Concrete structures are very resistant to fire. However, prolonged fire conditions cause various effects. Obvious signs of distress are cracking and spalling of the surface, caused by differential expansion of successive layers of the concrete as it is heated and by internal pressure as moisture in the concrete is turned to super-heated steam. Secondly, the material properties change. These are described in Concrete Society Technical Report 68, Assessment, design and repair of fire-damaged concrete structures.
Heating concrete above about 300oC reduces its compressive strength; the reduction is approximately linear with temperature, with all strength being effectively lost at about 1000oC. Similarly there will be a reduction in the strength of any reinforcing steel, which may have a significant effect on the performance of the structure.
The temperatures reached in a fire may cause the affected parts of the structure to expand significantly more than the amount for which they were designed. These movements are likely to result in cracking or structural damage, either to the member itself or to connected parts of the structure.
Most fire-damaged structures can be successfully repaired, this being the more cost-effective solution than demolition and reconstruction. Further information may be found in the following entries:
Defects and deterioration/Load related defects/ Assessment of fire-damaged structures
Design/Approach/Redesign of fire-damaged structures
Repair and strengthening/Repair techniques/Repair of fire-damaged structures.