Zhuojin Pipe Fitting Co., LtdAxial Compression Test Study of CFRP-Steel Tube RPC Short Columns
Literature Overview
This paper published in Journal of Building Materials (2020, Vol. 23, No. 4, pp. 852-857) by Cui Lishi, Jiao Chujie, Li Song, Tan Shuzhen, and Jian Chao from Guangzhou University presents experimental research on 12 CFRP-steel tube reactive powder concrete (CRST) short columns subjected to axial compression loading. The research was supported by the National Natural Science Foundation of China (Grants 51778158, 51478128), Guangdong Provincial Water Science and Technology Key Project (2017-32), and Ministry of Housing and Urban-Rural Development research projects (2010-K3-27, 2010-K4-18). The study investigates the dual-confinement effect of CFRP wraps and steel tubes on the high-strength RPC core.
Experimental Program and Test Results
Twelve short column specimens were tested, with core concrete strengths ranging from 100 to 160 MPa, steel tube wall thicknesses of 2 and 4 mm, and 2 layers of CFRP wrap as external reinforcement. The specimens represent a parametric study of the key design variables influencing the load-bearing capacity and failure behavior of CRST short columns.
Test Specimen Configuration
| Parameter | Variation Range | Number of Levels |
|---|---|---|
| RPC compressive strength | 100-160 MPa | 3 levels |
| Steel tube wall thickness | 2 mm, 4 mm | 2 levels |
| CFRP layers | 2 layers (constant) | 1 level |
| Total specimens | 12 | - |
The load-displacement curves of the CRST specimens can be divided into four distinct stages: linear elastic stage, elastoplastic stage, failure stage, and plateau stage. The plateau stage is a distinctive feature of the CFRP-steel tube confined RPC columns, indicating the sustained load-carrying capacity after initial concrete crushing due to the combined confinement of CFRP and steel tube.
Failure Mode Characteristics
| Failure Mode | Description | Governing Parameters |
|---|---|---|
| Shear failure | Diagonal cracking and shear failure of concrete core | Low confinement ratio, high RPC strength |
| End compression bulging | Lateral expansion at column ends | Steel tube yielding, CFRP debonding |
| CFRP rupture | Tensile failure of CFRP wrap | Excessive confinement pressure |
| Steel tube local buckling | Inward buckling of steel tube wall | Thin wall thickness, high concrete pressure |
Analytical Model Development
The study proposes an influence coefficient (IF) to quantify the improvement in ultimate load-bearing capacity due to the dual confinement of CFRP and steel tube. The IF is determined through linear regression fitting of the experimental data, providing a practical tool for design calculations.
Additionally, assuming that the steel tube has yielded at the ultimate load-bearing capacity, a simplified calculation method based on limit equilibrium analysis is developed. This method considers the confinement pressure from both the steel tube and CFRP wrap, the resulting increase in concrete compressive strength, and the direct load-carrying contribution of the steel tube and CFRP.
Design Formula Parameters
| Symbol | Description | Typical Value |
|---|---|---|
| f_c | RPC compressive strength | 100-160 MPa |
| f_y | Steel tube yield strength | 235-355 MPa |
| f_f | CFRP tensile strength | 3000-3500 MPa |
| t_s | Steel tube wall thickness | 2-4 mm |
| n | Number of CFRP layers | 2 |
| D | Column diameter | Specimen-dependent |
| IF | Influence coefficient | Derived from regression |
Engineering Practice Implications
The CRST short column system offers significant advantages for applications requiring high compressive strength in limited cross-sectional areas, such as bridge piers, building columns in high-rise structures, and offshore platform supports. The dual-confinement mechanism provides:
- Substantial improvement in concrete compressive strength through the combined lateral confinement of CFRP and steel tube.
- Enhanced ductility and energy dissipation capacity compared to unconfined or singly confined concrete columns.
- The plateau stage in the load-displacement curve indicates excellent post-peak performance, beneficial for seismic applications.
For fabrication and quality control, the following considerations are critical:
- The CFRP wrapping must be applied with uniform tension and proper overlap to ensure consistent confinement pressure around the column circumference.
- The steel tube must be precisely fabricated with uniform wall thickness, as variations in thickness create uneven confinement pressure and potential weak points.
- The interface between CFRP and steel tube requires careful surface preparation to ensure proper bond and prevent delamination under high confinement pressures.
- The RPC concrete placement must achieve full density without voids, as the high strength of RPC is sensitive to porosity and compaction quality.
Key Reflections and Study Insights
This research demonstrates the significant potential of combining CFRP external reinforcement with steel tube confinement for enhancing the performance of high-strength RPC concrete columns. The dual-confinement mechanism creates a synergistic effect where the steel tube provides initial confinement and ductility enhancement, while the CFRP wrap provides additional confinement pressure at higher strain levels.
The plateau stage observed in the load-displacement curves is particularly significant for seismic design, as it indicates that the column can sustain loads at a nearly constant level after reaching peak capacity. This behavior is highly desirable for performance-based seismic design, where structures are expected to undergo large inelastic deformations without catastrophic failure.
The proposed influence coefficient approach provides a practical and efficient method for design calculations, bridging the gap between detailed finite element analysis and simplified empirical formulas. However, engineers should be aware that the IF is derived from a specific range of parameters and should not be extrapolated beyond the validated domain without additional experimental verification.
The RPC concrete, while offering exceptional compressive strength, exhibits increased brittleness compared to ordinary concrete. The external confinement provided by CFRP and steel tube effectively addresses this brittleness concern, transforming a brittle material into a ductile composite system. This transformation is achieved through the lateral confinement that prevents concrete cracking from propagating and spalling, maintaining the structural integrity of the column even under severe loading conditions.
In summary, the CFRP-steel tube RPC short column system represents an advanced structural solution combining the highest strength concrete with dual external confinement mechanisms. The experimental results and analytical models developed in this study provide a solid foundation for the practical application of CRST columns in demanding structural environments. Engineers should leverage the proposed design formulas while maintaining rigorous quality control during fabrication, particularly for CFRP application quality, steel tube dimensional accuracy, and RPC concrete placement density, to ensure the full realization of the predicted structural performance.