What happens to concrete under extreme heat conditions?

Under extreme heat conditions, concrete can undergo spalling, which is a critical behavior of the material when exposed to high temperatures. Spalling occurs when the heat causes the moisture within the concrete to expand rapidly, leading to the cracking or flaking off of surface layers. This process compromises the structural integrity of the concrete, as the loss of surface material can weaken the entire structure and reduce its overall load-bearing capacity.

Concrete is primarily composed of aggregates, cement, and water, and while it is a durable material, it cannot withstand extreme heat indefinitely. Under intense heat, the physical changes that occur can further lead to the creation of micro-cracks and increase the porosity of the material, although these processes do not fully account for the sudden loss of surface material seen in spalling. The other options, such as melting or liquefying, do not accurately represent how concrete behaves under such conditions, as concrete is not a melting material like metals; instead, it can lose its coherence and begin to crumble or break apart through spalling.