Zhuojin Pipe Fitting Co., LtdEccentric Compression Performance of Circular Steel Tube Recycled Concrete Short Columns with Embedded CFRP Tubes
Literature Overview
This study analyzes the eccentric compression behavior of short columns constructed with circular steel tubes filled with recycled aggregate concrete (RAC), with an additional embedded carbon fiber reinforced polymer (CFRP) tube placed concentrically within the steel tube. The double-tube configuration creates a multi-layer confinement system where both the outer steel tube and the inner CFRP tube contribute to the lateral confinement of the recycled concrete core. This innovative structural system addresses two critical engineering challenges: the sustainable use of recycled aggregates in structural concrete and the enhancement of structural performance through hybrid composite confinement.
Core Technical Configuration
Structural Details and Material Properties
The column cross-section consists of three concentric layers: the outer carbon steel tube, the recycled concrete annulus, and the inner CFRP tube which may be hollow or partially filled. The recycled concrete is produced by replacing natural coarse aggregate with crushed recycled concrete aggregate at varying replacement rates.
| Parameter | Symbol | Typical Value | Notes |
|---|---|---|---|
| Steel tube grade | — | Q345 | f_y = 345 MPa |
| CFRP tube strength | f_cfrp | 1500–2500 MPa | Tensile strength |
| CFRP tube modulus | E_cfrp | 120–150 GPa | Longitudinal |
| Recycled concrete strength | f_c,r | 30–50 MPa | 20–40% lower than natural |
| Recycled aggregate replacement | R | 30–100% | By weight of coarse aggregate |
| Eccentricity ratio | e/D | 0.1–0.5 | D = outer diameter |
| D/t ratio | — | 30–60 | Outer tube |
Load-Displacement and Moment-Curvature Behavior
The eccentric compression test results reveal that the embedded CFRP tube significantly improves the post-peak behavior of the columns. While the peak load is enhanced by 15–30% compared to steel tube recycled concrete columns without CFRP, the most notable improvement is in the ductility and energy absorption capacity. The moment-curvature curves exhibit a well-defined plateau region, indicating excellent rotational capacity suitable for seismic applications.
Confinement Mechanism and Interaction Analysis
Multi-Layer Confinement Model
The double-tube system creates a hierarchical confinement mechanism. The CFRP tube provides initial high-stiffness confinement due to its high elastic modulus, while the steel tube provides high-strength confinement in the plastic range. The interaction between the two tubes is governed by the equilibrium of radial forces at each interface.
- The CFRP tube confinement pressure: f_l,CFRP = t_CFRP * f_CFRP / (D_CFRP / 2)
- The steel tube confinement pressure: f_l,steel = t_steel * f_y,steel / (D_steel / 2)
- The total effective confinement: f_l,total = f_l,CFRP + f_l,steel (with interaction factor η < 1)
Effect of Eccentricity on Confinement Distribution
Under eccentric loading, the confinement pressure is no longer uniform around the circumference. The compressed zone experiences higher lateral pressure from the concrete, intensifying the confinement effect on that side, while the tension zone may see reduced confinement or even local separation. The CFRP tube, being more rigid, distributes the uneven confinement more effectively than the steel tube alone.
| Eccentricity Ratio (e/D) | Peak Load (kN) | Ultimate Curvature (1/m) | Energy Absorption (kJ) | Failure Mode |
|---|---|---|---|---|
| 0.1 | 1250 | 0.045 | 85 | Concrete crushing |
| 0.2 | 1100 | 0.038 | 72 | Concrete crushing + steel yielding |
| 0.3 | 950 | 0.032 | 58 | Steel local buckling |
| 0.4 | 800 | 0.025 | 42 | Steel fracture |
| 0.5 | 650 | 0.018 | 30 | Steel fracture + concrete spalling |
Recycled Concrete Considerations
The use of recycled aggregate concrete introduces additional complexity due to the variable quality of recycled aggregates. Recycled aggregates typically have higher water absorption, lower density, and a weaker interfacial transition zone (ITZ) compared to natural aggregates. These factors can reduce the bond strength between the recycled concrete and the steel tube wall, potentially diminishing the confinement efficiency.
Quality Control for Recycled Concrete
- Water absorption test: Ensure recycled aggregate absorption is below 5% to avoid excessive mix water demand.
- ITZ treatment: Consider surface treatment of recycled aggregates with sodium silicate or polymeric admixtures to strengthen the ITZ.
- Mix design optimization: Adjust water-to-binder ratio and admixture dosage to compensate for the higher water demand of recycled aggregates.
- Strength verification: Conduct compressive strength tests on recycled concrete cubes before use in structural applications.
Standards and Design Implications
The design of these hybrid composite columns draws from multiple standards: GB 50017 for steel structures, GB/T 25177 for recycled concrete, and ACI 440 for CFRP-reinforced concrete. However, the interaction between these three material systems is not covered by any single standard. Engineers must exercise judgment in combining provisions from different codes.
Design Recommendations
- Use a conservative interaction factor (η = 0.7–0.85) when combining CFRP and steel confinement pressures.
- Account for the reduced bond strength of recycled concrete by applying a reduction factor of 0.85–0.90 to the confinement efficiency.
- For seismic design, verify that the column meets the required ductility ratio (μ ≥ 3.0) based on the moment-curvature analysis.
- Consider the long-term behavior of CFRP, including creep and potential degradation under UV exposure if the tube is not fully enclosed.
Study Insights and Implications
This study presents a promising approach to combining sustainability goals with enhanced structural performance. The embedded CFRP tube not only compensates for the strength reduction associated with recycled concrete but also significantly improves the ductility and seismic capacity of the column. The multi-layer confinement concept could be extended to other composite systems, such as steel tube with steel fiber reinforced recycled concrete or with geopolymer recycled concrete. For steel pipe manufacturers, this research highlights the potential market for hybrid composite columns in sustainable construction, particularly in regions with abundant recycled concrete waste and strict environmental regulations.