Zhuojin Pipe Fitting Co., LtdNonlinear Finite Element Analysis of Axial Compression Performance of Square Steel Tube Steel-Reinforced Recycled Concrete Composite Short Columns
Literature Overview
This paper by Ma Hui, Zou Changming, Wang Defa, Hu Guangbin, and Dong Jikun, published in the Chinese Journal of Computational Mechanics in 2018, presents a nonlinear finite element analysis of square steel tube steel-reinforced recycled concrete (RCFST) composite short columns under axial compression. The research was supported by multiple funding sources including the National Natural Science Foundation of China (51408485), the China Postdoctoral Science Foundation (2015M572584), and the Ministry of Housing and Urban-Rural Development Science and Technology Program (2015-K2-011). The study builds upon experimental research and extends the analysis through parametric investigation using Abaqus finite element software.
Research Context and Motivation
The use of recycled concrete in structural applications is driven by environmental concerns related to construction waste disposal and the depletion of natural aggregate resources. Recycled concrete, which incorporates crushed concrete waste as aggregate, typically exhibits lower strength and stiffness compared to conventional concrete but offers significant sustainability benefits. The combination of recycled concrete with steel tubes and steel sections creates a composite column that can compensate for the reduced concrete properties through enhanced confinement and steel contribution.
The square steel tube steel-reinforced configuration provides additional advantages over circular sections, including easier connection detailing with steel beams and better utilization of rectangular structural spaces. However, the square geometry introduces corner effects and potential local buckling at the corners that require careful analysis.
Methodology
Finite Element Model Development
| Component | Element Type | Material Model |
|---|---|---|
| Recycled concrete | Solid (C3D8R) | Concrete damage plasticity (CDP) |
| Square steel tube | Shell (S4R) or solid | Kinematic hardening plasticity |
| Steel section (H-beam) | Solid or shell | Kinematic hardening plasticity |
| Steel-concrete interface | Contact pair | Frictional contact |
The finite element model was developed in Abaqus and calibrated against experimental results to ensure accuracy. The concrete damage plasticity model captures the cracking and crushing behavior of recycled concrete, while the kinematic hardening model for steel accounts for the Bauschinger effect under cyclic or non-proportional loading.
Parametric Study Variables
| Parameter | Range Studied | Engineering Significance |
|---|---|---|
| Recycled aggregate replacement rate | 0% to 100% | Sustainability vs. strength trade-off |
| Square tube width-thickness ratio | Variable | Local buckling resistance |
| Steel section steel ratio | Variable | Composite action efficiency |
| Recycled concrete strength | Variable grades | Material variability |
Key Results and Technical Analysis
Failure Mode and Stress Distribution
The finite element analysis revealed that the failure mode of the composite short column is characterized by:
- Local buckling of the square steel tube walls: The flat walls of the square tube buckle inward under the confining pressure from the concrete core.
- Yielding of the steel section: The embedded steel section yields at the critical cross-section, contributing to the ultimate load capacity.
- Crushing of recycled concrete: The concrete core crushes at the column ends and mid-height, with the crushing pattern influenced by the confinement provided by the steel tube and steel section.
The stress distribution shows that the steel tube walls carry significant compressive stresses, with stress concentrations at the corners and at the interfaces with the embedded steel section. The recycled concrete experiences triaxial compression in the confined regions, which enhances its ductility compared to unconfined conditions.
Load-Strain Relationship
The load-strain curves exhibit a characteristic pattern:
- Initial elastic stage: Linear relationship between load and strain, with stiffness determined by the combined contribution of steel and concrete.
- Cracking stage: Recycled concrete begins to crack, causing a slight reduction in stiffness.
- Yielding stage: Steel components yield, and the load-strain curve becomes nonlinear.
- Post-peak stage: Progressive crushing of concrete and local buckling of steel tube lead to load degradation.
Parametric Effects
| Parameter | Effect on Axial Bearing Capacity | Trend |
|---|---|---|
| Recycled aggregate replacement rate | Decreases capacity | Higher replacement rate → lower strength |
| Square tube width-thickness ratio | Increases capacity | Thicker walls → better confinement |
| Steel section steel ratio | Increases capacity | More steel → higher contribution |
| Recycled concrete strength | Increases capacity | Higher grade → better performance |
The reduction in bearing capacity with increasing recycled aggregate replacement rate is expected, as recycled concrete has lower compressive strength and elastic modulus due to the presence of adhered mortar on recycled aggregate particles and higher porosity. However, the composite action between the steel tube, steel section, and recycled concrete provides significant confinement, which mitigates the strength reduction.
