Intumescent is a reactive coating which swells as a result of heat exposure, thus increasing in volume and decreasing in density. Specifically, an intumescent paint is a coating that reacts to heat by swelling in a controlled manner to many times its original thickness, producing a carbonaceous char formed by a large number of small bubbles that act as an insulating layer to protect the substrate.
The scope of intumescent products is the prevention of the structural collapse of the building, which can occur if load bearing steel elements reach a critical state.
For steel, this is linked to the critical temperature, defined as the temperature at which the load bearing capacity becomes equal to the effect of the applied loads (so the steel element is very close to the collapse). Critical temperature of steel can vary from 350 °C to 750 °C, depending mainly on the loading scheme, but in most of the cases between 500 °C and 620 °C.
For concrete, critical state is linked to the critical temperature of the reinforcing bars (normally from 350 °C to 500 °C) and the reaching of a temperature of 500 °C inside the concrete element.
For wood, critical state is linked to the residual section of the timber load-bearing element after burning.