Complex Stiffness Gradient Estimation of Field-Aged Asphalt Concrete Layers Using the Direct Tension Test

The characterization of viscoelastic properties of the aged field asphalt mixtures has been a challenge for pavement engineers. Instead of characterizing full mixtures from the field cores, only the binder was extracted from the mixture and was evaluated for its aging properties. Because the binder is only one component of the mixture, the properties of the aged binder may not clearly indicate the properties of the aged field mixtures. This study presents a novel method to calculate the complex stiffness gradient of a field-aged specimen using a direct tension test. Because the field asphalt mixtures are not aged uniformly with the pavement depth, there is a modulus gradient through the thickness of the asphalt layer. The asphalt mixture is stiffer at the surface. As a result, rectangular specimens cut from field cores when tested in a direct tension test in electrohydraulic servo machines with feedback frequency tend to oscillate. An analytical method has been developed to analyze the oscillating behavior of the specimen and to produce the stiffness gradient function as it varies with loading frequency and position relative to the pavement surface. Furthermore, a finite-element simulation of the test is conducted to verify the validity and robustness of the proposed method. This method was successfully used to characterize aged viscoelastic properties of field cores obtained from different roads in Texas.