Study On The Bond Failure Of Corroded Deformed Steel Bar And Concrete Based On Digital Image Correlation Method

The cohesiveness of reinforced concrete structures is an important factor in determining the overall bearing capacity and stiffness of the structure.There is good adhesion to the premise of ensuring the normal operation of reinforced concrete members.However,the reinforced concrete members are in service,the steel bars are easily corroded due to environmental changes.When external loads are applied to the structure,the reinforced concrete interface is prone to occur.Relative sliding causes damage to the bond and affects of structural integrity.Therefore,this paper studies the bond failure of corroded deformed steel bars and concrete,and the main contents include the following aspects:(1)Experiments on electric corrosion of bare bars.As the steel bar is the object of corrosion,test of electric corrosion is carried out on bare steel bars to explore influence of the mode of electrification on the design of corrosion rate.At the same time,the cross-sectional parameters of steel bars,including inner diameter,longitudinal rib height and transverse rib height,before and after corrosion research are discussed.The results show that the steel bar presents non-uniform corrosion,its' inner diameter is the largest corrosion degree,the second is the transverse rib and the longitudinal rib is the smallest.(2)Experiments on bond failure of corroded reinforced concrete with or without stirrups.Considering the difference of diameter of deformed steel bar,concrete strength,thickness of stirrup protective layer and stirrup spacing,C40D22,C40D16,C40D12 and C25D22 without stirrups are poured.The stirrup protective layer with stirrups of 17 mm,the spacing of 80 mm,50 mm and 110 mm,and the protective layer of 27 mm.Eight batches of C40D22 specimens with spacing of 80 mm are electrically corroded,and 0%,3%,5% and 7% of the stirrup-free specimens are corroded respectively.Rate,0%,1% and 3% corrosion rate of specimens with stirrups were pulled out and observed by digital image correlation method.(1)Failure mode.Splitting failure,pull-out failure and Splitting with pull-out failure occur in specimens without stirrups;only pull-out failure occurs in specimens with stirrups.(2)Distribution of surface strain of concrete in different drawing stages.The surface strain of concrete concentrates on cracks and occurs instantaneously during splitting failure;the surface strain of concrete remains unchanged during pull-out failure;the surface strain of concrete concentrates on cracks and extends slowly after splitting and pull-out failure.(3)The relationship between the surface point strain of concrete at the free end of reinforcement and the pull-out force.In the curves of principal strain and pull-out force at the surface point of concrete at the free end where splitting failure occurs on the specimens without stirrups,when the sudden change of principal strain at a certain point happens to be the maximum pull-out force Pmax,then the point locate in the crack area or near.In the analysis of the relationship between the principal strain of three points equidistant distributed on the concrete surface in front of the longitudinal rib of the free end of the steel bar and the drawing force of the corroded specimens with stirrups,it is found that the principal strain of three points increases sharply at the same time when Pmax is applied,and then the three principal strains remain basically unchanged.(4)Bond-slip curve of specimens after rusting.The bond strength of corroded specimens with and without stirrups increases first,and then decreases with the augment of corrosion rate.The bond strength of corroded specimens without stirrups increases most at 3% corrosion rate,while that of corroded specimens with stirrups increases most at 1% corrosion rate.The bond strength ratio of specimens with stirrups is larger than that of specimens without stirrups.(3)Prediction model of bond strength of corroded reinforced concrete.According to the results of pull-out test of reinforced concrete without and with stirrups,the bond parameter database is established,and the GA-BP network model is established by MATLAB.The test results are verified and predicted.The results show that the difference between the real value and the predicted value is not more than 0.3,which can better predict the test results.