Effects of Natural Drying and Carbonation on a Method for Investigating Fire-Damaged Concrete Using Phenolphthalein Solutions

Assessing the depth of the heat impact caused by fire damage is important for planning the repair and reinforcement of fire-damaged reinforced concrete (RC) structures. One method used to investigate fire-damaged concrete structures is measuring the depth of carbonation, which has been reported to increase after fire damage. And, previously, using an anhydrous phenolphthalein (PP) solution had believed to identify regions heated to around 500 degrees C (areas with CaO formation). Few studies have investigated or analyzed the chemical changes in cement hydrates in detail. In this study, heating experiments were conducted under N-2 using an electric furnace, focusing on the chemical changes in cement hydrates due to high-temperature heating, the coloring of PP solutions with and without water, and the moisture transport in concrete caused by high temperatures. Upon heating cement paste, areas where CaO was formed or carbonated by thermal decomposition could not be determined by observing the color of the anhydrous PP solution, whereas areas heated above approximately 150 degrees C almost dried out could be successfully evaluated. In heating experiments on concrete cores taken from carbonated RC structure before being exposed to fire, the PP solution with water enables judging carbonated areas where CaCO3 pyrolyzed to form CaO (i.e., areas heated above approx. 600 degrees C) in addition to determining dry areas. However, this method is not applicable to concrete where carbonation has not progressed significantly.