Zhuojin Pipe Fitting Co., LtdEccentric Compression Performance of Square Steel Tube Recycled Concrete Long Columns
Literature Overview
The paper by Wang Chenggang, Yuan Quan, and Zhang Zeyang, published in the Journal of Hefei University of Technology (Natural Science Edition) (2022, Vol. 45, No. 2, pp. 231–237), presents a comprehensive experimental study on the eccentric compression behavior of square steel tube recycled aggregate concrete (RAC) long columns. Supported by the Anhui Provincial Natural Science Foundation (JZ2019AKZR0220) and the Anhui Provincial Housing and Urban-Rural Construction Science and Technology Plan (2016YF-12), this research extends the understanding of composite column behavior from circular to square cross-sections and incorporates supplementary materials such as fly ash and expansive agents.
Experimental Program and Key Parameters
Test Matrix
The study involved 15 square steel tube RAC long columns with four varying parameters:
| Parameter | Levels Tested | Purpose |
|---|---|---|
| Recycled coarse aggregate replacement rate | 0%, 30%, 50%, 70% | Assess material substitution effect |
| Steel tube wall thickness | 3 mm, 5 mm, 7 mm | Evaluate confinement effectiveness |
| Slenderness ratio (L/D) | 8, 12, 16 | Study geometric stability |
| Eccentricity ratio (e/D) | 0.1, 0.2, 0.3 | Characterize bending-axial interaction |
Supplementary Material Effects
| Additive | Effect on Load Capacity | Mechanism |
|---|---|---|
| Fly ash | Overall increase | Pozzolanic reaction improves ITZ quality; reduces permeability |
| Expansive agent | Moderate increase | Compensates for shrinkage; enhances bond between steel and concrete |
| Recycled aggregate | Decrease | Higher porosity and weaker ITZ reduce concrete strength |
Core Technical Findings
Failure Modes
Two major failure categories were identified:
- End compression buckling failure: The steel tube wall buckles locally near the loaded end under combined axial and bending stresses. This mode is more common in columns with lower slenderness ratios and higher eccentricity ratios.
- Overall flexural instability failure: The entire column undergoes global bending instability, typically observed in slender columns with high slenderness ratios.
Load Capacity and Stiffness Relationships
| Parameter | Effect on Load Capacity | Effect on Initial Axial Stiffness |
|---|---|---|
| Increasing slenderness ratio | Decreases | Decreases |
| Increasing eccentricity | Decreases | Decreases |
| Increasing wall thickness | Increases | Increases |
| Increasing recycled aggregate rate | Decreases | Decreases |
| Adding fly ash | Increases (overall) | Moderate increase |
| Adding expansive agent | Moderate increase | Slight increase |
Confinement Effectiveness
A notable finding is that square steel tubes provide effective confinement to recycled concrete under compression. The corner regions of square tubes inherently provide enhanced confinement compared to circular tubes at equivalent wall thickness, due to the reduced curvature radius at the corners. This geometric advantage is particularly beneficial for recycled concrete, which typically has lower tensile strength and higher cracking tendency.
Technical Interpretation
Stress Distribution in Square Cross-Sections
The square cross-section introduces non-uniform stress distribution compared to circular sections. The corners experience higher confinement pressure due to the geometry, while the mid-span of each face experiences lower confinement. This creates a non-uniform triaxial stress state in the concrete core. From a design perspective, this means:
- The effective confinement pressure should be calculated using the corner-to-face ratio rather than a uniform assumption.
- The concrete strength enhancement factor should account for the non-uniform confinement distribution.
- Local buckling of the steel tube faces between corners may occur before corner buckling, requiring appropriate stiffener design.
Recycled Aggregate and ITZ Quality
The interfacial transition zone (ITZ) between recycled aggregate and cement paste is the critical weak link in recycled concrete. Recycled aggregate retains adhered old mortar, which creates a porous and weak ITZ. The addition of fly ash addresses this issue through:
- Filling of pores in the adhered mortar layer through pozzolanic reaction products.
- Refinement of the ITZ microstructure, improving bond strength.
- Reduction of permeability, enhancing long-term durability.
This explains why fly ash addition partially or fully compensates for the strength loss caused by recycled aggregate substitution.
Engineering Practice Integration
Design Considerations for Square Steel Tube RAC Columns
For practical engineering application, the following design guidelines can be derived:
- Replacement rate limit: For critical structural members, recycled aggregate replacement rates should not exceed 50% without supplementary measures such as fly ash addition.
- Wall thickness optimization: The wall thickness should be selected to provide sufficient confinement while avoiding excessive material usage. A thickness-to-width ratio of 1:50 to 1:80 is recommended for general applications.
- Slenderness control: Slenderness ratios exceeding 16 should be avoided for recycled concrete columns, as the combined effect of reduced material stiffness and geometric instability leads to disproportionate capacity reduction.
- Fly ash supplementation: When using recycled aggregate at rates above 30%, fly ash addition of 15–25% by cement weight is recommended to maintain structural performance.
Quality Control Considerations
| QC Parameter | Acceptance Criteria | Testing Method |
|---|---|---|
| Recycled aggregate absorption rate | ≤ 3% | Standard water absorption test |
| Concrete compressive strength | Meets design grade | Cube/cylinder compression test |
| Steel tube dimensional tolerance | ±0.5 mm | Caliper/measurement |
| Bond strength (steel-concrete) | ≥ 2.0 MPa | Pull-out test |
| ITZ microstructure | No visible microcracks | SEM analysis |
Key Reflections
The extension of recycled aggregate concrete research from circular to square steel tube columns is a significant contribution, as square and rectangular cross-sections are more common in building construction due to their efficient use of space. The finding that square tubes provide effective confinement to recycled concrete is encouraging for sustainable construction practices.
One area that requires further investigation is the long-term behavior of these columns under sustained loading. Recycled concrete is known to exhibit higher long-term creep and shrinkage compared to virgin aggregate concrete. The interaction between sustained loads, creep, and the non-uniform confinement in square sections could lead to progressive stress redistribution that may not be captured by short-term tests.
Additionally, the fire resistance of square steel tube RAC columns under eccentric loading deserves attention. Recycled concrete has lower thermal conductivity and higher thermal expansion coefficient compared to virgin concrete, which could affect the thermal stress distribution during fire exposure.
Summary
This study provides comprehensive experimental data on the eccentric compression behavior of square steel tube recycled aggregate concrete long columns, demonstrating that square steel tubes effectively confine recycled concrete and that supplementary materials such as fly ash and expansive agents can significantly mitigate the adverse effects of recycled aggregate substitution. The findings support the practical application of recycled aggregate concrete in square steel tube composite columns for building construction, with appropriate design adjustments for slenderness ratio, wall thickness, and material composition. The research contributes meaningfully to the advancement of sustainable structural engineering practices.