Evaluating Diffusion and Aging Mechanisms in Blending of New and Age-Hardened Binders During Mixing and Paving

As virgin pavement material sources become scarcer and costlier, the use of higher quantities of reclaimed asphalt pavement (RAP) in the production of new asphalt mixes becomes desirable. A major concern associated with the use of high percentages of RAP in new mixes is the interaction between age-hardened and new binders because the performance of the mix can be highly influenced by the properties of the composite binder. The objective of this study was to use asphalt binder testing and diffusion and aging theory to investigate the evolution of blending of virgin and RAP binders during asphalt mix production, storage, and placement. The rheological properties of a two-layer asphalt binder sample composed of virgin and simulated RAP binder and conditioned over time at different temperatures were measured with a dynamic shear rheometer. The conditioning process simulated the hot-mix asphalt (HMA) and warm-mix asphalt (WMA) time–temperature paths during mixing, storage, and placement. The diffusion and aging coefficients for the composite binder were estimated by comparing measured shear stiffness values with those predicted with a diffusion model and considering asphalt binder aging over time. The diffusion model was solved numerically with the finite control volume approach. On the basis of the preliminary laboratory test results, full blending of the age-hardened binder and the new binder was observed during HMA mixing and construction, while only partial blending was achieved during WMA production and construction. The blending of age-hardened and new binders could potentially be improved with the aid of warm-mix admixtures, rejuvenating agents, or both.