable platform for use by two men and large enough for a two-bag batch of concrete (i.e., a batch requiring two sacks of cement) is shown in Fig. 24. It is made from twenty- FIG. 24. Tools Used in Hand Mixing. two pieces of 1" X 6" tongued and grooved flooring, each 10 feet long, held together by five 2" X 4" cleats about 9 feet 6 inches long. The upper side of the flooring should be surfaced in order to make shoveling easy. The boards should be driven tightly together to prevent leakage of the water and cement. Any small knotholes may be covered by cleats on the bottom side. Around the upper side of the board should be nailed a 2" X 2" strip to keep the grout from running off the edges. For a MIXING AND HANDLING CONCRETE 75 crew of four men, a board 10 feet by 12 feet, which is large enough for a four-bag batch, may be desirable. Measuring the Materials. A measuring box should be made to hold the quantity of sand to be used for each sack of cement, and if the amount of stone used is not just twice that of sand, another box should be made to hold the quantity of stone used for each sack of cement. One bag of cement may be consid- ered as one cubic foot; hence, for a 1:2J:5 concrete the box should be made to hold 2| cubic feet. If made 21" long, 16" wide, and 10" deep inside, it will hold just this amount. If preferred, the boxes may be made large enough to hold mate- rials for a two-bag batch. The measuring box should be made bottomless. The two longer sides may be allowed to project about six inches past the ends and may be cut down to form handles, as shown in the illustration. When the materials must be wheeled in barrows it is a com- mon practice to measure into the barrows the proper amounts of sand and stone, and thereafter to fill them to the same height. If care is taken in filling the barrows, and if a check is made by using the boxes occasionally, this method will give good re- sults with a saving in labor, but the proportions will usually be less exact than when the boxes are used. Mixing the Materials by Hand. The mixing board should be set, if possible, so that the concrete, after mixing, can be shoveled directly into place without hauling, and the piles of materials should be so placed that they can be shoveled directly on to the board without the necessity of loading into barrows. Often, however, it will be impossible to arrange the work so conve- niently. In such cases, it will probably be desirable to place the board so that the materials can be shoveled directly on to it from the piles, and so that there will be but a short distance to wheel the concrete after it is mixed. The mixing platform should be blocked up so that it is level and will not sag under the weight of men and materials. Board runways should be built for the wheelbarrow from the mixing platform to the place of depositing the concrete, and to the piles of sand and stone, as these will very much lighten the work of 76 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the wheelers. They are best made of two-inch plank about 12 inches wide, but one-inch lumber can be used where the runs are on even ground. Runs elevated above the ground should be at least two feet wide. The mixing platform and runs having been placed in posi- tion, and the measuring box being ready, the method of mixing is as follows: The measuring box is placed on the platform near one of the ten-foot sides and is twice filled level full of sand. The sand is then leveled down to a nearly uniform depth of three or four inches. Two bags of cement are spread as evenly as possible over the sand. Two men stationed at opposite sides of the pile then begin to "turn it over" with shovels, beginning at the edge of the pile farthest from the side of the platform and gradually working through the pile. With a little care in empty- ing the shovels, the mixing of the materials may be made much more efficient than if the materials are carelessly shoveled over into another pile. The shovel should be turned completely over in emptying it and it should be drawn toward the shoveler with a sweeping stroke so as to distribute the material over consid- erable space, instead of depositing it all in a heap. Further, the mixture should not all be shoveled together into a pile, but should be distributed in a layer of uniform thickness, similar to the one it occupied before commencing to turn it over. Care should be taken not to leave a strip of materials in the middle of the pile, unturned. After the mass has all been turned over, the shovelers turn it back again so that it occupies the same position on the board as at first. Usually a third turning will be required be- fore it is ready for the addition of stone and water. If a third man is available for mixing, he may use a garden rake or a hoe on the pile upon which the other mixers are shoveling. He should give special attention to those portions which are least thoroughly mixed. After the mass has been turned the third time, it should be of uniform color, with the cement uniformly distributed through the sand. If it is not, mixing should be continued until this result is obtained. The mass should now be spread out to a MIXING AND HANDLING CONCRETE 77 uniform depth ready for the stone. The measuring box may be placed directly on top of the mixture, filled level full, and emptied. After the proper amount of stone is added, it should be leveled down to a uniform depth over the cement and sand mixture, and the water added. The water should be measured in buckets, the required amount being determined for the first batch by trial and the same amount being used for subsequent batches. Only about three-fourths of the water that will be required for the batch should be added at first, as otherwise some of it is likely to flow off the pile, washing away the cement with it. The mass is now turned again, as directed for the cement and sand, water being added to the dry spots until the required amount of water has been used, or the desired consistency has been obtained. Three turnings of the mass after the stone has been added will usually be sufficient, but if this does not make the mass of the same consistency throughout, or if the mortar is not uniformly distributed through the stone, the mixing should be continued until these results have been accomplished. The concrete is now ready to be shoveled into place, or into wheel- barrows or carts if it must be hauled. Some concrete workers vary the method by shoveling the cement and sand mixture on top of the stone, instead of plac- ing the stone on top of the cement and sand mixture, and some prefer to wet the sand and cement mixture before adding the stone. It is believed, however, that the method given above will be most generally satisfactory. The details of the process may be varied so long as the ultimate object is ob- tained, the thorough mixing of the materials into a homo- geneous mass. Machine Mixing. Machines suitable for use on small con- crete jobs may be divided into two general classes: (1) batch mixers, and (2) continuous mixers. In the former, the proper quantities of the materials for a batch of concrete are intro- duced into the machine, mixed, and discharged, before addi- tional materials are added. In the latter the materials are continuously fed into one end while the concrete is discharged from the other end. 78 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Continuous Mixers. A continuous mixer usually consists of a trough or cylinder, often inclined, in which the concrete is mixed by means of revolving blades or paddles, which continu- ously work the materials toward the discharge end of the FIG. 25. A Continuous Mixer. trough, and at the same" time mix them together. A continuous mixer is usually driven by a gasoline engine which is mounted with the mixer on a four-wheeled truck, so that it may readily be moved from place to place. The materials may be measured into the upper end of the trough automatically, by means of a system of cupped rolls which rotate continuously beneath hop- pers or by other devices, or they may be shoveled into the trough, the proportions depending on the number of shovelfuls of each material supplied. Both methods of measurement are open to the objection that they 'are likely to be inaccurate, the first on account of clogging of the pockets of the revolving rolls, or uneven feeding due to the varying moisture in the sands, and the second on account of unevenness in the size of shovelfuls taken. In either case it is necessary to check the proportions MIXIXG AXD HAXDLIXG COXCRETE 79 at frequent intervals by actual measurement, if satisfactory results are to be obtained. The machines are usually made so that the materials will be mixed dry in the upper end of the trough before they have reached the point where the water is sprayed in. The lower end of the trough is often provided with a hood in which the mixed concrete may be caught while the barrows or carts in which it is hauled away from the machine are changed. FIG. 26. A Batch Mixer of the Revolving Drum Type. There are on the market a number of these machines which will give good results if careful attention is paid to their operation. They are usually small and easily moved from place to place, and are of low first cost. They are used mostly on small jobs, such as sidewalks, curbs and gutters, cellar floors, and house founda- tions. Certain machines of this class are well adapted to mortar mixing and some are not provided with means for feeding stone, being used only for work in which no stone is required. Batch Mixers. A batch mixer consists usually of a revolv- ing cylinder or drum, inside of which blades or vanes are pro- vided to cut and mix the material as it is rolled over by the 80 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES turning of the drum. The drum may be driven either by a steam or gasoline engine, or by an electric motor. The large machines are usually driven by steam engines, steam from the boilers being used to heat the mixing water in cold weather. Most of the small machines are driven by gasoline engines. The engine and mixer may be mounted on a four-wheeled truck, or may be mounted on skids if it is not required that the machine be readily portable. These machines are often provided with a hopper,, or skip, of size sufficient to hold one batch of materials, with a hoisting rig, so that this may be ele- vated from the ground for charging the mixer. The skip may then be filled again while the previous batch is being mixed. Batch mixers are often provided with water drums with auto- matic means for measuring the same amount' of water for each batch. In the larger machines the materials are usually intro- duced at one end of the drum, and after these are mixed the concrete is drawn out of the other end, either partly or com- pletely, before the next batch is introduced. Several small machines have been recently brought out which employ a drum closed at one end, the materials being introduced into the open end on one side of the machine. After these are mixed, the drum is tipped over so as to discharge on the op- posite side. This results in simplifying the machine and cut- ting down the first cost to a point where it can compete with continuous mixers for small jobs, while maintaining the advan- tages of batch mixing. The author believes that this type of machine will grow in favor and will to a considerable extent displace the continuous mixer for small work. The advantages of the batch mixers are that the proportions can be accurately measured and the materials can be mixed as long and as thoroughly as is desired. The measurement of materials is usually by wheelbarrows, filled to the proper height, as 'was discussed in the treatment of hand mixing. A very common size of batch is one requiring one bag of cement, commonly known as a one-bag batch. This is a con- venient size, as it does not require the division of the packages of cement, the bags of the latter being emptied directly into the mixer or into the loading skip. Larger machines use two-bag MIXING AND HANDLING CONCRETE 81 batches. A convenient method of measurement for smaller ma- chines is by the use of 14-quart buckets, the cement being first emptied into a box, from which it can be conveniently dipped by the bucket. Care should be taken to see that the buckets are filled to the same height with the cement as with the sand and stone, and that the buckets of cement are lightly packed or shaken, while the buckets of sand and stone are loosely filled. FIG. 27. A Small Batch Mixer of the Revolving Tub Type. Transporting Concrete. On small jobs the concrete can sometimes be shoveled directly into place, from the mixing board; or, if a machine is used for mixing, it can sometimes be so located that the concrete can be dumped, or spouted directly into place. More often it will be necessary for the concrete to be hauled from the mixing platform or the mixer to the forms. Steel wheelbarrows or steel carts are usually used for this purpose. For the small contractor or concrete user, the barrows are preferable on account of their being more generally serviceable. For thin, high walls and columns, gal- vanized buckets will be found useful. 82 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES On jobs of considerable magnitude, towers are sometimes built in some central place, in which the concrete is elevated to a considerable height above the work. It is there dumped into a hopper, from which it is spouted to the various parts of the work. The advantage of this method is that the work of trans- porting the concrete is all done at one place, the tower, so that a hoisting engine or a team can be used, instead of the expensive hand labor of wheeling. On jobs below ground level, such as cellar floors, the method of spouting the concrete into place can be used, but without the necessity of first hoisting it. Care should be taken to see that the materials of the con- crete do not separate while being wheeled or spouted into place. Separation is most likely to occur if the materials are too wet or too dry. When mixed to a medium consistency, concrete can be wheeled or spouted long distances with little trouble from this cause. The spouts should preferably be only steep enough to permit the concrete to run freely. If it is necessary that they be steep, they should be entirely en- closed to avoid waste of materials and to assist in preventing separation of the stone from the mortar. In case separation should occur, the materials should be thoroughly remixed be- fore the concrete is allowed to harden. A limited amount of remixing can be done in the forms by thorough spading. Depositing Concrete. All concrete should be placed in the forms promptly after being mixed. Any handling or working after it begins to stiffen will result in a loss of strength. The practice of retempering concrete, i.e., remixing it with water after it has begun to set, is to be discouraged. All concrete must be carefully and thoroughly worked into place and compacted to get the best results. If a rather dry mixture is used, it should be placed in layers about six inches thick and heavily rammed until all air has been expelled and some water has flushed to the surface. With a somewhat wet- ter mixture, lighter ramming will be sufficient. A satisfactory rammer for this purpose can be made by nailing two 1"X 3" pieces 4 feet long for handles, on a 4"X 4" stick about 30 inches long, or a 2" X 4" stick about 6 feet long will do. A spade or a sharp-edged stick should be used to force the stones away from MIXING AND HANDLING CONCRETE 83 the forms and allow mortar to flow in, as otherwise air pockets will be left next to the forms, and when the latter are removed the surface of the concrete will present a " popcorn effect." If the mixture is made wet, or if the mass is fairly heavy, a spade will be satisfactory for compacting it. The spade should be worked through the concrete to expel all air and to work each successive batch down into that previously placed. Concrete can be deposited in water with good results if proper precautions are taken. It will harden and become as strong in water as in air. It is only necessary to get it into place in some manner such that the cement will not be washed out by the water. Perhaps as satisfactory a method as any is to pass it, in a continuous flow, through a sheet- metal tube about eight inches in diameter. The concrete within the tube should be kept con- stantly at a higher level than that of the water outside. This can easily be accomplished by allowing the lower end of the tube to rest on the mass which has been deposited, and raising it only as is necessary to allow sufficient concrete to flow out. Forms or cofferdams will usually be required to keep the concrete in place and pre- vent the cement from being washed away. These need not be water-tight, however. Curing of Concrete. If freshly poured con- crete is exposed to the direct action of the sun and wind, it may dry out so rapidly as to weaken it. In hot, dry weather it should be shaded from the sun for the first week or ten days by allow- ing the forms to remain in place, or by covering it with canvas, burlap, or sand. If this covering ^ IG 2 s. Tube is kept wet, the conditions for curing will be ideal, for Depositing It is especially important that dry mixtures be C ncrete under kept sprinkled. Green concrete should be protected from excessive loads or blows which might result in injury. Light loads which do not cause vi- bration may be applied, if the resulting stress is pure compression, 84 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES when the concrete is but a few days old. When the loads pro- duce bending stresses, a much longer period should be allowed. Bonding Old and New Concrete. Fresh concrete will not bond readily to concrete which has set. In warm weather even an hour is sufficient time for the concrete to stiffen up suffi- ciently so that fresh concrete will not bond to it readily, espe- cially if a scum has formed over the surface. This results in a seam being formed at this section, which is particularly objec- tionable in structures which must be water-tight, because liquid will seep through at the seam. To avoid this trouble, the surface of the concrete should be left rough, when work is stopped; and when it is begun again, the scum should be care- fully cleaned off. It will often be desirable, especially if a water- tight bond is desired, to tear up the old surface with a pick so as to expose the aggregate. A wash of neat cement and water, or a rich mortar of about 1:1, mixed to a creamy consistency, should be applied to the surface of the old concrete just before concreting is resumed. If the old concrete is dried out before the new is to be placed, it should be thoroughly soaked up with water before the cement wash is applied. Steel rods embedded for half their depth in the old concrete assist in bonding the old and the new work, and in heavy walls large stones are some- times embedded for half their depth before work is stopped, the projecting part acting as a dowel to tie the new and old work together when work is resumed. Contraction and Expansion Joints. Concrete, like all other materials, contracts and expands with changes of temperature. The amount of this change is practically the same as for steel with equal temperature differences. This is very fortunate, as it permits these two materials to be used together in reinforced concrete work without causing high stresses. The coefficient of expansion of concrete is about 0.000,006, which means that for each degree change of temperature, a piece of concrete will change in length 0.000,006 inches for each inch it is long. Thus the change in length of a concrete sidewalk 200 feet long,, when the temperature changes from 100 degrees in the summer to in the winter, is 100 X 200 X 12 X 0.000,006 = 1.44 inches, MIXING AND HANDLING CONCRETE . 