Zhuojin Pipe Fitting Co., LtdMechanical Behavior and Strength Calculation of Steel Tube Confined Recycled Concrete
Literature Overview
This paper, published in the China Civil Engineering Journal in 2013, presents a systematic experimental and theoretical investigation into the behavior of recycled concrete confined by steel tubes. The authors — Chen Zongping, Ke Xiaojun, Xue Jianyang, and Su Yisheng from Guangxi University and Xi'an University of Architecture and Technology — designed 33 specimens (22 circular steel tube and 11 square steel tube) subjected to axial compression. The study was supported by multiple national and provincial research grants, reflecting the significance of recycled materials in sustainable construction.
Core Experimental Design and Parameters
The experimental matrix varied three key parameters: recycled coarse aggregate replacement rate, cross-sectional shape (circular vs. square), and the confinement index (the ratio of steel tube cross-sectional area to concrete cross-sectional area). Each specimen was loaded under axial compression until failure, with load-displacement curves, peak stress, and peak strain recorded throughout the process.
| Parameter | Variation Range | Purpose |
|---|---|---|
| Recycled coarse aggregate replacement rate | Multiple levels | Assess impact on mechanical response |
| Cross-sectional shape | Circular, Square | Compare confinement effectiveness |
| Confinement index | Multiple levels | Evaluate steel tube contribution |
| Total specimens | 33 (22 circular, 11 square) | Comprehensive coverage |
Key Technical Findings
Failure Modes
The failure morphology differed distinctly between circular and square specimens. Circular steel tube recycled concrete specimens exhibited a barrel-shaped diagonal shear-compressive failure pattern, characteristic of effective lateral confinement. Square steel tube specimens, by contrast, showed a diagonal compressive failure mode, indicating less uniform confinement at the corners and edges. This difference is fundamentally tied to the geometry-dependent stress distribution within the concrete core.
Load-Displacement Response
All specimens followed a similar three-stage load-displacement curve: an initial ascending branch to the peak, a descending branch after peak load, and a secondary ascending branch. This re-hardening behavior is a hathe writing systemark of effective steel tube confinement, where the steel tube continues to carry increasing load after concrete crushing through hoop tension. The peak stress and peak strain of both circular and square specimens were significantly higher than those of plain recycled concrete, with circular tubes demonstrating notably greater improvement.
Effect of Recycled Aggregate Replacement Rate
The replacement rate influenced the failure mechanism but did not dramatically alter the overall confinement behavior. This is an important practical finding — it suggests that recycled concrete can be used in steel tube confined columns without fundamentally changing the design approach, provided the confinement index is properly selected.
Theoretical Analysis
The authors employed two analytical approaches:
- Full-process analysis method: A mathematical expression for the complete stress-strain curve was derived, capturing the ascending, descending, and re-ascending branches. This approach is valuable for nonlinear finite element modeling of steel tube confined recycled concrete members.
- Ultimate strength analysis method: An ultimate load formula was developed based on equilibrium and compatibility conditions. The theoretical predictions showed good agreement with experimental results.
Engineering Practice Implications
From a steel pipe manufacturing and welding perspective, the study reinforces several practical considerations:
- Steel tube geometry matters: Circular cross-sections provide more uniform confinement than square sections. In fabrication, the welding quality of longitudinal seams in circular tubes is critical, as localized weakness at welds can initiate premature buckling.
- Confinement index selection: Engineers should target a confinement index that ensures the steel tube yields before concrete crushes completely, maximizing ductility. Typical design ranges for the confinement index in steel tube concrete columns are 0.03 to 0.08 for structural applications.
- Weld quality control: For fabricated square steel tube columns, corner welds are stress concentration points. Full-penetration welds with appropriate heat input control are essential to prevent defects that could compromise confinement effectiveness.
Study Insights and Reflections
This paper is notable for bridging sustainable materials (recycled concrete) with structural engineering (steel tube confinement). The finding that replacement rate has limited effect on failure mechanism is encouraging for industry adoption. However, I would note that the study focuses on short-column axial compression; the behavior of slender members with bending moments and shear forces would require additional investigation. The theoretical models developed here provide a foundation for code provisions, but engineers should remain cautious when extrapolating beyond the tested parameter ranges. The study exemplifies how fundamental research on confined concrete behavior can directly inform practical design of steel tube concrete structures using recycled materials.