Zhuojin Pipe Fitting Co., LtdComparative Analysis of Axial Compressive Performance of Square and Circular Recycled Concrete-Filled Steel Tube Short Columns After High-Temperature Water-Spray Cooling
Literature Overview
This paper by Chen Zongping and Zhou Ji from Guangxi University, published in Industrial Construction (2020, Vol. 50, No. 2, pp. 150–157), investigates the axial compressive performance of square and circular recycled concrete-filled steel tube (R-CFST) short columns after exposure to high temperatures followed by water-spray cooling. The research was supported by the National Natural Science Foundation of China (Grant No. 51578163), the Guangxi Natural Science Foundation Key Project (Grant No. 2016GXNSFDA380032), and the Bagui Scholar Special Fund Project ([2019]79). The study involved axial compression tests on 27 square and 27 circular R-CFST specimens with varying maximum exposure temperatures, recycled coarse aggregate replacement ratios, and cooling methods.
Core Technical Points
Research Background and Motivation
The fire resistance of concrete-filled steel tube (CFST) columns is a critical design consideration for structural safety. When CFST columns are exposed to high temperatures during a fire, the mechanical properties of both the steel tube and the concrete core degrade, leading to a reduction in load-bearing capacity and ductility. Water-spray cooling is a common fire protection measure that can reduce the temperature of the structural member after the fire is extinguished. However, the cooling process itself can introduce additional stresses and potential damage to the structure, particularly in composite members with different thermal expansion coefficients.
The use of recycled aggregate concrete (RAC) in CFST columns adds another layer of complexity because RAC has different thermal properties and post-fire behavior compared to conventional concrete. The recycled aggregate particles have a higher porosity and water absorption, which can affect the thermal conductivity, thermal expansion, and post-fire strength retention of the concrete.
Experimental Program
The experimental program involved the following parameters:
| Parameter | Levels | Description |
|---|---|---|
| Cross-section shape | Square, Circular | Two geometric configurations |
| Maximum exposure temperature | Multiple levels | Varying the fire severity |
| Recycled coarse aggregate replacement ratio | Multiple levels | Varying the recycled content |
| Cooling method | Water-spray, Natural cooling | Two cooling approaches |
A total of 54 specimens (27 square + 27 circular) were tested under axial compression after the thermal exposure and cooling procedures.
Key Findings
The study reveals several important findings:
- Effect of cooling method on square specimens: The initial axial stiffness and energy dissipation capacity of square specimens are significantly affected by the cooling method. Water-spray cooling introduces additional thermal stresses that can reduce the initial stiffness and energy dissipation capacity.
- Effect of cooling method on circular specimens: The load-bearing capacity and ductility of circular specimens are significantly affected by the cooling method. Water-spray cooling can reduce the load-bearing capacity and ductility of circular specimens.
- Temperature effect on square specimens: As the maximum exposure temperature increases, the load-bearing capacity degradation, ductility variation, and energy dissipation capacity variation of square specimens are more pronounced compared to circular specimens.
- Temperature effect on circular specimens: Circular specimens exhibit more stable performance degradation with increasing temperature compared to square specimens.
- Initial axial stiffness: At lower temperatures, square specimens exhibit greater initial axial stiffness degradation compared to circular specimens. At higher temperatures, the trend reverses, with circular specimens showing greater stiffness degradation.
- Effect of recycled aggregate replacement ratio: The replacement ratio has a greater effect on the load-bearing capacity of square specimens and the initial axial stiffness of circular specimens. As the replacement ratio increases, the ductility coefficient and energy dissipation coefficient of square specimens increase, while those of circular specimens decrease.
