Fire-Resistant Properties of Building Materials - Assignment Example

A Comparison of the Fire-Resistant Properties of Building Materials: Timber, Steel and Concrete It is essential to consider the fire-resistant properties of materials used in the construction of buildings and homes. The material used must not be subject to disintegration at high temperatures associated with combustion. To ensure the stability of a structure, the material must not expand at high temperatures and should also contract minimally when cold water is used in fire-fighting applications. The. material should be fire retardant and not subject to combustion. This report summarizes the properties of concrete, steel and timber used in the construction of buildings and homes with regard to the effects of fire on their physical and chemical properties. In addition, a recommendation will be made on the building materials best suited for the construction of a proposed building. Fires generally produce temperatures of approximately 1000C which can seriously affect the structural integrity of a building. Among the effects of fire are the loss of strength of the building material which can result in building collapse. In addition, thermal expansion associated with high temperatures may affect structures that are not directly in the vicinity of the fire. With the exception of timber, all materials used in building construction will begin to lose strength at temperatures exceeding 250C. This effect may be permanent. Metals begin to form droplets beginning at 300-350C. Concrete changes color to pink at 250-300C. Chemical effects occur at 650C for aluminum and its alloys, and 1100-1200C for copper and iron (Prentice, 1990). Timber is one of the oldest materials used in building construction. The properties of timber are dependent on the type of wood used in construction, and how it has been treated. Wood responds well to tensional and compression forces, but can bend due to the fibrous nature of this material. Wood can be a fire resistant material as it is subject to charring, which means that the wood in the center may be protected during combustion to retain the structural integrity of the building. For the most part, however, timber is rapidly consumed by fire and can enhance the intensity of a fire since the material is subject to combustion. Timber used in building materials can be made more fire-resistant by coating the surface of the timber with fire-resistant chemicals such as ammonium phosphate and soleplate, borax and boric acid (Jones, 1986). Heavy wood timbers may be fire-resistant since the wood burns slowly inward causing charring of the exterior while the inner wood layers remain protected for significant periods of time. Timber will begin to turn brown at 120-150C. At 200-250C blackening will occur and vapors resulting from combustion will be produced at 300C. Above 400-450C the timber will ignite and char at a steady rate (Jones, 1986). Concrete is a mixture of cement and fly ash, slag cement and aggregates of gravel, crushed rocks (limestone or granite) and a fine aggregate such as sand. These are mixed with water to form concrete. The solidification of concrete when it is mixed with water is the result of hydration which bonds all the composite materials together to form a strong, solid structure. Concrete is the most commonly used building material in the world. Concrete is a structural material that is a very commonly used as a building material because of its durability, strength and fire resistance. It is a non-linear, non-elastic substance that is highly compressible but has weak tension capacity. As a result, concrete used in building materials is a composite reinforced material added to the basic components of sand, cement and water. The concrete is reinforced with steel bars called rebars or in the form of steel plates or fibers. The steel reinforcement decreases the brittleness of the concrete and increases its ability to withstand tensional forces. Concrete is the most fire resistant material used in building construction. Temperatures up to 250C do not affect the strength of concrete. Reinforced concrete structures can typically withstand 1000C temperatures for approximately one hour without a significant loss of strength or structural integrity. The position of the steel in reinforced concrete can affect the fire-resistant behavior of this material (Nilson et al, 2004). The compressive strength of concrete can vary according to the temperature and also in response to other factors, such as the rate of heating, its duration and the composition of the material. The type and size of the concrete aggregate, the percent of cement paste and the amount of water used to prepare the concrete can affect the sensitivity of concrete’s compressive strength to temperature increases. Fires generally affect the compressive strength of concrete. The strength of this material will recover upon cooling unless the temperatures exceed 300C, in which case the damage to compressive strength may be permanent. Since concrete has a low thermal diffusivity, (1 mm