In highway engineering, subbase is the layer of

aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...

material laid on the

subgrade In transport engineering, subgrade is the native material underneath a constructed road,http://www.highwaysmaintenance.com/drainage.htm The Idiots' Guide to Highways Maintenance ''highwaysmaintenence.com'' pavement or railway track (US: railroad ...

, on which the base course layer is located. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles. Subbase is often the main load-bearing layer of the pavement. Its role is to spread the load evenly over the

. The materials used may be either unbound granular, or

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixe ...

-bound. The quality of subbase is very important for the useful life of the road and can outlive the life of the surface, which can be scraped off and, after checking that the subbase is still in good condition, a new layer can be applied. Unbound granular materials are usually crushed

stone In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition, and the way in which it is formed. Rocks ...

, crushed

slag Slag is a by-product of smelting ( pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/base metals (by-p ...

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

, or

slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. ...

. Cement-bound materials come in multiple types. Mass

is used where exceptional loads are expected, with thickness usually , and optional reinforcement with steel mesh or polymer fibers. Other

cement bound material A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixe ...

s (CBM), with less strength but also lower cost, are used. They are rated by strength, from the weakest CBM 1 (also formerly known as soil cement) through CBM 2 to CBM 3, 4, and 5, which are more similar to concrete and are called "lean mix". In the UK, the specification for aggregate used as a subbase in the construction of driveways and roads includes MOT Type 1 Stone. The thickness of subbase can range from for garden paths through for driveways and public footpaths, to for heavy used roads, and more for

highway A highway is any public or private road or other public way on land. It is used for major roads, but also includes other public roads and public tracks. In some areas of the United States, it is used as an equivalent term to controlled-access ...

s. Low quality subbase material, including large pieces of rock and concrete, which was hardly acceptable heretofore, can now be re-used when crushed in-situ with conventional milling machines to obtain a homogenous grain size. It may then be treated normally with hydraulic binders, augmented by specific polymer formulations. The subgrade or base course is properly prepared for the required grade and camber of WBM road. The potholes and the depressions on the surface of the road are properly filled up and compacted.

References

{{Reflist Pavement engineering Pavements Road infrastructure