Fire resistance is the ability of a material to confine a fire while providing its structural function. This determined by timing the start of the fire up to the point where the material can no longer function as required. Timber is known to be vulnerable to fire whether in a wet or dry state. In a Cross Laminated Timber (CLT) structure, fire resistance is provided through ‘charring’.
Charring is a chemical process of incomplete combustion of certain solids when subjected to high heat. In wood, charring rate is affected by density, wood moisture content, heat intensity and the oxygen concentration of the surrounding air. How?
Density is the mass of an object compared to its volume. In simple terms, it is the compactness of a substance. An object of higher density, due to its compactness has a high fire resistance. When the timber panel is exposed to a fire of about 4000C, the surface timber-layer ignites and burns at a steady rate forming a black layer of ‘char’. The char becomes an insulating layer preventing an excessive rise in temperature within the unburnt core of the panel. It is the unaffected section which continues to function for the period of the fire resistance.
