Experimental Study On Axial Compressive Behavior Of CFRP Strengthened Square CFST Stub Columns With Mortar Segment

With the widely application of concrete-filled steel tube(CFST)in civil engineering,there is a potential reinforcement requirement for concrete-filled steel tubular structures or components because of corrosion,increase in the use of load,and changes in the use of the structure.The application of CFRP(Carbon fiber cloth)reinforcedconcrete-filled steel tubecolumn not only improves the bearing capacity but also improves the corrosion resistance.It has a good engineering application prospect.However,the problem of CFRP reinforced concrete-filled square steel tube columns is that the square steel tube cannot be chamfered like an ordinary concrete column.The corner stress concentration caused by outsourcing CFRP cannot be eliminated.In this paper,cross-sections are treated by wrapping a circular-arc mortar surfaceto reduce the stress concentration at the corner of the steel tube and enhance the restraining effect of the CFRP.Based on the existing research results,the main research contents are as follows.(1)Axial pressure test of 16 confined concrete specimens(including 12 CFRPstrengthened square CFST stub columns with circularized section by cement mortar,2 CFRP-confined square concrete-filled steel tubular short columns and 2 square concrete-filled steel tubular short columns)was carried out for axial compression bearing performance.Based on the experiments,the effects of the concrete strength,the number of CFRP layers and the height of the mortar layer on the bearing capacity and deformation performance of the specimen were studied.Increasing the number of CFRP layers and the height of the mortar layer to increase the bearing capacity of the specimens are more effective.According to the load-displacement curve of the specimen,the failure process was analyzed.The effects of different mortar heights on the fracture position,effective circumferential tensile strain,and the area of the effective restraint area of the carbon fiber were analyzed.(2)The method of determining the effective tensile strain coefficient and the area of the effective restraining area under different heights of the mortar layer was given.Based on the superposition theory and the existing constrained concrete strength calculation model,a calculation formula for the bearing capacity of CFRPstrengthened square CFST stub columns with circularized section by cement mortar was established.Comparing the formula prediction results with the experimental measured values,they are in good agreement.(3)Existing constrained concrete stress-strain relationship expressions and test data are used as the basis.Combining the stress mechanism and experimental stress-strain curve characteristics of CFRPstrengthened square CFST stub columns with circularized section by cement mortar,the Teng model is selected.The stress-strain relationship expression in this paper is expressed in a piecewise manner.The joint constraints of CFST and CFRP-confined concrete were considered to determine the characteristics of each stage.