A sand which soils the hands when rubbed between them, or which has little of the gritty feeling, should not be used. Another method of determining whether the sand is clean is to drop a quantity into a pail of clear water. If the water in two minutes is clear enough to enable you to see the sand at the bottom, the sand is clean. Still another method is to put some of the sand into a fruit jar with some water, shake well, and let the jar stand till all the materials have settled. If a layer of mud is seen over the sand, the sand is not clean, and should not be used. A small percentage of dirt, such as fine clay or other mineral matter, is not necessarily harmful; and for lean mortar, the presence of such impurities—up to, say, 10 per cent—may be actually beneficial. For rich mixtures, on the other hand, an excess of dirt or fine material spells weakness, and setting may be seriously delayed.
The presence of any considerable proportion of mica will destroy the value of any sand for concreting purposes. Its effect is more injuri ous upon fine sands than upon coarse. It greatly increases the amount of voids, and the smooth surfaces of the mica particles do not permit of a good bond with the surrounding cement.
In a great many sections of the country, the sands contain so much mica as to be almost worthless. The coarser sands seldom contain over 3 or 4 per cent; but some of the finer sands, which, at best, will not make a concrete more than one-third as strong as the coarser, contain 10 to 12 per cent of mica. Tests show that as mica increases, voids increase, weight dimin ishes, and there is loss of strength at any age of the mixture. A sand containing as high as 20 per cent of mica, will have 67 per cent of voids, will require three times the amount of water for mixing that sand without mica would require, and will show a decrease of 20 per cent in weight, and a loss of two-thirds in the strength of the mixture.
The weight of sand is to some extent an indi cation of its quality. A heavy sand is generally denser, and therefore better, than a light sand, as it ordinarily indicates coarseness and good grading of particles, with correspondingly low percentage of voids. This, however, is a very unsafe guide to follow, on account of the fact that the presence of moisture may cause as much as 20 per cent variation in weight, and fine sands require more water than coarse.
Tests made by Mr. Sanford E. Thompson on a certain natural sand, showed 38 per cent of voids in the sand when dry, and 52 per cent in the same sand when moist. The dry sand aver aged 103 pounds to the cubic foot; but after being wet by rain, allowed to drain for two days, and then shoveled into a measure and weighed in exactly the same way, it averaged only 83 pounds. This author therefore lays but little stress on the determination of voids in the sand, preferring to make the determination after the sand has been mixed with cement. That sand which, with the same percentage of cement, shows the lowest total proportion of voids, both of air and of water, and consequently gives the heaviest and densest mixture, is the best.
No absolute statement can be made as to the value of sand as compared with crushed stone screenings. Here also, aside from the quality of the stone itself, a proper grading of coarse and fine particles is of prime importance. In some cases, the strength of the mixture is greatly added to, because of the screenings themselves having a certain degree of hydraulic activity.
Natural sand or screenings usually contain sufficient fine material for rich mortars, such as 1 : 1, 1 : 2, or 1 : (that is, where the sand or screenings do not exceed two and a-half times the volume of cement); but for leaner mixtures, such as 1 :3, 1 : 4, or 1 :5, an addition of fine particles is of advantage by assisting the cement to fill the voids.
It is better to err on the side of coarseness than of fineness, for, other things being equal, a coarse sand will give a denser and stronger concrete or mortar; but a judicious proportion ing of the fine and the coarse will insure the best results all around; and, in the case of water tight work, it seems to be the preferred practice to make a slight sacrifice of the theoretical requirements of strength, by increasing the pro portion of fine material—except, as already noted, in the case of concrete to be deposited under sea water.