Moisture Migration and Temperature Discrepancies During Concrete Fire Tests: A Case Study

As previously discussed, the migration of moisture in concrete over a short period can be disregarded. However, water and vapor that quickly overflowed on the outer surface of the ceiling migrated inside the concrete through cracks. During migration, water and vapor were cooled as they moved from high temperature areas to low temperature areas. In the initial stages of the fire test, the temperature of the concrete was low, the internal cracks were small, and the vapor pressure inside the concrete was not high. Therefore, the migration speed of water and water vapor to the low temperature area was relatively slow. A small amount of vapor was easily cooled by concrete and condensed into liquid water. At this stage, only a small amount of liquid water overflowed from the concrete cracks, and the temperature did not significantly increase. The water overflow on the ceiling surface is shown in Fig. 5, and the infrared image of the ceiling surface is shown in Fig. 12.

During the fire test, the temperature of the concrete gradually increased. Under high temperature, moisture in the concrete vaporized rapidly, and the pressure inside the concrete increased significantly. High-pressure vapor inside the concrete was released through cracks. At this time, the internal cracks in the concrete were relatively large, and the vapor pressure could drive the water and vapor in the concrete to quickly migrate to the low temperature area through the cracks. Water and water vapor lost less heat during the rapid migration to the low temperature area, resulting in a large amount of high-temperature water and water vapor overflowing from the concrete cracks at this stage, as shown in Fig. 15. At 95 minutes, boiling water and vapor overflowed from the concrete cracks, but the temperature monitored by infrared was 85.7ﾰC. This was due to the high-temperature water vapor being cooled by low-temperature water and concrete during the migration to the low temperature area.