Fibres for precast concrete elements: types, dosage and production gains

Fibres for precast concrete fall into five main types: synthetic macro fibres (indicatively 2 to 6 kg/m³) for walls, stairs and small units; steel fibres (10 to 40 kg/m³, by calculation) for pipes, tunnel segments and heavily loaded elements; micro fibres (0.6 to 1.0 kg/m³) against plastic shrinkage; basalt for heat-exposed elements; and AR glass for thin façade panels.

In fibre reinforced precast concrete, the fibres replace the traditional reinforcement mesh wholly or largely. That saves, indicatively, 30 to 90 minutes of steel fixing and inspection per element and lowers the Environmental Cost Indicator sharply: for a precast internal wall from €17.51 to €1.13 ECI, a saving of 93%. Only lifting provisions and any edge reinforcement usually remain steel in liftable elements.

Precast fibres compared: which fibre for which element?

The right precast fibre depends on what the element has to carry, how thin it is and whether the face stays visible. The table gives indicative ranges per fibre type; the dosage for a specific element always follows from the performance it has to deliver.

• Synthetic macro fibre: a synthetic macro fibre such as TwistR GREEN HYBRID is the first choice for walls, non-load-bearing floor slabs, stairs and external works units. TwistR is a 48 mm hybrid macro fibre of 100% virgin polypropylene (a twisted monofilament plus a fibrillating network fibre), dosed at 2.0 to 6.0 kg/m³ and supplied with CE marking and an EPD: structural, rust-free and with the greatest ECI gain.

• High-tensile steel fibre: steel fibres for precast elements such as the MPWG HT+ 50/0.90 (50 mm, double hooked ends, tensile strength 2,100 N/mm², performance class 56, 10 to 35 kg/m³) deliver the high residual strengths that heavily loaded elements such as load-bearing floor slabs, beams and tunnel segments demand. Dosages and prices per m³ are in steel fibre concrete: applications, dosage and price.

• Shorter steel fibre: for thinner sections and tunnel segments, a 35 mm hooked-end fibre such as the MPZG HT+ 35/0.55 (35 mm, Ø 0.55 mm, 10 to 35 kg/m³) is an option; the choice between a 35 mm and a 50 mm fibre depends on the section thickness and the residual strength the element requires.

• Micro fibre: a micro fibre for precast surfaces such as Promicro, a 12 mm polypropylene monofilament of EN 14889-2 class Ia dosed at 0.6 to 1.0 kg/m³, limits plastic shrinkage cracks and improves surface quality. It is not a mesh replacement; in structural elements it works alongside a macro fibre or bar reinforcement.

• Basalt fibre: Basalt Wave (50 mm, 3D wave profile) for edge beams, bridge elements and elements with temperature requirements, as a mineral, rust-free alternative with a high temperature resistance.

For thin architectural façade panels, alkali-resistant (AR) glass fibre is the established choice (glass fibre reinforced concrete, GRC); it is not part of our range, and how it relates to the other fibre types is set out in steel, synthetic, glass and basalt fibres compared. Whether steel or synthetic suits a particular element best is a trade-off between residual strength, corrosion, weight and ECI, covered in full in steel fibres or synthetic macro fibres. General dosage rules and calculation steps are in concrete fibre dosage per fibre type.

The performance of steel and polymer fibres is declared under EN 14889 (part 1 for steel, part 2 for polymer fibres); for structural applications, the residual strength is determined with the beam test to EN 14651. When selecting fibres, always ask for those performance values, so the structural engineer works with validated figures.

What is fibre reinforced precast concrete?

Fibre reinforced precast concrete is concrete cast in a factory mould in which short fibres, mixed in with the other constituents, take over all or part of the reinforcement mesh, so the reinforcement sits throughout the whole cross-section of every element.

Precast concrete itself is cast in a controlled factory environment rather than on the construction site. The elements (walls, floor slabs, stairs, retaining walls, beams and bridge sections) are produced in steel moulds, cured, stored and transported just in time to the site. That offers clear advantages over in-situ concrete: execution does not depend on the weather, no scaffolding or pouring crews are needed on site, and the build time is shorter thanks to serial production.

Within that tightly organised process, traditional reinforcement has remained a strikingly labour-intensive step. Reinforcement mesh and bars have to be transported and stored, reinforcement drawings read, cages tied, spacers placed and the cover checked: exactly the kind of manual work prefabrication aims to take out of the building process. Fibre reinforcement tackles that last manual link: the reinforcement goes into the mixer as fibre and thereby sits automatically throughout the full cross-section of every element.

Why fibre reinforcement suits precast concrete so well

• Optimised production process: structural fibres largely replace the traditional reinforcement in precast internal walls, for example; at most, steel edge reinforcement and lifting provisions remain necessary to tilt and lift the element safely.

• Higher production speed: no time is lost tying and positioning reinforcement mesh, so capacity per mould and per day increases. In large-scale, serial production that gain per element adds up fast.

• Slimmer, lighter elements: with precast façade elements, transport and craneage are a considerable cost item. Where no mesh needs covering, fibre reinforcement, like prestressing, enables slimmer cross-sections and so limits the weight per element.