85 or nearly an inch and a half. As friction prevents the sidewalk from slipping over the ground, the walk must crack, unless cracks have already been provided. For this reason it is de- sirable to provide contraction joints in such cases, so that the cracks may be straight and uniform instead of being crooked and irregular. These joints should be placed at such intervals that the contraction due to temperature change will not cause the walk to crack between them. The joints should pass entirely through the walk, not merely through the wearing surface, as otherwise when the crack comes it may not follow the joint. For sidewalks, the blocks should not be much larger than five feet square. Objects, such as cellar floors, which are not subject to extreme changes of temperature, may be laid in larger blocks without danger of cracking. In long stretches of concrete it is desirable to allow expan- sion joints as well as contraction joints, to permit of the expan- sion of the concrete in hot weather. These differ from the contraction joints in that a small space is left between adjacent sections of concrete. To keep this space from becoming filled with material which will prevent the expansion of the con- crete, it is often filled with some substance, such as tar, which can easily be squeezed out in hot weather when the expansion occurs. Expansion joints may be placed much further apart than contraction joints, because the compressive strength of concrete is much greater than its tensile strength. If a small space is al- lowed at each contraction joint, this will take care of expansion; otherwise spaces about one-fourth of an inch wide should be left at intervals of about fifty feet, in sidewalks and similar work. Setting of Concrete. The setting of concrete is different from that of lime mortar in several respects. The early strength of the latter is due to the drying out of the mortar. This dried mortar then slowly absorbs carbon dioxide from the air and is partly changed to calcium carbonate. The process goes on most rapidly at the surface and very slowly in the interior. The setting of concrete, however, is not at all a drying process, but a chemical change. This can easily be seen from the fact that concrete will set and harden under water as readily as in air. Furthermore, concrete contains within itself all the sub- 88 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES stances necessary for its hardening, and it is unnecessary for it to absorb anything from the air, as in the case of lime mortar. The hardening, therefore, goes on at about the same rate ail through the mass, however large. It is impossible to say just how long it takes concrete to gain its ultimate strength. It is known, however, that it may continue to get stronger for many years. The rate of growth in strength will differ with the materials, the proportions, and the consistency of the concrete, and with the temperature. As a rough approximation, it may be said that, under average conditions, the compressive strength at the end of the first month will be about one-third more than at the end of the first week, and at the end of six months it will be about one-third more than at the end of one month. After this the increase in strength continues, but at a slower rate, the strength ultimately becoming about one-half more than at the end of one month. Since setting and hardening of concrete are due to chemi- cal changes, the rate at which they occur is dependent to a considerable extent on the temperature. At temperatures near freezing setting and hardening go on very slowly, while at high temperatures the rate is very rapid. Manufacturers of concrete building blocks, bricks, tile, and other articles often place their products in a closed room, heated to a high temperature and kept moist by the use of steam, in order to accelerate the hardening. In this way the concrete will attain as much strength in twenty-four hours as in several days at ordinary temperatures. The effect of temperature on the setting of concrete has an important bearing on the time of removal of forms. In cold weather, especially in freezing weather, the forms must be left in place much longer than in warm weather, and con- crete structures, especially reinforced ones, must not be loaded so early in the former case as they may in the latter. Failure to ap- preciate this fact has caused the collapse, when the forms were removed, of a number of concrete structures built in cold weather. Strength of Concrete. Tests of the compressive strength of concrete are now usually made on molded cylinders, 8" in diam- eter by 16" long, though many tests have been made on cubes from 8" to 12" on a side. The cylinders are to be preferred, as MIXING AND HANDLING CONCRETE 87 the conditions of stress are more like those usually met in concrete structures. The cylinders will usually show a unit FIG. 29. Testing a Concrete Cylinder. strength of about 0.73 times as much as is shown by the 12- inch cubes. Figure 29 shows an 8" X 16" concrete cylinder being FIG. 30. Results of Tests to Illustrate the Effect of the Richness of the Mixture on the Strength of Concrete. tested in the laboratory of the Kansas State Agricultural Col- lege. The machine shown can apply and weigh any load up 88 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES to 200,000 Ib. This is sufficient to develop a stress of about 4000 Ib. per square inch on an 8-inch cylinder. The breaking strengths that may properly be expected of standard concrete cylinders made of good materials properly handled are indicated in Table V, which gives maximum values to be used as the basis for design, as recommended by the Joint Committee from the national engineering societies. Indi- vidual tests may show values considerably in excess of those indicated, when conditions are favorable, but it would not be safe to count on the higher values. TABLE V CRUSHING STRENGTH OF DIFFERENT MIXTURES OF CONCRETE (In pounds per square inch, at 28 days) Aggregate 1:1:2 1:1:3 1:2:4 1:2^:5 1:3:6 Granite, trap rock 3300 2800 2200 1800 1400 Gravel, hard limestone and hard sandstone . ......... 3000 2500 2000 1600 1300 Soft limestone and sandstone. Cinders 2200 800 1800 700 1500 600 1200 500 1000 400 The safe working values to be used for the strength of con- crete may be obtained from the table by dividing the amounts given by the proper factor of safety. In its resistance to tensile and bending stresses concrete is relatively very weak, and it should not be used where it is subjected to severe stresses of this kind without being rein- forced with steel. In pure tension, the strength which may be expected of good concrete at the end of one month will be about as follows: 1:2:4 concrete 1:3:6 concrete 150-175 Ib. per sq. in. 100-125 Ib. per sq. in. Comparing these values with those given for the compres- sive strength above, we see that concrete is only from one-tenth to one-fifteenth as strong in tension as in compression. MIXING AND HANDLING CONCRETE 89 The strength of concrete in which clean gravel is used as the coarse aggregate will usually be a little less than that made in the same proportions with a good grade of broken stone, though the difference is not great and tends to become less as the con- crete grows older. A thin film of dirt on the gravel will greatly reduce the strength of the concrete. The hardness and strength of the stone has an important effect on the ultimate' strength of concrete. In tests of speci- mens which have had time to become thoroughly hardened, most of the stones break off at the planes of fracture instead of pulling out. The resistance to failure would therefore evi- dently be increased by using a stronger stone. Trap rock and granite make very strong concretes, hard sandstones and lime- stones give somewhat lower strengths, while soft limestones, sandstones, shales, and cinders make weak concrete. The strength of concrete which has thoroughly hardened is not materially affected by weak acids such as are found in sew- age and silage. Neither has manure nor animal nor vegetable oil any material effect on concrete after it has thoroughly hardened. It should be kept free from these substances, how- ever, while it is green. Sea water and the alkaline waters of some of the semiarid regions have a disintegrating effect on concrete and in time will cause its destruction. The best method of retarding this action is by making the concrete impervious to water by using a rich mixture, and, perhaps, by the use of waterproofing. Effect of Freezing on Concrete. Natural cement concretes and mortars are very seriously affected by freezing before they have set. Alternate freezing and thawing will cause almost complete disintegration. Portland cement mortars and concretes are much less affected. It is a disputed question as to how much injury is done to them by freezing, but it is probable that if proper precautions are taken, the injury is practically confined to the surface of the concrete, this injury often being manifested by a scaling off of a thin crust from the surface. It should be remembered, however, that the setting of the concrete is very slow in cold weather, and that little, if any, setting can take place when 90 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the concrete is frozen. Forms should therefore be left in place, and concrete should not be heavily loaded in cold weather until it has been given plenty of time to harden. If the concrete is kept from freezing until it has once set, it may be subjected to very low temperatures without injury. The rate of gain in strength will be lessened, but the concrete will ultimately get fully as strong as if maintained at normal temperatures. As a general rule, it may be said that concreting in freezing weather should be avoided. However, if the extra expense and care are justified, the work may be done successfully in temperatures as much as 20 degrees or more below freezing. Foundations and heavy walls, the face appearance of which is unimportant, may be laid without special precautions other than to see that the materials are warm enough when mixed so that the water will not freeze in a film on the aggregate or cement particles. This would prevent the adhesion of the cement to the surface and would of course cause failure. All frozen dirt and scum must be broken off the surface of the frozen concrete before fresh concrete is laid. Because of its resemblance to con- crete, frozen dirt may easily escape notice unless a careful ex- amination is made. For structures which must not be permitted to freeze, the temperature must be artificially maintained above the freez- ing point, or the freezing point of the concrete must be arti- ficially lowered. The former can be accomplished by enclosing the structure by means of canvas or otherwise, and by the use of stoves. If the temperature is only a few degrees below the freezing point, heating the materials may enable the con- crete to get its set before it freezes. The freezing point of the concrete can be artificially lowered by the addition of common salt. This is most conveniently added to the water. An amount of salt up to 10 per cent of the weight of the water may be used without injury to the ultimate strength, though the strength at short periods is re- duced; but it is not necessary to use so much except in very cold weather. As one cannot tell in advance how cold it will get before the concrete has set, an arbitrary proportion of salt may be used. MIXING AND HANDLING CONCRETE 91 About half a pound of salt to each gallon of water used, cor- responding to about two pounds for each bag of cement, will be sufficient for temperatures several degrees below freezing. To assist in retaining the heat of the mass, cement sacks, can- vas, or straw should be thrown over- the work when concreting is finished. PART III REINFORCED CONCRETE CHAPTER VII GENERAL PRINCIPLES Necessity for Reinforcing. While concrete is strong in com- pression, it is weak in tension and is brittle. In order to render it capable of resisting bending stresses, steel rods are often em- bedded in it. This produces what is known as reinforced con- crete. The steel reinforces, or strengthens, the concrete by resisting the tensile stress, or pull, while the concrete resists the compressive stress, or push. In this way concrete beams can be made strong enough to carry a load many times as heavy as they could carry if not reinforced, and concrete can be used in many places in which it would otherwise be wholly unsuitable. Materials Used for Reinforcement. Steel is the only mate- rial in common use for reinforcing concrete, though wrought iron may be used to a limited extent. Much of the material often called iron is really steel, there being comparatively little of the former material in use for any purpose at the present time. Steel is stronger and cheaper than iron, and consequently for most purposes it has very largely replaced the latter. Grades of Steel Used. The steel usually carried in stock in blacksmith shops and country hardware stores is too soft and weak to be an economical reinforcing material, though it is used to some extent. Two grades of steel are commonly used for reinforcing, known respectively as medium steel and high carbon steel. High carbon steel is stronger than medium steel, but is more brittle. Engineers are divided in their opinions of the relative merits of these two grades, but the use of high carbon steel is increasing. Either grade may be used with confidence if it will meet the requirements of the Standard Specifications of the American Society for Testing Materials. The physical properties and tests therein specified are as follows: 95 96 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES EXTRACT FROM STANDARD SPECIFICATIONS FOR BILLET-STEEL CONCRETE REINFORCEMENT BARS (Adopted by the American Society for Testing Materials, 1914) III. PHYSICAL PROPERTIES AND TESTS 8. Tension Tests. (a) The bars shall conform to the following requirements as to tensile properties. (See table on opposite page.) (b) The yield point shall be determined by the drop of the beam of the testing machine. 9. Modifications in Elongation. (a) For plain and deformed bars over f in. in thickness or diameter, a deduction of 1 from the per- centages of elongation specified in Section 8 (a) shall be made for each increase of f in. in thickness or diameter above f in. (b) For plain and deformed bars under yg in. in thickness or diame- ter, a deduction of 1 from the percentages of elongation specified in Section 8 (a) shall be made for each decrease of y 1 ^ in. in thickness or diameter below -j 7 ^ in. 10. Bend Tests. The test specimen shall bend cold around a pin without cracking on the outside of the bent portion, as follows: TABLET VI BEND TEST REQUIREMENTS Thickness or diameter of bar Plain bars Deformed bars Cold- twisted bars Structural steel grade Inter- mediate grade Hard grade Structural steel grade Inter- mediate grade Hard grade Under f inch . . . 180 d-t 180 d = 2t 180 d-St 180 d-t 180 d = 3t 180 d = 4t 180 d = 2t f inch or over. . . 180 d-t 90 d = 2t 90 d = 3t 180 d = 2t 90 d=.3t 90 d = 4t 180 d = 3t EXPLANATORY NOTE. d = the diameter of pin about which specimen is bent; t = the thickness or diameter of the specimen. 11. Test Specimens. (a) Tension and bend test specimens for plain and deformed bars shall be taken from the finished bars, and shall be of the full thickness or diameter of bars as rolled; except that the specimens for deformed bars may be machined for a length of at least 9 in., if deemed necessary by the manufacturer to obtain uniform cross-section. GENERAL PRINCIPLES 97 ed: de g n O T~ 2-3 8 "* 3 I I I *- t- oo S" 98 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES (b) Tension and bend test specimens for cold-twisted bars shall be taken from the finished bars, without further treatment, except as specified in Section 2 (b). 1 12. Number of Tests. (a) One tension and one bend test shall be made from each melt of open-hearth steel, and from each melt, or lot of ten tons, of Bessemer steel; except that if material from one melt differs f in. or more hi thickness or diameter, one tension and one bend test shall be made from both the thickest and the thinnest ma- terial rolled. (b) If any test specimen shows defective machining or develops flaws, it may be discarded and another specimen substituted. (c) If the percentage of elongation of any tension test specimen is less than that specified in Section 8 (a) and any part of the fracture is outside the middle third of the gage length, as indicated by scribe scratches marked on the specimen before testing, a retest shall be allowed. IV. PERMISSIBLE VARIATIONS IN WEIGHT 13. Permissible Variations. The weight of any lot of bars shall not vary more than 5 per cent from the theoretical weight of that lot. One class of high carbon steel reinforcing material is made by heating worn-out steel rails, and re-rolling them into bars. This is generally known as re-rolled steel or rail steel. It is usually considerably cheaper than that made from new steel, but many engineers are opposed to its use on the ground that it is likely to contain flaws, and therefore is unsafe. The author believes that, if it is carefully inspected and tested, it may well be used, but that it should not be used for important struc- tures without test. The Specifications given by the American Society for Testing Materials for the physical properties and tests of re-rolled steel are essentially identical with those for the hard grade of^billet steel, as heretofore given, except that no mention is made of cold-twisted bars, and hot-twisted bars are specified to have the same properties as other deformed bars. Forms of Reinforcing Steel. The forms in which steel is most used for reinforcing are: 1 Section 2 (b). If desired, cold-twisted bars may be purchased on the basis of tests of the hot-rolled bars before twisting, in which case such tests shall govern and shall conform to the requirements specified for plain bars of structural-steel grade. GENERAL PRINCIPLES 99 1. Smooth round bars or wire, 2. Twisted square bars, 3. Round or square bars with corrugations or projections rolled on their surfaces, 4. Woven or welded wire fabric, 5. Expanded sheet-metal fabric. Examples of the deformed bars and of the steel fabrics are shown in Figs. 31, 32, and 33. FIG. 31. Typical Deformed Reinforcing Bars. The advantages possessed by deformed bars (twisted or cor- rugated) over smooth bars is that the former will not slip through the concrete so readily as the latter, when they are FIG. 32. Woven Wire Reinforcing Fabric. heavily stressed. This is an important matter in short bars of large diameter, but is much less important in small bars of considerable length, where the surface of the bar gripped by 100 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the concrete is much greater in comparison with the tension in the bar. In doubtful cases, the resistance of smooth bars to slipping may be considerably increased by bending back three or four inches of the ends into loops. Twisted square bars have been very extensively used for reinforcing concrete, but they are now being displaced by other types of deformed bars. Tests made at the University of Illinois show that the twisted square bar begins to slip in the concrete at an even lower load than the plain round bar, although it will support a greater load before it pulls out entirely. Other types of deformed bars gave much more favorable results in the tests. FIG. 33. Expanded Metal Reinforcing Fabric. Woven wire and expanded metal fabrics are much used for reinforcing floor slabs, tanks, silos, and similar structures. When these materials are used, the sectional area of the steel extend- ing in the proper direction to resist the stresses should be made practically equal to the area that would be required if rods were used. Those fabrics which have the. principal wires or strands straight, without loops, kinks, or twists of any kind, are to be preferred. Expanded metal made from light sheets of steel is now much used as lath for cement stucco and plaster work. A special form of expanded metal lath is provided with deep corruga- tions, or ribs, of unexpanded metal at intervals in the width of GENERAL PRINCIPLES , ,101 the sheet. These ribs make the sheet stiff and rigid and also give it considerable reinforcing value. When this, material is used for floors and roof's, no forms are necessary, as the sheets are stiff enough, with a few braces, to support the weight of the concrete, and the meshes are fine enough to prevent the concrete from running through, if it is mixed to a medium FIG. 34. Expanded Metal Lath with Deep Ribs for Stiffening It. consistency. After the concrete on the upper surface has set, the lower side is plastered like ordinary lath. When the ribs do not provide enough reinforcing steel, small bars may be used in addi- tion to the metal lath. This material is also used for walls, both straight and curved, the concrete in such cases being usually ap- plied as a plaster. Figure 34 illustrates a typical lath of this kind. Classes of Structures in which Reinforced Concrete is Used. The principal kinds of structural elements in which rein- forced concrete is used are hollow cylinders, beams and slabs, walls, columns, and arches. Illustrations of hollow cylinders subjected to internal pressure are water tanks and silos. Drain- age tile, sewers, and some culverts illustrate hollow reinforced concrete cylinders subjected to external pressure, the pressure of the earth. Beams, slabs, walls, and columns of reinforced concrete are used in buildings and bridges. Reinforced concrete arches are often used in bridges, culverts, sewers, and tunnels and sometimes in buildings. In all the cases mentioned, the principal purpose of the steel is to resist the tensile stresses. In some cases, especially in 102 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES columns and arches, it is useful also in resisting compression. To enable the steel to fulfil its purpose it is necessary that (1) a sufficient amount of it be provided; (2) that it be in a form suitable for use; and (3) that it be properly placed in the con- crete. Those not familiar with the fundamental principles of beam action may use much more steel than is necessary, but, not placing it properly, may be unsuccessful. Hollow Cylinders Subjected to Internal Pressure. When hollow cylinders are subjected to an internal pressure equally distributed around the circumference, as is the case in water tanks and silos, the stress is almost wholly one of tension, so that concrete without reinforcing would be entirely unsuitable for the purpose. The walls would have to be very thick to resist the outward pressure, and the cost would be prohibitive. Hence steel reinforcing is depended on to take practically all the stress, the concrete serving only to hold the steel in place, to protect it from rusting, and to retain the contents of the cylinder. In fact, reinforced concrete cylinders are very similar to wooden tanks, the steel reinforcing of the one corresponding to the hoops of the other, and the concrete of the one to the wooden staves of the other. There is this difference, however. The concrete has the steel embedded in it, so that the latter is protected from the action of air and water and does not rust, and the concrete itself does not decay like the wood. The reinforced concrete tank is therefore permanent instead of temporary. Placing the Steel. Since the stresses in concrete cylinders are not bending but tension, it is not