Experimental study on bond behavior between inner steel member and concrete in steel reinforced concrete-filled steel tubes

To study the bond behavior between inner steel member and concrete in steel reinforced concrete-filled steel tubes (SRCFST), 18 specimens were designed based on the orthogonal experimental principle and subjected to push-out tests, including two types: SRCFST with inner I-section steel and SRCFST with inner steel tube. The research parameters include cross-sectional dimensions, bond length, concrete strength, and expansive agent content. Comparative analysis was conducted on failure modes and bond mechanisms, while the stress distribution characteristics of inner steel members were systematically analyzed. The influence of individual factors on bond strength was quantitatively evaluated. The results indicate that the bond stress-slip curves for both inner I-section steel and inner steel tube include the adhesion stage, nonlinear initial slip stage, and slip stage, and the curves exhibit four types after the peak point. For inner I-section steel, the primary-to-tertiary factors influencing ultimate bond strength are cross-sectional dimensions, bond length, and concrete strength. For inner steel tube, the primary-to-tertiary factors influencing ultimate bond strength are cross-sectional dimensions, expansive agent content, and bond length. Under peak load conditions, the bond stress distribution curves for two types of steel members exhibit a negative exponential distribution. The interface bond performance between the inner steel tube and concrete is superior to that of inner I-section steel. Based on experimental data, two theoretical formulas for the ultimate bond strength of SRCFST structures with inner steel member were proposed. The accuracy of these formulas was validated through comparison with theoretical formulas proposed by other scholars.