Durability And Finishing Performance Evolution Of Fair-faced Concrete In Coastal Metro Station

Due to the advantages of environmental protection,economy and aesthetics,fair-faced concrete has been used more and more in engineering.Due to the lack of protection from decorative materials,fair-faced concrete is more susceptible to corrosion by corrosive media such as chloride ions,sulfate ions and carbonization in the service environment.In addition to posing a threat to the durability of fair-faced concrete materials,these corrosive materials may also adversely affect the color consistency of fair-faced concrete surfaces.Based on the actual service environment of Qingdao Metro Line R3,this paper studies the basic properties of four proportioned fair-faced concrete materials,the law of chloride ion and sulfate ion transmission under real sea exposure environment,and the rule of steel corrosion under constant potential acceleration,the surface morphology evolution of fair-faced concrete under carbonation,salt spray and sulfate corrosion environment was studied by introducing image gray scale standard deviation,the strength,carbonation resistance and chloride ion penetration resistance of C40 and C50 concrete under actual service conditions were studied.The main results are as follows:(1)C40,C50,C50 S and C55 fair-faced concrete all have good workability,the slump of fresh concrete is above 180 mm,the expansion is above 380 mm,and there is no bleeding phenomenon.The concrete has good fluidity,adhesion,water retention,and strength development.(2)The order of carbonation resistance,salt spray and sulfate corrosion resistance of concrete specimens with different proportions is C55 > C50 S > C50 > C40.With the decrease of water binder ratio,the durability of concrete is improved.(3)It can be found by constant potential accelerated test that the corrosion of reinforcement occurs near the protective layer,the pitting corrosion of reinforcement occurs,and the corrosion spots of concrete are unevenly distributed,which is similar to the natural corrosion of reinforced concrete.The corrosion behavior of steel bars in concrete can be traced by Electrochemical Impedance Spectroscopy,the charge transfer resistance and corrosion current density can be obtained by impedance spectroscopy fitting,the deactivation and corrosion time of steel bars in concrete can be judged,According to the sudden change point of corrosion current,the time of concrete rust expansion and cracking can be determined,the area of rusty spot of reinforced concrete can be calculated by image processing software.(4)According to the experimental data of surface chloride concentration,chloride binding capacity,chloride diffusion coefficient,the experimental results in the marine atmospheric zone,tidal zone and splash zone are in good agreement.The order of the chloride resistance of concrete with different proportions from strong to weak is C55 > C50 S > C50 > C40,this indicates that reducing the water-to-binder ratio can significantly improve the chloride resistance of concrete.Secondly,at the same corrosion age,the corrosion degree of concrete in the splash zone is more serious than that in the tidal zone and the atmospheric zone.The chloride ion concentration in the surface layer of concrete increases with exposing age in power function,the free chloride ion concentration in the surface layer of concrete increases rapidly in the early period of service and slows down gradually in the later period of service.The apparent chloride ion diffusion coefficient of concrete shows a power function relationship with the exposure age,with the extension of corrosion age,the chloride ion diffusion coefficient of concrete decreases gradually,and the relationship between the speed of reduction is C55 > C50 S > C50 > C40.(5)According to the distribution of total sulfate ion concentration and the reaction coefficient of sulfate ion in atmospheric zone,tidal zone and splash zone,the sulfate corrosion degree in the splash zone is more serious than that in the atmospheric zone and tidal zone at the same corrosion age.The order of the resistance of concrete to sulfate ion from strong to weak is C55 > C50 S > C50 > C40,reducing the concrete water-to-binder ratio has a significant effect on improving the resistance to sulfate attack.(6)The results of salt spray,carbonization and sulfate corrosion tests on 250mm×250mm×100mm fair-faced concrete show that salt spray corrosion does not adversely affect the surface color consistency of fair-faced concrete,and the standard deviation of image gray scale does not occur before and after corrosion of fair-faced concrete specimens with different proportions,the standard deviation of grey scale is not related to the chloride corrosion resistance of concrete.Carbonation will have a certain adverse effect on the surface color consistency of fair-faced concrete,with the increase of carbonization age and carbonization depth,the grey standard deviation of the surface image of fair-faced concrete increases to a certain extent,but the degree of increase is not very obvious.Sulfate corrosion will significantly destroy the surface color consistency of fair-faced concrete.(7)The results of SEM and EDS microscopic tests show that the increase of color difference on the surface of fair-faced concrete under sulfate corrosion environment is mainly caused by the denudation of sulfate on the surface of fair-faced concrete.(8)The surface color of C40 and C50 fair-faced concrete test wall is uniform,which meets the standard requirement of fair-faced concrete.The strength of C40 and C50 fair-faced concrete test wall meets the design standard.The fair-faced concrete has good carbonation resistance and chloride ion penetration resistance under the actual engineering service environment.