Should sand require washing the simplest way for small quantities is to build a loose board platform from 10 to 15 feet long, with one end higher than the other. On the lower end and sides nail 2 by 6 inch boards. Spread the sand over the platform in a layer 3 or 4 inches thick and wash with water. The water may be supplied by any means which will cause agitation of the sand and allow the lighter material to run off with the water. When pressure or a head is obtainable the water is most easily applied by means of a garden hose. The washing should be started at the higher end and the water allowed to run through the sand and over the 2 by 6 inch piece at the bottom. [Figure 1] illustrates a convenient trough for washing larger quantities.
A small amount of clay, provided it is not in lumps, does not injure sand, but amounts over 10 per cent should be washed out.
COARSE AGGREGATE (STONE, GRAVEL, ETC.).
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.
GRAVEL.
Gravel which is too dirty for use usually can be detected by observation. It may be washed in the same manner as sand. Lumps of clay should be eliminated and care should be taken to see that the gravel is not coated with a film of clay or loam which will prevent the bonding of the cement.
BROKEN STONE.
Broken stone should be clean, hard, and of a size suited to the character of the work, and the same care in grading should be exercised as in the case of gravel. Trap, granite, hard limestone, and hard sandstone are commonly used. The composition and physical character of the stones should be considered, as some possess qualities that limit their use under certain conditions (see Substitutes for gravel).
Field stones are common in many localities and their use, when crushed, may be economical. The finer particles, after the dust is removed, can be used as sand. Small stone crushers, operated by three or four horsepower gasoline engines, can be purchased at a relatively low price and may prove profitable if a large quantity of stone is needed.