## 2 Coupling Action of Fatigue Load and Cl\({}^{-}\) Erosion
### Durability Performance of Reinforced Concrete under Fatigue Load Coupled with Cl\({}^{-}\)  
With the increase in corrosion problems in offshore structures in recent years, researchers have started to pay increasing attention to the effects of multiple factors acting together in the durability performance of RC, among which the most serious problem is caused by Cl\({}^{-}\). Cl\({}^{-}\) accumulates on the concrete surface and penetrates into the concrete pores, reaching the reinforcement surface and destroying the passivation film (2018), which eventually leads to internal reinforcement corrosion as the erosion time increases. This corrosion reduces the cross-sectional area of the load bearing of the reinforced steel and the bonding capacity of the reinforced steel to the concrete, and it also leads to the cracking of the concrete's protective layer after rusting and the expansion of the reinforced steel surface. After the fatigue load was applied, stress concentration occurred in the corroded part of the RC reinforcement, leading to a reduction in the load-carrying capacity of the RC and eventually to it reaching the end of its total fatigue life. The effect of dynamic loading on the fatigue degradation of the RC was considerably greater than that of static loading. [PERSON] et al. (2018) used the four-point static and dynamic deflection loading method in Figure 2 and obtained the conclusion that, compared with RC beams subjected to a static load, RC beams subjected to a dynamic load have a faster rate of corrosion of the reinforcement caused by Cl\({}^{-}\) intrusion, as well as a faster decrease in concrete load-carrying capacity by simulating the degree of corrosion of the reinforcement in the tidal zone of offshore structures under the combined effect of four-point bending fatigue load and an artificial seawater environment. [PERSON] et al. (2018) conducted an experimental study on the fatigue performance and Cl\({}^{-}\) permeability of RC beams and found that the fatigue load was more likely to cause Cl\({}^{-}\) to enter into the concrete than static loads. [PERSON] et al. [27] found that the process of RC fatigue damage was basically the same, but the rate of damage development was different. They concluded that the different development rate was probably related to the fatigue load of RC by Cl\({}^{-}\) erosion and stressed concentration in the reinforcement area, and the coupling of Cl\({}^{-}\) erosion and bending fatigue load accelerated the accumulation and development of damage. [PERSON] [28] found that the damage accumulation in the members with internal reinforcement corrosion under fatigue load was greater than that in the members with uncorroded reinforcement, and this damage accumulation was not a simple superposition relationship. [PERSON] [29] concluded that there was almost no difference in the load-carrying capacity between the columns with less Cl\({}^{-}\) corroded reinforcement and uncorroded columns under fatigue load, but in the RC columns with greater Cl\({}^{-}\) corrosion, the ultimate load-bearing capacity of the members was significantly reduced in RC columns with a higher degree of Cl\({}^{-}\) corrosion. [PERSON] et al. [26] further considered the effects of reinforcement rate and working environment, and they found that the combined effect of initial fatigue damage and Cl\({}^{-}\) corrosion led to the corrosion of the tensile reinforcement, which greatly reduced the total fatigue life of RC beams. However, in other studies, such as [30], it was found that hoop reinforcement in re-cantilever piers under Cl\({}^{-}\) corrosion conditions tended to rust first, and the increase in the volume of corrosion products led to cracks in the concrete protective layer, followed by the intrusion of Cl\({}^{-}\) from the cracks to accelerate longitudinal reinforcement corrosion, which eventually led to structural damage. It has been suggested that the fatigue load process leads to cracks in concrete structures, and the generation of cracks accelerates the corrosion of rebar steel in concrete.