Dynamic modulus prediction of HMA mixtures based on the viscoelastic micromechanical model

Dynamic modulus (vertical bar E-*vertical bar) of hot-mix asphalt (HMA) mixtures is one of the fundamental engineering properties measured by the simple performance tester and has also been incorporated as a basic input into the American Association of State Highway and Transportation Officials Mechanistic-Empirical Design Guide for flexible pavements. Although direct laboratory testing and empirical equations (such as the Witczak model and the Hirsch model) provide two ways to obtain the values of dynamic modulus of HMA mixtures, a predictive model based on the microstructure of HMA mixtures is more desirable. This paper presents a viscoelastic micromechanical model to predict the dynamic modulus of HMA mixtures based on the elastic-viscoelastic correspondence principle. In this model, HMA mixtures are treated as a composite by embedding the mastic (or asphalt binder)-coated aggregate particles into the equivalent medium of HMA mixtures. Using the proposed model, a solution was obtained to predict the elastic modulus of HMA mixtures. Based on the elastic-viscoelastic correspondence principle, a viscoelastic equation was derived to predict the complex modulus and subsequently the dynamic modulus of HMA mixtures. The developed equations had the capability of taking into account both aggregate gradation and air void size distribution. Laboratory experiments were conducted to verify the developed model. The dynamic modulus values of mastics and HMA mixtures were obtained through direct laboratory testing. The dynamic modulus of mastic was then used to predict the dynamic modulus of laboratory-prepared HMA mixtures with the newly developed model. Laboratory test results showed that a discrepancy exists between the calculated and measured dynamic moduli. The reason for the discrepancy between measured and predicted dynamic moduli and the factors affecting the dynamic modulus were also explored in the paper.