Particle clustering phenomena in hot asphalt mixtures with high content of reclaimed asphalt pavements

•Formation of RAP clusters was investigated.•The specific surface to be coated with new bitumen is reduced as a result of clusters.•RAP particles are coated with an active binder film that is covered by a more aged and stiffer binder crust.•Formation of temperature dependent clusters was verified using ESEM and EDX.•A clustering index was introduced as ratio between the modulus of a binder with clusters and without.

One challenge in efficient recycling of asphalt concrete is the adherence of the reclaimed asphalt pavement (RAP) particles or clustering that inhibits fulfilling the aggregate gradation requirements. In order to investigate this phenomenon two types of mixtures containing virgin aggregates, bitumen and 50% and 90% RAP were investigated at different mixing temperatures. The formation of clusters in the mixture was verified using rheological properties (DSR) and imaging (ESEM, EDX). The virgin binder in the mixture was detected using imaging techniques by means of a titanium dioxide tracer and it was shown how the virgin binder surrounded the clusters of RAP depending on mixing temperature.