Fibre-reinforced concrete
Concrete with short steel, polypropylene or glass fibres to limit shrinkage cracking in slabs and screeds, never to replace engineered reinforcement.
What is fibre-reinforced concrete?
Fibre-reinforced concrete, called vláknobetón in Slovak, is ordinary concrete with short discrete fibres mixed into the batch so they are spread through the whole volume of the slab, screed or panel. The fibres do not take over the main structural job that steel bars do in a beam. Their job is to bridge small cracks as they open, so the cracks stay fine instead of splitting into wide, visible gaps.
The need comes from how concrete behaves. It is strong under compression but weak in tension, and most of the cracking on a finished floor comes from tensile stress created as the material shrinks while it hardens. Fibres work at the scale of those small cracks, so they are most useful where the problem is shrinkage and surface cracking rather than bending under a designed load.
Which types of fibre are used in concrete?
Four families cover almost all practice. Steel fibres are short lengths of drawn wire, often with hooked or deformed ends to grip the surrounding matrix. They add the most toughness after cracking and are the usual choice for industrial floors. Polypropylene fibres are synthetic filaments in two classes. Micro fibres are very fine and very numerous, and they mainly help while the concrete is still plastic. Macro fibres are heavier and add toughness in the hardened state. Glass fibres must be alkali-resistant, because ordinary glass is attacked by the high alkalinity of cement, and they are mostly found in thin precast panels rather than in site-cast slabs.
| Fibre type | Main role | Typical use in a house | Main limitation |
|---|---|---|---|
| Steel | Toughness and residual load capacity after cracking | Garage floors built for heavy use | Can rust at the surface; can stiffen the mix |
| Polypropylene micro | Controls plastic shrinkage while the concrete is fresh | Screeds, thin slabs, balconies | Little effect on load capacity once hardened |
| Polypropylene macro | Residual toughness in hardened concrete | Ground-bearing slabs, sometimes in place of light mesh | Does not replace designed bar reinforcement |
| Alkali-resistant glass | Crack control in thin sections | Thin precast panels rather than site slabs | Needs the alkali-resistant grade; uncommon on site |
How do fibres control shrinkage cracking?
Shrinkage cracking starts when the concrete wants to contract but is restrained by the sub-base, by neighbouring bays, or by its own stiffness. Plastic shrinkage happens in the first hours after placing, when water leaves the surface faster than it can be replaced and the top contracts while the mass beneath is still soft. Fine polypropylene fibres hold the surface together through that window and reduce the number of cracks that open. Drying shrinkage develops over weeks and months. Fibres spread that contraction into many fine cracks rather than a few wide ones, but they do not stop the total movement.
That is why fibre concrete still needs control joints formed where the designer chooses. The joints decide where a crack is allowed to open, while the fibres limit how wide the uncontrolled cracks become between the joints.
Where does fibre-reinforced concrete make sense in a house?
In a family house the sensible uses are elements that sit on the ground and carry loads without significant bending. The clearest example is a betónová mazanina under a garage, a workshop or a utility room. A thin concrete screed laid over underfloor heating, exposed to daily temperature changes, is another good case. Industrial-style floors in a garage, where tyres, tools and abrasion are expected, benefit from steel or macro fibres because the floor must resist wear and crack less visibly.
Fibres are less useful in elements that bend. Suspended slabs, beams, lintels and foundation elements that span between supports need bar reinforcement placed in the tension zone, and no fibre dosage will replace it. Fibre also does not replace careful concrete curing, because a slab that dries too fast will still shrink.
Can fibres replace reinforcement designed by an engineer?
No. Where a structural engineer has designed bar reinforcement, the design sets bar size, spacing, cover and position from the bending moments, the shear and the requirements of the structure. A builder cannot check fibre dosage against those demands by looking at the floor, and the contribution of fibres to load capacity depends on the product, the dosage, the mix and how the concrete was placed. Fibre concrete is therefore a way to manage cracking in elements whose loads are modest and understood. Where the structural design calls for a fibre-reinforced slab, the fibre type and dosage come from that design, not from a supplier's default recommendation. A fibre layer added on top of a poor reinforced concrete layout does not repair it.
How is fibre-reinforced concrete different from reinforced concrete?
Reinforced concrete uses bars or mesh placed at a designed position in the tension zone, and the steel carries the tension. Fibre-reinforced concrete mixes many short fibres through the whole batch, so there is no fixed position to check. Bars give predictable capacity that an engineer can calculate. Fibres give crack control and some toughness, and the result depends heavily on how well the fibres were mixed and how the surface was finished.
| Aspect | Reinforced concrete (bars or mesh) | Fibre-reinforced concrete |
|---|---|---|
| Placement | Positioned by the designer at a set cover | Mixed into the batch and spread through the volume |
| Main purpose | Carries tension and bending | Controls cracking and adds toughness |
| Design basis | Structural calculation by an engineer | Product documentation and dosage; not a bending design |
| Suitable for | Beams, suspended slabs, frames, foundations | Ground-bearing slabs, screeds, industrial-style floors |
| Site check | Bar position and cover before the pour | Mixing, dosage and surface finish after the pour |
What standards and checks apply to fibres on site?
Fibres for concrete are specified against the EN 14889 series of product standards, which treats steel fibres and polymer fibres in separate parts. Ask the supplier for the product documentation and the dosage recommended for the intended use, and ask the ready-mix supplier to confirm that the mix is compatible with the chosen fibre. A mix that is too stiff leaves fibres balled up, and a mix that is too wet lets them settle, so the batch needs checking before the pour.
On site, check that the fibres are distributed evenly through the truck load, that the finished surface is not overworked with the trowel, and that curing starts as soon as finishing is complete.
What are the common misconceptions about fibre concrete?
The first misconception is that fibres make a slab strong enough to skip bar reinforcement. They do not. The second is that a fibre floor will never crack. It will still crack, but the cracks are finer and more numerous, and the control joints remain the means of directing where cracks open. The third is that fibres can rescue a poor pour. Rapid drying, poor compaction and premature sealing still cause defects, and fibres only redistribute the stress they cause.
A fourth misconception concerns the surface. Steel fibres that reach the surface can rust and leave stains, so cover and finishing matter. Polymer fibres do not rust, but they can show at the surface if the finish is poor.
Frequently asked questions
- Can polypropylene fibres replace the mesh in a garage floor?
- Sometimes, but only where the structural designer agrees, because mesh and macro fibres contribute differently to crack control and toughness. A simple floor with a clear design basis can be specified that way, while an uncertain floor should keep the reinforcement the design calls for.
- Does a fibre-reinforced slab still need control joints?
- Yes. Fibres narrow the cracks between joints, but they do not remove the contraction, so the joints still decide where cracks open.
- Will steel fibres rust through a floor?
- Fibres inside the concrete are protected by its alkalinity. Fibres that reach the surface can rust and stain, so cover and finishing matter.
- Why do micro fibres help during the first day?
- While the concrete is still plastic, water leaves the surface faster than it can be replaced. Fine fibres hold the surface together through that window, so fewer cracks open.
- Are glass fibres a practical choice for a house floor?
- Usually not on site. Glass fibres need an alkali-resistant grade and are mainly found in thin precast panels, so ordinary site floors rarely use them.