Study On Chloride Ion Transport In Unsaturated Concrete Under Cyclic Compressive Loading

Concrete bridges in salt fog zone are affected by durability degradation,among which chloride ion erosion is the most important factor.Exploring the transport behavior of materials in unsaturated concrete under repeated loads plays a vital role in improving the durability of concrete bridges in salt fog zone.In this paper,theoretical analysis and experimental study on the transport of water and chloride ions in unsaturated concrete under cyclic compression load are carried out,and the effects of stress level and loading times on the transport behavior of water and chloride ions are analyzed.The specific work and conclusions are as follows:(1)According to the transport mechanism of chloride ion in unsaturated concrete,the mathematical expression of chloride ion transport in unsaturated concrete is derived based on Fick's second law,extended Darcy's law and the law of mass conservation observed by chloride ion in the process of transport.Based on the existing theoretical basis and research results,considering the influence of cyclic compression load on water diffusion coefficient and chloride diffusion coefficient,the chloride ion transport model of unsaturated concrete under cyclic compression load is established,and the solution method of the theoretical model is proposed.(2)Based on the principle of connector,a real-time measuring device for concrete liquid absorption,the liquid storage tank,is designed to realize the coupling of cyclic compression load and chloride ion erosion.And at the same time,the liquid absorption rate of concrete can be quantitatively observed.The chloride ion penetration test of concrete under cyclic compressive loading was carried out for 150 mm X 150 mm X 550 mm concrete specimen,and the effects of stress level and loading times on the cumulative liquid absorption per unit area of concrete and capillary liquid absorption rate were investigated.(3)Based on the experimental results,a water diffusion coefficient model under cyclic compressive loading is established.It is found that the value parameter D0 of water diffusion coefficient varies in three stages with the number of loading times:In the first stage(N<2592,000 times),D0 is only related to the stress level,which decreases first and then increases with the increase of stress level.In the second stage(2592,000 times<N<500,000 times),because the secondary absorption rate is significantly less than the initial absorption rate,there is a sudden change in D0.Then when the stress is not zero,D0 increases with the increase of loading times,whereas D0 decreases with the increase of loading times.In the third stage(N>500,000 times),D0 decreases with the increase of loading times and increases with the increase of stress level.(4)The governing equations of capillary water absorption and chloride ion transport in unsaturated concrete under repeated compressive loads are solved.The theoretical calculation results of chloride ion concentration distribution are compared with the experimental results.The results show that they are in good agreement,which shows that the chloride ion transport model in this paper has certain rationality.Through the model calculation and analysis,it can be seen that with the increase of stress level,the penetration depth and the relative volume water content and chloride ion concentration at the same position in concrete decrease first and then increase.When the stress level is greater than 0.2,the growth rate of penetration depth,relative volume water content and chloride ion concentration at the same location in concrete increase continuously.With the increase of loading times,the penetration depth,the relative volume water content and chloride ion concentration at the same location in concrete increase,but the growth rate decreases.