Which material is prone to spalling under fire conditions?

Concrete is known to be prone to spalling under fire conditions due to its composition and structure. When exposed to high temperatures, the moisture trapped within the concrete can turn into steam. As the steam expands, it creates internal pressure. If this pressure exceeds the tensile strength of the concrete, it causes the surface layer to break away or spall. This phenomenon can be particularly dangerous as it compromises the structural integrity of the concrete, which is often used in building frameworks and fire-resistive construction.

In contrast, fiberglass is typically more resistant to high temperatures and does not undergo spalling in the same way as concrete. Steel may distort or weaken under fire conditions, but it does not spall. Wood, while it can char and burn, does not exhibit spalling either; instead, it may simply combust under extreme heat. Thus, concrete is the material that specifically demonstrates the spalling behavior during fire exposure, making it the correct answer in this scenario.