Axial Compression Performance Comparison of CFRP-Constrained Hollow Sandwich Carbon Steel and Stainless Steel Pipe Concrete Short Columns

Literature Overview

This study by Yao Yun, Peng Hongbin, Ma Yu, Peng Ganghui, and Tang Hongyuan, published in Concrete (2025, No. 8, pp. 1–8), compares the axial compression performance of CFRP (Carbon Fiber Reinforced Polymer) constrained hollow sandwich steel pipe concrete short columns with different outer pipe materials—carbon steel and stainless steel. The research combines experimental testing with finite element analysis to reveal differences in load-bearing capacity, confinement effect, stress distribution, and axial force distribution between the two configurations.

Core Technical Findings

The experimental and numerical results demonstrate that CFRP wrapping effectively enhances the axial compression performance of hollow sandwich steel pipe concrete columns:

The stainless steel outer pipe configuration exhibits a steeper slope in the load-displacement curve during the secondary ascending stage compared to the carbon steel configuration, indicating superior strain hardening behavior. This suggests that stainless steel provides a more effective confinement mechanism for the sandwich concrete under high deformation conditions.

Technical Analysis of Confinement Mechanism

The CFRP-constrained hollow sandwich steel pipe concrete column operates through a multi-layer confinement mechanism:

1. Primary confinement: The outer steel pipe (carbon steel or stainless steel) provides initial lateral confinement to the sandwich concrete.
2. Secondary confinement: The CFRP wrapping provides additional lateral restraint, particularly effective in the post-peak stage when the steel pipe begins to buckle.
3. Interaction effect: The combined action of steel pipe and CFRP creates a synergistic confinement effect that exceeds the sum of individual contributions.

The difference in strain hardening between carbon steel and stainless steel configurations can be attributed to the material properties:

• Stainless steel exhibits a higher strain hardening exponent and lower yield ratio (fy/fu), allowing it to sustain higher stresses at larger deformations. This results in a more gradual and sustained confinement pressure on the sandwich concrete.
• Carbon steel has a more abrupt yield behavior and lower strain hardening capacity, leading to earlier loss of confinement effectiveness after yielding.

Finite Element Analysis Insights

The FEA models validated against experimental results reveal important stress distribution and axial force distribution patterns:

The FEA results indicate that stainless steel configurations achieve more uniform stress distribution in the sandwich concrete, leading to better utilization of the concrete's compressive strength. The axial force is more evenly distributed between the outer pipe and the sandwich concrete in stainless steel configurations, whereas carbon steel configurations tend to transfer more load to the CFRP wrapping prematurely.

Engineering Practice Implications

For structural engineers considering CFRP-constrained steel pipe concrete columns:

• Material selection: Stainless steel outer pipes provide superior performance in terms of strain hardening, ductility, and CFRP utilization. However, the cost of stainless steel is significantly higher than carbon steel. The economic benefit of enhanced performance should be evaluated against the additional material cost.
• CFRP layer optimization: The 41.85% improvement from 2 to 4 CFRP layers suggests diminishing returns beyond a certain layer count. Engineers should optimize the number of CFRP layers based on the required load-bearing capacity and ductility targets.
• Corrosion resistance: Stainless steel's inherent corrosion resistance eliminates the need for additional protective coatings, which is particularly advantageous in aggressive environments (marine, chemical, or de-icing salt environments).
• Welding considerations: The outer steel pipe requires circumferential and longitudinal welds. Stainless steel welding requires careful control of intergranular corrosion and sensitization, while carbon steel welding is more straightforward but requires corrosion protection.

Key Questions and Reflections

Several important aspects require further investigation:

• The study focuses on short columns; the slenderness ratio effect on CFRP confinement effectiveness is not addressed. For slender columns, buckling may govern the failure mode, and CFRP confinement may be less effective.
• The long-term durability of CFRP under sustained loading and environmental exposure (UV, temperature cycling, moisture) is not evaluated. CFRP performance degradation over time could significantly affect the long-term structural integrity.
• The fire resistance of CFRP-constrained columns is not considered. CFRP loses most of its strength at temperatures above 200°C, which could be critical in fire scenarios.
• The effect of different stainless steel grades (e.g., 304, 316, duplex) on the confinement performance is not investigated. Different grades exhibit varying mechanical properties and corrosion resistance.

Study Insights and Practical Recommendations

The most significant finding is that CFRP confinement is an effective means of enhancing the axial compression performance of hollow sandwich steel pipe concrete columns, with the outer pipe material playing a crucial role in determining the strain hardening behavior and overall ductility. Stainless steel configurations offer superior performance but at a higher cost.

For practical applications, engineers should consider the following recommendations:

1. Use stainless steel outer pipes for applications requiring high ductility and corrosion resistance, such as seismic zones or marine environments.
2. Use carbon steel outer pipes for cost-sensitive applications where moderate ductility is acceptable and corrosion protection can be provided.
3. Optimize the number of CFRP layers based on the required performance level, recognizing that additional layers provide diminishing returns.
4. Incorporate fire protection measures (intumescent coatings, fire-resistant wraps) to protect the CFRP from thermal degradation.

The study provides valuable comparative data that can guide material selection decisions for CFRP-constrained steel pipe concrete structures, balancing performance requirements against economic considerations.