Aggregate may be defined as the solid materials contained in concrete which play no part in the chemical reactions that cause the concrete to set.
Aggregates can be classified as Normal weight or Lightweight.
Normal weight aggregates generally consist of various combinations of naturally occurring sands, gravel and stones, and of different sizes of crushed rock. Their relative densities usually lie between 2.4 and 2.9 and their strength is at least as great as that of normal cement mortar.
Lightweight aggregates are generally manufactured (for example expanded clay or expanded shale) but can also be natural rock (for example pumice). Their relative densities usually lie between 1.2 and 2.0 and their strength is normally less than that of normal cement mortar.
It is usual to consider the fine aggregate and the coarse aggregate in a concrete mix separately. The DOE Method, used by Firstmix, was developed for aggregates conforming to BS 882 (see References 5) and the fine aggregate is defined as containing a high proportion of particles passing a 5mm (0.197 in.) sieve, while the coarse aggregate is defined as containing a high proportion of particles retained by a 5mm (0.197 in.) sieve. In the US and elsewhere a 4.75mm (0.187 in.) sieve is used as the limit, but this difference is not so great as to prevent the use of the DOE Method for aggregates which are so classified.