Road layers

A road is a layered structure that spreads concentrated wheel loads down to a level the natural soil can carry. Each layer is stronger and costlier than the one below.

At the bottom is the subgrade, the compacted natural soil or fill. Its strength, often measured by the California Bearing Ratio, sets how thick everything above must be. Above it, in a flexible pavement, come the granular sub-base, which also drains water, then the base course of crushed stone, wet mix macadam or a stabilised material. On top are bituminous layers: a binder course and a wearing course that gives skid resistance and seals out water. Thin prime and tack coats bond the layers.

A rigid pavement puts a concrete slab over a dry lean concrete base and sub-base. The slab spreads load by bending, so the layers below can be thinner.

Water is the main enemy of a pavement. Camber, side drains and drainage layers keep the structure dry, because a wet subgrade loses much of its strength.