Dosing concrete fibres: practical guidelines

The correct dosage is decisive for the outcome of fibre reinforced concrete: too few fibres and the desired effect fails to materialise, too many and workability is compromised. This article provides practical guidelines per fibre type.

What determines the correct dosage?

The correct dosage depends on several factors:

• Fibre type: steel, synthetic (macro or micro) or basalt each have a different density and mode of action, and therefore a different typical dosage.

• Fibre parameters: length, diameter, shape and bonding behaviour. In particular, the ratio of length to diameter (l/d ratio) partly determines performance: the higher the l/d ratio, the better the performance generally is — but longer fibres are also harder to mix.

• Application and intended effect: structural strength, shrinkage crack control or fire resistance each call for a different dosage.

• Mix composition: proper adjustment of the concrete mix (particularly in the fine fraction) is needed to allow optimal coating and bonding of the fibres.

Guidelines per fibre type — steel fibre

For steel fibre reinforced concrete (SFRC), a typical dosage of around 0.3 to 1 volume percent of the concrete mix applies, which works out at roughly 25 to 50 kg/m³. Higher dosages are possible for special, more heavily loaded applications. For walls, for example, steel fibres with a length of 40 to 60 mm and a diameter of 0.6 to 0.9 mm are often used — cold-drawn, galvanised steel wire fibres with hooked ends for extra anchoring.

Macro synthetic fibre

Macro synthetic fibres, which — like steel fibre — can fully or partly replace traditional reinforcement, generally require a calculation based on the intended reinforcement plan. The exact dosage differs per product and application.

Micro synthetic fibre

Micro synthetic fibres (usually polypropylene) are applied in much greater numbers but at a much lower total weight: typically between 500 and 2,500 grams per m³. These fibres are very thin and around 10 to 20 mm long, and are particularly effective against shrinkage cracks thanks to the enormous number of fibres introduced per m³.

Why too many fibres is a problem

A higher dosage does not automatically mean a better result. At an excessive dosage, problems can arise such as blockage of the dosing equipment at the concrete plant, reduced workability of the concrete mix, and an increased risk of fibre balling during mixing — which can actually create weak spots in the concrete rather than stronger ones.

Homogeneous distribution is crucial

Besides the correct dosage, it is at least as important that the fibres are homogeneously distributed throughout the concrete mix. A good fibre does not settle out or float to the surface of the mix and does not form balls during mixing. This is determined partly by the mixing process at the concrete plant and partly by the quality of the fibre itself.

Who determines the final dosage?

For applications where fibres must (also) deliver structural strength, the dosage is generally determined by the structural engineer in collaboration with the fibre supplier, who uses specific calculation software for this purpose. For non-structural applications (such as shrinkage crack control), a standard dosage based on practical experience and the supplier's product guidelines is often sufficient.

In conclusion

The correct dosage of concrete fibres is bespoke: it depends on the fibre type, the application, the desired properties and the mix composition. Too low a dosage delivers insufficient effect, too high a dosage brings handling risks — so always agree the dosage with your supplier and, for structural applications, your structural engineer.