necessary that the steel be placed close to one face or the other as in most other cases of reinforced concrete. Probably the best place for the steel is near the middle of the wall, or perhaps a little nearer the outer edge, in order to give it the best protection from corrosion. It is not necessary that the ends of the reinforcing rods be fastened together as in the case of hoops for wooden tanks, but they must be lapped a distance of about sixty times the diame- ter of the rods, if smooth bars are used.. If deformed rods are used, or if the ends of the rods are bent back for three or four inches in the form of a hook, thirty times the diameter of the rods will be sufficient lap. Thus | inch rods should be lapped GENERAL PRINCIPLES 103 60 X I = 38 inches if smooth bars are used, or 30 X | = 19 inches if deformed or hooked bars are used. Beams and Slabs. Beams and slabs may be considered together, as a slab is really a wide, shallow beam. The load which must be supported tends to bend the beam or slab. This bending tendency causes, and is resisted by, compressive stresses on one side of the beam and tensile stresses on the other side. This can easily be demonstrated in a timber beam, by making a saw cut near the middle. If the cut is made from the one side, it tends to close up and pinch the saw, thus showing that this side of the beam was in compression. If the cut is made from the opposite side, the cut tends to open up, allowing the Load Reinforcing Rods . on Lower Side FIG. 35. Simple Beam Properly Reinforced. saw to move freely, thus showing that the portion cut away by the saw held the parts together, or was in tension. In beams supported at both ends, with the load pushing or pulling down- ward, the compression will be at the upper side of the beam, while the tension will at at the lower side. If one end of the beam is free, the other end being fastened so that it cannot move, with the load pushing or pulling down on the free end of Load Reinforcing Rods ' on Upper Side FIG. 36. Cantilever Properly Reinforced. the beam, the compression will be at the lower side of the beam while the tension is at the upper side. This kind of beam is called a cantilever. In either case the steel reinforcing must be placed near the surface, on the tension side of the beam. Fig- ures 35 and 36 show the proper arrangement in these two 104 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES cases. Figure 38 shows the effect of improper location of the steel on the strength of beams. Continuous Beams. Sometimes beams will extend over more than two supports, as is shown in Fig. 39. The dotted lines show in an exaggerated manner the form which the beam takes FIG. 37. Testing a Reinforced Concrete Beam. because of the action of the load. It can easily be seen that ten- sion will exist on the lower side of the beam near the middles of the spans and on the upper side of the beam over the supports. Hence the reinforcing steel must be placed close to the lower side of the beam near the middles of the spans and close to the upper side near the supports. In such cases it is common practice to bend up half or two-thirds of the steel rods from the bottom of the beam to the top, at an angle of about 45 degrees, starting from the bottom at points about one-third of the length of the span from each end. Enough additional rods are provided over the supports near the top of the beam to make the total number of bars here equal to the number of bars near the lower surface at the middle of the span. These bars should extend one-third of the span length on each side of the supports. GENERAL PRINCIPLES 105 It can be seen that the effect will be the same as in the case illustrated in Fig. 39, if heavy girders, instead of columns, form the supports for the ends of the beams. In any case, a FIG. 38. Results of Tests to Illustrate the Effect of Improper Location of Steel on the Strength of a Beam. little consideration will serve to show on which side the concrete will tend to pull apart, and the steel should be placed near the surface of the beam on that side. w FIG. 39. Continuous Beam Properly Reinforced. Depth of Embedment of Steel. It is desirable that the steel be placed as close to the surface as is safe, since it is much more effective in resisting bending when near the surface than when near the middle of the beam. On the other hand, if the 106 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES rods are placed too near the surface, there will be danger of cracking or splitting off the concrete below the rods when the beam is loaded. There will also be danger of the rods rusting and becoming so weakened that the beam will lose its strength. In buildings and other places where the concrete may be endan- gered by fire, the steel must be placed deep enough so that it will not be materially weakened by the heat. A very thin film of cement, provided it is unbroken, will pre- vent corrosion of the steel. On account of the danger of the formation of open pockets in a few places, however, it is desir- able never to use less than half an inch of concrete outside the rods, to prevent corrosion, especially if water is likely to come in contact with the surface. To prevent the concrete from splitting off, and to permit it to be well worked into place between and below the rods, the latter should be spaced so there is a distance of not less than one and one-half times the diameter of the rods, in the clear, between adjacent rods, with a distance of not less than one diameter in the clear between the rods and the bottom of the form. Thus if f-inch rods are used, they should be spaced not less than 1J X f = 1J inches apart in the clear, or 1| + f = If inches from center to center, while the clear distance between the bars and the bottom of the beam should be not less than f inch. If the rule given in the above paragraph is followed, with a minimum distance of one-half inch between the rods and the forms, this will be satisfactory for most cases coming within the scope of this book. In important structures subject to fire risk, the embedment given above would be insufficient for pro- tection from fire, a minimum depth of one and one-half to two inches being required. The steel should be well secured in its proper position, so that it may not be displaced by the concrete, or by the work- man while the concrete is being placed. The rods in beams should be well wired to each other and to the forms. Special care should be taken that the slab rods do not sag too low. Wooden laths may be used to hold up the rods, if they are re- moved as the concrete reaches them. A better way is to use a few steel rods of the proper size, placed at right angles to the GENERAL PRINCIPLES 107 main reinforcing, or some form of patented support, which may be left in place when the concrete is poured. Figure 40 shows such a support. If the cross rods are well wired to the main rods, supports will be needed at only a few points, and these may be supplied by pieces of flat stone or gravel. Columns. The principal stress in columns is of course com- pressive, and, as concrete is strong in compression, it might seem that steel reinforcement would not be needed. In fact, short concrete col- umns or piers are not infrequently built without reinforcing. If, how- ever, there should be any side thrust on the column, or if the load should not act exactly through the center of the column, a bending would be produced which would tend to make the column buckle sidewise. This would give rise to tensile stresses in the concrete, on the side buckling outward. To resist these stresses, FIG. 40. Support for Holding Reinforcing Steel in Position. some steel is ordinarily used in the columns. As the column may buckle in any direction it is necessary that it be rein- forced on all sides. A rod is often placed in each corner. The steel also aids in carrying the compressive stresses, and the col- umn may be made smaller than if no steel were used. This is often an important matter in buildings where floor space is very valuable. The action of the steel in concrete columns is similar to that in beams, and therefore the same remarks will apply as to the distance it should be placed from the surface of the concrete. If the clear distance of the rods from the surface is made one and one-half times the diameter of the rod, this should be sufficient, except where the necessity for fire protection requires a greater distance. Sometimes the steel columns are reinforced by means of wire hoops or spirals. These tend to prevent the bulging out which necessarily takes place when the columns shorten under 108 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the action of loads. The hoops are not effective in resisting a tendency to bend; hence it is always desirable that some rods be used with the hoops. These rods are usually placed just in- side the hooping and often serve as spacers to hold the latter the right distance apart. Arches and Hollow Cylinders Subjected to External Pressure. In arches, the stress is a combination of bending and com- pression. Steel is often used near both the outer and the inner surfaces, to carry the tension due to bending, as this may be first on the one side and then on the other. In sewers and drainage tile, the load is due chiefly to the weight of the earth above the tile. This tends to cause the top and the bottom to flatten, and the sides to bulge out. This produces a tension or tendency to crack on the inside of the wall at the top and bottom, and on the outside at the sides, as shown in Fig. 42. This tension is provided for by placing the steel in either of the manners shown in Fig. 43. Differences in Materials and Methods Used for Reinforced Concrete. -- The stresses in reinforced concrete are usually much higher than in plain concrete. It is therefore important that only the best ma- terials and rich mixtures be used, and that every care be taken to get a strong dense concrete, free from all open pockets and FIG. 41. Spiral Col- other imperfections. The coarse aggregate should be a firm umn Reinforcing. hard stone, or a clean gravel, screened over a f-inch screen. The size of the stone or gravel should be somewhat smaller than is usually necessary for plain concrete, on account of the difficulty in getting it worked into place around the reinforcement so that no open pockets will be left. The thickness of the members is frequently much less than is usual in unreinforced work, and this is an additional reason for using a smaller coarse aggregate. In GENERAL PRINCIPLES 109 average work a stone or gravel which would pass a 1-inch mesh screen is satisfactory, and even a little coarser aggregate will sometimes do, if care is taken in working it into place. Still finer aggregate may, in some cases, be desirable. The concrete for reinforced work, in order that it may be easily worked into place around the steel and into the corners, should usually be mixed to the consistency designated in Chap- ter VI as a wet mixture. Com- pacting should be done with a spade, which should be worked up and down in the concrete sufficiently to free the latter from all air, and to insure that the concrete has entirely filled the space below and around the steel. Care should be taken, how- FIG. 42. Method of Collapse of Sewer under Earth Pressure. ever, not to displace the latter. Columns should, if possible, be poured complete at one time and then a few hours or more should be given for these to set- tle before the floor above is poured. When work on floors is FIG. 43. Methods of Reinforcing Concrete Tile. stopped, the beams and slabs should be stopped near the middle not at one end as might be supposed. If the work is stopped near the ends of the beams, diagonal cracks which may be dan- gerous are likely to form at these places. The full depth of all beams and slabs must be poured at one time as far as the work is carried. 110 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES In much reinforced concrete work, as in floors, the weight of the concrete itself causes a large part of the total load which the member is required to carry. In such cases it is highly important that the forms be left in place until the concrete has had ample time to harden. At least two weeks should be allowed, and more if a load is applied on top of the floor. Full loads should not be applied to reinforced structures until at least one month after the concrete is poured. Most structures are designed on the basis of the strength the concrete will possess at the end of one month. Hence, if they are loaded earlier, the concrete may not yet be strong enough to with- stand the stresses developed. If it is necessary to apply the load earlier, as it may be at times, braces should be put uader the middle of the slabs, beams, and girders. t If the forms are left in place in such cases until the end of one month, they will offer material assistance. CHAPTER VIII STRENGTH OF REINFORCED CONCRETE WHILE the design of complex reinforced concrete structures is a complicated matter which should be left to competent concrete engineers, and a discussion of which is therefore be- yond the scope of this book, yet enough may be given here to enable the reader to make intelligent use of steel for reinforcing the simpler structures most often met with in rural communities. Stresses Used in Reinforced Concrete. It is usual in rein- forced concrete design to base the stresses in the concrete on the strength that the latter will possess at the end of four weeks, as it will usually be necessary to use the structure at about this time. If the maximum load must be applied at an earlier date than this, then the strength of the concrete at the end of the shorter period should form the basis of design. The increase in strength in longer periods will then give added security to the structure. In Chapter VI the crushing strength of 1:2:4 concrete at the end of 28 days was given as about 2000 to 2200 Ib. per sq. in. when gravel or the harder stones are used. It would not do, however, to use such high values in the design, for, if the con- crete were not quite so strong as expected, or if the load were a little greater, or if the structure received a jar, then failure would occur. Also, repetitions of the load would cause failure at stresses much lower than the value given, just as a piece of wire or a bar of steel can be broken by repeated bending back and forth. For these reasons, the values of strength found by tests must be divided by some number, called a factor of safety, to get values for the safe stresses to be used in design. This factor differs for different materials and for different types of structural elements. As it is impossible to tell in advance just how strong a given ill 112 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES concrete will be, it is customary to assume a strength of 2000 Ib. per sq. in. for good 1:2:4 concrete at the end of 28 days, and to apply the factors of safety to this value. If in any case there is reason to believe that a lower strength will be obtained, then the working stresses should be correspondingly reduced. The factor of safety for the concrete of columns and piers is usually taken at about 4.5. The working stress is therefore 2000 j-jT or, say, 450 Ib. per sq. in. For beams and slabs a fac- 4.o 2000 tor of about 3 is used, giving -r , or about 650 Ib. per sq. i 3 in., as the safe working stress. The factor of safety usually used for steel in tension is from 4 to 5, giving maximum values for working stresses of 12,000 to 16,000 Ib. per sq. in. for medium steel, and from 16,000 to 18,500 Ib. per sq. in. for high carbon steel. It is not generally thought desirable to use higher stresses than 16,000 Ib. per sq. in., and 14,000 Ib. per sq. in. is probably better for medium steel. Table VIII will be found convenient in selecting the sizes of rods to use. Hollow Cylinders Subjected to Internal Pressure. In hol- low cylinders subject to internal pressure, the steel is depended on to take care of all the principal stresses by hoop tension. It can easily be seen that more steel will be required near the bottom of a cylinder than near the top, on account of the greater pressure existing here. The following rule may be used to find the stress in the steel, at any distance from the top of a cylinder, due to pressure from a liquid: Multiply the height, in feet, of the liquid above the section under consideration, by the weight of a cubic foot of the liquid; multiply this by the diameter of the cylinder in feet; and divide the product by twice the area of the cross-section of steel in square inches per foot of height of the wall. The spacing of the reinforcement should not be made greater than two and a half to three times the thickness of the wall. The rule may be expressed alge- braically as follows: STRENGTH OF REINFORCED CONCRETE 113 9 3 "- g.a si COO5COOO OOi ii ( O iO O O CO i ( '-i'-iiM(MCOT^ O5 O5 O iO CO o'o'o'o'do'o'fHi-Hi-HC or the column must be 13.3 or, say, 13 inches square. This makes the length equal to = 14.2 times one side, which is allowable. Use about 1 per 102 cent of steel, or .01 x 13| x 13 = 1.82 sq. in. This requires one ^-inch square, or one f-inch round, rod near each corner. If the length of the column had been 20 feet, the size would have had to be increased to =r= = 16 inches square, in order to keep the length from being 10 more than 15 times the width of one side. In buildings where there is considerable fire risk, the outer one and one-half inches all around the column should be re- garded as fire-proofing only, and should not be taken into account in computing the safe load. In such cases the reinforc- ing bars should be placed two or two and one-half inches in the clear from the outside of the column, so that they will be pro- tected from the fire. For illustration, in problem 2, if the column is to be used in a fire-proof building it must be made 13 J + 2 X 1J = 16 J inches square, to carry safely 40 tons in a hot fire. Beams and Slabs. In determining the strength of beams and slabs, the concrete below the center of the steel is neglected and the effective depth is taken as equal to the distance from the upper surface to the center of the steel. The amount of steel generally used is 1 per cent of the area of the beam or slab above the steel, or a little less. Thus if a beam is 8 inches wide and 15 inches deep, with the center of the steel 2 inches above the bottom of the beam, the effective depth of the beam is only 13 inches, and the area of steel used should be about .01 X 8 X 13 = 1.04 sq. in. This would correspond to two -fl- inch round rods, which have a combined area of. 1.037 sq. in. Similarly, if the total depth of a slab is 5 inches and the STRENGTH OF REINFORCED CONCRETE 117 center of the steel is f inch above the bottom, the effective depth is only 4J inches; and 0.01 X 4J X 12 = 0.51 sq. in. of steel may be used per foot of width, corresponding to J-inch round rods 4J inches from center to center or | inch round rods 7J inches on centers. Rectangular Beams. Where not more than 1 per cent of steel is used, and where the length is not less than twelve times the effective depth, the safe uniformly distributed load which can be carried by a simple rectangular beam supported at both ends, of 1:2:4 concrete not less than one month old, can be found by the following rule. It is based on a stress of 14,000 Ib. per sq. in. in the steel and about 650 Ib. per sq. in. in the concrete. Multiply the depth of the steel from the surface of the beam by 8100 times the area of the steel, and divide the product by the length of the beam in feet. The result will be the safe load in pounds, including the weight of the beam. To find the load which can safely be placed on top of the beam, subtract the weight of the beam itself. The weight of the concrete may be taken as 150 Ib. per cubic foot. The rule may be expressed algebraically as 8100A s d w = W -L where w = the safe uniformly distributed load, in pounds, which can be placed on top of the beam, A g = area of the cross-section of the steel, in square inches, d = depth from the top of the beam to the center of the steel, in inches, L = length of beam in feet, and W = weight of the beam itself, in pounds. It should be especially noted that this rule does not apply to a beam containing more than 1 per cent of steel nor to a beam the length of which is less than twelve times its effective depth. Problem 1. Find the safe uniformly distributed load which can be placed on a concrete beam 9 inches wide, 16 inches deep, and 15 feet long, if it is reinforced by means of two |-inch round rods, the center of which is 2 inches above the bottom of the beam. 118 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Solution. The effective area of the beam is equal to 9 x 14 = 126 sq. in. The area of steel is 1.203 sq. in., so that the percentage of steel is izo = 0.95. The length is -^-j^ = 12.8 times the effective depth. As the reinforcement is less than 1 per cent, and the beam is not too short, the above rule will apply. Therefore the total load which can be carried, including the weight of the beam, will be equal to 81QO X I- 203 X 14 = 9100 pounds. The 10 weight of the beam itself will be 150 x ^y^T X 15 = 2250 pounds. Conse- quently the safe load which can be placed on the beam is 9100 - 2250 = 6850 pounds. If the size of a beam to carry a given load is to be found, it may be assumed, and the strength calculated by the above rule. If this beam is found to be too weak or too strong, a larger or a smaller size should then be assumed, and the strength again calculated, this process being continued until a size is found which is safe, and yet not excessive. It will usually be desirable to make the breadth of rectangular beams about