Various types of short, chopped polypropylene fibres are available. Typically they may be added to concrete at a rate of about 0.9 kg/m3. Their primary role is to modify the properties of the fresh concrete. They increase the homogeneity of the mix, stabilising the movement of solid particles and blocking bleed water channels. This reduces the bleed capacity of the concrete and slows down the bleed rate, helping to reduce plastic settlement. The matrix of filements also helps reduce plastic shrinkage cracking which can occur when the concrete surface is allowed to dry out rapidly.
Polypropylene fibres have a limited effect on the properties of the hardened concrete. They do not provide any significant post first crack ductility. Their ability to reduce bleed and segregation assists in maintaining the original water/cement ratio of the surface mortar, which can lead to improvements in the surface layer thereby increasing resistance to abrasion. Polypropylene fibres may be effective in distributing impact stresses and providing some enhancement to frost resistance. They have also been shown to reduce the spalling of concrete in a fire.
Polypropylene fibres are also used in sprayed concrete, to improve the initial properties and to reduce sloughing and rebound.
It is necessary to distinguish between the short polypropylene fibres and the larger synthetic macro-fibres, which provide some structural benefits similar to steel fibres, depending on dosage.
One note of caution is fibres can reduce the slump of concrete as it acts as thickening agent.