What is Aggregate?

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and cement, are an essential ingredient in concrete. A desirable concrete mix consists of clean, hard aggregates free of chemicals or clay and other fine materials that could cause the concrete to deteriorate.

Fine and Coarse (Sand and gravel)

Aggregates are divided into two distinct categories — fine and coarse. Fine aggregates generally consist of natural sand with most particles passing through a #4 sieve. Coarse aggregates are any particles greater than that, but generally range between 3/8 and 1.5 inches in diameter. Gravels and crushed limestone constitute the majority of coarse aggregate used in concrete.

Characteristics to consider in regard to selection

  • Grading — the particle-size distribution for aggregate.
  • Particle shape and surface texture — Particle shape and surface texture influence the properties of freshly mixed concrete more than the properties of hardened concrete.
  • Unit weights and voids — The void content between particles affects the amount of cement paste required for the mix. Larger sizes of well-graded aggregate and improved grading decrease the void content.
  • Absorption and surface moisture — The amount of water in the concrete mixture must be adjusted to include absorption and the moisture conditions of the aggregate.