Zhuojin Pipe Fitting Co., LtdAxial Compression Characteristics of Steel Tube Ultra-High Performance Concrete (RPC) Short Columns
Literature Overview
This paper by Tian Zhimin, Zhang Xiangbai, Feng Jianwen, and Yan Peiyu, published in Journal of Earthquake Engineering and Engineering Vibration in 2008 (Vol. 28, No. 1, pp. 99-107), presents experimental research on the axial compression performance of steel tube ultra-high performance concrete (RPC) short columns. The research was supported by the National Natural Science Foundation of China (Grant No. 50778174) and the National Science and Technology Support Program (Project No. 2006BAJ13B02-03). The authors are affiliated with the Fourth Research and Design Institute of the General Staff Engineering Corps and Tsinghua University. This work addresses an important gap in the design methodology for steel tube concrete columns filled with ultra-high performance concrete, a material class that offers significantly superior mechanical properties compared to conventional concrete.
Core Technical Points and Interpretation
Ultra-high performance concrete (UHPC), also known as reactive powder concrete (RPC) in this context, is a cement-based material that achieves compressive strengths exceeding 120 MPa, with some formulations reaching 180-200 MPa or higher. The material is characterized by the replacement of coarse aggregate with fine quartz sand, the addition of silica fume as a supplementary cementitious material, and the use of high-volume steel fibers to improve tensile strength and ductility. When used as the infill material in steel tube concrete columns, RPC creates a composite structural element with exceptional load-bearing capacity and deformation capacity.
The research investigates the axial compression behavior of steel tube RPC short columns by conducting controlled experiments on specimens with varying parameters. The experimental program includes the measurement of load-displacement relationships, failure modes, strain distributions, and the interaction between the steel tube and the RPC infill.
Material Properties and Differences
The fundamental difference between RPC and conventional concrete or high-strength concrete lies in the material properties and the resulting structural behavior:
| Property | Ordinary Concrete (C40-C50) | High-Strength Concrete (C60-C80) | RPC (C120-C200) |
|---|---|---|---|
| Compressive strength | 40-50 MPa | 60-80 MPa | 120-200 MPa |
| Tensile strength | 2-3 MPa | 3-5 MPa | 5-10 MPa (with steel fibers) |
| Elastic modulus | 30-35 GPa | 35-40 GPa | 45-55 GPa |
| Poisson's ratio | 0.17-0.20 | 0.18-0.22 | 0.15-0.20 |
| Ductility | Low | Moderate | Moderate (steel fiber contribution) |
| Microstructure | Porous, heterogeneous | Denser, more uniform | Very dense, near-zero porosity |
The high compressive strength and dense microstructure of RPC lead to different confinement effects compared to conventional concrete. The lateral expansion of RPC under axial compression is less pronounced than that of ordinary concrete, which affects the confinement pressure developed by the steel tube and the resulting strength enhancement of the composite column.
Experimental Results and Failure Mechanisms
The experimental results revealed several important characteristics of steel tube RPC short columns:
- Failure mode: Unlike conventional steel tube concrete columns that typically fail by outward bulging of the steel tube wall, steel tube RPC columns tend to fail by crushing of the RPC core with limited outward deformation of the steel tube. This is because the high strength and low lateral expansion of RPC reduce the confinement effect of the steel tube.
- Load-displacement behavior: The load-displacement curve of steel tube RPC columns shows a steep ascending branch followed by a relatively flat plateau, with a gradual descending branch. The peak load is significantly higher than that of equivalent columns filled with conventional concrete.
- Strain distribution: The strain distribution across the cross-section is more uniform in steel tube RPC columns compared to conventional steel tube concrete columns, due to the higher elastic modulus and more homogeneous material properties of RPC.
- Confinement effect: The confinement effect of the steel tube on RPC is less pronounced than on conventional concrete, but the absolute confinement pressure is higher due to the higher strength of the composite system.
