Coarse aggregate is derived from solid rock formations, or from crushed gravels. The rock formation can be of igneous, sedimentary, or metamorphic origin. Recycled materials, principally slag and crushed concrete are frequently used as well. A special type of 'artificial' lightweight aggregate is derived from either vermiculite, expanded slag or expanded, shale, clay, or sometimes slate.
Coarse aggregate is usually defined as material retained on 1/4" sieve ( mm). The gradation or size distribution is determined by use. Normally, the gradation requirements are such that the a given mass of aggregate occupies the smallest volume, i.e., produces the densest mass. On the other hand, for drainage purposes, the gradation peaks on one desired sieve size, giving the most porous and permeable mass.
The density of mass aggregate is determined by the density of the parent rock, but also by the shape of the particles. Equidimensional particles will pack most densely, whereas particles with unequal dimensions will result in loose packing. The particle shape is determined largely by the properties of the rock, but occasionally by the crushing method used.
