302. In determining the value of a certain material for aggregate, at least six characteristics are to be considered,— the strength and durability of the stone, the size and shape of the fragments, the volume of the voids, and the character of the surface to which the cement must adhere. As gravel is usually from the igneous rocks, its strength and durability are not often open to question. This may or may not be so in the case of broken stone, but the question of relative value of gravel and broken stone, which is so frequently conclusively settled either one way or another, seldom hinges on this point. As to the average size of the fragments, it is evident that as a general proposition it must be allowed that by proper screening either broken stone or gravel may be obtained of any desired size.
Of the three remaining characteristics, the shape of the fragments, volume of voids, and character of surface, the first is probably the least important and the third of the greatest moment. The round pebbles of the gravel slide readily one on another, and do not interlock to give a good bond. The angular fragments of broken stone give a better bond, but on the other hand, if not thoroughly tamped, are likely to bridge, or arch, and thus leave holes in the mass. On account of the shapes of the fragments and because the sizes are usually more varied in gravel, the latter has generally a smaller percentage of voids; thirty to thirty-seven per cent, voids in gravel, and forty to fifty per cent, in broken stone, may be considered to give, in a general way, some comparative figures. Coming now to the character of surface, cement will not usually adhere so firmly to the smooth surface of the gravel as to the freshly broken surface of the fragments of stone, but this cannot be considered a universal rule, for the strength in adhesion is not simply a matter of smoothness or roughness as it appears to the eye or the touch. The adhesion to limestone may be very much stronger than to a sandstone which has a rougher appearance.
303. Summing up the relative advantages, we find that the gravel is suitable for concrete because, first, it is not likely to bridge and leave holes in the concrete; if mixed rather wet, very little tamping is required to compact it; and second, the usual smaller percentage of voids makes it possible to secure a compact concrete with a smaller amount of mortar than would be required for broken stone. On the other hand, the angular fragments of broken stone will knit together, as it were, to form a strong concrete if properly tamped, and the very important question of a suitable surface for adhesion is usually in favor of the broken stone. It is evident, then, that this matter must resolve itself into a question of relative Cost and suitability, and a general statement that either gravel or broken stone is superior, is not tenable. One experimenter using a small percentage of mortar in the concrete, so that the voids in the broken stone are not nearly filled, may conclude that gravel is the better, while another experimenter using a larger amount of mortar, filling the voids in the broken stone but giving a large excess of mortar for the gravel, will conclude that broken stone is much to be preferred.