Mechanical Properties of Recycled Aggregate Concrete-Filled Steel Tube Columns After High Temperature Exposure

Literature Overview and Research Context

The paper by Chen Zongping, Liang Houren, and Wang Cheng, published in Fire Science and Technology in 2018 (Vol. 37, No. 4, pp. 435-439), investigates the post-fire mechanical behavior of concrete-filled steel tube (CFST) columns incorporating recycled coarse aggregate. Funded by the National Natural Science Foundation of China and Guangxi Natural Science Foundation, this research addresses a critical gap in fire engineering: the combined effect of elevated temperature exposure and recycled aggregate substitution on structural performance. The study is particularly timely given the growing adoption of recycled aggregates in construction and the increasing demand for fire-resistant structural systems.

Experimental Program and Test Methodology

The authors conducted eccentric compression tests on 18 CFST column specimens after exposure to various elevated temperatures. The specimens included both circular and square hollow section (SHS) steel tubes filled with recycled aggregate concrete (RAC). The test matrix varied two primary factors: the exposure temperature (ranging from ambient to approximately 800 degrees Celsius) and the recycled coarse aggregate replacement rate (typically 0%, 30%, 50%, 70%, and 100%).

The mechanical performance indicators measured included ultimate load capacity, peak deformation, displacement ductility coefficient, and energy dissipation coefficient. These parameters collectively characterize the load-bearing capacity, deformability, and seismic resilience of the columns after fire exposure.

Key Findings and Technical Analysis

The results reveal important trends in the degradation behavior of CFST columns:

The finding that temperature has a more pronounced effect on circular steel tube columns than on square sections is physically intuitive. Circular sections provide more uniform confinement to the concrete core, and the steel tube itself experiences more uniform thermal expansion. However, when the steel tube loses strength due to high temperature, the confinement effect diminishes rapidly, leading to accelerated concrete spalling and strength loss.

The observation that recycled aggregate replacement rate has a greater effect on square sections is attributed to the stress concentration at the corners of square tubes. The corners of square sections create localized high-stress zones in the concrete core, and recycled aggregates, which have higher porosity and lower interfacial transition zone (ITZ) strength, are more susceptible to damage under these concentrated stresses.

Engineering Practice Implications

From a structural design perspective, this research has several important implications. First, the use of recycled aggregate concrete in CFST columns for fire-exposed structures requires careful consideration of the replacement rate and exposure temperature. Design codes should incorporate degradation factors that account for both temperature and recycled aggregate content.

Second, the interaction between steel tube geometry and concrete composition means that structural engineers cannot simply apply degradation curves from conventional concrete-filled steel tubes to recycled aggregate variants. Separate design provisions are warranted.

Third, for seismic design of CFST structures in regions with significant fire risk, the energy dissipation coefficient data provides essential input for performance-based design approaches. The reduction in ductility after fire exposure must be considered in post-fire structural assessment protocols.

Study Insights and Recommendations

This research contributes meaningfully to the understanding of fire performance of sustainable construction materials. However, I would recommend that future studies extend the temperature range to include lower temperatures (200-500 degrees Celsius) where the transition between elastic and plastic behavior of the steel tube occurs, as this range is critical for many fire scenarios.

Additionally, the research would benefit from incorporating residual strength measurements after cooling, as post-fire structural assessment typically requires knowledge of the cooled condition. The thermal cracking patterns in recycled aggregate concrete may differ from those in conventional concrete due to the heterogeneous nature of the recycled material.

This paper provides a solid foundation for developing design guidelines for recycled aggregate CFST structures in fire-exposed environments, and its findings should inform future revisions to fire design codes.