More concrete can be made from given volumes of aggregates if the gravel used is graded from fine to coarse than if the particles are too nearly of one size, because the small stones help to fill the voids between the larger ones and less sand-cement mortar is required. The extra mortar thus adds to the volume of the concrete.

A common mistake to be guarded against is to assume that the volume of concrete produced is equal to the quantity of sand plus the gravel as indicated in the proportion. For instance a 1:2:4 mixture will not produce 6 cubic yards of concrete, if 2 yards of sand and 4 yards of gravel are used, because the sand will lodge in the voids between the pebbles. If 6 cubic yards of concrete are desired it will be necessary to use 2.7 cubic yards of sand and 5.34 cubic yards of gravel.

Table 1 shows the quantity of cement, sand, and gravel required under average conditions for the indicated proportions.

Table 1.—Materials for 1 cubic yard of rammed concrete.