Technical
How to read an EN 14651 test report: what fR1, fR4 and LOP actually mean
What the CMOD, residual strength and limit of proportionality figures in a fibre reinforcement test report mean, and how they should shape your project decision.
When a fibre reinforcement supplier hands you a test report, a handful of numbers in it tell you everything you need to know about the product. This piece is about reading those numbers.
How the test works
EN 14651 is the standard that measures the flexural behaviour of fibre reinforced concrete. It uses prismatic specimens of 150×150×500 mm. A 25 mm deep notch is cut at mid-span — which is how you decide where the crack will form.
The specimen is loaded in three-point bending over a 500 mm span. The test is closed-loop and deformation-controlled: you control the opening of the crack, not the load. A gauge set into the notch records how far the crack mouth opens (CMOD — Crack Mouth Opening Displacement).
LOP: before the crack
LOP (Limit of Proportionality) is the point at which the load–displacement curve departs from linear behaviour. In practice it is the flexural tensile strength at the moment the concrete first cracks.
The critical point here: LOP is largely a property of the concrete, not of the fibre. Across four different dosages in the İTÜ reports, LOP stays around 5.0–5.1 MPa. That is the expected result — the fibre does not engage until the concrete has cracked.
fR values: after the crack
This is the heart of it. Once a crack has formed, what holds the concrete together is the fibre. fR1, fR2, fR3 and fR4 give the residual flexural tensile strength at four different crack widths:
| Value | CMOD | What it tells you | |---|---|---| | fR1 | 0.5 mm | Behaviour under service conditions | | fR2 | 1.5 mm | Moderate crack | | fR3 | 2.5 mm | Advanced crack | | fR4 | 3.5 mm | Ultimate condition, load capacity |
In design, fR1 is generally used for the serviceability limit state and fR3 for the ultimate limit state.
The effect of dosage
In the tests carried out on POLY-PRO 54 M at İTÜ's Building Materials Laboratory, fR4 rose with dosage as follows:
- 2 kg/m³ → 0.9 MPa
- 3 kg/m³ → 1.3 MPa
- 4 kg/m³ → 1.7 MPa
- 6 kg/m³ → 2.5 MPa
That the fR values roughly triple while LOP stays flat shows the fibre working exactly where it is expected to — in post-crack behaviour.
What to check on any report
- Which standard was used? It should be EN 14651.
- How many specimens? One is not enough; the mean of four is standard practice.
- Is the concrete class stated? fR values only mean something alongside the concrete.
- Is the dosage stated? An fR value without a dosage says nothing.
- Is the report from an independent laboratory?
All of our own reports — one per dosage — can be downloaded from our certificates page.
