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One of the main reasons why portland cement concrete is so widely used in building construction is that it can help satisfy a cardinal need for public safety in face of the hazards of fire better than most of its competitors. Concrete is incombustible and a reasonable insulator against the transmission of heat. These qualities alone help confine the fire and limit the extent of the damage, and though the surface may crumble or spall, the essential engineering properties of the body of the concrete remain intact. A temperature of 649°C (1200°F) may be reached before 25 percent of the original compressive strength of a calcareous aggregate concrete is lost (the corresponding temperature for a siliceous aggregate concrete is about 427°C (800°F)). However, the main role of concrete in a fire is to protect any embedded steel for as long as possible against a rise in temperature to the point where its physical properties are reduced significantly, causing excessive structural deflections that might lead ultimately to collapse. Fifty percent of the yield strength of mild steel is lost by about 593°C (1100°F) (the corresponding temperature in the case of the tensile strength of prestressing wire is about 427°C (800°F)).
Director, Ministry of Transportation and Communications, Toronto, Ont.