EFFECTS OF FWD LOAD DISTRIBUTION ON BACKCALCULATED MODULUS VALUES FOR ASPHALT PAVEMENTS

The elastic moduli of structural layers of road pavements and subgrade soils can be evaluated using a nondestructive method that measures surface deflections, called a falling weight deflectometer (FWD). In the backcalculation process the FWD peak load application on the load plate is assumed uniformly distributed at the road surface. This study evaluates load distribution of FWD load at the surface of asphalt roads. The study shows that the distribution of FWD's plate load depends on flexural strength of the upper pavement layers. When the bituminous layer is thin, the flexural strength of pavements is generally low and deflection bowl is deep. When load distribution is nonuniform, the backcalculated modulus values of bound layers are unrealistically high. FWD maximum deflections measured on thinly paced asphalt roads were corrected to Benkelman beam deflections before backcalculation, which resulted in reasonably accurate modulus values. The FWD plate load distribution was also evaluated by deflection measurements at different pavement temperatures.