Experimental Study And Mesoscopic Numerical Simulation Of Chloride Diffusion In Cement Based Materials

In recent years,the phenomenon of reduced structural life caused by durability problems has attracted increasing attention,and the main reason for durability problems is the corrosion of steel bars caused by the penetration of chloride ions.In thesis,a new electrodeless test method was adopted to test saturated slice samples and standard solution in series,non-contact electrical resistivity method(NC-ER method),combined compressive strength test and rapid chloride migration test(RCM method),etc.The main research contents of the chloride ion penetration and diffusion performance of cement-based materials are as follows:(1)According to the Nernst-Einstein equation,the calculation formula of the chloride diffusion coefficient of hardened concrete was derived from the analysis of slice resistivity,chloride ion migration number and other parameters.The size effect of chloride diffusion coefficient and the correlation with compressive strength were explored.(2)Based on the test data of water-cement ratio of 0.3,0.4 and 0.5,the influence of water-cement ratio on the diffusion performance of chloride ions was explored.And based on the data of 3d,7d and 28 d age,the impact of curing age on the diffusion performance of chloride ions was explored.The results showed that the chloride diffusion coefficient increased with the increase of water-cement ratio and decreased with the increase of age.Based on the experimental data of 12% and 22% of aggregate volume fraction,the influence of aggregate on chloride ion diffusion performance was explored.The chloride diffusion coefficient decreased almost linearly with the increase of aggregate volume fraction.(3)Comparing the difference between the NC-ER method and the RCM method,the NC-ER method was a non-contact test method with no electrode reaction,and had the advantages of accurate test results,good repeatability,and greatly shortened test time,but only one sample can be tested at a time;The RCM method test phenomenon was obvious,and the obvious penetration depth of chloride ions can be observed,but the early test results are too large,and the test results of the aggregate sample were more discrete.(4)The COMSOL software was used to conduct meso-scale numerical simulation of the chloride ion penetration process of cement-based materials to explore the influence of aggregate shape and volume fraction,interface transition zone thickness and diffusion coefficient values,and model dimensions on chloride ion diffusion performance.Aggregate had an obstructive effect on the penetration of chloride ions.The more aggregates,the stronger the obstruction to the penetration of chloride ions.The interface transition zone promoted the diffusion of chloride ions.The simulation results of the three-dimensional aggregate model and the two-dimensional aggregate model are not much different,but the calculation time for the three-dimensional model was dozens of times that of the twodimensional model.It was better to use the two-dimensional aggregate model to simulate the chloride ion diffusion from the perspective of improving efficiency.