Moisture Migration in Concrete During Fire Tests: An Examination of Temperature and Pressure Effects

As discussed earlier, 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 their migration from high temperature to low temperature areas, they were cooled. In the initial stage 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. As a result, the migration of water and water vapor to the low temperature area was relatively slow. A small amount of vapor condensed into liquid water and was quickly cooled by the concrete. Consequently, only a small amount of liquid water overflowed from the concrete cracks, and the temperature did not increase significantly. Figures 5 and 12 show the water overflow on the ceiling surface and the infrared image of the ceiling surface, respectively.

With the progression of the fire test, the concrete temperature gradually increased. Under the influence of high temperature, the moisture in the concrete vaporized rapidly, and the pressure inside the concrete increased rapidly. High-pressure vapor inside the concrete was released through cracks. At this point, the internal cracks in the concrete were relatively large, and the vapor pressure could propel 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 their rapid migration to the low temperature area. Therefore, a significant amount of high-temperature water and water vapor overflowed from the concrete cracks at this stage, as shown in Figure 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 its migration to the low-temperature area.