Comparison with Experimental Results
The finite element model predictions showed good agreement with experimental results, validating the model's accuracy for predicting the axial compression behavior of square steel tube steel-reinforced recycled concrete composite short columns. The model captures the overall trend of load-strain behavior, including the peak load, post-peak degradation, and failure mode.
Welding and Fabrication Considerations
The fabrication of square steel tube steel-reinforced recycled concrete composite columns involves several critical welding operations:
Welding Sequence and Procedure
- Square steel tube fabrication: The steel plate is formed into a square section with longitudinal welds. The weld process should be selected based on plate thickness:
- For plates < 10 mm: ERW or HFW welding
- For plates 10-25 mm: SAW welding with flux-cored wire
- For plates > 25 mm: SAW welding with multiple passes
- Steel section attachment: The H-beam or other steel section is welded to the interior of the square tube. This requires:
- Precise fit-up between the steel section and the tube walls
- Full-penetration welds at the flange-to-tube interfaces
- Fillet welds at the web-to-tube interfaces
- End plate welding: End plates are welded to the tube ends for load application and connection to other structural elements.
Weld Quality Requirements
| Weld Location | Weld Type | Testing Requirement | Acceptance Criteria |
|---|---|---|---|
| Tube longitudinal weld | Full penetration | RT or UT | No Class II defects |
| Steel section flange-to-tube | Full penetration groove | MT + UT | No linear indications |
| Steel section web-to-tube | Fillet weld | MT | No surface cracks |
| End plate-to-tube | Full penetration | RT | No Class II defects |
The weld metal should be selected to match or exceed the base metal properties. For recycled concrete composite columns, the concrete strength may be lower than conventional concrete, but the steel components should still meet their specified grade requirements.
Residual Stress Management
The welding of multiple steel components in close proximity creates significant residual stresses that can affect the structural performance:
- Preheating to 100-150°C for thick sections to reduce cooling rate
- Interpass temperature control to limit thermal cycling
- Post-weld stress relief at 550-650°C for critical components
- Shot peening of weld toes to introduce compressive residual stresses
Study Insights and Engineering Reflections
This paper makes a valuable contribution to the sustainable construction field by demonstrating that recycled concrete can be effectively used in composite columns with adequate structural performance. The parametric analysis provides design guidance for optimizing the balance between sustainability (high recycled aggregate replacement rate) and structural efficiency (high bearing capacity).
The finding that the square steel tube width-thickness ratio has a significant effect on bearing capacity is particularly important for design. A higher width-thickness ratio (thicker walls) provides better confinement of the recycled concrete, which compensates for the lower concrete strength. However, excessively thick walls increase material cost and may lead to local buckling at higher slenderness ratios.
The good agreement between finite element predictions and experimental results validates the use of Abaqus with appropriate material models for the analysis of composite columns. The concrete damage plasticity model is particularly well-suited for capturing the nonlinear behavior of recycled concrete, including cracking, crushing, and the influence of confinement.
From a practical standpoint, the steel section steel ratio is a key design parameter that offers flexibility in achieving target bearing capacity. Increasing the steel ratio is an effective way to compensate for the strength reduction caused by high recycled aggregate replacement rates. However, this increases the cost of the steel components and may complicate the welding and fabrication process.
The use of recycled concrete in composite columns represents a significant step toward sustainable construction. The environmental benefits include reduced construction waste, lower carbon footprint from concrete production, and conservation of natural aggregate resources. The structural performance, while slightly reduced compared to conventional concrete, remains adequate for most structural applications when properly designed with sufficient steel reinforcement.
Reference Value and Outlook
This research provides a comprehensive framework for the design of square steel tube steel-reinforced recycled concrete composite columns. The parametric analysis offers practical guidance for engineers seeking to incorporate recycled materials into structural design without compromising safety. Future research should explore the behavior of these columns under eccentric loading, cyclic loading, and fire exposure, as well as the long-term durability of recycled concrete in aggressive environments. The development of design codes specific to recycled concrete composite columns would further facilitate their widespread adoption in engineering practice.