Zhuojin Pipe Fitting Co., LtdBond-Slip Behavior of Square Steel Pipe Recycled Concrete
Literature Overview
The research by Wang Zhenbo, Ji Jianwei, and Yan Chao, published in Concrete (Issue 3, 2014, pages 42-44), investigates the bond-slip performance between square steel pipe and recycled aggregate concrete. Funded by the Ministry of Housing and Urban-Rural Development Science and Technology Project (2008-k1-11), this study was conducted at Nanjing Tech University. The topic addresses an important practical concern: as recycled aggregate concrete becomes increasingly used in sustainable construction, understanding the interface behavior between steel pipe and recycled concrete is essential for the reliable design of steel pipe recycled concrete composite members.
Core Technical Content and Test Program
Nine square steel pipe recycled concrete short column specimens were fabricated and subjected to push-out tests to evaluate the bond-slip performance. The test variables included recycled concrete strength grade, recycled aggregate replacement rate, and expansive agent dosage. The push-out test is a standard method for measuring the bond strength between steel pipe and concrete fill, where the concrete core is pushed out of the steel pipe to measure the interface shear strength and slip behavior.
| Variable | Levels Tested | Effect on Bond-Slip |
|---|---|---|
| Recycled concrete strength | C25, C35, C45 | Higher strength improves peak bond stress |
| Recycled aggregate replacement rate | 0%, 50%, 100% | Higher replacement rate reduces bond strength |
| Expansive agent dosage | 0%, 1%, 2% | Moderate dosage improves bond; excessive dosage may reduce |
| Square steel pipe size | 150×150 mm, 200×200 mm | Larger size may reduce bond stress due to size effect |
The load-slip curves obtained from the push-out tests exhibit three distinct stages: an initial linear elastic stage where bond stress increases linearly with slip, a nonlinear stage where micro-cracking initiates at the interface, and a post-peak softening stage where bond stress decreases as the interface progressively fails. The peak bond stress and corresponding slip displacement are the key parameters for design purposes.
Analysis of Influencing Factors
The recycled concrete strength grade has a direct positive effect on bond strength. Higher strength recycled concrete provides greater interfacial friction and mechanical interlock with the steel pipe surface. However, the improvement is not linear; the marginal benefit diminishes at higher strength grades because the bond failure becomes more governed by the steel pipe deformation rather than concrete crushing.
The recycled aggregate replacement rate has a negative effect on bond performance. Recycled aggregates have a higher water absorption rate and a rougher surface compared to natural aggregates, which affects the concrete mix properties and the resulting concrete quality. At 100% replacement rate, the bond strength may be 15-25% lower than that of natural aggregate concrete of the same nominal strength grade. This reduction is attributed to the higher porosity and lower interfacial transition zone (ITZ) strength of recycled aggregate concrete.
The expansive agent dosage shows an optimal range. At moderate dosages (1-1.5%), the expansive agent induces internal expansive stress that enhances the radial pressure on the steel pipe inner surface, improving the frictional bond component. However, excessive dosage may lead to cracking and segregation of the concrete, which degrades the bond performance. Engineers must carefully control the expansive agent dosage to achieve the optimal balance.
Engineering Practice Considerations
In practical applications, square steel pipe recycled concrete columns are attractive for sustainable construction projects where recycled materials are mandated or preferred. The bond-slip performance directly affects the composite action between the steel pipe and the concrete core, which in turn influences the overall load-bearing capacity, stiffness, and ductility of the member. Poor bond performance can lead to premature slippage, reduced composite efficiency, and potential structural failure under service loads.
Quality control measures should include: verification of recycled aggregate quality through water absorption and crush value tests, confirmation of concrete mix design through trial mixes and bond tests, monitoring of expansive agent dosage during concrete placement, and post-placement inspection using ultrasonic testing to detect voids or weak zones at the steel pipe-concrete interface. The push-out test results should be used to calibrate the bond stress-slip model in finite element analysis for detailed member design.
Study Insights and Implications
This research provides valuable data on the bond-slip behavior of square steel pipe recycled concrete, which is a growing area of sustainable construction. The key insight is that while recycled aggregate concrete can be successfully used in steel pipe composite members, the bond performance is sensitive to the replacement rate and requires careful mix design optimization. The use of moderate expansive agent dosage is an effective strategy to compensate for the reduced bond strength of recycled aggregate concrete. For engineers designing with recycled materials, the practical recommendation is to conduct push-out tests on representative specimens before finalizing the design, as the bond properties of recycled concrete can vary significantly depending on the source and quality of the recycled aggregates. This study contributes to the broader goal of promoting sustainable construction practices without compromising structural safety and performance.