• No cover errors: fibres are distributed throughout the entire volume, so a shifted mesh or too little cover above the reinforcement simply cannot occur. That lowers failure costs and the risk of concrete decay through corrosion over the service life.

• Less handling damage: fibres throughout the volume hold edges and corners together during demoulding, tilting, stacking and transport, so fewer elements are chipped or rejected.

• 3D reinforcement: the fibres also work in directions a single mesh layer does not cover, for example through the thickness of a wall or at the re-entrant corners of a staircase.

Prefabrication is thus one of the strongest applications within our overview of applications and target groups for fibre reinforcement.

Types of precast concrete element and the role of fibres

Not every element lends itself equally well to fibre reinforcement. The overview below runs through the common precast concrete elements, from simple to heavily structural.

Precast concrete walls and internal walls

Precast concrete walls (internal walls, party walls and fire walls) are the classic fibre application. The loading is predominantly distributed across the plane, so structural macro fibres can largely replace the reinforcement mesh. A free-standing precast concrete wall as a boundary fence or façade plinth also falls into this category. For fire walls, fibres are particularly relevant: polypropylene fibres improve behaviour in fire by forming melt channels that counteract spalling of the concrete.

Precast concrete floors

In a precast concrete floor (solid floor slabs and the structural toppings of system floors), fibres control shrinkage cracking and can replace part of the mesh reinforcement. For load-bearing floor slabs with bending as the governing load, a calculation by the structural engineer remains decisive for the ratio of fibres to bar reinforcement; producing entirely mesh-free is not always feasible here.

Stairs and balconies

Precast concrete stairs are poured in complex moulds in which bending and fixing mesh takes relatively long. Fibres reach every part of that awkward geometry without extra work. The ECI comparison below also shows that the concrete staircase yields some of the greatest environmental gains: 79% lower environmental costs compared with traditional reinforcement.

Retaining walls and precast concrete blocks

Concrete retaining walls (the L- and T-shaped units used for earth retention, silage clamps and yard layouts) are almost always supplied precast and are a growth market for fibre reinforcement: fibres in the base and the stem limit shrinkage cracks and increase impact resistance during daily loading and unloading work. Precast concrete blocks (stacking blocks, often cast from leftover concrete) also benefit from fibres: they increase edge stability without any reinforcement having to go into the leftover concrete. More detail is in precast concrete retaining walls: types, dimensions and prices.

Small precast units: fence posts, copings and kerbs

Fence posts, copings, kerbs and similar small precast concrete units are cast in large numbers in simple moulds, where tying a small cage for every unit takes a relatively large amount of time. A synthetic macro fibre, alone or with a micro fibre for crisp edges, replaces the light mesh, cannot corrode near the surface and makes the units less prone to chipping. For slender units that carry bending, such as fence posts under wind load or lintels, the structural engineer checks whether fibres alone suffice or a bar is still needed.

Precast bridge elements and infrastructure

A precast concrete bridge, from cycle and foot bridges to edge beams and fibre-reinforced overlays on existing bridge decks, places high demands on durability: de-icing salt, moisture and frost make corrosion of reinforcing steel the dominant failure factor here. Fibre reinforced precast concrete rusts little or not at all, enabling slimmer edge beams and thinner overlays. For edge beams and bridge sections with elevated temperature or durability requirements, a basalt fibre for precast and edge beams is a mineral alternative: its 3D wave profile anchors optimally in the concrete matrix and the fibre has a high temperature resistance. When replacing timber bridge decks, fibre concrete is one of the three common options; the trade-off is covered in replacing a timber bridge deck: composite, plastic or fibre concrete.

Pipes, manholes, tanks and tunnel segments

Steel fibre concrete has been used for decades in precast pipes, manhole rings, tanks and tunnel lining segments. In Europe, EN 1916 covers steel fibre concrete pipes alongside unreinforced and reinforced ones, and in the United States ASTM C1765 is a dedicated product standard for steel fibre reinforced concrete pipe. Manufacturer reference projects indicatively mention around 25 kg/m³ of steel fibre for pipes and segments and around 40 kg/m³ for tanks; the actual dosage always follows from the calculation. The fibres also protect the edges and corners of segments during demoulding, handling and installation.

For septic tanks, inspection chambers and pits where corrosion is the main concern, synthetic macro fibres are used instead. To limit explosive spalling in a tunnel fire, polypropylene micro fibres are the established measure, at a dosage set by the fire design and usually higher than for shrinkage control: they melt and leave channels through which vapour pressure can escape. Basalt does not melt at those temperatures and is no substitute for them. Basalt Wave does list tunnels among its applications, for its temperature resistance and because it cannot corrode.

