When does concrete melt?

Melting point of concrete

Concrete is an artificial building material. It consists of a mixture of cement, water and aggregate (sand and gravel). Hence, there is no precise temperature at which concrete melts. Depending on its composition, concrete melts at temperatures between 1150°C and 1200°C. By the way carbon steel melts at temperatures between 1425°C and 1540°C.

The exact melting process is hardly dealt with in the literature. According to Schneider (in German; page 18) it is probably the case that the cement stone matrix melts first and then the aggregate stones. The photo below shows a small test body made of concrete that I examined at temperatures of up to 1200°C. It can be seen that the concrete has partially melted.

Fire brigades and technical organizations use high temperatures, for example, to separate concrete (and other materials). This is possible with an thermal lance. In the video you can see this from the 40th second.

Spalling

For example, before a concrete column melts, parts of the column can spall off. There is always some unbound water in concrete. Depending on the type of construction, the proportion is between approximately 2% to 5% (with a normal reinforced concrete column) up to 10% (with a steel composite column). In the event of a fire, the free water heats up and evaporates at 100°C.

If the water cannot escape through the capillary pores of the concrete, a lot of pressure builds up. Because concrete can only absorb low tensile stresses, explosive spalling can occur. These can be prevented or reduced with suitable reinforcement. You can find further information, for example, in the dissertation of Dr.-Ing. Meyer-Ottens.

By the way, high-strength concrete is particularly at risk of spalling in the event of a fire. It has a denser structure than normal-strength concrete and therefore fewer capillary pores. This makes it difficult for steam to escape. For this reason, there are special reinforcement rules for high-strength concrete in Eurocode EN 1992-1-2.

Can concrete burn?

Concrete melts, but it doesn't burn. In the building material class, it is therefore considered a non-flammable building material.