| Summary: | Thermal instability of high-performance concretes is a topical subject. Research in this area uses experimental and modelling approaches which have two main aims: understanding the mechanisms of thermal instabilities and attempting to predict their occurrence. This study falls into the first category. It examines the spalling behaviour of instrumented slab specimens of high-performance concrete and ultra-high-performance concrete which are equipped with thermocouples and subjected to standard fire thermal cycles on their undersides. During testing, all of the specimens suffered explosive thermal-induced spalling. The bursts were generally accompanied by audible sound production, and often manifested as sudden variations in the measured temperature-time curves, meaning that their occurrence and progression could be traced. Some of the thermocouples embedded in the concrete displayed temperature curves with a plateau at 100 °C; this reflects the presence of liquid and water vapour, and suggests a water saturation front in the concrete. The study focused on the times at which spalling began and the temperatures recorded at these points. When results were analysed, the majority could be explained by considering the microstructure and mechanical properties of the materials involved. Comparison of the results with those in the literature indicated that the first bursts occurred at relatively low vapour pressure, therefore suggesting the predominance of thermo-mechanical processes.
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