LAWS OF PROPORTIONS OF CONCRETE.
The proper proportioning of the ingredients of a concrete is an important matter. The ideal proportion would be that which secures the least cost, the greatest strength, and the maximum density. The cost varies chiefly with the proportion of cement used; and for the same amount of cement in a unit of volume of concrete, the strength and the density vary with the relative proportions of sand and stone, and with the gradation of the sizes of each. Im proper proportions may greatly increase the cost, or decrease the strength, or both. The first step toward an understanding of the correct theory of proportioning is to study the law governing the density of a concrete.
The density of concrete is an important factor in its strength and cost, and is the most important element affecting its permeability. For a method of determining the density of con crete when the metric system of weights and volumes is used, see f 234. The following example will illustrate the method when pounds and cubic feet are used.
What is the density of a 1 :3 :6 concrete which required 25 pounds of portland cement, 75 pounds of sand, 150 pounds of broken limestone, and 13 pounds and 14 ounces (13.88 pounds) of water to make 2,821.8 cubic inches of rammed concrete? The specific gravity of the cement was 3.09, of the sand 2.64, and of the stone 2.99. The weight in pounds of the cement, for example, divided by its specific gravity gives the weight in pounds of a volume of water equal to the volume of the solid particles of the cement; and this divided by the weight in pounds of a cubic inch of water will give the volume in cubic inches occupied by the solid particles of the cement. The weight in pounds of a cubic inch of water is equal to the weight of a cubic foot divided by the number of cubic inches in a cubic foot; or 62.3+1728=0.0360 lb.
The density of concrete is chiefly dependent upon the gradation of the sizes of the sand and the stone. The density increases with (1) the proportion of sand, (2) the proportion of stone, (3) the size of the stone, and (4) with the increase in the 'specific gravity of the stone. Any reasonably well-proportioned concrete will have a density between 0.80 and 0.84, and a carefully proportioned concrete
may have a density of 0.84 to 0.88.* The whole theory of the proper proportions of concrete is comprised in two well-established laws which are similar to those. governing the proportioning of cement mortar (§ 236), viz.: 1. For the same sand and the same coarse material, the strongest concrete is that containing the greatest per cent of cement in a unit of volume of concrete.
2. For the same per cent of cement and the same aggregate, the strongest concrete is made with that combination of the sand and the coarse material which gives a concrete of the greatest density.
The second law is equivalent to saying that the cement should fill the voids of the sand and the resulting mortar should fill the voids of the coarser aggregate. If the cement does not fill the voids of the sand, or if the mortar does not fill the voids of the aggregate, the concrete will obviously be less dense and also weaker than when the voids are filled. If the cement more than fills the voids of the sand, or if the mortar more than fills the voids of the aggregate, the con crete will be less dense than though the voids were just filled, since both the paste and the cement mortar have a less density than ordinary concrete; and hence the strength due to the increased amount of cement may be neutralized by the decrease in density, but the possibilities of this depend upon the plasticity of the mortar, the amount of tamping, the character of the sand and the stone, and the gradation of the sizes.