Study On The Diffusion Pattern Of Chloride Ions In Coagulation Under Chloride Salt-sulfate Erosion

This study is based on the background of a large photovoltaic field pile foundation project of a power group in Tianjin.The project site is located in a salt field of a sea crystal group in Tianjin,which is in a compound erosion environment of chloride salt and sulfate.In the chloride salt erosion environment,the main cause of structural failure of reinforced concrete is the permanent loss of structural durability caused by the rusting and swelling of reinforcing steel.In the sulfate erosion environment,the main cause of durability deterioration failure of concrete structures is sulfate crystallization swelling and cracking damage.For the above two aspects,domestic and foreign scholars have conducted a lot of research respectively.At present,there are relatively few studies on the durability of reinforced concrete structures carried out by chloride and sulfate compound erosion.The coexistence of chloride salts and sulfate in the same erosion environment and their mutual influence produce coupling effect on the diffusion pattern of chloride ions in concrete,which aggravates the damage and durability loss of reinforced concrete members,and the related research is of great significance.For the pile foundation project of large photovoltaic field developed based on the salt field environment rich in chloride ions and sulfate ions,the study of chloride ion diffusion law in concrete under the action of chloride salt-sulfate coupling erosion and chloride ion diffusion through the protective layer to the surface of reinforcing steel and accumulation to reach the induction period of reinforcing steel corrosion is carried out.Using environmental erosion simulation experiments,the effect of chloride salt-sulfate coupling and erosion time on the distribution law of chloride ions and on the diffusion coefficient was systematically studied,a new diffusion model was proposed based on Fick’s second law,the diffusion process of chloride ions in concrete was simulated using COMSOL finite element analysis software,and the life induction period was predicted and calculated.The research contents and main results are as follows:(1)According to the concrete design regulations and the requirements of marine environment,the marine concrete adapted to the link is prepared,and the influence of the main factors such as water-cement ratio,fly ash admixture and mineral powder admixture in the preparation of marine concrete is analyzed by orthogonal experiments,and the proportional optimization design of marine concrete is carried out.The strength,electric flux and diffusion coefficient of concrete are guaranteed to meet the usage requirements of the corresponding environment.(2)For the erosion environment of the pile foundation project site,experimental studies on the diffusion of chloride ions in concrete under chloride salt erosion alone and chloride salt-sulfate coupled erosion were carried out.The effect of sulfate ion concentration on chloride ion concentration was investigated,and the binding capacity,diffusion coefficient,and erosion time of chloride ions under different erosion environments such as single salt and compound salt were compared and analyzed.The complex salt test shows that sulfate ion and chloride ion have interaction,and the sulfate ion will inhibit the diffusion of chloride ion in a short period of time in the early stage of the experiment;in the later stage of the test,when the chloride ion concentration is lower or higher,the sulfate ion will inhibit or promote the diffusion of chloride ion,respectively.The diffusion coefficient is modified by introducing the sulphate corrosion resistance coefficient of concrete under the influence of sulphate,chloride ion binding capacity,considering temperature,relative humidity and time decay coefficient.in order to establish the modified diffusion model under the comprehensive factors.(3)Considering the influence of environmental conditions,protective layer thickness and force condition of the actual project,a model of chloride ion diffusion in concrete under this environment was established based on Fick’s second law,and the durability life induction period of pile foundation of a large photovoltaic project of Tianjin Power Group built in salt field was predicted by using COMSOL finite element software,and the results showed that when the thickness of structural protective layer is not less than 65 mm,the project can be guaranteed to reach the durability design service life.