In contrast to other construction materials such as steel or concrete, wood is a combustible material. This combustibility is certainly a negative feature of wood because of its ability to contribute to the development and propagation of fire. Moreover, depending on the particular type of timber and on other factors, wood can behave very differently once exposed to a source of heat and/or naked flame. This different behaviour is called its reaction to fire. Put simply, reaction to fire means the degree to which a combustible material takes part in the fire to which it is exposed. This phenomenon is influenced by a number of factors:
- flame propagation (the speed at which the flame front expands);
- flammability (the ability of a substance to maintain combustion);
- heat build-up (the quantity of heat produced in a certain time);
- droplets (the release of droplets of incandescent substances during and after exposure to flame and/or heat);
- smoke products;
- toxic gas production.
Improving its reaction to fire, regardless of the type of wood and the use to which it is put, means reducing the wood’s ability to ignite and to propagate flame. Changing the fire reaction capacity of a material does not however, prolong its stability over time (R) in the case of widespread fire. Indeed, the temperatures recorded during a fire are too high to enable systems intended to improve reaction to fire to carry out their function to the full. For wooden structural elements therefore, it is necessary to assess and check their fire resistance capacity (R).
Also in contrast to other construction materials, wood is able to defend itself from the action of a fire. Combustion initially affects the outer surface of the wood when it comes into contact with a heat source. Combustion then proceeds, gradually affecting the inner parts, continuing ever deeper until the entire woody mass is involved. This process requires a time lapse that depends on the section of the wood and during which the combustion creates a carbonaceous layer that, together with wood’s high insulation power, protects the inner part that has not yet been attacked by the fire, maintaining its mechanical properties. In normal ventilation conditions, surface ignition of the wood happens in temperatures between 260°C and 280°C. The speed of combustion is favoured by the following factors.
- High surface/volume ratio
- Low density, limited mass by volume
- Low humidity
- High content of inflammable resins
- The presence of inflammable finishing products
- Raw, uneven, loose and split surfaces.
In any of the above conditions, different types of wood behave differently, with different times taken for this process to happen, known as carbonisation speed.
To repeat, fire can be divided into the following two phases.
- Ignition phase strictly linked to the combustible nature of the material (reaction to fire)
- General burning phase with uncontrolled propagation of the fire, smoke and hot gasses (resistance to fire)
Specific protection systems are to be used for each phase.