Axial Compression Performance of GFRP Tube-Stone Powder Geopolymer Concrete-Steel Tube Composite Short Columns

Literature Overview

The paper by Wang Tongkui, Zhao Qi, Lei Ting, and Yang Wenwei (2023), published in Materials Reports (Vol. 37, No. 23, pp. 250-258), investigates a novel composite column system designated DSTC-GC (Doubly Reinforced Steel-Concrete with Geopolymer Concrete). Funded by the National Natural Science Foundation of China (52168025) and multiple regional research programs, the study was conducted at Ningxia University and Hezhou University. The DSTC-GC system replaces conventional concrete with stone powder geopolymer concrete (SPGC) — a sustainable material utilizing waste marble powder as a raw material — and combines it with both a glass fiber-reinforced polymer (GFRP) tube and a steel tube for structural reinforcement.

Technical Background and Material System

The DSTC-GC column represents a triple-reinforced composite system:

Stone powder geopolymer concrete is an alkali-activated binder system that uses industrial waste stone powder (primarily marble powder) as the main solid precursor. The geopolymerization reaction produces a three-dimensional aluminosilicate network that provides mechanical strength without requiring Portland cement. The replacement ratio of stone powder studied in this research reaches up to 50% by mass, with the optimal performance observed at this level.

Experimental Program and Results

Eleven specimens were fabricated with varying parameters including GFRP tube diameter and thickness, steel tube diameter and thickness, concrete type, and section hollow ratio. The axial compression tests revealed three typical failure modes:

1. Steel tube-dominated failure: The steel tube buckles locally under high compressive stress, with the GFRP tube providing secondary confinement.
2. GFRP tube-dominated failure: The GFRP tube exhibits delamination or fiber fracture, typically at locations of stress concentration.
3. Combined failure: Simultaneous failure of both the steel and GFRP tubes with concrete core crushing.

The load-strain curves exhibited a characteristic bilinear hardening behavior. A notable finding was the appearance of a gradual yielding plateau in the load-steel strain curve of DSTC-GC specimens, which was not observed in conventional DSTC specimens. This plateau suggests a more gradual transition from elastic to plastic behavior, potentially due to the interaction between the geopolymer concrete's different deformation characteristics and the dual-tube confinement.

Bearing Capacity Model Development

The authors established functional relationships between the key design parameters:

• f_cc / f_c0: Confined concrete strength ratio
• ε_cc / ε_c0: Confined concrete strain ratio
• λ_t: Confinement ratio (related to tube geometry and material properties)

Upon comparison with existing models, the Gao et al. model was found to be most applicable for DSTC-GC column bearing capacity calculations. This finding is significant because it suggests that the Gao model's underlying assumptions about the confinement mechanism are compatible with the triple-reinforced DSTC-GC system, despite the different material properties of geopolymer concrete compared to conventional Portland cement concrete.

Welding and Fabrication Considerations

The fabrication of DSTC-GC columns involves several welding-related considerations:

• Steel tube preparation: The inner steel tube must be precisely dimensioned and surface-treated to ensure adequate bonding with the geopolymer concrete. Welded joints in the steel tube (if assembled from sections) must be ground flush to avoid stress concentrations in the concrete core.
• GFRP tube installation: The GFRP tube is typically manufactured as a continuous spiral-wound or filament-wound tube and does not require field welding. However, the interface between the GFRP tube and the concrete-steel assembly must be carefully controlled to ensure load transfer.
• Geopolymer concrete casting: The alkaline environment of geopolymer concrete (pH typically 12-14) can be aggressive to unprotected steel. The steel tube must be properly coated or passivated to prevent corrosion at the concrete-steel interface. This is analogous to the chloride-induced corrosion concerns in conventional reinforced concrete but with different chemical mechanisms.
• Quality control: Non-destructive testing of the steel tube welds should follow GB/T 11345 or equivalent standards. The geopolymer concrete should be tested for compressive strength per GB/T 50081 or the relevant geopolymer concrete standard.

Study Insights and Implications

The DSTC-GC column system represents a meaningful step toward sustainable structural engineering by utilizing industrial waste materials (marble stone powder) and combining them with advanced composite materials (GFRP). The experimental results demonstrate that the system achieves comparable or superior performance to conventional DSTC columns, with the added benefit of corrosion resistance from the GFRP outer layer. The identification of the Gao et al. model as the most appropriate analytical tool provides a practical design basis for engineers. However, several areas require further investigation: long-term durability of the geopolymer concrete under environmental exposure, fire performance of the GFRP tube (which loses strength above 200°C), and the behavior of the composite column under combined loading conditions (axial compression plus bending). For welding engineers, the primary concern is the alkaline corrosion environment at the steel tube interface, which necessitates appropriate surface protection strategies during fabrication.