f to J of their effective depth. Problem 2. Design a beam to carry safely a uniformly distributed load of 10,000 pounds on a span of 18 feet. Solution. Since this beam must carry a larger load, with a longer span, than the beam of the example above, it must be heavier. Assume a width of 12 inches and an effective depth of 18 inches. This depth is just ^ of the span. If 1 per cent of steel were used, this would be .01 x 12 x 18 = 2.16 sq. in. Try two one-inch square rods with an area of 2.0 sq. in., a little less than 1 per cent. The total safe load, including the weight of the beam, is by the rule given above, 8100 x 2 x 18 1COAA - = 16,200 pounds. -1 The weight of the beam itself, if two inches of concrete are allowed below the steel, making the total depth of the beam 20 inches, is 150 x 4 x 18 = 4500 pounds. Hence the safe load which can be placed on top of the beam is 16,200 - 4,500 = 11,700 pounds. As this is more than is required, a somewhat smaller size, say 11 inches by 18 inches, may be assumed, and its strength calculated. Proceeding as before, we find that, with three f-inch round rods, the safe load on top of this beam is 10,500 pounds. This is close enough, and the beam may be used. STRENGTH OF REINFORCED CONCRETE 119 If beams are required to be a little deeper than one-twelfth of their length, the method given above may be used in designing them, provided several stirrups of J-inch or f-inch rods are placed near each end and spaced a distance of about half the depth of the beam from each other, with the first one an equal distance from the end of the beam. One-third to one-half of the main reinforcing rods should also be bent up at an angle of about 45 degrees, beginning at points from one-fourth to one- third the length of the span from the ends. Deformed bars should be used in all such deep heavy beams. , Sometimes the beams must be designed to carry a load concen- trated at the middle of the span, instead of uniformly distributed. In such cases one may proceed to find the safe uniformly dis- tributed load which can be put on the top of the beam, and then divide this by 2 to find the safe load concentrated at the middle. Floor and Roof Slabs. Floor and roof slabs may be consid- ered equivalent to a large number of beams laid side by side, and hence the rule given above may be used to find their safe loads. It is convenient to make the calculations on the basis of a width of strip of 12 inches, though any other width would give the same results. Slabs will rarely be shorter than twelve times their effective depth, so stirrups will seldom, if ever, be required. The spacing of the main reinforcing rods should never be greater than 2| to 3 times the effective depth of the slab. It is desirable to have some light rods at right angles to the main reinforcing, to assist in distributing concentrated loads, and to resist stresses caused by shrinkage and changes of tem- perature. Round rods J inch to f inch in diameter, spaced about 24 inches on centers, are commonly used. This steel is not taken into account when the percentage of reinforcement is figured. Problem 1. Find the safe uniformly distributed load on a flat slab floor of 4^ inches total thickness and 10 feet span, if it is reinforced with 1 per cent of steel, with a distance of f inch from the center of the steel to the bottom of the slab. Solution. Consider a section of the slab one foot wide, and treat it as a beam. The effective depth of the slab is4-f = 3 inches, and the effective area of the concrete in this width is, therefore, 3f x 12 = 45 sq. in. Hence 120 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES the area of steel required per foot of width of the slab is 1 per cent of this, or 0.45 sq. in. Table VIII shows that this corresponds to -inch round rods 5j inches from center to center, or f-inch round rods 8 inches on center. Then, applying the rule given above, the total load the slab can carry for each foot of width, including the weight of the slab, is - : ^' 45 X 3 * = 1370 pounds, Al x 1O The weight of this width of slab is 150 x ~-/AA X 10 = 560 pounds, so the load which can be safely put on top of the floor is 1370 - 560 = 810 pounds, for each foot of width, or - 8 T V =81 pounds per square foot. Problem 2. r Find the depth of slab and amount of reinforcing required to carry a load of 150 pounds per sq. ft., in addition to the weight of the slab, on a span of 12 feet. Solution. Assume a total depth of slab of 7 inches, with the center of the steel. 1 inch above the bottom. Then the effective depth is 7 - 1 = 6 inches. One per cent of steel will require for each foot of width of slab .01 x 6 x 12 = 0.72 sq. in., which can be supplied by f-inch round rods spaced 1\ inches on centers. Then the safe load, including the weight of the slab, which each foot of width can carry is, 8100x0.72 X6 12 The slab will weigh, for this width, = 2920 pounds. 150 x ; X 12 = 1050 pounds, J.TX making the safe load on top of the slab 2920 - 1050 = 1870 pounds per foot of width or pr- = 156 pounds per square foot. This is close enough to the given load for all purposes. Had the safe load come out too low or too high, another calculation on a slightly stronger or weaker assumed size would have been required. Continuous Beams. Beams and slabs which are continuous over several supports should have about half or two-thirds of the steel bent up at an angle of 45 degrees at about half or two-thirds of the distance from the center to the end of the span, as discussed in Chapter VII, and should have additional rods, of length equal to two-thirds the span, provided near the upper surface, above the supports, to make the total amount of steel here equal to that in the middles of the spans. If this is not done, unsightly and possible dangerous cracks are likely to form over the supports. In slabs reinforced with wire fabric or expanded metal, all this is usually bent up over the supports. STRENGTH OF REINFORCED CONCRETE 121 The strength of continuous beams and slabs, reinforced as directed, is 25 to 50 per cent greater than where the reinforcing over the supports is not provided. FIG. 44. Interior of Reinforced Concrete Building, Showing Arrangement of Slabs, Beams, Girders and Columns. T-Beams. Concrete beams are very frequently cast in one piece with the slabs. Such beams, called T-beams, may be given a much larger amount of steel than ordinary rectangular beams of the same size, and their strength is correspondingly greater. It is usually necessary to use stirrups with these beams, and the method of calculation is too complex to be discussed here. Arches and Hollow Cylinders Subjected to External Pressure. The design of arches and hollow cylinders subject to exter- nal pressure is a complex matter and beyond the scope of this book. 122 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES PROBLEMS 1. Compute the amount of reinforcing required in a cylindrical water tank 12 feet in diameter and 8 feet high at points 2, 4, 6, and 8 feet below the top. Select suitable bars and spacing for the reinforcement of the tank from top to bottom, so as to provide approximately this reinforcing. What thickness of wall would you use? 2. Find the amount of reinforcing required in a concrete silo 18 feet in diameter and 40 feet high, at points 10, 20, 39, and 40 feet from the top, if the pressure of silage be assumed to be equivalent to that of a liquid weighing 11 pounds per cubic foot. Select suitable bars and spacings for these points. 3. Find the safe load on a concrete column 16" x 16" x 12' long. What reinforcing would you use in the column? What would be the safe load on this column in a building required to be fire-proof? 4. Find the size of a concrete column to carry a load of 40 tons applied centrally, if the length is 12 feet. What reinforcing should be used? 6. Solve problem 4, assuming the length of the column to be 20 feet; assum- ing it to be 5 feet. 6. What is the safe uniformly distributed load on a beam 10 inches wide by 16 inches deep, to the center of the steel, and 18 feet long, if it is reinforced by two 1-inch round rods? Would stirrups be required in this beam? What would be the safe load concentrated at the middle of the beam? 7. Design a rectangular reinforced concrete beam to carry a safe load of 10,000 pounds concentrated at the middle of a 16-foot span. 8. What safe load per square foot can be placed on a floor slab 6 inches thick, reinforced with -inch square rods spaced 5| inches on centers, if the center of the steel is one inch above the bottom of the slab and the span is 10 feet? 9. Design a floor slab to carry a safe load of 200 pounds per square foot, on an 8-foot span. 10. What will be the cost of the steel for the floor of problem 8, if the width is 16 feet, and the steel costs 2 cents per pound? PART IV MISCELLANEOUS MATTERS CHAPTER IX CONCRETE SURFACE FINISHES Methods of Treatment. As concrete is a plastic or semi- fluid substance when it is first placed, it will show all the irregularities of the forms when the latter are removed. Fre- quently, every joint between the boards, every imperfection in the forms, and even the grain of the wood can be traced on the surface of the concrete. The honeycombed places which sometimes appear, even in excellent work, together with the rough places left where mortar ran out into the cracks between boards, give the surface an unfinished appearance. No special treatment is necessary where the appearance is unimportant, or where the surfaces are to be covered, except to patch the open pockets. This should be done as soon after removing the forms as possible, using a mortar mixed in about the same proportions as were used for the cement and sand in the original concrete. For many purposes the unfinished appearance of the surface is objectionable, and some kind of treatment is desirable. The principal methods used are: 1. Providing a rich mortar face, which may be troweled or floated after the form is removed. 2. Brushing the surface with a cement grout. 3. Scrubbing the surface with a stiff brush and water. 4. Rubbing the surface with a hard brick and sand, or a carbo- rundum block. 5. Working over the surface with a cutting tool. 6. Plastering the surface. Mortar Face. In molded articles of concrete, such as bricks and building blocks, it is common practice to use a rich mortar face, while the body of the block is made of a leaner mixture. This is easily accomplished by making the block face down and putting about an inch of rich mortar in the mold first, 125 126 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES then using the leaner mixture. The dry mixture, with a metal mold, makes a fairly satisfactory surface, and usually no special treatment is attempted. A mortar face may also be used in walls with little trouble. Just before the concrete is poured, the surface of the form may be plastered with a rich mortar. If this is mixed to the right consistency, it will adhere to the forms fairly satisfactorily. The concrete is then poured, and worked into thorough contact with the mortar face, care being taken that no stones are forced through this face so as to show on the surface when the forms are removed. Another method of accomplishing the same results is by means of a sheet-metal gage, as shown in Fig. 45. This has Handles Riveted to Flared Edge FIG. 45. Sheet Metal Gage for Making a Mortar Face on a Concrete Wall. riveted upon its face two or more angles, with the outstanding legs equal in width to the thickness of the mortar facing desired. This is placed against the form, the concrete is poured outside of it, and the mortar is put between it and the form. The upper edge of the sheet is bent back, as shown in the figure, to make it easy to get the mortar into place. When the concrete has been filled on the outside, and mortar on the inside, to the top of the gage, the latter is raised a distance nearly equal to its width, the concrete is thoroughly compacted and worked into contact with the face, and the placing of concrete and mortar is resumed. By either of the above methods there is very little danger of the concrete cracking off, as there is when the mortar is plas- tered to the concrete after the forms have been removed. CONCRETE SURFACE FINISHES 127 Since both the facing and the backing are placed at the same time, a thoroughly good bond is secured. If the forms are taken off as early as possible, the mortar face may be floated down to a uniform finish, all of the form marks being thus removed. FIG. 46. A Reinforced Concrete House. Brushed Surface. One of the cheapest and most easily applied finishes is obtained by brushing the surfaces with a thin coat of neat cement, or equal parts of cement and fine sand, mixed with water to a creamy consistency. A whitewash brush or even an old broom will answer for this purpose. The brushing should be done as soon after the concrete is poured as the forms can be removed. Unless the concrete is still very green, the surface should be thoroughly saturated with water before the brush coat is applied. Surface imperfections should always be removed before the grout is applied. All projections should be nibbed off and all pockets should be patched. This treatment is very effective in filling up the surface pores and helping to make the concrete water-tight. A second coat should 128 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES usually be applied over the first as soon as the latter has set sufficiently. This finish is likely to check if it is allowed to dry too rapidly, or if it is exposed to the direct action of the sun. Wet cloths laid over the surface or frequent sprinkling during the first few days will help to prevent checking. Scrubbed or Etched Surface. Another method of treatment which is perhaps as satisfactory as any, considering the results secured and the cost, is to scrub the surface of the concrete as soon as the forms are removed, with a stiff brush and water to remove the thin film of cement outside the aggregate. The length of time which must elapse after pouring the concrete before the surface can be scrubbed will vary with circumstances from about 12 to 36 hours. If the scrubbing is done too soon, the aggregate will be torn out; if not soon enough, the cement film cannot be removed. Instead of a brush one may use a carborundum block, or a hard brick with sand for a cutting agent, water being used as before to wash away the material removed. With the carbo- rundum block or the brick and sand, the scrubbing can be done when the concrete is a little harder than it can be when the brush is used, and they are probably to be preferred to the brush. By using selected aggregates, such as pebbles or colored sand or stones, especially pleasing effects can be produced. A modification of the above method is to apply a dilute solu- tion of muriatic acid to the surface. This will eat out the cement, leaving the aggregates exposed. After the action has gone far enough, the excess acid should be thoroughly washed from the surface. When limestone has been used in the aggre- gates, these will also be attacked by the acid and the result is likely to be unsatisfactory. Any of the above methods of scrubbing or etching the sur- face can be used successfully in connection with the mortar face. Tooled Surface. A very satisfactory surface can be made by picking or cutting the surface of the concrete by means of a stone cutter's bush hammer, toothed chisel, or pick. The con- CONCRETE SURFACE FINISHES 129 crete should be allowed to set two or three weeks before this is done. If the work is attempted earlier, the particles of ag- gregate near the surface will be broken loose, while, if a longer time is allowed, the concrete will be unnecessarily hard and the work will be difficult. Plastered Surface. A cement plaster is often used on the surface of concrete, with varying degrees of success. If the work is properly done, the results will often be entirely satis- factory, but great care is required or the plaster coat will eventually crack and scale off. Generally other methods of fin- ishing are to be preferred, especially by those not skilled in this kind of work. Forms for concrete which is to be plastered should not be oiled, nor coated with a soap solution, but instead should be thoroughly wet before the concrete is placed in them. A mortar composed of one part of cement to one and one- half or two parts of sand is generally used. This will be im- proved by the addition of hydrated lime in any amount up to about one-tenth the volume of the cement. For best results, the plaster should be applied while the concrete is still green. The forms should be removed as early as possible. In small work like curbs and steps, this can be done in a very short time if a medium or a dry mixture has been used. If the concrete has dried out, the surface should be roughened, and should be thoroughly saturated with water before plaster is applied, so that the water will not be absorbed from the latter too rapidly. A wash of neat cement and water mixed to a creamy consistency should be applied, followed im- mediately by the plaster. Best results will be obtained if the plaster coat on green con- crete is made as thin as possible, preferably not more than T V to | inch thick. If a thicker plaster is necessary, it may be placed in successive coats as directed for stucco in the next chapter. The surface of the plaster may be finished by any of the methods there given for stucco finish. Plaster work on concrete must be kept from drying out too quickly. Frequent sprinkling of the surface or protection by wet burlap is effective. 130 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Interior walls and ceilings of concrete may be plastered with the hard wall plasters in the same manner as brick or stone. There is some danger that the plaster will not adhere to the concrete on account of the smoother surface. Roughening the surface, and saturating it with water before applying the plas- ter, will aid materially in securing a good bond. CHAPTER X STUCCO AND PLASTER WORK Uses of Stucco. The term stucco as used in this book refers to cement mortar used as an exterior coat of a building or other structure. The term is sometimes used to designate gypsum, lime, or even mud plasters, either in interior or in exterior work, but will not be so used here. FIG. 47. Stucco Garage on Deep Rib Metal Lath. Good Portland cement stucco serves as an excellent outer coat for a building, as it is cheap, durable, and pleasing in ap- pearance, if properly applied, and there is no cost for painting or repairs. The use of stucco has been increasing rapidly, and there is every reason to believe that it will continue to grow in favor. This material is used in two general classes of work: (1) renovating old buildings of wood, brick, stone, and concrete; and (2) new buildings, chiefly on frame, brick, or tile walls. Stucco is especially well adapted to recoating old houses. It 131 132 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES may be applied to metal lath or furring strips nailed directly to the weather boarding. On old brick or stone walls it is best applied over metal lath, but it may be used without the lath if the precautions indicated in the specifications at the end of this chapter are observed. It is not recommended for general use on concrete walls, some other method of finishing, as indicated in the last chapter, being more often desirable. On new work, stucco is much used in place of weather- boarding in frame structures. The construction is carried out exactly the same as in the ordinary frame building as far as the studding, care being taken that the latter are well braced. The sheathing may. be applied to the studs in the usual manner and the lath and stucco applied outside of this, or the lath and stucco may be applied directly to the studs. Where sheath- ing is used with stucco, it is desirable to have it run diagonal^, so as to better brace the studding and thus prevent possible crack- ing of the stucco. The use of stucco on new brick walls permits the use of common brick throughout the wall, thus avoiding the expense and delay involved in the use of facing brick. Hollow tile is a material admirably adapted for use with a stucco face. The stucco may be applied directly to the tile without lath or furring, while the inside of the tile may also be plastered direct without the use of lath or furring, and with- out danger of dampness working through the wall. The con- struction is cheap, substantial, dry, sanitary, warm in winter and cool in summer, and fire-proof, and so has many advan- tages over frame construction. Stucco Surface Finishes. A wide variety of finishes may be had in stucco work, differing both in color and in the rough- ness of surface. Variations in the color may be obtained by selection of the aggregates, by the addition of lime or colored minerals, or by the use of white cement. These methods of coloring are all discussed in Chapter XII. Methods of obtain- ing the different surface finishes are given in paragraphs 31 to 38 of the specifications following. Specifications for Stucco. The following specifications, adopted by the American Concrete Institute, give full direc- tions for the preparation and application of stucco. FIG. 48. Stippled Surface. FIG. 49. Sand-Floated Surface. FIG. 50. Sand-Sprayed Surface. FIG. 51. Rough Suction Surface. FIG. 52. Splatter Dash Surface. FIG. 53. Pebble Dash Surface. 134 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES STANDARD SPECIFICATIONS FOR PORTLAND CEMENT STUCCO (Adopted June, 1913) Paragraphs marked "a" apply only to back-plastered walls without sheathing. Paragraphs marked "b" apply only to walls with sheathing. All other paragraphs apply to both forms of construction. MATERIALS 1. Cement. The cement shall meet the requirements of the Stand- ard Specifications for Portland Cement of the American Society for Testing Materials, and adopted by this Institute (Standard No. 1). 