Zhuojin Pipe Fitting Co., LtdAxial Compression Behavior of Steel Tube-Recycled Brick Aggregate Concrete Columns
Literature Overview
The experimental study by Huang Liang and colleagues, published in the Journal of Railways and Transportation Science and Engineering (2020, Vol. 17, No. 3), investigates the axial compression performance of circular steel tube columns filled with recycled brick aggregate concrete (RBAC). This research, funded by the National Key R&D Program of China (2017YFC0703305), addresses the growing need for sustainable construction materials by evaluating the structural performance of recycled concrete within steel tube confinement. Twenty circular column specimens were tested, with systematic variation of recycled coarse aggregate replacement rate and confinement ratio. The work was conducted at Hunan University's School of Civil Engineering and the Ministry of Education Key Laboratory of Building Safety and Energy Efficiency.
Core Technical Content
Test Matrix and Specimen Configuration
| Parameter | Variation Range | Number of Specimens |
|---|---|---|
| Recycled aggregate replacement rate | Multiple levels | Systematically varied |
| Confinement ratio (hoop coefficient) | Multiple levels | Systematically varied |
| Total specimens | - | 20 |
| Geometry | Circular steel tube columns | Uniform |
Failure Mode and Stress-Strain Behavior
The failure process of steel tube-recycled brick aggregate concrete (ST-RBAC) columns closely resembles that of conventional CFST columns, indicating that the recycled aggregate does not fundamentally alter the structural behavior mechanism. The stress-strain curves exhibit the characteristic three-stage behavior: elastic stage, plastic hardening stage, and descending stage. Both the recycled aggregate replacement rate and the confinement ratio influence the ultimate load capacity and deformation characteristics.
Load-Bearing Capacity Calculation Comparison
The study evaluates twelve calculation formulas from three theoretical frameworks against experimental results:
| Theoretical Framework | Accuracy | Safety Margin |
|---|---|---|
| Confinement concrete theory | Closest to experimental values | Moderate |
| Unified strength theory | Conservative | Moderate |
| Superposition calculation theory | Most conservative | Largest safety margin |
The confinement concrete theory provides the most accurate predictions, while the superposition calculation theory offers the greatest safety margin. The Chinese code GB 50936-2014 formula, which combines reasonable accuracy with adequate safety, was selected for modification.
Modified Calculation Formula
A modification coefficient θ was proposed to account for the influence of recycled coarse aggregate replacement rate on the load-bearing capacity calculation. The modified formula was validated against experimental data, demonstrating improved prediction accuracy for ST-RBAC columns.
Technical Analysis and Quality Considerations
Recycled Aggregate Effects on Concrete Properties
The incorporation of recycled brick aggregate introduces several material property changes that must be considered in structural design:
- Reduced compressive strength: Recycled aggregates typically exhibit lower strength than natural aggregates due to residual mortar and microcracking.
- Increased water absorption: The porous nature of recycled brick aggregate increases concrete permeability and affects workability.
- Modified interface zone: The bond between recycled aggregate and cement paste may be weaker than with natural aggregate, affecting concrete durability.
- Variable quality: Recycled aggregate properties may vary between batches, requiring quality control measures.
Confinement Effect Enhancement
The steel tube confinement effectively mitigates the adverse effects of recycled aggregate on concrete performance. The lateral confining pressure provided by the steel tube restrains the lateral expansion of concrete under axial compression, creating a triaxial stress state that enhances both strength and ductility. The confinement ratio (ratio of steel tube area to concrete area) is a critical design parameter that determines the degree of confinement effect.
Engineering Practice Integration
The use of recycled brick aggregate in CFST columns represents a significant step toward sustainable construction. Construction waste, particularly brick debris from demolition activities, can be effectively utilized as concrete aggregate, reducing landfill burden and natural resource consumption. The experimental results demonstrate that ST-RBAC columns can achieve structural performance comparable to conventional CFST columns when properly designed with appropriate confinement ratios.
For practical implementation, several considerations must be addressed:
- Aggregate quality control: Recycled brick aggregate must be properly cleaned, sorted, and graded to ensure consistent concrete properties.
- Mix design optimization: The water-cement ratio and admixture dosage may require adjustment to accommodate the higher water absorption of recycled aggregate.
- Durability assessment: Long-term durability testing, including carbonation resistance, chloride penetration, and freeze-thaw resistance, should be conducted for service-life assessment.
- Code compliance: The proposed modification coefficient θ provides a basis for incorporating recycled aggregate effects into existing design codes.
Study Insights and Implications
This research contributes meaningfully to the advancement of sustainable structural engineering by demonstrating the viability of recycled brick aggregate in steel tube-concrete composite columns. The finding that the failure behavior of ST-RBAC columns closely mirrors conventional CFST columns is particularly encouraging, as it suggests that existing design methodologies can be adapted with relatively minor modifications. The proposed modification coefficient θ offers a practical tool for engineers to account for recycled aggregate effects in load-bearing capacity calculations. Future research should extend to larger-scale specimens, long-term durability studies, and seismic performance evaluation to provide comprehensive design guidance for the widespread adoption of recycled aggregate in CFST structural systems.