Zhuojin Pipe Fitting Co., LtdComparative Axial Compression Behavior of CFRP Confined Hollow Sandwich Steel Pipe Concrete Short Columns with Carbon Steel and Stainless Steel Outer Tubes
Literature Overview
This paper by Yao Yun, Peng Hongbin, Ma Yu, Peng Ganghui, and Tang Hongyuan, published in Concrete (2025, Issue 8, pp. 1-8), investigates the axial compression performance of circular hollow sandwich steel pipe concrete (SHSC) short columns confined by carbon fiber reinforced polymer (CFRP) sheets. The study compares two types of outer tubes—carbon steel and stainless steel—and evaluates how the outer tube material influences confinement effectiveness, load-bearing capacity, and stress distribution. The work is supported by the National Natural Science Foundation of China (Grant No. 52178169) and the Dazhou Municipal Construction Environment Engineering Technology Research Center (Grant No. SDJ2025ZB-02).
Core Experimental Setup and Configuration
The experimental specimens consist of circular hollow sandwich steel pipe concrete short columns. The "hollow sandwich" configuration implies a three-layer structure: an outer steel tube, an inner steel tube, and a concrete layer sandwiched between them. CFRP sheets are externally wrapped around the outer tube to provide additional hoop confinement. The key variable is the outer tube material—carbon steel versus stainless steel—while CFRP layer counts vary from 2 to 4 layers.
Key Technical Findings
CFRP Confinement Effectiveness
The study demonstrates that CFRP wrapping significantly enhances the axial compression performance of hollow sandwich steel pipe concrete columns. When CFRP layers increase from 2 to 4, the peak strength of the sandwich concrete increases from 557.60 kN to 790.94 kN, representing a 41.85% improvement. This confirms that the confinement mechanism operates primarily through hoop stress restraint, which restrains the lateral expansion of concrete under axial loading and thereby elevates the concrete's effective compressive strength.
| CFRP Layers | Peak Strength (kN) | Improvement Rate |
|---|---|---|
| 2 layers | 557.60 | Baseline |
| 4 layers | 790.94 | +41.85% |
Carbon Steel versus Stainless Steel Outer Tube Performance
A critical finding is the superior confinement effect provided by stainless steel outer tubes compared to carbon steel outer tubes. In the secondary ascending stage of the load-displacement curve, the slope of the CFRP-confined stainless steel specimen is markedly steeper than that of the CFRP-confined carbon steel specimen. This indicates that stainless steel provides a stronger constraint effect and exhibits better strain hardening behavior.
The engineering significance of this finding is substantial. Stainless steel's higher ductility and superior strain hardening capacity mean that it maintains load-bearing capability even after the concrete core has begun to degrade. Carbon steel, while adequate in the elastic range, tends to yield earlier and provides diminishing confinement as plastic deformation accumulates.
Stress Distribution and Axial Force Allocation
Finite element analysis (FEA) validated against experimental results reveals differences in stress distribution and axial force allocation between the two outer tube materials. The stainless steel outer tube distributes hoop stresses more uniformly due to its higher ductility, while the carbon steel outer tube shows more localized stress concentrations, particularly near the CFRP wrap edges. The axial force allocation between the steel tubes and the concrete core also differs: stainless steel specimens show a more gradual transfer of load from the concrete core to the steel tubes as deformation progresses.
Engineering Practice Implications
Material Selection Guidance
For applications where high confinement efficiency and post-peak ductility are critical—such as seismic retrofits of existing steel pipe concrete columns or high-pressure hydraulic cylinder housings—stainless steel outer tubes combined with CFRP wrapping represent a superior structural solution. However, the cost differential between carbon steel and stainless steel must be weighed against the performance gains.
CFRP Layer Optimization
The 41.85% strength improvement from doubling CFRP layers suggests diminishing returns beyond 4 layers. Engineers should consider economic optimization: the marginal benefit of additional CFRP layers decreases as the number of layers increases, particularly when the outer tube material already provides significant confinement.
Design Considerations for Hollow Sandwich Configurations
The hollow sandwich steel pipe concrete concept offers an interesting alternative to solid concrete-filled steel tubes. The hollow center reduces self-weight and material cost while maintaining structural efficiency through the sandwich concrete layer. However, the design must account for the interaction between the two steel tubes and the concrete, which the FEA models in this study capture but which requires careful attention in practical design.
Critical Reflections and Questions
One question that arises from this study is the long-term durability of CFRP-concrete interfaces, particularly under cyclic loading or corrosive environments. The study focuses on monotonic axial compression, but real-world structures often experience repeated loading. Additionally, the study does not address the fire resistance of CFRP-wrapped columns, which is a critical concern for structural steel applications. CFRP loses significant strength above 200°C, which could compromise the confinement effect in fire scenarios.
Another point worth noting is the absence of discussion regarding the bond quality between CFRP sheets and the outer steel tube surface. Surface preparation quality, adhesive type, and environmental conditions during bonding all significantly affect the effectiveness of CFRP confinement. In engineering practice, these factors often determine whether the theoretical confinement benefits are realized.
Study Insights and Reference Value
This paper contributes meaningfully to the understanding of hybrid confinement systems in steel pipe concrete structures. The comparative approach between carbon steel and stainless steel outer tubes provides valuable guidance for material selection in retrofit applications. The combination of experimental testing with validated FEA modeling offers a reliable basis for extending the findings to different geometries and loading conditions. For practicing engineers involved in structural retrofitting or innovative column design, this work highlights the potential of combining metallic confinement with composite reinforcement to achieve enhanced structural performance.