Base layers are the foundation.

The base layer of a road lays the foundation for the upper pavement structure. It must offer excellent bearing capacity, be capable of withstanding a broad range of different climatic conditions, and remain functional over several decades. What materials are used for the construction of base layers? What properties do they need to possess?

A strong base for the upper road structure

Roads must be capable of withstanding the loads imposed by traffic irrespective of weather conditions. The body of a road is constructed from different layers of different thicknesses, depending on the anticipated traffic load.

A layer for the tough job

The lowest layer in the upper road structure is built mainly from granular mineral aggregate. The main function of base layers is to reduce the loads induced by traffic to such an extent that the underlying subgrade is protected from deformation. The required bearing capacity is achieved by using a mixture of gravel, chippings and crushed sand that needs to be compacted to the required density.

A gravel layer to protect against frost

Roads are exposed to particularly high stresses when the water contained in the pavement structure begins to freeze. Water expands when freezing, which can lead to frost damage that will sooner or later have an impact also on the road surface. This is prevented by a so-called frost blanket which usually consists of a mixture of gravel and sand, supplemented by crushed mineral aggregate. When compacted, these layers of frost-resistant materials conduct water away from the upper pavement layers, reducing tensions very effectively at the same time.

The mix makes all the difference

Granular base layers are often overlaid with a bituminous or hydraulically bound base layer, with either bitumen, or cement or lime respectively, used as binding agents. A well-graded mixture of mineral aggregate with a fairly high percentage of crushed stone is characteristic of bound base layers.

Bituminous bound base layers

When using mixes containing bitumen as a binding agent, the resulting base layer is called an asphalt base layer in hot or cold application – depending on whether hot mix or cold mix is used. Bituminous base layers in hot application consist of well-defined mineral aggregate mixtures and road bitumen.

Hydraulically bound base layers

When the mineral aggregate mixture is bound with cement or lime, the resulting base layer is called a hydraulically bound base layer. Mineral aggregate mixtures used for this type of base layer consist of uncrushed gravel or coarse aggregate, chippings, and crushed or natural sand. Ever more frequently, these mixes also contain a percentage of recycled construction materials.