| Concrete filled steel tube(CFST)columns are used in civil engineering because of their cost performance,excellent ductility and fire resistance.However,the performance of Concrete-filled steel tube(CFST)columns corroded locally under freeze-thaw conditions has hindered its wide application in practice.This paper focuses on circular concrete filled steel tubular short columns with local corrosion in freeze-thaw environment.According to the suggestions of previous studies,the local corrosion of specimen steel tubes is simulated by artificial notch.A total of 168 specimens were designed in this experiment,including 6round steel tube concrete specimens without local corrosion and freeze-thaw cycle,36 round steel tube concrete specimens only subject to local corrosion,18 round steel tube concrete specimens only subject to freeze-thaw cycle,and 108 round steel tube concrete specimens first subject to local corrosion and then freeze-thaw cycle.The study of its axial pressure performance and variation law is carried out,and the main conclusions are as follows:(1)The bearing capacity of locally corroded specimens decreased by 4%,6% and11%,respectively,compared with that of unopened specimens,DO,RV and DV notched specimens.The variation of ultimate bearing capacity of DH,RH and RO notched specimens is ±1% compared with that of intact specimens.The failure mode of the unopened specimen is the local buckling of the outer steel tube,and the failure mode of the locally corroded specimen is the bulging,closing and local buckling of the notch.Because the overall buckling of unopened specimens is similar to that of horizontal openings,the horizontal openings are slightly improved by sacrificing part of ductility in the process of early closure,and the vertical and inclined notches result in a significant decrease in the mechanical properties of specimens compared with unopened specimens.(2)When freeze-thaw cycles were 15,30 and 45 days,outward buckling was observed in the outer steel tubes of all samples.With the increase of frozen-thawing days,the attenuation ratio of ultimate bearing capacity is as follows: mix ratio A is 0.78%,1.32%,5.11%,mix ratio B is 1.28%,1.69%,3.28%,mix ratio C is 0.01%,0.02%,0.79%.The ultimate bearing capacity of CFST specimens with mix ratio A and B showed A decreasing trend,while the mechanical properties of CFST specimens with mix ratio C were stable.The ductility coefficients showed A downward trend,and the ductility of CFST of mix ratio A decreased the most.(3)Under the coupling effect of local corrosion followed by freeze-thaw cycle,the strength attenuation ratio of the specimen under the conditions of 270 freeze-thaw cycles in45 days and DH,DO and DV local corrosion is as follows: the mix ratio of specimen A is1.1%,14.9% and 17.1%,respectively.The proportions of B samples were 2.5%,8.5% and16.9%,respectively.The mixture ratios of C specimens were 4.5%,11.8% and 17.2%,respectively.When the concrete filled steel tube is vertically opened,the ultimate bearing capacity of the specimen decreases most obviously,and the ultimate bearing capacity of the specimen decreases with the impact of notch and the increase of freeze-thaw days.The ductility shows a decreasing trend,and the elastic stage increases with the increase of freeze-thaw days,while the elastic stage shortens due to the impact of notch.(4)Based on the effective section area of the notched steel tube and the degree of freeze-thaw cycle damage of the notched steel tube,this paper proposes a calculation formula that can predict the axial compression bearing capacity of concrete filled circular steel tube columns,and compares the test results with the calculated results of the derived prediction formula.The results show that the deviation of the predicted ultimate bearing capacity of CFST columns is less than 10%,which can be used to predict the ultimate bearing capacity of CFST columns. |
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