Axial Compression Performance of CFRP-Constrained Steel Tube Reactive Powder Concrete Short Columns

Literature Overview

This study by Jiao Chujie, Li Song, Cui Lishi, Wang Zhiren, and Jian Chao, published in Acta Materiae Compositae Sinica (2021, Vol. 38, No. 2, pp. 439–448), investigates the axial compression behavior of circular CFRP (Carbon Fiber Reinforced Polymer)-confined steel tube Reactive Powder Concrete (RPC) short columns. The research was supported by the National Natural Science Foundation of China (Grants 51478128, 51778158), the Guangdong Provincial Water Science and Technology Innovation Key Fund Project (2017-32), and the Guangzhou University Graduate Innovation Ability Cultivation Fund (2018GDJCD08).

The experimental program included 12 CFRP-confined steel tube-RPC short columns, 4 steel tube-RPC short columns, and 4 steel tube short columns, with CFRP wrapping layers and steel tube wall thickness as the primary parameters.

Core Technical Findings

The study provides comprehensive data on the load-displacement behavior, ultimate load capacity, CFRP strain efficiency, and ductility coefficient of the hybrid confinement system. Key findings include:

• CFRP confinement effectively enhances both the load-bearing capacity and deformation capacity of steel tube-RPC short columns.
• Compared to CFRP-confined steel tube-concrete columns, CFRP-confined steel tube-RPC columns exhibit reduced CFRP strain efficiency and lower ductility.
• A practical axial compression bearing capacity model was proposed based on the enhancement coefficient and confinement ratio relationship.

Steel Tube-RPC Interface Behavior

The interaction between the steel tube and RPC is a critical aspect of this hybrid system. RPC, known for its exceptional compressive strength (typically exceeding 100 MPa) and low permeability, presents unique challenges for steel tube confinement:

• Interface bond: The bond between the steel tube inner surface and RPC is influenced by the steel tube's surface finish, RPC mix design, and casting method. Poor interface bonding would lead to delamination under compressive loads, reducing the effectiveness of the confinement.
• Steel tube buckling: Under high compressive loads, the steel tube may experience inward local buckling, which is resisted by the RPC core. The interaction between steel tube buckling and RPC confinement is a complex coupled problem.
• Welding considerations: If the steel tube is fabricated from welded plate, the weld seam must be inspected for defects. Any weld deficiency could serve as an initiation site for buckling or fracture under compressive loading.

CFRP Confinement Efficiency Analysis

The CFRP strain efficiency—the ratio of actual CFRP strain at failure to the ultimate CFRP strain—is a critical metric for evaluating the effectiveness of the CFRP confinement. The study found that CFRP strain efficiency decreases in steel tube-RPC systems compared to steel tube-concrete systems. This reduction can be attributed to:

1. Higher concrete strength: RPC's higher compressive strength means that the concrete core can sustain higher stresses before requiring confinement from the CFRP, resulting in lower CFRP strain at failure.
2. Lower concrete deformability: RPC's lower strain capacity compared to normal concrete means that the CFRP jacket does not reach its full strain potential before the concrete fails.
3. Steel tube interaction: The steel tube provides partial confinement, reducing the demand on the CFRP and consequently lowering CFRP strain efficiency.

Engineering Practice Implications

For engineers considering CFRP-confined steel tube-RPC columns in practice, the following considerations are important:

1. Cost-benefit analysis: While CFRP confinement enhances load capacity, the reduced CFRP strain efficiency in RPC systems means that more CFRP layers may be required to achieve the same enhancement compared to normal concrete systems. Engineers should perform detailed cost-benefit analyses.
2. Ductility requirements: The lower ductility of CFRP-confined steel tube-RPC columns may be a concern for seismic applications. Engineers should verify that the ductility coefficient meets the requirements of the applicable design code.
3. Steel tube fabrication quality: The steel tube must be manufactured to high standards, with verified wall thickness, straightness, and weld quality. Any geometric imperfection or weld defect could initiate premature failure.
4. CFRP application quality: The CFRP wrapping must be applied with proper surface preparation, resin application, and curing. Void content in the CFRP laminate should be minimized through quality control.

Bearing Capacity Model Discussion

The proposed bearing capacity model relates the enhancement coefficient to the confinement ratio, providing a practical tool for design. The model's applicability depends on the range of parameters tested in the experimental program. Engineers should verify that their design parameters fall within the tested range before applying the model.

Key limitations of the model include:

• The model is based on short column tests and may not accurately predict the behavior of slender columns where buckling effects are significant.
• The model assumes perfect CFRP confinement, which may not be achieved in practice due to application quality issues.
• The model does not account for long-term effects such as CFRP aging, steel tube corrosion, or RPC durability.

Study Insights and Conclusions

This research provides valuable data on the hybrid confinement system combining CFRP, steel tube, and RPC, which represents a promising approach for high-strength column applications. The finding that CFRP strain efficiency is lower in RPC systems is an important design consideration that engineers must account for. The proposed bearing capacity model offers a practical tool for design, but its applicability should be verified against the tested parameter range. For steel pipe manufacturers, the study highlights the importance of producing high-quality steel tubes with precise dimensions and sound welds, as the steel tube serves as a critical component of the hybrid confinement system. The research contributes to the advancement of high-performance concrete and composite confinement technologies, providing a foundation for future work on optimizing material combinations and confinement strategies for specific structural applications.