Axial Compression Behavior of Circular Steel Tube Geopolymer Recycled Concrete Short Columns

Literature Overview

This paper, published in the journal Progress in Steel Building Structures (2022, Vol. 24, No. 8, pp. 61-69), presents an experimental investigation into the axial compressive mechanical performance of circular steel tube geopolymer recycled concrete (GRC) short columns. The research was conducted by Chen Yufeng and colleagues from Fujian Construction Engineering Group and Fujian Agriculture and Forestry University, supported by the National Natural Science Foundation of China (Grant No. 51878176) and the Fujian Provincial Department of Science and Technology (Grant No. 2019Y0049). The study is particularly significant in the context of sustainable construction, as it explores the use of recycled aggregate combined with geopolymer binder technology within steel tube confinement systems, addressing both environmental concerns and structural performance requirements simultaneously.

Core Technical Content and Experimental Design

The experimental programme consisted of 8 specimens in total: 6 circular steel tube geopolymer recycled concrete short columns and 2 circular steel tube geopolymer ordinary concrete short columns used as reference specimens. The primary variable was the recycled aggregate replacement rate, which was systematically varied to evaluate its influence on structural performance. All specimens were subjected to monotonic axial compression loading until failure.

The following table summarizes the key experimental parameters and variables:

Failure Mode Analysis

One of the most important findings is that the failure modes of circular steel tube geopolymer recycled concrete columns show no significant difference from those of conventional steel tube concrete columns. This observation is critical from a design perspective, as it implies that existing design methodologies based on conventional steel tube concrete can be extended to geopolymer recycled concrete applications without major modifications to failure mode assumptions. The failure typically involves concrete crushing at the mid-span region, accompanied by outward bulging of the steel tube, which is consistent with the well-established confinement failure mechanism in steel tube concrete members.

Load-Deformation Behaviour

The load-deformation relationship curves reveal an important mechanical characteristic: compared with ordinary steel tube concrete specimens, the descending branch of the load-deformation curve for circular steel tube geopolymer recycled concrete specimens is more gradual. This finding indicates enhanced post-peak energy dissipation capacity, which is a desirable property for structures subjected to seismic loading or impact events. The more gradual descending branch suggests that the geopolymer matrix, combined with the recycled aggregate and steel tube confinement, provides a more ductile failure mechanism after the peak load is reached.

From a metallurgical and materials science perspective, this behaviour can be attributed to several factors:

• The geopolymer binder phase, typically composed of aluminosilicate gels, may exhibit different fracture characteristics compared to Portland cement hydration products, potentially providing more energy absorption during crack propagation.
• Recycled aggregate, due to its attached old mortar layer, may create additional interfaces that contribute to progressive microcracking rather than sudden brittle failure.
• The steel tube confinement restrains lateral expansion of the concrete core, and the interaction between the confined core and the steel tube shell continues to provide load-carrying capacity even after the core begins to degrade.

Effect of Recycled Aggregate Replacement Rate

The relationship between recycled aggregate replacement rate and structural performance presents an interesting trade-off:

As the recycled aggregate replacement rate increases, the ultimate bearing capacity of the specimens decreases. This is consistent with the general understanding that recycled aggregate has lower strength and higher water absorption compared to natural aggregate, leading to a weaker interfacial transition zone between the aggregate and the geopolymer matrix. However, the ductility shows an increasing trend with higher replacement rates, which is counterintuitive but can be explained by the increased porosity and microcracking capacity of the recycled aggregate, which allows for more progressive deformation before final failure.

Comparison with Existing Design Codes

The authors compared the experimentally measured ultimate bearing capacities with values calculated using existing design formulas provided in codes for ordinary steel tube concrete. The results indicate that all code-based calculations are conservative relative to the experimental values. This finding has direct practical implications:

1. Existing design codes can be safely applied to circular steel tube geopolymer recycled concrete columns without modification, ensuring structural safety.
2. The conservative nature of the code predictions suggests that there may be room for optimization, potentially allowing for lighter or more economical designs if future research establishes reliable reduction factors.
3. The conservatism likely arises because the code formulas do not account for the enhanced post-peak ductility and the specific confinement effects of the geopolymer recycled concrete system.

Engineering Practice Implications

From an engineering practice standpoint, this research provides valuable guidance for the following applications:

• Sustainable infrastructure projects where recycled materials are mandated or preferred for environmental compliance.
• Short column applications in building structures, such as columns in multi-storey buildings, where the slenderness ratio is low and axial compression is the dominant loading condition.
• Seismic regions where enhanced ductility is a critical design requirement, as the geopolymer recycled concrete system demonstrates superior post-peak deformation capacity.
• Projects where geopolymer technology is already being adopted for other structural elements, as this research provides evidence that steel tube confinement can effectively enhance the structural performance of geopolymer recycled concrete.

Key Reflections and Critical Analysis

The experimental sample size of 8 specimens, while sufficient for preliminary investigation, is relatively small for establishing definitive design relationships. The study focuses exclusively on short columns under monotonic axial compression, and does not address more complex loading conditions such as combined axial and bending moments, cyclic loading for seismic assessment, or fire exposure. Furthermore, the long-term durability of geopolymer recycled concrete within steel tube confinement, including resistance to chloride penetration, carbonation, and freeze-thaw cycles, remains an important question that should be addressed in future research.

The finding that existing code formulas are conservative is encouraging but also raises the question of how much conservatism exists and whether this conservatism is uniform across different replacement rates and steel tube dimensions. A parametric study with a larger sample size would be beneficial to quantify this conservatism more precisely.

Study Insights and Conclusions

This research contributes meaningfully to the growing body of knowledge on sustainable steel tube concrete systems. The key insight is that geopolymer recycled concrete can be effectively used within circular steel tube confinement to produce short columns with acceptable load-bearing capacity and enhanced ductility, while existing design codes provide a conservative safety margin. The more gradual descending branch of the load-deformation curve is particularly noteworthy, as it suggests that these columns may perform well under seismic or impact loading. For engineers involved in sustainable construction, this research provides experimental evidence supporting the use of recycled aggregates and geopolymer binders in steel tube concrete applications, provided that appropriate design factors are applied. Future research should expand to long-term durability testing, cyclic loading behaviour, and parametric studies to develop more precise design models that can reduce the conservatism of current code-based approaches.