When concrete heats up during fire, at some point in time the steel reinforcement will heat up as well. The heating of the steel reinforcement is delayed by the concrete cover since the thermal transfer through the concrete is relatively slow. Moreover, at temperatures in a range of roughly 200-800 ºC the concrete itself loses its load-bearing capacity because of internal micro-cracking and chemical transformation (dehydration) of the cement phase. Typically, design standards simplify this gradual strength loss by assuming a limiting temperature of 500 ºC for concrete. Whenever the concrete cover thickness is insufficient to keep the steel reinforcement temperatures low enough to avoid collapse, or when spalling of concrete could occur or when the concrete itself is deteriorated, fire protection is needed.
