Study On The Performance Evolution And Mechanism Of Steel Fiber Reinforced Concrete Under Chloride Ion Attack

Steel fiber-reinforced concrete(SFRC)is fiber-reinforced composite material,with excellent characteristics compared to ordinary concrete,such as tensile strength,flexural toughness and crack resistance performance,which has broad application prospects in water conservancy,ocean,port,bridges and tunnels.However,the performance of SFRC will deteriorate due to corrosion in harsh Marine environment.At present,the limited research on chloride corrosion of SFRC in a marine environment results in an unclear performance evolution law and deterioration mechanism of SFRC after corrosion,which limits the application and development of SFRC to some extent.In this work,the chloride content distribution and mechanical properties evolution of SFRC,as well as the corrosion mechanism of steel fibers suffered from chloride attack were investigated.The main contents and conclusions are as follows:(1)According to the service environment characteristics of concrete structure in the marine environment,the total immersion,drying-wetting cycles and salt fog method were used to simulate the corrosion behavior of SFRC in the marine environment.The transport and concentration distribution of chloride ions in SFRC under the influence of different factors were studied.The results show that the chloride ion transport depth of SFRC increases but the chloride diffusion coefficient decreases with the exposed time increasing;except for the specimens curing in salt fog environment,the chloride depth of other specimens was more than 20mm after 24 months of corrosion.The W/B ratio significantly affected chloride penetration in the SFRC,a lower W/B ratio of the SFRC showed lower chloride ion content and lower chloride diffusion coefficientl The addition of fly ash and slag showed a reasonable inhibition effect on chloride permeability;moreover,the SFRC with 20%fly ash presented higher chlorine resistance than that with 20%fly ash and 40%slag.The chloride ion transport depth and content of SFRC exposure in drying-wetting cycles condition were the highest but the lowest in salt fog condition.(2)The compressive strength,axial compression and bending properties of SFRC were tested at different corrosion time.The results show that the steel fibers near the surface of SFRC were seriously corroded,but no visible sign of rust was observed on the steel fibers embedded in the SFRC.The chloride erosion has no effect on the mechanical performance of SFRC,the compressive strength of SFRC exposed in immersion and salt fog conditions increases with the exposure time increasing,and the bending toughness of SFRC decreases slightly under the drying-wetting cycles condition.The deterioration degree of the SFRC subjected to three types of chloride ion conditions was drying-wetting cycles condition,immersion condition,salt fog condition in descending order.(3)The chloride corrosion test of steel fiber was designed to test the changes of diameter,tensile ultimate load and microstructure of corroded steel fiber;moreover,the influence of diameter of steel fiber,alkalinity and concentration of chloride solution on the corrosion of steel fiber were considered.The results show that the corrosion degree as well as the surface roughness of steel fiber increase with the corrosion time increasing,accompanied by the significant decrease of diameter;however,the resulting corrosion product layer will delay the corrosion process of steel fiber.The smaller the diameter of steel fiber,the greater the degree of corrosion.The lower alkalinity of chloride solution showed the higher corrosion rate and attenuation degree of steel fiber;especially,the steel fiber was hard to be corroded when the alkalinity of chloride solution is more than 12.The steel fiber was prone to be corroded with the increase of the chloride solution concentration.(4)The electrochemical detection technology was used to monitor the corrosion process of steel fiber for the real-time.The open circuit potential,Tafel polarization and Electrochemical impedance spectra(EIS)of steel fibers under different factors were obtained by means of open circuit potential method,Tafel polarization method and EIS.The results show that the corrosion process of steel fibers in chlorine solution is fast and the time of desaturation is short,some steel fibers were found broken after 62 days.The diameter of steel fibers has little effect on electrochemical corrosion.The steel fibers with larger diameter exhibit a higher corrosion tendency at the early stage of corrosion,while the steel fibers with smaller diameter exhibit a greater corrosion degree at the later stage of corrosion.The corrosion tendency and corrosion rate of steel fibers are the highest when pH of chloride solution is 7,and the corrosion resistance of steel fibers is still strong when pH is 13.The higher the chloride ion concentration is,the faster the electrochemical corrosion process of steel fibers will be.The corrosion rate and corrosion degree of steel fibers will be greatly improved when the chloride ion concentration exceeds 4%.