Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Steel Fiber Recycled Concrete-Filled Steel Tube Short Columns
Literature Overview
This paper by Su Cuiping, Yang Feng, Liang Jiongfen, and Miao Yujie from East China University of Technology, published in Concrete (2021, Issue 8, pp. 96-98), investigates the axial compression behavior of short columns using steel fiber reinforced recycled concrete (SFRC) filled in steel tubes. The study was supported by multiple funding sources including the National Natural Science Foundation (51868001), Jiangxi Provincial Science Foundation, and the Ministry of Education Engineering Research Center.
Experimental Program
Nine steel tube steel fiber recycled concrete short columns were tested under axial compression. Two primary variables were investigated: recycled concrete replacement rate and steel fiber dosage.
| Parameter | Levels Tested | Purpose |
|---|---|---|
| Recycled concrete replacement rate | Multiple levels (up to 100%) | Evaluate recycled aggregate effect |
| Steel fiber dosage | Multiple levels (up to 0.6%) | Assess fiber reinforcement effect |
| Steel tube parameters | Standardized | Control variable |
| Test type | Axial compression | Determine capacity and ductility |
Specimen Configuration
The test matrix was designed to isolate the effects of each variable while maintaining practical relevance. The steel tube dimensions and concrete mix proportions were selected to represent typical structural applications, ensuring that test results could be directly applied to design calculations.
Core Technical Findings
Steel Fiber Effects
The steel fiber addition demonstrated consistent positive effects on both ductility and load-bearing capacity across all replacement rates tested. The fibers bridge micro-cracks that develop in the concrete matrix, preventing crack propagation and maintaining load-carrying capacity beyond the initial concrete crushing.
| Steel Fiber Dosage | Effect on Capacity | Effect on Ductility | Mechanism |
|---|---|---|---|
| 0% (baseline) | Baseline | Baseline | No crack bridging |
| 0.2-0.4% | Moderate improvement | Noticeable improvement | Crack bridging initiated |
| 0.6% | Maximum improvement | Best ductility | Optimal crack bridging network |
Optimal Combination
The most significant finding is that the combination of 100% recycled concrete replacement rate with 0.6% steel fiber dosage achieved the maximum structural capacity and best ductility performance. This result is particularly important because it demonstrates that recycled concrete, when properly reinforced with steel fibers, can fully replace natural aggregate without performance penalty, and in some aspects even outperform natural aggregate concrete.
Recycled Concrete Behavior
Recycled concrete typically exhibits lower strength and higher porosity compared to natural aggregate concrete due to the residual mortar on recycled aggregate particles creating a weaker interfacial transition zone. The steel fibers compensate for this weakness by:
- Bridging the weaker interfacial transition zones between recycled aggregate and mortar.
- Providing post-cracking tensile resistance that maintains composite action.
- Improving energy absorption capacity during the post-peak softening phase.
- Reducing the risk of catastrophic brittle failure characteristic of recycled concrete.
Finite Element Model Verification
Although the paper primarily presents experimental results, the findings have important implications for numerical modeling:
| Modeling Aspect | Recommendation | Basis |
|---|---|---|
| Concrete constitutive model | Modified with fiber contribution | Fiber bridge effect |
| Steel tube model | Elastic-plastic with hardening | Confined behavior |
| Interface model | Bond-slip with fiber enhancement | Recycled aggregate interface |
| Failure criterion | Modified for fiber-reinforced concrete | Post-peak behavior |
Engineering Practice Implications
Sustainability Benefits
The use of 100% recycled concrete with steel fiber reinforcement in CFST columns offers substantial environmental benefits:
- Diverts construction and demolition waste from landfills.
- Reduces demand for natural aggregate extraction.
- Steel fibers can be manufactured from recycled steel scrap.
- The composite action of steel tube confinement and fiber reinforcement ensures structural reliability despite recycled material variability.
Quality Control Considerations
From a quality control perspective, several additional measures are required when using recycled concrete:
- Recycled aggregate cleanliness must be verified through chemical analysis to ensure absence of harmful substances (sulfates, chlorides, organic matter).
- Steel fiber dispersion in the concrete mix must be verified through test slab examination to ensure uniform distribution.
- Concrete filling procedures must ensure complete filling of the steel tube without voids, requiring proper vibration or pumping procedures.
- Welding of steel tube components must maintain structural integrity, with weld quality directly affecting confinement effectiveness.
Welding and Fabrication
The steel tube fabrication for these columns requires attention to:
- Weld seams that provide full confinement pressure to the concrete core.
- End plates welded to tube ends for load application and boundary condition control.
- Potential longitudinal welds in welded tubes that may affect confinement uniformity.
- Surface preparation and coating compatibility between steel tube and concrete.
Key Questions and Reflections
The paper's conclusion that 100% recycled concrete with 0.6% steel fiber achieves optimal performance raises important questions about long-term durability. Recycled concrete is more susceptible to moisture penetration and carbonation due to higher porosity. The steel fiber addition may partially mitigate this by reducing crack widths, but long-term exposure testing is essential before widespread adoption in aggressive environments.
Additionally, the workability of recycled concrete with 0.6% steel fiber addition may present challenges for filling into steel tubes, particularly for longer columns. The pumping and vibration requirements must be carefully specified to ensure complete filling without fiber segregation or balling.
Study Insights and Conclusions
This research demonstrates that recycled concrete-filled steel tube columns with 0.6% steel fiber reinforcement achieve structural performance equivalent to or exceeding natural aggregate concrete-filled steel tube columns, providing a viable pathway for sustainable construction using recycled materials. The key mechanism is the synergistic interaction between steel fiber crack-bridging and steel tube confinement, which together overcome the inherent weaknesses of recycled concrete. For engineering practice, the optimal combination of 100% recycled replacement with 0.6% steel fiber should be adopted as a standard specification, with appropriate quality control measures for aggregate cleanliness, fiber dispersion, and concrete filling procedures to ensure reliable structural performance in service.