Wavelet-based characterisation of aggregate segregation in asphalt concrete X-ray computed tomography images

Aggregate segregation, defined as the non-uniform distribution of fine and coarse aggregates within asphalt concretes (ACs), is recognised as a serious pavement construction problem that reduces pavement life. This paper describes a mathematical approach for quantifying the degree of directional aggregate segregation in ACs. It utilises a wavelet approach for separating coarse from fine aggregates and the normalised wavelet energy to quantify aggregate concentration (i.e. white grey scale intensity). A directional segregation index (SI) is defined as the ratio of fine-to-coarse aggregate normalised wavelet energy. This index is shown to be a good indicator of the degree of segregation observed in X-ray CT images of AC samples. The approach demonstrated using laboratory compacted samples can be applied to quantify segregation in 2D images of composite materials.