Microstructure investigation, strength assessment, and thermal modelling of concrete exposed to different heating cooling regimes

Building fires are sometimes unavoidable due to unexpected accidents. During the fire, the structural elements are exposed to temperatures above 1000 °C. Structural concrete loses its strength and stiffness during the heating and cooling cycle. In most cases, during a real fire, the structural elements are cooled by water quenching. It is rare scenario that, buildings subjected to fire are cooled by natural air without forced water spraying. However, structural concrete’s residual strength (RS) in both cooling cases after the fire incident is entirely different. Therefore, an experimental study was conducted to understand concrete’s heating–cooling (H–C) behaviour subjected to standard fire temperature. Concrete with different grades, namely C20, C30, C40, and C50, is considered in the present investigation. Specimens were subjected to higher temperatures following ISO 834 guidelines, and then, temperature-exposed specimens were either cooled by natural air or spraying water. Investigations are carried out to evaluate fire-exposed concrete’s hardened, micro-structural, and thermal performance. The damage intensity of the concrete samples is investigated by observing the thermal crack pattern, crack width, and porosity. Relationships between the temperature exposure and RS of concrete were proposed to predict the RS of fire-affected air- and water-cooled specimens. Based on the results, it is found that water-sprayed specimens exhibit higher strength loss (SL) than natural air cool (AC). The difference in the RS of the two cooling regimes decreases with increase in heating durations. Damage level and intensity of thermal cracks are higher for water-cooled (WL) specimens having higher strength grades.