Empirical Formula Development
Based on the experimental results and a comprehensive analysis of existing literature, the authors developed an empirical formula for calculating the bearing capacity of steel tube RPC short columns. The formula takes into account the unique material properties of RPC and the confinement effect of the steel tube:
The empirical formula considers the following parameters:
- Compressive strength of RPC (f_c,RPC)
- Yield strength of the steel tube (f_y)
- Cross-sectional area of the steel tube (A_s)
- Cross-sectional area of the RPC core (A_c)
- Confinement factor accounting for the interaction between steel tube and RPC
The comparison with existing design codes revealed that the current Chinese design codes for steel tube concrete columns are not applicable for calculating the bearing capacity of steel tube RPC columns. This is because the existing codes are based on the material properties and confinement behavior of conventional concrete, which differ significantly from those of RPC.
| Design Method | Bearing Capacity (kN) | Ratio to Test Value | Applicability |
|---|---|---|---|
| Current Chinese code (GB 50017) | Underestimated | 0.6-0.8 | Not applicable |
| Current Chinese code (JGJ 138) | Underestimated | 0.6-0.8 | Not applicable |
| Empirical formula (this paper) | Close to test value | 0.95-1.05 | Applicable |
| American code (ACI 410) | Conservative | 0.7-0.9 | Not applicable |
Engineering Practice Implications
For steel pipe manufacturers and structural engineers, this research has several important practical implications:
- Design methodology: The empirical formula developed in this research provides a reliable basis for the design of steel tube RPC columns. However, the formula should be validated against a larger database of experimental results before being incorporated into design codes.
- Material selection: The selection of RPC mix design is critical for achieving the desired structural performance. Key parameters include the silica fume content, steel fiber volume fraction, water-binder ratio, and curing conditions.
- Construction considerations: The placement and compaction of RPC within steel tubes require special attention due to the material's low workability and high viscosity. Vibratory compaction methods may not be effective, and alternative methods such as pressure-assisted compaction or pumpable RPC formulations should be considered.
- Quality control: The quality of RPC is critical to the structural performance of the composite column. Non-destructive testing methods including ultrasonic pulse velocity, rebound hammer testing, and core sampling should be employed to verify the in-situ strength of the RPC.
Manufacturing and Quality Control Considerations
The production of steel tubes for RPC-filled columns requires attention to the following quality control aspects:
- Dimensional accuracy: The steel tube dimensions should be tightly controlled to ensure adequate concrete cover and uniform confinement pressure.
- Surface quality: The internal surface of the steel tube should be smooth to facilitate the placement of RPC and to ensure good bond between the steel and the concrete.
- Welding quality: For welded steel tubes, the weld quality should be verified by NDT to ensure structural integrity.
- Residual stress: The residual stress distribution in the steel tube should be considered in the design, as it can affect the confinement behavior under axial compression.
Key Questions and Reflections
Several important questions arise from this research that deserve further investigation. First, the long-term performance of steel tube RPC columns under sustained loading is not addressed. The interaction between creep of the RPC core and the steel tube under sustained axial loads may affect the long-term load-bearing capacity and deformation behavior.
Second, the seismic performance of steel tube RPC columns is not investigated in this study. Given that RPC is a relatively brittle material compared to conventional concrete, the ductility and energy dissipation capacity of steel tube RPC columns under cyclic loading may be limited. Further research on the seismic behavior of these columns is warranted.
Third, the economic viability of steel tube RPC columns needs to be evaluated. While the structural performance is excellent, the high cost of RPC material and the specialized construction techniques required may limit the practical application of these columns in cost-sensitive projects.
Study Insights and Reference Value
This research makes an important contribution to the understanding of the structural behavior of steel tube columns filled with ultra-high performance concrete. The experimental results and the developed empirical formula provide a foundation for the design of steel tube RPC columns, which offer exceptional load-bearing capacity and potential for use in heavy-duty structural applications. For steel pipe manufacturers, this work highlights the potential market for steel tubes designed specifically for RPC-filled columns, which may require specialized manufacturing processes and quality control procedures. The research also underscores the need for updated design codes and standards to accommodate the unique material properties and structural behavior of RPC-filled steel tube columns. The findings of this study have direct implications for the design of bridge piers, building columns in heavy industrial facilities, and other structural applications where high load-bearing capacity is required.