Stiffening Mechanisms of Asphalt–Aggregate Mixtures

Hot-mix asphalt (HMA) is a particulate composite of mineral aggregates and asphalt binder in which the aggregate particles reinforce the soft asphalt binder phase, as evidenced by the several orders of magnitude of difference in the stiffness of HMA compared with the binder. To develop a mechanistic way of predicting the stiffness of HMAs, an experimentally constructed database of the stiffness evolution, as the volume concentration of the aggregate particles progressively increases (from asphalt binder to coarse HMA mixture), is essential. In this study, a unique database of the complex modulus (E^*) was collected with a hollow cylinder tensile test device; asphalt–aggregate mixtures with varying volume concentrations of aggregate particles were tested over a range of temperatures and loading frequencies. The complex modulus master curve data for these mixtures were then compared with micromechanical model predictions, on the basis of a differential scheme effective medium theory that is known to capture accurately the reinforcing effect of stiff particles placed in a soft matrix. Stiffening mechanisms, such as volume replacement and aggregate interlock, were explored in light of this new data set and the model predictions. The volume replacement stiffening mechanism was quite pronounced; a considerable amount of aggregate interlock effect was also observed at certain aggregate volume concentrations and testing frequencies and temperatures. A detailed analysis of the data that sheds light on stiffening mechanisms in asphalt concrete mixtures is provided.