Prediction For Corrosion Life Of Reinforced Concrete Structures Under Salt Freeze-thaw Environment

Freezing-thawing and chloride ion erosion are the important factors affecting the corrosion life of reinforced concrete structures,while freezing-thawing damage and environmental temperature and humidity have significant effects on chloride ion transfer in concrete.The existing research on the transport characteristics of chloride ions in concrete is mainly for the normal temperature environment under constant humidity conditions,but there are few studies on the positive and negative temperature alternation and relative humidity change environment common in the northern region,and related research often predicts the rusting time of steel bars by the time when the chloride ion concentration reaches the critical concentration in the depth of the concrete protective layer,not considering the cumulative effect caused by the diffusion of chloride ions to the steel surface due to diffusion,which leads to the early rusting time of the steel bars.At present,China’s industry standard "Standards for Durability of Concrete Structures"(CECS220:2007)does not consider the influence of steel bar retardation in the calculation of steel rusting time prediction under chloride ion erosion environment.Obviously,the predicted results is dangerous in engineering.In view of this,this paper focuses on the substance of rust life prediction of reinforced concrete structures in salt-freezing environment,and proposes a modified equation based on time-varying temperature,relative humidity,freeze-thaw damage and concrete chloride ion diffusion coefficient.The chloride ion transport model in concrete under freeze-thaw environment was constructed,and the model was verified by the existing literature.The results show that it has high calculation accuracy.Furthermore,the chloride ion transport model considering the cross-sectional shape and the steel bar retardation effect in the freeze-thaw environment was established by using the COMSOL Multiphysics.The evolution of the chloride ion concentration distribution on the steel surface in the member with different cross-section shapes with erosion time was analyzed.Quantitative analysis of the sensitive factors affecting the retardation effect of steel bars was carried out,and the effects of freeze-thaw damage and steel block retardation on the corrosion life of the structure under different cross-sectional shapes were discussed.The main conclusions are as follows:1)The tensile strength of concrete decreases exponentially with the increase of the number of freeze-thaw cycles;The average chloride ion diffusion coefficient of concrete increases exponentially with the increase of the number of freeze-thaw cycles;The apparent chloride ion diffusion coefficient of concrete decreases first and then increases with the increase of erosion depth,and finally stabilizes;The chloride ion concentration of concrete surface increases exponentially with the increase of erosion time after freeze-thaw.2)The freeze-thaw damage and the retardation effect of steel bars have an important influence on the corrosion life of concrete structures under cold chloride environment.The model presented in this paper can be used to simulate the transport of chloride ions in concrete and predict the corrosion life of the structure.3)In the environment of chloride salt erosion,the steel bar in the concrete member has an influence on the chloride ion transport process and its concentration distribution.The chloride ion accumulates on the erosion surface of the steel bar,and the back erosion surface concentration is delayed.When other conditions are the same,the concentration of chloride ion on the steel bar surface at the same position in the circular section is higher than that in the rectangular section.4)The distribution of chloride ion concentration on the surface of the non-corner area in the circular and rectangular sections is relatively stable,and the overall shape is elliptical,and the peak concentration is always kept closest to the surface of the member;The shape of chloride ion concentration distribution on the surface of the rectangular section in the rectangular section varies with the erosion time.The early approximation is a heart shape,that is,the concentration peak is closest to the positive and side surfaces of the reinforcement.Later,the concentration peak gradually shifts to the diagonal area.The distribution shape is obliquely elliptical.5)When other conditions remain unchanged,the retarded effect of reinforcement on chloride ion transfer in concrete increases with the increase of reinforcement diameter,weakens with the increase of concrete protective layer thickness,and attenuates with the increase of erosion time.For the rectangular section of the rectangular section,the most significant point of the retardation effect occurs at the 45° center angle of the reinforcing bar,and the most significant point of the concentration hysteresis occurs at the center angle of the reinforcing bar at 225°.The retardation effect is not sensitive to the value of the chloride ion concentration on the concrete surface,and it is considered to be unaffected by the surface chloride ion concentration.6)Regardless of the freeze-thaw damage and the retarding effect of the steel bar,when the other conditions are the same,the corrosion life of the concrete structure is significantly reduced after considering the freeze-thaw damage and the retarding effect of the steel bar.