Study On The Evolution Of The Microstructure And Chloride Ion Diffusion Coefficient Of Concrete Under Cyclic Loading

When concrete is used in marine environments,excessive cyclic loads can lead to the connectivity of pores and the expansion of microcracks within the concrete,thereby increasing the risk of harmful ions entering the internal structure of the concrete,reducing its durability and causing wastage of resources.Therefore,in order to ensure the stability and durability of concrete structures,this study investigates the effects of different cyclic loading conditions on the microstructure of concrete and its chloride ion diffusion coefficient.Cyclic axial compression and tension tests were conducted on concrete specimens,respectively,to study the energy dissipation and damage characteristics of concrete with different water-cement ratios under cyclic loading.A fatigue life model and a cyclic damage model for concrete were established.Subsequently,the microstructure evolution of concrete specimens under cyclic loading was studied through mercury intrusion porosimetry and equilibrium water content tests,and a pore size distribution prediction model was established based on the adsorption and transport mechanism of water in the pores.Finally,a multiscale model considering the influence of cyclic loading on the porosity and chloride ion diffusion coefficient of concrete was established.The model was validated using data from rapid chloride migration tests,and the effects of model parameters such as initial porosity,tortuosity,and axial strain on the chloride ion diffusion coefficient of concrete were further explored.The specific work and main conclusions of this study are as follows:1)The deformation characteristics,energy dissipation evolution,and dynamic elastic modulus development of concrete under different cyclic loading regimes were analyzed through cyclic loading tests.The study showed that as the number of cycles increased,the distance between adjacent stress-strain hysteresis loops exhibited a development pattern from sparse to dense,and the hysteresis curves continuously shifted towards the direction of increasing stress.When all the cyclic stages were completed,the dynamic elastic modulus was about 75-97%of the initial elastic modulus.During the continuous cyclic loading process,slowly increasing the stress level,compared to directly applying a higher stress level load,can make the energy transfer inside the concrete more uniform,thereby reducing the damage to the concrete material and prolonging the service life of the concrete structure.A fatigue life model of concrete under cyclic loading was established,and a cyclic damage evolution model of concrete was established based on fatigue life.The damage evolution curve fitted by the model was in good agreement with the experimental results.2)Using adsorption isotherm and mercury intrusion porosimetry methods,the pore structure parameters and pore size distribution of concrete specimens under different conditions were analyzed to further investigate the evolution of pore volume for different types of pores such as gel and capillary pores.The results of the mercury intrusion test show that as the water-to-cement ratio increases,the porosity of the concrete gradually increases,while the change in average pore size and median pore size is relatively small,around 20 nm and 10 nm,respectively.The cyclic load increases the porosity and pore size of the concrete,and the average pore size of the concrete increases with the number of load cycles and stress level,while the median pore size remains stable at around 10～15 nm.The predicted results of the pore size distribution model show that the small capillary pore peak of the tensile and compressive specimens is located at around 7 nm and 11 nm,respectively,while the large capillary pores are mainly distributed after 100 nm.Under different conditions,the volume fraction of small capillary pores in the specimens is the largest.After cyclic loading,the volume fraction of small capillary pores decreases slightly,while the volume fraction of large capillary pores increases significantly,about four times that of the undamaged specimens.In addition,the content of gel pores also increases slightly,with a growth rate of about 16.7%.3)Based on multiphase sphere theory,a model for the multi-scale chloride diffusion coefficient of concrete is established.The model considers concrete at a microscopic scale as a three-phase composite material consisting of a cement mortar phase,an aggregate phase,and an interfacial transition zone phase.At the microscopic scale,the cement mortar is regarded as a two-phase composite material composed of a pore-free mortar matrix and pores.The accuracy of the multi-scale model is verified by the results of RCM experiments,which show that the model can predict the evolution of chloride diffusion coefficient of concrete under cyclic loading well.The study of model parameters reveals that the damage porosity of dry mortar is higher than that of saturated mortar when the initial porosity of mortar increases.The chloride diffusion coefficient of mortar increases with the increase of initial porosity,and the effect of cyclic loading on the chloride diffusion coefficient of mortar is greater when the initial porosity is smaller.The chloride diffusion coefficient of concrete increases with the increase of shrinkage and the decrease of tortuosity coefficient.When the volume fraction of coarse aggregate is the same,the concrete to mortar diffusion coefficient ratio D_c/D_m increases with the increase of the interface transition zone to mortar diffusion coefficient ratio D_I/D_m.As the volume fraction of coarse aggregate increases,the development of D_c/D_m is relatively flat and approximately equal to 0.86.when D_I/D_m=15;when D_I/D_m decreases,D_c/D_mgradually decreases and the slope gradually increases.