Fibre reinforcement in precast concrete elements

In precast concrete elements, fibre reinforcement replaces 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.

What are precast concrete elements?

Precast concrete is concrete poured not on the construction site but in a controlled factory environment. 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

• Optimisation of the 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. 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.

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, the combination with fibres is extra interesting: 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 fill that awkward geometry effortlessly. The ECI comparison further on moreover 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 — L- and T-shaped retaining walls for earth retention, silage clamps and site layout — 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. Types, dimensions and prices are covered separately in concrete retaining walls: types and prices.

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 is heat-resistant up to around 700 °C. 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.

Façade panels and architectural concrete

For architectural façade panels, glass fibre reinforced concrete is also used. Because glass fibre gives the concrete a considerably higher splitting tensile strength, the material can be cast in very thin, light panels — down to a few millimetres thick — in virtually any shape, colour and surface texture, including complex 3D forms. That makes it suitable for façade cladding where low weight and aesthetics come first. Where heat resistance counts alongside aesthetics, basalt fibre is the mineral alternative.

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 differs per factory and element type; so calculate your own situation based on the current reinforcement hours per element.

ECI worked example: precast internal wall and concrete staircase

For tenders with environmental requirements — MPG for buildings, ECI (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 2–6 kg of macro fibre per m³ replaces tens of kilograms of reinforcing steel — including the transport and galvanising that come with it. With a macro fibre such as TwistR, 100% polypropylene with a twisted structure for optimal anchorage, this gain per element can be calculated 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.

Which fibre for which precast element?

• Synthetic macro fibre — fibres for precast concrete such as TwistR (2–6 kg/m³) are the first choice for walls, non-load-bearing floor slabs, stairs and site elements: 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) 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.

• Basalt fibre — Basalt Wave for edge beams, bridge elements and elements with elevated temperature requirements (heat-resistant up to around 700 °C), as a mineral, rust-free alternative.

The performance of concrete and steel fibres is documented to EN 14889; for structural applications, the residual strength is determined with the beam test to EN 14651. When selecting fibres, always ask for those performance classes — then the structural engineer works with validated values.

Lifting reinforcement and structural limits

One nuance belongs in every conversation about fibre-reinforced precast: the lifting phase. An element lies horizontally in the mould, is 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 structural, load-bearing precast elements, moreover: a static calculation in accordance with 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.

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 seamlessly 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.

Preferably start with a trial pour per 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.