The experimental results of five concrete mixes subjected to high temperatures are presented in this paper with the aim of highlighting explosive spalling. The specimens were subjected to heating-cooling cycles from room temperature to 150, 300, 450, or 600°C. Two levels of heating rates (i.e., 0.1 and 1°C/min) with a fixed time of one hour at the maximum temperature and a free cooling were imposed. The experimental results showed a link between the evolution of the thermal gradient in the specimens and the specimens’ weight losses. The results also showed that the thermal stability of the concrete mixes was a function of the water to cement ratio, heating rate, and specimen dimensions. The scale effect on the spalling phenomenon was demonstrated, and a critical temperature was determined. The factors leading to the explosive spalling of the concrete were studied and identified as (1) low permeability, (2) weak water departure from the concrete, and (3) an increase of the thermal gradient. It appeared tha...