Repair of fire damaged structures
Most fire-damaged structures can be successfully repaired, this being the more cost-effective solution than demolition and reconstruction. Before carrying out any repair it is necessary to determine the extent of the damage to the concrete, and hence how much must be removed and replaced. Similarly the effect on the reinforcement must be determined and hence the requirement for any additional steel. These aspects are covered in the separate entries;
Defects and deterioration/Load related defects/Assessment of fire-damaged structures
Design/Approach/Redesign of fire-damaged structures.
The methods used for the repair of fire-damaged concrete are no different from those used for the repair of concrete damaged by corrosion of the reinforcement, with advice available from various sources, including Concrete Society Technical Reports 26, Repair of concrete damaged by reinforcement corrosion and 38, Patch repair of concrete subject to reinforcement corrosion.
For reinforced concrete, the main processes to be undertaken are as follows:
• Removal of damaged or weakened concrete
• Replacement of weakened reinforcement
• Replacement of concrete both to reinstate the original form and to provide adequate structural capacity, durability and fire resistance. In some circumstances there may also be a requirement for the reinstatement of special finishes and appearance.
Before finalising remedial works specifications, the concrete should be thoroughly assessed to ensure that repairs and reinstatements address any inherent or pre-existing problems, such as low covers, excessive levels of chloride or depths of carbonation; advice is available from Concrete Society Technical Report 54, Diagnosis of deterioration in concrete structures. In addition, the effects of any contaminants derived from the fire on the long-term durability of the structure should be considered.
Before breaking out is undertaken, it is necessary to be certain that the reduction in structural section will not over-stress the member. In some cases it may be necessary to remove any heavy load (e.g. equipment or plant) supported by the member and reinstate it after repair. An alternative may be to prop the structure. Propping is essential, with full removal of the load, if a full structural repair is required, i.e. in cases where the new concrete or mortar is expected to carry its full share of the load in the repaired member.
Further guidance on appropriate repair methods is given in Concrete Society Technical Report 68, Assessment, design and repair of fire-damaged concrete structures, which includes design examples.