Axial Compression Behavior of Steel Tube High-Strength Recycled Concrete Composite Columns

Literature Overview

This paper by Niu Haicheng et al., published in the Acta Compositae Sinica in 2022, investigates the axial compression performance of composite columns combining steel tubes with high-strength recycled concrete (CFSTRCC). The study was funded by the National Natural Science Foundation of China (U1904188) and the Henan Provincial Natural Science Foundation (182300410247). The authors conducted axial compression tests on five specimens: two columns with ordinary high-strength concrete cores and three columns with high-strength recycled concrete cores. The research variables include concrete type, steel tube cross-sectional shape, and the presence of cross-shaped tie bars within square steel tubes.

Core Technical Findings

The experimental results reveal several important conclusions regarding the structural behavior of CFSTRCC members:

• The damage evolution process and failure modes of recycled concrete CFSTRCC are similar to those of ordinary concrete CFSTRCC, indicating that recycled aggregate substitution does not fundamentally alter the failure mechanism.
• Recycled concrete CFSTRCC exhibits higher load-bearing capacity and energy dissipation capacity compared to ordinary concrete counterparts.
• However, the peripheral reinforced concrete experiences severe spalling, and ductility is inferior to that of ordinary concrete columns.
• Cross-shaped tie bars installed within square steel tubes significantly enhance the confinement effect of the steel tube on core concrete, markedly improving ductility, load capacity, and energy dissipation.
• Under conditions of equal steel tube cross-sectional area and comparable material strength, circular steel tube CFSTRCC outperforms square steel tube CFSTRCC in terms of load-bearing capacity, energy dissipation, and ductility.

Key Technical Parameters and Design Considerations

The cross-shaped tie bars serve as critical internal reinforcement elements that improve the composite action between the steel tube and core concrete. From a welding and fabrication perspective, these tie bars must be properly welded to the steel tube interior, requiring careful attention to weld quality, penetration depth, and heat-affected zone control to avoid compromising the steel tube's structural integrity.

Interpretation of Material Behavior

The superior load-bearing capacity of recycled concrete CFSTRCC can be attributed to the interlocking effect of recycled aggregate particles, which have rougher surfaces than natural aggregates. This creates better mechanical bonding with the cement paste, resulting in higher compressive strength. However, the higher porosity and weaker interfacial transition zone (ITZ) in recycled concrete lead to more brittle failure characteristics and increased peripheral spalling.

The ductility improvement achieved through cross-shaped tie bars is particularly significant. These internal restraints effectively distribute the confining pressure more uniformly across the core concrete, delaying local crushing and promoting more uniform strain distribution. The observation that peak strain increases with tie bar installation indicates more complete material utilization before failure.

Engineering Practice Implications

For engineers designing CFSTRCC structures, several practical recommendations emerge:

1. Circular steel tubes should be preferred over square tubes when ductility and energy dissipation are critical design requirements.
2. Cross-shaped tie bars should be installed within square steel tubes to compensate for the inherently lower confinement efficiency of square sections.
3. The design load capacity calculation formulas from domestic and international codes provide reasonable predictions for CFSTRCC axial capacity, as validated by the 26-specimen database in this study.
4. Welding quality of internal tie bars to steel tubes must be controlled to ensure effective composite action, with particular attention to weld throat thickness and absence of undercut defects.

Study Insights and Reflections

This research demonstrates that recycled concrete can be effectively utilized in composite steel tube columns without compromising load-bearing performance. The key challenge lies in managing the reduced ductility through proper detailing, particularly the use of internal tie reinforcement. From a steel pipe fabrication standpoint, the installation of internal tie bars adds complexity to the manufacturing process, requiring either pre-welding of tie bar assemblies or post-fabrication insertion with welding operations inside the tube. This necessitates specialized welding access and potentially robotic welding solutions for consistent quality.

The finding that recycled concrete provides higher peak loads but more severe spalling suggests that protective measures for the peripheral reinforced concrete are essential in seismic design applications. Engineers should consider additional confinement reinforcement or fiber-reinforced polymer wrapping to mitigate spalling damage.

Summary

This study provides valuable experimental evidence that high-strength recycled concrete can be successfully incorporated into steel tube composite columns with improved load-bearing capacity and energy dissipation. The critical design parameter is the steel tube cross-sectional shape, with circular tubes offering superior ductility, while square tubes require internal tie bar reinforcement to achieve comparable performance. Welding quality of internal reinforcement connections is essential for ensuring the predicted composite action and structural reliability.