How does fibre reinforcement work in concrete?

Fibre reinforced concrete (also called fibre concrete) is concrete to which loose steel, synthetic, glass or basalt fibres have been added — typically 25–50 kg of steel fibre or 0.5–6 kg of synthetic fibre per m³. The fibres bridge cracks and take over the tensile force the moment the concrete cracks. Failure behaviour thereby shifts from brittle to ductile and the concrete retains load-bearing capacity: the residual strength.

What is fibre reinforced concrete?

Concrete fibres are short, loose fibres — of steel, synthetic material (polypropylene), glass or basalt — that are distributed through the fresh concrete during mixing. The result is a homogeneous material containing thousands to millions of small reinforcing elements per cubic metre, in every direction and at every location. Internationally this material is known as FRC: fibre reinforced concrete.

That is precisely the fundamental difference from traditional reinforcement. Reinforcing steel is placed deliberately where the structural engineer expects tension; fibres are everywhere, and therefore exactly where a crack happens to form. To understand why that is so valuable, it helps to first look at the weak point of plain concrete.

Concrete is strong in compression, weak in tension

Concrete can carry enormous compressive forces, but in tension it is weak and brittle: the tensile strength is, indicatively, only around 10% of the compressive strength. As soon as the tensile stress exceeds the (low) tensile strength, a crack forms and unreinforced concrete fails almost immediately and without warning. For floors and beams the relevant measure is the flexural tensile strength: the tensile stress in the underside of a deflecting element at the moment the first crack forms.

Traditionally this is addressed with reinforcing steel at the locations where tension occurs. Fibre reinforcement tackles the same problem differently: instead of a few large bars, a large number of small fibres are distributed evenly throughout the entire concrete mass. How that approach compares with bars and mesh is covered in fibre concrete versus traditional reinforcement.

The principle: crack bridging

The core of fibre reinforcement is crack bridging. As long as the concrete is uncracked, the fibres do little; the matrix carries the load. The moment a crack forms, that changes. Every concrete mix shrinks during curing and responds to temperature changes and loading; wherever the tensile stress exceeds the tensile strength of the hardened cement paste, a micro-crack forms. Without reinforcement, such a micro-crack propagates unhindered into a visible, structural crack.

In fibre reinforced concrete, thanks to the homogeneous distribution achieved during mixing, there are always fibres in the immediate vicinity of where a small crack starts. As soon as the crack plane reaches a fibre, that fibre begins to take up force: the tensile stress that was first concentrated at a single point is spread via the fibre across a larger volume of concrete. The fibres crossing the crack tension across the crack faces and carry the tensile force from one side to the other — countless small bridges instead of a single fracture plane.

That bridging works through the anchorage and friction between fibre and concrete. For a crack to open further, the fibres must be pulled out of the matrix. That pull-out costs energy — and it is precisely that energy absorption that ensures the concrete does not fail all at once, but gradually and in a controlled manner.

Three effects of crack bridging

• Delayed crack initiation — because the fibres act from the very first micro-cracks, it takes longer for a crack to become visible and structural.

• Limited crack width — where unreinforced concrete forms one wide crack, fibre reinforced concrete often develops several finer cracks that each carry less force. Finer cracks let through less water and fewer aggressive substances, which benefits the durability of the structure.

• Post-cracking behaviour — the characteristic from which fibre reinforced concrete derives its added value: because fibres bridge crack faces, the concrete retains tensile capacity after cracking, instead of failing immediately as unreinforced concrete does.

From brittle to ductile: residual strength after cracking

The most important property fibres add is called residual strength (or post-cracking strength): the load the concrete can still carry after the first crack has formed. In unreinforced concrete that residual strength is virtually zero. In fibre concrete a significant load-bearing capacity remains, because the fibres continue to bridge the crack. This translates into higher toughness, better impact resistance and gradual, predictable failure behaviour.

How high that residual strength is depends on the fibre type, the dosage and the combination of properties in the hardened concrete — not on a single isolated factor. That is exactly why fibre concrete is assessed with a standardised bending test, covered further on.

Two scales: micro and macro fibres

Fibres work at two different moments in the life of the concrete, and two types of fibre correspond to those moments.

• Micro fibres (thin, usually polypropylene, such as micro fibres against plastic shrinkage cracks) are mainly active in fresh concrete. They form a fine-meshed network that counters early-age shrinkage cracks in the first hours after pouring, when the concrete barely has any tensile strength yet. Typical dosage: 0.5–1 kg/m³.

• Macro fibres (thicker, of steel or robust synthetic material, such as the structural macro fibres that take over the tensile force) do their work in hardened concrete. They provide the residual strength after cracking and can thereby contribute structurally to the load-bearing capacity.

Both types are sometimes combined (a hybrid solution), so that both early-age shrinkage cracking and behaviour under load are covered. An overview of all fibre types and their properties can be found in types of concrete fibres compared.

Steel fibres and synthetic fibres: difference in how they work

Steel fibres — for example hooked-end steel fibres for residual strength after cracking — are generally used to genuinely take up tensile forces and allow the structure to carry heavier loads; they therefore function as a form of reinforcement. Common dosages lie between 25 and 50 kg/m³.