2. Fine Aggregate shall consist of sand, crushed stone, or gravel screenings, graded from fine to coarse, passing when dry a screen hav- ing ^-in. diameter holes, shall be preferably of siliceous materials, clean, coarse, free from loam, vegetable, or other deleterious matter. 3. Lime. The lime shall be thoroughly hydrated either by the manu- facturer, or the contractor. If hydrated by the contractor, it shall be slaked in sufficient water to make a soft paste and allowed to stand at least one week before being applied to the wall. 4. Hair or Fiber. There shall be used only first quality long cow hair, free from foreign matter, or a long cocoanut fiber well combed out. 5. Coloring Matter. Only mineral colors shall be used, but no coloring matter which is affected by lime, Portland cement, or the elements is permissible. 6. Water shall be clean, free from oil, acid, strong alkalies, or vegetable matter. PREPARATION OF MORTAR 7. Mixing. The ingredients of the mortar shall be thoroughly mixed to a uniform color, sufficient water added to obtain the desired consistency, and the mixing shall continue until the cement and lime are uniformly distributed and the mass is uniform in color and homogeneous. The hair or fiber shall be added during the process of wet mixing. 8. Measuring Proportions. Methods of measurement of the pro- portions of the various ingredients, including the water, shall be used which will secure separate uniform measurements at all times. All proportions stated are by volume. A barrel of cement shall be assumed to contain 4 cu. ft. Lime when used shall be measured in the form of putty. Hydrated lime shall be made into putty before being measured. STUCCO AND PLASTER WORK 135 9. Quantity. There shall not be mixed at one time more mortar than will be used within one hour. Mortar which has begun to stiffen or take on its initial set shall not be used. 10. Hand Mixing. The mixing shall be done on a water-tight plat- form and the materials shall be turned until they are homogeneous in appearance and color. 11. Consistency. The materials shall be mixed so as to provide sufficient water to insure a proper bonding and a dense mortar free from voids. 12. Retempering. Retempering mortar, i.e., remixing with water after it has partially set, shall not be allowed. STRUCTURE 13. Framing. Studs spaced at 12-in. centers wherever possible shall be run from foundation to rafters without any intervening hori- zontal grain in the wood. These studs shall be tied together just below the floor joists by 6-in. boards which will be let into the studs on their inner side, so as to be flush and securely nailed to them. These boards will also act as sills for the floor joists, which in addi- tion will be securely spiked to the side of the studs. 14. Bracing. The frame of the building shall be so rigidly con- structed and braced as to avoid cracking the stucco. (a) * At least once between each two floors, brace between the stud- ding with 2" X 3" bridging. Or (b) ! Bracing may be omitted, as the sheathing boards act as bracing. 15. Sheathing. (a) The lath is to be fastened direct to the stud- ding and back-plastered, and no sheathing boards are to be used. Or (b) Sheathing boards shall be not less than 6 in. or more than 8 in. wide, dressed on one or both sides to a uniform thickness of f in. They shall be laid diagonally across the wall studs and fastened with two nails at each stud. 16. Inside Waterproofing. (a) The faces of the studs and for one inch back of the face on each side where the plaster may come in con- tact with them shall be thoroughly waterproofed with tar or asphalt. Or (b) Over the sheathing boards shall be laid in horizontal layers, beginning at the bottom, a substantial paper well impregnated and thoroughly waterproofed with tar or asphalt. The bottom strip shall lap over the base board at the bottom of the wall, and each strip shall lap the one below at least 2 in. The paper shall lap the flash- ings at all openings. When required, the lower horizontal edge of each 1 See note at the beginning of these specifications. 136 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES strip shall be cemented with hot or liquid tar or asphalt compound, to the strip below and to the grounds of flashings at all openings. All tacking shall be within 2 in. of the top horizontal, edge, where tacks will be covered by the lap of the strip above. 17. Furring. When furring strips form an integral part of the metal lath to be used, then separate furring strips as described in this paragraph are to be omitted. (a) Galvanized or painted ^-in. crimped furring strips, not lighter than 22 gage or other shape giving equal results, shall be fastened direct to the studding, using l^-in. X 14 gage staples, placed 12 in. apart. Or (b) Fasten ^-in. galvanized or painted crimped furring, not lighter than 22 gage or other shape giving equal results over the sheathing paper and directly along the line of the studs, using l|-in. X 14 gage galvanized staples, placed 12 in. apart. The same depth of furring should be adhered to around curved surfaces, and furring strips shall be placed not less than 1^ in. or more than 4 in. on each side of and above and below all openings. 18. Preparation of Original Surface. All roof gutters 'shall be fixed and down-spout hangers and all other fixed supports and fasteners shall be put up before the plastering is done, so there will be no break made in the plastering where they are permanently fixed. Wall copings, balustrade rails, chimney caps, cornices, etc., shall be built of concrete, stone, tile, or metal with ample overhang drip grooves or lip, and water-tight joints, to keep water from behind the plaster. If wood sills are used, they should project well from the face of the plaster and have ample drip groove or lip. Metal lath shall be stopped far enough above the level of the ground to be free from ground moisture. Care should be taken to provide for placing all trim the proper distance from the studding or furring to show its right projection after the plaster is on. 19. Lath. The lath shall be not thinner than 24 gage, galvanized or painted, expanded metal lath weighing not less than 3 Ib. to the square yard, or woven wire lath, galvanized or painted, 19 gage 2| meshes to the inch with stiffeners at 8-in. centers. 20. Application of Lath. Place lath horizontally over the furring strips, driving galvanized staples 1 in. X 14 gage 8 in. apart over the furring strips into the studding. The sheets of lath shall be locked or lapped at least 1 in. and tied at joints between studs both verti- cally and horizontally with 18-gage wire. STUCCO AND PLASTER WORK 137 21. Corners. There shall be 6-in. strips of metal lath bent around the corners and stapled over the lath, or the sheets of metal lath shall be folded around the corners a distance of at least 3 in. and stapled down as applied. Galvanized corner bead may be applied over the lath. 22. Insulation. (a) After the lath on the outside has been back- plastered, the air space may be divided by applying heavy building paper, quilting, felt, or other suitable insulating material between the studs, fastening it to the studs by nailing wood strips over folded ends of the material. This insulation should be so fastened as to clear the bridging, leaving the preponderance of the air space next to the plaster. Care must be taken to keep the insulating material clear of the outside plaster and to make tight joints against the wood fram- ing at the top and bottom of the spaces and against the bridging where the face intercepts. Or (b) When quilting, felt, or other insulating material is to be used it shall be applied to the sheathing boards under the inside water- proofing. 23. Brick, Tile, or Cement Block Surfaces. Existing surfaces to be stuccoed shall have all loose, friable, or soft mortar removed from the joints to a depth of not less than \ in. All dirt, dust, or any other for- eign matter shall be removed by means of a wire brush, stiff broonv or other effective means. 1 In case the surface has been painted, is oily, or otherwise in such condition that the stucco will not firmly adhere, then metal furring and lathing shall first be applied. New Surfaces shall have ample roughness to assure a strong bond and key between the stucco and the surface. The mortar joints shall not be less than f in. thick and the mortar shall be omitted from or raked out of the joints for at least ^ in. back from the face to which the stucco is applied. Before placing the scratch coat the surface shall be brushed clean from all dust, dirt, or loose particles and thor- oughly wetted. MORTAR COATS 24. Plaster. (a) The first coat shall contain not more than two and one-half (2^) parts of sand to one (1) part of Portland cement by volume. If lime putty is added, it shall not be in excess of one-third (l) of the volume of cement. Hair or fiber may be added in sufficient quantity to bond the mortar. 2 1 The wall may be cleaned by washing with a solution of one part muriatic acicT to five parts of water. After this is done it must be thoroughly washed with water before applying the stucco. Author. 2 Use about one pound of good cow hair to each bag of cement. Author. 138 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Or (b) The first coat shall contain not more than two and one-half (2^) parts of sand to one (1) part of Portland cement by volume. If lime putty is added it shall not be in excess of one-third (|) of the vol- ume of cement. No hair, fiber, or similar material of any kind or in any quantity shall be added to the mortar. For second coat, the proportion of sand to cement shall not be greater than 1\ to 1 by volume, nor shall more than \ part of lime putty be added. For third coat, the proportion of sand to cement shall not be less than 2 to 1 nor more than 2| to 1, by volume, nor shall more than | part of lime putty be added. 25. Application. The plastering should be carried on continuously in one general direction, without allowing the plaster to dry at the edge. If it is impossible to work the full width of the wall at one tune, the joint should be at some natural division of the surface, such as a window or door. Metal Lath, (a) The first coat shall be applied to the outside of the lath and pushed through sufficiently to give a good key. Over the face of the studs the plaster shall be forced well through the lath in order to fill entirely the space between the lath and the stud. The backing coat shall be applied to the back of the lath and shall be thor- oughly troweled so that the lath shall be entirely covered. The final coat shall be applied to the face of the first coat. Or (b) The first coat shall be applied to the lath and thoroughly pushed through against the inside waterproofing so as to completely embed the metal of the lath on both sides. Special care shall be taken to fill all voids around furring strips and where laths lap. Brick, Tile, or Cement Block Surfaces. The first coat shall be forci- bly and thoroughly troweled into the depressions of the previously saturated surface so as to make a firm bond. Care shall be taken to insure the complete filling by the mortar of all crevices and pores. The intermediate and final coats shall be applied in order and well troweled on to insure good contact with previous coats. 26. Roughing. Soon after applying and before the initial set has taken place, the surface of the coats which are to receive succeeding coats shall be roughened with a saw-toothed paddle or other suitable device. 27. Dampening. Before applying mortar the surface of the preced- ing coat shall be thoroughly wetted to prevent absorption of water from the fresh mortar. 28. Thickness of Coat, (a) The first coat shall be at least f in. STUCCO AND PLASTER WORK 139 thick over the face of the lath and project through behind the lath about f in. The backing coat shall increase the thickness behind the lath to not less than f in. The final coat shall be not less than f in. thick. Or (b) The first coat shall have a minimum thickness over the lath at any point of not less than | in. The intermediate coat shall have a thickness of not less than | in. or more than f in. The final coat shall have a thickness of j in. when placed over an intermediate coat, or of f in. when placed directly on the scratch coat. 29. Drying Out. The final coat shall not be permitted to dry out rapidly, and adequate precaution shall be taken, either by sprinkling frequently after the mortar has set hard enough to permit it or by hanging wet burlap or other material over the surface. 30. Freezing. Stucco should never be applied when the tempera- ture is below freezing. FINISH 31. Smooth-troweled. The finishing coat shall be troweled smooth with a metal trowel with as little rubbing as possible. 32. Stippled. The finishing coat shall be troweled smooth with a metal trowel with as little rubbing as possible, and then shall be lightly patted with a brush of broom straw to give an even, stippled surface. 33. Sand-floated. The finishing coat, after being brought to a smooth, even surface, shall be rubbed with a circular motion of a wood float with the addition of a little sand to slightly roughen the surface. This floating shall be done when the mortar has partially set. 34. Sand-sprayed. After the finishing coat has been brought to an even surface, it shall be sprayed by means of a wide, long fiber brush a whisk broom does very well dipped into a creamy mix- ture of equal parts of cement and sand, mixed fresh every 30 min- utes and kept well stirred in the bucket by means of the whisk broom or a paddle. This coating shall be thrown forcibly against the surface to be finished. This treatment shall be applied while the finishing coat is still moist and before it has attained its final set, i.e., within 3 to 5 hours. To obtain lighter shades add hydrated lime of 5 to 15 per cent of the volume of the cement. 35. Splatter Dash or Rough Cast. After the finishing coat has been brought to a smooth, even surface and before attaining final set, it shall be uniformly coated with a mixture of one part cement and two parts of sand thrown forcibly against it to produce a rough surface of uniform texture when viewed from a distance of 20 ft. Special care shall be taken to prevent the rapid drying out of this finish. 140 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES 36. Pebble Dash. After the finishing coat has been brought to a smooth, even surface, and before attaining initial set, clean round pebbles or other material as selected, not smaller than \ in. or larger than f in., previously wetted, shall be thrown forcibly against the mortar so as to embed themselves in the fresh mortar. They shall be distributed uniformly over the surface of the final coat and may be pushed back into the mortar with a clean wood trowel, but no rubbing of the surface shall be done after the pebbles are embedded. 37. Exposed Aggregates. The finishing coat shall be composed of an approved, selected coarse sand, marble dust, granite dust, or other special material, in the proportion given for finishing coats, and within 24 hours after being applied and troweled to an even surface, shall be scrubbed with a stiff brush and water. In case the cement is too hard, a solution of one part hydrochloric acid in four parts of water by volume can be used in place of water. After the aggregate particles have been uniformly exposed by scrubbing, particular care shall _be taken to remove all traces of the acid by thorough spraying with a hose. 38. Mortar Colors. When it is required that any of the above fin- ishes shall be made with colored mortar, not more than 6 per cent of the weight of Portland cement shall be added to the mortar in the form of finely ground coloring matter. A predetermined weight of color shall be added dry to each batch of dry fine aggregate before the cement is added. The color and fine aggregate shall be mixed together and then the cement and lime mixed in. The whole shall then be thoroughly mixed dry by shoveling from one pile to another through a |-in. mesh wire screen until the entire batch is of uniform color. Water shall then be added to bring the mortar to a proper plastering consistency. MACHINE STUCCO 39. Stucco may be applied by a machine provided the results ob- tained are equal to those produced by hand work. OVERCOATING During recent years there has come into vogue a method of remod- eling old frame houses. This overcoating, as it is called, is used exten- sively in all sections of the country, and the following practice is recommended : 40. Where a furring strip is used so deep that the space back of the lath is not entirely filled with plaster, some provision must be made for extending the old window and door frames to correspond with the increased thickness of the wall. In some cases the plaster is brought STUCCO AND PLASTER WORK 141 over the old frames in such a manner that a recessed window or door opening is made. In case the furring strips are fastened to the stud- ding, it is not necessary to provide for extending the window and door frames, as the new stucco finish will have the same relations as the old weather-boarding. 41. Preparation of Original Surface. If the weather-boarding is in poor condition it should be removed and furring strips and metal lath applied over the sheathing, to which waterproof paper has previously been fastened. It may be advisable also to tear off the sheathing, in which case the furring strips can be fastened direct to the studding after bracing between the studs. Another method would be to fasten the furring strips direct over the weather-boarding over which the metal lath is applied. In preparation for any of these methods the house should be gone over carefully to determine if the framework is well enough preserved to justify the improvement. The doors should be looked after, the studding inspected, partitions and outside walls lined up and brought into plumb. 42. Furring. Fasten galvanized or painted f-in. crimped furring, or other shape giving equal results, vertically over the original surface, whichever of the above may be adopted. 43. Lathing and Plastering. Follow the above specifications for stucco. CHAPTER XI WATERPROOFING AND COLORING CONCRETE Necessity for Waterproofing. Unless special precautions are taken, concrete is likely to be porous to such an extent that it will permit the percolation of water or the penetration of damp- ness. For certain purposes for which concrete is otherwise admirably adapted, this tendency is a serious drawback, and much thought and effort have been devoted to the development of methods for damp-proofing or waterproofing. In some cases all that is necessary is to reduce the percolation of water so there is no appreciable leakage, as in water tanks and sewers, while in other cases it is necessary to prevent the penetration of dampness, as in residence walls, in certain cases, heavy pressures must be resisted, while in others all that is necessary is to prevent the penetration of moisture by capillary action. In rural structures the pressures will not usually exceed those due to a head of a few feet of water. Precautions to be Observed in Water-tight Work. In any structure that is to be waterproof, it is desirable to pour the whole mass in a continuous run when possible. Unless care is taken to bond the new concrete to the old, joints will occur between successive days' work which will permit seepage. If it is impossible to complete the mass in one run, the precau- tions to be observed in making the joint are as follows: (1) When work is stopped for the day, the surface should be left rough, and all scum should be removed. Often this can best be accomplished by allowing the concrete to set until all surplus water has disappeared and then skimming off the upper layer to the thickness of half an inch, or whatever is necessary to remove all scum. The surface can then be roughened with a trowel or a stick. (2) If the wall is of sufficient size to permit it, when work is 142 WATERPROOFING AND COLORING CONCRETE 143 stopped angular stones may be embedded at intervals, to about half their length, leaving the remainder to project up into the new work. Iron or steel rods or scrap may also be used in this manner. (3) Just before new concrete is deposited, the old surface should be drenched with water and should then be slushed with neat ce- ment, or equal parts of neat cement and sand, mixed with water to a creamy consistency. In case the scum was not all removed before the old concrete had set or if the surface was left smooth, it should be picked over before this slush coat is placed. Methods of Waterproofing. Three general principles are employed in waterproof structures: (1) Reducing the porosity of the concrete. (2) Using a water-repellent substance in the concrete. (3) Using impervious washes or coatings on the surface. One of the best methods of producing a water-tight concrete is to use a rich mixture, with a well graded aggregate. The water-tightness of the concrete increases very rapidly with an increase in the amount of cement used. All structures which are to be water-tight should therefore be made of a rich mixture, with proportions ordinarily about 1:2:4. Proper grading of the stone or gravel, and proportioning of the sand to the coarse aggregate to produce the greatest density (see Chapter IV) are of the greatest importance in this connection. The water tightness of concrete is affected to an even greater degree than is the strength by proper proportioning. Use of Hydrated Lime. Various substances may be added to the materials ordinarily used in making concrete, to reduce its porosity. One of the best of these is hydrated lime. Being in the form of a very finely divided powder, it helps to fill up the voids of the concrete and so prevent the percolation of water. The lime also seems to lubricate the wet concrete so that the particles slide over each other more easily and thus it reduces the number of open pockets. The amount of lime used may be equal to about 10 per cent of the weight of the dry cement for 1:2:4 concrete or 15 per cent for 1 : 2J : 5 or leaner mix- tures. This amount of lime will not reduce the strength, but may increase it slightly in the leaner mixtures. 