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Prediction of Phase Angles from Dynamic Modulus Data and Implications on Cracking Performance Evaluation

Mirkat Oshone, Eshan Dave, Jo Sias Daniel, Geoffrey M. Rowe


The need for viscoelastic characterization of hot mix asphalt (HMA) is increasing as advanced testing and modelling is incorporated through mechanistic-empirical pavement design and performance based specifications. Viscoelastic characterization includes measurement of the mixture stiffness and relative proportion of elastic and viscous response. The most common method is to measure the complex modulus, where dynamic modulus represents the stiffness and the phase angle represents the relative extent of elastic and viscous response. Determination of phase angle from temperature and frequency sweep tests has been challenging, unreliable, and prone to error due to a high degree of variability and sensitivity to signal noise. There is also a large amount of historical dynamic modulus data that is either missing phase angle measurements or has poorly measured phase angle data that inhibits their use in further evaluation. This paper evaluates the robustness of phase angle estimation from stiffness data for asphalt mixtures. The objectives of the study are to: (1) evaluate phase angle determination from the slope of log-log stiffness master curve fitted with a generalized logistic sigmoidal curve and compare it with lab measurements and the Hirsch model; (2) assess the effect of measured and predicted phase angle on mixture Black space diagram; (3) evaluate the effect of using predicted phase angles on S-VECD fatigue analysis, particularly on damage characteristics curves and fatigue coefficients; and (4) evaluate the impact on LVECD pavement fatigue performance evaluation due to the use of predicted phase angles. Three sets of independent mixtures were evaluated in this study comprising a wide range of mixture conditions. The results indicate a good agreement between measured and predicted phase angle values in terms of shape and peak master curve values. In terms of magnitude, the values from both matched very well for a certain sets of mixtures and subsequently manifested in similar performance predictions. However, for other sets of mixtures a considerable difference was observed between measured and predicted phase angle values as well as S-VECD and LVECD results. The differences may be attributed to the use of different types of LVDTs (loose core versus spring loaded). Another possible explanation of the difference could be the contribution of plastic strain, which may create a difference in phase angles of 1 to 2 degrees.

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