## 4 Durability of Concrete under Dry-Wet Cycles, Fatigue Load, and Cl\({}^{-}\) Erosion
[PERSON] [86] simulated the actual working conditions of RC bridge structures in a coastal environment and undertook wet and dry cycles of seawater with a dry-wet ratio of 7:1 for 180 days at different stress levels and different numbers of fatigue cycles to determine the Cl\({}^{-}\) content of concrete in the tension and compression zones. It was found that the Cl\({}^{-}\) content in the tension and compression zones of fatigue-damaged RC beams was significantly higher than that of undamaged RC beams. When the stress level was \(\leq\)0.3, the chloride content of the concrete in the tension zone increased with the increase in the upper load limit; when the stress level was 0.4 times higher than the tensile strength and the number of fatigue loads was \(\leq\)800,000 times, the chloride content of the concrete in the tension zone increased with the increase in the number of fatigue loads. The chloride content of the concrete in the compression zone did not change significantly with the upper load limit, but it did increase with the increase in the number of fatigue loads. However, [PERSON] [87] simulated the actual working conditions of concrete bridge structures in a coastal environment and chose a test similar to that found in the literature [86] and discovered that the Cl\({}^{-}\) diffusion rate in the compression zone was higher than that in the tension zone for a fatigue stress level of 0.3 times higher than the tensile strength under the combined action of dry-wet cycles, fatigue load, and Cl\({}^{-}\) erosion. Figure 9 shows the variation in Cl\({}^{-}\) content with depth for beams subjected to 800,000 fatigue cycles at different fatigue stress levels. Figure 10 shows the effect of the upper fatigue load limit on the diffusion coefficient at different measurement points, where B represents the beam. As observed in Figures 9 and 10, the Cl\({}^{-}\) content in the tension zone, as reported in the literature [86], was significantly greater than the Cl\({}^{-}\) content in concrete in the compression zone at a stress level of 0.3 times higher than the tensile strength, and it was seen that the Cl\({}^{-}\) diffusion coefficient in the tension zone was greater than the Cl\({}^{-}\) diffusion coefficient in the compression zone. The literature [87] showed that the Cl\({}^{-}\) diffusion coefficient in the compression zone was distributed along the span of the beam at a stress level of 0.3 times higher than the tensile strength, which was significantly greater than that in the tensile zone. The tests in the literature [86,87] were similar, and the opposite conclusions were obtained, indicating that the internal changes in concrete under the combined effects of fatigue load, wet and dry cycles, and Cl\({}^{-}\) erosion were complex, so they led to differences in the conclusions, and more relevant tests were needed to obtain conclusions regarding Cl\({}^{-}\) transport in concrete under the combined effects of multiple factors. The combination of multiple factors affects the durability of concrete, resulting in modeling that is often not comprehensive. [PERSON] et al. [88] established a Cl\({}^{-}\) erosion model for concrete under fatigue load and dry-wet cycles based on the migration model of Cl\({}^{-}\) in dry-wet cycle environments, and they analyzed the influence law of different factors on Cl\({}^{-}\) migration in concrete through indoor tests. [PERSON] [83] established a Cl\({}^{-}\) migration model considering diffusion, convection, and capillary modes; calculated the Cl\({}^{-}\) diffusion coefficient under fatigue load and dry-wet cycles; and obtained the relationship between Cl\({}^{-}\) diffusion coefficient and stress level. The results of the model analysis were basically consistent with the experimental results. [PERSON] [89], after proposing a model applicable to convection and diffusion, found that, at higher stress levels, the Cl\({}^{-}\) erosion rate would increase.