Effect Of Crack On The Mechanism And Features Of Chloride Ion Diffusion And Carbonation In Concrete

The cracks will unavoidably appear in the structure of concrete during the construction and service period.The existence of cracks speeds up the diffusion of chloridion and carbonization degree in the concrete.It accelerates the rate of steel corrosion and reduces the durability of concrete structure.This paper focuses on the experimental study and theoretical analysis of the features of chloride ion diffusion and carbonization in concrete with cracks to explore its mechanism,thus establishes the chloride ion diffusion model and carbonization model in concrete with cracks.It provides theoretical basis for life prediction of concrete structure.Using the method of theoretical analysis combining experimental research and numerical simulation,this paper completes the following research:1.Study on the Preparation Method of CrackA new method of non-destructive preparation is proposed,and a high-precision and non-destructive preparation equipment which can control the width and depth of crack is designed.Cracks’preparation with arbitrary width and depth in different experimental studies can be realized by changing the size and form of testing model and the width and thickness of steel disc.2.Experimental Study（1）Experimental study on the diffusion of chloride ion:the paper adopts non-destructive method and bends loading method to make the test pieces of mortor with cracks and concrete with cracks,and takes the length,depth and interval of crack as parameters;altogether 36 test pieces are made.Taking the water cement ratio and test time as parameters,we prepared 18 test pieces in total.Through chloride salt solution soaking,RCM and electric flux,experiments and studies were conducted on the chloride ion diffuse process.The experimental results show that three kinds of test results were in good consistency.Then we come to the conclusion that the influence of the depth of a crack is greater than that of the width of a crack in the process of chloride ion transport.Compared to the saturated state,the diffusion depth value of chloride ion under the non-saturated state is obviously larger.（2）Experimental study on the carbonization:taking the length,depth and interval of crack as parameters,24 test pieces are made in total.And taking the water cement ratio and test time as parameters,we prepared 12 test pieces in total.The bidirectional carbonization formed at the junction of cracks and the surface of concrete enlarges the area around the cracks which is the result of carbonization,making the carbonization area present an inverted"V"shape.In the diffusion process of carbon dioxide,the influence which is brought by the width of cracks is greater than that of the depth of cracks.3.Theoretical Study（1）Theoretical study on chloride ion diffusion:based on the chloride ion diffusion test results,the concrete structure model with crack is built from a mesoscopic view,and the diffusion coefficient of chloridion in the concrete with cracks is calculated on a mesoscopic scale.The diffusion of chloride ion in the concrete with cracks under unsaturated condition is analyzed.The diffusion process of chloride ion was simulated by ANSYS software.Test results and simulation results have appeared a favorable consistency.（2）Theoretical study on carbonization characteristics:based on the concrete carbonization test results,the equivalent diffusion model of CO₂ in concrete with cracks is built,and further the two-dimensional diffusion control equations of CO₂ in concrete and cracks are established respectively according to the difference of CO₂ diffusion coefficient between concrete and cracks.The transmission process of chlorine ion was simulated by ANSYS software.Test results and simulation results have appeared a favorable consistency.4.Coupling Effect StudyTo truly simulate the complex environmental condition where the concrete with cracks structure is located,we carried out a study on the coupling effect among chloride ion diffusion,carbonization and freeze-thaw cycle of the cracked concrete.With the increase of carbonization time,diffusion depth of chloride ion of concrete with cracks,on the contrary,decreases.The chloride ion diffusion enhances the carbonization resistance of concrete to some extent,while freezing and thawing cycle speeds up the carbonization process of concrete.This research has provided both basis for forecasting the using life of concrete structure accurately and references and guiding significance for engineering practice by studying the cracks’influence on the durability of concrete structure.