The components of a flexible pavement are shown in fig. 1: subgrade
or prepared roadbed, the subbase, the base, and the wearing surface. The
performance of the pavement depends on the satisfactory performance of each
component, which requires proper evaluation of the properties of each component
separately.
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| Fig. 1: Components of a Flexible Pavement |
Subgrade (Prepared Road Bed)
The subgrade is usually the natural material located along the
horizontal alignment of the pavement and serves as the foundation of the
pavement structure. It also may consist of a layer of selected borrow
materials, well compacted to prescribed specifications. It may be necessary to
treat the subgrade material to achieve certain strength properties required for
the type of pavement being constructed.
Base Course
The base course lies immediately above the subbase. It is
placed immediately above the subgrade if a subbase course is not used. This
course usually consists of granular materials such as crushed stone, crushed or
un-crushed slag, crushed or un-crushed gravel, and sand. The specifications for
base course materials usually include more strict requirements than those for
subbase materials, particularly with respect to their plasticity, gradation,
and strength. Materials that do not have the required properties can be used as
base materials if they are properly stabilized with Portland cement, asphalt,
or lime. In some cases, high-quality base course materials also may be treated with
asphalt or Portland cement to improve the stiffness characteristics of
heavy-duty pavements.
Surface Course
The surface course is the upper course of the road pavement and
is constructed immediately above the base course. The surface course in flexible
pavements usually consists of a mixture of mineral aggregates and asphalt. It
should be capable of withstanding high tire pressures, resisting abrasive
forces due to traffic, providing a skid resistant driving surface, and
preventing the penetration of surface water into the underlying layers. The
thickness of the wearing surface can vary from 3 in. to more than 6 in.,
depending on the expected traffic on the pavement. The quality of the surface
course of a flexible pavement depends on the mix design of the asphalt concrete
used.
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