Axial Compression Behavior of FRP-Constrained Square Concrete-Filled Steel Tube Columns

Literature Overview

This study by Zhang Yitian and Xiao Yan, published in the Journal of Hunan University (Natural Sciences) in 2019, investigates the axial compression performance of eight square concrete-filled steel tube (CFST) short columns confined with different fiber-reinforced polymer (FRP) materials. The research is funded by the National Natural Science Foundation of China (Key Program 51438010) and the Hunan Provincial Natural Science Foundation (2019JJ50093). The work addresses a critical gap in the understanding of hybrid confinement systems where FRP wrapping provides supplemental lateral restraint to steel tubes already acting as internal confinement for concrete cores.

Core Technical Findings

The experimental program comprised eight specimens using three types of FRP materials: carbon fiber reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP), and basalt fiber reinforced polymer (BFRP). The key findings are summarized below:

The fundamental conclusion is that the greater the confinement strength provided by the FRP material, the higher the axial load-bearing capacity and the better the ductility of the confined CFST column. When confinement strength is held constant, BFRP-confined specimens exhibit superior ductility compared to GFRP-confined specimens, suggesting that the mechanical properties of the FRP matrix and fiber-matrix interface play a decisive role in post-peak behavior.

Technical Interpretation of Confinement Mechanism

The confinement mechanism operates through a two-stage process. In the initial elastic stage, the steel tube and FRP wrapper share the lateral restraint responsibility proportionally based on their respective stiffness values. As the concrete core begins to dilate under increasing axial load, the FRP wrapper engages progressively, providing an active lateral pressure that counteracts the concrete expansion. The CFRP material, with its highest tensile strength among the three FRP types, generates the greatest effective confining pressure, resulting in the most pronounced improvement in both peak load and post-peak ductility.

A critical engineering insight is the interaction between the steel tube's inherent confinement and the FRP's supplemental restraint. The steel tube acts as a rigid confinement shell that prevents early concrete spalling, while the FRP wrapper provides an elastic confinement layer that maintains integrity even after the steel tube has undergone significant local buckling. This sequential failure mechanism—where the FRP continues to confine the concrete after steel tube local buckling initiates—is what produces the enhanced ductility observed in CFRP-confined specimens.

Proposed Load-Carrying Capacity Formula

The authors developed an analytical expression for the axial load-bearing capacity of FRP-confined square CFST columns. The formula accounts for:

• The unconfined concrete strength and its confined equivalent through the effective confining pressure
• The steel tube contribution including its compressive and bending resistance
• The FRP confinement pressure derived from the FRP tensile strength, thickness, and geometric parameters
• The interaction factor between steel tube and FRP confinement systems

Validation against the authors' own test data and results from other researchers demonstrated good agreement, with deviations generally within acceptable engineering tolerances. This suggests the formula can be reliably applied for preliminary design calculations of FRP-confined CFST structural members.

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, several practical considerations emerge:

1. Material selection hierarchy: For applications requiring maximum strength enhancement, CFRP is the preferred choice despite higher cost. For applications where ductility is paramount at equivalent confinement levels, BFRP offers a cost-effective alternative to CFRP.
2. Square section challenges: Square CFST sections exhibit more complex confinement behavior than circular sections due to corner effects. The FRP wrapping must be designed to accommodate the geometric discontinuities at corners, where stress concentrations develop during concrete dilation.
3. Quality control requirements: The effectiveness of FRP confinement depends critically on the bond quality between the FRP wrapper and the steel tube surface. Surface preparation, adhesive selection, and wrapping technique are all critical process variables that must be controlled in field applications.
4. Damage detection: Unlike steel tubes which exhibit visible local buckling, FRP failure can be sudden and catastrophic. Non-destructive testing methods such as ultrasonic testing (UT) should be incorporated into the inspection protocol for FRP-confined CFST members.

Key Reflections

This research highlights the potential of FRP as a retrofit and enhancement material for existing CFST structures. In engineering practice, where existing steel tube structures cannot be replaced but need capacity improvement, FRP wrapping represents a viable solution that does not significantly alter the member's cross-sectional dimensions. The finding that BFRP performs comparably to CFRP in ductility at equal confinement levels is particularly significant for cost-sensitive applications in developing regions, as BFRP is generally less expensive than CFRP while offering adequate fire resistance properties.

Concluding Summary

The study provides valuable experimental data and a validated analytical model for FRP-confined square CFST columns under axial compression. The clear hierarchy of CFRP > BFRP > GFRP in terms of strength and ductility enhancement, combined with the practical analytical formula, offers structural engineers a reliable basis for designing and evaluating these hybrid confinement systems. Future research should extend to cyclic loading conditions and investigate the long-term durability of FRP confinement under environmental exposure, as these factors will ultimately determine the service life and reliability of FRP-enhanced CFST structures in practical engineering applications.