Mechanical Behavior of Eccentrically Loaded Square CFST Columns with Recycled Concrete After High Temperature Exposure

Literature Overview

This paper by Chen Zongping, Zhou Wenxiang, Xue Jianyang, and Li Ling, published in the Industrial Construction journal in 2014 (Vol. 44, No. 11, pp. 25-31), investigates the post-fire mechanical performance of eccentrically loaded square concrete-filled steel tubular (CFST) columns incorporating recycled coarse aggregate concrete. Funded by multiple research grants including the National Natural Science Foundation of China (Grant 51268004), the study examines nine test specimens subjected to fire exposure followed by monotonic static loading.

Core Technical Content

Experimental Program

The experimental matrix investigates two primary variables:

Each specimen was a square CFST column subjected to:

1. Controlled fire exposure to achieve target peak temperature
2. Natural cooling to ambient temperature
3. Eccentric monotonic static loading to failure

Test Results and Key Findings

The study reveals several important patterns in the post-fire behavior:

Failure characteristics:

• Failure process and mode are similar to normal (non-fire-exposed) CFST columns
• Three distinct stages observed: elastic stage, yielding stage, and failure stage
• Progressive degradation of load-bearing capacity with increasing temperature

Temperature effects:

Recycled aggregate effects:

• Minimal influence on mechanical properties compared to temperature effects
• Recycled aggregate replacement rate has negligible impact on post-fire performance
• This finding is significant for promoting recycled material use in structural applications

Cross-Sectional Strain Distribution

The measurement of cross-sectional strain distributions provides insight into the composite action after fire exposure:

1. Steel tube experiences higher strains due to higher thermal expansion coefficient
2. Concrete core shows non-uniform strain distribution due to eccentric loading and thermal gradients
3. Interface behavior between steel and concrete is affected by differential thermal expansion

Engineering Practice Implications

Fire Design Considerations for CFST Columns

The findings have direct implications for fire design of CFST structures:

Steel Pipe Selection for Fire-Exposed Applications

For steel tubes in fire-prone environments:

• Steel grade: Higher strength steels may experience proportionally greater strength loss at elevated temperatures. Q345 steel maintains better post-fire performance characteristics compared to higher grades.
• Wall thickness: Thicker walls provide better thermal mass and slower temperature rise in the concrete core, potentially preserving more capacity.
• Protective coatings: Intumescent coatings or fire-resistant wraps can delay temperature rise in the steel tube, preserving structural capacity longer during fire exposure.

Recycled Concrete in Structural Applications

The finding that recycled aggregate replacement rate has minimal impact on post-fire performance is particularly significant:

1. Sustainability: This supports the use of recycled aggregates in structural applications without significant fire performance penalties.
2. Material specification: Recycled concrete can be specified for CFST applications with confidence in post-fire behavior.
3. Quality control: Standard quality control procedures for recycled aggregate concrete are sufficient for fire-exposed applications.

Key Questions and Reflections

Temperature Thresholds and Design Limits

The study provides data on temperature effects, but several questions remain:

1. Critical temperature: What is the maximum temperature beyond which structural integrity cannot be assured? Typical critical temperatures for steel structures range from 500°C to 600°C.
2. Cooling rate effects: The study uses natural cooling, but rapid cooling (water cooling) may introduce thermal shock effects not captured.
3. Duration effects: The study considers peak temperature exposure, but prolonged exposure at lower temperatures may produce different degradation patterns.
4. Multiple fire cycles: Structures may be exposed to multiple fire events over their service life; cumulative damage effects are not addressed.

Interface Behavior After Fire

The steel-concrete interface is particularly critical after fire exposure:

• Differential thermal expansion may cause debonding at the interface
• The bond strength degradation with temperature is not explicitly quantified in this study
• Interface debonding could reduce composite action effectiveness in post-fire assessment

Study Insights and Engineering Recommendations

This research provides valuable data on the post-fire behavior of CFST columns with recycled concrete, confirming that these structures maintain acceptable performance characteristics after fire exposure. The similar failure modes to ambient temperature conditions suggest that existing design methodologies can be adapted with appropriate temperature-dependent reduction factors.

The finding that recycled aggregate replacement rate has minimal impact on post-fire performance is particularly encouraging for sustainable construction practices. Engineers can confidently specify recycled concrete for CFST applications without significant concerns about fire performance degradation.

For practical engineering, the key recommendations are:

1. Apply temperature-dependent strength and stiffness reduction factors in fire design of CFST columns.
2. Ensure adequate steel tube wall thickness to provide thermal protection to the concrete core.
3. Consider fire-resistant protective measures for critical structural elements.
4. Use recycled aggregate concrete with confidence in fire-exposed applications, subject to standard quality control requirements.

The work contributes to the growing body of knowledge on sustainable structural materials and their performance under extreme loading conditions, supporting the transition toward more environmentally responsible construction practices.