## 3 Coupling of Dry-Wet Cycles and Cl\({}^{-}\)
### Transport Properties of Cl\({}^{-}\) under the Action of Dry-Wet Cycles
Figure 5 shows the variation in the surface Cl\({}^{-}\) content in concrete with depth for different dry-wet cycling regimes (refer to Table 1). Cl\({}^{-}\) generated a peak at 2 mm on the surface, identifying this region as the convective zone of the concrete. Additionally, the effect of the dry-wet ratio and number of dry-wet cycles on Cl\({}^{-}\) erosion in concrete was not negligible. Both the wet and dry ratio and the number of cycles directly affected the process of Cl\({}^{-}\) transport. [PERSON] et al. [52] found that, during drying, Cl\({}^{-}\) lodged deeper in the concrete remains in the saturated pore solution for free diffusion, and lengthening the drying cycle could accelerate Cl\({}^{-}\) infiltration. Drying led to the formation of crystalline compounds being adsorbed deeper, and after the next wetting cycle, Cl\({}^{-}\) increased in the concrete. Additionally, the infiltration rate was found to be linearly related to the square root of the number of dry-wet cycles. [PERSON] [53] also assessed the effect of wet and dry ratio on Cl\({}^{-}\) infiltration, with significant differences in \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Code** & **G1S1** & **G3S1** & **G5S1** & **G1S3** & **G1S5** & **QJPZ** & **DS-G5S1** & **DG-G1S5** \\ \hline Ratio of dry and wet & 1:1 & 3:1 & 5:1 & 1:3 & 1:5 & & 5:1 & 1:5 \\ Single time ratio/h & 24:24 & 36:12 & 40:8 & 12:36 & 8:40 & 0:48 & 60:12 & 12:60 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dry–wet cyclic test system [51]. Figure 5: Transport properties of Cl\({}^{-}\) in concrete under different wet and dry regimes. (**a**) 30 times. (**b**) 40 times [51]. Figure 6: Distribution curve of Cl\({}^{-}\) concentration in test concrete after days of exposure in a simulated environment (**a**) 200 d; (**b**) 600 d [46]. Cl\({}^{-}\) concentration and depth within the concrete with different wet and dry ratios. [PERSON] [51] observed that the convective area gradually disappeared as the number of wet and dry cycles increased in samples with a dry-wet ratio of less than 1. As shown in Figure 7, for each wet/dry ratio case at different cycle days, the Cl\({}^{-}\) diffusion coefficient in specimens with a dry to wet ratio of \(\geq\)1 was slightly greater than that in specimens with a dry to wet ratio of \(<\)1. At present, some results have been obtained from the numerical simulation of concrete subjected to Cl\({}^{-}\) erosion under dry-wet cycle conditions domestically and abroad, and these results, in some cases, verified the conclusions obtained from the experiments and explained what was being microscopically observed. Recently, researchers [47; 54] have provided a life expectancy for concrete structures in a dry-wet cycle environment. [PERSON] [55], on the basis of [PERSON]'s second law, examined the convective effect of moisture and derived convection-diffusion control equations that suit the model of ion transport inside concrete under the operation of dry-wet cycles more closely. [PERSON] et al. [56] established the non-saturated state of concrete, and [PERSON] [57] used an existing coupled ion-moisture transport model. After updating the relevant parameters in the model, it was highly consistent with the free and total Cl\({}^{-}\) concentration curves measured in the laboratory under dry-wet cycles, avoiding the problem that the traditional Cl\({}^{-}\) transport performance needed to consider porosity. [PERSON] [58] designed a model to simulate the Cl\({}^{-}\) diffusion in concrete under dry-wet cycles considering factors such as Cl\({}^{-}\) concentration difference and convection and capillary action due to pore space, and they discussed the accuracy of the model in the context of practical situations.