## 3 Coupling of Dry-Wet Cycles and Cl\({}^{-}\)
### Transport Properties of Cl\({}^{-}\) under the Action of Dry-Wet Cycles
\[C(x,t)=C_{0}+(C_{s,\Delta x}-C_{0})\times\left[1-erf\frac{x-\Delta x}{2\sqrt{ D(t)t}}\right] \tag{1}\]  
\(C(x,t)\) is the chloride content at depth \(x\) and exposure time \(t\), \(C_{0}\) is the initial chloride content, \(C_{s,\Delta x}\) is the chloride content at depth \(\Delta x\) in the convective zone, _erf(_) is the error function, and \(D(t)\) is the chloride diffusion coefficient. Regarding the coupling effect of dry-wet cycles and Cl\({}^{-}\), researchers have offered, through a combination of experimental and numerical analysis, a more detailed analysis of concrete during dry-wet cycles and the Cl\({}^{-}\) erosion state of Cl\({}^{-}\) transport. Most researchers concluded that the effect of Cl\({}^{-}\) intrusion rate and concrete depth on wet-dry cycles was greater than under full immersion conditions. In order to further study the dry-wet cycle effect of Cl\({}^{-}\) transport performance, researchers have studied the changes in the dry-wet ratio, but other conclusions of coupling effects were not presented. Durability Performance of Reinforced Concrete under the Coupling Action of Dry-Wet Cycles and Cl\({}^{-}\) Erosion  
Dry and wet cycles allow Cl\({}^{-}\) to enter RC and corrode the reinforcement, ultimately reducing the load-carrying capacity of RC. [PERSON] et al. [59; 60] found that dry-wet cycles favor the penetration of Cl\({}^{-}\) into the concrete. Cl\({}^{-}\) intrusion into the interior reinforcement surface did not directly cause reinforcement corrosion but accumulated at the reinforced concrete  
Figure 7: Variation of Cl\({}^{-}\) diffusion coefficient under different wet and dry cycle regimes [51]. interface until it reached a certain Cl\({}^{-}\) concentration, i.e., the critical Cl\({}^{-}\) concentration and rebar corrosion were accelerated [61; 62; 63]. As shown in Figure 8[63], (a) the local corrosion of the reinforcement started, and the scanning electron microscope (SEM) was used to scan the corrosion site of the reinforcement, and it was observed that the triangular area in (b) was black corrosion products (Fe\({}_{3}\)O\({}_{4}\)), the elliptical area was red corrosion products (Fe\({}_{2}\)O\({}_{3}\)-xH\({}_{2}\)O), and the rectangular area was the passivation film on the surface of the reinforcement. It can be seen that the formation of a protective film mostly on the surface of the reinforcement starts to be gradually replaced by corrosion products and the reinforcement gradually rusts. [PERSON] [64] used SEM to assess the coupling effect of dry-wet cycles and Cl\({}^{-}\) erosion to clearly observe reinforcement rusting and the loss of the oxide film of the reinforcement surface, followed by cracks around the reinforcement leading to the destruction of the concrete protective layer. [PERSON] et al. [65] assumed that Cl\({}^{-}\) reaches the surface of the reinforcement at a constant rate, and a good linear relationship existed between the rate of reinforcement corrosion and the Cl\({}^{-}\) content at the reinforcement-concrete interface when the rate of reinforcement corrosion was low, from which a critical Cl\({}^{-}\) concentration could be deduced, which was not present for the reinforcement inside the already broken concrete. [PERSON] [66] and [PERSON] [67] found that, as the number of dry-wet cycles in the case of Cl\({}^{-}\) corrosion increases, Cl\({}^{-}\) accumulates rapidly on the surface of the reinforcement and corrodes it rapidly, reducing the bonding properties between the reinforcement and the concrete, leading to a significant reduction in RC residual strength and ductility and a consequent decrease in the ultimate load-carrying capacity. The effect of fissures in reinforcement under wet-dry cycle was also considered [68]. [PERSON] [69] comparatively studied the erosion of cracked and uncracked cementitious composites under dry-wet cycles and found that the depth of Cl\({}^{-}\) invasion was significantly higher in the cracked group than in the uncracked group, while the cracked group was more likely to cause the rusting of the reinforcement.