Simulation Study On Diffusion Of Chloride Ions In Concrete With Heterogeneous Stress Distribution

The service life of reinforced concrete structures in a chloride-laden environment needs to be determined based on the diffusion of Cl~-in the cover concrete under external loads.From the perspective of Cl~-diffusion,concrete should be regarded as a heterogeneous material,which will produce discontinuity microstructural changes and damage under loads,and further influence the interior Cl~-diffusion.Moreover,for structures in service,the undertaken loads are usually diverse,complex and with uncertain magnitude,which lead to the diffusion of Cl~-in concrete is very complicated,and difficult to be accurately quantified via laboratory tests.Therefore,use numerical simulation methods to explore the Cl~-diffusion processes within concrete is of great importance.Simulation work and corresponding conclusions obtained in this thesis are listed as follows:Based on the laboratory-measured Cl~-ingress results for mortar or interfacial transition zone samples under axial tensile and compressive loads,the change in Cl~-diffusion coefficients due to volume changes of microstructure D(θ)and damage D(φ)were obtained.Subsequently,a heterogeneous stress-diffusion coupling model based on Fick’s second law of diffusion was established,which has been verified for accuracy and reliability via the laboratory tested Cl~-ingress results under axial compressive/tensile loads.By comparing the ingress of Cl~-obtained from models considering the heterogeneous or equivalent distribution of stress in mortar and ITZ,it can be conducted that once the stress level was relatively low,the Cl~-distribution in the two types of models did not present significant difference.On the contrast,at a higher stress level,the difference was more pronounced,which relates to multiple factors,including the tensile or compressive loads applied and properties of concretes tested.The effects of volume fraction,shape,gradation for aggregate and ITZ thickness on the internal Cl~-diffusion process under different axial load ratios were studied by orthogonal experimental design.The statistical results show that the degree of influence of each parameter is related to the different load levels.Compared with the three aggregate-related parameters,the effect of ITZ on the Cl~-diffusion under axial loads is significantly weaker than the aggregate volume fraction.Under the effect of a certain load within the scope of failure,the Cl~-diffusion depth and the immersion duration have a good exponential relationship.However,the higher the load value,the less obvious the exponential relationship.The applicability of the coupling model in concrete beams subjected to bending loads was discussed,and the effects of heterogeneous stress distribution and damage caused by aggregates and steel bars on the Cl~-diffusion process were investigated.The results show that:the load ratio has little effect on the diffusion performance of the concrete beam in the compression zone.Aggregate changes the form of damage in the tensile zone of the loaded concrete leads to a quicker ingress of Cl~-to the interior region.The stress redistribution caused by the nonelastic properties of reinforced concrete makes the Cl~-concentration of the steel bars at the same depth in the tension zone and compression zone significantly different,and the maximum difference can reach 4%in the relevant calculations.