Measuring the binder film residues on black rock in mixtures with high amounts of reclaimed asphalt

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Reclaimed asphalt pavement (RAP) and virgin materials were mixed and analysed using electron microscopy and image analysis techniques. Titanium dioxide was added as a tracer in the virgin bitumen to detect the effect of mixing temperature on the “RAP binder film thickness” which is the stiffer binder in the mixture in comparison to the virgin binder. The analysis conducted with the environmental scanning electron microscopy and X-Ray spectroscopy has shown that the RAP binder, instead of completely blending with the new binder, forms a layered structure coating the RAP aggregates. This structure is composed of an inner layer of RAP binder that is not blended with the virgin bitumen indicating the existence of the “black rock”. The RAP binder film thickness is a function of mixing temperature, local aggregate curvature and size of aggregates. The higher the mixing temperature, the thinner the layer of RAP binder tends to be. RAP binder film thickness was measured at micro-scale and subsequently statistically analysed showing that along the surface of the aggregate it is not homogenous; it depends on the local curvature of the aggregate, with positive curvature resulting in a smaller film thickness and negative curvature in larger film thickness.