Concrete is one of the best fire-resistant products found in the construction of buildings and flatwork. However, intensive heat not only affects concrete, it can weaken it to the point where the only solution is replacement. After all soot is removed from the concrete, the color beneath is an indicator of the extent of the damage. Slight damage may appear as a lightly mottled gray/orange while extensively damaged concrete will often take on a darker orange/red shade. Your choice of repair depends upon the surviving microstructure of the concrete.
Things You'll Need
Concrete rebound hammer
Determine what kind of repair is necessary by using a concrete rebound hammer. These hammers, designed to measure the remaining structural integrity in the concrete, employ a rubber impact system that measures the amount of elastic rebound when a rubber insert impacts the surface of the concrete. A digital rebound hammer will indicate when the level of impairment registers in the danger zone. (See Resources)
Blast the soot from the surface of the concrete with either a dry ice mixture or sand. The least damaging removal method uses dry ice although locating the necessary equipment can be difficult. For a very small area of damage, you may use a wire brush and a solution of warm water and Tri-Sodium Phosphate (TSP), found at hardware stores.
Consult an engineer if you are unsure about the structural integrity of the remaining concrete. If the fire was small, you may paint or apply a surface-strengthening bond after removing all of the soot. If there is extensive structural damage, a portion or all of the concrete may require demolition and re-construction.
Apply a surface strengthening bond product to clean concrete to protect and seal it. Because extreme heat makes the concrete porous, a surface-bonding agent will protect it from moisture and eventual crumbling. Follow the manufacturer's instructions when applying a surface strengthener. (See Resources)
Remove small areas of spalling and patch with concrete mortar. This repair is feasible if the concrete surface suffered slight damage with spalling but the integrity of the concrete structure is still intact. Mix concrete mortar with water as directed on the package and apply to the surface with a large flat trowel, smoothing it evenly as you go. If the concrete beneath is very dry--spray it down every three hours in the preceding 24-hour period to reduce premature drying of the mortar.
Demolish concrete that no longer is safe and rebuild the structure. On large or especially hot fires, concrete may suffer damage that is beyond repair. In these cases, the only option is to remove the entire concrete structure.
Contact your local building authority. In some communities, a qualified structural engineer must inspect all fire-damaged concrete before repairing.
Avoid trying to repair extensively damaged concrete. Because the porosity level is much higher, the structural integrity of the concrete is now unsafe.