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.