Fundamental and comprehensive characterization of the mobilisation and blending of aged and unaged binders in reclaimed asphalt pavement (RAP) mixtures

Abstract

The use of reclaimed asphalt pavement (RAP) in new bituminous mixtures has become a routine practice in many parts of the world as it is known to provide considerable environmental and economic benefits. However, the mixture design associated with it is complicated, with one of the concerns being the attributes of the resulting binder that is formed in the mix. Specifically, asphalt binders in mixtures with RAP consist of segments of both aged (RAP binder) and unaged (virgin) binders, intended to exist as a single entity. A significant apprehension in the design of such mixes is the undetermined extent of aged binder that is mobilised during mixing, and the ensuing blending of aged and unaged binders. The level of this blending is theorized to control the extent of homogeneity of the binder in the mix, and consequently has an important role in the overall mixture performance. This dissertation research is concerned with systematically investigating and improving the fundamental understanding of the many facets involved in the mobilisation of RAP binder and the subsequent interaction of RAP binders and unaged binders. The first part of this dissertation was focused on developing a novel technique to evaluate RAP binder mobilisation using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) as a major assessment tool. Following this, the effects of important variables in mixture design such as mixing temperature, incorporation of warm mix asphalt (WMA) additives and different RAP materials were assessed. Chemical tests such as saturates, aromatics, resin and asphaltenes (SARA) fractionation and microstructural analysis using optical microscopic techniques were then used to concurrently identify the possible mechanisms dictating the mobilisation process. The analysis results suggested that RAP binder mobilisation is greatly dependent on the mixing temperature, and the inclusion of certain WMA additives will also influence the extent of mobilisation. Moreover, the degrees of mobilisation were seen to vary based on the specific type of RAP used in the mix. Based on these results, it was proposed that viscosity could be a useful parameter for practitioners to assess and standardize RAP variability. In the second part, the compatibilities between unaged and aged binders were studied to fully capture the interaction mechanism of binders in RAP mixtures. An extension of the Hansen solubility parameter (HSP) model of solubility was used to approximate the relative molecular interactions of binders in terms of dispersive interactions, hydrogen bonding interactions and polar interactions. It was seen that some unaged binders and aged binders could markedly vary in terms of certain molecular interactions. This, in turn, is expected to have compatibility-based implications for the mobilisation of RAP binder and its blending with unaged binders. An investigation was also conducted into the chemistry and internal stability of combined blends of RAP binder and unaged binder. It was observed that higher concentrations of aged binders incorporated into unaged binders could change the internal stability of the blends, and plausibly affect the polydispersity of molecules. Hence, some consideration of the characteristics of the intermixing binders could be essential to design the most appropriate mix.