Highway Engineering

Highway Pavement Materials and Design 193

Roadbase

The roadbase is the main structural layer whose main function is to withstand
the applied wheel stresses and strains incident on it and distribute them in such
a manner that the materials beneath it do not become overloaded.

Surfacing

The surfacing combines good riding quality with adequate skidding resistance,
while also minimising the probability of water infiltrating the pavement with
consequent surface cracks. Texture and durability are vital requirements of a
good pavement surface as are surface regularity and flexibility.
For flexible pavements, the surfacing is normally applied in two layers – base-
course and wearing course – with the basecourse an extension of the roadbase
layer but providing a regulating course on which the final layer is applied.
In the case of rigid pavements, the structural function of both the roadbase
and surfacing layers are integrated within the concrete slab.
In broad terms, the two main pavement types can be described briefly as:

Flexible pavementsThe surfacing and roadbase materials, bound with
bitumen binder, overlay granular unbound or cement-bound material.

Rigid pavementsPavement quality concrete, used for the combined surfac-
ing and roadbase, overlays granular cement-bound material. The concrete
may be reinforced with steel.

The general layout of these two pavement types is shown in Figs 7.1 and 7.2.
Pavements are thus composed of several layers of material. They can consist
of one or more bitumen or cement-bound layers overlaying one or more layers
of unbound granular material which in turn is laid on the in-situ soil (if the
highway is in cut) or imported soil/granular material (if the highway is con-
structed in fill) which exists below formation level (HD 23/99) (DoT, 1999).

Surfacing

Foundation

Wearing course

Basecourse

Roadbase

Subbase (unbound / cement based)

Subgrade

Formation level

Figure 7.1Layers within a typical flexible highway pavement.