The above method is subject to considerable error, since it is difficult to eliminate the air bubbles,—particularly if the sand is fine or has been rammed. Further, if the sand is dirty and the water is poured upon it, there is liability of the clay's being washed down and puddling a stratum which will prevent the water's penetrating to the bottom. If the bubbles are not excluded, or if the water does not penetrate to the bottom, the result obtained is less than the true proportion of voids.
Hence, to determine the voids more accurately, put part of the water into the vessel and sprinkle the sand slowly into the water, so as to give opportunity for the air bubbles to escape. The sand should not drop through any considerable depth of water, as there is a liability that the sand may become separated into strata having a single size of grains in each, in which case the voids will be greater than if the several sizes were thoroughly mixed. Add water from time to time, and continue to drop in the sand until the vessel is full of water and the sand is at the top of the water. Finally, as before, the quantity of water in the vessel with the sand, divided by the amount of water alone which the vessel will contain, is the pro portion of voids in the sand.
Effect of Moisture on Voids. A small per cent of moisture has a surprising effect upon the volume and consequently upon the per cent of voids. For example, fine sand containing 2 per cent of moisture uniformly distributed has nearly 20 per cent greater volume than the same sand when perfectly dry.* This effect of moisture increases with the fineness of the sand, and decreases with the amount of water present, and with the amount of tamping. When saturated, sand will have a bulk less than the original dry volume.
A knowledge of the amount and of the effect of moisture present in the sand is important in proportioning mortar. For example, with ordinary sand 3 or 4 per cent of water will increase the volume so that a mortar consisting of 1 volume of cement to 4 volumes of damp sand is equivalent to a 1 to 3 mortar of dry sand.
Data on Voids. Table 19, page 93, shows the voids of a number of both artificial and natural sands. An examination of the table shows that the voids of natural sand when rammed vary from 30 to 37 per cent. Sands No. 10, 11, and 12 are fairly good, although they are finer than the first four in Table 18, page 89; but sands No. 13 and 14 are too fine to give a strong mortar, although they have a fairly low per cent of voids. All five of these sands are frequently employed in making mortar and concrete for important work.
The following observations may be useful in investigating the relative merits of different sands: The proportion of voids is independent of the size of the grains, but depends upon the gradation of the sizes, and varies with the form of the grains and the roughness of the surface. A mass of perfectly smooth spheres of any uniform size packed as closely as possible would have 26 per cent of voids; but if the spheres are packed as loosely as possible the voids would be 48 per cent. A promiscuous mass of bird-shot of nominally one size has about 36 per cent of voids. The difference between this and the theoretical minimum per cent for perfectly smooth spheres is due to the variation in size, to the. roughness of the surface, and to not securing in all parts of the mass the arrangement of the shot necessary for minimum voids.
If the mass of sand consists of a mixture of two sizes of grains such that the smaller grains can occupy the voids between the larger, then the proportion of voids may be very much smaller than with a single size of grains. The proportion of any particular size should be only sufficient to fill the voids between the grains of the next larger size.
The finer the sand the more nearly uniform the size of the grains, and consequently the greater the proportion of voids. The advantage of coarse sand over fine increases as the proportion of cement de creases, since with the smaller proportions of cement the voids are not filled.