=Rich mixture.=--Used for concrete subject to high stresses or where
exceptional water-tightness and resistance to abrasion are desired:
1:1-1/2:3; i. e., 1 part cement, 1-1/2 parts sand, and 3 parts gravel.
=Standard mixture.=--Used generally for reinforced concrete and
water-tight work: 1:2:4; i. e., 1 part cement, 2 parts sand, and 4 parts
gravel.
=Medium mixture.=--Used for plain concrete of moderate strength: 1:3:5;
i. e., 1 part cement, 3 parts sand, and 5 parts gravel.
Leaner mixtures are sometimes used after a test has proved them to be
suitable for the work at hand.
It will be noticed that always in indicating the proportions the first
number refers to the cement, the second to the sand, and the third to the
gravel. The three materials must be measured by volume, using the same
unit. The cubic foot is a convenient measure, because a sack of cement,
weighing 94 pounds, is considered to contain 1 cubic foot.
When the coarse aggregate (gravel, etc.) is omitted the mixture is
generally spoken of as mortar and the proportions are indicated thus,
1:2, meaning 1 part cement and 2 parts sand. Mortar is used for
plastering, stucco, top coats of floors, and for laying masonry.