Characterization of compatibility between aged and unaged binders in bituminous mixtures through an extended HSP model of solubility

Asphalt binders in bituminous mixtures with reclaimed asphalt pavement (RAP) comprise fractions of both aged and unaged binders, existing as a single entity. The degree of blending between the aged and virgin binders dictates the extent of homogeneity of the binder in the mix and consequently its mixture performance. However, their mechanism of interaction has not been well understood yet. In this study, a fundamental measure of compatibility between aged binders and unaged binders was evaluated exercising an extension of the Hansen Solubility Parameter (HSP) model of solubility. Nine binders consisting of unaged binder, field aged binders and artificially aged binders were tested using a method of turbidimetric titrations, to identify the internal stability of the binders and relative extents of different intermolecular interactions in terms of dispersive forces, hydrogen bonding and polar interactions. The results show that some aged and unaged binders noticeably differ in terms of polar interactions, which may lead to low dispersibility of molecules and increased molecular associations. These findings were further corroborated by chemical composition analysis based on saturates, aromatics, resins and asphaltene (SARA) fractionation which verified that the content of the highest polar fraction, i.e., asphaltene, was considerably higher in the aged binders. Overall, the study indicated that there are noteworthy differences in internal stability and composition between aged and unaged binders. Consequently, mixing of certain virgin-aged binder pairs could be more compatible than others depending on their solubility parameters. To attain a homogenous blend while incorporating RAP, a more careful and detail-oriented approach that considers chemistries and inter-solubility of these components is recommended.