Performance verification of various bulk density measurement methods for open- and gap-graded asphalt mixtures using X-ray computed tomography

This paper presents a study aiming to verify the accuracies of various bulk density measurement methods for open-graded and gap-graded asphalt mixtures using X-ray Computed Tomography (CT) scanning. The industrial X-ray CT scanning was performed on laboratory prepared open-graded and gap-graded mixture specimens. The histograms of gray values from the CT scanning images showed distinct peaks of air voids and asphalt materials, based on which the threshold value for calculating the voxels of air voids was determined by the ISO50 threshold method. In this way, the actual air void contents of the testing specimens were determined successfully and then compared to the values calculated from the bulk densities measured using the selected methods, including the dimensional analysis, automatic vacuum sealing, Parafilm coating and saturated surface dry methods. The results from the statistical analyses showed that for the open-graded mixture, only the automatic vacuum sealing method produced air void contents, which were not significantly different from the CT scanning results. For the gap-graded mixture, neither the results from the automatic vacuum sealing method nor those from the Parafilm coating method showed significant difference compared with the CT scanning results.