Coupled Carbonation–Chloride Effects on Reinforced Concrete: Service Life Predicting Under Marine Tidal Zone Cyclic Drying–Wetting Conditions

In marine engineering, reinforced concrete corrosion and deterioration in tidal zones are severe. It is thus important for marine concrete subjected to cyclic wet-dry conditions in tidal environments to investigate structural service life prediction. Corrosion of steel bar caused by chloride ion intrusion is one of the most important factors affecting the durability of marine concrete, and it is also a key to investigating the life prediction of concrete in tidal zone, while the influence of carbonation on chloride ion erosion cannot be ignored. In this study, the data were incorporated into a chloride ion ingress model considering carbonation effects on ion transport via statistically analyzing and fitting tidal cycle data from a coastal area in the Southeastern China. A probabilistic service life prediction model was established based on the coupled carbonation-chloride interaction, using critical chloride concentration for steel bar corrosion as the boundary condition. The corrosion probabilities of reinforced concrete at different elevation levels were investigated and compared with actual engineering data.

According to the statistical data, China, the tidal level information of the southeast coast was counted, and the tidal equation was obtained. The chloride diffusion coefficient D of concrete with different carbonation degrees was determined by RCM. According to the standard GB T50082—2009, China, the carbonation depth Δl and the change rate of chloride ion diffusion coefficient ΔD were calculated at 0, 3, 7, 14, 21 d and 28 d, respectively. To investigate the critical conditions of carbonization and steel corrosion, 5 groups of solutions with different pH values were prepared, the specimens after demoulding were immersed in the solution, a certain amount of NaCl was dripped into each solution for every 48 h, and the rust was detected regularly.

Based on Fick 's second diffusion law, Darcy 's convection law and the model proposed, a life prediction model of chloride ion concentration C related to time t was obtained. The results of durability life prediction of reinforced concrete under different carbonization degrees were calculated by random sampling, and the Weibull probability model was fitted to analyze the influence of different carbonization rates at high and low water–binder ratios. The dry–wet ratio at different height positions was counted after setting H=[1,6], the influence of different height positions on life was analyzed. According to the test report of a bridge in the southeast coast, the chloride ion diffusion coefficient D at different heights of the bridge was calculated, and the 30 a corrosion rate of the calculated value of the model and the actual value of the pier was counted, and the height development law and redundancy of the calculated value and the actual value of the bridge model were analyzed.

The law of tidal change at different heights belongs to the typical harmonic. The fitted tidal level equation solves the dry–wet ratio data closer to the reality. The low water-binder ratio concrete’ D firstly decreases, and then increases and decreases, and high water-binder ratio concrete’ D firstly decreases and then increases with the increase of carbonization depth. Compared with the carbonation depth of 0 cm, the final carbonation can reduce low water-binder ratio concrete’ D, but increase high water-binder ratio concrete’ D.

The life prediction results of high water-binder ratio are mostly concentrated in 30–40 a with 0% carbonization. The frequency and distribution of life expectancy are approximately a normal distribution, and the 95% life expectancy distribution is 21.9 a. The predicted service life of low water-binder ratio concrete is longer than that of high water-binder ratio, the predicted service life of high water-binder ratio concrete is mostly between [25, 30], and its 95% predicted service life is reduced to [0, 19] with 50% carbonization. Compared with 0% carbonization, the predicted life of low water-binder ratio with 50% carbonization increases. The life prediction of concrete is gradually shorter, and the predicted life with 95% reliability is 22, 17 a and 7 a, respectively, as carbonization increases. However, the 100% carbonization exceeds the thickness of the protective layer, and the predicted life decreases significantly. The predicted life of low water-binder ratio concrete with 100% carbonization is greatly reduced, but it is nearly 3 times higher than that of high water-binder ratio concrete.

The high water-binder ratio’ D firstly decreases and then increases, the low water–binder ratio’ D decreases continuously, and the dry–wet ratio firstly increases and then decreases, the predicted life of high water–binder ratio concrete firstly increases and then decreases, and the predicted life of low water–binder ratio concrete firstly decreases, and then increases and decreases as the height position H increases. The durability trend of the two in the range of 2–5 m in the middle of the tidal zone is opposite.

Compared with the data of a bridge in the southeastern coast for the carbonization and concrete strength grade, the carbonization degree is far from the steel bar. In the case of 30 a service, the corrosion rate is 13.872%–17.386%, and the average corrosion probability is 15.5%. The relative risk coefficient of steel corrosion firstly increases and then decreases with H. The calculated value of the bridge model is consistent with the actual value, and the calculated value shows a high redundancy.

The tidal variation patterns at different elevations belonged to typical harmonic wave types. Carbonation reduced chloride diffusion coefficients in low water-binder ratio concrete but increased it in high water-binder ratio concrete. The initial carbonation had a minimal impact on the reinforced concrete durability, but the concrete durability decreased rapidly when carbonation penetrated the protective layer. The service life of high water–binder ratio concrete showed an initial increase and a subsequent decrease, while low water-binder ratio concrete exhibited a sequential decrease–increase–decrease pattern as the height increased. A comparison of actual engineering data and model calculation results indicated that the computed values of the model could be close to actual steel corrosion probabilities, and the relative risk coefficients of corrosion probabilities at different heights were consistent with statistical results from engineering practice, demonstrating a reasonable simulation effectiveness of the model.