Comparative Analysis of Square vs. Circular Sections
| Parameter | Square Specimens | Circular Specimens |
|---|---|---|
| Cooling method sensitivity | Initial stiffness, energy dissipation | Load capacity, ductility |
| Temperature sensitivity | More pronounced degradation | More stable degradation |
| Replacement ratio effect | Load capacity, ductility, energy dissipation | Initial stiffness, ductility, energy dissipation |
| Overall performance | More variable, sensitive to parameters | More stable, predictable behavior |
Engineering Practice Implications
From a steel tube fabrication and welding perspective, this research has several important implications:
- Welding and fire exposure: The welding process introduces residual stresses and heat-affected zone (HAZ) modifications in the steel tube. These pre-existing conditions can interact with the thermal stresses introduced during fire exposure, potentially leading to crack initiation or propagation. For welded tubes (ERW, HFW, LSAW, SSAW), the weld seam represents a region of potential weakness where the combined effects of residual stresses, HAZ modifications, and fire-induced thermal stresses can lead to premature failure.
- Post-fire inspection and repair: The study highlights the importance of post-fire inspection and assessment of CFST columns. The water-spray cooling process can introduce additional damage that may not be visible on the exterior of the column. Non-destructive testing (NDT) methods such as ultrasonic testing (UT), magnetic particle testing (MT), and radiographic testing (RT) should be employed to assess the internal condition of the column after fire exposure and cooling.
- Material selection for fire-prone applications: The study suggests that circular CFST columns exhibit more stable and predictable behavior under fire exposure and water-spray cooling compared to square CFST columns. For applications where fire resistance is a critical design consideration, circular CFST columns may be preferred over square CFST columns, particularly when using recycled aggregate concrete.
- Welding procedure optimization for fire-prone structures: For structures in fire-prone environments, the welding procedure should be optimized to minimize residual stresses and HAZ modifications. Low-heat-input welding processes, such as gas tungsten arc welding (GTAW) or laser welding, may be preferred over high-heat-input processes such as shielded metal arc welding (SMAW) or flux-cored arc welding (FCAW). Post-weld heat treatment (PWHT) may also be considered to relieve residual stresses and improve the toughness of the HAZ.
- Quality control of recycled aggregate concrete: The quality of the recycled aggregate concrete is critical for the post-fire performance of the CFST column. The recycled aggregate must be properly cleaned, graded, and stored to ensure consistent quality. The water absorption and porosity of the recycled aggregate should be measured and controlled to ensure that the thermal properties of the concrete are within acceptable limits.
- Design considerations for recycled CFST columns: The study provides valuable data for the design of recycled CFST columns in fire-prone environments. The design should account for the reduced load-bearing capacity, ductility, and energy dissipation capacity of the column after fire exposure and cooling. The design should also consider the potential for additional damage introduced by the water-spray cooling process.
Study Insights and Reflections
This paper provides valuable experimental data on the post-fire performance of recycled CFST columns with different cross-section shapes and cooling methods. The most significant finding is the differential sensitivity of square and circular specimens to the various parameters — temperature, replacement ratio, and cooling method. This finding has important implications for the design of recycled CFST columns in fire-prone environments.
From a metallurgical and welding engineering perspective, the paper highlights the importance of considering the combined effects of welding, fire exposure, and cooling on the structural performance of CFST columns. The welding process introduces residual stresses and HAZ modifications that can interact with the thermal stresses introduced during fire exposure, potentially leading to crack initiation or propagation. The water-spray cooling process can introduce additional thermal stresses that may exacerbate the damage caused by the fire exposure.
The paper also underscores the importance of quality control in the fabrication of recycled CFST columns. The quality of the recycled aggregate, the welding quality of the steel tube, and the quality of the concrete infill all contribute to the overall structural performance of the column. Any compromise in quality at any stage of the fabrication process can lead to a reduction in the post-fire performance of the column.
The experimental data presented in this paper should be incorporated into design codes and standards for recycled CFST columns in fire-prone environments. The data provide a basis for developing design formulas and safety factors that account for the reduced performance of recycled CFST columns after fire exposure and cooling. Further research is needed to investigate the long-term durability of recycled CFST columns after fire exposure and cooling, particularly in terms of corrosion resistance and fatigue performance.