Experimental Study on Axial Compressive Properties of Circular Steel Tube Geopolymer Recycled Concrete Short Columns

Literature Overview

This experimental study by Chen Yufeng et al., published in Progress in Steel Building Structures (2022, Vol. 24, No. 8), investigates the axial compressive behavior of circular steel tube columns filled with geopolymer recycled aggregate concrete (GRAC). The research was supported by the National Natural Science Foundation of China (Project No. 51878176) and the Fujian Provincial Department of Science and Technology (Project No. 2019Y0049). Eight specimens were tested: six with varying recycled aggregate replacement ratios and two control specimens with ordinary geopolymer concrete.

Core Technical Approach

The experimental program focused on the recycled aggregate replacement ratio as the primary variable. The geopolymer binder system eliminates Portland cement, offering significant carbon emission reductions compared to conventional concrete. The circular steel tube provides uniform confinement pressure around the concrete core, making it an ideal configuration for studying the interaction between recycled aggregate concrete and steel confinement.

Test Matrix

Key Experimental Findings

Failure Modes

The failure modes of circular steel tube geopolymer recycled concrete columns showed no significant difference from conventional steel tube concrete columns. The steel tube yielded uniformly around the circumference, and the concrete core exhibited radial cracking and outward bulging. This consistency in failure patterns is encouraging from a design perspective, as it means existing design methodologies for steel tube concrete can be adapted with appropriate modifications.

Load-Deformation Behavior

A notable finding is that the descending branch of the load-deformation curve for geopolymer recycled concrete specimens is more gradual compared to conventional steel tube concrete specimens. This indicates improved post-peak ductility, which is attributed to the geopolymer binder system's inherent characteristics. The geopolymer matrix provides better crack bridging capability, delaying sudden strength degradation after peak load.

Effect of Recycled Aggregate Replacement Ratio

The ultimate bearing capacity decreases with increasing recycled aggregate replacement ratio due to the lower strength of recycled aggregates and the weaker ITZ. However, the ductility improves, which is attributed to the more flexible deformation characteristics of recycled aggregate concrete under confinement.

Standards Comparison and Design Verification

The researchers compared experimental results with existing design formulas from current codes for ordinary steel tube concrete. All code predictions were on the safe side, meaning the calculated capacities were lower than experimental values. This suggests that current design provisions, while conservative, are applicable to geopolymer recycled concrete steel tube columns without major modifications.

Engineering Practice Integration

From a steel pipe manufacturing standpoint, this research has direct relevance to the production of steel tube concrete components. The circular cross-section is the most common form for steel tube concrete columns, typically manufactured as ERW or HFW welded pipes, or seamless pipes. The uniform wall thickness distribution is critical for achieving uniform confinement, and manufacturing tolerances per standards such as API 5L or GB/T 8163 directly affect structural performance.

For welding engineers, the fabrication of steel tube concrete columns involves seam welding (for large-diameter pipes) or butt welding (for spliced columns). The welding process must ensure full penetration and adequate heat input to achieve proper fusion without excessive distortion that could affect the internal concrete placement quality. Post-weld inspection using ultrasonic testing is essential to ensure weld integrity before concrete filling.

Key Questions and Reflections

The research addresses the question of whether geopolymer technology can be successfully applied to recycled aggregate concrete in confined structural applications. The positive findings suggest that the geopolymer binder system effectively compensates for some of the inherent weaknesses of recycled aggregates. However, questions remain regarding long-term durability, particularly regarding carbonation resistance and chloride penetration in geopolymer recycled concrete under confinement.

Study Insights and Implications

This work represents a significant step toward sustainable structural engineering by combining two green technologies: geopolymer binders and recycled aggregates. The finding that ductility improves with higher recycled aggregate replacement ratios is particularly valuable for seismic design applications, where energy dissipation capacity is paramount. Engineers should consider this technology for applications where environmental sustainability is a design priority, while ensuring that quality control protocols address the inherent variability of recycled aggregates. The conservative nature of existing code formulas provides a safety margin that supports the practical adoption of this technology in current engineering practice.