## 3 Coupling of Dry-Wet Cycles and Cl\({}^{-}\)
### Transport Properties of Cl\({}^{-}\) under the Action of Dry-Wet Cycles  
Concrete under dry-wet cycles experiences repeated wet expansion and dry shrinkage, thus affecting its durability. Dry-wet cycles can directly change the pore structure of concrete to a certain extent, leading to the accumulation of internal cracks and eventually leading to a decrease in the permeability resistance of concrete. [PERSON] [40] found that hydration increased the internal compactness and strength of concrete at the start of the dry-wet cycles. [PERSON] [41] found that pores were the key to determine the permeability performance of concrete, and the pores that affect the permeability performance of concrete were mainly large capillary pores (100 nm-1000 nm), but as the dry-wet cycle time increased, the proportion of pores less than 100 nm in concrete increased. [PERSON] [42] discovered that the quantity of Cl\({}^{-}\) entering the concrete depends on the open porosity. Unlike the free diffusion of ions in full immersion, the mode of transport in concrete in the dry-wet cycles state is divided into free diffusion, convection, and capillary action. The complex mode of transport also has different effects on the final Cl\({}^{-}\) erosion results. [PERSON] et al. [43] and [PERSON] et al. [44] showed that the Cl\({}^{-}\) erosion rate under the action of dry-wet cycles was significantly greater than that in the fully immersed concrete, with the difference increasing the longer the age of erosion and gradually decreasing as one moves deeper into the concrete. This suggests that the Cl\({}^{-}\) erosion mechanism varies considerably in the concrete's surface layer, while the erosion mechanism was closer in the deeper concrete. [PERSON] [45] found that Cl\({}^{-}\) accumulates rapidly in the early stage of dry-wet cycles, and as the number of dry-wet cycles increased, Cl\({}^{-}\) accumulated in the concrete and combined with calcium ions. Eventually, CaCl2 precipitation could be observed in the internal pores and cracks. The Cl\({}^{-}\) distribution method in concrete during dry-wet cycles has been studied in depth. The change in Cl\({}^{-}\) concentration in concrete with the increase in depth during wet and dry cycles increases first and then decreases, and the depth of this peak Cl\({}^{-}\) concentration to the concrete surface is called the concrete convection zone [46; 47]. Figure 6 shows the variation in free Cl\({}^{-}\) content with depth for concrete with different water-to-ash ratios (w/c). [PERSON] et al. [46] found that concrete showed significant variation peaks in Cl\({}^{-}\) content with depth during dry-wet cycles in both laboratory simulations and field exposure tests. The peak occurrence was attributed to the dry and wet cycles, and this location was also identified as the concrete convection zone. [PERSON] [48] found that both dry-wet cycles of the specimens showed convective zone in the surface layer of concrete and found that, as a result of the dry-wet cycles, the convective zone was related to the total porosity, critical pore size, and open porosity. [PERSON] et al. [49] found that, while the convection zone was observed under dry-wet cycles and Cl\({}^{-}\) erosion, the concrete convection zone gradually deepened with the increase in the number of dry-wet cycles, indicating that the migration of Cl\({}^{-}\) to the inside of the concrete was accelerated under the action of dry-wet cycles. [PERSON] [50] found that the convection zone was caused by the simultaneous effect of Cl\({}^{-}\) scattering and convection, and that the depth of the convection zone increased the drying time. [PERSON] [51] found that the transport depth under dry-wet cycle conditions was lower than that of full immersion, but the Cl\({}^{-}\) content at the same depth within the surface layer was greater than that in the full immersion case, and the generation of a convective zone was observed after ten dry-wet cycles.