| With the implementation of China’s marine power strategy,the infrastructure construction of marine engineering has reached an unprecedented scale,and more and more attention has been paid on the durability of concrete exposed to coastal environment.The chloride-induced corrosion of steel bar has been the most important factor that affects the durability deterioration of reinforced concrete exposed to marine environment.The speed of chloride migration into concrete is not only related to the composition of concrete materials,but also is closely dependant to the marine erosion zone.For the drying-wetting alternation action in the marine tidal zone,the chloride ingress into the concrete is relatively fast driven by multiple complex mechanisms,such as the coupling action of convection and diffusion caused by temperature,humidity and concentration gradients,resulting in the most serious corrosion of reinforced concrete subjected to drying-wetting cycle.Therefore,according to the experiments of real marine exposure and artificial indoor simulation,the behavior of chloride ingress into oridinary concrete(C30,C50),strain hardening cement-based composite(SHCC)and alkali-activated concrete(AAC)exposed to marine tidal zone was analyzed.The influence of dryingwetting cycle time ratio on the performance of chloride ingress into concrete was discussed.Based on the convection-diffusion model of coupled water and chloride transport,the numerical simulation analysis of chloride transport in unsaturated concrete was carried out,and the similarity of chloride transport property between real marine exposure test and indoor simulation test was analyzed.The main work and conclusions of this paper are shown as follows:(1)The long-term real marine exposure and indoor accelerated simulation tests of chloride ingress into concrete exposed to tidal zone were conducted,and the effects of concrete type(C30,C50,SHCC and AAC),exposure time(exposed in real marine evnironment for 90 d,180d,360 d,720d and in indoor simulation for 30 d,90d,180d)and time ratio of drying-wetting cycles(3:1 and 85.4:1)on the key parameters for chloride ingress process were contrastively analyzed.Based on Fick’s second law,the time-varying law of apparent chloride diffusion coefficient considering the influence of convection zone was analyzed.Futher,the apparent chloride diffusion coefficient was modified by considering the influence of temperature,humidity,time and binding ability on chloride penetration process.The results show that within the range of 10 mm penetration depth,the degree of chloride ingress into different kinds of concrete was SHCC > C30 > C50 >AAC,respectively.Among them,AAC exhibites the best resistance to chloride ion attack.As the exposure time increases,the increasing range of free chloride content obviously decreases after 360 days of real marine exposure and 180 days of indoor simulated environment.The influence of drying-wetting cycles time ratio on the free chloride content of SHCC was greater than that of ordinary concrete and AAC.(2)The convection-diffusion model of chloride ion transport in unsaturated concrete was established,and the finite difference method was used to numerically calculate the process of chloride transport in concrete.The mesoscale simulation of chloride transport under different time ratios of drying-wetting cycles was conducted by means of finite element method and validated by comparing with the experimental results of real marine exposure and indoor simulation.The overall distribution of free chloride content calculated by the numerical simulation agree well with the obtained experimental results,indicating that the proposed numerical simulation method is feasible to simulate the process of chloride penetration into concrete exposed to marine tidal zone,which provides a basis for the analysis of chloride ingress mechanism in marine tidal zone.(3)Based on the three theorems of similarity theory,the similarity index was derived through the convection diffusion equation and the harmonic time criterion was established.According to the concentration criterion obtained from the analytical solution of Fick’s law,the similarity model of chloride ingress between the real marine exposure test and the indoor simulation test was established,and the prediction of durability service life of concrete in marine tidal zone was carried out.Based on the indoor simulation test data,it respectively takes about 2500 d,1800d and 1600 d for C30,C50 and SHCC under the real sea exposure environment when the free chloride content at the penetration depth of15 mm reaches 0.4%,while it is hard to reach the critical value of 0.4%,exhibiting the better resistance to chloride ingress into AAC.The results show that the relationship curve of harmonic time criterion and concentration criterion calculated by the similarity model was in good agreement with the measured data,and it can accurately describe the distribution change of chloride content in the real marine exposure and indoor simulation environments. |
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