Though they posses some inherent fire resistance capabilities, in the presence of fire, structural reinforced concrete members will in a longer term degrade and lose their loadbearing capacity. Pre-stressed reinforced concrete, thin concrete elements, older structures with small concrete covers are the cases of major concern. Collapse will generally occur by failure of the steel reinforcing bars, however spalling of concrete may dramatically accelerate the destruction process. To prevent collapse when the existing concrete cover is not enough for the target fire resistance time, a protective material must be applied. Intumescent coatings are the best possible option in many cases.
CHAR 21 is tested and approved for fire protection of reinforced concrete and pre-stressed reinforced concrete structures according to the European standard EN 13381-3. Fire resistance ratings can be up to 120 or 180 minutes, however the actual performance depends on the cooperation of the existing concrete cover and additional layer of intumescent coating. In the perspective of EN 13381-3, performance to fire is actually measured in terms of "equivalent concrete cover thickness" provided by the coating film. Tests are performed on shapes (a concrete beam, results of which are valid for both beams and columns) and flat surfaces (an horizontal slab, results of which are valid for both ceiling slabs and concrete walls).
Fire protection can be engineered by calculating the concrete cover thickness needed for a certain performance and making up the needed thickness adding the actual concrete thickness and the "virtual" thickness brought in by the protective material. The method applies for both passive protection materials (renderings) and reactive materials (intumescent coatings). In simple cases, following the prescriptive approach, the needed concrete cover can be simply found in fire codes or building codes. However concrete structures often come in complex shapes, concrete cover thickness is not evenly distributed and inner reinforcing steel bars have different critical conditions, all in all the system has a complex static behaviour. Therefore fire safety structural calculation for concrete are often performed with the help of modeling tools.