Frequency determination from vehicular loading time pulse to predict appropriate complex modulus in MEPDG

The complex modulus is used by the Mechanistic-Empirical Pavement Design Guide (MEPDG) for hot-mix asphalt (HMA) time and temperature dependency simulation. The correctness of the design process essentially depends on the accuracy of the conversion from the time domain to the frequency domain since the complex modulus is measured in the frequency domain while vehicular loading is applied in the time domain. That the frequency is calculated as the inverse of the loading time is assumed by the current MEPDG approach. That the frequency can be determined from the angular frequency is suggested by another approach. These two techniques are analyzed in the paper, and it is demonstrated that they only represent approximations for specific cases not encountered in pavement systems. There is presentation of a novel approach for accurate determination of vehicular loading frequency based on a detailed Fourier analysis. A direct and reliable technique to obtain moving load frequency spectrum based on the time pulse data is provided by the proposed Fourier approach. Data from the Virginia Smart Road were used to evaluate and validate this approach. There was analysis of the field loading frequency spectra at different depths and under various vehicle speeds. Analysis results indicated that the MEPDG approach is associated with a 40 to 140% error range in frequency estimation depending on depth of calculation and the vehicle speed. There is estimation of the natural frequency of an HMA layer and investigation of the delayed-response association with HMA viscous properties during the unloading phase based on this method.