Investigation of anisotropic flow in asphalt mixtures using the X-ray image technique: pore structure effect

Abstract

The importance of moisture flow as the main contributor to premature deterioration of asphalt pavements has long been recognised. However, a precise quantification of anisotropic flow and their relationships to complex pore structures in asphalt mixtures are yet to be determined. The objective of this study was to (1) quantify the anisotropic flow in asphalt mixtures and (2) investigate the pore structure effect on anisotropic flow. Three asphalt mixtures with contrasting pore structures were selected for this study. A custom-made permeameter in accordance with the UNI EN 12697/19 procedure was used to analyse the anisotropic flow in asphalt pavement. Results illustrated that the vertical flow rate is quantitatively smaller, even much smaller, than that of the horizontal flow rate which is influenced strongly by the hydraulic gradient and mixture type. Within the same mixture type, the flow under a high hydraulic gradient always has a lower anisotropy than that under a low hydraulic gradient. Within the same hydraulic gradient, stone mastic asphalt mixtures display a higher overall anisotropy than open-graded friction courses and asphalt concrete. Then, the pore structure of asphalt mixtures was quantified by X-ray computed tomography. The analysis of variance indicated that pore distribution characteristics, including air voids content, void diameter and tortuosity, contribute slightly to anisotropic flow, while the anisotropy of pore connectivity, such as connective void content/ratio, is the main factor influencing the anisotropic flow in asphalt mixtures.