HMA pavements are
flexible pavements. Flexible pavements are so named because the total pavement structure
deflects, or flexes, under loading. A flexible pavement structure is
typically composed of several
layers of material. Each layer receives the loads
from the above layer, spreads them out, then passes on these loads to the next
layer below. Thus, the further down in the pavement structure a particular
layer is, the less load (in terms of force per area) it must carry (see Figure
1).
In order to take maximum advantage of this property, material layers are
usually arranged in order of descending load bearing capacity with the highest
load bearing capacity material (and most expensive) on the top and the lowest
load bearing capacity material (and least expensive) on the bottom. A typical flexible pavement structure (see
Figure 2) consists of:
Surface Course. The layer in contact with traffic loads.
It provides characteristics such as
friction,
smoothness, noise control,
rut resistance and
drainage. In addition, it prevents entrance of surface water into the underlying base, subbase
and subgrade (NAPA, 2001). This top structural layer of material is
sometimes subdivided into two layers: the wearing course (top) and
intermediate/binder course (bottom).
Base Course. The layer immediately beneath
the surface course. It provides additional load distribution and
contributes to drainage and
frost resistance. Base courses are usually
constructed out of aggregate or HMA.
Subbase Course. The layer between the
base course and subgrade. It functions primarily as structural support
but it can also (1) minimize the intrusion of
fines from the subgrade into the
pavement structure, (2) improve drainage and (3) minimize frost action damage. The subbase generally consists of lower quality materials than the base
course but better than the subgrade soils. A subbase course is not always
needed or used.