There are many different materials one can use to build structures. The most common are cement-based materials (e.g. concrete), metals (e.g. steel), clay (e.g. bricks), wood, plastics (e.g. PVC), composites (e.g. GRP) and many more.

 

When an engineer designs a new structure or object, the choice of construction material will be important for a number of reasons. The material’s strength-to-weight ratio can for example be crucial when designing an aircraft. The tensile strength of low carbon steel (365 MPa) and of aluminium alloy (310 MPa) may seem reasonably close, but considering that the density of steel (7.87 g/cm³) is 2.9 times higher than the density of aluminium (2.70 g/cm³), an airplane made of steel would be almost three times as heavy as one made from aluminium.

 

Typical considerations when selecting a material for construction are:

• Its strength (to carry weight)
• Its flexibility (to handle vibration / movements)
• Its weight (to move / handle the object)
• Its ability to be formed into the required shape
• Its durability (against environmental conditions, e.g. corrosion & weathering)
• Its need for maintenance work
• The cost of the material
• Life Cycle Cost (a combination of many of the above factors)

 

In this chapter, we will focus on metals used for construction, and in particular:

• Iron and its alloys
• Aluminium and its alloys
• Copper and its alloys
• Zinc
• Titanium

General

Iron in its various forms is a very common material on Earth. Pure iron (Fe) is not suitable for constructions, but mixed with carbon it forms steel, which is the second most used construction material in the world (after concrete).

 

All steel is composed of iron and carbon. It is the amount of carbon, and the additional alloys that determine the properties of each grade. While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium.

 

According to the American Iron & Steel Institute (AISI), Steel can be categorized into four basic groups based on the chemical compositions:

• Carbon Steel
• Alloy Steel
• Stainless Steel
• Tool Steel

 

There are many different grades of steel that encompass varied properties. These properties can be physical, chemical and environmental.

 

A. Carbon Steel

Carbon Steel has a carbon content of up to 2%.  The carbon content will influence the properties of the steel, such as:

• Hardness
• Flexibility
• Brittleness
• Welding suitability

 

Steel can be categorized by its carbon content into four main categories:

• Low less than 0,3% carbon, called mild steel
• Medium 0,3 – 0,6% carbon, increased hardness
• High 0,6 – 0,99% carbon, difficult to weld
• Ultra-high 1 – 2% carbon, for tools, springs, etc.

 

Corrosion resistance will be increased as the carbon content increases. Once the carbon content is more than 2% and less than 4%, the steel is called cast iron.

 

Low Carbon Steel (Mild Steel)

Typically contain 0.04% to 0.30% carbon content. This is one of the largest groups of Carbon Steel. It covers a great diversity of shapes; from Flat Sheet to Structural Beam. Depending on the desired properties needed, other elements are added or increased. For example: Drawing Quality (DQ) – The carbon level is kept low and Aluminium is added, and for Structural Steel the carbon level is higher and the manganese content is increased.

 

Medium Carbon Steel

Typically has a carbon range of 0.31% to 0.60%, and a manganese content ranging from .060% to 1.65%. This product is stronger than low carbon steel, and it is more difficult to form, weld and cut. Medium carbon steels are quite often hardened and tempered using heat treatment.

 

High & Ultra High Carbon Steel

Commonly known as “carbon tool steel” it typically has a carbon range between 0.61% and 2%. High carbon steel is very difficult to cut, bend and weld. Once heat treated it becomes extremely hard and brittle.

 

B. Alloy Steel

Alloy steel is steel that has had small amounts of one or more alloying elements (other than carbon) such as manganese, silicon, nickel, titanium, copper, chromium and aluminium added. This produces specific properties that are not found in regular carbon steel. Alloy steels are workhorses of industry because of their economic cost, wide availability, ease of processing, and good mechanical properties. Alloy steels are generally more responsive to heat and mechanical treatments than carbon steels.

 

Low alloy Steels

Iron with less than 0.25% carbon (often below 0.15%), with alloying elements totalling only 2 – 3%. Typical alloy elements include:

• Carbon significantly increases mechanical strength
• Nickel increases ductility and toughness, it also reduces the corrosion rate
• Chromium gives stainless steels their basic corrosion resistance
• Molybdenum significantly increases the resistance to both uniform and localized corrosion. It increases the hardness at higher temperatures
• Manganese is generally used to improve the metals ability to be formed when hot
• Silicone increases resistance to corrosion and increases strength

 

The corrosion resistance of carbon steel and low alloy steel is more or less the same. They are used to construct objects such as ship’s hulls, buildings, bridges, cars, etc.

 

Weathering steels

Weathering steel, best-known under the trademark COR-TEN steel (sometimes written without the hyphen as corten steel), is a group of steel alloys which were developed to eliminate the need for painting, and form a stable rust-like appearance if exposed to the weather for several years.

 

It exhibits increased resistance to atmospheric corrosion when compared to unalloyed steels by forming a firmly adhering protective rust-coloured oxide layer, or patina, on its surface, as can be seen on certain buildings.

 

Cor-Ten steel is traditionally used in the construction of shipping containers, although usually overcoated by special container paint.

