Mechanical Properties of Square Steel Tube Recycled Concrete Short Columns with Embedded CFRP Circular Tubes

Literature Overview

This paper, published in Journal of Shenyang Jianzhu University (2025, Vol. 41, No. 1, pp. 39-46) by Xu Wei, Cheng Jiabao, Tong Zhou, and Zhang Haixia, investigates the mechanical properties of composite columns consisting of square steel tubes filled with recycled concrete and embedded with CFRP (carbon fiber reinforced polymer) circular tubes. The research was funded by the National Natural Science Foundation of China (Project No. 52278196).

Research Background and Motivation

This research addresses two contemporary engineering challenges simultaneously:

1. The need for sustainable construction materials (recycled concrete from demolished structures)
2. The demand for enhanced structural performance through composite design

The combination of square steel tubes, recycled concrete, and CFRP circular tubes creates a hybrid composite system that leverages the strengths of each component while mitigating their individual weaknesses.

Test Parameters and Configuration

Core Technical Findings

Effect of CFRP Layer Number

The CFRP circular tube provides substantial improvement in both axial and eccentric compression capacity, though the effect on eccentric compression capacity is significantly less pronounced than on axial compression.

Effect of Eccentricity Ratio

As eccentricity increases, both capacity and ductility decrease. However, under the same cross-sectional area, square section specimens exhibit more stable eccentric compression behavior compared to circular section specimens.

Key Comparative Finding

The embedded CFRP circular tube improves the mechanical properties of square steel tube recycled concrete short columns, but the improvement is less significant for eccentrically loaded columns than for axially loaded columns. This is attributed to the asymmetric stress distribution under eccentric loading, which reduces the effectiveness of the CFRP confinement.

Engineering Practice Integration

Material and Fabrication Considerations

From a steel pipe and composite engineering perspective, several practical considerations arise:

1. Recycled concrete quality: The aggregate replacement ratio in recycled concrete affects its mechanical properties, workability, and bond with the steel tube. Quality control of recycled aggregate (particle size distribution, water absorption, strength) is essential.
2. CFRP tube installation: The CFRP circular tube must be precisely positioned within the square steel tube before concrete placement. Any eccentricity in CFRP tube placement will affect the composite behavior.
3. Steel tube fabrication: The square steel tube requires precise welding of the four sides, with attention to weld quality at the corners where stress concentrations develop.
4. Concrete placement: Filling recycled concrete into a square tube with an internal CFRP circular tube creates a complex geometry that challenges concrete flow and compaction.

Welding and Connection Design

The square steel tube sections require transverse welds to form the closed section. These welds are critical for:

• Structural integrity of the outer tube
• Confinement effectiveness
• Load transfer between tube walls

Finite Element Modeling Validation

The ABAQUS finite element model was validated against axial compression test results before being applied to eccentric compression analysis. This validation approach is sound engineering practice, ensuring that the numerical model accurately captures the complex interaction between the steel tube, recycled concrete, and CFRP tube.

Key Questions and Reflections

The relatively modest improvement in eccentric compression capacity (5.97%) compared to axial compression capacity (30.44%) raises questions about the applicability of this composite design for columns subjected to significant bending moments. In real structures, columns rarely experience pure axial loading; the presence of bending moments due to frame action, eccentric loads, or second-order effects means that the actual benefit of CFRP embedding may be less than the axial compression improvement suggests.

The finding that square sections perform more stably under eccentric loading than circular sections (despite the CFRP tube being circular) is counterintuitive and warrants further investigation. This may be related to the different stress distribution patterns in square versus circular sections under eccentric loading.

Sustainability Implications

The use of recycled concrete addresses environmental concerns, but the incorporation of CFRP introduces high-performance composite materials with their own environmental footprint (energy-intensive production, non-recyclability). A full life-cycle assessment would be needed to evaluate the net environmental benefit of this composite system.

Study Insights and Implications

This research demonstrates a viable pathway for enhancing the performance of recycled concrete columns through CFRP embedding, contributing to both structural efficiency and material sustainability. The significant improvement in ductility (up to 41.11% for axial compression) is particularly valuable for seismic applications, where energy dissipation capacity is critical. Engineers should consider this composite approach for applications where axial compression dominates and where the additional cost of CFRP can be justified by the performance gains. The finite element methodology developed in this study provides a reliable tool for design optimization and parameter study.