## 2 Coupling Action of Fatigue Load and Cl\({}^{-}\) Erosion
### Transport Properties of Cl\({}^{-}\) under Fatigue Loading  
There are two types of fatigue load: by compression or flexion. The stress level of fatigue load is the ratio of the actual stress applied to the breaking strength and is an important factor affecting the effect of fatigue load. The diffusion rate and the Cl\({}^{-}\) content in concrete under various fatigue loads are, thus, different [4]. Under the coupling effect of compressive fatigue load and Cl\({}^{-}\), the change in the pores and the diffusion of Cl\({}^{-}\) into concrete are directly related to the stress level of the compressive fatigue load. [PERSON] [5] and [PERSON] [6] conducted compressive strength tests at various stress levels and clearly observed the effect of the stress level on the diffusion coefficient of Cl\({}^{-}\). After applying compression fatigue load (for five times, the stress level was equal to 0.4 times the level of compressive strength), the resistance to Cl\({}^{-}\) permeability was reduced; after applying compression fatigue load (for five times, the stress level was equal to 0.8 times the level of compressive strength), the resistance of concrete to Cl\({}^{-}\) permeability was further reduced, and it showed that the fatigue load significantly reduced the Cl\({}^{-}\) resistance of concrete. The degree of stress had a direct effect on the transport of Cl\({}^{-}\) in the concrete. The greater the accumulated fatigue damage, the more severe the deterioration of the concrete's durability and the higher the Cl\({}^{-}\) diffusion coefficient. [PERSON] [7] found that the compression fatigue load increased the velocity and depth of the diffusion of Cl\({}^{-}\). When the stress level of the compression fatigue load did not exceed 0.4 times the level of compressive strength, the effect on the Cl\({}^{-}\) diffusion rate and depth was small, while the Cl\({}^{-}\) diffusion rate and depth increased significantly when the stress level exceeded 0.6 times the level of compressive strength. [PERSON] et al. [8] also reported the results of tests conducted with a critical stress level of 0.4 times the level of compressive strength. When the stress level of the fatigue load applied to concrete was lower than the critical stress level, the damage was smaller, the stress level was greater than the critical stress level of the concrete, and the free Cl\({}^{-}\) content at a certain depth in the concrete was significantly increased. The higher the stress level, the greater the Cl\({}^{-}\) content at the same depth in the concrete. [PERSON] et al. [9] considered that Cl\({}^{-}\) erosion on concrete mainly relies on the permeation of water through pores. They studied the diffusion law using a pore solution in concrete under specific stress levels and compression fatigue conditions, and it was found that the increase in stress level and amount of fatigue leads to the development of microcracks, which increases the efficiency of water transfer in concrete and thus leads to an increase in the Cl\({}^{-}\) diffusion coefficient. At low stress levels, the compressive fatigue load can interfere with Cl\({}^{-}\) erosion. In the study of the effect of uniaxial compressive stress on the porosity of cement paste, [PERSON] et al. [10] found that the fatigue load not only did not cause damage to the concrete microstructure, but also acted as a dense porous material, making it difficult for the solution to penetrate into the concrete, thus decreasing Cl\({}^{-}\) diffusivity. In the bending fatigue test, the stress level and fatigue count also influenced the durability performance of the concrete. [PERSON] et al. [11] pointed out that the bending fatigue load reduced the Cl\({}^{-}\) resistance of concrete, while the bending fatigue load increased the Cl\({}^{-}\) diffusion rate, which increased significantly after a stress level greater than 62% of the tensile strength. The service life of concrete structures is shortened due to the coupling effect of the bending fatigue load and Cl\({}^{-}\).