One of the challenges in designing recycled asphalt mixtures with a high amount of reclaimed asphalt pavement (RAP) is estimating the blending degree between RAP binder and added virgin bitumen. The extent of blending is crucial because asphalt concrete response is influenced by the final binder properties. This paper focuses on the evaluation of interaction and extent of blending between RAP binder and virgin bitumen by studying the microstructures of the blending zone with atomic force microscopy (AFM). AFM is used to probe the change of microstructural properties from a RAP binder and virgin bitumen to the blending zone of these two. Averaged microstructural properties have been observed in thin-film blends of RAP binder and pure bitumen. The morphology of the blending zone (spatial extent of about 50 μm) exhibits domains of a wide range of microstructure sizes (160 nm to 2.07 μm) and can be considered to be a completely blended new material that has been observed directly for the first time. The fully blended binder properties are found to be between those of the two individual binders, as could be inferred from the averaged microstructural properties derived from AFM images of the blending zone. This finding is also consistent with the results of mechanical tests by dynamic shear rheometer on the same materials. Finally, a design formula is proposed that relates the spatial dimensions of the blending zone to temperature and mixing time, which will eventually allow the results of this study to be extended from small-length scales up to the engineering level.

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In this paper the results of a model study are reported in which glass beads covered with artificially aged bitumen are mixed with superheated bold beads and virgin bitumen. The goal of the study was to simulate how aged binder and added virgin binder would distribute over RAP and virgin aggregates. Furthermore the goal of the study was to determine whether high preheating temperatures would affect the rheological properties of the binder covering the RAP and virgin aggregates. The study showed that when preheating temperature was 300 or 400°C, the shear modulus of the binder recovered from the super-heated bold particles was about the same as the G * of the binder that was recovered from the particles that were covered with aged binder before mixing. At 300°C the log pen rule seemed to be applicable which was not the case when the preheating temperature was 400°C.

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