Rebar (reinforcing bar) is an important component of reinforced concrete. It is usually formed from ridged carbon steel; the ridges give frictional adhesion to the concrete. Rebar is used because although concrete is very strong in compression it is virtually without strength in tension. To compensate for this, rebar is cast into it to carry the tensile loads on a structure.
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. The embodied energy values of reinforcing steel are based on the energy used to melt and reform it. The energy input per tonne of reinforced steel is less than half of that for structural steel.
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.

Sustainability credentials

CO2 Manufacture of steel reinforcement bars for reinforced concrete could be a course of significant energy consumption and a large contributor to embodied CO2. However, the UK industry uses the Electric Arc Furnace process, which generates up to six times less CO2 than those emanating from the Basic Oxygen Steel making system that is used for making other UK steel. 
Recycling UK produced reinfrocement for concrete is manufactureed from 100 per cent recycled UK scrap steel. Scrap steel reinforcement from demolished buildings is recycled to manufacture raw steel reinforcement. Two thirds of reinforcement used in the UK is produced in the UK. The majority of imported reinforcement is also produced from scrap steel by Electric Arc Furnace.
Resource depletion The use of Electric Arc Furnaces allows reinforcement steel to be made from 100% scrap metal, reducing the specific energy (energy per unit weight) required to produced the steel, but also relieving pressure on the Earth's natural ore resources. The UK is a net exporter of scrap steel.
Source: Concrete Credentials: Sustainability, MPA - The Concrete Centre, 2010
Links to further information

Visit The British Association of Reinforcement at

For more information on reinforcement and concrete, download the following publication from The Concrete Centre: