Laws of Proportions of Concrete

These experiments seem to prove that the strength of concrete varies as the quantity of cement, provided the voids of the coarse material are filled with mortar.

The same conclusion is proved by the data summarized in Fig. 11, page 140. The diagram presents the results of forty-eight experiments on 4-inch cubes.* Each point represents two experiments, the age of the mortar in one being 7 days and in the other 28 days. The points with one circle around them represent the strength of broken-stone concrete, and the points with two circles gravel concrete. Both the sand and the gravel employed in these experiments were very coarse, and consequently the amount of cement per cubic yard is unusually great.

Relation between Strength and Density.

Table 25, page 141, shows that with the same aggregate and the same proportion of cement, the densest mortar is the strongest. Notice that the strength decreases more rapidly than the density and that the strength of the weakest concrete is only one eighth of that of the strongest. In the first line of Table 25, the mortar is just enough to fill the voids; but in subsequent lines the proportion of mortar is too great to fill the voids, and consequently both the density and the strength are less than in the line above. The decrease in strength by substituting one part of sand for an equal weight of stone is due to the fact that the sand and the stone have approximately the same per cent of voids, while the sand has the greater and also the smoother surface; and the decrease in density is due to the fact that the sand has a less density than the stone which it displaced.

Table 25 is a striking illustration of the advantage of properly proportioning the ingredients of concrete. The fact that the densest concrete is the strongest affords a convenient means of comparing different sands and stone and also of determining the best proportions for any particular sand and stone, as will be explained later (see 301).

The following data illustrate the fact that an improper proportion or gradation of the ingredients of concrete may neutralize the effect of an increased amount of cement.

Table 26 shows that a concrete containing a considerable pro portion of pebbles may be stronger than the mortar alone, although the mortar contains 1 to 2 times as much cement as the richer concrete and 2 to 21 times as much as the leaner. These anomalous results are probably due to two causes: (1) the larger aggregate gives the greater strength; and (2) the mortar probably had abnor mally small density owing to the coarseness of the sand and the dryness of the mixture, while the concrete probably had an abnor mally great density.

The average crushing strength of twenty cubes of concrete ranging from 4 to 16 inches on a side composed of 1 volume of cement, 3 volumes of sand, and 6 volumes of stone, was 20 per cent more than that of an equal number of similar cubes made of the mortar alone.* Two other sets of the same experiments gave somewhat similar results.f Substantially the same general results are shown by the following data:$ Each value is the mean of three to five 12-inch cubes tested when three months old. The stone was 11-inch trap.

The preceding results differ among themselves owing to the differences in the materials, the methods of proportioning, etc.; but all show the advantage possible by properly proportioning the ingredients of concrete.

There are four methods in more or less general use in determining the proportions to be ployed, which may be briefly designated as follows: (1) by arbitrary assignment; (2) by the voids; (3) by trial; and (4) by sieve-analysis curves. These methods will be considered separately in the order. Proportioning by Arbitrary Assignment. The most common and least scientific method of proportioning concrete is to assume the relations as a matter of judgment, without much, if any, sideration of the character of, the aggregate; that is, the proportions are assigned without any reference to the fineness or coarseness of the sand and the stone, or to the gradation of the sizes of each. This method is frequently employed regardless of whether the broken stone to be used is crusher-run having sizes varying from } to 21 inches and having 20 per cent voids, or whether it is screened to practically one size and has 50 per cent of voids. Again, if a strong concrete is desired, a proportion of 1 : 2 : 4 may be adopted without any consideration whether some other proportion of the same ingredients might not give a cheaper and also a stronger concrete. Often concrete proportioned by this process can be greatly improved by substituting coarse aggregate for a portion of the sand. For example, in Table 25, page 141, each concrete is stronger than the one in the line below it, simply because in the latter a portion of the stone has been replaced by sand.