Methods of Waterproofing Concrete

Instead of using alum and soap as above, it is both cheaper and more effective to substitute aluminium sulphate for the alum (see f 374).

Grout Wash.

A cream of neat cement spread on with a whitewash brush is quite effective, if well rubbed in. One or two coats will make ordinary concrete practically impermeable under a 10-foot head of water. The grout wash is most effective if it is put upon the water side of the concrete.

Other Washes.

Any of the materials mentioned in * 377 may be used as a wash; but they are more expensive and not as effective as the alum and soap or as the grout wash. The surface of the concrete is sometimes painted with linseed oil. Most, if not all, of the proprietary compounds (* 376) may be used as a wash as well as an ingredient to be mixed in the concrete. Sodium silicate ("soluble glass") and also paraffine dissolved in naphtha are some times used, but both are more expensive and no more effective than the alum and soap or the grout washes.

Waterproof Plaster.

Very frequently the attempt is made to waterproof a concrete or masonry wall by applying an impervious plaster. The plaster is made waterproof by the use of the alum and soap mixture or some of the proprietary compounds. The pro portions of the plaster coat are usually 1 part cement to 2 or 3 parts of sand. It is usual to apply a 1-inch coat to floors and a finch coat to vertical surfaces.

The difficulty is to make the plaster stick (see 4 356). The wall should be thoroughly cleaned before the coating is applied, and the plaster should be troweled with considerable pressure; and if the wall is under water, the pressure should be relieved by drainage or by pumping until the plaster has set.

Most failures with this method of waterproofing occur because the wall was not absolutely clean. The plaster can usually be made to adhere if the concrete wall is thoroughly and repeatedly washed with water; but if the wall has a dense hard surface of nearly neat cement, it may be necessary to wash it with dilute hydrochloric acid to dissolve out the cement and roughen the surface. Of course the acid must be thoroughly washed off. A coat of ordinary white wash, or a layer of laitance, or an almost invisible film of the oil or grease used to keep the concrete from sticking to the forms, will keep the plastering from adhering unless they are scrupulously removed.

Sometimes the surface is cleaned and then covered with a coat of tar; and while the tar is still tacky, the impervious plaster is troweled on. The tar should be made thick by boiling and should be applied very hot, when it will adhere to the smoothest surface. The tar could be used alone, if it were not for its color and if it did not become brittle by oxidation.

Asphalt Coating. Where its color is not objectionable, asphalt is sometimes used to make concrete or masonry water-tight. The asphalt should not flow at 180° to 200° F., and should not be brittle at 0° F. The surface of the concrete should be dry and warm, or should first be coated with paint made by dissolving asphalt in naphtha. The asphalt should be heated to about 450° F. but not more, and should be applied without unnecessary cooling.

Bituminous shield.

This method of making concrete water proof consists in surrounding the structure with an impervious shield which keeps the water away from the concrete; and hence, strictly speaking, is not a method of rendering concrete impervious. The waterproof diaphragm consists of several, usually four or more, thicknesses of tarred paper or felt (usually the former) cemented together and covered with tar. Of course, the several sheets should break joints, and every precaution should be taken to make the shield continuous and to prevent its being punctured during sub sequent building operations.

Sometimes the shield is put outside of the wall, in which case it is usual to protect it by building a brick wall against it. With this construction, if, after the building is completed, the diaphragm is not water-tight, as is frequently the case owing to imperfect work manship or to its having been punctured, it is sometimes necessary to tear out a considerable portioh of the original wall to discover and stop the leak; and hence the water-tight shield is sometimes placed on the inside of the main wall and a brick protecting wall is built inside of the waterproof diaphragm, so that if repairs of the water proofing are required it is necessary to tear down only the lighter protecting wall. However, other things being the same, such a shield is most effective on the outside.