Steel Bar Corrosion In Concrete And Time-dependent Reliability Analysis Of Reinforced Concrete Structures Under Chloride Attack

Chloride-induced steel bar corrosion is one of the main causes of the durability failure of reinforced concrete structures in marine environments.The durability failure process of the reinforced concrete structures mainly includes chloride diffusion into the concrete,the accumulation of chloride ions on the surface of the steel bar to a critical value resulting in the initiation of rebar corrosion,and the concrete cover cracking due to the propagation of the rebar corrosion.First of all,a large number of chloride diffusion models considering the influence of various factors have been established.Secondly,many semi-empirical models were established to predict the corrosion rate of steel bars,which were based on the assumption that the corrosion rate was controlled by the oxygen diffusion.However,more discussions about the chloride diffusion model considering the time-dependent chloride binding capacity,determination of chloride threshold for corrosion initiation,corrosion mechanisms and corrosion rate prediction model,time-dependent reliability analysis of overall structures are needed.Therefore,lab experiments were applied to simulate the chloride diffusion and the rebar corrosion in the concrete exposed to marine environments.The degradation mechanism of rebar corrosion in the concrete was discussed.Based on the experimental results,prediction models of chloride ion diffusion and rebar corrosion rate were established.In addition,the time-varying reliability analysis of the members and the overall structure of reinforced concrete structures are analyzed using numerical calculation methods to provide a reference for the durability design and evaluation of reinforced concrete structures.The main research contents and conclusions of the present work are as follows:(1)A Q-FOG salt spray apparatus and an independently developed artificial marine environment automatic simulation system were used to simulate the marine environments of salt spray area and tidal zones,which can realistically simulate the chloride diffusion in the concrete.Results show that the chloride binding effect can be described by a linear isothermal adsorption curve and the degradation of chloride binding capacity can be simplified to be a power function of the exposure time.A chloride diffusion model considering the time-varying chloride binding capacity was thus established to predict the chloride concentration in concrete.(2)A test method combining accelerated chloride migration test(ACMT)and artificial natural diffusion test(ANDT)was proposed to simulate the steel bar corrosion process in concrete to investigate the chloride threshold for corrosion initiation and the corrosion rate of steel bars in the concrete.Comparing to the traditional experimental methods,this test method can not only shorten the time for chloride ions to reach the surface of the steel bar and start the corrosion but also try to protect the real corrosion status of rebars.(3)The chloride threshold for corrosion initiation is defined as the critical chloride ion concentration on the surface of the steel bar required for activating the depassivation of steel bar and maintaining the aggressive pitting corrosion.A multi-criteria assessment was proposed to determine the corrosion initiation.Results show that the chloride threshold will decrease with the increase of the thickness of the concrete cover,but when the thickness is greater than 25 mm,the chloride threshold will tend to be stable.Moreover,longitudinal rebars(ribbed steel bars)have a higher chloride threshold than stirrups(plain steel bars).Therefore,according to the design parameters of the reinforced concrete structures in the marine environment,the chloride threshold and the critical p H were suggested for the stirrups and longitudinal rebars in the concrete,respectively.(4)The electrochemical analysis was used to explore the control factors of the steel bar corrosion process in the concrete exposed to the salt fog zone and the tidal zone marine environments,and to predict the corrosion rate.Results show that the corrosion process is mainly controlled by the anode diffusion.Based on the assumption of anode diffusion control,a semi-empirical model was established to evaluate the corrosion rate of single steel bars or steel bar cages in uncracked concrete in the above two marine environments.The physical meanings of its parameters are clear and the equation is simple.It is easy to calculate and is in good agreement with the experimental data.(5)Randomness of relevant parameters in the process of durability degradation of reinforced concrete structures was considered.Taking a typical high-piled wharf as an example,a Monte Carlo stochastic simulation method was used to establish a time-dependent resistance probability model for each member of the high-pile wharf;a Latin hypercube sampling method and a finite element model were used to establish a probabilistic model of time-dependent overall bearing capacity of a high-pile wharf.Based on the segmented time section and the time-dependent reliability theory,the JC method was used to evaluate the time-dependent reliability of the members and the overall structure.The results show that the initial corrosion time of the steel bar and the cracking time of the concrete cover approximately follow the lognormal distribution.Also,the time-dependent coefficient of both mean value and the standard deviation of member resistance and overall bearing capacity can be described by the quadratic function of service time.The above experimental results not only provide a new test method to simulate the reinforced concrete degradation,but also propose more reasonable prediction models to describ the durability degradation of reinforced concrete under chloride attack.Moreover,the time-dependent reliability analysis of members and the overall structure,based on the experimental results,also provides a reference for the durability design and evaluation of reinforced concrete structures.