Sand containing loam or earthy material is cleansed by wash ing with water, either in a ma-chine specially designed 'for the purpose, or by agitating the sand with water in boxes provided with holes to permit the dirty water to flow away.
Very fine sand may be used alone, but it makes a weaker con crete than either coarse sand or coarse and fine sand mixed. A mortar consisting of very fine sand and cement will not be so dense as one of coarse sand and the same cement, although, when measured or weighed dry, both contain the same proportion of voids and solid matter. In a unit measure of fine sand, there are more grains than in a unit measure of coarse sand, and therefore more points of contact. More water is required in gauging a mixture of fine sand and cement than in a mixture of coarse sand and the same cement.
The water forms a film and separates the grains, thus producing a larger volume having less density.
The screenings of broken stone are sometimes used instead of sand. Tests frequently show a stronger concrete when screenings are used than when sand is used. This is perhaps due to the vari able sizes of the screenings, which would hare a less percentage of voids.
SO. Percentage of Voids. As before stated, a mortar is strongest when composed of fine and coarse grains mixed in such proportion that the percentage of voids shall be the least. The simplest method of comparing two sands is to weigh a certain gross volume of each, the sand having been thoroughly shaken down. Assuming that the
stone itself of each kind of sand has the same density, then the heavier volume of sand will have the least percentage of voids. The actual percentage of voids in packed sand may be approximately determined by measuring the volume of water which can be added to a given volume of packed sand. If the water is poured into the sand, it is quite certain that air will remain in the voids in the sand, which will not be dislodged by the water, and the apparent volume of voids will be less than the actual. The precise determination involves the measurement of the specific gravity of the stone of which the sand is composed, and the percentage of moisture in the sand, all of which is done with elaborate precautions. Ordinarily such precise deter minations are of little practical value, since the product of any one sandbank is quite variable. While it would be theoretically possible to mix fine and coarse sand, varying the ratios according to the vary ing coarseness of the grains as obtained from the sand-pit, it is quite probable that an over-refinement in this particular would cost more than the possible saving is worth. Ordinarily sand has from 28 to 40 per cent of voids. An experimental test of sand of various degrees of fineness, .1.21per cent of it a No. 100 sieve, showed only 22 per cent of voids; but such a aline is Of only theoretical interest.