IntroductionIn the context of marine water fluctuation environment, the transport of chloride ions in concrete involves two mechanisms, i.e., diffusion due to concentration gradients and capillary adsorption due to moisture gradients. Most of the existing studies focus on diffusion and horizontal capillary adsorption of chloride ion transport patterns in water fluctuation environments, while neglecting vertical capillary adsorption. A few studies on vertical capillary adsorption are in static water level environments, which are not considered for water level changes. Vertical capillary adsorption exists in concrete under oceanic water level fluctuations, which may have an impact on the chloride ion transport process. It is thus necessary to carry out a research on the effect of vertical capillary adsorption on the chloride ion transport process under the environment of marine water level change.MethodsTo investigate the chloride ion transport patterns within concrete considering vertical capillary absorption in water level fluctuation environments, this study utilized a homemade marine tidal cycle simulation device. Two sets of comparative experiments were arranged, i.e., one accounting for vertical capillary absorption and another not. Concrete samples were ground in an active grinding machine at intervals of 1mm per layer within the 0-5 mm range from the diffusion surface, and at intervals of 2 mm per layer within the 7-21 mm range. The chloride ion content in the powdered samples in concrete for both sets of experiments was measured by a model ZDJ-5B-5/4B/4A conductometric titrator. The location of the most severe chloride ion erosion in concrete under water level fluctuation conditions was identified, and the effect of vertical capillary absorption on chloride ion concentration distribution, surface chloride ion concentration, and diffusion coefficients in concrete were evaluated.Results and discussionBased on the results of the chloride ion concentration distribution in concrete, the maximum chloride ion concentration in the capillary group appears in the area above tidal zone, while it in the control group appears in the area where the dry time accounted for 13/14 of the cycle. Comparing with the control group, the capillary group has higher chloride ion concentrations above the tidal zone but lower chloride ion concentrations in the tidal zone. This is due to the vertical capillary absorption on concrete. In the drying phase of the wet-dry cycle, vertical capillary absorption causes the concrete to absorb moisture from the bottom, maintaining a certain level of humidity. This reduces the intensity of capillary absorption during subsequent wetting, thus lowering the surface chloride ion concentration in the tidal zone. Also, vertical capillary absorption promotes the transfer of chloride ions from the tidal zone to the area above tidal zone, resulting in higher chloride ion concentrations above the tidal zone and lower chloride ion concentrations in the tidal zone.In the tidal zone, the diffusion coefficients for chloride ions in the capillary test group are lower than those in the control group. For instance, after 200 d exposure, the diffusion coefficient for the capillary group is 2.11×10^-12 m2/s when the dry time ratio is 13/14, compared to the diffusion coefficient of 2.33×10^-12 m2/s for the control group. The surface chloride ion concentration (C_s) for the capillary group is 1.63% (in mass fraction), while that for the control group is 1.80% (in mass fraction). The surface chloride ion concentration and the diffusion coefficient in the capillary group both are approximately 10% lower than those in the control group.In the area above the tidal zone, vertical capillary absorption significantly increases both the diffusion coefficient (D_a) and the surface chloride ion concentration (C_s). For instance, in the 0-50 mm segment above the highest water level, the surface chloride ion concentration and diffusion coefficient in the capillary group both are higher than those in the control group throughout the entire exposure period.Based on Fick's second law, an empirical model for chloride ion transport in concrete considering the impact of vertical capillary absorption is established, and the predicted data by the model are in reasonable agreement with the experimental results.ConclusionsVertical capillary absorption increased the chloride ion concentration above the tidal zone and reduced the chloride ion concentration in the tidal zone. The location with the maximum chloride ion concentration was within the 0-50 mm range above the highest water level due to the influence of vertical capillary absorption. In the tidal zone, vertical capillary absorption reduced the surface chloride ion concentration and the diffusion coefficient of chloride ions, leading to a less pronounced variation in chloride ion diffusion coefficient with wet-dry ratio. In the area above the tidal, vertical capillary absorption increased the surface chloride ion concentration and the diffusion coefficient of chloride ions within the 0-100 mm range above the highest water level. The effect of vertical capillary absorption diminished with increasing elevation, and the most significant impact occurred within the 0-50 mm range above the highest water level. A time-variable model for chloride ion transport considering vertical capillary absorption in marine water level fluctuation environments was established. The predicted data by the model were within ±15% deviation, compared to the measured results, validating the rationality and accuracy of the model.