Simulation Of Chloride Ion Transport In Concrete Under Bending Load Coupled With Weting-drying Cycle

The study of the transport properties of chloride ions in concrete under the coupling of bending load and drying-wetting cycle is an urgent issue facing the current marine concrete.The experimental simulation research of traditional laboratories often succumbs to changes in test conditions and environmental conditions,which makes the test results relatively discrete.It is difficult to objectively reflect the transmission characteristics of chloride ions in concrete,and it is difficult to accurately predict the service life of structural concrete.With the increase of computing speed,theoretical model and computer simulation based on structural concrete microstructural characteristics are the most scientific research methods to realize chloride ion transport characteristics in complex environment.Based on Darcy′s law and Fick′s second law,respectively,the two-dimensional variable coefficients of the constitutive equation of water and of chloride ion diffusion-convection in concrete in the unsaturated state are established.A model of the relationship between the bending load and tortuosity and porosity is established.Finally,a variable coefficient constitutive model for chloride ion transport under coupled bending and drying-wetting conditions is established.Key factors such as the porosity,tortuosity and saturation changes during the transport of chloride ions in concrete are fully considered in this model.The reliability of the model of chloride ion transport under a bending load and drying-wetting cycles is verified by experiment and by data from the literature.The results predicted by the model in this paper are consistent with the experimental results;the maximum error is 15%,which proves the accuracy of the model.Theoretical studies show that the two-dimensional variable coefficient constitutive equations of water and chloride ion transport can be solved by the Alternative Direction Implicit(ADI)method,which satisfies the unconditional convergence condition.The ADI discretized equation is programmed and calculated with MATLAB software.The operating efficiency is significantly improved,and its calculation error is small.The effects of the ratio of drying time to wetting time(D/W),the stress ratio,and the surface concrete saturation on the concentration distribution of chloride ions in concrete are analyzed theoretically.The results show that the chloride ion content in concrete decreases and transport mechanism of chloride ion has convection control gradually turning to diffusion control with an increase in D/W;With the increase of bending load,the tortuosity of chloride ion transport changes significantly,and the total amount of chloride ion entering concrete increases;With the increase of surface concrete saturation,surface capillary pore pressure decreases,resulting in an increase in the total amount of chloride ions entering concrete.