per second), the temperature increase of the material during a fire may be restricted to the outer periphery, leaving the interior unaffected. The modulus of elasticity also decreases with increasing temperature, but has been found to recover so long as the coefficient of thermal expansion is 10.5 per degree Celsius (Hosford, 2005). Steel is an alloy of iron that contains between 0.2-1.7% carbon. Due to its high strength and low cost, it is a very popular material used in construction. Steel responds well to high compression and tensional forces. Unlike concrete, steel is not very fire resistant. The effect of temperature on the strength of steel is extremely important. The yield strength of this material is reduced by one-half at temperatures of 550C and falls to 10% at 1000C. It is necessary to insulate steel due to its high thermal conductivity as unprotected steel will reach temperatures approximately equal to the fire temperature. The loss of strength at high temperatures causes steel to lose its load bearing capacity which can result in structural collapse. In addition, unreinforced steel will suffer significant thermal expansion as a consequence of fire. The coefficient of expansion is 10.5 per degree temperature increase (Celsius). At temperatures above 500C, the loss of strength may be permanent. At temperatures above 600C steel bars lose their tensile strength and will melt at temperatures of 1400C and above. Thus, the structural integrity of a steel building may fail in fires at these temperatures. The material becomes soft and weak at these high temperatures and may collapse (Blank et al, 1993). A client wants to build a shopping mall and asks for a recommendation for the best building material for the construction. In addition to retail shops, there will be several restaurants and a movie theatre. Due to the size and nature of the planned building, the use of fire-resistant building construction materials is essential. As concrete is generally considered to comprise the most fire-resistant building material, this is the recommended choice. Due to its high heat capacity, a fire will not cause a rapid increase in temperature of this building material. This may limit overall damage and fire spread which would facilitate evacuation in an emergency. It is necessary to choose a concrete made with the most fire-resistant aggregate composition as the effects of fire will depend on the type of concrete used in building construction. It would be better to use concrete made of carbonate aggregates such as limestone or dolomite or concrete consisting of lightweight aggregates such as shale, clay or slag which will retain their compressive strength at temperatures up to 650C, rather than concrete comprised of siliceous aggregates containing granite or quartz which will retain only half their compressive strength at this temperature. It will be essential to reinforce the concrete with steel to provide the needed tensile strength to support the structure. The use of steel makes the building somewhat less fire-resistant; however, embedding the steel bars within the concrete structure will provide a protective effect since the concrete has low thermal conductivity. This will provide a shield between the heat source and the steel building reinforcement. The steel bars used as reinforcement for tensile strength can withstand temperatures of 550C. This combination will provide for the structural needs of the building which require high compressive and tensile strength as well as maximal fire protection based on the primary use of concrete with fire-resistant composition as the primary building material. Using this construction plan, it is anticipated that the building will resist the rapid of a fire that could occur in a kitchen or within the facility in general. The fire should remain localized due to the installation of a sprinkler system and most especially due to the insulating capacity of concrete. Due to its low thermal conductivity, it will resist temperature increases and concomitant structural damage. This will minimize spread of the fire in the structural domain of the building. In addition, the shielding effect of the concrete will protect the steel reinforcement bars located internally that are more fire-sensitive. The use of this building material for a shopping mall will provide the best overall fire protection as it will help to prevent the spread of localized fires through the structural building components and will provide a structure resistant to damage and loss of compressive or tensile strength in the case of fire. References Blank, Alan; McEvoy, Michael; Plank, Roger , 1993. Architecture and Construction in Steel. Taylor & Francis. Hosford, William F., 2005. Mechanical Behavior of Materials. Cambridge University Press. Jones, W.W., 1986. A multicomponent model for the spread of fire, smoke and toxic gases, Fire Safety Journal, 9, pp. 55-80. Nilson, Arthur H.; Darwin, David; Dolan, Charles W., 2004. Design of Concrete Structures. McGraw-Hill. Prentice, John E ,1990. Geology of Construction Materials. Springer. Read More