MAKING CONCRETE WATERPROOF vv By IRA O. BAKER, M. Am. Soc. C. E. Consulting Engineer Professor of Civil Engineering University of Illinois Urbana, 111. Reprint from "The Technograph” (the annual of the Engineering Societies of the University of Illinois), No. 23, 1908-09. NOTICE: Return or renew all Library Materials! The Minimum Fee for each Lost Book is $50.00. The person charging this material is responsible for its return to the library from which it was withdrawn on or before the Latest Date stamped below. Theft, mutilation, and underlining of books are reasons for discipli- nary action and may result in dismissal from the University. To renew call Telephone Center, 333-8400 UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN Z. I Ap.’l3 <5-, 'B^4uAx 3 [ 1 TTU MAKING CONCRETE WATERPROOF. 1 By Ira 0. Baker, 2 M. Am. Soc. C. E. Concrete is very largely employed in many building constructions, and in some situations it is very important that it should be at least practically waterproof. There are several methods of rendering concrete more or less impermeable to water, some of which are simple and free to any one, and some of which consist in using secret or patented compounds. Of the former the simplest method consists in mixing hydrated lime or finely divided clay with the concrete, thus at least partially filling the voids or interstices in the concrete and decreasing the percola- tion of water. Another simple method consists in mixing powdered alum with the cement and dissolving ordinary soap in the water to be used in mixing the concrete. Al- though the latter materials have been used for more than 60 years as a wash for rendering masonry impervious to water, and although in recent years they have frequently been employed as ingredients of concrete to make the entire mass impermeable, the proportions to be employed i 1 From “The Technograph” (the annual of the Engineering Socie- ties of the University of Illinois), No. 23, 1908-09. 2 Professor of Civil Engineering, University of Illinois, Urbana, 111. 1 and the reason for the effect seems to have had little or no scientftfc consideration, .at least the proportions, used in practice vary very widely. For the purpose of securing data for the use in the revised edition of the writer’s Treatise on Masonry Construction, now in preparation, certain investigations and experiments, have been made ; and it is the object of this article to present these results more fully than can be done in the book mentioned. Alum and Soap Waterproofing Compound To use the alum and soap method of decreasing the permeability of mortar or concrete, the alum in powdered form may be mixed with the dry cement or the sand, and the soap may be dissolved in the water employed in mix- ing the concrete; or both the alum and the soap may be dissolved in the water. The former is probably the safer method in practice, since with the latter method the water must be thoroughly stirred while the two are being mixed or the precipitate may form in large masses which it is practically impossible to break up ; and further the water must be stirred continuously to prevent the compound from accumulating on the surface. These are conditions that it is not always easy to be certain of securing. How- ever, the alum is more easily dissolved than the soap ; and hence the alum may be dissolved in, say one fifth of the water and the soap in the remaining four fifths, and then the two portions may be mixed together, being careful to stir them as the mixing progresses. The alum and the soap combine and form a finely-divided, flocculent, insoluble, 2 water-repelling compound wliicli fills the pores of the con- crete and decreases its permeability. The best proportions are : alum 1 part and hard soap 2 parts, both by weight. Soap varies in its chemical com- position, and hence a single proportion can not be stated which will be chemically exact for all cases. The above proportion is in round number the relative combining weights of alum and average hard soap; and hence it is the best proportion to use, although widely different pro- portions have been used in practice with success. Any reasonably pure soap will do; but if soft soap is em- ployed, a greater amount should be used in proportion to the amount of water in it. It is difficult to dissolve more than about 3% of hard soap in cold water; and hence this practically limits the amount of alum to 1.5% and of soap 3%. These amounts will give a precipitate equal to about 3% of the weight of the total water. The amount of precipitate formed in the pores of the mortar or concrete will de- pend upon the amount of water used in the mixing. Of course, if it were desired to use a greater quantity of soap and alum, the soap could be first dissolved in a simi- lar quantity of hot water, which is afterwards mixed with the water used in making the mortar ; but this is hardly practicable, nor necessary, as