## 3 Coupling of Dry-Wet Cycles and Cl\({}^{-}\)
### Transport Properties of Cl\({}^{-}\) under the Action of Dry-Wet Cycles
[PERSON] [70] considered cracks as a favorable area for the initiation of corrosion under dry-wet cycles through which chlorides rapidly enter the concrete interior and concluded that the effect of Cl\({}^{-}\) on reinforcement in concrete under dry-wet cycles was greatly influenced by the relative humidity inside the concrete. In addition to the corrosion of the reinforcements caused by Cl\({}^{-}\), the increase in the oxygen content during drying in wet-dry cycles also resulted in an increase in the corrosion rate of the reinforcements. [PERSON] [71] found a direct relationship between reinforcement corrosion and relative humidity in RC and different rates of the corrosion of reinforcement at different relative humidity values because of the limitation of oxygen transport due to high humidity. [PERSON] [72] investigated reinforcement corrosion in concrete under dry-wet cycling conditions by means of a hypothetical relationship between Cl\({}^{-}\)-corrosion-induced current changes in a variable humid environment and concluded that oxygen diffusion was an important factor affecting reinforcement corrosion during dry-wet cycling. [PERSON] [73] examined the diffusion of oxygen and Cl\({}^{-}\) inside concrete during dry-wet  
Figure 8: (**a**) Rusted condition of the reinforcement. (**b**) SEM morphology of the surface of the reinforcement at the rusted/unrusted junction [63]. cycles: firstly, Cl\({}^{-}\) accumulated on the surface of the reinforcing bars with diffusion, and when a critical amount of Cl\({}^{-}\) concentrates in the reinforcement started to corrode it, and as the wet and dry cycles occurred, the oxygen content in the pores increased during drying, leading to the accelerated corrosion of the reinforcement. [PERSON] [74] conducted a study of Cl\({}^{-}\) erosion under dry-wet cycles using electrochemical methods and also found that the ion diffusion was intense during the wet phase and the corrosion rate of the reinforcement increased rapidly during the drying phase. In addition to the environmental factors that require special attention when modelling the degradation of RC under the action of dry-wet cycles, the corroded state of internal reinforcement also requires attention [75]. In order to create a model of reinforcement erosion by Cl\({}^{-}\) under timed conditions, [PERSON] [76], in a study on the corrosion of reinforcing bars in RC under dry-wet cycles, found that the corrosion of the reinforcing bars was significantly increased with the increase in the crack width under timed conditions, which responded better to the reinforcement corrosion process as a result of the combined effect of dry-wet cycles and Cl\({}^{-}\). Other researchers have simulated the durability of RC under dry-wet cycles and Cl\({}^{-}\) erosion [77; 78], and all of them compared these simulations with actual situations, but it was found that they could not be adapted to most numerical studies of the durability of RC under dry-wet cycles and Cl\({}^{-}\) erosion. The coupling effect of wet-dry cycles and accelerated Cl\({}^{-}\) RC damage, as well as corrosion of the armatures, were more readily observed at the cracks rather than at other sites. In addition to the dry-wet cycles that make Cl\({}^{-}\) penetrate rapidly into the concrete interior and lead to reinforcement corrosion, the presence of oxygen would also accelerate the reinforcement corrosion, while the drying process led to the evaporation of water in the pores, so that oxygen entered the interior of the pores to rust the reinforcement, and the corrosion rate of reinforcement was accelerated under the double corrosion effect, which eventually led to the loss of RC strength. In terms of numerical analysis, it could also reflect the damage of RC structure as a result of dry-wet cycles and Cl\({}^{-}\) coupling, but there were still problems in trying to apply a single model to a wide range of applications.