144 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Use of Water-repellent Compounds. Ordinary concrete has a capillary attraction for water. By means of certain sub- stances it can be made water-repellent, much as a greasy sub- stance is water-repellent. Substances commonly used are oil, alum and soap, and lime and soap. Either of these combina- tions forms an insoluble compound which, when distributed through the mass of concrete, is effective as a waterproofing. In using alum and soap, dissolve one pound of alum in two gallons of water and, separately, 2| pounds of soap in eight gallons of water. The two solutions may then be mixed, being stirred frequently to prevent the compound from accumulating on the surface. This solution should be used instead of water in mixing the concrete. If preferred, the alum may be pulver- ized and mixed dry with the cement, the soap being dissolved in the water. This method of waterproofing decreases the strength of the concrete somewhat, perhaps about 20 per cent. If lime and soap are used, dissolve a quarter of a pound of soap per gallon of water and add half an ounce of unslaked lime. Stir the mixture thoroughly to keep. in suspension the calcium soap which is formed. This solution is to be used instead of water in mixing the materials. If desired, hydrated lime may be mixed dry with the cement instead of with the water, and this may be used with a saturated solution of soap in cold water. A considerable excess of lime will do no harm, for the lime alone is effective waterproofing. Various waterproofing compounds are being sold under trade names. Some of these are in the form of liquids to be added to the water used in mixing, others are powders which are to be mixed with the cement, and still others are sold mixed with cement ready for use. The compositions of these substances are trade secrets, but calcium soap, such as is formed by the lime-soap process described above, seems to be the essential waterproofing element in several of them. Some of them are good, but others soon lose their effectiveness. Tests made by the Office of Public Roads of the U. S. De- partment of Agriculture l indicate that the addition of a mineral 1 U. S. Dept. Agr. Bulletin No. 230, " Oil-mixed Portland Cement Concrete." WATERPROOFING AND COLORING CONCRETE 145 oil to concrete or mortar is very effective in rendering it damp- proof and waterproof under small pressures, and that the amount of oil necessary does not seriously decrease the strength of the concrete. For most purposes where damp-proofing is required, an amount of oil equal to 5 per cent of the weight of the cement is sufficient. After the cement and sand are mixed dry, water is added and the mortar is mixed to a uniform mushy consis- tency. The oil is then added and the mass is remixed until no trace of the oil is visible on the surface of the mortar. The previously wetted stone or gravel is then added and the mass is again thoroughly mixed. Specifications for the oil, as given by the Office of Public Roads, are given below. The purpose of the specifications is to eliminate certain oils which would be injurious to the concrete. SPECIFICATIONS FOR OIL TO BE USED IN OIL-CEMENT CONCRETE (1) The oil shall be a fluid petroleum product and shall contain no admixture of fatty or vegetable oils. (2) It shall have a specific gravity not greater than 0.945 at a temperature of 25 C. (3) It shall show a flash point of not less than 150 C. by the closed-cup method. (4) When 240 c.c. of the oil is heated in an Engler viscosimeter to 50 C., and maintained at that temperature for at least three minutes, the first 100 c.c. which flows out shall show a specific viscosity of not less than 15 nor more than .30. (5) When 1 part of the oil is shaken up with 2 parts of hundredth normal caustic soda, there shall be no emulsification, and upon allowing the mixture to remain quiet the two components shall rapidly separate in distinct layers. Surface Coatings. The above methods of waterproofing are applicable only while the concrete is being mixed, and hence cannot be used with existing structures. Methods used to render such structures waterproof consist of surface applica- tions to prevent the penetration of water, either by rendering the surface of the concrete impervious and water-repellent, or 146 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES by applying an impervious layer to the surfaces. In any case all projections should be removed and all open pockets filled before the surface coating is applied. Methods commonly used are as follows: (1) Cement Wash. The methods of mixing and applying the wash are explained in detail in Chapter IX. Before the first coat is dry, a second coat should be applied, and this should be kept moist for several days. This method gives a pleasing surface finish to the con- crete at the same time that it waterproofs it, but the wash is likely to craze-crack if it is exposed to the action of the sun and rain, espe- cially during the first few weeks after it is applied. The wash is very satisfactory for water tanks, silos, sewer tile, and all similar structures. It is most effective if applied on the side of the concrete exposed to the water. (2) Sylvester's Wash. Sylvester's wash has long been used for waterproofing brick work and concrete which has hardened and dried out. It consists in the alternate applications of alum and of soap solutions to the face of the wall. The alum solution is made by dis- solving eight ounces of alum per gallon of water, and the soap solution by dissolving one and one-half pounds of hard soap per gallon of water. The surface should be clean and dry so that the solutions will be readily absorbed. The air temperature should not be less than 50 F. The soap solution should be applied boiling hot, while the alum solution should be about 70 F. A coat of the soap solution is first applied, using a whitewash or other convenient brush and rubbing it well into the surface but taking care not to produce a froth. This is left for 24 hours or until the surface is entirely dry. A coat of the alum solution is then applied and allowed to dry for another 24 hours. This is followed with another coat of soap and another of alum at simi- lar intervals. Two pairs of coats should be sufficient for any ordinary case, though additional ones may be applied if required. The effect of this treatment is to form an insoluble compound of calcium soap in the outer pores of the concrete, this soap filling the pores and acting as a water-repellent. It is one of the most effective treatments which can be given a concrete surface. (3) Paraffine Coating. A paraffine coating may be applied either hot or cold. In either case the surface should be thoroughly clean and dry. If the coating is applied hot, the walls to be treated must first be heated. The melted paraffine wax is then thoroughly rubbed in. In the cold process, the paraffine is dissolved in a volatile liquid, WATERPROOFING AND COLORING CONCRETE 147 such as naphtha, and applied with a brush. The volatile liquid evapo- rates and leaves the surface appearance the same as before, but with the outer pores filled with paraffine. At least two coats should be given. Sometimes asphalt dissolved in naphtha is used in the same manner as the paraffine. The materials may be bought ready pre- pared for use under various trade names. (4) Sodium Silicate Coating. Sodium silicate, or water glass, which can usually be purchased at drug stores, may be applied to the surface of concrete with a brush, to waterproof it. Two coats are usually suffi- cient. The treatment is more expensive than the soap and alum treat- ment and is no more effective. (5) Plaster Coat. A plaster coat of rich mortar, with or without waterproof compounds, may be used to render a concrete wall water- proof. It is recommended that a mixture of one part of cement, two parts of sand, and one-fourth part of hydrated lime be used. For details of the method of application, see Chapter IX. Bituminous Shield. An impervious shield may be placed around the concrete so as to prevent the entrance of water. It usually consists of several layers of tarred paper or felt, cemented together and covered with tar or asphalt. This shield is then usually protected by another layer of concrete or a brick wall. The method, while highly effective, is expensive and not gen- erally available for work of the character considered in this book. COLORING CONCRETE Materials commonly used for coloring concrete are: (1) Colored aggregates (2) Mineral Pigments (3) Paints or washes applied to the surface Colored Aggregates. The use of colored aggregates is one of the most effective and satisfactory means of coloring con- crete, as the colors will be permanent, and will not appear forced. If sand and gravel, or broken stone and screenings, of the color desired can be obtained, and the surface of the con- crete is etched with acid or is tooled (see Chapter IX) the result will be very effective. When a rich facing is used, the colored aggregate is required in the facing, only. Light gray concrete can be made by the use of white Port- 148 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES land cement, mixed either with white sand, or with crushed quartz, marble, or gray limestone screenings. Hy'drated lime, to the amount of one-fourth of the weight of cement used, will lighten the color considerably. Mineral Pigments. Only pure mineral pigments should be used, as others are likely to be injurious or not to be perma- nent. Different . tints can be produced by the use of different proportions of the same pigment, so that care should be taken to measure up each batch accurately. Colors will usually, be considerably darker while the concrete is wet than after it dries out, and the colors are likely to grow somewhat lighter with age. Hence considerably more pigment should be used than is necessary to bring the wet concrete or mortar to the desired shade. White Portland cement used with the colors will make it possible to get some tints not otherwise obtainable. The following table may be used as a rough guide to the materials and proportions required: TABLE IX MATERIALS USED FOR COLORING MORTARS Color of hardened mortar Mineral Pounds of color to each bag of cement Gray Germantown lamp black i Black Manganese dioxide 12 Black Excelsior carbon black 3 Blue Green Red Bright red Brown Buff Ultramarine blue Ultramarine green Iron oxide Pompeian or English red Roasted iron oxide or brown ochre Yellow ochre 5 6 6 6 6 6 Ordinary lamp black should not be used, as it is likely to run and fade. The blue and green colors will gradually fade out. The others should be practically permanent if high-grade colors are obtained. Lighter tints may be obtained by using smaller amounts of the minerals. The colors should be mixed dry with the cement before the aggregate is added. The color- WATERPROOFING AND COLORING CONCRETE 149 ing materials, in the quantities designated, will not seriously re- duce the strength of the concrete. Painting Concrete Surfaces. It is not generally desirable to paint concrete surfaces, as other methods of finishing give better and more permanent results. Ordinary oil paints can be used successfully, if the concrete is allowed to become thor- oughly dry and is given a priming coat of a solution of eight pounds of zinc sulphate per gallon of water. Various prepared paints which do not use oil as a base, and which serve also as waterproof coatings, can be purchased. A wide range of colors is available. CHAPTER XII CASTING IN MOLDS SEPARATELY molded concrete units are widely used in the forms of building bricks and blocks, including sills, lintels, cor- nice blocks, columns, and other building elements; of fence and other posts; and of hollow tile for drains and sewers. When they are properly made and used, cast concrete speci- mens are entirely satisfactory for the above purposes, and have much to recommend them. Many unsatisfactory blocks, posts, and tile have been made and sold, either from ignorance of proper methods of manufacture, or from a desire to cheapen the product. The unsuccessful attempts to imitate stone through the use of the so-called "rock face" and of artificial coloring have helped to give to concrete blocks the impression of cheapness, and to bring them into disrepute. As proper methods of manufacture and use become more generally em- ployed, there will undoubtedly be a great growth in the use of these materials. Building Blocks. The advantages claimed for building blocks are as follows: 1. They are often much cheaper in first cost than brick or stone. 2. They can be manufactured near the building site, thus saving transportation charges. 3. They can be more cheaply laid into the wall than brick, because of the larger size of the blocks. 4. On account of fewer joints than in brick work, a considerable saving in mortar results. 5. The air spaces in the walls make the building cool in summer and warm in winter. 6. The air spaces help to prevent water from soaking through the wall so that with well-made blocks the plastering may be done directly on the inner face of the wall, without furring. 150 CASTING IN MOLDS 151 7. The air spaces permit pipes and wires to be concealed in the walls. 8. The construction is substantial and fire-proof. Processes of Manufacture. Concrete blocks may be made by either the wet process or the dry process. In the wet process enough water is added to the mixture to make a mushy consistency, similar to that used in ordinary poured concrete work. The blocks must be left in the molds until they harden, and hence many molds are required. For this reason this method is little used, notwithstanding the fact that it gives better blocks than the dry process when the same proportions are used. In the dry process only enough water is added to make the concrete of the consistency of damp earth. This is heavily rammed or compressed into the mold, which can then be removed immediately, the concrete retaining its shape. By this means but one mold is required to carry on the work continuously. As much water should be used as is possible without causing the block to fail when the mold is removed. Otherwise the blocks will be weak and porous. Block Machines. Many concrete block machines are on the market designed to facilitate (1) the handling of the blocks, (2) the use of cores for making the blocks hollow, and (3) the use of a rich mortar for the faces of the blocks. They may be classified as: (1) Vertical face, or upright, machines, (2) Face-down machines, (3) Machines for making two-piece blocks, (4) Machines for use with wet concrete. The first three types use the dry process. In the first, the mold consists of a wooden or metal rectangular frame, so hinged and fastened that the block can readily be released after it is made. Upright cores are used for making the blocks hollow. If it is desired to use a richer face than body, or one of a dif- ferent color, a vertical parting plate is used. This is inconven- ient, and care must be taken to get a satisfactory bond between the face and the body of the block. For this reason these 152 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES FIG. 54. Vertical Face Mold for Column Blocks. machines are much less satisfactory for faced blocks than are the face-down machines. Wooden or cast-iron bottom plates are used, some machines being designed to be lifted bodily from the block and others being stationary so that the blocks are lifted from the machine on their bottom plates. (See Fig. 54.) In the second class, or face-down machines, the face plate forms the bot- tom of the mold, the bot- tom board forms one side, and the cores are horizon- tal. If the blocks are to have a special facing ma- terial, this is thrown into the mold first and leveled off. The body mixture is added to the height of the core, and tamped. The core is then put into place, and the filling and the tamping are continued. The machine is then rolled over so that the bot- tom plate is below the block, the cores and sides of the mold are released, and the block is removed on its bottom plate. Machines of this type are very widely used. Figure 55 shows one of them. Machines of the third class are built to manufacture blocks nearly automatically, the shape of the blocks being such that they can be compacted by pressing instead of by tamping. The machines are large and complicated and are designed for a large output. A wall of two-piece blocks is shown in Fig. 56. The mixture is shoveled or dropped into the mold, pressure is applied, and the block is at once released. If facing is desired, this is applied in the top of the mold before pressing. The pressure being applied directly to the face, the latter is made very dense and hard and a good bond is obtained with the backing. In the fourth class of machines, a large number of wooden or metal forms and cores are provided of the shape and size de- sired for the blocks, and these are filled with a mushy concrete. After twelve to twenty-four hours, the side forms and the cores CASTING IN MOLDS 153 FIG. 55. Face-down Block Machine. can be removed and again used. The blocks are likely to be less porous and stronger than those made by the dry process. Kinds of Blocks. A large variety of shapes and sizes is used for the blocks, and for the cored spaces. Common outside dimensions are 12, 16, 20, or 24 inches long, 4, 8, or 12 inches high, and 8, 10, or 12 inches thick. The larger sizes are rather heavy, so that the 16-inch blocks 8 inches high are being much used. The use of various sizes in the same wall avoids the monoto- nous effect which sometimes FlG . 56 . _ Wa n of Two .p ieC e Concrete results when all are of the Blocks. 154 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES same size. (See Fig. 57.) Curved blocks are made for use in silos and other round structures. FIG. 57. Broken Ashlar Effect from Concrete Blocks. The arrangement of cores should be such as to give the block sufficient strength and still make it light, and prevent the pas- sage of moisture to the inside of the wall. Staggered air spaces are very effective in preventing the passage of moisture, but the blocks are likely to be weaker than those with webs extend- ing through the block. Waterproofing and Coloring of Blocks. The same methods can be used for waterproofing and coloring blocks as for other forms of concrete. These are discussed elsewhere in this book. An impervious partition may be formed in the blocks made by the face-down machine by placing a thin layer of rich mortar in each of the web spaces after the cores have been put into place. In this way, but little of the rich mixture is required, and moisture is effectively prevented from passing through the wall. Curing of Blocks. Since blocks are usually made with but little water, it is especially important that they should not be exposed to wind or sun and that they should be frequently sprinkled. The first sprinkling should be given as soon as is possible without washing out the cement, and the blocks should be kept moist for at least a week. They should be left on their bottom boards for forty-eight hours or until they can be handled without injury, and should be allowed to cure for about a month before being built into walls. Some manufacturers cure their blocks in a closed room which is kept moist and heated to a CASTING IN MOLDS 155 high temperature by the use of steam. Blocks cured in this manner will become as strong in twenty-four hours as they would in several days at ordinary temperatures. Cost of Concrete Blocks. Concrete block walls will, where conditions are favorable, be considerably cheaper than either brick or stone. For one hundred blocks 8" X 8" X 16", of 1: 3: 4 concrete, with J inch of 1 : 2 facing, allowing J of the volume of the blocks for cores, the cost may be estimated as follows: 8| bags cement @ $0.40 = $3.40 I cu. yds. sand @ $0.75 = 0.65 1 cu. yd. gravel @ $1.25 = 1.25 Total cost of materials = $5.30 Labor, 2 men, 6 hrs. @ 20^ per hour = $2.40 Total cost of 100 blocks at factory - $7.70 Allowing 50 per cent for profit and incidental expenses, the blocks could be sold for about 12 cents each at the factory. Allowing 3 cents each for hauling and 7 cents for laying will make the total cost per block in the wall 22 cents, or about 25 cents per square foot of wall face. The prices given above will vary in different localities and at different times, but may be taken as a fair average. The following specifications may be used as a guide to the construction and use of first-class concrete building blocks, suitable for any walls where brick or stone might be used. RECOMMENDED PRACTICE FOR CONCRETE ARCHITEC- TURAL STONE, BUILDING BLOCKS, AND BRICKS (Adopted by National Association of Cement Users, May, 1912.) GENERAL 1. This Recommended Practice is intended to cover the general requirements for the manufacture and testing of concrete architec- tural stone, building blocks, and brick. MATERIALS 2. Cement. The cement shall meet the requirements of the Stand- ard Specifications for Portland cement of the American Society for Testing Materials, and adopted by this Association. (Standard No. 1.) 