Backcalculation of Flexible Pavement Layer Moduli from Traffic Speed Deflectometer Data

In the past few years, state transportation departments in the United States have been investigating the potential use of the traffic speed deflectometer (TSD) in routine network-level pavement structural evaluation, including efforts to compute parameters typically obtained with the falling weight deflectometer (FWD). However, differences in loading configuration (single plate versus dual tires) and mechanism (impact versus moving load) between the FWD and the TSD necessitate modification of conventional analysis methodologies for use with TSD data. The TSD is a trailer truck that uses Doppler lasers to measure the pavement deflection velocities at normal traffic speed. Deflection velocities are then converted to surface deflections with a computational algorithm. This study proposes two approaches to backcalculating layer moduli from the TSD data. The first method uses 3D-Move software and deflection velocities measured directly by the TSD. The 3D-Move software can closely simulate TSD loading by considering moving load, nonuniform contact pressure of the dual tires, and viscoelastic material properties of asphalt layers. The second method uses TSD-computed surface deflections in a linear elastic analysis backcalculation program modified to incorporate a dual-tire configuration. The two methods were evaluated with field data from the MnROAD test facility and two in-service pavement sections in Pennsylvania and Idaho. The results show that the second method can be reliably used in network-level applications until the first method becomes computationally practical. The limitations of use of the TSD deflection measurement in the conventional FWD backcalculation program was also demonstrated.