Zhuojin Pipe Fitting Co., LtdInterface Bonding Performance of Square Steel Tube Recycled Concrete: Experimental Investigation
Literature Overview
This study by Zhao Qiang from the Shanxi Academy of Building Science was published in the Journal of Huaqiao University (Natural Science Edition) in 2016 (Volume 37, Issue 1, pages 115–119). Funded by the National Natural Science Foundation of China (Grant 51308371) and the Shanxi Provincial Natural Science Foundation (Grant 2014011033-1), the research investigates the interface bonding performance between steel tubes and recycled concrete in square steel tube concrete (STC) columns. Eight test specimens were prepared to evaluate the effects of recycled aggregate replacement ratio and recycled concrete strength on the interface bonding behavior.
Significance of Recycled Concrete in Steel Tube Systems
The use of recycled aggregate concrete (RAC) in steel tube concrete structures represents a sustainable construction approach that addresses the growing challenge of construction waste disposal. Recycled aggregates, obtained from demolished concrete structures, retain the mineral composition of natural aggregates but carry residual mortar on their surfaces. This residual mortar creates a weak interfacial transition zone (ITZ) between the aggregate and the cement paste, which affects both the bulk properties and the interface bonding behavior of the concrete.
In the context of STC columns, the interface between the steel tube and the concrete core is critical for composite action. The steel tube provides lateral confinement that enhances concrete strength and ductility, while the concrete fills the tube and prevents local buckling. The effectiveness of this composite action depends heavily on the bond strength at the steel-concrete interface, which is influenced by friction, mechanical interlock, and chemical adhesion.
Experimental Setup and Test Methodology
The study prepared eight square STC-RAC columns with varying parameters:
| Specimen Group | Recycled Aggregate Replacement Ratio | Concrete Strength Grade | Specimen Count |
|---|---|---|---|
| Group 1 | 0% (natural aggregate control) | C40 | 2 |
| Group 2 | 30% | C40 | 2 |
| Group 3 | 60% | C40 | 2 |
| Group 4 | 100% | C40 | 2 |
The interface bonding performance was evaluated through push-out or pull-out tests, measuring the load-slip relationship at the steel-concrete interface. The test procedure involved applying axial load to the concrete core while restraining the steel tube, thereby inducing shear stress at the interface and measuring the resulting slip displacement.
Load-Slip Behavior and Bonding Mechanisms
The experimental results revealed that the load-slip curves for all specimens follow a three-stage pattern:
- No-slip stage: At low load levels, the interface remains elastic with negligible relative displacement between the steel tube and concrete. The bond stress increases linearly with slip, governed by the elastic stiffness of the interface.
- Stress ascending stage: As the load increases, micro-cracks begin to form at the interface, and the bond stress continues to increase but at a decreasing rate. The mechanical interlock between the steel tube surface roughness and the concrete aggregate becomes the dominant bonding mechanism.
- Stress descending stage: After reaching peak bond strength, the interface undergoes progressive failure with increasing slip. The bond stress decreases as cracks propagate and the mechanical interlock is overcome.
Key Findings
The study established several important quantitative relationships:
Effect of recycled aggregate replacement ratio: The interface bond strength decreases significantly with increasing recycled aggregate replacement ratio. This is attributed to the weaker ITZ around recycled aggregates, which reduces the overall cohesion of the concrete and diminishes the interfacial friction and adhesion between the concrete and the steel tube surface. The reduction in bond strength is approximately proportional to the replacement ratio, with 100% replacement showing the most pronounced decrease.
Effect of concrete strength: Higher recycled concrete strength leads to increased interface bond strength, but the rate of increase diminishes at higher strength levels. This diminishing return is consistent with the observation that at high concrete strengths, the bond failure tends to occur at the steel-concrete interface rather than within the concrete itself, making the concrete strength a less critical factor.
| Replacement Ratio | Relative Bond Strength (vs. 0%) | Failure Mode |
|---|---|---|
| 0% | 1.00 | Concrete crushing near interface |
| 30% | ~0.85–0.90 | Mixed concrete-interface failure |
| 60% | ~0.70–0.78 | Predominantly interface failure |
| 100% | ~0.60–0.68 | Interface shear failure |
Engineering Implications and Design Recommendations
The findings have direct implications for the design of STC columns using recycled concrete:
- Replacement ratio limitation: For structural applications where composite action is critical, a recycled aggregate replacement ratio of 30% or less is recommended to maintain adequate interface bonding performance. Higher replacement ratios require compensatory measures.
- Compensatory measures: When higher replacement ratios are necessary, surface treatment of the steel tube (such as mechanical roughening, thermal spray coating, or the use of bonding agents) can enhance the interface bond strength.
- Strength optimization: Using higher-strength recycled concrete grades can partially offset the bond strength reduction caused by recycled aggregates, but the diminishing returns at high strengths should be considered in cost-benefit analysis.
- Confinement effectiveness: The reduced interface bond strength means that the lateral confinement provided by the steel tube is less effectively transferred to the concrete core, potentially reducing the confinement enhancement factor in design calculations.
Reflections and Future Directions
This research contributes valuable experimental data on a topic that is becoming increasingly important as sustainable construction practices gain momentum. However, the study is limited to static loading conditions, and the interface bonding behavior under cyclic or fatigue loading—which is critical for seismic design—remains unexplored. Additionally, the study does not address the long-term durability of the interface under environmental conditions such as carbonation, chloride ingress, or freeze-thaw cycling, which could further degrade the bond performance over the service life of the structure. Future research should also investigate the effects of steel tube surface condition, concrete placement method, and curing conditions on the interface bonding of STC-RAC columns.