Zhuojin Pipe Fitting Co., LtdMechanical Properties of Recycled Concrete-Filled Square Steel Tubes After High Temperature
Literature Overview
This study, published in the Journal of Shenyang University of Technology (2015, Vol. 37, No. 3), investigates the post-fire mechanical behavior of concrete-filled square steel tube (CFST) short columns containing recycled aggregate concrete under eccentric compression. Using ABAQUS finite element software, the authors analyzed the effects of exposure temperature, recycled aggregate replacement rate, and eccentricity ratio on residual load-bearing capacity. The research was funded by the National Natural Science Foundation of China (Grant No. 51308347) and Shenyang Science and Technology Project (F13171900).
Core Technical Findings
The parametric study revealed three distinct degradation mechanisms:
- Temperature effect: Ultimate load-bearing capacity decreases progressively with increasing exposure temperature, reflecting the combined degradation of both steel tube material properties and recycled concrete strength.
- Replacement rate effect: When subjected to the same fire exposure, increasing recycled aggregate replacement rate causes a gradual but not dramatic reduction in ultimate load capacity, indicating that recycled aggregate content has a secondary influence compared to temperature.
- Eccentricity effect: Residual load-bearing capacity decreases significantly with increasing eccentricity ratio, demonstrating that eccentric loading amplifies the adverse effects of fire exposure on CFST members.
Technical Parameter Analysis
| Parameter | Range Studied | Effect on Residual Capacity | Relative Severity |
|---|---|---|---|
| Exposure temperature | Multiple levels | Progressive capacity reduction | Primary degradation factor |
| Recycled aggregate replacement rate | Multiple rates | Gradual capacity decrease | Moderate effect |
| Eccentricity ratio | Increasing values | Significant capacity reduction | Amplifying factor |
Welding and Material Considerations
From a welding engineering perspective, this study has important implications for the design and fabrication of CFST structural members in fire-exposed environments. The steel tube material properties after fire exposure directly affect the residual mechanical behavior of welded joints connecting these members. Engineers must consider that:
- Steel tubes typically lose yield strength above 500°C, with significant ductility degradation above 600°C
- The thermal expansion mismatch between steel tubes and concrete cores can induce additional stresses at welded connections
- Recycled concrete's higher water content and porous structure may cause more severe spalling during fire exposure, potentially exposing steel tube surfaces to direct flame contact
- Post-fire welding repair procedures must account for the altered material properties of both parent metal and heat-affected zones
Engineering Practice Implications
For structural engineers designing CFST members in fire-exposed applications, this research provides critical data for post-fire assessment and rehabilitation decisions. The finding that eccentricity significantly amplifies fire damage effects suggests that members subjected to combined axial and bending loads require more conservative fire protection measures. In practical terms, this means that column members at building corners or near openings — which are more likely to experience eccentric loading under fire-induced deformation — should receive enhanced fire-resistant protection.
Study Insights
This literature contributes valuable quantitative data for the fire engineering design of recycled concrete-filled steel tube structures. The use of finite element modeling validated against experimental results provides a reliable analytical tool for predicting post-fire behavior. The relatively minor effect of recycled aggregate replacement rate on post-fire performance is encouraging for sustainable construction practices, suggesting that recycled concrete can be used in CFST members without substantially compromising fire resilience. However, engineers should note that the study focuses on short columns; slender CFST members may exhibit different post-fire behavior due to increased buckling sensitivity.