There are three general types of roadway pavements, namely flexible, rigid, and composite. Flexible pavements typically consist of asphalt concrete placed over granular base/subbase layers sup-ported by the compacted soil, referred to as the subgrade. Some asphalt-paved surfaces consist of a simple bituminous surface treat-ment (BST), while other, lighter-duty asphalt-surfaced pavements are too thin, to be considered as flexible pavements, (i.e., combined layer thicknesses less than 15 cm). Rigid pavements typically consist of a portland concrete layer placed over the subgrade with or without a middle base layer. Composite pavements are typically the result of pavement rehabilitation, whereby portland concrete is used to cover damaged asphalt concrete or vice versa.
The terms flexible and rigid relate to the way asphalt and port-land concrete pavements, respectively, transmit stress and deflection to the underlying layers. Ideally, a flexible layer transmits uniform stresses and nonuniform deflections, while the opposite is true for a rigid layer. In practice, the stress and deflection distributions in asphalt concrete and portland concrete pavements depend on the relative stiffness of these layers with respect to those of the under-lying granular layers. This ratio is much lower for asphalt concrete
than portland concrete, which justifies their generic designation as flexible and rigid, respectively. As described in later chapters, this affects significantly the way these two pavement types are analyzed and designed.
Figure 1.1 shows a typical cross section of a flexible pavement. The asphalt concrete layer, which may consist of two or more sublayers, or lifts, is placed on top of the granular base/subbase layers, which are placed on top of the subgrade. A tack coat layer may be applied to provide adhesion between layers, while a seal coat may provide a pavement surface barrier. A fabric or other geotextile placed between the base and the subgrade prevents migration of fines between them, and maintains their integrity. The base layers can be either compacted gravel, referred to as, simply, granular, or incorporate cement, referred to as stabilized. Typically, the asphalt concrete layer is designed with no interconnected voids (i.e., mix air voids 4–8%), and hence relies on surface runoff for precipitation drainage. Alternatively, asphalt concretes with interconnected voids, (i.e., mix air voids higher than 12%) allow drainage through the surface. This design requires a lower impermeable asphalt concrete layer, to prevent water from penetrating the base layer. Water runoff led to the edge of the pavement can be removed by surface evaporation, ditches, or drainage pipes.