Due to its low thermal conductivity, a layer of concrete is frequently used for fireproofing of steel structures. However, concrete itself may be damaged by fire. An example of this was the 1996 Chunnel fire, where the fire reduced the thickness of concrete in an undersea tunnel connecting France with England. For this reason, common fire testing standards, such as ASTM E119, do not permit fire testing of cementitious products unless the relative humidity inside the cementitious product is at or below 75%. Otherwise, concrete can be subject to significant spalling.

Up to about 300 C (572 F), the concrete undergoes normal thermal expansion. Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses. Up to about 500 C (932 F), the major structural changes are carbonatation and coarsening of pores. At 573 C (1,063.4 F), quartz undergoes rapid expansion due to phase transition, and at 900 C (1,652 F) calcite starts shrinking due to decomposition. At 450-550 C (842-1,042 F) the cement hydrate decomposes, yielding calcium oxide. Calcium carbonate decomposes at about 600 C (1,112 F). Rehydration of the calcium oxide on cooling of the structure causes expansion, which can cause damage to material which withstood fire without falling apart. Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonatation from carbon dioxide which is reabsorbed.

Concrete exposed to up to 100 C (212 F) is normally considered as healthy. The parts of a concrete structure that is exposed to temperatures above approximately 300 C (572 F) (dependent of water/cement ratio) will most likely get a pink color. Over approximately 600 C (1,112 F) the concrete will turn light grey, and over approximately 1000 C (1,832 F) it turns yellow-brown.[5] One rule of thumb is to consider all pink colored concrete as damaged that should be removed.

Fire will expose the concrete to gases and liquids that can be harmful to the concrete, among other salts and acids that occur when gases produced by a fire come into contact with water.

If concrete is exposed to very high temperatures very rapidly, explosive spalling of the concrete can result. In a very hot, very quick fire the water inside the concrete will boil before it evaporates. The steam inside the concrete exerts expansive pressure and can initiate and forcibly expel a spall.[6]