How is macro fibre dosage calculated? Start with the load, not the catalogue

The most common question about macro synthetic fibre reinforcement is: "How many kilograms per cubic metre?" That the answer is not a single number bothers most people. But that is precisely the right answer: dosage is a property of the project, not of the concrete.

What the range means

Our product page says 1–10 kg/m³. That is not a recommendation — it is the range in which the product works. For ground slabs and sprayed concrete the range we use in practice is 2–4 kg/m³. Four variables decide where in that band you sit.

1. Loads

Forklift traffic, rack leg loads, or foot traffic only? Wheel load and repetition count directly drive the fatigue behaviour of the slab. Point loads under rack legs need a separate check at edges and corners.

2. Slab thickness

Fibre works together with section depth. A 12 cm ground slab and a 20 cm industrial floor will not behave the same at the same dosage. Increasing dosage and reducing thickness lowers total cost on some projects — but that decision belongs with the structural design.

3. Exposure class

Coastal structures, waste-water plants, cold stores and floors with chemical exposure are calculated differently. Fibre's immunity to corrosion is the decisive advantage here: with steel mesh, concrete cover and corrosion risk sit at the centre of the calculation; with fibre that line disappears entirely.

4. Pour method

Pumped or discharged straight from the mixer? Sprayed concrete? Mixing time and method decide whether the fibre disperses homogeneously. A dosage that does not disperse evenly is correct on paper and worthless on site.

What the test data says

Four separate dosages were tested to EN 14651 on notched beam specimens at Istanbul Technical University's Building Materials Laboratory. Residual flexural tensile strength at CMOD 3.5 mm (fR4) came out as:

• 2 kg/m³ → 0.9 MPa
• 3 kg/m³ → 1.3 MPa
• 4 kg/m³ → 1.7 MPa
• 6 kg/m³ → 2.5 MPa

The relationship is close to linear, which shows the dosage decision is calculable rather than arbitrary. All of the reports can be downloaded from our certificates page.

How we work in practice

Starting a project, we ask for: the floor plan, the load schedule, the slab section, the sub-base report and the pour programme. With those we produce a written dosage recommendation within one working day — stating the standard and the test data it rests on.

A number copied from a catalogue comes back as a crack by the end of the first winter.