A review of microstructure characterization of asphalt mixtures using computed tomography imaging: Prospects for properties and phase determination

The analysis of microstructural characteristics and evolution of asphalt mixtures is essential due to their impact on the macroscopic performance of the pavement. X-ray Computed Tomography (CT) imaging is an advanced non-destructive technique to investigate the three-dimensional (3D) details of the internal structure of asphalt mixture paving the way for improved mixture design. These 3D details include defects (e.g. pore, voids, cracks), aggregates, bitumen, water and other additives (e.g. rubber, capsules, and fibers). Analyses of the void spaces in the asphalt mixture (qualitative and quantitative visualization) by CT technique is obtained by utilizing the initial contrast and segmenting the CT images where thresholding is frequently used to delineate the pore spaces and other components boundaries. This allows the investigation of physical parameters related to the voids space such as permeability, deformation and cracks. The determination of permeability utilizing the CT approach is based on the analysis of spatial anisotropy of the voids structure such as the shape and content, the average diameter, the pore throat and tortuosity dimensions and the pores interconnectivity. Using CT imaging can reasonably monitor and investigate processes of damage and treatments in asphalt mixtures by analyzing the changes in pores size (expansion or compaction), the coalescing of isolated pores, and the development of new pores. Also, it is feasible to analyze and identify the dimensions, distribution, direction, and propagation of cracks in asphalt mixtures by tracking changes in the number and size of air voids. Furthermore, the distribution of components and various phases including aggregates, fibers, additives, and moisture can be characterized by image analysis based on the difference in brightness level between components of asphalt mixtures such as thresholding, which is based on a combination of atomic mass and physical density. As a three-dimensional imaging approach, CT is rapidly advancing in various fields, such as civil engineering. With the increased advancement and availability of CT systems, especially industrial CT and digital image processing, CT imaging is becoming increasingly popular.