Microscopy-based approach for measuring asphalt film thickness and its impact on hot-mix asphalt performance

The thickness of the asphalt binder’s film surrounding the aggregate particles in Hot-Mix Asphalt (HMA) plays a significant role in the durability of the pavement. This study introduces a new approach for determining the film thickness of the asphalt binder using a reflective digital stereo microscope. Image analysis was performed using the thresholding process to distinguish the aggregates, the voids, and the asphalt binder from each other using ImageJ software. Six different mixtures were investigated using this approach. The same materials were used to fabricate those mixtures but with different aggregate gradations. The thickness of the asphalt film was correlated to the dynamic modulus (E*) and rutting measured by flow number. The analysis showed that asphalt film thickness plays a major role in the dynamic modulus measurements at intermediate and high-temperature ranges. The correlation of asphalt film thickness with rutting showed a significant decrease in the flow number as the asphalt film thickness increased. This was more significant for gap-graded and Bailey’s gradation asphalt mixtures compared to the dense-graded asphalt mixtures. Finer dense gradations were slightly less affected by the asphalt film thickness increase compared to the coarser gradations.