Fractal Study Of Chloride Ion Transport And Erosion Law In Undersea Tunnel Lining Concrete

Since the Belt and Road Initiative and the Marine power strategy were put forward,a large number of Marine buildings with reinforced concrete structure as the main structure have been developed rapidly,among which the undersea tunnel is a very common Marine building.However,the service environment of the undersea tunnel is very complicated.The chloride ions in the sea will erode the concrete structure of the undersea tunnel and reduce its structural bearing capacity.The transport law of chloride ions in the lining is closely related to the microstructure of concrete.Although the influence of single pore structure of concrete on chloride ion transport has been considered in the existing studies,few have considered the influence of the change of concrete structure during the erosion process of chloride ions,and it is difficult to accurately reveal the multi-scale transport law of chloride ions in concrete.In this thesis,concrete is regarded as a double porous medium.Based on the fractal theory of porous media and the non-uniform corrosion model of reinforcement,a fluid-solid-chemical multi-field coupling model is established considering the mass transfer of chloride ions in pores and cracks under water pressure and the effect of corrosion pressure caused by reinforcement corrosion and expansion on concrete.The correctness of the model is verified by comparison.Based on Shantou Bay Tunnel,the influences of water pressure and concrete initial porosity on chloride ion concentration distribution,chloride ion diffusion coefficient ratio to its initial value,maximum corrosion thickness of steel bar and pore structure of concrete were analyzed.The main conclusions of this thesis are as follows:(1)The diffusion coefficient of chloride ions increases with the increase of pore size ratio and crack length ratio,and increases with the increase of pore size fractal dimension and crack length fractal dimension.The maximum fracture length ratio contributed more to the diffusion coefficient of chloride ion.The diffusion coefficient of chloride ion decreases with the increase of pore tortuosity and increases with the increase of matrix porosity.The diffusion coefficient of chloride ion decreases with the increase of fracture zigzag degree.The diffusion coefficient of chloride ion is positively correlated with the porosity of fracture.(2)The larger the fractal dimension of the initial pore size and the fractal dimension of the crack length,the larger the chloride ion concentration and diffusion coefficient will be,and this influence relationship is nonlinear.The larger the fractal dimension of initial pore tortuosity,the smaller the concentration and diffusion of chloride ions.When the effective stress and pore water pressure are considered,the diffusion coefficient of chloride ion increases rapidly first and then tends to be stable.The three initial fractal dimensions have strong nonlinear effects on chloride ion concentration,diffusion coefficient and initial pore size fractal dimension,and the weakest one is fracture length fractal dimension.(3)The higher the seawater pressure is,the faster the chloride ion concentration increases.In the area close to the erosion interface,the chloride ion concentration growth rate decreases with time,while in the area far away from the erosion interface,the chloride ion concentration growth rate increases with time.In the presence of water pressure,the ratio of chloride diffusion coefficient to its initial value increases first,then remains unchanged,and finally begins to decline.The positive correlation between the water pressure and the maximum corrosion thickness of the steel bar is nonlinear.The larger the initial porosity,the larger the chloride ion concentration,but the increment of both shows nonlinear attenuation.There is a negative correlation between the initial porosity and the ratio of chloride diffusion coefficient to its initial value.The smaller the initial porosity,the more significant the change of chloride diffusion coefficient to its initial value.The corrosion thickness of reinforcement is positively correlated with the initial porosity,and the correlation increases nonlinearly.The smaller the initial porosity is,the more significant the fractal dimension changes.