Axial Compression Performance of Square Steel Tube Recycled Concrete Columns After High-Temperature Water Spray Cooling

Literature Overview

The paper by Chen Zongping, Zhou Ji, Wang Cheng, and Liu Jingmin, published in the Journal of Disaster Environments (Volume 29, Issue 1, 2020, pp. 25–37), investigates the mechanical behavior of square steel tube recycled concrete (STRC) short columns subjected to high-temperature exposure followed by water spray cooling. This study was supported by the National Natural Science Foundation of China (Grant No. 51578163), the Guangxi Key R&D Program (Guike AB17292083), and the Bagui Scholar Special Fund. The research addresses the dual challenges of sustainability (recycled concrete) and fire safety (post-fire structural assessment) in modern construction.

Core Technical Points

The experimental program consisted of 27 square steel tube recycled concrete specimens subjected to various combinations of parameters:

The specimens were loaded to failure after fire exposure and cooling, and the following data were collected:

• Load-displacement curves
• Peak loads
• Axial stiffness
• Ductility index
• Energy dissipation capacity
• Visual inspection of damage patterns

Key Experimental Findings

1. Temperature Effect: As the maximum exposure temperature increased, the damage to the core concrete became more severe. Peak load and axial stiffness generally decreased with increasing temperature. However, at 800°C, the specimens exhibited the best ductility, suggesting that high temperatures cause a transition from brittle to ductile behavior in the concrete core.
2. Recycled Aggregate Replacement Ratio Effect: The peak load did not show a clear monotonic trend with increasing replacement ratio. Interestingly, specimens with 75% replacement ratio exhibited the highest peak load. Axial stiffness showed fluctuating behavior, while ductility was more noticeably affected by the replacement ratio. The energy dissipation capacity was less sensitive to the replacement ratio.
3. Cooling Method Effect: Different cooling methods had a significant effect on ductility but minimal influence on other mechanical properties such as peak load and stiffness. Water spray cooling, due to the thermal shock it induces, appears to create additional microcracking in the concrete core, which may paradoxically improve ductility by allowing more gradual crack propagation.

Comparison with Existing Codes

The authors introduced material strength reduction coefficients and compared the predicted capacities with existing codes:

The study confirms that existing code methods, when properly calibrated with appropriate strength reduction coefficients, can reasonably estimate the post-fire axial compression capacity of square steel tube recycled concrete columns.

Interpretation and Engineering Practice Integration

Steel Tube Manufacturing Considerations

The use of recycled concrete in steel tube composite members raises several manufacturing concerns:

• Recycled aggregate has higher water absorption than natural aggregate, which affects the concrete mix design and pumping characteristics during column filling
• The permeability of recycled concrete is generally higher, which may affect the long-term durability of the concrete core
• The steel tube provides confinement that partially compensates for the lower strength of recycled concrete, making the composite system more robust than the concrete alone

Welding and Fabrication Quality

For square steel tube recycled concrete columns, the following welding quality aspects are critical:

• The steel tube must be manufactured with tight dimensional tolerances to ensure proper concrete fill and confinement
• Welds at tube corners (for square sections) must be full-penetration to prevent water ingress and corrosion
• Post-weld heat treatment may be necessary for high-strength steels to relieve residual stresses that could affect post-fire behavior
• The weld quality directly affects the confinement effectiveness, which is the primary mechanism for improving post-fire performance

Thermal Shock and Material Degradation

The water spray cooling scenario is particularly relevant for post-fire structural assessment, as firefighters typically use water to cool structures. The thermal shock from rapid cooling can:

1. Induce additional cracking in the concrete core
2. Cause spalling of the concrete cover (if present)
3. Create differential thermal stresses between the steel tube and concrete core
4. Affect the bond between the steel tube and concrete

The study's finding that cooling method significantly affects ductility but not peak load is practically important for post-fire structural assessment protocols.

Key Questions and Reflections

1. The study focuses on short columns. How do the findings translate to slender columns where buckling may govern the failure mode? The interaction between thermal degradation and buckling behavior is complex and deserves dedicated investigation.
2. The recycled aggregate replacement ratio of 75% showing the highest peak load is counterintuitive. This may be related to the specific properties of the recycled aggregate used, and the finding should be validated with different sources of recycled aggregate.
3. The long-term durability of post-fire recycled concrete columns is not addressed. Repeated fire exposure cycles, or fire exposure followed by long-term service in corrosive environments, may present additional challenges.
4. The bond behavior between the steel tube and recycled concrete after fire exposure is not explicitly studied. The confinement effect depends on this bond, and its degradation could significantly affect post-fire performance.

Study Insights and Implications

This research bridges two important areas of sustainable construction: recycled materials and fire safety. The finding that existing code methods can reasonably predict post-fire capacity, when properly calibrated, is encouraging for engineering practice. For steel pipe manufacturers, the key takeaway is that steel tube concrete members using recycled aggregate require careful attention to concrete mix design, steel tube quality, and welding integrity to ensure both structural performance and fire resistance. The study also highlights the importance of considering cooling methods in post-fire structural assessment, as the choice between natural cooling and water spray cooling can significantly affect the residual ductility of the structure. Future research should extend to long-term durability studies, repeated fire exposure scenarios, and the development of specific design guidelines for recycled concrete composite members in fire-exposed applications.