Microstructure characterization of cold in-place recycled asphalt mixtures by X-ray computed tomography

This study investigated the microstructure features of cold in-place recycled (CIR) asphalt mixtures by the X-ray computed tomography (CT) and digital image processing (DIP) methods. A variety of CIR asphalt mixtures were prepared considering four types of aggregate gradation and three type of compaction method. The air void distribution characteristics of CIR specimens were analyzed based on the concept of air void gradation. A two-parameter Weibull function was used to fit the air void gradation, of which the statistical parameters are sensitive to both the aggregate gradation and compaction method. The concept of thickness spectrum was then developed to characterize the thickness distribution of cement asphalt (CA) mortar. Lognormal function was observed of good fitness of thickness spectrum. Finally, the uniformity index (UI) was built to evaluate the homogeneity of CIR mixtures and found the UIs of CIR mixture decreases gradually with the gradation from fine to coarse, and among the three compaction methods, the Superpave gyratory compaction is the best in terms of homogeneity. The findings of this study provide a solid foundation in exploring the internal structure and guiding the mix design of CIR mixtures.