## 2 Coupling Action of Fatigue Load and Cl\({}^{-}\) Erosion
### Transport Properties of Cl\({}^{-}\) under Fatigue Loading
[PERSON] [12] found that, when concrete was exposed to a low stress level, the Cl\({}^{-}\) diffusion coefficient of concrete first decreased, which was due to the extrusion of pores at a low stress level, but the Cl\({}^{-}\) content inside the concrete increased significantly and Cl\({}^{-}\) resistance decreased as the stress level and the amount of fatigue increased in the concrete. In contrast to the compression fatigue load, the bending fatigue load is not the same in the tensile and compression areas of concrete. [PERSON] et al. [13] concluded that bending fatigue load caused damage to the concrete interior and decreased the anti- Cl\({}^{-}\) erosion performance of concrete, and the free Cl\({}^{-}\) content in concrete increased with the increase in fatigue damage variability. The free Cl\({}^{-}\) content in the concrete tension zone was higher than that in the compression zone after fatigue damage, and the damage in the concrete tension zone was more severe than that in the compression zone under the same stress level. The effect of fatigue load on the durability of concrete has been attributed to changes in residual strain. [PERSON] et al. [9] observed that compressive fatigue load had a significant effect on residual strain, and the concrete was subjected to compressive fatigue damage through residual strain in three stages; concrete damage accumulated more rapidly as the residual strain increased. [PERSON] et al. [14], [PERSON] et al. [15], and [PERSON] et al. [16] concluded that the Cl\({}^{-}\) permeability of concrete after fatigue load also mainly depends on the residual strain of concrete, which gradually increases as the residual strain increases. The stress level required, when the Cl\({}^{-}\) permeability of concrete started to increase significantly after unloading by multiple fatigue loads, was lower than that of concrete after unloading by one load, which is consistent with the gradual accumulation of internal damage in concrete under fatigue load. [PERSON] [17] and [PERSON] [18] studied in detail the relationship between the residual strain and Cl\({}^{-}\) diffusion coefficient of concrete under bending fatigue load and Cl\({}^{-}\) coupling, and they found that the Cl\({}^{-}\) diffusion coefficient increased after the residual strain increased. Figure 1 shows the variation of Cl\({}^{-}\) diffusion coefficient with residual strain for (a) plain concrete and a (b) High-Performance Concrete (HPC) and High-Performance Fiber-Reinforced Cement Composite (HPFRCC). The Cl\({}^{-}\) diffusion coefficient increased significantly after the residual strain reached a certain critical value. This critical value was identified as the starting point for the concrete resistance to Cl\({}^{-}\) diffusion. Some researchers conducted a numerical analysis of the microstructure of fatigue load in concrete. Based on the study of concrete fracture mechanics, [PERSON] et al. [19] investigated  
Figure 1: Comparison of Cl\({}^{-}\) diffusion coefficients in concrete for different degrees of flexural fatigue damage (**a**) plain concrete; (**b**) HPC and HPFRCC [17; 18]. the mechanism of fatigue crack growth under cyclic loading and observed a slow crack expansion rate at low stress levels. As the strain level increased, the crack dilation rate increased. [PERSON] et al. (2018) concluded that the cracks generated by the fatigue load had a greater effect on the pore size of the concrete around them, and if there were continuous cracks, the effect of penetration would be greater than that of diffusion. Cl\({}^{-}\) erosion has been incorporated into concrete fatigue loading models to study the durability of concrete. [PERSON] (2018) concluded that the permeability of concrete at low stress levels that produce deformation was closely related to concrete pores and found that it was the initial porosity of concrete that was the main cause of Cl\({}^{-}\) diffusion.