Zhuojin Pipe Fitting Co., LtdExperimental Study on Interfacial Bond Strength of Square Steel Tube Concrete
Literature Overview
This concise but technically significant paper by Liu Yongjian and Chi Jianjun, published in Building Technology in 2005, investigates the interfacial bond strength between steel tubes and concrete cores in square steel tube concrete (SSRC) members. The research employed push-out tests to measure the load-slip relationship at the steel-concrete interface, providing fundamental data for understanding the composite action mechanism in square SRC structural elements. The classification code TU398 places this work within the broader context of composite steel-concrete structural research.
Technical Methodology and Bond Mechanism
The push-out test is the standard experimental method for characterizing the interfacial bond between steel and concrete in composite members. In this test, a steel tube is embedded in a concrete cylinder, and a steel rod is inserted through the tube to apply axial tension, forcing the tube to slide relative to the concrete. The resulting load-slip curve reveals the bond stress transfer mechanism.
Bond Strength Characteristics in Square vs. Circular Tubes
| Aspect | Circular Steel Tube | Square Steel Tube |
|---|---|---|
| Bond stress distribution | Uniform along perimeter | Non-uniform; concentrated at corners and flat faces |
| Corner effect | Not applicable | Stress concentration at re-entrant corners |
| Slippage initiation | Uniform along circumference | Initiated at flat face midpoints |
| Ultimate bond strength | Higher per unit area | Lower per unit area due to corner effects |
| Post-slip behavior | Gradual degradation | More abrupt degradation possible |
The geometry of the square cross-section introduces corner effects that fundamentally alter the bond stress distribution compared to circular tubes. At the flat faces, the steel tube provides uniform lateral constraint to the concrete, promoting effective interfacial friction and mechanical interlock. However, at the corners, the curvature discontinuity creates localized stress concentrations that can initiate micro-cracking in the concrete at the interface, potentially reducing the effective bond length.
Welding and Fabrication Considerations
For square steel tube concrete members used in structural applications, the fabrication and welding processes directly influence the interfacial bond performance:
- Corner weld quality: Square tubes are typically fabricated by roll-forming and side welding. The weld seam is usually located at one flat face. The weld HAZ exhibits different thermal expansion characteristics from the base metal, and residual stresses from welding can create micro-gaps at the steel-concrete interface that reduce initial bond stiffness.
- Surface preparation of steel tubes: The surface roughness of the steel tube inner surface directly affects the mechanical interlock component of bond strength. Shot blasting or acid pickling to achieve a controlled surface profile is recommended to enhance bond performance.
- Concrete placement and compaction: The presence of the steel tube interior creates a confined space where concrete vibration must be carefully managed. Over-vibration can cause honeycombing near the steel surface, while under-vibration leaves voids that reduce effective bond area.
Study Insights and Reflections
The load-slip relationship obtained from push-out tests provides the fundamental input for interface modeling in finite element analysis of SSRC members. The non-linear bond-slip behavior must be captured through appropriate constitutive models, such as the Menegotto-Pinto model or the bilinear bond-slip model with elastic, plastic, and residual branches. From a quality control perspective, the interfacial bond strength is ultimately governed by the cleanliness and preparation of the steel tube inner surface, the concrete mix design including aggregate size relative to the tube dimensions, and the quality of concrete placement and curing. Engineers should recognize that the bond strength is not a fixed material property but rather a system property dependent on fabrication quality, construction practices, and environmental exposure conditions throughout the service life of the structure.