The danger of fire is present always and everywhere. The imminent danger depends upon actual exposure, and naturally differs if the threatened construction is a pedestrian subway, a roadway tunnel or a subterranean garage in a skyscraper. Concrete is the loadbearing material in nearly all built structures and is therefore at high risk, since the entire structure would collapse upon its material failure.
Concrete must therefore, independent of the danger scenario, be properly formulated or protected by external measures, in order to hinder failure at high temperature in case of fire. The capillary and interstitial water begins to evaporate at temperatures around the boiling point of water (100 oC). Steam needs more space and therefore exerts expansion pressure on the concrete structure. The cement matrix begins to change at temperatures of about 700 °C. The effect of the aggregates is mainly dependent on their origin and begins at about 600 °C.Fire resistance is defined as the ability of a structure to fulfill its required functions (load bearing function and/or separating function) for a specified fire exposure and a specified period (integrity).Fire resistance applies to building elements and not the material itself, but the properties of the material affect the performance of the element of which it forms a part. In most cases fire temperature increases rapidly in minutes, leading to the onset of explosive spalling as the moisture inherent in the concrete converts to steam and expands.
Concrete with high fire resistance is used for:
- Emergency areas in enclosed structures (tunnel emergency exits)
- General improved fire resistance for infrastructure
- Fire resistant cladding for structural members
Production of concrete with high fire resistance
- The concrete production does not differ from standard concrete
- The mixing process must be monitored due to the fibers normally included
- It is beneficial to the future fire resistance of this concrete if it can dry out as much as possible
Constituents for the production of concrete with high fire resistance
- Achievement of maximum fire resistance is based on the composition of the aggregates used
- The resistance can be greatly increased by using special aggregates
- The use of special plastic fibers (PP) increases the resistance considerably
- The use of selected sands improves the resistance of the cement matrix
The most severe fire scenario modelled is the RWS fire curve from the Netherlands and represents a very large hydrocarbon fire inside a tunnel. There are various options available to improve the fire resistance of concrete. Most concretes contain either Portland cement or Portland blended cement which begins to degrade in respect to important properties above 300 °C and starts to lose structural performance above 600 °C.
Of course the depth of the weakened concrete zone can range from a few millimeters to many centimeters depending on the duration of the fire and the peak temperatures experienced. High alumina cement used to protect refractory linings reaching temperatures of 1'600 °C has the best possible performance in a fire and provides excellent performance above 1'000 °C.
The choice of aggregate will influence the thermal stresses that develop during the heating of a concrete structure to a large extent. Aggregates of the carbonate type such as limestone, dolomite or limerock tend to perform better in a fire as they calcine when heated, liberating CO2. This process requires heat, so the reaction absorbs some of the fire’s exothermic energy.
Aggregates containing silica tend to behave less well in a fire. Finally as the heat performance is related to the thermal conductivity of the concrete, the use of lightweight aggregates can, under certain conditions, enhance the fire performance of the concrete.
Polymer or polypropylene monofilament fibers can significantly contribute to the reduction of explosive spalling and thus improve the ‘fire resistance’ of the concrete. In a fire, these fibers melt at around 160 °C, creating channels which allow the resulting water vapor to escape, minimizing pore pressures and the risk of spalling. Under conditions in which the risk of structural collapse is unacceptable, designers examine other ways to protect the concrete from the effects of fire. Alternatives range from local thickening of the concrete, cladding using heat shields coated with intumescent paint, use of protective board systems and spray-applied lightweight mortars. The purpose of these passive fire protection systems depends on the type of tunnel as well as the form being protected.
