###### Abstract  
This paper aims to characterize the three-dimensional (3D) internal structure evolution of asphalt mixtures under freeze-thaw cycles. Asphalt mixtures with three levels of design void content (3%, 5%, and 7%) were prepared in the laboratory. Subsequently, X-ray computed tomography (CT) tests were conducted to capture two-dimensional (2D) images of the internal structure of samples before and after freeze-thaw testing. A set of image processing techniques for reconstructing 3D images of the internal structure were utilized to extract the internal structure properties, which were then used to analyze the changes in the air void distributions and to evaluate the internal structure evolution under freeze-thaw cycles. 3D images reconstructed from X-ray CT images illustrated a dramatic degradation in the internal structure after cyclic freeze-thaw exposure. The change in internal structure occurs mainly in three ways: (1) expansion of existing individual voids, (2) combination of two separated air voids, and (3) generation of new voids. In addition, the parametric analysis of the three-dimensional reconstructed voids revealed that the asphalt mixture void ratio increased with the number of freeze-thaw cycles, while the larger the initial void content, the more pronounced the increase in the specimens. Therefore, asphalt mixture freeze-thaw resistance should be optimized in relation to the design void content. asphalt mixtures; X-ray computed tomography; 3D internal structure; freeze-thaw cycles; design void content +
Footnote †: journal: Applied  
19 April 2021  
Academic Editor [PERSON]  
Received: 19 April 2021  
Accepted: 6 May 2021  
Submitted: 10 May 2021  
**Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. [PERSON] et al. proved that specimens with the same macro-scale air void volume may have diverse aggregate and air void distributions, resulting in distinct performance behavior under freeze-thaw cycles [10; 11]. X-ray CT is a useful advanced technology that allows porous media to be characterized on the basis of their micro-level structure distribution. X-ray CT requires no sample treatment before scanning and is non-destructive to the sample for further experiments, which is essential for studying the evolution of internal structure before and after freeze-thaw cycles [12; 13]. Researchers have conducted numerous studies to characterize the internal structure of asphalt mixtures via 2D and 3D imaging, for specific types of asphalt mixture. They used image analysis to evaluate orientation of the aggregate particles, aggregate gradation, and air void distribution [14; 15; 16; 17]. [PERSON] et al. [18] studied the relationship between the permeability of asphalt mixture and 3D distribution of voids. They quantitatively analyzed the effect of irregular flow paths, effective void fraction, and void surface areas on permeability by using a modified Kozeny-Carman equation. Moreover, the internal structure of asphalt mixtures under mechanical loading was also evaluated by various researchers. [PERSON] et al. [19] defined three parameters to describe the spatial distribution of voids in order to characterize the internal structural damage of asphalt mixtures and quantitatively analyze void content and void size. [PERSON] et al. employed void parameters, including void content and void connectivity, to characterize the moisture content and saturation of asphalt mixtures. The authors correlated these parameters with the relative stiffness obtained from mechanical testing to demonstrate the negative effect of moisture on the performance of asphalt mixtures [20; 21]. Meanwhile, [PERSON] et al. [9] reported that the effect of design of air voids and water saturation on the internal structure evolution of asphalt mixtures before and after freeze-thaw cycles was not negligible. They found out that low air void content and air voids in partially saturated asphalt mixtures possessed strong resistance to freeze-thaw damage.