Metals are not generally considered appropriate materials for low-cost constructions in developing countries as they are usually expensive, in most cases imported, and very often require special tools and equipment. However, only a very small percentage of buildings are constructed without the use of metals, either as nails, hinges, roofing sheets or reinforcement in concrete components.
Metals used in construction are divided into two main groups:
• Ferrous metals: irons and steels
• Non-ferrous metals: aluminium (Al), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), tin (Sn), zinc (Zn).
• All ferrous metals are made from pig iron, which is produced by heating iron ore, coke, limestone and some other materials, in a blast furnace.
• Cast irons are alloys of iron, carbon (in excess of 1.7 %), silicon, manganese and phosphorus. They have relatively low melting points, good fluidity and dimensional stability.
• Wrought iron is pure iron with only 0.02 to 0.03 % carbon content, is tough, ductile and more resistant to corrosion than steel, but is expensive and unsuitable for welding, so that it has almost completely been replaced by mild steel.
• Steels are all alloys of iron with carbon contents between 0.05 and 1.5 %, and with additions of manganese, silicon, chromium, nickel and other ingredients, depending on the required quality and use.
• Low carbon steels, with less than 0.15% carbon, are soft and used for wire and thin sheet for tin plate.
• Mild steels, with 0.15 to 0.25 % carbon, are the most widely used and versatile of all metals. They are strong, ductile and suitable for rolling and welding, but not for casting.
• Medium carbon steels, with up to 0.5 % carbon, are specialist steels used in engineering.
• High carbon steels, with up to 1.5 % carbon, have high wear resistance, are suitable for casting, but difficult to weld. They can be hardened for use as files and cutting tools.
• Aluminium, the the most common element, but difficult to recover as a metal (produced with very high energy input and high costs), is the lightest metal, has good strength, high corrosion resistance, high thermal and electrical conductivity, and good heat and light reflectivity. Aluminium and its alloys have numerous applications in building construction, but their high costs and limited availability in most developing countries makes them less appropriate building materials.
• Copper is an important non-ferrous metal, available in its pure form, or as alloys, such as brass, bronze, etc., and suitable for a large number of special uses, but with few applications in low-cost constructions.
• Lead, mainly used in its pure form, is the densest metal, but also the softest, and thus weakest metal. Its good corrosion resistance makes it useful for external applications, eg in roofing (flashings, gutters, etc.), but rarely in low-cost constructions. Its high toxicity makes it a less recommended material, especially where alternatives are available, as for pipes and paint pigments.
• Cadmium, chromium, nickel, tin, zinc and a few other metals are mainly used as constituents of alloys to suit a variety of requirements, or as coatings on less resistant metals to improve their durability, a common example being galvanization (zinc coating) of corrugated iron sheets (gci).
• Structural steel components (columns, beams, joists, hollow sections, etc.) for complete framed structures, or individual elements, such as lintels, trusses, space frames and the like.
• Sheets, usually corrugated for stability, for roofs (mainly galvanized corrugated iron, less commonly corrugated aluminium sheets), walls (infill panels or cladding), sun-shades, fencing, etc.
• Plates, strips or foil for flashings (eg steel, copper, lead), fastenings (as in timber trusses) and facing (for protection against physical damage or for heat reflection).
• Steel rods, mats, wire mesh for reinforcement in concrete and ferrocement. The use of deformed bars (twisted or ribbed) gives higher mechanical bond between steel and concrete, reducing construction costs by up to 10 %. Mild steel wires of 6.5 to 8 mm, drawn through a die at normal temperatures, producing 3,4 or 5 mm wires, have twice their original tensile strength and low plasticity, and are used (predominantly in China) in making prestressed concrete components, saving 30 to 50 % of the steel.
• Wire of various types and thicknesses, eg steel wire for tying steel reinforcements or other building components together, copper wire for electrical installations and thick galvanized
steel, aluminium or copper wire for lightning conductors.
• Galvanized steel wire mesh or expanded metal (made by slotting a metal sheet and widening the slots to a diamond shape) as a base for plaster or for protection of openings.
• Nails, screws, bolts, nuts, etc., usually galvanized steel, for connections of all kinds of construction components, formwork, scaffolding and building equipment.
• Rolled steel sections or extruded aluminium sections of various profiles for door and window frames, shading devices, fixed or collapsible grilles.
• Ironmongery of all kinds, eg hinges, handles, locks, hooks, various security devices, handrails, etc.
• Pipes, channels, troughs for sanitary, electrical, gas installation.
• Construction tools and equipment.
• Miscellaneous metal components for tanks, furniture, outdoor facilities.
• Most metals have high strength and flexibility, can take any shape, are impermeable and durable.
• Prefabricated framed construction systems of steel or aluminium are assembled extremely quickly. With strong connections, such systems can be very resistant to earthquake and hurricane destruction.
• Roofing sheets are easy to transport without damage, easy to install, require minimum supporting structure, permit large spans, are relatively light, are wind- and waterproof, and resistant to all biological hazards. In most developing countries they have a high prestige value.
• Many concrete constructions are only possible with steel reinforcements.
• Similarly, there are often no alternatives to certain uses of metals, eg electrical installations; screws, bolts, etc.; tools; security devices.
• High costs and limited availability of good quality metal products in most developing countries. As a result, inferior quality products are supplied, eg extremely thin roofing sheets, insufficiently galvanized components.
• With regard to roofing sheets: lack of thermal insulation (causing intolerable indoor temperatures, especially with extreme diurnal temperature fluctuations); condensation problems on the underside of roofs (causing discomfort, unhealthy conditions and moisture related problems, such as corrosion and fungal growth); extreme noise during rainfall; tendency of thin sheets to be torn off at nailed or bolted points (particularly those without or with only small washers) under strong wind forces; havoc caused by whirling sheets that have been ripped off in hurricanes.
• Poor fire resistance of most metals: although they are non-combustible and do not contribute fuel to a fire or assist in the spread of flames, they lose strength at high temperatures and may finally collapse.
• Corrosion of most metals: corrosion of ferrous metals in the presence of moisture and some sulphates and chlorides; corrosion of aluminium in alkaline environments; corrosion of copper by mineral acids and ammonia; corrosion of various metals by washings from copper; corrosion by electrolytic action due to contact of dissimilar metals.
• Toxicity of some metals: lead poisoning through lead water pipes or paints containing lead; toxicity caused by fumes emitted when welding metals coated with or based on copper, zinc, lead or cadmium.
• Cost reduction by limited use of metals and design modifications which permit the use of cheaper alternative materials.
• To counteract heat and condensation: avoidance of sheet metal roofs in areas of intense solar radiation and large temperature fluctuations; double layer roofs with ventilated air space and absorptive lower layer; reflective outer surface.
• To prevent corrosion: avoidance of use in moist conditions; periodic renewal of protective coating; in case of dissimilar metals, prevention of contact with non-metallic washers; avoidance of contacts between aluminium and cement products (mortar or concrete).
• For noise reduction: shorter spans and coating of bitumen on underside of roofing sheet; also careful detailing of suspension points, and application of insulating layers or suspended celling.
• For resistance to uplift: thicker gauged sheets and stronger connections.
• To reduce toxicity: avoidance of lead or lead compounds where they may come into contact with food or drinking water; good ventilation of rooms in which toxic fumes are produced.
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