Various types of fibre may be added to the concrete to improve its properties, either in the fresh (unhardened) state or in service. The main fibres used are steel, polypropylene and glass; see separate entry Constituents/Fibres for details of their properties.
Steel fibre concrete is used extensively for industrial ground-supported floors, with between 20 and 50 kg per m3. The fibres provide some degree of control over the formation of early thermal and shrinkage cracks. They give the hardened concrete some post-cracking residual strength; depending on the type of fibre used and the dosage, the residual strength may be about 50% of the tensile strength. Short steel fibres are also used in sprayed concrete to make the material more cohesive and to reduce rebound as well as crack control.
Various types of short polypropylene fibre may be added to concrete. Typically the dosage rate is around 0.9 kg per m3. The fibres are used to reduce the risk of crack formation in young concrete (plastic cracking). They do not significantly affect the properties of the hardened concrete, though some increase in freeze-thaw and impact resistance is claimed for some materials.
In the past few years, larger diameter synthetic fibres have been introduced, known as macro synthetic fibres, which can give similar post-cracking strength to steel fibres. They are used in similar applications, e.g. paving, sprayed concrete and precast units, particularly when there are concerns about durability.
A recent innovation has been the use of steel or macro synthetic fibres to replace the nominal reinforcement in composite slabs on metal decking (see Structural elements/Suspended floors/Steel concrete composite suspended floors). Note that only specific combinations of fibres and decking have been approved for use.
Alkali-resistant glass fibres are used in glassfibre reinforced concrete (GRC), see separate entries.
Further information on the use of fibres in concrete may be found in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete and Technical Report 65, Guidance on the use of macro-synthetic-fibre-reinforced concrete.