 

Weathering steel is a special type of low alloy steel:

• Better resistance against atmospheric corrosion than ordinary structural steels
• A result of having been alloyed with slightly higher amounts of chromium, phosphorus and copper
• Will corrode during an initial period of 1,5 – 4 years, like carbon steel
• Will form a dense and dark surface layer (rust coat) called “PATINA”, which protect material and provide decorative effects
• When used on building facades it is usually not painted and left with its patina, but unforeseen changes in the climatic conditions may cause unexpected severe corrosion

 

High alloy Steels

Mostly called stainless steel, with alloying elements totalling 15 – 30%

 

• Stainless steel is defined as a steel alloy with a minimum 11% chromium content by weight
• Steel is alloyed primarily with Chromium and Nickel
• Molybdenum may be added for increased corrosion resistance
• A thin protective chromium-oxide film is formed on the surface of the metal, preventing rust from appearing
• Stainless steels are not immune to corrosion, the protective oxide layer may be damaged or attacked by chlorides, the environment, lack of oxygen and other influences which may lead to extensive corrosion / pitting

 

There are many different stainless steel qualities and naming systems in use globally based on various published standards, such as EN-standard and AISI / ASTM standards. Examples:

• ASTM 304 (18-20% Cr, 8-10,5% Ni) - plain
• ASTM 316 (16 – 18,5% Cr, 10 - 14% Ni, 2 -3% Mo) – more seawater resistant
• ASTM S31254 (19,5-20,5% Cr, 17,5-18,5 Ni, 6 -6,5% Mo) – even more seawater resistant

Modern steelmaking can be broken down into six steps:

 

1. Iron-making: In the first step, the raw materials iron ore, coke and lime are melted in a blast furnace. The resulting molten iron - also referred to as ‘hot metal‘ - still contains 4-4.5% carbon and other impurities that make it brittle.
2. Primary Steelmaking: Primary steelmaking methods differ between BOS (basic oxygen steelmaking) and EAF (electric arc furnaces) methods.
   • EAF methods feed recycled steel scrap through high power electric arcs (temperatures up to 1650 °C) to melt the metal and convert it to high-quality steel.
   • BOS methods add recycled scrap steel to the molten iron in a converter. At high temperatures, oxygen is blown through the metal, which reduces the carbon content to between 0 - 1.5%.
3. Secondary Steelmaking: Secondary steelmaking involves treating the molten steel produced from both BOS and EAF routes to adjust the steel composition. This is done by adding or removing certain elements and/or manipulating the temperature and production environment. Depending on the types of steel required, the following secondary steelmaking processes can be used:
   • stirring
   • ladle furnace
   • ladle injection
   • degassing
   • CAS-OB (Composition Adjustment by Sealed argon bubbling with Oxygen Blowing)
4. Continuous Casting: In this step, the molten steel is cast into a cooled mould causing a thin steel shell to solidify. The shell strand is withdrawn using guided rolls and fully cooled and solidified. The strand is cut into desired lengths depending on application; slabs for flat products (plate and strip), blooms for sections (beams), billets for long products (wires) or thin strips.
5. Primary Forming: The steel that is cast is then formed into various shapes, often by hot rolling, a process that eliminates cast defects and achieves the required shape and surface quality.
6. Manufacturing, Fabrication, and Finishing: Finally, secondary forming techniques give the steel its final shape and properties. These techniques include:
   • shaping (e.g. cold rolling)
   • machining (e.g. drilling)
   • joining (e.g. welding)
   • coating (e.g. galvanizing)
   • heat treatment (e.g. tempering)
   • surface treatment (e.g. carburizing)

 

Steel rolling

Rolling is a metal forming process in which metal stock is passed through pairs of rollers in order to re-crystallize, and modify, strengthen or to form an expected shape and thickness

• Hot rolling is rolling that occurs above the re-crystallization temperature of the material (usually above 600^oC)
• Cold rolling occurs with the metal is below its re-crystallization temperature (usually at room temperature)

 

Hot Rolling

• The hot cast steel is forced between rollers under high pressure and thereby given new shapes and dimensions
• By using combinations of multiple rollers in different configurations, hot rolled products are formed into flat products (steel plates), long products, seamless tubes, various profiles (H-beam, I-beam, L-shape, etc.) and specialty products.
• During hot rolling, a special form of iron oxide is formed on the surface of the steel, called Mill Scale
• Mill Scale is cathodic to the base metal (steel)
• Mill scale is bluish-black in colour and forms initially a barrier to corrosion of the steel
• Mill scale is brittle and will eventually crack and flake off, meaning the steel is no longer protected and will suffer from accelerated corrosion due to the remaining noble mill scale fragments

 

Cold Rolling

• Cold rolled steel does not have mill scale
• Cold rolled steel is thinner than hot rolled steel (usually below 4 mm) and has better strength

• Aluminium (Al) is a silvery-white, soft, nonmagnetic, ductile metal. Aluminium is the third most abundant element in the Earth‘s crust (after oxygen and silicon) and is its most abundant metal.
• Aluminium metal is so chemically reactive that pure metal is very rare to find in the nature. Instead, it is found combined in over 270 different minerals. The chief ore of aluminium is bauxite.
• Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and structures, such as building facades and window frames

• Light-weight metal with silver colour
• Typically alloyed with copper, magnesium, manganese, silicon, and zinc
• Active metal, but passive oxide film (0,01 µm) hinders corrosion
• Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required
• Aluminium can corrode when the protective oxide layer is damaged by impact, abrasion or chemical attack

Copper and all its common alloys have good corrosion resistance.