will appear presently. An excess of alum does no harm, since alum alone is a fair waterproofing material. An excess of soap does no harm; and an excess is better than a deficiency, since 3 the excess will unite with the free lime of the cement and form calcium soap— a finely-divided, water- repelling compound which is apparently the essential element of several of the proprietary waterproofing compounds. The above is the explanation why such diverse proportions of alum and soap give reasonably successful results in actual practice. Effect of Water-Repelling Compound. A film of oil on the wires of a moderately fine sieve makes it nearly, if not quite, water-tight. The question then naturally occurs : Can a water- repelling compound in the concrete act in the same way as oil on a sieve? Or in other words, can a volume of water-repelling com- pound less than the volume of the voids in the concrete decrease the permeability of the concrete in a greater ratio than the per cent of the voids filled? If so, then a water-repelling compound is more efficient in decreas- ing the permeability of a concrete than mere void-filling material. To test the effect of a water-repelling ingredient upon the permeability of concrete, a series of experiments were made under the writer’s direction by Mr. B. L. Bowling in the Cement Laboratory of the University of Illinois. The experiments consisted in molding a series of port- land cement mortar disks in short lengths of 6-in. pipe, subjecting these disks to water under pressure, and measuring the amount of percolation. For convenience in making the experiments, mortar was used instead of 4 concrete. The mortar for the treated and the untreated disks was alike except for the alum and soap compound. However, the mortars containing the alum and the soap were invariably drier than those made without these in- gredients; but this probably has no significance in this connection. An annulus, 1-in. wide, at the circumference of both the top and the bottom faces of the disk was coated with hot asphalt, and also covered with a rubber gasket against which a flat casting was pressed by holts through an external flange; and consequently the flow was through a disk 4 ins. in diameter and 2 ins. thick. The percolating water was caught in a tin funnel, the top of which fitted closely against the lower casting and the neck of which passed through a perforated rubber stop- per into a bottle. The water pressure varied from 40 to 45 lb. per sq. in. The data and the results of the experiment are given in the table on page 7. Tests 1 and 2 were made together, as also tests 3 to 6, and 7 to 9. The intention was to have four tests in each series, but through one reason or another some of the tests were of doubtful value or were useless, and hence are not reported. The mortar used in making the disks was 1 : 6, which is unusually lean and porous, but it was purposely made porous the better to test the effect of the alum and soap compound. Known weights of cement, sand, and water were used, and the volume of the mortar produced was measured; and then knowing the specific gravity of the 5 several ingredients, the density of the resulting mortar was computed by a process not necessary to explain here. The voids in each disk are stated in the table, and ranged between 23.4 and 24.0%. If the sand had not been well graded, the per cent, of voids would have been considerably greater, and also the individual voids would have been larger. A 1 : 2 : 4 concrete made of well-graded sand and stone would have only about 13 to 15 % of voids. The amount of water used in making each of the sev- eral disks is stated in the table. Knowing the percentage of alum and soap in the water and the amount of water used in mixing, it is easy to compute the amount of the precipitate in the mortar. The amount of precipitate in each disk is given in the table. The percolation for each of several successive 24-hour periods is given in the table. The results show that the alum and soap precipitate is quite effective in decreas- ing the percolation. In the first series of experiments., No. 1 and 2, the alum and soap compound equal to 1.2% of the cement, stopped a little over 76% of the percola- tion during the first 24 hours ; and the corresponding re- sults for the other series are 54 and 69%, respectively. The variation in the several series is surprisingly large, but it is certain that the experiments were carefully made; and the range in the results probably indicates the effect of undetected inequalities in the materials, the proportions, and the mixing. 6 TABLE SHOWING EFFECT OF ALUM AND SOAP ON PERMEABILITY OF CEMENT MORTAR. CM ™ i-i r-* 00 CO CM CM VO m co CM 1-5 VO rf tx CM O CO rj* co ro CO t— t »— < o i— t i— < I i— • CM »-i 1-1 I rt O CO I th o t>I CM O rt CO H 00 *« «-< CM «-i 1 *H CO i-i I CM M* i-t CO 1 -H to 00 5 r ^ 5 1 £ § ( ° \ v I 53 Ih i qj 0 qj « T3 co >'S ^ ju