3. Aggregates. The aggregates shall be clean, coarse, hard, dur- able materials, and shall be free from dust, soft, flat or elongated par- 156 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES tides, loam, vegetable or other deleterious matter. In no case shall aggregate containing frost or lumps of frozen material be used. (a) Fine Aggregate. The fine aggregate shall consist of sand, crushed stone or gravel screenings, preferably of siliceous material, graded from fine to coarse, and passing, when dry, a screen having one-quarter () fnch diameter holes; not more than twenty (20) per cent shall pass a sieve having fifty (50) meshes per linear inch, and not more than six (6) per cent pass a sieve having one hundred (100) meshes per linear inch. Fine aggregate shall be of such quality that mortar composed of one (1) part Portland cement and three (3) parts fine aggregate by weight when made into briquettes will show a tensile strength at least equal to the strength of 1:3 mortar of the same consistency made with the same cement and Standard Ottawa sand. (b) Coarse Aggregate. The coarse aggregate shall consist of gravel, crushed stone, or other suitable material graded in size, which is re- tained on a screen having one-quarter (j) inch diameter holes. In.no case shall the maximum dimension be greater than one-half the mini- mum width of any section of the finished product. 4. Coloring Matter. Where color is required, only the most per- manent and durable mineral colors shall be used and shall be consid- ered as aggregate in measuring proportions. 5. Water. The water shall be clean, free from oil, acid, strong alkalies or vegetable matter. PROPORTIONS 6. Proportions. The proportions of cement to aggregate shall be such as require at least the minimum amount of cement to produce the strength and density specified in the Standard Specifications for Concrete Architectural Stone, Building Blocks, and Bricks. 1 The pro- portions of the various sizes of aggregates and cement to aggregates shall preferably be made by weight. If by volume, a bag of Portland cement shall be considered one (1) cubic foot. 7. Measuring Proportions. Methods of measurement of the pro- portions of the various ingredients shall be used which will secure uniform measurements at all times. MIXING 8. Mixing. The ingredients of concrete shall be thoroughly mixed dry, sufficient water added to obtain the desired consistency, and the mixing shall continue until the cement is uniformly distributed and 1 Proportions of 1: 2: 4, or 1: 3: 5 may generally be used for the body, and 1 : 2 for the face. AUTHOR. CASTING IN MOLDS 157 the mass is uniform in color and homogeneous. The mixing shall preferably be done with a machine mixer of a type which insures the proper mixing of the materials throughout the mass. 9. Consistency. (a) Wet Process. The concrete must have at least a sufficient amount of water to make it so soft that it must be handled quickly to prevent it running off the shovel, but not so thin as to cause segregation of the materials. (b) Semi-wet Process. The material shall be mixed with a maximum amount of water permissible, and must have sufficient water so that the mixture will hold its form after being compressed in the hand. 10. Retempering. Retempering, that is remixing mortar or concrete partially hardened with additional water, or using mortar or concrete forty minutes after being mixed, shall not be permitted. REINFORCEMENT 11. Reinforcement. All lintels, bearing stones, and other members subjected to cross bending shall be reinforced by means of rods placed about one and one-half inches from their tension surface, and the total sectional area for the reinforcement shall not be less than 0.8 of 1 per cent of the cross-sectional area of the concrete in the member reinforced. When any member exceeds in any dimension eight times its least dimension, it shall be reinforced to insure safety in handling. CURING 12. Natural Curing. The concrete products shall be protected from the sun and strong currents of air for a period of at least 7 days. During this period they shall be sprinkled at such intervals as is nec- essary to prevent drying, and maintained at a temperature of not less than 50 F. Such other precautions shall be taken as to enable the hardening to take place under the most favorable conditions. After 7 days the products may be removed to the yard, but in no case used before they are 21 days old. 13. Steam Curing. The products shall be removed from the molds as soon as conditions will permit and shall be placed in a steam-curing chamber containing an atmosphere of steam saturated with moisture for a period of at least 48 hours. The curing chamber shall be main- tained at a temperature between 100 and 130 F. The products shall then be removed and stored for at least 8 days. (This does not apply to high pressure steam curing.) FINISHING, MARKING, AND HANDLING 14. Finish. Concrete products may have exposed surfaces treated by any of the various methods proposed by this Association in the 158 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Report on Treatment of Concrete Surfaces. 1 All surfaces and arrises of stone must be true and without imperfections. 15. Marking. All concrete products of full standard size shall be marked for purpose of identification, showing name of manufacturer or brand, and date (day, month, and year) made. 16. Handling. All concrete products shall be handled with utmost care. When transported and subjected to rough handling they shall be crated and packed in non-staining material in such a way as to insure no damage from chipping or abrasion. All large and heavy stone shall be provided with hooks for lifting. When necessary, stone shall be provided with metal bonds for the purpose of tying to^ the masonry backing. PROPOSED STANDARD BUILDING REGULATIONS FOR THE USE OF CONCRETE ARCHITECTURAL STONE, BUILDING BLOCKS, AND BRICK (Adopted by National Association of Cement Users, May, 1912.) I. GENERAL 1. Class of Buildings. Concrete Architectural Stone, Building Blocks, and Brick meeting the requirements set forth in the Standard speci- fications and Standard Recommended Practice may be used hi build- ing construction, subject to the usual form of approval required of other materials of construction by the Bureau of Building Inspection. 2. Height of Buildings. The height of buildings constructed of concrete building products shall be limited by the requirements in these regulations. II. DETAILS OF CONSTRUCTION 3. Thickness of Walls. (a) Bearing Walls, 25ft. Span. The thickness of bearing walls in such buildings as garages, stables, office buildings, hotels, tenements, boarding and lodging houses, and residences shall be as given in the table below, for buildings in which the maximum dis- tance between bearing walls or columns does not exceed 25 feet. Thickness of wall in inches No. of stories Basement 1st story 2d story 3d story 4th story 1 8 8 2 10 8 8 3 12 10 8 8 4 16 12 10 8 8 1 See Chapter IX for methods of finishing surfaces. CASTING IN MOLDS 159 160 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES (6) Bearing Walls, More than 25ft. Span. In buildings not covered by the above, the thicknesses of the bearing walls shall be determined according to the limit of loading specified in paragraph 6. In no case, however, shall any outside bearing wall be less than eight (8) inches thick. (c) Party Walls. Hollow concrete blocks used in the construction of party walls shall be filled solid with concrete placed on the job. (d) Curtain or Partition Walls. For curtain walls or partition walls, the requirements shall be the same as in the use of hollow tile, terra cotta, or plaster blocks. 4. Walls, Laying, etc. (a) Bonding. Where the face only is of hollow cement block, and the backing is of brick, the facing of hollow block must be strongly bonded to the brick, either with headers pro- jecting four (4) inches into the brick work, every fourth course being a header course, or with approved ties, no brick backing to be less than eight (8) inches thick. Where the walls are made entirely of con- crete blocks, but where said blocks have not the same width as the wall, every fifth course shall extend through the wall, forming a secure bond, when not otherwise sufficiently bonded. (b) Portland Cement Mortar shall be made of Portland cement and sand in the proportions of one (1) part cement and not more than two and one-half (2) parts sand, and shall be used immediately after being mixed. (c) Portland Cement and Lime Mortar shall be made of Portland cement and sand in the proportions of one (1) part cement, not more than two and one-half (2) parts sand and not more than one-quarter (I) part hydrated or thoroughly slaked lime. 5. Girders or Joists. Wherever girders or joists rest upon walls so that there is a concentrated load on the block of over two (2) tons, the blocks supporting the girder or joists must be made solid for at least eight (8) inches from the inside face. Wliere such concentrated load exceeds five (5) tons, the blocks for at least three courses below, and for a distance extending at least eighteen (18) inches each side of each girder shall be made solid for at least eight (8) inches from the inside face. Wherever walls are decreased in thickness, the top course of the thicker wall shall afford a full solid bearing for the webs and walls of the course of blocks above. 6. Limit of Loading. No wall composed of hollow concrete block when laid up in a Portland cement and lime mortar shall be loaded at any point to an excess of 167 Ib. per sq. in., equivalent to twelve (12) tons per square foot of the superficial areas of such blocks as used in CASTING IN MOLDS 161 the wall, including the weight of the wall. When the blocks are laid up in a Portland cement mortar, this limit of loading may be increased to 200 Ib. per sq. in. In buildings where most of the floor loads, etc., are carried by pilasters, said pilasters may be made of hollow concrete building blocks and the air spaces filled in solid with slush concrete placed on the job. Such pilasters shall not be loaded to exceed 300 Ib. per sq. in. of gross cross-sectional area. FIG. 59. A Round Concrete Block Barn. Note the Silo in the Center. 7. Strength of Blocks. No blocks shall be used in bearing walls that have a crushing strength of less than 1000 Ib. per sq. in. of gross cross- sectional area at the age of 28 days. 8. Hollow Space. The hollow space in building blocks used in bear- ing walls shall not exceed 33 per cent except where blocks containing a greater percentage shall be proved by actual tests to meet all the test requirements herein specified to the satisfaction of the Bureau of Building Inspection. 162 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES PROPOSED STANDARD SPECIFICATIONS FOR CONCRETE ARCHITECTURAL STONE, BUILDING BLOCKS, AND BRICK , (Adopted by National Association of Cement Users, May, 1912.) I. TEST REQUIREMENTS 1. Concrete architectural stone, building blocks, and brick must be subjected to compression and absorption tests. The test samples must represent the ordinary commercial product of the regular size and shape used in construction 2. Compression. (a) Solid Concrete Stone, Block, and Brick. In the case of solid concrete stone, block, and brick the ultimate compressive strength at 28 days must average 1500 Ib. per sq. in. of gross cross-sectional area of the stone as used in the wall, and must not fall below 1050 Ib. per sq. in. in any case. (6) Hollow and Two-Piece Building Blocks. The ultimate com- pressive strength of hollow and two-piece building blocks at 28 days must average 1000 Ib. per sq. in. of gross cross-sectional area of the block as used in the wall, and must not fall below 700 Ib. per sq. in. in any case. (c) Area of Hollow Blocks. In the case of hollow building blocks the gross cross-sectional area shall be considered as the actual wall area including the block and air space displaced by the block. (d) Area of Two-Piece Blocks. In the case of two-piece blocks the blocks shall be tested in pairs consisting of the front and rear blocks as used in the wall. The compressive strength shall be regarded as the sum total sustained by the two blocks divided by the product of the length of the blocks and the width of the wall. 3. Absorption. The percentage of absorption at 28 days (being the weight of the water absorbed, divided by the weight of the dry sample) must not exceed five (5) per cent when tested, as hereinafter specified. II. STANDARD METHODS OF TESTING 4. General. (a) Laboratory. All tests required for approval shall be made in some laboratory of recognized standing. (b) Samples. For the purpose of the tests at least nine samples or test pieces must be provided. Such samples must represent the ordi- nary commercial product, and shall be selected from stock. In cases where the material is made and used in special shapes or forms too large for testing in the ordinary machines, smaller size specimens shall be used as may be directed. CASTING IN MOLDS 163 (c) Tests. Tests shall be made in series of at least three. The re- maining samples are kept in reserve in case duplicate or confirmatory tests be required. All samples must be marked for identification and comparison. 5. Compression Tests. The compression tests shall be made as follows : (a) Solid Concrete Stone, Block, and Brick. When testing solid con- crete stone, block, and brick, the net area shall be considered as the minimum area in compression. (6) Hollow and Two-piece Building Blocks. Whenever possible such tests shall be made on full-size blocks. When such tests must be made on portions of blocks, both pieces of the block must be tested and both must contain at least one full web section. The samples must be care- fully measured, then bedded flatwise in plaster of Paris to secure uni- form bearing in the testing machine and crushed. The net area shall be regarded as the smallest bearing area of the piece being tested. The total compressive strength shall be divided by the net area to obtain the compressive strength in Ib. per sq. in. of net area of each piece. The sum of the two results shall then be averaged to obtain the average strength in Ib. per sq. in. of the net area of the total block. The entire block shall be carefully measured to determine the maxi- mum air space prior to breaking the block for the compressing tests, and the net compressive strength obtained shall then be reduced to compressive strength in Ib. per sq. in. of gross area, this being figured from the actual air space of the block determined above. 6. Absorption Tests. The sample is first thoroughly dried to a con- stant weight at not to exceed 212 F. and the weight carefully re- corded. When dried, the sample is to be immersed in a pan or tray of water to a depth of 2 in., resting on two strips not over 1 in. in width to allow the water to have free access to face. At the end of 48 hours from the time it is placed in water, the sample shall be re- moved, the surface water wiped off, and the sample carefully weighed. Concrete Brick. Many machines are now on the market for manufacturing concrete brick to be used in place of the ordi- nary clay brick. The usual size is about 2J X 3J X 8| inches, or about the same as clay brick. The methods of manufacture and treatment are much the same as in the case of concrete blocks, though the compacting is usually done by pressure and no cores are used. Figure 60 shows a small brick machine. 164 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES Some of the advantages claimed for these brick as compared with clay brick are: FIG. 60. A Concrete Brick Machine. 1. They are true and uniform in size, thus comparing favorably with pressed-clay bricks. 2. They are cheaper than pressed-clay brick and are even cheaper than common brick in many localities. 3. They are more durable. 4. They are more pleasing in appearance than common brick. 5. They are laid better and more cheaply on account of their uniformity in size. 6. They can easily be made of any color or design. 7. They absorb less moisture. 8. They resist frost better. Building Trim. Window and door sills and lintels of con- crete are suitable for use with brick, stone, or concrete buildings. They are very cheaply and easily made, and are satisfactory in use. Smooth-faced blocks are preferable for this use, and these may readily be made on the site of the work, or even in place in the wall, simple board forms being used for the pur- pose. Sills and caps should be reinforced with steel rods, these being placed near the upper surface for the sills and near the lower surface for the caps. Reinforcing to the amount of about 1 per cent of the cross-section of the block should be used. For example, suppose a concrete window cap 8" thick and 12" high is to be built. The amount of reinforcing to be used is CASTING IN MOLDS 165 about .01 X 8 X 12 = 0.96 sq. inches. Two f-inch round rods placed 1J inches above the bottom would answer. FIG. 61. A Concrete Block House with Concrete Trim. Many forms of concrete ornaments are made for use in build- ings and elsewhere, including columns with their bases and capi- tals, balusters, railings, cornices, lattice work, vases, fountains, and statuary. Molds for ornamental casting may be made from wood, cast iron, sand, glue, plaster of Paris, or any other substance which will hold its shape and resist the pressure of the semifluid concrete. In much of the higher grade of ornamental work, white Port- land cement and white sand or marble dust are used. Drain Tile. The concrete pipe industry has developed to large proportions in this coun- nnv i f FIG. 62. Hand Mold for Making Con- try . The sizes used run from crete Tile with Bell and Spigot Ends a few inches to many feet in diameter. Pipes larger than about three feet in diameter are 166 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES usually cast in place, while the smaller sizes are cast separately in units similar to the clay tile in common use. All but the smaller sizes of tile are usually reinforced, the saving due to the decreased thickness of walls being greater than the cost of the steel. FIG. 63. Power Concrete Tile Machine. Among the advantages of concrete pipe over clay pipe is the fact that it can be manufactured almost anywhere and the freight charges saved, so that it is cheaper, as well as stronger and truer, than clay pipe. Some concrete pipe appears to have been injured by the alkali in the soils of some western states, but it has been demonstrated that if the pipe is made imper- vious to water, the trouble from this source is greatly reduced. CASTING IN MOLDS 167 Very simple forms may be used for the manufacture of tile, consisting essentially of outer and inner collapsible cylinders be- tween which the concrete is rammed. A semi-dry mixture like that used for concrete blocks is used, so that immediately after the compacting, the mold may be removed and used again. FIG. 64. Curing Yard for Concrete Tile. When large quantities of tile are desired, power-driven ma- chines are used, in which the operations are automatic, all that is necessary being to supply the machine with concrete and to remove the finished tile. The same care should be taken in the curing of tile as of concrete blocks, keeping it for a week in the curing shed, where it is protected from the sun and wind, and moistening it by sprinkling. It should not be laid till it is a month or more old. The following data on concrete tile are given by the Besser Manufacturing Company. (See table on the following page.) Reinforced Concrete Pipe. Several systems of separately cast reinforced concrete pipe have been developed for use where 168 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES TABLE X CONCRETE TILE DATA AND COSTS "8 J s o g o o >> o" h -2 T5 1, ^"S J a sj | . 5* 1 iS a 6 J *o JJ i ^n -"S is Islt Is $4 a fl -Is Is i !