Concrete on its own performs well in compression but not in tension. Steel reinforcement is used to deliver tensile capacity where it is needed. Hence reinforced concrete uses different materials very efficiently. This minimisation of material use is often taken for granted but is a major contributor to sustainability.
Rebar (reinforcing bar) is usually formed from ridged carbon steel; the ridges give frictional adhesion to the concrete. Whilst any material with sufficient tensile strength could conceivably be used to reinforce concrete, steel is used in concrete as they have similar coefficients of thermal expansion. This means that a concrete structural member reinforced with steel will experience minimal stress as a result of differential expansions of the two interconnected materials due to temperature changes.
The steel used in UK reinforced concrete utilises 100% recycled scrap steel as feedstock. At the end of its life, all reinforcing steel can be recovered, recycled and used again.
Embodied carbon
The embodied energy values of reinforcing steel are based on the energy used to melt scrap metal and reform it. Although all steel manufacture is an energy-intensive process, the energy needed to produce one tonne of steel from scrap steel is as low as one third of that needed to make one tonne of steel from iron ore. All the reinforcement made in the UK is made from recycled scrap steel. Equally, reinforcing steel itself can be recovered, recycled and re-used at the end of a building or structure’s service life.
There are two common steelmaking processes used for steel in the UK market. These are Basic Oxygen Steelmaking (BOS) and Electric Arc Furnace (EAF) steelmaking. The BOS route is the most widely used steelmaking process worldwide and involves the smelting of iron ore, coal and other raw materials in a two-stage process. The EAF production process involves passing an electric charge through scrap metal, melting it; thus enabling it to be recycled into new products.
The EAF process normally uses approximately 98% scrap metal as the raw material. An EAF furnace generally produces 0.5 to 1.0 million tonnes per annum, making it ideally suited to smaller-scale steel making operations typically used for the manufacture of reinforcing steel. EAF production sites typically include specialised rolling mills producing long products such as reinforcing bar. UK-produced reinforcement uses this energy efficient EAF process.
Non-steel reinforcement
Some construction cannot tolerate the use of steel. For example, MRI (Magnetic Resonance Imaging) machines have huge magnets, and need to be housed in nonmagnetic buildings. For these purposes some structures have been constructed using fibre-reinforced plastic rebar, grids or fibres. The 'plastic' reinforcement can be as strong as steel.
Fibres are often also used in concrete to produce fibre-reinforced concrete, which is a concrete mix that contains short discrete fibres that are uniformly distributed and randomly oriented. Types of fibres include steel, glass, synthetic and natural fibres. Within these different fibres that character of fibre reinforced concrete changes by varying the concrete's fibre materials, geometries, distribution, orientation and densities.