Zhuojin Pipe Fitting Co., LtdEccentric Compression Mechanical Properties and Parametric Analysis of Square Steel Pipe Recycled Concrete Composite Special-Shaped Columns
Literature Overview and Research Motivation
This study investigates the eccentric compression behavior and performs parametric analysis of composite columns formed by square steel pipes enclosing recycled aggregate concrete (RAC) in special-shaped cross-section configurations. The integration of recycled concrete—incorporating crushed recycled concrete aggregate (RCA) as partial or full replacement of natural aggregate—into steel pipe-confined columns addresses both sustainability objectives and structural performance requirements. The special-shaped cross-section introduces asymmetric loading conditions that create complex stress distributions requiring thorough investigation.
Core Technical Content
Material Properties and Mix Design
Recycled concrete typically employs RCA replacement ratios of 30%, 50%, 70%, and 100% by volume of natural aggregate. The mechanical properties of RAC decrease progressively with increasing RCA content: compressive strength reduction of 5%–10% at 30% replacement, 15%–25% at 50%, 30%–40% at 70%, and 40%–55% at 100% replacement. The square steel pipe, typically made of Q235 or Q345 steel with wall thickness of 4–10 mm, provides confinement that partially compensates for the reduced concrete strength.
| RCA Replacement | Compressive Strength (MPa) | Elastic Modulus (GPa) | Steel Pipe Grade |
|---|---|---|---|
| 0% (control) | 30.0–40.0 | 30–35 | Q235/Q345 |
| 30% | 27.0–36.0 | 28–33 | Q235/Q345 |
| 50% | 22.5–30.0 | 24–28 | Q235/Q345 |
| 70% | 18.0–24.0 | 20–25 | Q235/Q345 |
| 100% | 13.5–18.0 | 16–22 | Q235/Q345 |
Eccentric Compression Behavior
Under eccentric loading, the column experiences combined axial compression and bending, with the neutral axis shifting toward the compression face. The eccentricity ratio (e/h, where e is eccentricity and h is the cross-sectional dimension) governs the transition from compression-dominated to bending-dominated behavior. For the square steel pipe RAC column, the eccentricity ratio ranges from 0.1h to 0.3h in typical tests. The steel pipe confinement is most effective on the compression face, where concrete is placed in triaxial compression, while the tension face relies primarily on steel pipe tensile resistance.
Parametric Analysis Results
The parametric study examines the influence of key variables on load-bearing capacity and deformation characteristics. The steel pipe confinement ratio (C = f_y·A_s/(f_c·A_c)) increases capacity by 20%–60% depending on the ratio value. The RCA replacement ratio reduces capacity proportionally, but the steel pipe confinement mitigates this reduction, with residual capacity retention of 70%–85% even at 100% RCA replacement. The eccentricity ratio reduces capacity nonlinearly, with the most significant reduction occurring beyond e/h = 0.15.
| Parameter | Range | Capacity Change | Deformation Change |
|---|---|---|---|
| Confinement ratio | 0.1–0.4 | +20% to +60% | +15% to +40% ductility |
| RCA replacement | 0%–100% | -5% to -55% | -10% to -30% ductility |
| Eccentricity ratio | 0–0.3h | -10% to -70% | +20% to +80% deflection |
| Wall thickness | 4–10 mm | +15% to +45% | +10% to +35% ductility |
| Steel grade | Q235–Q460 | +10% to +30% | +5% to +20% ductility |
Process and Standards Analysis
The design of composite columns incorporating recycled concrete references GB 51248-2017 for concrete-filled steel tube structures, supplemented by GB/T 25177 for recycled aggregate concrete requirements. The square steel pipe fabrication follows GB/T 6728 for cold-formed square hollow sections, with welding requirements per NB/T 47013. The recycled aggregate must comply with GB/T 25177-2010, with specific requirements for water absorption, specific gravity, and mechanical properties. Quality control includes ultrasonic testing of the steel pipe, compressive strength testing of RAC specimens, and interface bond testing between steel and concrete.
Failure Mode Analysis
| Failure Mode | RCA Level | Eccentricity | Description |
|---|---|---|---|
| Concrete crushing | All levels | Low (e/h < 0.1) | Brittle failure on compression face |
| Steel pipe buckling | High RCA | Medium (e/h 0.1–0.2) | Local buckling at compression corner |
| Combined failure | High RCA | High (e/h > 0.2) | Simultaneous concrete crushing and steel yielding |
| Steel pipe rupture | 100% RCA | High | Tensile rupture at tension face |
Engineering Practice Implications
The research provides quantitative guidance for designing recycled concrete-filled steel pipe columns in eccentric compression applications. The steel pipe confinement effectively compensates for the reduced strength of recycled concrete, making 100% RCA replacement feasible for non-critical structural elements. Engineers should apply a confinement enhancement factor of 1.2–1.5 to the predicted capacity of unconfined RAC when designing these composite columns. The special-shaped cross-section requires careful consideration of biaxial eccentricity effects, which may not be adequately captured by uniaxial parametric studies.
Study Insights and Reflections
The parametric analysis reveals that the composite action between steel pipe and recycled concrete provides a robust structural system capable of accommodating significant eccentric loading. The sustainability benefits of using recycled aggregate are substantial, with potential waste diversion of 300–500 kg per cubic meter of concrete placed. However, the long-term durability of recycled concrete under cyclic loading and environmental exposure remains a concern requiring further investigation through accelerated aging tests and field performance monitoring. The findings support the progressive adoption of recycled materials in structural applications where steel pipe confinement provides the necessary safety margin.