Merits and Demerits of Weathering Steel - Research Paper Example

Weathering steel: Introduction: Weathering steel is a kind of alloyed steel whose properties are decided by the proportion of different alloys present in it. Weathering steel is known for its unusual characteristic features that are often quite favorable to the requirements of a profitable business. What makes the weathering steel unique is its remarkably innovative manner of preventing corrosion. The irony is that it prevents rusting by self-rusting i.e. forming a protective layer of oxide around its body, also known as the “protective oxide film” or “patina”, whose thickness is almost equal to that of a thick coat of paint. (Laffrey et al, 2000, p. 1). When weathering steel is exposed to the atmosphere, oxygen in the atmosphere reacts with steel in the presence of atmospheric moisture to form a cover of rust. This is similar to the behavior exhibited by other types of steel which undergo rusting to form a loose layer of crystalline rust. This layer is not very stable and provides room for more water to seep through the pores allowing the corrosion to penetrate deeper into the body of steel. Thus the steel continues to corrode and loose its strength. But the procedure does not hold true for weathering steel. Although weathering steel starts to corrode in the similar manner as other types of steel, yet the rust layer it develops is not as unstable as that of unalloyed steels. The protective oxide layer once formed stays and keeps air and water from penetrating inside. Although the oxide film is rusty itself, yet it serves to preserve the strength of the underlying base metal. Merits of weathering steel: Weathering steel is preferred over other types of steel because of a number of reasons some of which are mentioned below: Naturally finished surface: Weathering steel, with its naturally beautiful rust color, provides the structure with a finished surface thus eliminating the need for the structure to be painted. The protective oxide film that forms around the body of the weathering steel is stable enough to prevent corrosion from penetrating deep into the body of the base metal. Therefore, there is no need to paint the structure and the natural protective oxide coating provides a finished surface for the weathering steel. Cost effective manufacturing: Weathering steel is cheap to manufacture as compared to the conventional carbon and manganese steel that goes through various complicated procedures from exploration to refining and treatment before arriving at the site, because it is chiefly made up of previously recycled steel. The costs associated with the treatment of recycled steel are considerably lesser than those incurred in the course of production of new steel. The costs are further minimized because weathering steel is never wasted. It is instead recycled to produce more weathering steel. Reduced construction time: As we all know, “time is money”, the sooner the structure is built, the sooner it can be put to use and the profits earned can be enlarged. Because there is no need to paint the weathering steel structure, the time required to paint the structure is saved and is utilized to earn the revenues. Besides, pre-cast weathering steel members are also readily available and can be used to save the time and hence, the cost of construction. Reduced construction cost: Weathering steel is considered very favorable for enlarging the profit on a business as it serves to cut down the cost of paint, which is needed on the surface of unalloyed steel structures to prevent the structure from rusting, inherent in the nature of the unalloyed steel. Paint is considered as one of the most expensive items when it comes to the finishing of a structure. The annual cost of corrosion was estimated to be in billions of dollars according to a research sponsored by the Federal Highway Administration (FHWA), while the FHWA worked out the costs incurred due to the annual painting of present steel bridges alone to be a minimum of $500 million. (rampartllc.net, 2006). Thus using weathering steel in the construction of structures exposed to open air tends to minimize the construction costs both in the initial phase as well as throughout the life of the structure. “The use of uncoated weathering steel typically provides initial cost savings of 10 percent or more, and life cycle cost savings of at least 30 percent over the life of the structure.” (www.steel.org, 2010). Prolonged life of the structure: Weathering steel, with its high strength and minimum corrosion tends to elongate the life of a structure. “The corrosion rate is so low that bridges fabricated from unpainted weathering steel can achieve a 120 year design life with only nominal maintenance.” (Corus, 2005). A time period of 120 years is big enough to justify the stability of patina. Nominal maintenance: In order to make sure that the weathering steel bridge is absolutely safe and functional, all what is required is a periodic check and cleaning. Owing to the minimal maintenance that is required for weathering steel bridges, they find their biggest use in mega-structures, typically bridges, where access for heavy maintenance is not easy to develop and can be quite cumbersome and expensive. Maintenance of weathering steel structures basically comprises washing the members with water whose pressure should be low enough to keep the patina intact, removal of any grown weeds or vegetation and tracing and repairing leaks developed in the structure over time. The maintenance required for weathering steel structures is considerably low as compared to other carbon manganese steel bridges which may even require deeply corroded parts to be reinstalled as a part of maintenance. The low maintenance required for weathering steel bridges also tends to cut down both the direct and indirect maintenance costs, hence increasing the profitability. Health and safety benefits: Replacing ordinary carbon manganese steel with weathering steel is a step that fully corresponds to the increased emphasis placed on the workplace health and safety in the modern age. Use of weathering steel in mega-structures brings health and safety benefits in a number of ways a few of which are discussed below: 1. VOC free environment: Elimination of the need to paint the structure keeps the environment free of the harmful Volatile Organic Compounds (VOCs) that are emitted from the paint as gases. “VOCs are emitted by a wide array of products numbering in the thousands. Examples include: paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings”. (www.epa.gov, 2010). In addition to that, the environment is saved from the contamination caused by blast cleaning debris given the simple nature maintenance work required for the weathering steel bridges. 2. Reduction in the number of accidents: In a vast majority of cases, falls are the most fundamental cause of accidents happening on the construction sites, while a majority of falls from scaffolding occur while plastering and/or painting. Elimination of the need to paint serves to save the workers against the biggest cause of falls, i.e. fall from scaffolding while painting and accordingly, the number of accidents are minimized. Similarly because of reduced amount of physical work required for carrying out the maintenance of weathering steel structure, accidents are minimized. Best alternate for ordinary steel in bridges: Weathering steel has found its biggest use in the construction of bridges. Although bridges have not been the first area of application of weathering steel, over 4000 bridges have been made of weathering steel subsequently after the construction of first weathering steel bridge in the middle of 1960s. (rampartllc.net, 2006). Thermal efficiency enhancer: Weathering steel has a capability to both enhance and weaken the thermal efficiency of a building, depending upon the way it is used. For instance, “When used as a protective rain screen on a facade, it can substantially reduce solar heat gain on the primary structure and also serve as a trellis for a vegetative wall.” (Manfra, 2007, p. 19). Demerits of weathering steel: Certain disadvantages are associated with everything. However, if the advantages of weathering steel are weighed against its disadvantages, the former will outweigh the latter. Also, the disadvantages associated with the use of weathering steel as a construction material can be significantly minimized by careful handling and treatment of weathering steel. Poor performance in certain environments: Although weathering still has been noticed to tackle with the harshness of weather well, yet there are certain environments which impose limits to its stability. Some of them are discussed below: Marine environment: Self protective nature of weathering steel can be adversely affected by air rich in chloride ions and salts. The concentration of the chloride ions can be higher in air. In other cases, weathering steel structures, typically the bridges built over seas or rivers come into contact with splashing waves bringing high amounts of salts and chlorides to the structure. While the salts stay on the structure, it is their hygroscopic nature that serves to maintain continuous dampness over the protective oxide film of the weathering steel. “Weathering steel should not be used for bridges within 2km of coastal waters, unless it can be established that chloride levels do not exceed the salinity classification of S2 (cl . “Volatile Organic Compounds (VOCs)”. An Introduction to Indoor Air Quality. 23 April 2010. Web. 26 June 2010. . Read More