## 1 Introduction
[PERSON] et al. studied the mechanical properties of asphalt pavement under F-T cycles. The results showed that the compressive strength and elastic modulus of asphalt pavement decreased with the increase in cycles, and the rate of degradation slowed after eight cycles [15]. The performance of asphalt mixture is closely related to the adsorption of asphalt, the content of pore distribution in the framework and the structure of the asphalt mixture. Research on the development process of volume and its morphology and distribution characteristics can provide a valuable experimental basis for further understanding the damage mechanism and performance evolution law of asphalt mixtures. By analyzing the homogeneity index of asphalt mixtures, the weak position of the internal structure of the specimens can be judged. The void condition of three gradation types (AC, SMA and OGFC) is compared, and an accurate void prediction method for asphalt mixtures is established [16; 17]. [PERSON] et al. have studied different asphalt mixtures (AC-13, AC-16, AC-20 and SMA-13). The results show that SMA-13 asphalt mixture has better compaction performance and high temperature stability than AC-13 asphalt mixture, AC-13 asphalt mixture has better high temperature stability and compaction performance than the AC-16 and AC-20 mixtures [18]. The results show that there is a linear relationship between the equivalent diameter of micro-voids and the properties of materials [19]. [PERSON] et al. created a finite element model of asphalt mixture structure randomly. The mechanical properties of the model were evaluated by viscoelastic constitutive relation. The effects of aggregate gradation and void fraction on the mechanical properties of the asphalt mixture were studied [20]. [PERSON] et al. studied the void ratio, indirect tensile strength and indirect tensile stiffness modulus of modified asphalt mixture under a F-T cycle. The results showed that diatomite and SBS could reduce the void ratio of asphalt mixture [21]. In recent years, graphics processing technology has been widely used in engineering research. The method of obtaining material microstructure data by accurate processing, identification and detailed statistical analysis of the obtained graphics has been adopted by many road material researchers. [PERSON] et al. obtained a nondestructive image of asphalt mixture samples through X-ray tomography, then extracted the parameters such as voids, average void size, connectivity porosity and void number by digital image processing technology, and established the relationship between information entropy and pore structure [22]. The internal microstructure of the specimens was established by linear tomography, and the three-dimensional model was processed using digital image processing technology. The shape and distribution of the voids and the expansion of voids were obtained. The mechanism of high temperature damage of asphalt mixture was studied [23]. [PERSON] et al. analyzed the dispersion of recycled rubber asphalt mixture by CT and IPP image processing software. The microstructure of the asphalt mixture was studied [24]. [PERSON] et al. carried out F-T cycle experiments on three kinds of asphalt mixture. The distribution of interstitial space and changes in the internal structure of the asphalt mixture samples were analyzed by X-ray tomography and image processing technology during F-T cycles. In this paper, the base matrix asphalt mixture, nano-TiO\({}_{2}\)/CaCO\({}_{3}\) (NTC)-modified asphalt mixture, basalt fiber (BF)-modified asphalt mixture and nano-TiO\({}_{2}\)/CaCO\({}_{3}\) and basalt fiber (NTC-BF) composite modified asphalt mixture were studied. Based on image processing technology, the CT scanning images before and after F-T cycles were obtained and processed. The pore distribution and morphology of four kinds of asphalt mixtures were studied. The influence of F-T cycles on the performance of test asphalt mixtures was analyzed from the micro point of view.