Synthetic fibre concrete: when is it the right choice?

Alongside steel fibre, synthetic fibre — polypropylene in particular — is a widely used type of reinforcement in concrete. Where steel fibre is mainly deployed for structural strength, the strength of synthetic fibre lies elsewhere: shrinkage crack control, corrosion resistance and ease of handling. This article explains when synthetic fibre is the right choice.

What is synthetic fibre concrete?

Synthetic fibre concrete is concrete to which polypropylene fibres have been added during mixing. These fibres are generally very thin and around 48 mm long. They are added in dosages that typically range between roughly 2.00 and 6.00 kg per m³.

Why synthetic fibre instead of steel?

No corrosion. Synthetic fibres are not corrosive, unlike steel. This makes them attractive for applications with heavy moisture exposure, outdoor floors, hydraulic engineering structures or exposed floors where rust staining is undesirable.

Shrinkage crack control. Adding polypropylene fibres can achieve various effects, strongly dependent on the diameter, length, surface structure and shape of the fibre. The main effect: limiting plastic shrinkage cracking at an early stage of curing, before the concrete has built up its full strength. This is particularly relevant for large, flat surfaces such as floors and slabs, where shrinkage during curing is a common cause of cracking.

Ease of handling. At moderate dosages, the concrete mix remains readily pumpable, which makes synthetic fibre suitable for projects where the concrete has to be applied with a pump.

Lower weight. Synthetic fibres are considerably lighter than steel, which simplifies transport and handling.

Shape and type of synthetic fibre

There are various forms of synthetic fibre, each with its own mode of action:

• Monofilament: single, smooth fibres that disperse well and are particularly effective against fine shrinkage cracks.

• Fibrillated (mesh-like) fibres: fibres with a mesh structure that provide greater bonding to the cement matrix, and can therefore absorb more force after cracking.

When is synthetic fibre the right choice?

Synthetic fibre is particularly suitable when:

• Corrosion is a concern, for example with outdoor floors, hydraulic engineering or damp environments.

• Shrinkage crack control is the priority, such as with screeds, terraces and large flat surfaces.

• Lightweight handling is desired, for example to avoid heavy reinforcement mesh that has to be moved into place — relevant for smaller projects such as the floor of a garden shed or a new subfloor during a renovation.

• Pumpability is important for execution.

When is steel fibre or traditional reinforcement the better choice?

For heavily mechanically loaded floors where structural strength is the primary requirement — such as assembly halls with intensive forklift traffic — steel fibre generally offers more structural value per volume. In heavily reinforced, self-supporting structures, structural reinforcement also remains necessary; always seek advice from your structural engineer for this.

In conclusion

Synthetic fibre is not necessarily "better" or "worse" than steel fibre — it is a different tool for a different part of the problem. Where steel adds structural strength, synthetic fibre focuses mainly on shrinkage crack control, corrosion resistance and ease of handling. In practice, the two fibre types are also combined to get the best of both worlds.