Base course

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Base Course in pavements refers to the sub-layer material of an asphalt roadway and is placed directly on top of the undisturbed soil so as to provide a foundation to support the top layer(s) of the pavement. Generally consisting of a specific type of construction aggregate, it is placed by means of attentive spreading and compacting to a minimum of 95% relative compaction, thus providing the stable foundation needed to support either additional layers of aggregates or the placement of asphalt concrete which is applied directly on top of an asphalt sealed Base Course, all resulting in a roadway pavement.

Aggregate Base (AB) is typically made of a recipe of mixing different sizes of crushed rock together forming the Aggregate which has certain desirable properties. 3/4 inch Aggregate Base, Class 2, is used in roadways and is an aggregate made of a specific recipe of different sizes and quality of rock inclusive of 3/4 in (19.05 mm) to fine dust. An aggregate is normally made from newly quarried rock, or it is sometimes allowed to be made from recycled asphalt concrete and/or Portland cement concrete.

Aggregate Base is used as the base course under asphalt concrete pavement roadways, under Portland cement concrete slabs and structural foundations, and as backfill material for underground pipelines and other underground utilities within a roadway.^{[citation needed]}

Aggregate Base Course is often referred simply as AB.^{[citation needed]}