Post-Fire Mechanical Performance of Square Steel Tube Recycled Aggregate Concrete Eccentric Columns

Literature Overview

This experimental study by Chen Zongping, Zhou Wenxiang, Xue Jianyang, and Li Ling, published in Industrial Construction (2014, Vol. 44, No. 11, pp. 25-31), investigates the mechanical behavior of square steel tube recycled aggregate concrete (SRAC) eccentric columns after exposure to high temperatures. Funded by the National Natural Science Foundation (Grant No. 51268004) and several Guangxi Provincial research programs, the research is conducted at Guangxi University and Xi'an University of Architecture and Technology. Nine specimens were subjected to fire exposure followed by monotonic static loading, with recycled aggregate replacement rate and maximum exposure temperature as the two variable parameters.

Core Technical Content and Experimental Findings

The experimental program systematically examines how fire exposure affects the post-fire structural performance of SRAC eccentric columns. The specimens are square steel tube columns filled with recycled aggregate concrete (RAC), loaded under eccentric compression after being heated to various temperatures and then cooled. The test parameters include recycled coarse aggregate replacement rates and maximum exposure temperatures ranging from ambient to 800 degrees Celsius.

The failure process and modes of post-fire SRAC eccentric columns are found to be similar to those of conventional steel tube concrete (SRC) eccentric columns, progressing through elastic, yielding, and failure stages. However, the temperature significantly affects the ultimate bearing capacity and compressive stiffness, while the recycled aggregate replacement rate has minimal influence on the mechanical properties.

Test Matrix and Key Results

Interpretation of Technical Points

The finding that recycled aggregate replacement rate has minimal influence on post-fire mechanical properties is particularly noteworthy. This suggests that the interfacial transition zone (ITZ) between recycled aggregates and cement paste, which is typically the weakest link in RAC, does not become the dominant failure mechanism under post-fire conditions. Instead, the temperature-induced degradation of the cement paste matrix and the steel tube governs the post-fire behavior.

The significant reduction in bearing capacity and stiffness with increasing temperature is attributed to multiple mechanisms: degradation of concrete compressive strength due to decomposition of calcium hydroxide and loss of internal water, thermal cracking of concrete, reduction of steel yield strength at elevated temperatures, and potential buckling of steel tube walls due to thermal gradients and loss of concrete support.

Temperature Effects on Material Properties

Integration with Engineering Practice

From a steel tube manufacturing perspective, this research highlights the importance of steel tube material selection for fire-prone structures. The steel tube serves as both a structural element and a fire protection barrier for the concrete core. The steel grade and wall thickness must be selected to maintain adequate structural capacity at elevated temperatures. For critical structures in fire-prone environments, fire-resistant steel grades with better high-temperature strength retention may be warranted.

The welding connections in SRAC columns are particularly vulnerable to fire exposure. Welded joints typically have lower heat resistance than the base metal due to the heat-affected zone (HAZ) microstructural changes. Post-fire inspection and assessment of welded connections should be a standard part of fire damage evaluation protocols. According to GB 51249 and ISO 22899, post-fire structural assessment should include visual inspection, ultrasonic testing of welds, and material property verification at representative locations.

Post-Fire Assessment Protocol for SRAC Columns

Key Questions and Reflections

A critical practical question is the residual capacity assessment methodology. The experimental data provides ultimate capacity values for specific temperature exposures, but real fire scenarios involve complex temperature distributions with varying exposure times. The development of reliable residual capacity prediction models that account for fire exposure duration, temperature gradient, and cooling conditions is essential for practical post-fire assessment.

The minimal influence of recycled aggregate replacement rate on post-fire performance raises an important question about the applicability of RAC in fire-prone structures. If the post-fire behavior of RAC is comparable to conventional concrete, then RAC can be confidently used in fire-prone applications without additional fire design considerations. However, this conclusion should be verified with larger-scale tests and full-scale structural tests before widespread adoption.

The study focuses on monotonic loading after fire exposure, but real structures may experience cyclic loading (such as seismic events) after fire damage. The interaction between fire-induced degradation and cyclic loading capacity is a complex issue that requires further investigation. The post-fire ductility reduction observed in this study suggests that fire-damaged SRAC columns may be more vulnerable to seismic damage.

Study Insights and Implications

This research provides valuable experimental data on the post-fire behavior of SRAC eccentric columns, demonstrating that temperature is the dominant factor affecting residual structural capacity while recycled aggregate replacement rate has negligible influence. The findings support the use of recycled aggregate concrete in steel tube concrete columns for fire-prone applications, provided that appropriate fire design considerations are incorporated. For steel pipe engineers, the key implication is that steel tube material selection and weld quality are critical for maintaining structural integrity during and after fire exposure, reinforcing the need for strict material control and welding quality assurance in fire-prone construction.