Experimental discussion on the mechanisms behind the fire spalling of concrete

International audienceThe behaviour of six concretes at high temperature (600 °C) and in particular the risk of fire spalling is studied. Two of the four ordinary concretes are made with calcareous aggregates (including one with polypropylene fibres) and two are made with silico-calcareous aggregates (including one with polypropylene fibres). In complement, tests are also done on two high performance concretes. Tests are performed with two sizes of samples: small samples (300 x 300 x 120 mm 3) and small slabs (700 x 600 x 150 mm 3). In addition to the constituents of concrete, other experimental parameters are studied. Different storage conditions (pre-drying at 80 °C, air storing and water storing) are used to highlight the effect of the initial water content. Thanks to different scenarios of heating, the influence of the heating curve and the maximum temperature are studied. Results obtained enabled to identify parameter that influence highly the risk of fire spalling: initial content of free water and permeability of concrete during the heating. The permeability of concrete can increase during heating due to the melting of the polypropylene fibres or a strong concrete thermal damage. This thermal damage is especially important when heating is violent (ISO 834 or increased hydrocarbon fire), or when concrete is made with silico-calcareous aggregates (in particular flint). The onset of fire spalling cannot be explained by either the only thermo-mechanical behaviour of concrete, or only by the appearance of high pore gas pressure during heating. Based on the recent theory of the critical zone explaining the occurrence of fire spalling, the formation of a saturated layer of liquid water is consistent with the results obtained