Study On Chloride Ion Erosion Resistance Of High Ductility Concrete Under The Combined Action Of Chlorine-wetting Cycle

High ductility concrete(HDC)is a new composite material developed based on micromechanics theory.Compared with ordinary concrete,HDC has better mechanical properties and corrosion resistance;Compared with other fiber reinforced concrete,it has high ductility and good energy dissipation capacity;Therefore,it is widely used in the reinforcement of building structures and the repair of damaged parts of roads,bridges and hydraulic structures.In some special geographical environments in China,concrete buildings suffer from external environmental erosion all year round.For example,structures such as coastal wharf,Salt Lake area buildings and concrete pavement often treated with deicing salt will be eroded by chlorine salt,which will affect the strength of concrete structure and shorten the service life.Using HDC to reinforce and repair the damaged parts by increasing the section and material replacement can improve the bearing capacity of the structure and achieve the reinforcement effect,but it will lead to HDC replacing the raw concrete material to directly contact with the chloride environment,making it withstand the test of external extreme environment and service life during service.Therefore,it is particularly important to study the resistance of HDC materials to external chloride ion erosion and the basic mechanical properties after erosion.Taking HDC as the research object,this paper studies the chloride ion corrosion resistance of HDC under the action of dry wet cycle from both macro and micro aspects by using the experimental mechanism of the corrosion dry wet cycle coupling effect of chloride solution with three concentrations(5%,10%and 20%).On this basis,the strength attenuation model,uniaxial compression constitutive model and the diffusion model of chloride ion in HDC are established,The main research contents and results are as follows:(1)The apparent phenomenon,mass loss rate and strength change law of HDC after going through the predetermined dry wet cycle in different concentrations of chlorine salt solutions are analyzed.It is found that there are different degrees of salt crystallization on the surface of HDC in the three groups of solutions.The higher the concentration is,the more obvious the phenomenon is,and the strength and quality first increase and then decrease.The uniaxial strain attenuation model is established based on the uniaxial compression stress attenuation curve of HDC.The results show that even after 300 days of erosion in 20%salt solution,the compressive and flexural strength of HDC remains more than 80%of that of non eroded HDC,and still has good ductility.(2)Based on Fick’s second law,considering the factors such as time deterioration,internal dry humidity,temperature,chloride ion binding capacity and surface chloride ion content of HDC,combined with the experimental data,the diffusion model of Cl-in HDC under dry wet cycle is established.The experimental values are compared with the calculated values of the model,and it is found that the two are in good agreement,The results show that the model can accurately reflect the transmission law of chloride ion in HDC under chloride salt dry wet cycle.(3)The damage layer thickness of HDC under the coupling action of Chloride Erosion and dry wet cycle is calculated and analyzed by ultrasonic horizontal measurement method.According to the geometric relationship between the damaged layer and the whole test block,the compressive strength of the damaged layer is calculated,and it is found that it has a quadratic polynomial relationship with the actual compressive strength.According to the variation law of relative dynamic elastic modulus,it is found that the internal compactness of HDC increases first and then decreases with the increase of erosion cycle.(4)XRD and SEM techniques were used to analyze the micro erosion mechanism of cl-on HDC under the coupling action of chlorine salt and dry wet cycle.It was found that clwould react with HDC matrix to produce Friedel salt,and f salt would gather in HDC pores with hydration product C-S-H gel and salt crystal.With the increase of solution concentration and cycle period,more and more erosion products would gather in the pores,causing salt swelling damage of HDC,resulting in internal micro cracks,This is also the reason for the macro mechanical change of HDC.In this paper,the action mechanism of PVA fiber under stress and erosion is analyzed from the micro point of view.It is found that PVA fiber can still maintain the crack resistance and integrity of the matrix and increase the ductility and durability of HDC after being eroded by chloride.