Hydrated lime may he used in cement mortar to replace a small per cent by weight of the cement without injury to the strength of the mortar. It is a very finely divided material, much more bulky than the cement, and therefore renders the mortar less permeable. Where the strength is sufficient, a somewhat leaner concrete may be used if hydrated lime is added. Experiments by Mr. Thompson 1 show that hydrated lime may be of considerable value in rendering concrete impervious under considerable heads. Mr. Thompson recommends the addition of hydrated lime in the following percent ages of weight of dry hydrated lime to the weight of Portland cement: For 1 part Portland cement; 2 parts sand; 4 parts stone, add S per cent hydrated limo.
For 1 part Portland cement; 21 parts sand; -1' parts stone, add 12 per cent hydrated lime.
For 1 part Portland cement; 3 parts sand; 5 parts stone, add 16 per cent hydrated lime.
Clay.—Finely divided clay in the sand used in making concrete has been found to lessen the permeability of the concrete. The clay must be free from vegetable matter and present in small propor tion, not more than 5 per cent of the weight of sand. Finely pulver ized rock has much the same effect. These materials are of use when the mortar is lean, though for rich mortars they may be detri mental to strength without materially affecting the permeability.
Alum end soul) solution hits sometimes been used to mix with the body of concrete and seems to have been fairly efficient as a Proceedings, American Society for Testing Materials, Vol. VIII, p. 501.
water-proofing medium. In applying, it is usually best to mix powdered alum with the cement and dissolve the hard soap in the water to be used in mixing. The soap may be about 3 per cent of the weight of water, and the alum about one-half the weight of the soap.
ced Concrete.—It has been proposed to use mineral oil as an integral water-proofing. The results of experiments by Mr. Page i indicated that the use of a small amount of asphaltic oil in mixing cement mortar decreased the permeability of the mortar, without materially decreasing its strength. He found the crushing strength to be somewhat reduced, when the weight of oil was 10 per cent that of the cement the crushing strength was reduced about 25 per cent.
The lubricating effect of the oil is such that it cannot he used for reinforced work with plain steel bars.
The oil recommended by Mr. Page is a fluid petroleum resiauay, and he suggests the use of 5 per cent of the weight of cement for water-proofing purposes.
89. Waterproof Coatings.—Various methods have been pro posed and are sometimes used for the treatment of concrete surfaces to make them waterproof. For ordinary work, as already stated,
the concrete may itself be made watertight and nothing is needed beyond care in the selection of materials and in preparing and placing the concrete. Under some circumstances, however, as in old work or where joints and cracks cannot be avoided, it may be neces sary to provide some means of protecting the surface of concrete against the penetration of water.
Layers of Waterproof Materials.—Probably the most effective method of protection is that of applying layers of waterproof paper or felt coated with asphalt or coal-tar pitch. The concrete is first coated with hot asphalt, layers of paper or felt are then placed, and each coated with the hot asphalt, the applications being made from 3-ply to 6-ply, depending upon the degree of protection needed—a method frequently employed on subways, and bridge floors with good results. Objection has been made to this method on account of it preventing the radiation of heat in subway work. Careful workmanship is necessary in placing such a protection; the layers must break joints properly, and be protected against being punctured after being placed.
Cement Grout.—Washing the surface of concrete with a grout of neat Portland cement may sometimes be of use on a surface exposed to water, serving to fill voids and cracks which may exist in the surface.
Plastering with cement mortar, or other materials mixed with cement, is sometimes adopted as a means of waterproofing. Usually such plastering needs protection against possible weather cracks, and sometimes the plaster does not adhere. On horizontal or inclined surfaces a troweled mortar finish, similar to that commonly used on sidewalks, makes a water-tight job, provided care be. taken to guard against cracks, and to insure the bonding of the mortar to the concrete.
Alum and Soap.—A solution of soap and alum is often used to wash the surface of concrete; it is of the same character as the mix ture employed in integral waterproofing, and is also sometimes used for mixing with cement mortar for plastering the surface.
Bituminous coatings consisting of one or more coatings of asphalt or tar painted on hot are sometimes employed, such applications being often used on the outside of the cellar walls of buildings. A number of proprietary compounds are available for use as surface washes, some of which seem quite effective, though in many cases they require renewal from time to time. Some interesting tests of a number of methods of waterproofing concrete surfaces were made by Mr. F. M. McCullough, and the results given in Bulletin No. 336 of the University of Wisconsin, on " Tests of the Permea bility of Concrete,"