Aggregate base is typically composed of crushed rock capable of passing through a rock screen. The component particles will vary in size from 3/4 inch down to dust. The material can be made of virgin (newly mined) rock, or of recycled asphalt and concrete.

Base is used as a base course in roadways, as a base course for cement pads and foundations, and as backfill material for underground pipelines and other underground utilities.

"Base course" refers to the sub-base layer of an asphalt roadway. Generally consisting of larger grade aggregate, spread and compacted to provide a stable base for further layers of aggregates or asphalt pavement. Aggregate base course is often referred to as ABC.