In-situ evaluation of material properties is very important for estimating the structural adequacy of a pavement section under traffic loading. The fundamental material property for the AC layer in a flexible pavement is the complex modulus |E*| or the equivalent relaxation modulus E(t). In-situ |E*| can be used as a quality control tool as well as in estimation of remaining service life of existing pavements, and it is a critical input to the new Mechanism-Empirical Pavement Design Guide (M-E PDG). In this research, a method is presented to backcalculate |E*| or E(t) for the AC layer, in addition to base and subgrade layer moduli, using Falling Weight Deflectometer (FWD) surface deflection time histories. First, the deflection time histories from FWD test data were separated as the dynamic response part, and the viscoelastic response part: The time delay of each sensor is related to wave propagation through the pavement system while the shifted deflection time history without time delay can be considered as the viscoelastic response of the pavement. Second, the time delay of each sensor was used to estimate the elastic modulus of the subgrade layer, based on the wave propagation theory. Third, a forward layered viscoelastic solution was developed based on Schapery's `quasi-elastic' approximation. Finally, backcalculation was numerically done by Newton's method, and followed by the MATLAB internal function `fminsearch', to match the predicted deflection time histories from estimated modulus of each layer against the shifted measurement from the FWD test. The backcalculation results show very good agreement with the actual modulus values for both numerical examples and field FWD test data. The error in the base and subgrade moduli is generally less than 3%. The AC layer relaxation and complex modulus curves match the actual functions within the ranges of 0.0001 to 0.1 sec, and 10 to 10,000 Hz, respectively. The sensitivity analysis shows that errors in the deflection time-histories are the most significant factor affecting the backcalculated AC relaxation and complex modulus curves. Also, the layer thicknesses should be as accurate as possible.
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