Recycled Aggregate Concrete-Filled Square Steel Tube Axial Compression Performance Study

Literature Overview

This study by Chen Mengcheng, Liu Jingjian, and Huang Hong from East China Jiaotong University investigates the axial compression behavior of square steel tube recycled concrete (RRCFST) columns. The research addresses an important sustainability concern in civil engineering: the increasing availability of recycled aggregates from demolished concrete and the need to incorporate them into structural systems without significant performance penalties. Five axially compressed short column specimens were designed and tested, supplemented by ABAQUS finite element simulations to capture the full load-deformation response.

Core Technical Findings

The study demonstrates that recycled aggregate concrete-filled square steel tubes exhibit mechanical behavior closely analogous to conventional concrete-filled steel tube (CFST) columns. The nominal stress-strain relationships, stiffness degradation patterns, damage accumulation characteristics, energy dissipation capacity, lateral deformation coefficients, and load-displacement/load-strain curves all follow similar trends to those of conventional CFST columns. This finding is significant because it suggests that recycled aggregates can be substituted for natural aggregates in CFST systems with minimal impact on structural performance.

The confinement effect coefficient (ξ) emerges as the dominant parameter governing specimen behavior. Specimens with higher ξ values demonstrate enhanced peak stress and corresponding strain, improved ductility, lower initial stiffness, and stronger energy dissipation capacity. During the late loading stage, the steel tube and core recycled concrete develop pronounced interaction, placing the recycled concrete under triaxial compression. This confinement substantially elevates the peak stress and strain of the recycled concrete, effectively mitigating the inherent deficiencies of recycled aggregates—namely lower strength and poor ductility.

Engineering Practice Implications

From a steel pipe manufacturing perspective, the study validates that standard square hollow sections (SHS) conforming to GB/T 6728 or EN 10219 can be directly applied in recycled concrete confinement systems without requiring special steel grades or modified cross-sectional geometries. The steel tube functions as a passive confinement element, and the recycled aggregate's lower strength is compensated by the triaxial stress state induced by the steel shell. This has direct implications for procurement decisions: recycled aggregate sourcing does not necessitate upgrading the steel tube specification, which is economically favorable for large-scale infrastructure projects.

The finite element modeling approach described in the paper provides a validated numerical framework for parametric studies. Engineers can leverage this framework to evaluate the performance of recycled CFST columns under eccentric compression, bending, or combined loading without extensive experimental programs. The ABAQUS model's ability to replicate the full load-deformation process—including local buckling of the steel tube, concrete crushing, and post-peak softening—makes it suitable for design-level analysis under GB 50935 or GB 50396 design provisions.

Critical Reflections

While the study confirms the viability of recycled aggregates in CFST systems, several limitations warrant attention. First, the study examines only short columns under pure axial compression, which represents the most favorable loading condition for confinement effectiveness. Slender recycled CFST columns under eccentric loading would exhibit more complex buckling behavior, and the recycled aggregate's lower modulus of elasticity could amplify second-order effects. Second, the study does not address long-term durability concerns—recycled aggregates retain higher porosity and weaker interfacial transition zones, which may accelerate carbonation or chloride ingress through the steel tube's micro-cracks over service life. Third, the welding joints connecting square steel tubes in multi-story structural systems were not examined, yet these joints are critical load paths in real structures and may govern overall system behavior under seismic or wind loading.

The study's practical value lies in providing confidence that recycled concrete-filled steel tube columns can be designed using existing CFST design methodologies with appropriate safety margins. For steel pipe suppliers and fabricators, this opens a market segment where recycled aggregates reduce material costs while maintaining structural adequacy, particularly for non-critical structural elements in buildings, bridges, and industrial facilities.