Postfire Behavior of Circular Concrete-Filled Steel Tubular Columns with Granulated Blast Furnace Slag as Fine Aggregate in Core Concrete

This study explores the axial compression behavior of concrete-filled steel tube (CFST) and concrete-filled double-skin steel tube (CFDST) columns made with granulated blast furnace slag (GBS) as a fine aggregate following exposure to elevated temperatures. A total of 20 circular column specimens were tested, in which 10 were CFST and 10 were CFDST subjected to axial compression after being exposed to fire. The concrete mix used GBS exclusively as the fine aggregate, eliminating natural sand and supporting sustainable construction efforts by conserving natural resources. The test program included both short and intermediate-length columns, which were heated to temperatures of 200°C, 400°C, 600°C, and 800°C for 2 h. After cooling to ambient temperature, the columns were tested under axial load using a displacement-controlled setup. A detailed three-dimensional finite-element model was developed to replicate the experimental results. The findings showed minimal reduction in load-carrying capacity for specimens exposed to temperatures up to 600°C, attributed to the resilient properties of the GBS-based concrete. Distinct failure modes were noted: local bulging in short columns and Euler-type buckling in intermediate ones. The numerical model demonstrated good agreement with the experimental data, accurately simulating the structural behavior of both fire-exposed and unexposed CFST and CFDST specimens.