Zhuojin Pipe Fitting Co., LtdTheoretical Analysis and Experimental Study of Recycled Concrete-Filled Steel Tube Short Columns After High Temperature Exposure
Literature Overview
This paper by Yang Youfu and Hou Rui from the State Key Laboratory of Coastal and Offshore Engineering at Dalian University of Technology (Journal of Disaster Prevention and Mitigation Engineering, 2012, Vol. 32, No. 1, pp. 71-76) investigates the post-fire behavior of recycled concrete-filled steel tube (RCFST) short columns under axial compression. Supported by the National Natural Science Foundation (50908034) and Liaoning Provincial Natural Science Foundation (20092154), the research combines experimental testing with numerical modeling to develop design-oriented analytical models.
Experimental Program and Material Behavior
The experimental study subjects RCFST short columns to controlled high-temperature exposure followed by axial compression testing. The recycled concrete incorporates recycled coarse aggregate with varying replacement ratios, introducing inherent variability in material properties that must be accounted for in post-fire design. The steel tube provides confinement to the concrete core, and the interaction between the two materials changes significantly after thermal exposure.
| Test Parameter | Typical Range | Effect on Post-Fire Capacity |
|---|---|---|
| Recycled coarse aggregate replacement ratio | 0% to 100% | Higher ratio reduces capacity |
| Maximum exposure temperature | 200°C to 800°C | Higher temperature reduces capacity |
| Confinement effect coefficient | Variable | Higher coefficient increases capacity |
| Steel tube thickness-to-diameter ratio | Design-dependent | Higher ratio improves confinement |
The authors propose a stress-strain relationship model for the recycled concrete core after high temperature exposure that accounts for both the recycled aggregate replacement ratio and the maximum experienced temperature. This model captures the degradation of concrete properties due to thermal damage while recognizing the inherent variability introduced by recycled materials.
Numerical Analysis and Simplified Formulas
The numerical method used to analyze the complete load-deformation response of RCFST short columns after high temperature exposure is validated against experimental results. The numerical model captures the nonlinear interaction between the degrading concrete core and the steel tube, including the progressive loss of concrete strength and the potential for steel tube buckling under elevated temperatures.
The parametric study systematically examines the influence of recycled aggregate replacement ratio, maximum exposure temperature, and confinement effect coefficient on the axial compression strength factor. Based on these parametric results, simplified calculation formulas are proposed for practical design use. The simplified formulas demonstrate good agreement with both experimental and numerical results, making them suitable for engineering application.
Design Implications for Steel Pipe Selection
From a steel pipe engineering perspective, this research has several important implications:
- Steel grade selection for post-fire applications must account for the degradation of steel properties at elevated temperatures, particularly the reduction in yield strength above 400°C.
- The confinement effectiveness of the steel tube depends on maintaining adequate wall thickness and preventing local buckling, which becomes more critical after thermal exposure.
- Weld quality in tubular columns becomes more important for post-fire performance, as weld defects may concentrate stresses in the thermally weakened material.
The research also highlights the growing importance of recycled materials in construction, which introduces additional complexity in predicting post-fire behavior due to the inherent variability of recycled concrete properties.
Study Reflections
This research contributes significantly to the understanding of fire performance in composite structures utilizing recycled materials, which aligns with sustainable construction goals. The development of simplified design formulas based on rigorous numerical analysis provides practical tools for engineers designing RCFST members in fire-exposed applications. For steel pipe manufacturers, the findings underscore the importance of providing consistent material properties and adequate quality documentation, as the post-fire behavior of composite members is highly sensitive to both concrete and steel material characteristics. The work also demonstrates that recycled materials, when properly characterized and designed for, can perform adequately in demanding structural applications including fire-exposed conditions.