Methods of Waterproofing Concrete

Several of the more common ingredients added to concrete to render it impermeable will be considered. Whatever the ingredient added, it should be uniformly incorporated; and the concrete should be of a plastic consistency, and should be thoroughly mixed, care fully placed, and well tamped. Waterproofing concrete, by adding void-filling materials, is proportionally more effective with lean than with rich concrete, since with the latter the cement furnishes enough fine material to fill at least the larger voids.

Lime.

Hydrated lime (* 107) is cheap, is easily obtained, is in fine particles, and is easy to mix with the concrete; and there fore it is an excellent material for reducing the permeability of concrete. Experiments* show that hydrated lime in the following percentages renders concrete made with the usual materials prac tically water-tight under a pressure of 60 pounds per square inch: for a 1 : 2 : 4 concrete add dry hydrated lime equal to 8 per cent of the weight of the dry cement, for a 1 : 2k : 4f concrete add 12 per cent, and for a 1 : 3 : 5 concrete add 16 per cent. Smaller per cents give satisfactory results under smaller pressures. Lime is more effective to resist pressures than high. This use of lime is most advantageous with lean concrete, and in localities where cement is unusually high-priced. Lime has a capillary attraction for water.

Slaked lime is equally as good as hydrated except for the diffi culty of getting it evenly distributed throughout the concrete.

Pozzolan Cement. Pozzolan cement (3: 122-24) being largely composed of lime acts substantially the same as lime in making concrete water-tight, except that lime adds practically nothing to the strength of the concrete while pozzolanic material adds consider able strength. Since pozzolan cement is not plentiful (§ 123), this method of making concrete waterproof is not of much practical importance. However, pozzolanic material is specially valuable for waterproofing concrete to be exposed to sea water, since it is not acted upon by the sulphates in the sea water (§ 391).

Clay.

It has long been known that the addition of clay in a finely divided state materially increases the water-tightness of concrete. Clay is ineffective with rich well-proportioned concrete, since the cement furnishes sufficient fine material to fill the voids; but with lean concretes 10 to 15 per cent of finely divided clay, either added directly or by the substitution of a dirty for a clean sand, increases the water-tightness without materially decreasing the strength. In this connection see § 189. It would not be easy

to add clay directly; but the clay naturally in a gravel may mate rially affect the waterproof quality of the concrete.* Clay has a capillary attraction for water, which is an undesirable quality for a waterproofing material.

Alum and Soap. These materials have been used for more than sixty years as washes for rendering masonry impervious to water (see 4 642); and in recent years they have frequently been used as ingredients of concrete to make the entire mass impervious. The alum in the form of a fine powder may be mixed with the cement, and the soap may be dissolved in the water used in mixing the con crete; or both the alum and the soap may be dissolved in the water. In the latter case the water must be frequently stirred to prevent the compound from accumulating in large masses on the surface of the water which it is not easy to break up. Since the alum is the more soluble, it 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, being careful to stir the water as the mixing progresses. The alum and the soap combine and form a flocculent, insoluble, water-repelling compound. This capilliary repellent compound not only partially fills the voids and thereby decreases the permeability of the concrete, but also by its water-repelling property still further decreases the permeability.

The best proportions are: alum 1 part and hard soap 2.2 parts, both by weight. Soap varies in its chemical composition and particularly in the water it contains; but the above proportions are the chemical combining weights for alum and the best hard soap, and are sufficiently exact for any good well-seasoned hard soap. Any reasonably pure soap will do, but if soft soap is used, a greater amount should be employed according to the amount of water in it. Soft soap contains from 50 to 90, or even 95, per cent of water. An excess of alum does no harm, since alum is itself a fair waterproofing material; but an excess of soap is better than en excess of alum, since the excess soap will unite with the free lime of the cement and form calcium soap—a finely divided, water-repelling compound (§ 375). The above is the reason why widely divergent proportions of alum and soap have given fairly successful results in practice.