Polypropylene fibres are added in far greater numbers, but at a much lower dosage by weight — indicatively 0.5–6 kg/m³, depending on whether they are micro or macro fibres. With micro fibres, the effect lies mainly in limiting (plastic) shrinkage cracks at an early stage of curing, before the concrete has reached its full strength; synthetic macro fibres additionally deliver residual strength in the hardened concrete and are immune to corrosion.

What determines effectiveness?

Not every fibre dosage delivers the same result. Performance is determined by an interplay of factors:

• Dosage — the quantity of fibres per m³ of concrete. More fibres generally means more residual strength, up to a practical limit beyond which workability (and pumpability) decreases.

• Slenderness (aspect ratio) — the ratio between length and diameter. More slender fibres bond and bridge more effectively, but are harder to mix in evenly.

• Fibre material and stiffness — stiff fibres such as steel already contribute substantially at small crack widths; more flexible synthetic fibres mainly deliver toughness and crack control at larger deformation.

• Anchorage and surface texture — profiled, ribbed or hooked fibres grip better in the matrix and pull out less easily than smooth fibres.

• Shape — single fibres (monofilament) and mesh-like, fibrillated fibres each have their own way of transferring force.

• Orientation and distribution — fibres work best when well distributed and favourably oriented relative to the crack direction.

Practical guideline values per application can be found in concrete fibre dosages; with the fibre calculator you can work out the quantity required for your project straight away.

What does and doesn't change in the concrete?

A common misconception is that fibres make concrete stronger across the board. In reality, the gain lies almost entirely in the cracked stage and in behaviour under tension — not in compressive strength. What fibres do and don't do, per property:

• Compressive strength — barely any effect: compression is carried by the concrete matrix itself.

• Tensile and flexural tensile strength (uncracked) — small effect: the first crack forms at roughly the same load as in unreinforced concrete.

• Behaviour after cracking — the biggest gain: the fibres bridge the crack and continue carrying the tensile force (residual strength).

• Toughness and energy absorption — strongly improved: failure proceeds gradually rather than suddenly and brittle.

• Crack width — finer, better-distributed cracks instead of a few wide cracks.

• Impact and fatigue resistance — clearly higher due to the energy absorbed as fibres are pulled out.

• Durability — narrower cracks limit the ingress of water and salts, which benefits service life.

This combination explains why fibre reinforced concrete performs particularly well against shrinkage, impact loading and cyclic loading — and is no miracle cure where higher compressive strength is the main requirement.

How is it measured and designed?

Because the added value lies in the cracked stage, fibre concrete is assessed on its residual strength, not just its compressive strength. The European standard EN 14651 prescribes a three-point bending test for this, on a beam with a sawn notch; during the test the crack mouth opening displacement (CMOD) is measured, and the limit of proportionality (LOP) and a series of residual strength values at set crack widths are determined.

The fibres themselves fall under EN 14889 — part 1 for steel fibres, part 2 for polymer fibres; what that standard and the associated CE marking mean is covered in EN 14889 and CE marking for fibre concrete. For structural design, the fib Model Code offers a framework that classifies fibre concrete into performance classes based on the measured residual strength. In the Netherlands, structural engineers additionally work with the CUR Recommendations for fibre reinforced concrete, such as CUR Recommendation 111 for steel fibre concrete industrial floors on piles. This allows the contribution of the fibres to be substantiated in the calculation.

Does fibre reinforcement replace traditional reinforcement?

Sometimes entirely, sometimes partially — it depends on the application and the load. For ground-bearing floors, sprayed concrete and many precast applications, fibre reinforcement can replace steel mesh or bars. For heavily or dynamically loaded structures, the two are often combined, or bar reinforcement remains the leading solution. That trade-off is for the structural engineer to make, based on the design load and the demonstrated residual strength — not on material choice alone.

For the designer or contractor this means that fibre type, dosage and application must always be considered together. A fibre that works excellently against shrinkage cracks in a thin floor finish is not by definition suitable to replace structural reinforcement in a heavily loaded foundation — and vice versa.

Applications and costs in practice

Typical applications of fibre reinforced concrete are industrial floors and ground-bearing slabs, precast yard slabs and precast elements, sprayed concrete, agricultural floors and hard standings. There, its working principle pays off twice: less cracking during curing plus residual strength under service loading, without the placing and checking of mesh.

What that costs depends on fibre type and dosage: an indicative overview is given in what fibre reinforced concrete costs. Answers to the most common practical questions are collected in the frequently asked questions about fibre reinforced concrete.

From working principle to fibre choice

Fibre reinforcement does not work by making concrete “stronger” in the usual sense, but by replacing brittle failure with tough, controlled behaviour. The fibres bridge cracks, keep them fine and well distributed, and retain load-bearing capacity precisely at the moment unreinforced concrete would give way.

Which fibre type and dosage are suitable depends on the application. More background on all fibre types and applications can be found in the pillar overview of fibre reinforced concrete; if you want to go straight to concrete advice for your project, the product finder guides you to the right fibre in a few questions.