Zhuojin Pipe Fitting Co., LtdBond-Slip Performance of Square Steel Pipe Recycled Concrete
Literature Overview
The paper by Wang Zhenbo, Ji Jianwei, and Yan Chao, published in Concrete (2014, Issue 3, pp. 42-44), investigates the bond-slip behavior between square steel pipes and recycled concrete. Funded by the Ministry of Housing and Urban-Rural Development Science and Technology Project (2008-k1-11), this research was conducted at Nanjing Tech University. The study examined nine square steel pipe recycled concrete short columns through push-out tests, considering the effects of concrete strength grade, recycled aggregate replacement rate, and expansive agent dosage on the bond-slip performance.
Core Technical Points
Bond-slip behavior at the steel-concrete interface is fundamental to the composite action of steel pipe concrete columns. The push-out test is the standard method for characterizing this interface behavior, where a steel pipe is embedded in a concrete cylinder and axially pulled out, measuring the load required to cause slippage at the interface. The resulting load-slip curve reveals the bond strength, slip capacity, and failure mode of the interface.
Recycled concrete introduces additional complexity due to the presence of recycled aggregates (typically recycled from demolished concrete), which have higher porosity, lower density, and weaker ITZ compared to natural aggregates. The bond between the steel pipe and recycled concrete is therefore expected to be weaker than that with normal-weight concrete, which has direct implications for the design and application of recycled concrete composite columns.
Influencing Factors and Experimental Results
| Factor | Range Studied | Effect on Bond-Slip Performance |
|---|---|---|
| Concrete strength grade | LC25, LC30, LC35 | Higher strength generally improves bond strength |
| Recycled aggregate replacement rate | 0%, 30%, 50%, 70% | Higher replacement rate reduces bond strength |
| Expansive agent dosage | 0%, 2%, 4% | Moderate dosage improves bond through micro-expansion |
| Square pipe dimensions | Various | Larger cross-section provides greater bond area |
The expansive agent plays a particularly interesting role. By inducing micro-expansion in the concrete, the expansive agent creates additional compressive stress at the steel-concrete interface, which increases the frictional resistance to slippage. However, excessive dosage may lead to cracking or spalling, which would be detrimental to the bond performance. The optimal dosage represents a balance between beneficial expansion and potential cracking.
Interface Behavior Analysis
The bond-slip curve typically exhibits four stages:
- Elastic stage: Linear relationship between bond stress and slip, with slip values typically less than 0.05 mm
- Plastic stage: Non-linear increase in bond stress with increasing slip, as micro-cracks begin to form at the ITZ
- Peak bond stress: Maximum bond resistance is achieved, typically at slip values of 0.1-0.3 mm
- Post-peak stage: Bond stress decreases as slip increases, due to progressive debonding and frictional sliding
For recycled concrete, the peak bond stress is generally reduced by 10-25% compared to normal concrete of similar strength, primarily due to the weaker ITZ of recycled aggregates. The slip at peak bond stress may also be reduced, indicating lower ductility of the interface.
Engineering Practice Considerations
From a steel pipe fabrication and welding perspective, the surface condition of the steel pipe significantly affects the bond performance. Surface treatments such as:
- Roughening: Mechanical roughening or shot blasting increases mechanical interlock
- Welded ribs: Welded steel ribs on the inner surface of the pipe provide positive mechanical connection
- Surface cleanliness: Removal of mill scale, rust, and contaminants is essential for chemical adhesion
The welding of ribs or other mechanical connectors to the steel pipe introduces additional quality control requirements. Each weld must be inspected for defects, and the weld metal composition must be compatible with the base steel to avoid galvanic corrosion or hydrogen-induced cracking.
Key Questions and Reflections
The long-term durability of the bond interface in recycled concrete composite columns is a critical concern. Recycled aggregates may contain residual chlorides or sulfates from previous exposure, which could accelerate corrosion of the steel pipe. Additionally, the higher permeability of recycled concrete may allow moisture and aggressive ions to penetrate more easily, potentially degrading the bond interface over time.
Another important consideration is the effect of environmental loading (temperature cycling, freeze-thaw, chemical exposure) on the bond performance. These factors may not be fully captured in short-term push-out tests, and long-term durability testing is essential for reliable design.
Study Insights and Implications
This research provides valuable data for the design and application of recycled concrete in steel pipe composite structures. The findings support the use of moderate recycled aggregate replacement rates (up to 50%) with appropriate expansive agent dosage for acceptable bond performance. For steel pipe manufacturers, the key takeaway is that surface preparation and treatment of the pipe interior should be optimized for recycled concrete applications, potentially requiring different surface treatments than those used for normal concrete. The push-out test data can be used to calibrate interface models in finite element analysis, enabling more accurate prediction of composite column behavior.