The method of proportioning concrete by surface areas is based on the theory that the strength of a concrete depends on the amount of cement used in proportion to the surface area of the aggregates.[[57]]
The proportioning of concrete on the basis of a water-cement ratio and a fineness modulus was introduced by Prof. D. A. Abrams.[[58]] It is based on the theory that with fixed conditions of aggregate, moisture, etc., the ratio of water to cement determines the strength of the concrete.
A method of proportioning concrete by determining experimentally the voids in mortars made up with a given amount of sand and definite proportions of cement, and then calculating the voids in the concrete made up by adding a definite amount of coarse aggregate to the mixture, has been developed.[[59]] The method is based on the theory that the strength of the concrete is a known function of the ratio of the volume of cement to the volume of the voids in the concrete. The effect of varying the proportion of the ingredients, including an increase in the amount of mixing water beyond that required to give the densest mixture, may be found by the method, and a comparison may be made of results obtainable with different classes of fine and coarse aggregates.
Arbitrarily selected proportions, proportions based on voids, and proportions based on trial mixtures are usually satisfactory for small jobs where the amount of materials involved is not large. Where the saving in materials will permit, more accurate methods should be used. The methods can be studied more fully by reference to the original articles quoted in the footnotes, or to the following texts:
Materials of Construction, Johnson, 5th Edition, 1918.
Materials of Engineering, H. F. Moore, 2d Edition, 1920.
Masonry Construction, I. O. Baker, 10th Edition, 1912.
Concrete Engineer’s Handbook, Hool and Johnson, 1918.
Concrete, Plain and Reinforced, Taylor and Thompson, 1916.
94. Waterproofing Concrete.—The waterproofing of concrete is most satisfactorily done by making dense mixtures. In practice such substances as hydrated lime, clay, alum and soap, and proprietary compounds such as Ceresit, Medusa, etc., are frequently mixed with the concrete under the theory that these very fine substances will fill any remaining voids and render the concrete impervious. The specifications of the Joint Committee issued on June 4, 1921, are much briefer and contain less detailed instruction than those issued earlier.[[60]] The earlier instructions follow.