Characterization of the Stiffness of Unbound Materials for Pavement Design: Do We Follow the Right Approach?

Two main approaches have been followed to determine the modulus of unbound materials in six asphalt pavement sections tested at the CEDEX test track. The first one consists on back-calculation from the deflection bowls measured under the load of a falling weight deflectometer. In the second approach, a nonlinear constitutive relationship is calibrated from repeated load triaxial testing, and layer modulus is estimated through this relationship for the stresses calculated under traffic loads by using a multilayer linear elastic model. Significant differences were found between the two procedures, with back-calculation resulting in higher modulus. The two approaches were evaluated by comparing model predictions to actual response measured under the load of the traffic simulation vehicles of the test track. The results indicate that back-calculation provided more realistic estimations of the stiffness of unbound materials than the laboratory approach. This last approach significantly underestimated the stiffness of granular materials located at shallow depths in the pavement structure.