Study On Chloride Diffusion Mechanism In Concrete At Meso-scale

In coastal,marine and deicing salt environments,chloride ion erosion is the main cause of steel corrosion in concrete structures.After the steel bar are being corroded,the bearing capacity of the reinforced concrete structure is degraded,which seriously affects its service life.Compared with the macroscopic level,the meso-scale concrete model considers the non-uniformity of concrete,which is closer to the actual situation.Therefore,this paper studies the diffusion mechanism of chloride ions in concrete at meso-scale,and the following work are carried out:Firstly,the numerical model of meso-concrete polygonal aggregate is established based on aggregate grading,and the numerical model is used to simulate the diffusion of chloride ion in concrete,the effects of different coarse aggregate shapes,interface transition zone thickness and diffusion coefficient,water-cement ratio and other factors on chloride ion diffusion are investigated.The results show that the shape of aggregate has little effect on chloride ion diffusion when the volume fraction of coarse aggregate is the same.The increase of the thickness and diffusion coefficient in the interfacial transition zone cause the chloride ion concentration to decrease when the depth is less than 1 Omm,and the chloride ion concentration becomes larger between 10mm and 40mm.When the water-ratio is increased by 1.33 times and 2 times,the chloride ion diffusion depth increases by 2 times and 4.6times respectively.Secondly,the numerical model of meso-cracked concrete polygonal aggregate is established,and the numerical model is used to simulate the diffusion of chloride ion in cracked concrete,and the diffusion law of chloride ion under different crack widths and crack spacings is analyzed.The results show that the diffusion region of chloride in cracked concrete is divided into uncracked zone and cracked zone.In the cracked zone,chloride ion diffusion is two-dimensional diffusion,and chloride ion diffusion is one-dimensional diffusion in the uncrack zone.The crack spacing has a superposition effect on the chloride ion concentration between the cracks,and when the crack spacing is greater than 50mm,the superposition effect is small.When the crack spacing is less than 50mm,the superposition effect decreases as the crack spacing increases.Thirdly,based on the three-dimensional composite sphere model and the elastic mechanics theory,the relationship between concrete stress and porosity of cement mortar and interfacial transition zone is established.The theoretical model of chloride ion diffusion coefficient under stress is established.The effects of three-phase volume fraction,initial porosity of cement mortar and interfacial transition zone on chloride ion diffusion coefficient are analyzed.The results show that when the concrete is under stress,the chloride ion diffusion coefficient in concrete is mainly affected by the initial state porosity of the interfacial transition zone and cement mortar,concrete stress and three-phase volume fraction.Under the condition that the volume fraction of coarse aggregate is constant,when the diffusion coefficient of the interfacial transition zone increases,the chloride diffusion coefficient in concrete increases.Under the condition that the initial porosity of cement mortar is constant,when the initial porosity of the interfacial transition zone increases,the chloride diffusion coefficient in concrete increases.Finally,according to the established mesoscopic numerical model and theoretical model,the initial corrosion time of steel bars in reinforced concrete beam is predicted.The effects of concrete cover thickness,surface chloride ion concentration,chloride ion diffusion coefficient and crack on the initial corrosion time of steel bars are analyzed.The results show that when the thickness of the concrete cover is increased by 25%,the initial corrosion time of the steel bar is delayed by 55.7%.When the surface chloride ion is doubled,the initial corrosion time of the steel bar is reduced by 3.5 times.When the chloride ion diffusion coefficient is increased by 3 times,the initial corrosion time of the steel bar is shortened by 50%.When the thickness of the concrete cover is 25 mm,the crack caused the initial corrosion time of the steel bar in the concrete to be shortened by 64.9%compared with the un-cracked concrete.