* 5 s!! - I P P g S 3 a n -.2-9 II C^jH II lli "0*0 ll Ill Itil a ll II 6 1J 10 $0.03 60 $0.02 90 $0.06 $0.11 $0.06 $0.15 8 11 6* .06 45 .02 80 .07 .15 .08 .20 10 If 5 .09 32 .03 70 .08 .20 .10 .30 12 3 .12 25 .04 65 .09 .25 .13 .36 15 if 2! .18 19 .05 57 .10 .33 .17 .55 18 If IT 9 7 .21 16 .06 50 .11 .38 .19 .80 20 2 lj .24 14 .07 45 .12 .43 .22 1.00 24 2| J^ .34 9 .11 40 .14 .59 .30 1.50 30 2 .45 7 .14 35 .16 .75 .38 2.00 36 3 .60 6 .18 30 .18 .96 .48 2.50 42 3 i .75 5 .20 17 .35 1.30 .65 48 4 i 1.15 3^ .30 10 .65 2.10 1.05 the loads are too heavy, or the diameters too large, for plain concrete to be used economically. In these the reinforcement FIG. 65. Reinforced Concrete Pipe, Meriwether System. is allowed to project from both ends of the pipe, so that when the latter is laid in the trench, the reinforcement of the abutting CASTING IN MOLDS 169 t * l ends overlaps. The joint is then slushed up with a rich mortar, which seals it and ties the ends together. The pipe, when laid, thus becomes practically one continuous piece. Woven wire or expanded metal fabric is usually used for reinforcing. Concrete Fence Posts. Concrete is being used to a considerable extent to replace wood for fence posts, hitching posts, and power transmission line poles. When properly made these are satisfactory, though some complaint has been made against them on account of the failure of poorly made posts. For best results, the posts must be made of the proper size, from good con- crete and with sufficient rein- forcing. Well-made concrete posts are permanent, and will justify a somewhat greater ex- pense than temporary wooden or metal posts, though in cer- tain places concrete posts can be made as cheaply as first- class wooden posts can be obtained. Molds for fence posts are made of cast-iron, galvanized sheet steel, or wood. Several patented forms made of cast iron or sheet steel are on the market, and, if one is going FIG. 66. Power Transmission Line into the business of manufac- turing posts on a large scale, the purchase of such forms is well worth while. Wooden forms can be made very cheaply, however, 170 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES and will answer every purpose for the farmer who wishes to make only his own posts and a few for his neighbors. The sizes for posts will depend somewhat on the purpose for which they are to be used. The shapes commonly made are rectangular, triangular, or combined rectangular and half-round. The posts usually taper from the bottom to the top. This is as it should be, for the place at which there is the greatest tendency to break off is at or near the ground. Rectangular fence posts for line use may be made five inches by six inches at the butt and three inches by four inches at the top. For ordinary use they may be made about seven feet long. A form which can readily be made by anyone is shown in Fig. 67. This will accommodate six posts. Before the forms FIG. 67. Wooden Forms for Concrete Fence Posts. are used, they should be given a coating of crude oil or soft soap to prevent adhesion of the concrete, and this coating should be repeated as often as necessary during the use of the molds. The broken stone or gravel should be of small size, not larger than J to J inch. A 1:2:4 mixture will usually be satis- factory. The concrete should be mixed to a mushy or semi- liquid consistency, so that it can be compacted by joggling it with a shovel. The tamping which would be necessary with a dryer mix would be likely to spring the forms and displace the reinforcing. CASTING IN MOLDS 171 The reinforcement should consist of steel rods about J inch in diameter. One of these rods should be placed near each corner, about J inch from each side. A high carbon steel may well be used for reinforcing fence posts, and that made by re-rolling rails should be entirely suit- able. This can be had from steel supply houses very cheaply and will make stronger posts at less cost than the softer steel. FIG. 68. Curing Concrete Fence Posts. Many posts have been made with small wires, either barbed or smooth, as reinforcement, and a good deal of the trouble with broken posts can be traced to this cause. Wire is all right for reinforcing posts, provided enough of it is used. In the judg- ment of the author, nothing smaller than No. 6 wire or two strands of No. 10 wire twisted together should be used. If the wire is bought cut to length and bundled up straight, like baling wire, it will be found much more convenient to use than when it is in coils. Plain wire should be used, as the galvanized is more expensive and no better. The directions given for curing blocks apply equally to fence posts. The posts should not be used for about a month after pouring, and care should be taken in handling them while they are green, not to subject them to shocks or to bending stresses. 172 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES They should be placed on end after they are removed from the molds. Corner and Gate Posts. Corner and gate posts must be made considerably heavier than line posts, as the pull of the fence wire comes directly on them. They should be well braced by diagonal reinforced concrete braces running downward at an angle of about 30 degrees from a point about three-fourths the height of the fence to a secure footing in the ground. (See Fig. 69.) The upper end of the braces may be made beveled FIG. 69. Concrete Fence Posts and Braces at Corner. to fit in a recess or against a bracket cast on the post. The lower end of the brace may be held in place by means of a small mass of concrete placed when the post is set. Diagonal wire ties may also be used on the post next to the corner or gate post. Braced posts should also be placed at intervals along the line of the fence. Corner and gate posts are usually made square in section and without taper. The sizes commonly used are 8 by 8 or 10 by 10 inches. They should be reinforced by half-inch rods, one in each corner, about three-quarters of an inch from each side. They are usually cast in place, in wooden or metal forms, as they are too heavy to be moved easily. CASTING IN MOLDS 173 Fastening Fence to Posts. - Various methods have been de- vised for attaching the fence to concrete posts. Probably the most satisfactory method is to pass a tie wire around the post on the side opposite the fence and then to twist its ends tightly FIG. 70. Forms for Corner Post and Braces about the wire of the fence. Two methods commonly used are shown in Fig. 71. A piece of strap iron bent and cut as shown in the figure is useful in making these twists. In either case the tie wires should be twisted up tightly to prevent their slipping along the post. Grooving the face of the post when it is poured will assist in holding the wires in place. The wider sides of rectangular posts should be placed at right angles to the line of the fence, as they are 'stronger when placed this way. Cost of Posts. The cost of concrete fence posts varies considerably under different circumstances, but the following estimate will serve as a guide in computing the cost in any given case. (See table on the following page.) 174 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES ' ""7 FIG. 71. Methods of Attaching Wire to Concrete Fence Posts. COST OF CONCRETE FENCE POSTS (3" x 4" at top and 5" xft" a,t butt, 7 ft. long, of 1: 2: 4 gravel concrete, with four \" rods 6 ft. 10 in. long) For 100 posts there will be required, 3.1 yds. gravel 1.6 yds. sand 21.3 sacks cement 456 Ibs. steel rods 30 hours labor Cost of 100 posts Cost of each post .20 $3.88 1.20 8.52 11.40 6.00 $31.00 31 cents The above does not include the cost of forms, as this may be distributed over many posts. Where the posts are made during spare time, and scrap steel is used for reinforcement, the cost may be considerably reduced. PART V TYPICAL APPLICATIONS OF CONCRETE CHAPTER XIII SIDEWALKS, FLOORS, AND ROADS THE principles brought out in the preceding chapters apply to the use of concrete for all purposes. The essential factors for success are careful selection of materials, use of suitable proportions, proper methods of mixing, handling, and placing the concrete, and the preparation of suitable forms. These matters have all been discussed. In the remaining chapters, more detailed directions will be given for the construction of a few typical structures selected from those widely used in rural communities. SIDEWALKS Preparation of Subgrade. The preparation of a suitable foundation for a sidewalk is very important. If the walk is to be laid on a new fill, the latter should be placed in layers not thicker than six inches, each of which should be thoroughly rolled or tamped to prevent future settlement. A liberal use of water on the fill will often help in compacting it. If the soil is stiff and clayey, the foundation should be well drained. This can be accomplished by placing the walk on a sub-base consisting of a layer of cinders, gravel, or broken stone from four to eight inches in thickness, depending on the char- acter of the soil, the climate, and the width of the walk. This material should be thoroughly compacted by tamping or rolling. If the soil is very stiff, drains of porous material or of tile should be placed at the lowest points to carry off any water which may accumulate in this sub-base. In mild climates where the soil is sandy, the sub-base may be omitted. Proportions. Walks are generally made of two courses, a thick base of concrete and a - thin wearing surface of rich mor- 177 178 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES tar. For widths of four feet or more, the base should be about four inches thick, and the top coat about three-fourths inches. For narrow walks the base may be made two and one- half to three inches thick, and the top coat about one-half inch. The proportions for the base should be about 1 : 2 J : 5 and for the top about 1:1J. With extra good materials 1:3:6 may be used for the base and 1:2 for the top. The coarse aggregate should not be larger than 1} inch. On account of the extra labor required in providing contrac- tion joints when broken stone or gravel is used, it is a common practice in many places to omit the coarse aggregate, or some- times to use a very fine gravel or broken stone. In such cases proportions of 1:4 or 1:3:4 may be successfully used for the base. With a good bank-run gravel the proportion of 1:5 is used. Single course walks are coming into use to some extent. In these the whole depth of the walk is made of a rich mixture with an excess of mortar, such as 1:2:3J. By tamping, the coarse aggregate is forced down, leaving only mortar on the surface. This is leveled down and finished in the usual way. One of the arguments advanced for this method is that it elimi- nates any danger of separation of the top coat from the base. Where driveways Cross walks, the thickness of the base should be increased to six inches and of the top to one inch or more. Forms. For straight walks forms are usually made of wooden two-by-fours laid on edge and held in place by stakes driven on the outside. The forms may be lightly nailed to these stakes to hold them at the proper height. For making curves, one-by-fours or one-half by four inch pieces are used. The forms on one side should be placed a little lower than on the other to insure that no water will stand on the walk during a rain. A drop of one-fourth inch for each foot of width will be sufficient. If coarse aggregate is used in the concrete, then cross forms will be needed to divide the walk into blocks. These may be made of two-by-fours so placed that alternate blocks can be SIDEWALKS, FLOORS, AND ROADS 179 poured. After the base of these blocks has been placed, the cross forms are removed and the intermediate spaces are filled. Marks should be made on the side forms at each joint of the base, so that the grooves in the top coat can be located accu- rately over these joints. Failure to do this has caused many unsightly cracks. Another method, which has much to recom- mend it, is to use sheet steel dividing plates about one-eighth inch thick between adjacent blocks. These are made with hooks on the ends, which hold the side forms in place. When the surface is ready to be finished, the dividing plates are re- moved and a groover is run over the joints. FIG. 72 Sheet Steel Sidewalk Forms. Sheet-steel side forms are being used to some extent and are proving very satisfactory. Figure 72 shows one of these forms. Mixing and Placing the Concrete. A concrete of dry con- sistency is often used for the base of sidewalks. It is better to use a mushy concrete, as less labor is required in compacting and better results are obtained. A straight-edged board, notched over the side forms to leave a proper depth for the top coat, may be used to level off the base. The top coat should be mixed wet, but not so wet as to give any surplus water when it is leveled off. It is a common fault to mix the top coat too wet. The top should be placed on the base as soon as possible after the latter is poured. If it is not placed within half an 180 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES hour, an imperfect bond between the two layers is likely to be obtained. This is one of the most common causes of the fail- ure of concrete sidewalks. Provision must be made for contraction joints, or the walk will crack irregularly. If alternate blocks are first placed and then the intermediate sections are filled in, a sheet of tar paper between the blocks will maintain a good joint. If steel parting plates are used, these will of course provide both contraction and expansion joints. When the coarse aggregate is omitted, it is a common practice to make these joints by cutting through both top and base with a trowel, after the concrete has stiffened up sufficiently. If this is done, great care must be taken to see that the base is cut entirely through, as otherwise the cracks may not follow 'the groove in the surface. The size of blocks should be limited to about six feet wide by five feet long, for walks 4f inches thick. If the walks are thin- ner, the maximum dimension of the blocks may be made about twelve times the thickness. Expansion joints about one-half inch wide should be left entirely through the walk at intervals of fifty feet or less in straight runs, and on each side of turns, unless a small space is allowed between each block. When metal parting strips are used between blocks, and are left in place until the top coat is placed and finished, no additional provision for expansion need be made. Surface Finish. The top of the wearing coat should be struck off even with the side forms with a straight edge imme- diately after it is placed. As soon as the free water has dis- appeared, it should be worked over with a wooden float to take out all irregularities and to compact it. This will leave a rough, pleasing finish which will not be slippery in wet or icy weather. An edging tool may be run next to the side forms, and a groover across the joints, to give a border to the blocks and to round the edges off. The wooden float can easily be made by nailing a cleat for a handle on a one-by-six inch board about fifteen inches long. The edger and groover can be bought very cheaply at hardware stores. If it is desired to have a smoother finish than the above, it can be obtained by means of a steel trowel, manipulated with a SIDEWALKS, FLOORS, AND ROADS 181 circular motion, while the surface is still rather soft. This will give a finish which is not so smooth as to be slippery, nor yet so rough as to be difficult to keep clean. FIG. 73. Concrete Sidewalk with Floated Finish. The smooth, glassy finish obtained by troweling the surface after it has become almost hard is objectionable for the reasons that it is slippery, that it gives a disagreeably intense reflection of the sun in hot weather, and that working the concrete after it has begun to set is injurious to it. Either the wooden- floated or the steel-floated finish is much to be preferred. Curing. In very dry, hot weather the surface of the walk should be covered with sand, earth, or other material, as soon as the concrete has hardened sufficiently, and this covering should be kept wet for about four days. In cool or cloudy weather the covering may be omitted, but the surface should be kept moist for a few days. No traffic should be allowed on the walk for at least forty-eight hours,. or longer in cool weather. If a rainstorm should 'come up before the walk is hardened sufficiently to resist pitting, it can be protected with an inch of sand. Cost of Sidewalks. The cost of materials and labor for concrete sidewalks, including the preparation of the sub-base, will usually range from 10 to 15 cents per square foot, depend- ing on local conditions. Walks on the grounds of the Kansas State Agricultural College cost about 8 cents per square foot, not including the preparation of the sub-base. 182 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES FLOORS Use of Concrete for Floors. Concrete is being used to a large extent for floors for all purposes, especially those placed directly on the ground. It is used for the first or basement floor of warehouses, factories, dwellings, garages, barns, dairies, and granaries, for feeding floors, for barnyard floors, and where- ever it is desired to have a cheap, clean, sanitary, and per- manent surface. The methods used for sidewalk construction will generally apply equally well for floors. Cellar Floors. Cellar floors do not usually require any po- rous foundation, and need be only three to four inches thick unless they are to receive hard usage. A one course floor mixed 1 : 2J : 4 may be used, or a top of 1 : 2 about half an inch thick may be placed on a 1 : 2J : 5 base. On account of the smaller temperature range, blocks may be a littler larger than for sidewalks, say about six or eight feet square. A smooth troweled finish will usually be preferred, but the troweling should be done before the surface has become too stiff. Barn Floors. Barn floors should be made like sidewalks, with a porous sub-base about six inches thick, while the floor itself is about five inches thick of either one or two courses. If two courses are used, the mortar top should be about one inch thick. Sufficient slope must be given to the floor to carry the liquids to the drains. A semi-rough finish should be used, so that animals will not slip on it. " --- J --- 3-6- --- H FIG. 74. Section of Concrete Floor for Dairy. Figure 74 shows a section of a concrete dairy floor for use with iron pipe stanchions. The feed trough can easily be made by building board forms of the proper height on the two sides, and scraping out the interior by means of a template made up SIDEWALKS, FLOORS, AND ROADS 183 of boards cut to the desired shape and supported by the forms on each side, as shown in Fig. 75. Means should be pro- vided to drain the feed trough in order that it may be used for watering the cattle also. Template of 1 In.Boards FIG. 75. Forms and Template for Feed Trough of Dairy Floor. Feeding Floors. Feeding floors are a profitable investment on farms where much live stock is kept. They may easily save FIG. 76. Interior of Dairy Barn with Concrete Floor and Iron Pipe Stanchions. 184 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES their cost in one year in the saving of feed and fertilizer and in the improved condition of the animals. Concrete is by far the most suitable material for their construction. Exactly the same materials and methods should be used for making feeding floors as for sidewalks. A wall about six inches thick and eighteen inches deep should be placed under the outer FIG. 77. Concrete Feeding Floor. edge of the floor, on all sides, to keep hogs from undermin- ing it, and to keep rats out. The floor should be given a slope of one-fourth inch per foot of width, to insure drainage. A gutter may be made at the lower edge to carry away the water. If desired, the gutter may be connected with a concrete manure pit, so that all the manure washed off by rains will be saved. Feeding troughs' for hogs may be made of concrete as a part of the feeding floors, as shown in Fig. 78. CONCRETE ROADS Concrete is being extensively used for road surfaces, both in the cities and in the country, and its use for these purposes will un- doubtedly be greatly extended in the future. It is much cheaper than brick in first cost, and in upkeep is considerably less ex- SIDEWALKS, FLOORS, AND ROADS 185 pensive than macadam. On account of the severe usage to which roads are subjected, and the expense of their construction, a competent engineer should always be employed to prepare plans FIG. 78. Feed Trough for Hogs, Built as Part of Feeding Floor. and specifications and to supervise their construction. 1 Many times the amount paid for his services may be saved in the longer life and lower expense for repairs of the road. Construction of Roads. The general methods used in road con- struction are the same as for sidewalks, but, on account of the more severe use to which roads are subjected, somewhat greater care must be taken in the selection of materials and in the prepara- tion of the sub-base, and richer and thicker concrete must be used. Good drainage must be provided and the- sub-grade and porous sub-base should be thoroughly rolled with a five or ten ton roller. All vegetable or other perishable matter must be entirely removed. The top of the sub-base should be flat for the full width of the concrete. The width to be paved will, of course, vary with local condi- 1 Standard specifications for concrete highways and for one- and two-course concrete pavements may be obtained from the American Concrete Institute, Philadelphia, Pa., the Association of American Portland Cement Manufactur- ers, Philadelphia, Pa., or the Office of Public Roads and Rural Engineering, U. S. Dept. of Agriculture, Washington, D. C. 186 CONCRETE CONSTRUCTION FOR RURAL COMMUNITIES tions. It should preferably be enough to enable vehicles to pass each other, but this is not absolutely necessary if the shoulders are well graded up on each side of the concrete. The Not more than: Slope |(Not more than-^r Flat Subgrade W up to 1Oit^GOt^<^O rHTH(M(NCOCOCOCO "* (N ol fe & & fc GO (M * ^ rH|N ft: rt|N fe 2 1-2 2; 2 2:2 2 ?! * & & & GOOOiO'-Hfe ^ * fe r-(rHT Ii lC5t^CDO 1 1 1 1 1 I 1 1 pasn sxeq jo aaq -uinu pun '[aa^s punoj /7 | aoj saipui ui guioBdg *>'%"* fc GO 00 (M * * * ^ ^IN i IT ll lOit^-OOT^ ! 1 1 1 1 1 1 1 For silos 14' to 17' inside diameter Spacing in inches and number of strands used for different gages of steel wire ii & ft: rHlOt-OOCOlOTtlT* T-(^-l-tOt^CD T-ii-Hi-i.CDlO i-ii-HtNcocoeococo !l 0^ ft: fc ft: ft: ft: ft: CO 1-H H|M ft: ft: rt|N H|N H|N rHi-Hl>-COO51>-?DlO 1 1 1 1 1 1 1 1 ^H^rH^HfMfMCqiM !| 0^ ft: ft: ft: % GO CO (N * * * *= H ^rtr^^^^T ! S 0| GOGOGOt^^ii-HO*= 1 1 1 1 1 1 1 P pasn sjeq jo aaq -ranu put? 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TABLE XXIII DIMENSIONS FOR WING WALLS FOR FLAT-TOP CULVERTS 2 Concrete: 1: 2f : 5 Amount of materials, including Height Depth apron, guard rail, and floor between wing walls Span = span + floor L = span G of thickness apron Concrete, Cement Sand Gravel cu. yds. bbls. cu. yds. cu. yds. 3'Q" 3' 6|" 3' 0'' 1' 8" *-o- 3.5 4.7 1.6 3.2 3' 6" 4' 1" 3' 6" 1' 10' S^^ 4.3 5.4 2.0 4.0 4'0" 4' 7-1" 4'0" 2' 0" 5'cl 5.8 7.2 2.7 5.4 4' 6" 5' 2|" 4' 6" 2' 0" fsP 7.1 8.9 3.3 6.6 5'0" 5' 9" 5'0" 2' 3" *o ^ g .iocOOCX)Oi(NCOOO 03 00000^-H-.BB patunssv s. >--)':.i J;B ui uouBpunoj uug o; pua'^xa ox T3 '3 1 g j k Tj5 ^ ? CQ (N ii ^^ H a TJ s II J" CVJ