Zhuojin Pipe Fitting Co., LtdBending Performance of Recycled Concrete Filled Rectangular Steel Tubes
Literature Overview
This research investigates the flexural behavior of rectangular steel tubes filled with recycled concrete, addressing both the structural performance and the sustainability implications of using recycled aggregate in composite structural members. Recycled concrete, produced by incorporating crushed waste concrete as partial or full replacement of natural aggregate, is gaining attention as a sustainable construction material. However, its use in structural applications requires rigorous performance evaluation, particularly under bending loads where the composite action between steel tube and concrete core is most critical.
Flexural Behavior and Failure Mechanisms
The study employs four-point and three-point bending tests on rectangular CFST specimens with recycled aggregate replacement ratios of 0%, 30%, 50%, 70%, and 100%. Key findings include:
- The ultimate bending moment capacity decreases by approximately 5-12% for 30% recycled aggregate replacement and 15-25% for 100% replacement, compared to natural aggregate CFST specimens.
- The deflection at peak load increases with recycled content, indicating reduced stiffness but potentially improved energy absorption capacity.
- Failure initiates at the compression zone of the concrete core, where microcracks propagate from the recycled aggregate interfacial transition zone (ITZ) and extend toward the neutral axis.
- The steel tube continues to provide lateral confinement even after concrete crushing, with the tube walls yielding in tension at the compression face and in compression at the tension face.
| Recycled Content (%) | Ultimate Moment (kN·m) | Peak Deflection (mm) | Flexural Rigidity (EI) | Energy Absorption (kJ) |
|---|---|---|---|---|
| 0 (control) | 100 (reference) | 100 (reference) | 100 (reference) | 100 (reference) |
| 30 | 90-95 | 105-115 | 92-97 | 108-118 |
| 50 | 82-88 | 115-125 | 85-92 | 115-125 |
| 70 | 75-82 | 125-135 | 78-85 | 120-130 |
| 100 | 75-85 | 130-145 | 72-82 | 125-140 |
Steel Tube Material and Welding Requirements
For rectangular CFST members, the steel tubes are typically fabricated by roll-forming with longitudinal and transverse welds. The bending performance is sensitive to weld quality because:
- Longitudinal welds along the compression face must resist the combined effects of compressive stress and hoop tension from concrete confinement.
- Transverse welds (if present) create stress concentration points that can initiate cracking under cyclic or impact loading.
- The rectangular cross-section geometry creates non-uniform stress distribution, with corner regions experiencing higher stress concentrations than flat regions.
Fabrication standards such as GB 51246 or EN 1993-1-1 provide guidance on acceptable weld imperfections, but the specific application of recycled concrete-filled tubes may require more stringent criteria due to the higher interfacial stresses at the recycled aggregate ITZ.
Sustainability and Engineering Trade-offs
The study highlights that up to 50% recycled aggregate replacement offers an acceptable compromise between structural performance and environmental benefit. Beyond this threshold, the increased variability in recycled aggregate properties leads to greater scatter in structural response, complicating design and quality assurance. The coefficient of variation in ultimate moment increases from approximately 3-5% for natural aggregate specimens to 8-12% for 100% recycled specimens, necessitating higher safety factors or more rigorous quality control.
From a welding perspective, the use of recycled concrete does not directly affect welding procedures, but the higher permeability and lower durability of recycled concrete may accelerate corrosion of the steel tube interior over time. This suggests that internal corrosion protection coatings or increased wall thickness may be warranted for long-term service in aggressive environments.
Study Insights
This research contributes valuable data for the progressive implementation of recycled materials in structural steel-concrete composite systems. The engineering takeaway is clear: recycled concrete-filled steel tubes can achieve acceptable structural performance with proper design considerations, but the increased material variability demands enhanced quality control at both the material and fabrication levels. The bending performance data provides essential input for design codes that are increasingly incorporating sustainability requirements into structural specifications.