 

Copper

Pure copper (Cu) is a reddish-brown metal. It is soft, has good electrical conductivity and ductility.

 

Typical use

• Plumbing
• Heat exchangers
• Process equipment
• Roofing
• Electrical wires

 

Patina on copper

The surface film which forms on copper during atmospheric exposure contains basic salts and is quite protective. The film has a pleasant green colour and is used for architectural applications, such as decorative details and on roof, gutters and downpipes on prominent buildings.

 

Bronze

Bronze is an alloy consisting primarily of copper, commonly with about 12% tin and often with the addition of other metals (such as aluminium, manganese, nickel or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus or silicon. These additions produce a range of alloys that may be harder than copper alone, or have other useful properties, such as stiffness, ductility or machinability. Lead and nickel may be added to achieve different properties.

 

Other elements such as phosphorus, aluminium, manganese, and silicon can be used to alloy with copper, and are also characterised as “bronze” (Nickel bronze, Aluminium bronze, etc.)

 

The archaeological period where bronze was the hardest metal in widespread use is known as the Bronze Age.

 

Typical use

• Engineering parts, such as:
  • Ships’ propellers
  • Boat and ship fittings
  • Springs, bearings, bushings, automobile transmission pilot bearings
• Bells (church & ships bells)
• Sculptures / statues
• Ornaments

 

Brass

It is an alloy of copper and zinc, where the zinc content may vary from about 5 to 45%

 

Typical use

• Brass is used for decoration for its bright gold-like appearance;
• for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition casings and valves
• for plumbing and electrical applications
• Used extensively in brass musical instruments such as horns and bells where a combination of high workability (historically with hand tools) and durability is desired.
• It is also used in zippers.
• Brass is often used in situations in which it is important that sparks not be struck, such as in fittings and tools used near flammable or explosive materials.
• Coins (most copper coins are actually brass)
• Medals

Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper. Zinc is a bluish-white, lustrous metal, though most common commercial grades of the metal have a dull finish. The metal is hard and brittle at most temperatures. Zinc is a fair conductor of electricity and has relatively low melting (419.5 °C) and boiling points (907 °C).

 

It is a moderately reactive metal. The surface of the pure metal tarnishes quickly, eventually forming a protective passivating layer of basic zinc carbonate, by reaction with atmospheric carbon dioxide. This layer helps prevent further reaction with air and water.

 

Apart from Zinc sheet metal used as roofing and cladding material on buildings, Zinc is not much used as a construction material. The main consumption of zinc within construction industry is used for galvanizing steel.

 

Galvanizing is the process of applying a protective zinc coating to steel or iron, to prevent corrosion. The most common method is hot-dip galvanizing, in which parts are submerged in a bath of molten zinc. Galvanizing protects in three ways:

• It forms a coating of zinc which, when intact, prevents corrosive substances from reaching the underlying steel or iron.
• The zinc serves as a sacrificial anode so that even if the coating is scratched, the exposed steel will still be protected by the remaining zinc.
• The zinc protects its base metal by corroding before iron.
• For better results, application of chromates over zinc is also seen as an industrial trend.
• Not typically used in Marine Immersion / Splash Zones
• Good corrosion resistance (~50 -100 years inland)

Titanium (Ti) is a lustrous transition metal with a silver colour, low density and high strength. It is highly resistant to corrosion.

 

Properties

• Low density
• Strong
• Metallic sheen
• Corrosion-resistant to sea water, aqua regia (a mixture of nitric acid and hydrochloric acid) and chlorine
• Noble metal, with good passive film to prevent corrosion
• Used in aggressive environments

 

Because of their high tensile strength to density ratio, high corrosion resistance, fatigue resistance, high crack resistance, and ability to withstand moderately high temperatures without creeping, titanium alloys are used in:

• About two thirds of all titanium metal produced is used in aircraft engines and frames
• Armour plating, naval ships, spacecraft, and missiles
• When alloyed with aluminium, zirconium, nickel, vanadium, and other elements: critical structural parts, fire walls, landing gear, exhaust ducts (helicopters), and hydraulic systems
• In the chemical and petrochemical industries: welded titanium pipe and process equipment (heat exchangers, tanks, process vessels, valves), used primarily for corrosion resistance
• Automotive applications, particularly in automobile and motorcycle racing where low weight and high strength and rigidity are critical
• Sporting goods: tennis rackets, golf clubs, lacrosse stick shafts; cricket, hockey, lacrosse, football helmet grills, and bicycle frames and components
• Jewellery (rings, necklaces, bracelets, etc.)
• Many medical uses, including surgical implements and implants, such as hip balls and sockets (joint replacement) and dental implants