Corrosion Behavior Of Steel Bars In The Negative Bending Moment Zone Of ECC-RC Continuous Beams In Chloride Environment

The construction method of reinforced concrete(RC)continuous beam bridge is to prefabricate reinforced concrete beams and erect them on the top of pier columns,and then cast concrete joints on site to make them continuous.This joint is called the negative moment zone of RC continuous beam bridge.The negative moment zone of RC continuous beam bridge is in tensile state under normal use,but the tensile strength of concrete is low and the ductility is poor.In the northern cold region,large vehicles running on the bridge will cause cracks in the concrete in the negative bending moment area.In order to ensure the safety of driving in snowy days,it is necessary to sprinkle snow melting agent(mainly composed of chloride salt)to clean up the snow.However,when the bridge is subjected to load in a chloride environment,the coupling effect will promote the diffusion of chloride ions,resulting in accelerated corrosion of steel bars and decreased bearing capacity of the bridge.In view of the mechanical characteristics and actual diseases of the negative moment zone of the precast RC continuous beam bridge,ECC material with excellent performance is used to replace the ordinary concrete in the negative moment zone of the RC continuous beam bridge to form the post-cast joint of ECC.Based on the coupling effect of dry-wet erosion and sustained load in chloride salt environment,combined with electrochemical test,steel bar tensile test,three-dimensional scanning technology and other technical means,10ECC-RC beams were poured to simulate RC continuous beam bridges under different sustained loads and different dry-wet cycles,and the corrosion behavior of steel bars in negative bending moment area was studied.The main work and results were as follows:(1)Through the electrochemical test of the steel bars in the negative bending moment zone of ECC-RC beams,it could be seen that the corrosion reaction tendency of the steel bars in the negative bending moment zone of ECC-RC beams was the most serious under the overload level II state,followed by the steel bars in the negative bending moment zone of ECC-RC beams under the overload level I state,and the corrosion reaction tendency of the steel bars in the negative bending moment zone of ECC-RC beams under normal use state was the lightest.The corrosion degree of steel bars in the negative moment zone of ECC-RC beams under normal service condition and overload grade I condition was small,and the corrosion rate was slow and stable.Although the corrosion degree of steel bars in ECC-RC beams under overload grade II was large,the corrosion rate was still decreasing.(2)Through the axial tensile test of the steel bar,the mechanical performance index and load-displacement curve of the steel bar were obtained.It could be seen that under the same sustained load,with the increase of the number of wet and dry cycles,the ultimate load of the steel bar in the negative bending moment zone of the ECC-RC beam gradually decreased,and the decrease gradually increased.In the normal use state of ECC-RC beams,the corrosion degree of steel bars was small and could maintain good plasticity.The degradation degree of mechanical properties of steel bars was much smaller than that of steel bars in the negative bending moment zone of ECC-RC beams under overload grade I and overload grade II.The reduction of ultimate load of steel bars in the negative bending moment zone of all ECC-EC beams in this experiment remained below 10%.(3)By measuring the mass corrosion rate of steel bars,it could be seen that the mass corrosion rate of steel bars in the negative moment zone of ECC-RC beams could be maintained below 3%under normal service condition and overload grade I condition.By establishing the correlation between the degradation coefficient of nominal yield strength,the degradation coefficient of nominal ultimate strength,the degradation coefficient of elongation after fracture and the mass corrosion rate,it could be seen that the mechanical properties of the steel bar will degenerate with the increase in the corrosion degree of the steel bar,and showed a linear relationship.Compared to the yield strength and ultimate strength of steel bars,the elongation after fracture was more significantly affected by the degree of steel corrosion.(4)Through the macroscopic and microscopic analysis of the fracture of the steel bar,it could be seen that the fracture form of the steel bar was serrated fracture macroscopically and dimple fracture microscopically.In this experiment,ductile fracture occurred in the tensile process of the steel bar.(5)Through the three-dimensional scanning test of steel bars and the establishment of the correlation between steel corrosion indicators,it could be seen that the three-dimensional scanning technology was reliable for the acquisition of steel corrosion information.As the dry-wet cycle progresses,the variation range of the remaining cross-sectional area distribution of the steel bar begins to gradually increased,and the distribution gradually became discrete.With the increase in corrosion degree,more local corrosion pits appeared on the steel bar,which resulted in the uneven corrosion of the steel bar surface.With the increase in the average corrosion rate,the growth rate of the non-uniformity of the steel surface corrosion would be more and more slow.(6)Through the statistical analysis of the remaining cross-sectional area parameters of the steel bar,it can be seen that the corrosion degree of the steel bar in the negative bending moment area of the ECC-RC beam under normal use was not large,and the remaining cross-sectional area is mostly concentrated between 76mm~2 and 77.5mm~2.The corrosion of the steel bar is more uniform and the corrosion pits are less.With the increase of the load of ECC-RC beams,the corrosion pits of steel bars began to increase,and the unevenness of corrosion on the surface of steel bars gradually became prominent.The corrosion degree of steel bar in the negative moment zone of ECC-RC beam was the largest under the overload II state,and the distribution of the remaining cross-sectional area was more dispersed.The smaller remaining cross-sectional area was mainly concentrated in the middle of the steel bar.