Pavements are deceptively complex systems, involving the interaction of numerous variables. Their performance is influenced by factors such as material properties, the environment, traffic loading, and construction practices. Pavement design procedures depend heavily on empirical relationships based on long-term experience and field tests such as the American Association of State Highway Officials (AASHO) Road Test.

The AASHO Road Test, performed in the late 1950s, is the basis for most pavement design procedures which utilize the American Association of State Highway and Transportation Officials (AASHTO) method of equivalency factors. The relationships between traffic loading and pavement performance obtained from the AASHO Road Test are recognized to apply only to the conditions under which they were developed. Extrapolation of these relationships to other conditions is quite problematic. The relative damage to pavements caused by new vehicle characteristics and configurations may be very different from that caused by the axle loads used at the AASHO Road Test.

There have been a number of attempts to derive new load equivalency factors to account for vehicle characteristics and other factors not considered in the AASHO Road Test. Many of these studies have produced new load equivalency factors which are intended to supplement or replace the AASHTO equivalencies currently used by most agencies. These methods for load equivalency factor determination can be divided into two broad categories: (1) empirical methods, which utilize observed loading and distress data to estimate pavement damage; and (2) mechanistic methods, which incorporate pavement mechanistic (primary) response parameters such as stress, strain or deflection to estimate pavement damage.

The mechanistic (primary) response parameters of pavements, required for damage prediction models, can be analytically evaluated. These parameters include: the vertical strain on top of the subgrade, the tensile strain at the bottom of the pavement layer, the surface vertical deflection, and the tensile stress in a concrete pavement. A variety of computer programs are available for the solution of the "boundary value" problem of a multilayered pavement system. These programs are divided into two major categories: (1) finite element; and (2) elastic layer theory. A variety of material constitutive models, such as: linear elastic, nonlinear elastic, viscoelastic, and elasto-viscoplastic models, can be employed to describe the behavior of the pavement materials.

For validation of the analytical model, the calculated mechanistic response parameters can be compared with the measured response parameters obtained from field tests. If accurate correlations between the calculated and the measured mechanistic response parameters can be obtained, then the analytical model can be used to directly estimate primary response load equivalency factors, utilizing any of the deflection-based or strain-based equivalency factor methods such as the Christison method (Christison, 1986).

This study illustrates the usefulness of the finite element method in the analysis of three-layer pavement systems subjected to different types of loading. It is shown that the method is capable of simulating the observed responses of pavements subjected to axle loads with different tire pressures, axle loads with different configurations, and axle loads traveling at different speeds. A variety of material constitutive models such as linear elastic, nonlinear elastic, and viscoelastic are employed in the analyses to describe the behavior of the pavement materials.