Study On Axial Compression Performance Of High Strength Concrete-Filled Steel Tube After Corrosion

Concrete filled steel tube is widely used in building structure,underground engineering,bridge structure,oil drilling platform and transmission tower because of its high strength,good toughness,excellent seismic performance and convenient construction.Concrete-filled steel tubes serving in coastal environment,wet and acid rain environment are prone to corrosion.Corrosion will weaken the cross-sectional area of steel and lead to the deterioration of mechanical properties,resulting in the reduction of bearing capacity and plastic deformation capacity.Therefore,the research on the mechanical properties of concrete filled steel tube after corrosion is one of the urgent problems to be solved in the engineering field,and it is also an important problem to further study the durability of concrete filled steel tube structure in service.At present,the research on the mechanical properties of concrete filled steel tubular members after corrosion mainly focuses on the mechanical properties such as axial compression,axial tension,bias and quasi-static force.The strength of steel mainly adopts Q235 and Q345,and the strength of concrete mainly adopts C30-C55.However,the research on the mechanical properties of high strength concrete filled steel tubular columns after corrosion is still very limited.Therefore,this thesis studies the axial compression mechanical properties of high strength concrete filled steel tubular columns after corrosion.The main research contents are as follows:(1)The accelerated corrosion test and tensile test of 12 groups of mediun and high strength steel specimens with different corrosion rates were carried out,and the mechanical properties and failure modes of high strength steel after corrosion were analyzed.By observing the corrosion process of steel,it is found that the actual corrosion occurs at the same time as uniform corrosion and pitting corrosion.There are obvious pitting corrosion on the surface of steel,the surface is not flat,and small irregular bulges or depressions can be seen with the naked eye.Through the uniaxial tensile test of corroded steel samples,it is found that there is no obvious yield platform for each material sample of high strength steel(Q460)before and after corrosion,and the yield platform of ordinary strength steel(Q235,Q345)disappears after corrosion.With the increase of corrosion rate,all the material samples change from ductile failure to brittle failure,the fracture strain decreases gradually,the yield strength and ultimate strength decrease gradually,and the strengthening section becomes shorter.Due to the randomness of corrosion pits,the fracture occurs at the minimum cross-sectional area or stress concentration.The yield strength,ultimate strength and elastic modulus of Q460,Q235,Q345 round steel pipe and Q460 square steel pipe after corrosion were linearly fitted,and the regression linear equation was obtained.The average goodness of fit was 0.860,and the regression linear fitting was good.(2)Based on the axial compression test of corroded concrete filled steel tubular columns and high strength concrete filled steel tubular columns,the finite element model was established to verify the correctness of the model,contact relationship and boundary conditions.Based on the finite element,88 finite element models of concrete filled steel tubular columns were established,and the effects of corrosion rate,steel strength,concrete strength and slenderness ratio on axial compression bearing capacity were considered.The results show that the greater the corrosion rate,the more obvious the degradation of the ultimate bearing capacity of high-strength concrete-filled steel tubular columns.Compared with the circle section,the axial bearing capacity of the square section is more affected by corrosion.Increasing the strength of steel can significantly improve the bearing capacity of concrete-filled steel tubular columns after corrosion.The bearing capacity of circular concrete-filled steel tubular columns is increased by 20%.When the strength of steel is increased from Q235 to Q460,the bearing capacity can be increased by 14.25%.With the decrease of concrete strength,the greater the corrosion rate,the more obvious the decrease of bearing capacity,and the decrease of square section concrete filled steel tubular column is greater.The corrosion rate of square concrete filled steel tubular is 20%.When the concrete strength decreases from C60 to C50 and C40,the bearing capacity decreases by 12.89% and24.72% respectively.The slenderness ratio increases from 8.75 to 16.63,which has little effect on the bearing capacity and obvious effect on the stiffness.(3)By introducing the effective yield strength after corrosion,the correction formula of axial compression bearing capacity of circular/square steel tube concrete after corrosion is obtained,and the feasibility of the correction method is proved according to the finite element calculation results.According to the modified formula,it is found that the difference between the calculated results of ACI 318-19 and CECS28-2012 and the finite element calculation is the largest,and the average error is greater than 5%.The modified British standard BS 5400 predicts that the axial compression bearing capacity of high strength concrete filled steel tube with circular section and square section after corrosion is more accurate,and the average errors are 2% and 1%respectively,which can be used as a reference for engineering practice.