Materials used in construction can burn due to their organic nature. Flame spread tests, such as the tunnel test, evaluate the rate of combustion. Standards are used to compare materials, with zero flame spread achieved by asbestos fiber cement board. Building codes use flame spread ratings to determine fire-resistant materials. Cost of fireproofing may depend on the weight of non-combustible materials.
Materials used in building construction can burn if they are organic, as they contain carbon which can ignite in the air. Various wood, plastic, and many fiber products are combustible, which means they will burn when ignited with a flame. One test used to determine the flammability of various materials is the degree of flame spread.
When a combustible product is ignited with a flame, it burns at a rate that depends on several conditions, including the amount of airflow and the materials used to make the product. Fiberboard can be made with various glues that can accelerate the burning rate of the flame. These factors have to be compared in a standard test and the evaluation of the spread of flame allows direct comparison of different materials.
A common flame spread test is the tunnel test. The material to be tested is fixed to the top of a tunnel-shaped room, with openings at each end only. Windows along the length of the tunnel compartment allow you to observe the spread of the flames. The specimen is at least 18 inches (46 centimeters) wide and 25 feet (7.6 meters) long. A timer starts when a flame is applied to one end of the sample and stops when the flame has burned off a specific length of the sample.
The rate at which the flame moves through the sample depends on the amount of airflow. The movement of air in the tunnel is carefully controlled so that the degree of flame spread can be directly compared for different building materials. One end of the tunnel contains an air control damper for the incoming air and any test smoke is removed at the other end with a fan.
To compare flame spread values for different materials, standards are needed. Standards are materials that burn consistently in several tests, giving a similar value for all tests. Zero flame spread is achieved by attempting to ignite asbestos fiber cement board, which will not burn. Red oak wood is assigned a value of 100, which provides a flammable range for all other materials to use.
Building code authorities use flame spread rating information to create standards for the construction of new buildings. For hospitals, in areas where fires are more common, such as residential garages, or in areas of the world where fires are a seasonal problem, low quality materials may be required. Class I materials are generally considered to be the most fire resistant and Class V are the most combustible.
Material cost may not be the only factor in determining the cost of fireproofing a building. Cement-based board may not be more expensive than wood board on a per-board basis, despite having very low fire resistance. The construction cost may depend more on the structural weight of the non-combustible materials. Cement or concrete materials can be quite heavy and can cost more due to the sturdier structures needed to support them and the increased manpower to install them.
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