Microscopic characteristics of interface transition zones of hot mix asphalt containing recycled concrete aggregates

The understanding of microscopic characteristics of interface transition zones (ITZs) between recycled concrete aggregates (RCAs) and asphalt mastic contributes to the treatment strategy of RCAs and the optimum design of hot mix asphalt. For this purpose, on ITZs samples cutting from stone mastic asphalt (SMA) mixture containing RCAs, scanning electron microscopy, nano-indentation and energy dispersive spectroscopy were performed to examine the micromechanical properties, morphological characteristics, and chemical composition properties. Results indicate that the addition of RCAs increased the high-temperature stability, and decreased the low-temperature cracking resistance, the water stability, and the indirect tensile cracking resistance of SMA. The microscopic characteristics of aggregate, residual mortar, asphalt mastic, and their ITZs show significant differences. In terms of micromechanical parameters, the ITZs between the aggregate and the residual mortar (ITZs-A-R) has a thickness of 25 μm and an average elastic modulus of 24.01 GPa while the ITZs between the residual mortar and the asphalt mastic (ITZs-R-A) own a thickness of 26 μm and an average elastic modulus of 28.68 GPa. Correspondingly, the asphalt mastic adjacent to the RCAs indicates a denser appearance than that unconnected to the RCAs. Therefore, the penetration of asphalt binder helps improve the strength of the porous attached mortar. The energy dispersive spectroscopy presents that the ITZs-A-R is dominated by substances containing calcium oxide and aggregates and the ITZs-R-A is principally composed of asphalt, mineral, powder, and substances containing calcium silicate. The thicknesses of ITZs determined by chemical composition are similar to that determined by nano-indentation. Incorporating RCAs leads to a more complex mechanical composition system and the performance degradation of hot mix asphalt. The microscopic characteristics of the ITZs should be taken into account during the strengthening treatments of RCAs and the measurement of the mechanical properties of hot mix asphalt incorporating RCAs.