Transport Performance Prediction And Mitigation Measures Of Structural Concrete Under Sulfate-chloride Coupling Attack

The large amount of damage spalling of structural concrete in western saline soil areas is mainly due to the transport through the pores of concrete under high concentration of sulfate-chlorate,and in order to accurately assess the service life of concrete,the process of sulfate-chlorate coupling transport reaction must be quantified and studied.However,most of the current theoretical and experimental studies are mostly qualitative and assume that the transport coefficients are constant.In addition,the coupled sulfate-chlorate transport focuses on the transport in one-dimensional direction,and the transport in two-dimensional direction,which is in line with the actual situation,is less studied.Therefore,it is important to study the two-dimensional variable coefficient coupled sulfate-chloride salt transport characteristics for structural concrete service performance as well as life prediction.The following important results were obtained.This thesis is supported by the National Natural Science Foundation of China（NSFC）under the grant"Damage deterioration mechanism and performance assessment and regulation of structural concrete under sulfate-chlorine salt coupling（5217083308）".（1）By conducting corrosion tests on cement mortar and net slurry immersed with different concentrations of sulfate-chloride compound solutions and using macro and microscopic testing methods,the time-varying patterns of macroscopic and microscopic properties of cement mortar and net slurry during coupled sulfate-chloride erosion were obtained,and the interaction mechanism of coupled sulfate-chloride transport in concrete was explored.In other words,the presence of different concentrations of chloride ions did not change the transport law of sulfate ions,which delayed the diffusion of sulfate ions within 180 d.The presence of sulfate ions also did not change the transport law of chloride ions.（2）A predictive model of two-dimensional coupled sulfate-chloride salt transport in saturation was developed,in which the influencing factors such as concrete porosity,curvature,cement hydration,physical phase content,and interaction between chloride ions and sulfate ions were fully considered,and the model was solved by the ADI format in the finite difference method,and the predictive model was confirmed to be reliable by both experimental and literature results,with a maximum error of 23.49%.Numerical simulations of sulfate-chloride salt transport in concrete showed that concrete with an initial porosity of 8%under the attack of 5%Na₂SO₄-3%NaCl composite solution produced initial expansion damage cracks after about 1300 d.（3）Based on the capillary adsorption and Fick’s second law,the moisture transport model was introduced and combined with the coupled sulfate-chlorine salt transport model in saturated state,and a two-dimensional coupled sulfate-chlorine salt transport model in unsaturated state was established,and the model was solved by using the ADI format in the finite difference method,and the reliability of the model was verified by experiments with a maximum error of 28.57%.The number of cycles and the cycle mechanism were found that the number of cycles as well as the drying time affected the accumulation of peak ion concentration and the rate of inward deflection,and the higher the degree of drying,the higher the number of cycles and the faster the inward deflection rate.（4）The results of macro and micro tests on mortar/net slurry doped with nano-SiO₂showed that nano-SiO₂improved the resistance of concrete to sulfate-chlorite attack.And at 3%admixture,the flexural and compressive strengths of specimens with water-cement ratio of 0.45 decreased by 18.92%and 19.48%respectively after 180 d of immersion;the most available pore size decreased by 65.91%;and the content of AFt decreased by 16.53%.