Seismic Performance of Circular Steel Tube Recycled Aggregate Concrete Columns

Literature Overview and Research Background

This study investigates the seismic behavior of circular steel tube columns filled with recycled aggregate concrete (RAC), a topic of growing importance in sustainable structural engineering. With the rapid depletion of natural aggregates and the increasing volume of construction and demolition waste, recycled coarse aggregate from demolished concrete has emerged as a viable alternative material. The research addresses a critical gap: while recycled aggregate concrete has been extensively studied in conventional reinforced concrete members, its performance within steel tube confinement under cyclic lateral loading remains insufficiently characterized. The circular steel tube provides lateral confinement that compensates for the lower elastic modulus and higher porosity of recycled aggregates, potentially restoring ductility and energy dissipation capacity comparable to natural aggregate concrete-filled tubes.

Key Technical Findings

The experimental program typically involves fabricating circular steel tube columns with varying recycled aggregate replacement ratios (commonly 0%, 30%, 50%, 70%, and 100%) and subjecting them to quasi-static cyclic loading protocols. The steel tubes generally conform to GB/T 3091 or API 5L Grade B specifications with typical wall thicknesses ranging from 4 to 8 mm and outer diameters of 150 to 300 mm. The recycled aggregate concrete mixtures are designed to achieve target strengths of C30 to C50 while maintaining workability suitable for placement inside the confined tube geometry.

The study demonstrates that columns with up to 70% recycled aggregate replacement maintain satisfactory seismic performance, exhibiting ductility factors exceeding 5 and energy dissipation capacities within 10–15% of natural aggregate counterparts. At 100% replacement, degradation in peak load capacity of approximately 12–18% is observed, accompanied by reduced initial stiffness and earlier onset of local buckling in the steel tube. The confining pressure exerted by the circular steel tube effectively mitigates the interfacial transition zone weakness inherent in recycled aggregate concrete, delaying concrete spalling and maintaining composite action throughout the loading cycle.

Defect Analysis and Failure Mechanisms

The failure modes observed in the experimental specimens follow a progressive sequence that can be analyzed through an FMEA (Failure Mode and Effects Analysis) framework. Initial micro-cracking occurs in the recycled aggregate concrete at approximately 30–40% of the peak load, propagating from the interface between old adhered mortar and the steel tube inner surface. As loading continues, localized concrete crushing initiates at mid-height where bending moments are maximal, followed by outward bulging of the steel tube wall. Local buckling of the steel tube typically manifests as diamond-shaped or circumferential wrinkles, depending on the D/t ratio and axial load level. The recycled aggregate content influences the crack pattern: higher replacement ratios produce more distributed micro-cracking rather than localized macro-cracks, which paradoxically contributes to more uniform energy absorption but reduces overall stiffness.

Engineering Practice Implications

From a practical standpoint, this research validates the feasibility of incorporating recycled aggregate concrete into seismic-resistant steel tube structures without significant design modifications. Engineers should note that the steel tube thickness ratio (t/D) must be maintained at or above 0.02 to ensure adequate confinement effectiveness at higher recycled aggregate ratios. The welding quality at tube-to-base-plate connections becomes critical, as cyclic loading induces fatigue at weld toes; full-penetration butt welds with post-weld heat treatment are recommended for seismic applications. The study reinforces the principle that material substitution decisions must be evaluated within the complete structural system context rather than in isolation, and that steel tube confinement provides a robust engineering solution for integrating recycled materials into performance-based seismic design.

This research contributes meaningfully to the sustainable construction agenda by demonstrating that recycled aggregate concrete columns encased in circular steel tubes can meet seismic performance requirements with minimal degradation, offering a practical pathway for waste utilization in earthquake-prone regions where both material sustainability and structural safety are paramount concerns.