Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Steel Tube Concrete Columns with Coarse Aggregate Ultra-High Performance Concrete
Literature Overview
This paper, published in Engineering Mechanics (Volume 39, Issue 10, 2022), investigates the axial compression behaviour of circular steel tube columns filled with coarse aggregate ultra-high performance concrete (CA-UHPC). The research was conducted by Zeng Yanqin, Xu Lihua, Wu Fanghong, Yu Min, and Chi Yin from Wuhan University and Foshan University of Science and Technology, supported by the National Natural Science Foundation of China Key Project (51738011).
Core Technical Findings
Test Matrix and Parameters
The experimental programme comprised 14 short column specimens designed to investigate the effects of three key parameters:
| Parameter | Symbol | Levels Tested | Purpose |
|---|---|---|---|
| Steel fibre content | Vf | Multiple levels | Evaluate toughening effect |
| Coarse aggregate replacement ratio | R | Multiple levels | Assess aggregate contribution |
| Steel tube wall thickness | t | Multiple levels | Determine confinement effect |
The confinement ratio ξ was calculated as the ratio of the steel tube cross-sectional area to the concrete core cross-sectional area, ranging from 0.217 to 1.431 across the test series.
Failure Modes and Confinement Ratio
The study identified a clear transition in failure mode based on the confinement ratio:
| Confinement Ratio Range | Dominant Failure Mode | Description |
|---|---|---|
| 0.217 ≤ ξ < 0.883 | Shear failure | Diagonal shear cracking in concrete core, with steel tube buckling |
| 0.883 ≤ ξ ≤ 1.431 | Drumming failure | Bulging of steel tube walls with concrete core confinement |
This transition is consistent with established understanding of confined concrete behaviour, where higher confinement ratios provide greater lateral restraint to the concrete core, shifting the failure mode from brittle shear to ductile drumming.
Influence of Coarse Aggregate
A key finding of this research is that the incorporation of coarse aggregate into the UHPC mix effectively improves the load-bearing capacity of the composite column at the same steel tube thickness. This is attributed to:
- Improved concrete strength: Coarse aggregate provides a stronger interlocking skeleton within the concrete matrix.
- Reduced shrinkage: The aggregate skeleton reduces the tendency of UHPC to shrink, maintaining better bond with the steel tube.
- Enhanced toughness: The combination of coarse aggregate and steel fibre provides multi-scale reinforcement, improving post-peak ductility.
Load-Deformation and Load-Strain Behaviour
The experimental results revealed the following characteristics:
- Initial stiffness: Increases with confinement ratio and concrete strength, as expected.
- Peak load: Significantly enhanced by coarse aggregate addition compared to conventional UHPC without coarse aggregate.
- Post-peak behaviour: Maintains residual load capacity due to the combined confinement of steel tube and internal reinforcement (fibre + aggregate).
- Strain distribution: More uniform strain distribution across the cross-section at higher confinement ratios, indicating better composite action.
Proposed Design Formula
Based on the experimental results and referencing existing steel tube concrete design codes, the authors proposed a new axial compression capacity formula for CA-UHPCFST short columns. The formula accounts for:
- The enhanced concrete strength due to coarse aggregate and steel fibre
- The confinement effect of the steel tube
- The interaction between steel and concrete under compression
Engineering Practice Implications
Material Selection and Mix Design
For engineers considering CA-UHPC in steel tube concrete applications, the following guidance is relevant:
| Consideration | Recommendation |
|---|---|
| Coarse aggregate size | Optimize for compatibility with steel tube inner diameter |
| Steel fibre content | Balance between toughness and workability |
| Water-binder ratio | Maintain UHPC characteristics while accommodating coarse aggregate |
| Pumping requirements | Ensure mix flowability for on-site placement through steel tube |
Fabrication and Construction Considerations
From a steel tube fabrication perspective:
- Tube straightness: The presence of coarse aggregate increases the stiffness of the concrete core, which is beneficial for maintaining column straightness during construction. However, the increased concrete density may require additional support during placement.
- Welding of tube segments: The enhanced concrete strength means that the steel tube weld joints must be designed to match the increased load capacity. Under-matched welds would become the weak link.
- Surface treatment: The steel tube inner surface should be clean and free of mill scale to ensure proper bond with the coarse aggregate UHPC. Shot blasting or chemical cleaning is recommended.
- Concrete placement: Coarse aggregate UHPC requires careful placement to avoid voids and ensure full compaction within the steel tube. Vibration may be challenging through the tube wall, so self-compacting properties should be maintained.
Quality Control Measures
- Perform ultrasonic testing (UT) on the concrete core to detect internal voids or poor compaction.
- Conduct pull-out tests on the steel tube-concrete interface to verify bond strength.
- Measure steel tube wall thickness at multiple locations to ensure uniform confinement.
- Perform compression tests on concrete cubes cast alongside the column to verify in-situ strength.
Study Insights and Reflections
The concept of incorporating coarse aggregate into UHPC for steel tube concrete applications represents a significant advancement in material science for structural engineering. Traditional UHPC, while offering exceptional strength, has been limited by high cost and restricted use to special applications. By introducing coarse aggregate, the material system becomes more economical while maintaining or even improving structural performance.
The confinement ratio threshold of approximately 0.883 for the transition from shear to drumming failure is a valuable design parameter. Engineers can use this threshold to determine whether a given column design will exhibit ductile or brittle failure, which is critical for seismic design where ductile behaviour is preferred.
The improvement in capacity from coarse aggregate addition at constant steel tube thickness suggests that the aggregate provides internal reinforcement that supplements the external confinement of the steel tube. This dual confinement mechanism — external (steel tube) and internal (aggregate + fibre) — creates a synergistic effect that enhances both strength and ductility.
One practical concern is the increased concrete weight due to coarse aggregate, which may affect the overall structural weight and foundation design. However, the improved load capacity may partially offset this by allowing for more efficient cross-sectional design.
This research opens new possibilities for the application of ultra-high performance concrete in steel tube structures, particularly in applications where high strength, durability, and ductility are all required simultaneously.