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SUMMARYThe behaviour of six concretes at high temperature (600°C) and in particular the risk of fire spalling is studied. Tests are performed with two sizes of samples: small samples (300 × 300 × 120 mm 3 ) and small slabs (700 × 600 × 150 mm 3 ). Different storage conditions (pre-drying at 80°C, air 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 is studied.Results enabled to identify parameters that highly influence the risk of fire spalling: initial water content and concrete permeability during heating. The permeability of concrete can increase during heating due to the melting of the polypropylene fibres or by thermal damage. This thermal damage is important when heating is violent (ISO 834 or increased hydrocarbon fire), or when concrete is made with silicocalcareous aggregates (flint).Fire spalling cannot be explained by either the only thermo-mechanical behaviour of concrete, or only by the appearance of high pore gas pressure. Based on the recent hypothesis of the critical zone, the formation of a saturated layer of liquid water is consistent with the results obtained.

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