Post-Fire Behavior of Steel Tube Recycled Concrete Short Columns

Literature Overview

The study by Yang Youfu and Hou Rui from Dalian University of Technology, published in the Journal of Disaster Prevention and Mitigation Engineering in 2012, investigates the mechanical behavior of steel tube recycled concrete (STRC) short columns subjected to elevated temperatures. This research is particularly significant given the growing emphasis on sustainable construction materials and the increasing concern for structural fire safety. The authors combined experimental testing with numerical analysis to develop predictive models for post-fire structural performance.

Experimental Program and Material Characterization

The experimental program involved testing STRC short columns after exposure to various elevated temperatures, simulating realistic fire scenarios. The recycled coarse aggregate (RCA) replacement ratio and maximum exposure temperature were the primary variables. The steel tube provided confinement to the core recycled concrete, and the interaction between these components under post-fire conditions was the central focus of the investigation.

Test Matrix and Parameters

The test results demonstrated that both RCA replacement ratio and exposure temperature significantly reduce the axial compressive strength of STRC columns. However, the steel tube confinement effect partially compensates for the strength loss, with the degree of compensation depending on the confinement coefficient (D/t ratio).

Stress-Strain Model Development

Based on the experimental data, the authors proposed a constitutive model for the core recycled concrete after fire exposure. This model incorporates both the RCA replacement ratio and the maximum experienced temperature as input parameters, providing a comprehensive description of the material behavior throughout the loading process.

Post-Fire Concrete Stress-Strain Relationship

The proposed model accounts for the following phenomena observed in the tests:

1. Reduced peak strength proportional to temperature exposure and RCA content
2. Modified ascending branch reflecting the degraded elastic modulus
3. Altered descending branch due to changes in the fracture characteristics of recycled concrete
4. Confinement enhancement from the steel tube, which becomes more pronounced at higher temperatures due to differential thermal expansion

The model was validated against experimental results and showed good agreement across the full range of tested parameters. The numerical analysis confirmed that the model accurately captures the load-deformation behavior of STRC short columns throughout the entire loading process.

Parametric Analysis and Simplified Formulas

Using the validated numerical model, the authors conducted a systematic parametric study examining the influence of RCA replacement ratio, maximum temperature, and confinement coefficient on the axial compressive strength coefficient of STRC short columns after fire exposure.

Parametric Study Key Findings

The parametric study revealed that temperature is the most influential factor on post-fire strength, with RCA replacement ratio providing a secondary but still significant reduction. The confinement effect from the steel tube is particularly beneficial at higher temperatures, where the differential thermal expansion between steel and concrete creates additional confinement pressure.

Simplified Calculation Formulas

The authors developed simplified formulas for predicting the axial compressive capacity of STRC short columns after fire exposure. These formulas were validated against experimental results and showed acceptable accuracy for practical design purposes. The simplified approach eliminates the need for complex numerical analysis while maintaining engineering accuracy.

Welding and Fabrication Considerations for Steel Tubes

From a welding and fabrication perspective, the steel tube used in STRC columns must meet specific requirements to ensure reliable confinement behavior, particularly after fire exposure. The following considerations are essential:

Steel Tube Fabrication Requirements

1. Seamless steel tubes are preferred over welded tubes for fire-exposed applications, as weld defects can propagate more readily under thermal cycling.
2. If welded tubes are used, the longitudinal weld must be fully inspected using UT or ET to ensure absence of laminations or incomplete fusion.
3. The steel tube ends must be properly capped or sealed to prevent concrete leakage during placement and to maintain confinement integrity.
4. Any field welding required for tube splicing must use processes that minimize residual stress, such as low-heat-input GTAW or PAW welding.

Post-Fire Weld Integrity Assessment

Engineering Practice Implications

The findings of this study have important implications for the use of recycled concrete in structural applications, particularly in regions where fire safety is a critical design consideration. The results suggest that STRC columns can maintain acceptable post-fire performance when properly designed with adequate steel tube confinement.

For engineers specifying steel tubes for STRC applications, the following recommendations emerge from this research:

• Select steel tubes with appropriate wall thickness to provide sufficient confinement, with D/t ratios of 40-60 being optimal for balanced strength and confinement
• Use steel grades with adequate post-fire strength retention, such as Q235 or Q345, avoiding high-strength steels that may experience more significant strength loss
• Ensure that tube fabrication quality is maintained to prevent premature failure of the confinement mechanism
• Consider the long-term durability implications of recycled concrete, including permeability and chloride ingress, which may be exacerbated by fire damage

Key Reflections and Study Insights

This research contributes significantly to the understanding of sustainable structural systems and their fire performance. The use of recycled aggregate in structural concrete is an important step toward reducing construction waste and carbon footprint, but it introduces additional variability in material properties that must be accounted for in design.

The development of practical constitutive models and simplified formulas represents a bridge between academic research and engineering practice, enabling designers to incorporate post-fire performance considerations into their structural designs without requiring advanced computational resources. The confinement effect of the steel tube is demonstrated to be a valuable mitigation strategy for the strength reduction caused by both recycled aggregate and fire exposure.

From a metallurgical perspective, the post-fire behavior of the steel tube itself is an important consideration that deserves further investigation. The microstructural changes in the steel tube after exposure to elevated temperatures, particularly in the heat-affected zones of any welds, can affect the confinement effectiveness and overall column performance. Future research should integrate metallographic analysis of the steel tube after fire exposure to provide a more complete picture of the system behavior.

In summary, steel tube recycled concrete columns offer a promising sustainable structural solution with acceptable post-fire performance when properly designed and fabricated, provided that the combined effects of material degradation and confinement are accurately predicted and accounted for.