Zhuojin Pipe Fitting Co., LtdSeismic Performance of Recycled Concrete-Filled Steel Tube Column-Composite Beam Spatial Frame Structures
Literature Overview
This study note addresses the seismic performance of spatial frame structures composed of recycled concrete-filled steel tube (RCFST) columns and composite beams. With the global construction industry facing increasing pressure to reduce waste and carbon emissions, recycled aggregate concrete (RAC) has emerged as a viable alternative to conventional natural aggregate concrete. The literature investigates how the substitution of natural aggregates with recycled concrete aggregates affects the seismic behavior of RCFST column-composite beam frame systems, providing critical data for the seismic design of sustainable building structures.
Core Technical Content
Recycled concrete aggregate, obtained from the crushing and processing of demolished concrete structures, typically contains residual adhering mortar that reduces its mechanical properties compared to natural aggregate. The literature evaluates recycled aggregate replacement ratios of 30%, 60%, and 100% by mass in the concrete used to fill steel tubes in frame columns. The steel tubes are fabricated from Q355B steel with outer diameters of 219 mm and wall thicknesses of 8 mm, while the composite beams consist of steel I-sections with concrete topping.
Material Properties of Recycled Concrete
The mechanical properties of recycled concrete at different replacement ratios are summarized as follows:
| Recycled Aggregate Replacement | Compressive Strength (MPa) | Elastic Modulus (GPa) | Split Tensile Strength (MPa) |
|---|---|---|---|
| 0% (Natural aggregate) | 48.5 | 38.2 | 4.2 |
| 30% | 45.2 | 36.5 | 3.9 |
| 60% | 40.8 | 33.1 | 3.5 |
| 100% | 35.6 | 29.8 | 3.1 |
The reduction in compressive strength at 100% replacement is approximately 27%, primarily attributed to the weaker interfacial transition zone (ITZ) between the recycled aggregate and the new cement paste. The residual mortar on recycled aggregate particles creates a porous and weak zone that reduces load transfer efficiency.
Seismic Testing Methodology and Results
The literature describes the pseudo-static cyclic loading tests performed on full-scale RCFST column-composite beam subassemblages. The specimens are subjected to displacement-controlled cyclic loading with drift ratios up to 4%, simulating seismic loading conditions. Key performance indicators include:
- Peak load capacity
- Energy dissipation capacity (calculated from the area enclosed by the load-displacement hysteresis loops)
- Ductility coefficient (ratio of ultimate displacement to yield displacement)
- Stiffness degradation rate
- Cumulative damage index
| Recycled Aggregate Replacement | Peak Load (kN) | Energy Dissipation (kN·m) | Ductility Coefficient | Stiffness Degradation at 3% Drift |
|---|---|---|---|---|
| 0% | 1850 | 42500 | 3.8 | 62% |
| 30% | 1790 | 40200 | 3.6 | 65% |
| 60% | 1720 | 37800 | 3.4 | 68% |
| 100% | 1650 | 35200 | 3.2 | 72% |
The data indicate that while recycled aggregate replacement reduces the peak load and energy dissipation capacity, the ductility characteristics remain acceptable, with all specimens maintaining ductility coefficients above 3.0, which exceeds the minimum requirement of 2.5 specified by most seismic design codes.
Failure Modes and Damage Progression
The literature documents the failure modes observed during cyclic testing:
- At low drift ratios (below 1%), the frame exhibits elastic behavior with no visible damage.
- Between 1% and 2% drift, flexural yielding initiates in the composite beam near the column face, accompanied by the first visible cracks in the concrete topping.
- At 2% to 3% drift, the RCFST column begins to exhibit local buckling of the steel tube, with concrete crushing visible at the column ends. The recycled concrete specimens show earlier crack initiation but comparable crack propagation rates.
- Beyond 3% drift, progressive damage accumulation occurs with significant concrete spalling, steel tube yielding, and bond deterioration at the steel-concrete interface. The 100% recycled aggregate specimens exhibit slightly more extensive spalling due to the lower tensile strength of the recycled concrete.
Interface Bond Behavior
The bond strength between the recycled concrete and the steel tube is a critical parameter affecting the composite action of the RCFST column. The literature reports:
- Average bond stress at 0% replacement: 3.8 MPa
- Average bond stress at 30% replacement: 3.5 MPa
- Average bond stress at 60% replacement: 3.1 MPa
- Average bond stress at 100% replacement: 2.7 MPa
The reduction in bond strength is attributed to the higher porosity and lower stiffness of recycled concrete, which reduces the contact pressure at the steel-concrete interface. However, the bond strength values remain above the minimum design values specified in relevant codes.
Engineering Design Implications
For the seismic design of RCFST column-composite beam frames, the literature recommends the following:
- Recycled aggregate replacement up to 60% can be adopted without significant compromise in seismic performance, provided that the design accounts for the reduced material properties through appropriate strength reduction factors.
- The 100% recycled aggregate option is feasible for low-to-moderate seismic zones but should be used with caution in high seismic zones where superior energy dissipation is required.
- Supplemental reinforcement in the form of steel fibers (0.5–1.0% by volume) can partially compensate for the reduced tensile properties of recycled concrete and improve post-crack energy absorption.
- The connection design between RCFST columns and composite beams should incorporate shear keys or dowel bars to enhance the moment transfer capacity and prevent premature separation.
Study Insights and Conclusion
This literature makes a significant contribution to the field of sustainable structural engineering by demonstrating that recycled concrete-filled steel tube columns can maintain satisfactory seismic performance even at high recycled aggregate replacement ratios. The systematic evaluation of material properties, seismic behavior, failure modes, and interface bond characteristics provides a comprehensive foundation for the seismic design of RCFST frames. The finding that 60% recycled aggregate replacement represents a practical and safe threshold for most seismic applications offers engineers a clear design guideline that balances environmental sustainability with structural safety. As the construction industry moves toward circular economy principles, this work provides the technical evidence needed to support the widespread adoption of recycled materials in seismic-resistant structural systems. The emphasis on composite action and interface behavior addresses a critical aspect that has been under-investigated in previous research on recycled concrete structures.