Investigation on the microstructure of recycled asphalt shingle binder and its blending with virgin bitumen

Abstract

Recycling waste roofing shingles into pavement construction has attracted more attention due to their high content of usable bitumen. Waste roofing shingles have gone through an air-blowing process during production and are exposed to severe weather for many years during service life, which yields extremely aged asphalt binder. The difference in nature between virgin binder and binder from recycled asphalt shingle (RAS) has led to concerns over binder blending and compatibility of asphalt paving mixtures containing RAS. Therefore, usage of RAS has been commonly limited to a maximum of 3–5% incorporation. Currently, there is very little information in the literature that has addressed the RAS–virgin binder blending issues. This paper used atomic force microscopy (AFM) to characterise microstructural properties of the selected virgin, post-manufactured RAS and post-consumer RAS (tear-off) binders, as well as the temperature dependence of microstructure in one type of tear-off RAS binder. Meanwhile, the mode of interaction while bringing together virgin and RAS binders, that is, the extent of mixing or blending, was first observed in this study. According to the observations, AFM proved to be capable of differentiating virgin binder from RAS binder in terms of microstructure. The microstructure of tear-off RAS binder was found to be temperature dependent at moderate temperatures, but changed very little between 60°C and 180°C. Virgin binders selected in this study could not blend through an RAS-binder layer of 300 µm within 30 min at 180°C. Based on the observations, RAS binder was found to interact with virgin binder through “mixing” rather than “blending”, in a mixing zone of 25–30 µm.