Fire-Resisting Properties of Materials
The fire resisting properties of the different materials commonly used for construction purposes are described below:
1. Timber: Timber has unique property of self-insulation and slow burning and offers considerable resistance to fire. When subjected to fire, timber first gets charred to certain depth and there after, this charred layer serves as insulation to check the spread of fire to the inner portion. Additional fire resistance is achieved through impregnation of timber with large quantities of fire retarding chemicals. The commonly used chemicals for this purpose are ammonium phosphate and sulphate, borax and boric acid, zinc chloride etc. This treatment retards increase in temperature during fire decreases rate of flame spread and enables easy fire control. Usually 32 to 48 kg per cubic meter for high protection. During the recent years, a number of fire resistant paints have also been introduced for rendering the timber fire resistant. Paints of asbestos, magnesium sulphate, ferrous oxide etc., have been found to be very much effective.
2. Brick: First class bricks molded from good clay can stand exposure to fire for a considerable length of time. The properties of bricks which render them fire resistant are the size of bricks, the method of construction and the component of fire resistive material in bricks i.e., clay. It has been well established that brick masonry construction is most suitable for safeguarding the structure against fire hazards.
3. Stone: The usage of stone in a fire resistive construction should be strictly restricted to the minimum. Hot stone when subjected to sudden cooling develops cracks and can lead to failure of structure. Granite when exposed to server fire, explodes and disintegrates. Lime stone is least recommended as it crumbles and gets ruined by an ordinary fire. Only compact sand stone having grains may be used as it can stand the exposure to moderate fire without serious cracks.
4. Steel: Steel although incombustible has a very low fire resistance value. With the increase in temperature, the co-efficient of elasticity of the metal falls appreciably rendering the structural members soft and free to expand. When the members in this state come in contact with water used for extinguishing the fire, they lead to contract, twist or distort and thus the stability of the entire structure in endangered. It has been noticed that unprotected steel when subjected to fire, causes the collapse of the structure. Hence in a fire resistant construction, structural steel members must be suitably protected by covering them with materials like brick, terracotta, concrete etc.
5. Concrete: The influence of fire on concrete varies with the nature of its coarse aggregate and its density. It has been found that aggregates obtained from igneous rocks containing higher calcero is content, tend to crack when subjected to fire. Coarse aggregates like foamed slag, cinder and brick are best suited for a concrete which has to be fire resistive. It has been noticed that in an average fire, the concrete surface gets disintegrated for a depth of about 25 mm. This is due to the dehydration of mortar in concrete by the fire. Hence, in a reinforced concrete fire resistant construction, the thickness of clear cover should be more.
6. Glass: Glass conducts heat faster than metal. Because of its low thermal conductivity, the change in volume on account of expansion or contraction is very small and as such it may be considered to be a good fire resisting materials. However, when subjected to sudden and extreme variation of temperatures, it fractures or cracks can be minimized. Even when the cracks are formed, the fractured glass remains in its original position, as the embedded wire holds the cracked portion and does not allow it to fall. The reinforced glass has a higher melting point than the ordinary glass, and as such it is commonly used for fire resisting doors, windows, sky-lights etc.
7. Asbestos: Asbestos is a fibrous mineral which is combined with Portland cement to form a material having great fire resistive value. Asbestos cement products are largely used for the construction of fire-resistive partitions, roofs etc. On account of its low co-efficient of expansion and property of in-combustibility, the structural members blended with asbestos cement possess great resistance to cracking, swelling or disintegration when exposed to fire.
8. Plaster or Mortar: Plaster or mortar is in-combustible and as such by suitable choice of the type of mortar, the walls or ceiling of a building can be made more fire resistant. Cement mortar is preferred to lime mortar as the latter is liable to calcine. The resistance of the plaster to fire hazards can be increased by using it in thicker layers or reinforcing the plaster with metal laths. Gypsum plaster when applied over structural members like steel columns etc., makes the latter have good fire resistive qualities.