Impact of reclaimed asphalt pavements on pavement sustainability

Highway pavements are one of the main building blocks of the United States infrastructure and economy. Asphalt concrete is the most common material to construct highway pavements. Billions of dollars are spent every year to maintain and rehabilitate two-million-mile the U.S. highway network. Asphalt pavement recycling is one of the few ways to reduce the amount of dollars spent on maintenance and new pavements construction. Reclaimed asphalt pavements (RAP), being a source of aggregates and asphalt binder, is the most recycled material in the U.S. However, incorporating high amount of RAP in asphalt mixtures can pose significant mix design issue and could compromise the pavement performance. Technical complications aside, for RAP to be considered a sustainable material, it is essential for it to be cost effective and socially and environmentally beneficial. The main objective of this study is to evaluate the feasibility of using high RAP in base-course asphalt mixtures. A holistic approach is taken to achieve the objective of the study; mixtures with high RAP contents are not only designed and characterized, their economic and environmental impacts have also been evaluated. The asphalt mixtures with high RAP content (up to 50%) are designed with desired and similar volumetrics as those of asphalt mixtures prepared with virgin materials, setting a great precedent for any future study conducted on high RAP content. The effect of RAP content as well as the effect of binder-grade bumping on the laboratory performance of asphalt mixtures was evaluated. Results showed that the asphalt mixtures with RAP can perform equal to the mixtures produced with virgin aggregate provided they are designed properly. The asphalt binder-grade bumping is found effective in helping to retain the original properties of the virgin mixture. An in-depth multiaxial viscoelastic characterization of the recycled mixtures is conducted by implementing a novel analytical approach. The new approach bypasses the controversial viscoelastic Poisson’s ratios and measures Young’s, shear, and bulk moduli directly in time domain. It has been shown that incorrect assumption of constant PRs for viscoelastic materials can lead to significant errors in estimating the moduli values. Use of Poisson’s ratios should be completely avoided in characterizing the asphalt concrete. The outcome of life cycle cost analysis (LCCA) and life-cycle assessment (LCA) conducted in this study showed viability of using high RAP content in asphalt mixtures. Significant reduction in cost as well as in energy consumption and global warming potential (GWP) have been observed. The economic and environmental LCA conducted under various performance scenarios highlighted the importance of achieving equivalent field performance for recycled mixtures to that of the virgin mixtures. The actual field performance of these mixtures would eventually dictate their net benefits over the virgin mixtures.