This is known as the “coarse aggregate” of concrete. Great care should
be used in its selection. The pebbles should be closely inspected
to see that there is no clay on their surface. A layer of such clay
prevents the “binding” of the cement. If necessary stone or gravel may
be washed in the same way as above described for sand. Indeed, it is
more easily done than sand, as the water flows through the larger voids
in the gravel more readily than through the voids in the sand. Dust may
be left in the crushed stone without fear of its interfering with the
strength of the cement, but care should be taken to see that such dust
is distributed evenly through the whole mass, and when dust is found in
stone, slightly less sand should be used than ordinarily.
As to the size of stone or gravel, this must be determined by the
form of construction contemplated. For foundations or any large thick
structure, use anything from ½ to 2½ inches in diameter. For thin walls
use ¼ to 1-inch stone.
The best results are obtained by the use of a mixture of sizes graded
from small to large. By this means the spaces or voids between the
stones or pebbles are reduced and a more compact concrete is obtained.
Moreover, this method makes it possible to get along with less sand and
less cement.