Fire resistance

Up to 250°C, concrete shows no significant loss of strength, but by 450°C, depending upon the duration of heating, the strength may be reduced to half and by 600°C little strength remains. However, as concrete is a good insulator, it may take four hours within a building fire for the temperature 50 mm below the surface of the concrete to rise to 650°C (Fig. 3.11).

The effect of heat on the concrete causes colour changes to pink at 300°C, grey at 600°C and to buff by 1000°C. The aggregates used within concrete have a significant effect on fire resistance. For fire protection, limestone aggregates may perform slightly better than granites and other crushed rocks, which spall owing to differential expansion. Where the concrete cover over reinforced steel is greater than 40 mm, secondary reinforcement with expanded metal gives added protection to the structural reinforcement. Lightweight-aggregates concretes, owing to their enhanced thermal properties, perform significantly better in fires with respect to both insulation and spalling.

Concrete manufactured without organic materials is Class A1 with respect to reaction to fire. If more than 1% of organic materials are incorporated into

Fig. 3.11 Temperature profile within dense concrete exposed to fire for 60 minutes magnesium and sodium sulfates, and the presence of chlorides and nitrates if the pH is less than 5.5 (acid).

Fig. 3.11 Temperature profile within dense concrete exposed to fire for 60 minutes the mix, then the material will require testing to the standard (BS EN 13501-1: 2002).

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