What is spalling in concrete, and why is it a concern in fires?

• A. The flaking or breaking of concrete surface due to heat
• B. The surface becoming smoother and more durable after heating
• C. The expansion of concrete surface due to heat
• D. The color change of concrete

Answer: (B)

Spalling is the flaking or breaking away of the outer concrete surface when it is exposed to high heat, such as in a fire. It happens because moisture trapped inside the concrete turns to steam under the heated surface and builds up pressure, causing pieces to crack and detach. This is a serious concern in fires because it erodes the protective concrete cover around reinforcing steel, allowing heat to reach the steel more quickly, weakening it, and increasing the risk of structural failure. It also creates falling debris that can spread fire or endanger occupants. The idea of the surface simply becoming smoother and more durable after heating isn’t correct, and spalling is distinct from mere expansion or a color change.