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One of the challenges in designing recycled asphalt mixtures with a high amount of RAP is to estimate the blending degree between RAP binder and the added virgin bitumen. The extent of blending is crucial in this case as 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 ?m) exhibits domains of a wide range of microstructure sizes from 160 nm to 2.07 ?m and can be considered to be a completely blended ‘new material’ which has been observed directly for the first time. The fully blended binder properties are found to be in between those of the two individual binders, as could be inferred from the averaged microstructural properties as derived from AFM-images of the blending zone. This 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. This relation will eventually allow for translating the results of this study from small length scales up to the engineering level.Structural EngineeringCivil Engineering and Geoscience

First Observation of Blending-Zone Morphology at Interface of Reclaimed Asphalt Binder and Virgin Bitumen