fireproofing

The Columbia Encyclopedia, 6th ed.

fireproofing

fireproofing, method of making normally combustible materials as nearly noncombustible as possible. Fireproofing generally applies to textiles and construction materials that are treated with a solution or coating of some substance that will tend to retard their ignition.

Fireproofing of Textiles

Textiles for use indoors are soaked in fire-retardant solutions made up mostly of boric acid and borax. Textiles for outdoors are soaked with chlorinated paraffin, chlorinated synthetic resins, or chlorinated rubber. The standard for effectiveness of these treatments is the weight of chemicals remaining after the materials dry. Large areas of textiles are brushed or sprayed, but they gain little resistance against severe fire exposure; the treatment is mainly a guard against lit cigarettes and short exposure to flame.

Rain, washing, or dry cleaning tends to leach the chemicals from the fabric; therefore latex is often added to waterproof the material. At one time asbestos could be mixed with natural fibers to increase their fire resistance, but now only glass or ceramic fibers are permitted. The fire resistance of a textile is generally expressed in hours of endurance to a standard temperature furnace.

Fireproofing of Construction Materials

In construction, heavy wood timbers have a relatively high fire resistance, because fire tends to burn very slowly inward from the surface, leaving enough sound timber in the center to prevent collapse. Wood framing can also be impregnated with ammonium phosphate solution or covered with special mastics. Stucco or other incombustible facing also gives a wood frame some protection from fire.

To be classed as fire resistive, buildings must be made of reinforced concrete or protected steel that will stand considerable fire with minor damage; even a building made of unprotected steel may be damaged. While steel retains its strength up to a very high temperature, it fails rapidly at temperatures over 1,000°F (540°C). Structural steel may be protected in a number of ways. It can be faced with brick, concrete, or tile; however, construction with these materials usually adds too much weight to a building. A protective layer of concrete over all surfaces of a beam or over the steel bars in reinforced concrete has to be at least 2.5 in. (6.4 cm) thick to be effective; hollow clay tile used to cover beams and girders has to be at least 4 in. (10 cm) thick. Thus most buildings use lightweight fireproofing such as gypsum, perlite, and vermiculite mixed in plaster, concrete, and mineral fiber; one inch (2.5 cm) of such materials will absorb an equivalent amount of heat as 2.5 in. (6.4 cm) of concrete.

Some recent buildings circulate water inside each column, protecting the structure against meltdown. Asbestos is no longer used, because inhalation of the fibers causes abestosis, a fatal lung disease; fireproof board made from a mixture of asbestos and cement is used only rarely. Concrete is still used, but mostly as a thin slab on floors. In urban areas, buildings must also provide protection against fire in neighboring buildings through fireproof exterior walls—preferably windowless, since windows are fire openings. Standards for fireproofing are set by organizations such as the American Insurance Association and the International Conference of Building Officials.

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