How does fibre reinforcement in concrete work? The mechanism explained

Why does a few kilograms of loose fibre per cubic metre of concrete stop a floor or foundation from cracking? The answer lies in how fibres work together with the concrete at the micro level to prevent cracking. In this article, we take a deeper look at the mechanism behind fibre reinforcement.

From crack initiation to crack control

Every concrete mix shrinks during curing and responds to temperature fluctuations and loading. At points where tensile stress exceeds the tensile strength of the cement matrix, a micro-crack forms. Without reinforcement, such a micro-crack continues unimpeded into a visible, structural crack.

In fibre reinforced concrete, thanks to the homogeneous distribution during mixing, fibres are always present in the immediate vicinity of where a crack starts to form. As soon as the crack face reaches the fibre, the fibre starts to absorb force: the tensile stress that was initially concentrated at one point is distributed via the fibre over a larger volume of concrete. This is called crack bridging.

Three effects of crack bridging

• Delayed crack initiation: because the fibres are already engaged from the very first micro-cracks, it takes longer before a crack becomes visible and structural.

• Limited crack width: whereas 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 aggressive substances, which benefits the durability of the structure.

• Post-crack behaviour: this is the characteristic from which fibre reinforced concrete (FRC) gets its name. Because fibres bridge crack faces, the concrete retains tensile strength after cracking — instead of failing immediately as unreinforced concrete does.

The role of fibre properties

Not every fibre works the same way. The degree to which crack bridging is achieved depends strongly on:

• Diameter and length of the fibre — a higher length-to-diameter ratio (aspect ratio) generally gives better bonding to the cement matrix.

• Surface structure — ribbed or hooked fibre ends anchor better than smooth fibres.

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

• Dosage — at moderate dosages, the concrete mix remains readily pumpable; excessive dosages can actually worsen workability.

Difference between steel fibres and synthetic fibres in operation

Steel fibres are generally applied to actually absorb tensile forces and increase the load-bearing capacity of the structure — they therefore function as a form of reinforcement. Polypropylene fibres are often added in much greater numbers, but at a lower dosage by weight; the effect then lies mainly in limiting (plastic) shrinkage cracking at an early stage of curing, before the concrete has reached its full strength.

What does this mean in practice?

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 automatically suitable for replacing structural reinforcement in a heavily loaded foundation — and vice versa.

In conclusion

The mechanism behind fibre reinforcement — crack bridging and force distribution — is universal, but the result depends strongly on the fibre type chosen, the dosage and the application. In the following articles in this series, we look at the specific properties and areas of application for each fibre type.