To study the bond performance between internal steel and concrete in concrete filled steel tube (CFST)

18 CFST push-out tests were conducted. The bond mechanism and bond stress distribution law between two kinds of internal steel and concrete were studied. The influencing law of parameters on bond strength was obtained and analyzed through orthogonal test

and empirical formulas of bond strength were proposed based on measured data. Test results show that bond stress-slip curves of both internal I-shaped steel and internal steel tube consisted of 3 stages

namely

adhesive stage

non-liner initial sliding stage

and sliding stage. For internal I-shaped steel

the parameters that influenced ultimate bond strength were in order of section size of internal I-shaped steel

bond length

and concrete strength in terms of importance. The ultimate bond strength of the internal I-shaped steel increased with increasing section size of I-shaped steel and concrete strength

while decreased with increasing bond length. For internal steel tube

the parameters that influenced ultimate bond strength were in order of section size of internal steel tube

mixing amount of expansion agent

and bond length in terms of importance. The ultimate bond strength of the internal steel tube increased with increasing section size of internal steel tube and mixing amount of expansion agent

but decreased with increasing bond length. Bond stress distributions of both internal I-shaped steel and internal steel tube exhibited negative exponential relations under peak load

and the average bond stress on external flange of I-shaped steel was about 1.77 times that of web. The bond performance between internal steel tube and concrete in CFST was better than that of internal I-shaped steel. A comparison with previous research results verified the better applicability of the proposed formulas.