We're involved in the repair of a fire-damaged building. Much of the concrete is charred, but otherwise appears to be in sound condition. What effect do elevated temperatures have on concrete? Does the concrete need to be replaced after a fire?
The effects that fire has on the compressive strength of concrete are outlined in ASTM C 856. Petrographers usually can determine the temperature that the concrete reached in a fire by examining color changes in the aggregate. According to Laura Powers-Couche, petrographer at Construction Technology Laboratories, the location within the concrete of the 700 F isotherm (a line connecting points of equal temperature) is important because, at this temperature, the reduction in compressive strength is about 50%. For practical reasons, however, the petrographer will locate the depth at which limestone aggregate is distinctly pink. This depth is the 570 F iso-therm and conservatively indicates concrete that is fire-damaged and probably will need replacing.