Evaluation of the Accuracy of Pavement ME Methodology in Calculating Equivalent Loading Frequency and Its Effect on Strain Response Predictions in Flexible Pavements

The prediction of strain responses under axle loadings is critical for flexible pavement design by the mechanistic-empirical approach. The mechanistic-empirical pavement design (pavement ME) method uses linear-elastic analysis to simulate the strain responses under axle loadings, and it relies on the concept of equivalent loading frequency to determine the elastic modulus of the asphalt concrete (AC) layer from the dynamic modulus master curve. The pavement ME method has a simplified procedure to calculate this frequency. The main goal of this study is to evaluate the accuracy of axle loading frequency calculated by the pavement ME method. This paper first introduces the concepts of predominant and equivalent frequencies, provides a brief explanation of the difference between them, and then proposes three predominant frequency methods for evaluation. The accuracy of the pavement ME method and the other methods of calculating the predominant frequency is evaluated in terms of frequency, modulus, and strain by comparing their results with those from dynamic viscoelastic analysis with moving loads. Results show that the time–frequency relationship for predominant frequency is closer to $f=1/(2t)$ than $f=1/t$, assuming that the pulse duration $t$ is accurate. Nevertheless, using $f=1/t$ with the approximate pulse duration as calculated by the pavement ME method gives reasonable predictions of the maximum tensile strain. On the other hand, while it gives reasonable predictions of vertical strains with increasing depth, the pavement ME method can underestimate them near the surface by up to 55%. Overall, even though the procedure for pavement ME frequency calculation is highly simplified, its general performance appears to be acceptable.

The two most typical distresses of flexible pavement are rutting and fatigue cracking, and they are related to two critical strains: vertical strains throughout the layers and horizontal strains at the bottom of the asphalt concrete later of the pavement. The commonly used design approach, the pavement ME method, utilizes the concept of equivalent loading frequency to predict critical strains within the pavement by linear-elastic analysis rather than the full dynamic viscoelastic analysis. This reduces the computational cost significantly. This paper investigates the accuracy of the equivalent loading frequency calculated by the pavement ME method by comparing the frequency values with several other methods, and, finally, comparing the critical strains predicted by the pavement ME method and the results from dynamic viscoelastic analysis, which is considered to be the truth. The results showed that the pavement ME method underestimates vertical strains near the surface of the pavement, which may lead to underestimating rutting (before calibration). However, calibration may be able to remedy the problem. On the other hand, the horizontal strain predicted by the pavement ME method is reasonable, which means the fatigue cracking can be reasonably predicted.