Micromechanics-based analysis of stiffness anisotropy in asphalt mixtures

The mechanical behavior of many bound granular materials such as asphalt mixtures is anisotropic in nature. However, the majority of the current mechanical tests and analytical models for asphalt mixtures are based on the assumption of isotropic material properties. This study investigates the stiffness anisotropy of asphalt mixtures using micromechanics-based models. The models' parameters are obtained by quantifying the internal structure anisotropy in terms of the preferred orientation of longest axes and contact normals of aggregates. Image analysis techniques are used to conduct the internal structure measurements. The orientations of the longest axes are found to be easier to measure, and better descriptors of anisotropy, than the contact normals. Finite-element analyses of the internal structure are also used to provide insight into the mixture stiffness anisotropy. The mixture properties are selected to represent a wide range of temperatures. The stiffness in the horizontal direction is shown to be as high as 30% more than the stiffness in the vertical direction. The stiffness anisotropy decreases with a decrease in the mixture temperature. The finite-element results are shown to have very good correlation with the results of the micromechanics model derived based on the orientation of the longest axes of aggregates.