Investigating the structural performance of high-strength concrete columns under elevated temperatures: the role of BFEGC as an insulating and strengthening system

The integrity of high strength reinforced concrete (HSRC) columns is highly affected under elevated temperatures, necessitating effective strengthening techniques. This work concentrates on the performance of basalt fiber sheets and basalt fiber-based engineered cementitious composites (BFEGC) in enhancing the resilience of square HSRC columns exposed to high temperatures – cooling. Specimens were categorized into three goups: control columns (CC), columns wrapped prior to fire exposure (WBF) as insulation, and columns wrapped post-fire exposure (WAF) as a strengthening. The temperature profile followed the cellulose ISO834 fire curve for three different intensities and durations: medium (300 °C – 30 min), high (600 °C – 20 min), and very high (900 °C – 15 min). The effectiveness of both WBF and WAF was assessed, under two distinct cooling regimes: gradual cooling (GC) and rapid cooling (RC). As rapid cooling was followed in this work, the exposure time at each temperature level was carefully regulated to avoid the specimens failing completely. Columns were tested to axial load up to failure. A comprehensive thermal-structural analysis was conducted numerically with ANSYS Workbench 2023. Results from the parametric analysis indicate that the wrapping system functioned effectively as an insulator up to 600 °C, regardless of the cooling pattern employed. Improvements were recorded in parameters such as ultimate load, confinement coefficient, strength index, energy ductility, and displacement ductility. Conversely, the strengthening application was effective only for columns that experienced sudden cooling, maintaining structural integrity up to 600 °C. Additionally, implementing an effective repair technique prior to wrapping would enhance the overall capacity of columns under varying intensities, duration, and cooling regimes. Basalt fiber sheet-based reinforced engineered cementitious composites can be utilized as an insulation system for HSC – columns, irrespective of intensities of fire and gradual cooling regime. The same could be adopted as a strengthening system for HSC – columns up to high intensity of temperature under rapid cooling condition.