What happens to concrete during exposure to various temperatures up to 1,000 degrees F?
Between 200 and 400 degrees F free moisture evaporates. If the concrete is heated locally and instantaneously to these temperatures spalling may result from the generation of high internal steam pressures. Degradation in compressive strength begins to occur between 400 and 750 degrees F. At 600 degrees F strength might be reduced to 60 to 85 percent of its original value. At temperatures of about 1,000 degrees F the compressive strength would be reduced to only 20 to 45 percent of its original value. All of these changes depend, however, on how rapidly and how long the concrete is heated. As concrete is cooled from 1,000 degrees F it loses another 20 percent of the strength it had while it was hot. Concrete expands at a normal rate in temperatures up to 300 or 500 degrees F but at higher temperatures it shrinks at an even faster rate.