Mechanistic Approach for Back-Calculating Asphalt Dynamic Modulus Master Curves from Falling Weight Deflectometer Time-History Data

Accurate in situ performance assessment of asphalt pavements is a critical step in developing cost-efficient maintenance and rehabilitation strategies. Back-calculation of pavement layer modulus from falling weight deflectometer (FWD) field-test data is a widely used method for estimating the load-bearing capacity of flexible pavements. However, traditional back-calculation methods assume purely elastic behavior and overlook the viscoelastic nature of asphalt materials, often resulting in inaccurate assessments. To address this limitation, this study introduces a mechanistic-based back-calculation technique to determine the dynamic modulus ( E *) master curve directly from FWD surface deflection time histories. The method integrates layered elastic theory with viscoelastic principles and uses differential-time Fourier transform analysis to determine the relaxation modulus from measured time-domain data. The corresponding frequency-domain response is computed and then fitted to a fractional viscoelastic model to construct the E * master curve. Validation against laboratory-measured E * values from field-cored specimens demonstrates that the proposed technique generates highly accurate E * master curves for all asphalt layers. Notably, the highest accuracy in the back-calculated E * master curve is achieved for the surface layer, followed by the intermediate and base layers. This framework offers a practical alternative to laboratory E * testing and improves the accuracy of in situ pavement structural evaluations.