Figure 16-5.Pavement deflection results in tensile and compressive stresses in pavement structure.
asphalt pavements is similar to the problem of designing
any other complex engineering structure. When asphalt
pavement was first being introduced, determining the
proper thickness was a matter of guesswork, rule of
thumb, and opinion, based on experience. Almost the
same situation once prevailed in determining the
dimensions of masonry arches and iron and steel
structures. However, these early techniques have long
since yielded to engineering analysis. Similarly, based
on comprehensive analysis of vast volumes of
accumulated data, the structural design of asphalt
pavements has now been developed into a reliable
There is no standard thickness for a pavement.
Required total thickness is determined by engineering
design procedure. Factors considered in the procedure
are as follows:
1. Traffic to be served initially and over the design
service life of the pavement
2. Strength and other pertinent properties of the
3. Strength and other influencing characteristics of
the materials available or chosen for the layers (or
courses) in the total asphalt pavement structure
4. Any special factors peculiar to the road being
Because weight and traffic volume normally
increase, pavement originally built thick enough to
handle immediate traffic volumes may not be thick
enough and strong enough to handle future needs. With
asphalt pavement, this problem can be met
economically by first building the thickness required,
then adding, when needed, layers of asphalt to increase
total pavement thickness. This procedure is called stage
It avoids excessive investment in the
beginning; and when a new layer of asphalt is added, the
wearing surface is equal to or better than the original.
Several methods for evaluating or estimating the
strength and supporting power of a subgrade are in use
today, including the following:
1. Loading tests in the field on the subgrade itself.
For example, the plate bearing test uses large, circular
plates, loaded to produce critical amounts of
deformation on the subgrade in place.
2. Loading tests in a laboratory using repre-
sentative samples of the subgrade soil. A test commonly
used by the Seabees is the California bearing ratio
(CBR) test, which is sometimes used on the subgrade in
place in the field.
3. Evaluations, based on classification of soil by
identifying and testing the constituent particles of the