## 2 Coupling Action of Fatigue Load and Cl\({}^{-}\) Erosion
### Durability Performance of Reinforced Concrete under Fatigue Load Coupled with Cl\({}^{-}\)
[PERSON] et al. [31] concluded that the load level directly affected the rate of the corrosion of reinforcement within concrete, and that microcracks generated by the fatigue load had a greater effect on the corrosion of reinforcement in concrete than macro-cracks, but to some extent, micro-cracks could heal and reduce the corrosion. [PERSON] et al. [32] also found that the presence of transverse fissures in the tensile surface of RC aggravated the corrosion of the reinforcements. [PERSON] et al. [33] found that the generation of longitudinal cracks was independent of both load stress level and load type, and they concluded that corrosion was not the only factor leading to the generation of cracks. The bonding capacity of reinforcement to concrete was then evaluated by studying the evolution of transverse cracks, and the stress level of bending fatigue load was determined. [PERSON] [28] also found that transverse cracks caused damage to the bonding properties of concrete and reinforcement. The fatigue load not only accelerated the Cl\({}^{-}\) erosion of RC, but also caused the corrosion products of the reinforcement to penetrate the cement paste through the cracks. [PERSON] [34,35] found that the Cl\({}^{-}\) invaded the interior of the concrete and corroded the armature, while the fatigue load led to the detachment of aggregates and cement paste from ordinary reinforced concrete (OPCC). Figures 3 and 4 show the three sites in OPCC and HPC where corrosion products enter the cracks. As shown in Figures 4 and 6, the opening and closing of cracks under fatigue load forced the flow of corrosion products from the reinforcement to the concrete bond into the cracks in OPCC and high-strength concrete (HPC) due to fatigue load. Thus, corrosion products diffuse from OPCC at the  
Figure 2: (**a**) Static load test apparatus. (**b**) Fatigue load test apparatus [25]. cracks caused by fatigue load into the cement paste, while in fatigue-loaded HPC corrosion products stayed at the cracks at the reinforced concrete interface. A numerical analysis was used to better study the causes of RC damage under fatigue stress and CI\({}^{-}\) erosion. [PERSON] [36] established a microstructure model of rust products based on measured data and found that the coupling effect of chloride-salt-fatigue-load led to an increase in the content of rust products, which had a serious negative impact on the RC structure. The stifter rust products also increased as the strain level of the fatigue load increased. The RC protective layer was more prone to cracking due to the coupling of fatigue load and CI\({}^{-}\) erosion, resulting in reduced durability. [PERSON] et al. [37] proposed a simplified model for RC bridge columns with damage due to the CI\({}^{-}\)-induced corrosion of reinforcement, explaining the reduction in the cross-sectional area of reinforcement due to CI\({}^{-}\) corrosion. Similarly, it was found that the rate of reinforcement corrosion changed with different stress levels. [PERSON] et al. [38] combined fatigue damage modelling and crack extension analysis methods, followed by crack and CI\({}^{-}\) diffusion modelling of RC for steel corrosion studies and suggested that the future modelling of steel corrosion and corrosion cracking must be investigated experimentally and numerically. [PERSON] [39] presented a numerical analysis of RC under bending fatigue load, where chloride corroded the reinforcing under fatigue conditions and ultimately affected the structural performance of concrete. Overall, the damage accumulation of RC under fatigue load was higher than that of the RC structure under a static load, and in the coupling of CI\({}^{-}\) erosion and fatigue load, cracks appeared on the surface of the RC structure under fatigue load to provide  
Figure 4: Distribution of corrosion products in dynamically loaded HPC concrete (**a**) 10 mm from concrete surface; (**b**) 18 mm from concrete surface; (**c**) 20 mm from concrete surface (Red arrows point to the location of visible corrosion products) [35].