Methods of Waterproofing Concrete

METHODS OF WATERPROOFING CONCRETE. In recent years a great deal of attention has been given to methods of rendering con crete waterproof, but there is no uniformity as to the best practice. The various methods employed may be divided into four classes as follows: (1) grading the aggregate and proportioning the cement so as to secure a concrete so dense as to be waterproof; (2) mixing some substance with the concrete to make it impermeable; (3) applying a waterproof coating to the concrete after it is in place; and (4) surrounding the structure with a bituminous shield to keep the water away from the concrete. For brevity these will be designated: (1) Dense Concrete; (2) Waterproofing Ingredients; (3) Waterproof Coating; and (4) Bituminous Shield.

Dense Concrete.

By grading the ingredients according to the ideal sieve-analysis curve (§ 302) it is possible to make concrete that is practically water-tight. The chief points to be considered in making impermeable concrete are: 1. The greater the proportion of cement the less the permeability, but in a much larger inverse ratio. If the aggregates are well graded, cement equal to from 12 to 15 per cent of the weight of the dry materials will usually give a water-tight concrete under high pressure.* 2. Gravel produces a more water-tight concrete than broken stone. 3. The larger the maximum size of the aggregate the less the permeability, although it is not so easy to get a uniform mixture with large stone as with small. 4. The finer sand grains should be slightly in excess of the proportion required by the ideal sieve-analysis curve for maximum density and strength. 5. The concrete should be mixed wet or at least so as to•quake freely when tamped and so as to leave no empty pockets. 6. The concrete should be mixed very thoroughly. 7. Fo: the best results the entire structure should be laid in one continuous operation; but if this is impracticable, particular care should be taken in joining the fresh concrete to that already set (see § 345). 8. The permeability will decrease with the time of flow, owing to the silting up of the mass by the soluble portions of the cement carried by the percolating water. 9. The permeability decreases with the age of the specimen, owing to a slight swelling of the par ticles of cement in hardening.

The proportions employed to prevent percolation usually are: with ordinary materials 1 : 2 : 4; and with carefully graded materials 1 : 2 : 6 or 1 : 3 : 5.

388. Troweled Finish. A particular case of the making of a concrete dense enough to resist the penetration of water, is the method of finishing the floors of basements, reservoirs, and tanks. A layer of ordinary concrete is placed, and upon it is immediately laid a coating of 1 : 1 or 1 : 2 plastic cement mortar which is then troweled. The troweling forms a rich dense film on the surface, which is nearly, if not absolutely, water-tight. This surface is frequently, but improperly, called a granolithic finish.

Obviously this method is not applicable to a vertical surface, since the forms can not be removed until the surface of the concrete is too hard to trowel. A mortar face, constructed as described in § 351, would add to the impermeability of the concrete; but there are better methods of securing the same result.

Waterproofing Ingredients.

The principle of this method is to mix in the concrete some finely divided material which will at least partly fill the voids and thereby reduce the permeability of the concrete. There are two methods of accomplishing this result: (1) adding a single inert void-filling substance, and (2) adding one or more substances which by action upon the cement or between themselves may produce a void-filling material.

There are two distinct classes of void-filling materials. (1) those that have a capillary attraction for water, and (2) those that have a capillary repulsion for water. The first reduces permeability by obstructing the voids, while the second acts by decreasing the volume of the voids and also by its repellent action for water. Examples of the first class of materials are lime, clay, and pozzolan cement; and of the second, wax, resin, alum and soap, and a number of proprietary articles. The difference in action between capillary attractive and capillary repellent void-filling materials seems not to have been investigated, except possibly by the originators of certain waterproofing compounds, and except as stated in I 373.