Research On The Corrosion Initiation Of Steer Bars In Concrete Under Atmospheric Environment

Concrete materials are the most widely used engineering materials.However,a large number of reinforced concrete structures have not reached the design life.The corrosion of steel bars is one of the major issues that cause the lack of durability,which can lead to structural failure.Therefore,establishing a more reasonable criterion for the corrosion initiation of rebars is one of the core issues in durability evaluation and life prediction of concrete structures.Carbonation-induced corrosion of reinforcing steel is one of the most important causes for the deterioration of reinforced concrete structures under an atmospheric environment.This paper takes the reinforcement corrosion mechanism as the starting point and takes the cover of concrete structures that have been naturally carbonated for several decades as the research object.The research is conducted at both the macro and micro levels.The main research contents and conclusions are as follows.(1)The cover depth,carbonation depth and corrosion status of steel bars were tested.The sample was prepared by cutting along the depths of the cover and actual pore solution of the sample was prepared by the expression method.The pore solution p H and phase compositions were tested layer by layer.The results indicate that when the carbonation depth reaches 1/3~1/2 of the cover depth,the steel bar in the concrete has begun to corrode.And the existence of semi-carbonation zone in concrete is the cause of this phenomenon.The range of fully carbonation zone,semi-carbonation zone and non-carbonated zone of the concrete was determined according to the corrosion status of the steel bar,the p H value of the pore solution and the contents of calcium carbonate and calcium hydroxide.(2)The TG,SEM and MIP were used to analyze the microstructure changes in different carbonated zones.The result indicates that when the p H is between 8 and 9.5,the concentration of calcium hydroxide in concrete is nearly zero,and the concentration of calcium carbonate is between 12%~18%.When the p H is between 9.5and 12.0,the concentration of calcium hydroxide increases from 0% to 4% and the concentration of calcium carbonate decreases from 13% to 6%.The result of SEM shows that the calcium carbonate formed by the carbonation reaction gradually precipitates in the pores of the concrete,resulting in the change of the porosity of the concrete.As the degree of carbonation increases,the total porosity of the concrete decreases and the most probable pore diameter increases,resulting in the increase of connectivity of the pores.As the degree of carbonation increases,the proportion of non-harmful pores decreases and the proportion of harmful pores increases.(3)Based on the carbonation mechanism of concrete,a theoretical mathematical model of carbonation reaction and its numerical solution method were established.Based on the semi-carbonation zone defined in this paper,the effect of water-cement ratio,cement dosage,relative humidity,carbonation time and carbon dioxide concentration on the length of the semi-carbonation zone was analyzed.A theoretical model of the length of the semi-carbonation zone was established.Furthermore,a theoretical model for the corrosion initiation of rebars in concrete under atmosphere environment was established.