Moisture Migration in Concrete Under Fire Conditions: A Detailed Analysis

The migration of moisture in concrete over a short time period is typically considered negligible. However, under fire conditions, water and vapor can rapidly accumulate on the outer surface of the ceiling and penetrate inside the concrete through cracks. During this process, water and vapor are cooled as they migrate from high to low temperature areas.

In the initial stages of the fire test, the temperature of the concrete is relatively low, the internal cracks are small, and the vapor pressure inside the concrete is not high. As a result, the migration rate of water and water vapor to the low temperature area is relatively slow, and a small amount of vapor is quickly condensed into liquid water by the concrete. At this stage, only a small amount of liquid water overflows from the concrete cracks, and the temperature does not increase significantly. The water overflow on the ceiling surface is depicted in Fig. 5, and the infrared image of the ceiling surface is shown in Fig. 12.

As the fire test progresses, the concrete temperature gradually increases, causing the moisture in the concrete to vaporize rapidly and the pressure inside the concrete to increase. High-pressure vapor inside the concrete is released through cracks, which are now relatively large. This vapor pressure drives water and vapor in the concrete to quickly migrate to the low temperature area through the cracks. Water and water vapor lose 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 min, boiling water and vapor overflowed from the concrete cracks, but the temperature monitored by infrared was 85.7ﾰC. This result was due to the high-temperature water vapor being cooled by low-temperature water and concrete during the migration to the low temperature area.

The results of the fire test demonstrate that the migration of moisture in concrete is a complex process that is dependent on temperature, pressure, and crack size. The findings of this study have important implications for the design and construction of buildings, particularly with regard to fire safety. By understanding the behavior of moisture in concrete under fire conditions, engineers and architects can develop more effective strategies for preventing or mitigating the effects of building fires.