What occurs to moisture in concrete when there is rapid heating in a fire?

When concrete experiences rapid heating during a fire, the moisture within it heats and expands. Concrete is a porous material that can contain water in the form of both free water and capillary water. Under extreme heat conditions, the water molecules begin to rise in temperature and the kinetic energy of these molecules increases, leading them to expand.

As the temperature continues to rise, the water can transform into steam, which occupies a larger volume than liquid water. This expansion raises the pressure inside the concrete structure, potentially causing spalling—where pieces of the surface break off due to the pressure buildup. Consequently, the correct understanding of this process is key for fire investigators to assess potential damage and structural integrity after a fire event.

The other potential answers do not accurately describe the behavior of moisture in concrete under rapid heating. For instance, moisture would not immediately evaporate at once without first heating and expanding, nor would it freeze—an unlikely scenario during a fire. Likewise, condensation does not apply here, as the rapid heating causes moisture to change phase rather than condense. Understanding these physical and chemical reactions is essential for effective fire investigation and analysis.