Use of X-ray computed tomography to evaluate the gradual behaviour of air voids in asphalt mixtures during permanent deformation

Abstract

ABSTRACT X-ray computed tomography (CT) as a non-destructive testing method has been used to evaluate the air voids and void-related distresses in asphalt mixtures. To evaluate the air void development in asphalt mixtures under loading, this study proposed a volume-equivalent maximal ball model based on CT to characterise the changes of void connectivity. The void distribution and shape features were described using the variation coefficient of modified annular-sector segmentation, positional eccentricity ratio, and sphericity. The results indicate that voids are relatively heterogeneous and discrete in stone mastic asphalt (SMA-13) mixture compared with dense-graded asphalt concrete (AC-13), particularly for those with volume smaller than 0.01 mm3. The void connectivity and distribution in SMA-13 seem to be more susceptible to load as its coarse aggregates tend to shift their positions during deformation. Rutting reduces the vertical inhomogeneity of AC-13 but increases that of SMA-13. Moreover, loading complicates the void geometry of AC-13 whereas the opposite is true for SMA-13, foreboding that SMA mixtures maintain the potential to resist rutting failure after 1 h of loading compared to AC types. In general, the topological characteristics of air voids within asphalt mixtures and their dynamic response during permanent deformation are principally dependent on mixture gradations.