All grains, small pebbles, or particles of broken stone are considered as
sand if they will pass through a wire screen with one-fourth inch meshes.
The particles or grains should be hard and well graded and should vary in
size, as a stronger concrete is thus obtained than when the size of the
grains is nearly uniform. If a large proportion of the sand is very fine
an extra quantity of cement should be used and if exceptionally fine it
is advisable to use 25 per cent more cement.
The sand should be clean; that is, free from vegetable matter, loam, or
any considerable amount of clay. If the hands are soiled when a small
quantity of sand is rubbed between them the following test should be
made: Put 4 inches of sand into a pint preserving jar, fill with clear
water to within an inch of the top, fasten the lid, and shake the jar
vigorously until the whole is thoroughly mixed. Set the jar aside and
allow the contents to settle. The sand will settle to the bottom with the
clay and loam on top of it. If more than three-eighths of an inch of clay
or loam shows, the sand should be rejected or washed. The difference in
fineness and color shows clearly the line of division between the clay or
loam and the sand.
[Illustration: Fig. 1.--Sand and gravel washing trough.]
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.