Shrinkage fibre for sand-cement screeds: limiting cracking

Sand-cement screeds are indispensable as a level, sturdy base for tile, parquet and other floor finishes — but they have one well-known weak point: shrinkage during drying can lead to cracking. Shrinkage fibre is one of the most widely used ways to limit this risk. This article explains how it works, when you need it, and how it compares to traditional shrinkage reinforcement.

What is a sand-cement screed?

A sand-cement screed (also called a cement screed, floated screed or mortar screed) is a mixture of sand, cement and water applied on top of an underlying load-bearing floor to level out unevenness, bridge height differences, or embed underfloor heating pipes. It is a commonly used substrate in both new-build and renovation, for both housing and commercial buildings.

Why does cracking occur?

As the sand-cement screed dries, the mix loses moisture, which leads to shrinkage. This shrinkage is a normal, unavoidable part of the curing process of cement-bound mortars. The problem is not the shrinkage itself, but the stress that arises when the floor cannot shrink freely — for example because it is fixed at the edges, or because the underlying insulation layer of a floating (loose-laid) screed moves unevenly with it. This build-up of stress is released as cracks.

What is shrinkage fibre and how does it work?

Shrinkage fibre consists of fine synthetic fibres (usually polypropylene) added directly to the sand-cement mix during mixing. Unlike traditional shrinkage reinforcement — a fine-mesh net of steel wire or synthetic material (chicken wire mesh) laid into the floor by hand — fibres are distributed evenly throughout the entire volume of the screed.

As soon as a micro-crack forms due to shrinkage, the fibres in the immediate vicinity bridge that crack plane. This does not prevent cracks from forming, but it does limit how wide these cracks become — comparable to the effect of fibre reinforcement in concrete, as we described earlier in our article on the mechanism of fibre reinforcement.

Shrinkage fibre versus shrinkage reinforcement (mesh)

It is important to understand the difference clearly: according to manufacturers, shrinkage fibre is a secondary reinforcement — it does not replace mesh, but strengthens its effect.

Neither method prevents cracking entirely — shrinkage is, after all, an intrinsic property of curing cement-bound material. Both limit crack width, which in practice makes the difference between invisible hairline cracks and noticeable, visible cracks.

When is shrinkage fibre (alone) sufficient, and when should it be combined with mesh?

• Floors with underfloor heating and/or floating (loose-laid) screeds: here, fibre addition is applied as standard, due to the additional risk factors (thermal movement from heating, an insulation layer that can shift).

• Floating screeds thinner than 65 mm: here it is recommended to apply shrinkage reinforcement (mesh) in combination with fibre reinforcement, because the reinforcement also helps with any deformation of the insulation layer.

• Floating screeds on a firm, barely compressible insulation layer: here fibre reinforcement alone can be sufficient, because no deformation occurs and the reinforcement is only needed to limit shrinkage.

• Screeds of 6 cm or thicker: here shrinkage reinforcement (a mesh) is required under current practice, unless a plasticiser is used to reduce the amount of mixing water — and thus shrinkage.

Practical points of attention

• Keep moist while drying. Besides reinforcement and fibres, it remains important to keep the floor moist for the first few days after laying, to limit overly rapid and uneven drying (and thus shrinkage).

• Movement joints for large areas. For larger floor areas, movement joints remain necessary to control shrinkage cracks, even when fibres or a reinforcement mesh are used.

• Alternative: shrinkage-reducing admixtures (SRA). Besides fibres and reinforcement, there is another route to limit shrinkage: shrinkage-reducing admixtures, which lower the surface tension of the capillary water in the mortar. Field studies show a shrinkage reduction of up to around 30-40%, depending on dosage and mix characteristics. This is a supplementary, not a replacement, measure.

In conclusion

Shrinkage fibre is an effective, easy-to-apply way to limit cracking in sand-cement screeds — particularly relevant for underfloor heating and floating screeds. For the best protection against cracking: fibre and mesh are not competitors but complementary measures, with the fibre limiting crack width throughout the whole volume and the mesh providing extra resistance at the point where stress is greatest.