Estimating Blending Level of Fresh and RAP Binders in Recycled Hot Mix Asphalt
Despite all advantages of using Recycled Asphalt Pavement (RAP) in new HMA mixtures, the lack of a clear understanding of the nature, and more importantly, the rate of blending between fresh and RAP material has been a major concern. Although it is expected that the degree of blending between the RAP and fresh binder is dependent on mixture temperature, contact time, and constituent binder chemistry, complete or near complete blending is often assumed in order to achieve the full cost savings potential of using RAP. Inadequate performance of some high-RAP mixtures has prompted several recent studies to call into question the degree of blending that truly occurs in high-RAP mixtures. Since the aged binder exists as a thin layer covering the aggregates in RAP material, the mechanical blending through mixing between fresh and RAP binder during mixing is not a sound assumption. A better assumption is that blending between fresh and aged binders in contact is achieved through molecular diffusion driven by molecular energy imbalance and polar attractions. This research presents an experimental-based method to quantify the time and temperature sensitivity of the diffusion rate and ultimate degree of blending that occurs between highly aged and fresh binders. Thin binder samples were conditioned at predetermined temperatures and time intervals to inspect the effect of these factors on the diffusion process between two binders. The rheological response of the binder samples were determined using the Dynamic Shear Rheometer (DSR), and was used in an analysis procedure developed to estimate the diffusion rate and blending capacity. Theoretical calculations for composite material were used to verify the diffusion rates and degree of blending estimated from laboratory measurements. The effect of binder compatibility was investigated using two binder sources with extreme differences in composition. In-service blending capacity was also demonstrated for two different RAP materials.