Evaluation of Witczak Dynamic Modulus Prediction Model (05-2112)**
Charles W. Schwartz, University of Maryland

The dynamic modulus |E*| is a fundamental property defining the response of hot mix asphalt (HMA) mixtures in flexible pavement systems and is one of the primary material inputs in the mechanistic-empirical design methodology developed in NCHRP Project 1-37A. An empirical predictive model for estimating |E*| for HMA has been developed based on work by Witczak and co-workers over many years (Andrei et al., 1999). This model relates |E*| to loading rate, temperature-dependent binder viscosity, and mixture volumetric and gradation parameters. The accuracy and robustness of the Witczak |E*| model is evaluated through a set of sensitivity and validation studies. Although the results from the validation study of 26 mixtures suggest that the model may overestimate |E*|, particularly at higher temperatures, the overall findings confirm that the Witczak model can provide sufficiently accurate and robust estimates of |E*| for use in mechanistic-empirical pavement performance prediction and design. Because of the strong dominance of temperature influences over other mixture parameters, however, the model may not be able to make fine distinctions between predicted performance for different mixtures under the same environmental and other conditions. Laboratory testing of |E*| may still be required for this purpose.