The larger particles used in concrete may be gravel, broken stone,
air-cooled blast-furnace slag, or other suitable materials. The coarse
aggregate should be sound and clean, that is, free from disintegrated or
soft particles, loam, clay, or vegetable matter. Air-cooled blast-furnace
slag should weigh at least 70 pounds per cubic foot. The best results
are obtained from a mixture of sizes graded from those retained on
a one-fourth inch screen to those passing a three-fourths to 2 inch
ring, depending upon the work. Ordinarily the greatest dimension of any
particle should not be over one-fourth of the thickness of the concrete
work.