That mixture in which all the spaces (called “voids”) between the stone
or gravel are filled with sand, and all the spaces between the sand
are filled with cement, is the ideal mixture. This mixture is rarely
attained, as the voids in each load of gravel and sand vary slightly,
and in order to be absolutely safe, it is well to use a little more
cement than will just fill the voids.
[Illustration: Fig. 1.—Quantities of cement, sand, and gravel in 1: 2:
4 concrete mixture, which means 1 part cement, 2 parts sand, 4 parts
crushed stone or gravel, and the resulting quantity of concrete, which
is only slightly greater in size than the gravel, the sand and cement
filling the voids in the gravel.]