Variability of rap binder mobilisation in hot mix asphalt mixtures

Assuming the full availability of aged binder in reclaimed asphalt pavement (RAP) is one of the considerations in the design of bituminous mixtures with RAP. However, many studies have shown that in reality only a part of this binder should be reflected in the mixing process. The amount of RAP binder mobilised during mixing could have a significant effect on the performance properties of mixtures. Very little is currently known regarding the variability of this mobilisation subject to the type of RAP used. In this study, the extents of RAP binder mobilisation for three different RAP sources were compared using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy as a major assessment tool. The results of the laboratory tests showed that there is considerable variability in mobilisation, of up to 50% depending on the source of RAP. Viscosity tests on the binders indicated that the workability of the RAP binders at the mixing temperature might play an important role in the mobilisation process. Further, chemical composition analysis through saturates, aromatics, resins and asphaltene (SARA) fractionation and microstructural characterisation using optical microscopy implied that the mobile fractions of the respective RAP binder will likely influence its ability to be active in a mixing process. Overall, it is recommended that practitioners take this variability into consideration. Provisions should be deliberated, such as improved specifications or accommodation of special mixing conditions, depending on the rheology of the RAP used.