Façade panels and architectural concrete

For architectural façade panels, glass fibre reinforced concrete (GRC) is also used. It contains alkali-resistant (AR) glass fibre at around 4.5 to 5% by weight in sprayed GRC and 2 to 3.5% in premix, which gives the thin section a high flexural and tensile strength. That allows thin-walled panels that weigh far less than conventional precast panels, in almost any shape, colour and surface texture, including complex 3D forms, which makes GRC suitable for façade cladding where low weight and aesthetics come first. Where heat resistance counts alongside aesthetics, basalt fibre is the mineral alternative. For the visible faces of conventional precast elements, a micro fibre such as Promicro helps limit plastic shrinkage cracks on the surface.

Production gains: what does fibre reinforcement save per element?

The business case for fibres in the precast factory revolves around labour hours and lead time per element, and that gain can be made concrete per production line:

• Reinforcement labour: tying, placing and checking a mesh or cage takes indicatively 30 to 90 minutes for an average wall element; more for complex geometry such as stairs. Fibres are dosed into the mixer in a few minutes per batch, regardless of how many elements are poured from that batch.

• Mould occupancy: without reinforcement assembly in the mould, the mould can be poured again sooner; over a week of production that means more elements per mould.

• Logistics: no storage bay for mesh and bars, no internal crane movements for reinforcement packages and no waiting on the reinforcement supplier when designs change.

• Failure costs: cover errors and shifted mesh disappear as grounds for rejection; the fibre dosage is registered per batch and is thereby directly traceable in quality control.

The exact gain varies by factory and element type, so base your own calculation on the current reinforcement hours per element.

ECI worked example: precast internal wall and concrete staircase

For Dutch tenders with environmental requirements (the MPG for buildings, the ECI or MKI for infrastructure), fibre reinforcement in precast is one of the quickest levers to pull. LCA-substantiated field examples with structural synthetic fibre (dosage around 3 kg/m³) show a substantial saving on the Environmental Cost Indicator compared with traditional reinforcement:

• Precast internal wall: ECI with traditional reinforcement €17.51, with synthetic fibre €1.13: a saving of 93%.

• Concrete staircase: ECI with traditional reinforcement €52.54, with synthetic fibre €10.82: a saving of 79%.

The explanation is simple: polypropylene is more than eight times lighter than steel, so a dosage of 2 to 6 kg of macro fibre per m³ replaces tens of kilograms of reinforcing steel, together with the steel production and transport behind it. With TwistR GREEN HYBRID, a 48 mm hybrid macro fibre of 100% virgin polypropylene with a twisted monofilament for anchorage, supplied with an EPD, this gain per element can be carried through into the project's environmental performance calculation. How you set up that calculation and what gain is realistic is covered in lowering MPG and ECI with fibre reinforcement.

Designing fibre reinforced precast elements: residual strength, lifting and limits

Structural fibre concrete is designed on its residual flexural strength after cracking. That strength is measured with the notched beam test to EN 14651, which yields the residual strength values fR1 and fR3. The fib Model Code 2010 and fib Bulletin 105 describe how designers use those values; in the UK, the Concrete Society reports TR63 (steel fibres) and TR65 (macro synthetic fibres) are the usual guidance. The second-generation Eurocode 2 (EN 1992-1-1:2023) includes an annex for steel fibre reinforced concrete design. Bear in mind that the dosage a supplier declares under EN 14889 is a minimum for CE marking, not a design dosage.

One nuance belongs in every conversation about fibre reinforced precast: the lifting phase. An element lies horizontally in the mould and is then tilted and lifted by lifting anchors, and those concentrated forces around anchors and edges are often more governing than the loading in the final situation. That is why liftable elements usually keep steel lifting provisions and local edge reinforcement, even when the rest of the element is fully fibre reinforced.

For heavily loaded, load-bearing precast elements, a static calculation to the applicable Eurocodes determines the extent to which fibres can replace the traditional reinforcement. Fibres are a fully fledged reinforcement medium, but the degree of replacement is always an outcome of the calculation, not of the brochure. Prestressed elements such as hollow-core slabs and long beams, as well as columns, keep their prestressing or bar reinforcement; there, fibres can at most supplement it.

Fibres in the precast factory: dosing and ordering

The changeover in the factory is limited: fibres are supplied in dissolvable bags or big bags and added to the mixer by hand or via a dosing machine, with a short extra mixing time for homogeneous distribution. Batch registration of the dosage fits into existing quality control. What the supply, dosing and mixing logistics look like in practice is described in fibres at the concrete plant: ordering and production integration; the process in a precast factory is virtually identical.

Mix design, workability and surface finish

Fibres reduce the slump or flow of the mix, macro and steel fibres more than micro fibres. Compensate with a superplasticiser rather than extra water, and check flow and fibre distribution in a trial batch; that matters all the more with self-compacting precast mixes. Compaction works with the usual external or table vibration.

On the surface, flexible synthetic fibres can show on trowelled or exposed faces; after hardening, they can be removed by brief flaming. Steel fibres at the surface can leave small rust spots, which are cosmetic only; for visible architectural faces, a synthetic or basalt fibre avoids this. Background on the mechanism is in how fibres work in concrete.

Ideally, start with a trial pour for each element type: one batch with the intended dosage, assessed for workability, surface and (for structural work) residual strength. For project-specific dosage advice and a price quotation per element type, you can request a quotation based on your element drawings.