Seismic Performance of Recycled Concrete-Filled Steel Tube Column-Composite Beam Spatial Frame

Overview of the Study

This research examines the seismic behavior of a three-dimensional frame structure composed of recycled aggregate concrete-filled steel tube (RCFST) columns and steel-concrete composite beams. The use of recycled concrete, incorporating 30% to 50% crushed waste concrete as coarse aggregate replacement, addresses environmental sustainability while raising concerns about mechanical property degradation and long-term durability. The spatial frame configuration introduces torsional coupling and out-of-plane effects that are absent in planar test specimens, making the seismic response more complex and realistic.

Core Technical Content

The recycled concrete typically exhibits lower compressive strength compared to natural aggregate concrete, with C40 recycled concrete achieving approximately 85% to 92% of the strength of equivalent natural aggregate C40. The steel tube confinement partially compensates for this strength reduction by providing lateral restraint that enhances the ductility and post-peak behavior of the concrete core.

Key findings from such spatial frame tests generally include:

• The overall seismic capacity of the RCFST column-composite beam frame is reduced by 8% to 15% compared to natural aggregate concrete-filled frames of identical geometry.
• The energy dissipation capacity, measured by the equivalent viscous damping ratio, decreases by approximately 10% to 18% due to the lower fracture energy of recycled concrete.
• The frame exhibits acceptable ductility, with a displacement ductility factor of 4.0 to 5.5 under peak ground acceleration of 0.40g.
• Torsional coupling effects increase the demand on diagonal columns by 15% to 25% compared to planar frame predictions.

Material and Structural Parameter Analysis

The steel tube sections used in such studies are typically square hollow sections (SHS) with dimensions ranging from 200mm × 200mm to 300mm × 300mm and wall thicknesses of 8mm to 14mm. The steel grade is Q345B per GB/T 1591, with yield strength of 345MPa and tensile strength of 470MPa to 630MPa.

Standards and Code Compliance

The design of recycled concrete-filled steel tube columns must comply with multiple standards:

A critical consideration is the confinement model used to predict the stress-strain behavior of recycled concrete within the steel tube. The Mander model, modified for recycled aggregate, is commonly employed, with the confinement strength expressed as:

f_cc = f_c0 × (1 + 2.5 × f_l / f_c0)

where f_c0 is the unconfined compressive strength of recycled concrete and f_l is the lateral confinement pressure provided by the steel tube.

Engineering Practice Integration

From a manufacturing and construction perspective, several practical considerations emerge:

1. The recycled aggregate must be properly cleaned and graded to remove adherent old mortar, which significantly affects the aggregate-mortar interface bonding. Ultrasonic pulse velocity testing per GB/T 50728 should be used to verify the quality of recycled concrete placement.
2. The steel tube fabrication process remains identical to natural aggregate CFST columns, but the concrete pumping and vibration parameters may need adjustment due to the higher viscosity of recycled concrete mixes. Slump values of 100mm to 150mm are recommended for vertical tube filling.
3. Welding quality control is unaffected by the concrete type, but the inspection schedule should be intensified at column bases and beam-column joints where inelastic deformation concentrates.
4. The spatial frame configuration requires careful attention to out-of-plane bracing and torsional restraint. Diagonal bracing or outrigger systems may be necessary to control torsional response under seismic loading.

Key Questions and Reflections

The primary question is whether the reduction in seismic capacity of RCFST frames can be fully compensated by increasing section sizes or steel thickness, or whether the fundamental material degradation of recycled concrete imposes inherent limitations. The study suggests that while moderate recycled aggregate replacement (30% to 40%) is acceptable with appropriate design adjustments, higher replacement levels (above 50%) may require supplementary measures such as fiber reinforcement or partial prestressing.

Another important reflection concerns the long-term durability of recycled concrete within steel tubes. The higher water absorption of recycled concrete may accelerate carbonation and chloride ingress, potentially reducing the corrosion resistance of the steel tube over time. This is particularly relevant for structures in marine or industrial environments where corrosion protection is a primary design concern.

Study Insights and Implications

This study demonstrates that recycled concrete-filled steel tube columns are a viable structural solution for seismic regions, provided that design parameters are appropriately adjusted for the reduced material properties. For steel pipe manufacturers, the key implication is that the structural performance of CFST columns is not solely dependent on the steel tube quality but also on the concrete infill characteristics. This necessitates close coordination between steel tube fabrication and concrete supply chains, with clear specifications on recycled aggregate quality and concrete mix design. The environmental benefits of recycled concrete use must be balanced against the modest reduction in seismic capacity, and future research should focus on optimizing recycled aggregate treatment processes to minimize property degradation.