Effect of Mineral Fillers on Ageing of Bituminous Mixtures

Abstract

Ageing of bitumen has been long-recognized as one of the major reasons responsible for the gradual deterioration of asphalt pavements. Age-hardening of the binder leads to the embrittlement of the overall asphalt mixture, which entails its increased susceptibility to traffic and environmental induced damage. As a result, high maintenance efforts as well as increased expenses are required to allow for an asphalt pavement to reach its expected service-life.
Past research efforts have established a solid background with respect to the implications of bitumen ageing on the overall response of an asphalt mixture as well as on the binder’s physico-chemical properties. In the same framework, studies have demonstrated that the ageing process of bitumen in the field is not solely a function of the bitumen type itself, but rather, added effects have been identified attributed to the mineral matter and the asphalt mixture design parameters.
This thesis attempts to provide a deeper understanding of the effect of mineral aggregates on the ageing of bitumen, and more specifically, the effect of a special fraction of the solid phase in asphalt mixtures, the mineral fillers. Six different mineral fillers were employed in this research, covering a wide range of physical and composition-related (i.e. elemental/mineralogical) properties. Bitumen-mineral filler blends were prepared, according to a single design protocol, and the resulting mastics along with neat bitumen were subjected to accelerated laboratory ageing by means of the Pressure Ageing Vessel.
The rheological (i.e. Dynamic Shear Rheometer) evaluation of the resulting materials and the derivation of ageing indices revealed the overall ability of the mineral fillers, regardless of their individual properties, to mitigate the ageing of bitumen incorporated in the mastics. The chemical (i.e. Fourier Transform-Infrared Spectroscopy) investigation of the materials showed that, in fact, the chemically active mineral fillers catalysed the oxidation of bitumen incorporated in the mastics. These results allowed for the identification of two mechanisms through which the effect of the mineral fillers on the ageing of bitumen occurs. The first one is related to the physical presence of the mineral matter in the mastics, whereas the second one to the developed physico-chemical interactions between the mineral fillers’ particles and the bitumen. Basic mineral fillers were found to be more efficient in reducing the age-hardening of mastics, compared to acidic ones, by allowing for more intensive interactions between the mastics’ constituent materials. Moreover, there are indications that the mineral fillers’ specific surface area also has a primary role to the developed interactions, and, by extension, to the ageing behavior of the mastics.
Finally, in addition to the aforementioned main findings, binders were extracted and recovered from the aged mastics, in an effort to derive further information on the effect of mineral fillers on ageing of bituminous mixtures by investigating the aged mastics’ constituent materials on their individual level. The rheological and chemical examination of the recovered materials did not lead to any further insight regarding the research questions of this study. Instead, features were revealed which manifest that the extraction and recovery of bitumen may not be a suitable approach for the investigation of the research problem addressed in this thesis.