Accurately predicting strain responses under axle loadings is crucial for the design of flexible pavements using the mechanistic-empirical approach, especially within the prevalent Pavement ME methodology. These strains are directly used in pavement damage calculation and predicting distresses. The stiffness of the top layer of flexible pavement, asphalt concrete (AC), is influenced by both loading frequency and temperature due to its viscoelastic nature. Typically, the mechanistic behavior of AC is characterized by the dynamic modulus (E*) master curve, derived from laboratory tests under uniaxial sinusoidal loadings. While a full dynamic viscoelastic analysis can precisely predict critical strains, it is computationally demanding. Consequently, Pavement ME employs a layered linear-elastic analysis, relying on the concept of "equivalent loading frequency" to determine the elastic modulus of the AC layer under specific axle loadings. However, this method has limitations in accurately predicting critical strains within the AC layer.This thesis introduces two novel frequency calculation methods: the "centroid of PSD" and the "equivalent frequency." The former computes frequency based on the weighted center of Power Spectral Density (PSD) of vertical stress pulses induced by axle loadings, while the latter iteratively adjusts frequency until it matches strains computed by dynamic viscoelastic analysis under moving loads. The accuracy of these methods, alongside the Pavement ME method, is evaluated against dynamic viscoelastic analysis results under moving loads. Findings reveal that while Pavement ME underestimates surface strains, it provides reasonable predictions with increasing depth for single and multiple axle configurations. Differences in loading frequencies between axle configurations are highlighted, and a correction method based on pulse width and equivalent frequency is proposed. Finally, both the original and corrected frequencies are implemented in MEAPA software to predict long-term pavement distress for real projects in Michigan. The results show that the difference between bottom-up fatigue cracking predicted by the original and corrected Pavement ME frequencies is negligible. The corrected frequency yields higher rutting predictions compared to the original Pavement ME method, ranging from approximately 15% to over 20% for AC rutting and 5% to 10% for total rutting, depending on pavement structures and traffic volumes.
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