Modeling Study On The Influence Of Cyclic Freeze-Thaw On Chloride Ion Migration In Concrete

Reinforced concrete structures in service are,more often than not,subjected to the combined actions of multiple factors.Cyclic freeze-thaw and chloride salt corrosion cause the most severe deterioration among the factors which have adverse effects on the performance of reinforced concrete structures.Due to the synergistic effect induced by the combined actions of both factors,reinforced concrete structures deteriorate at a faster rate than under the single action of either of them,prone to damage as a result of premature failure.The worst case of deterioration comes from rebar corrosion,which will be significantly accelerated under the influence of cyclic freeze-thaw.Therefore,conducting research on the migrating mechanism of chloride ions under cyclic freezethaw is of great engineering significance,providing a theoretical basis for the durability design of reinforced concrete structures.Targeted at the problem in question,this paper analyzes the chloride concentration distribution in the cyclic salt frost experiments conducted by previous researchers and explores the migrating mechanism of chloride ions under cyclic freeze-thaw on the basis of the micro-ice-lens theory in the subject of concrete and the research results of the migrating mechanism of salt in soil in the subject of soil mechanics.It assumes that chloride ion migration can occur during the phase change induced by cyclic freeze-thaw.Based on the hypothesized migrating mechanism of chloride ions,a pertinent rapid cyclic salt frost experiment is designed with different concentrations of chloride salt solution(2%,4%,6%)and different cycles of salt frost(25,50,75,100).With the way Fick's law was deduced for reference,an analogy-based analytical method is adopted to establish the partial differential equation of chloride ions migrating under cyclic freezethaw considering the microscopic commonality of ion migration and heat transfer.Also,the chloride migration coefficient is corrected on account of the periodic occurrence of phase change during cyclic freeze-thaw.Relevant parameters in the equation are fitted out of the data obtained in the experiment,and the fitted parameters are corrected versus the cycle of salt frost;a comprehensive mathematical model of chloride concentration versus corrosion time and corrosion depth is established that takes into account the combined actions of diffusion,convection and cyclic freeze-thaw.In the Coefficient PDE Module of FEM software COMSOL Multiphysics,the model of the migration of chloride ions under cyclic freeze-thaw is established.Twodimensional cloud pictures are presented that describe the chloride ion distribution after the fitted surface chloride concentration and the chloride migration coefficient are typed in.Finally,the reasonability of the comprehensive model is verified with the data obtained from the alternate experiment of chloride ingress conducted simultaneously when the research project was going on.