The Research On Transport Properties Of Chloride Ions In Cement Mortar During Drying Process

The durability of concrete structures has aroused extensive attention in the field of civil engineering,while the penetration of chloride is one of the important causes of steel corrosion and performance degradation of concrete structures.During the drying process in the concrete which suffered chloride intrusion,the evaporation of water causes the reverse convection of chloride ions,which leads to a condensation phenomenon of chloride in surface layer of concrete,which has a significant effect on the durability of concrete proved by a large body of research.Therefore,to study the transportation of chloride ions in drying process contributes to perfect the theory of chloride transport in cement-based materials and predict the service life of reinforced concrete structures.The existing researches on materials transport law in the drying process of cement-based materials are incomprehensive where the short testing period and high temperature environment did not correspond to reality.For above problems,experimental research and simulation analysis of transport properties of chloride ions in mortar during drying process were systematically carried out in this paper,as following:(1)In the test,two kinds of water-cement ratio of 0.4 and 0.6 were prepared.After pre-drying and salt-absorption,the specimens were divided into two groups,one group conducted a drying test that under the coupling effect of convection and diffusion while the other group conducted a free-diffusion test to investigate the influence of reverse convection and diffusion on chloride ions migration.(1)The drying test results showed that the drying time has an important influence on the chloride transport law.Reverse convection caused by water evaporation led a portion of chloride ions to migrate to the mortar surface.Surface chloride content increased at the beginning of test: Within 7 days of drying,because the outward convection is greater than the inward diffusion due to the high mortar saturation and strong evaporation,the chloride content near the surface of the specimen increased slightly;After drying for 28 days and 56 days,the chloride content in the shallow layers reduced gradually and increased in the deep layers as time went on when the inward diffusion of chloride ions was more effective than the outward convection.(2)The results of free diffusion test showed that chloride ions migrated only under the action of concentration gradient and the distribution curves of chloride contents became gentle after free diffusion: Since the chloride ions in the mortar diffused from the high concentration region to the low concentration region under the action of free diffusion,the surface chloride content decreased and increased in the deep layers.(3)According to the chloride diffusion coefficient extracted from the drying test and the free diffusion test,the chloride diffusion coefficients in the drying test were smaller than one in the free diffusion test on the same experimental condition.It is mainly because the convection effect generated by the drying test was opposite to the inward diffusion of chloride ions which prevented chloride ions from migrating to the interior.(2)Based on the Richards equation and Fick’s second law,mesoscopic numerical model coupling transportation of water and chloride in unsaturated cement-based materials were established.Crucial parameters of this model were specified by related experiments whose mix proportion was as same as the specimens above.Key parameters in water transport model were obtained by the isothermal adsorptiondesorption test and the constant-head permeability test.The test of surface chloride content of the one-dimensional salt absorption for 90 days and the RCM test were conducted to obtain the key parameters of the chloride transport model to ensure the accuracy of the subsequent numerical simulation.(3)The model coupling transportation of water and chloride was calculated and analyzed by using the Fluid Flow module and Transport of Diluted Species module in COMSOL.The distribution of saturation and the dynamic migration of chloride ions were obtained in the simulation.By comparing with the test results,the validity of the water-chloride coupling transmission model and the rationality of the parameters determining method were verified,which provided theoretical basis and model support for subsequent related research simulation.