QUANTITIES FOR A YARD OF MORTAR. Table 22, page 120, shows the approximate quantities of cement and sand required for a cubic yard of mortar by the three methods of proportioning described in 4 240. The table is based upon actual tests made by carefully mixing one half cubic foot of the several mortars; but at best data are only approximately applicable to any particular case, since so much depends upon the specific gravity, fineness, compactness, etc., of the cement; upon the fineness, humidity, sharpness, compact ness, etc., of the sand; and particularly upon the amount of water used in mixing. The consistency of the mortar in Table 22 is about that usually employed in laying brick or stone masonry. •In the preceding edition of this book was given a similar table showing the quantities required for dry mortar, that is, for mortar of such consistency that moisture flushed to the surface when the mortar was struck with the back of the shovel used in mixing. Dry mortar is less dense and requires less cement than plastic, since more air is entrained in the mixing. If the mortar is mixed wetter than that in Table 22, as wet, for example, as is usually employed in making concrete, it will be more dense and hence will require more cement and more sand.
The volume of the resulting mortar is always less than the sum of the volumes of the cement and sand, or of the paste and sand, because part of the paste enters the voids of the sand; but the volume of the mortar is always greater than the sum of the volumes of the paste and the solids in the sand, because of imperfect mixing and also because the paste coats the grains of sand and thereby increases their size and consequently the volume of the interstices between them. This increase in volume varies with the dampness
and compactness of the mortar. For example, the volume of a rather dry mortar with cement paste equal to the voids, when com pacted enough to exclude great voids, was 126 per cent of the sum of the volumes of the paste and solids of the sand; and the same mortar when rammed had a volume of 102 to 104 per cent. If the paste is more than equal to the voids, the per cent of increase is less; and if the paste is not equal to the voids, the per cent of increase is more. The excess of the volume of the mortar over that of the sand increases with the fineness of the sand and with the amount of water used in mixing.
The attempt is frequently made to compute the amount of mortar produced by mixing certain quantities of cement and sand, knowing only the per cent of voids in the sand; but the data in the preceding paragraph show that such computations at best are very crude.