Durability Study And Service Life Prediction Of Reinforced Concrete Specimens Under Salt Frozen Environment

Durability failure of reinforced concrete structure caused by salt frost action willlead to enormous economic losses and will affect the economic and social development.This highlights the importance of durability study of concrete structure under salt frostenvironment.The salt frost action could cause irreversible damage on concrete, speedup permeation and diffusion of corrosive medium, accelerate the corrosion of steel inconcrete and concrete cover corrosion crack of reinforced concrete structure, affect thenormal use and bearing performance. To study the concrete durability under salt frostenvironment and reinforcement corrosion rules caused by chloride ion erosion haveimportant theoritical significance and engineering value on the durability design ofnew construction and durability assessment of existing concrete structures.The reinforced concrete structure in deicing salt environment and cold regionmarine environment was taken as the research object to research the effect of salt froston concrete durability and steel bar initial corrosion and concrete cover cracking, thenpredict the durability service life of salt frost concrete structure. The main researchcontents are as follows:Firstly, through the rapid freezing thawing experiment with freezing medium of3.5%NaCl solution for the fly ash concrete, the effect of salt frost action on theconcrete external damage and internal damage were studied. Based on the influence ofsalt frost on the fly ash concrete static modulus of elasticity, the relationship betweenthe concrete static modulus of elasticity and salt frost damage degree with salt freezingdamage degree introduced. The effect of reinforcement on the concrete salt frozenexternal damage and internal damage were compared. At the same time, the salt frostdamage mechanisms of reinforced concrete caused by the uneven distribution of heatstress were analyzed during the course of salt freezing. All these will lay a foundation for the salt frost durability research and analysis of reinforced concrete member.Secondly, the experimental research of chloride ion diffusivity in reinforcedconcrete members after salt frost action has been developed. Through the analysis offly ash concrete surface structure and pore structure due to the effects of freezing andthawing, the free chloride ion diffusivity in concrete was studied and the relationmodel between the salt damage degree and the coefficient of chlorine ion diffusivitywas established. Based on Fick’s second law, considering the temperature and humidity,time dependence of chloride ion diffusivity, chloride ion binding properties, concretefreeze-thaw damage and the convection zone depth of chloride ion diffusion effect, themodel of chloride ion diffusivity was established under salt frost environment whichhave been verified by the actual case. To consider the existence of block effect ofreinforcement on the chloride ion diffusion in concrete by introduce the chloride ionconcentration increase factor can provide a scientific basis for the accurate predictionof steel bar surface chloride ion concentration and corrosion of rebar.Thirdly, through the analysis of steel rebar corrosion control in concrete under saltfrost environment, the steel corrosion rate prediction model has been established beforeconcrete cover crack with the comprehensive consideration of the influence of theconcrete salt frost damage on the chloride ion diffusion and steel bar corrosion. Thenwith the analysis of influencing parameters, all the effect of the concrete salt frostdamage, concrete resistance, chloride ion concentration, temperature et al. on the steelcorrosion rate before concrete cover crack have been analyzed. At last, the predictionmodel was verified.Fourthly, the accelerated corrosion test of concrete cover cracking conditionsunder salt frost environment has been developed. Both the effect of the concrete frostdamage degree and the ratio of concrete cover thickness to the reinforcement diameterinfluence factors on the concrete surface cracking morphology, steel bar corrosion rateand steel bar corrosion depth has been studied. Based on the strength reduction ofconcrete, static elastic modulus decreased, the concrete cover thickness decreased, thevariation of concrete crack expansion process and the critical expansion force has beenanalyzed. Then the steel corrosion rate model was established on the time of theconcrete cover cracking under salt frost environment, which could provide a theoretical basis for the concrete structure service life prediction at the severe environment.Finally, the study focused on the corroded reinforced concrete structure durabilitylife prediction. With the analysis of function relation between the number of freeze-thaw cycles and freezing thawing fatigue, the concrete freezing thawing damageprobability distribution characteristics has been obtained. Subsequently, the concretedurability life prediction method has been put forward in the actual freezingenvironment.Through the analysis of the steel corrosion initial conditions and concretecover cracking due to corrosion, the initial corrosion prediction life of reinforcedconcrete structure in the salt frost environment and concrete cover cracking lifeprediction method have been proposed on the basis of reliability theory. That canprovide a reference for the reinforced concrete structure durability design.