BROKEN STONE.
In masonry construction broken stone is used as one of the ingredients of concrete. Any hard and durable stone is suitable for use in making concrete. Trap, granite (not only true granite, but also syenite, diorite, gneiss, etc., which are frequently called granites), limestones, and the more compact sandstones make good broken stone for concrete; while the looser-textured sandstones, shales, and slates are poor for this purpose. The suitability of any particular stone for making concrete may be tested by using it in making a cube of concrete and crushing it at any age desired; if the fragments of the stone pull out of the mortar, the adhesion of +he cement limits the strength of the concrete, but if the fragments of stone are broken across, then the strength of the concrete is limited by the shearing strength of the stone. A stone which breaks in approximately cubical pieces rather than in long, thin, splintery fragments should be preferred, since the latter is liable to break under pressure or while being rammed into place, and thus leave two uncemented surfaces.
The stone should be broken small enough to be conveniently handled and easily incorporated with the mortar; but, other things being the same, the larger the stone, the stronger and the denser the concrete. For plain concrete the stone is usually broken to pass any way through a 2- or 2i-inch ring; but for rein forced concrete (Chap. VIII), the stone is broken to pass a f- or 1-inch ring, the smaller stone being used so the concrete may fit itself more closely around the reinforcing metal. The finer the stone is broken the greater the cost; and the finer the stone the greater the surface to be coated, and consequently the greater the amount of cement required.
Stone is sometimes screened to approximately one size. This is only a waste of labor and material, for the screened stone requires more cement and makes a weaker concrete.
The per cent of voids in a mass of broken stone may be determined by either of the two methods employed in finding the voids in sand (see § 194-95).
If the stone is porous, it is best to wet it, so as to determine the voids between the fragments; for the water absorbed by the material should not be included in the voids, since when the concrete is mixed the aggregate is usually dampened,—particularly if it is porous. Of course, in wetting the aggregate before determining the voids, no loose water should remain in the pile. The voids may be determined for the material either loose or compacted. The proportion of the voids is found to determine the amount of mortar that will be required to fill the voids of the concrete in place; and therefore it is better to determine the voids in the compacted mass, since the concrete is usually rammed when laid. The compacting may be done by shaking or by ramming, the latter being the better method, since it more nearly agrees with the conditions under which the concrete is used, and, further, since in compacting by shaking the smaller pieces work to the bottom and the larger to the top, which separation increases the percentage of voids.
The method of determining voids by direct measurement usually gives results slightly too small, owing to the difficulty of excluding all the air-bubbles. However, a high degree of accuracy can not be expected, since the material is neither uniform in composition nor uniformly mixed.