Fibre Reinforced Facade Encyclopedia: Materials
Fibre reinforced concrete is concrete containing short discrete fibres distributed through the mix, added to control cracking and to give the hardened material toughness after it has cracked. The fibre does not usually replace main reinforcement; it changes the behaviour of the concrete between and beyond the bars. Steel, synthetic, glass and natural fibres are all used, in dosages from well under one percent to several percent by volume, and each is chosen for a different job.
Plain concrete fails in tension the instant the first crack forms. The same concrete with 35 kg of steel fibre per cubic metre still carries a substantial load across an open 2.5 mm crack, and that residual capacity is what design codes use.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Steel fibre dosage | 20 to 60 kg/m³ | Roughly 0.25 to 0.75% by volume |
| Macro synthetic fibre dosage | 2 to 9 kg/m³ | Polypropylene or polyolefin, 40 to 60 mm long |
| Micro synthetic fibre dosage | 0.6 to 1.0 kg/m³ | Plastic shrinkage control only, no structural credit |
| AR glass fibre in GRC | 3 to 5% by weight of mix | A different material class; fibre is the only reinforcement |
| Fibre aspect ratio | 40 to 100 | Length divided by diameter; higher bonds better, mixes worse |
| Residual flexural strength fR1 | 1.5 to 5.0 MPa | Measured at 0.5 mm crack mouth opening, EN 14651 |
| Residual flexural strength fR3 | 1.0 to 4.5 MPa | Measured at 2.5 mm crack mouth opening |
| Effect on compressive strength | Negligible | Fibre works in tension, not compression |
| Effect on workability | Slump loss of 20 to 50 mm typical | Corrected with admixture, not with water |
What fibres actually do
Concrete is strong in compression and weak and brittle in tension. Fibres do not change that materially before cracking; the first crack stress rises only a few percent at normal dosages. What changes is everything after the crack. Fibres bridge the crack faces, transfer stress across the opening and pull out gradually rather than snapping, which converts a brittle failure into a ductile one and gives the section a measurable residual load capacity.
That residual capacity is the design quantity. It is why fibre reinforced concrete can replace light mesh in a ground slab, carry temporary loads in a sprayed tunnel lining and control handling cracks in a precast element.
The fibre types
- Steel fibre: 30 to 60 mm long, hooked, crimped or flattened ends, the highest residual strength per dollar, corrodes where it reaches a wet surface
- Macro synthetic fibre: polypropylene or polyolefin, 40 to 60 mm, lower residual strength than steel but no corrosion and lower density
- Micro synthetic fibre: fine polypropylene at low dosage, for plastic shrinkage cracking in the first hours only
- Alkali resistant glass fibre: high zirconia glass, used at much higher volume fractions in thin section GRC
- Cellulose and natural fibre: shrinkage control and toughness in fibre cement and some low grade applications
- Polypropylene microfibre in dense mixes: added specifically to relieve pore pressure and reduce explosive spalling in fire
Fibre choice follows the failure mode being managed. Steel for structural residual strength, macro synthetic where corrosion at the surface matters, micro synthetic for early age cracking, AR glass for thin sections with no bar reinforcement at all.
How it is specified and tested
Fibre reinforced concrete is specified by performance, not by dosage. The correct clause states the residual flexural tensile strength class, obtained from a three point bending test on a notched beam to EN 14651, reported as fR1 at a crack mouth opening of 0.5 mm and fR3 at 2.5 mm. The fib Model Code 2010 classification takes those two numbers and turns them into a strength class and a ductility letter that design equations use directly.
The North American equivalents are ASTM C1609 for flexural performance and ASTM C1116 for the classification of fibre reinforced concrete. Specifying a dosage in kilograms per cubic metre without a performance class transfers the risk to the specifier and gives the supplier no obligation to deliver anything in particular.
Where it is used
Slabs on ground and industrial floors, where fibre replaces light mesh. Sprayed concrete linings for tunnels and slope stabilisation, where steel fibre is standard practice. Precast elements including pipes, pits, segments and panels, where fibre handles demoulding and transport stresses. Ultra high performance concrete sits at the top of the range, using two to six percent steel fibre by volume in a very dense matrix.
Glass fibre reinforced concrete is best understood as a separate material class rather than a variety of fibre reinforced concrete. The fibre content is much higher by volume, there is no coarse aggregate, the section is 10 to 20 mm rather than 150, and the fibre carries the entire tensile load because there is no steel to help it. The glass fibre reinforced concrete entry covers it properly.
Limits and site issues
Fibre balling during mixing is the classic production problem, caused by adding fibre too quickly, by too high an aspect ratio or by insufficient mixing energy. Steel fibres at an exposed face rust and stain, so they are avoided where appearance matters. Pumping requires mix adjustment, and fibre orientation follows the direction of flow, which can help or hurt depending on where the design tension lies.
Standards and references
- EN 14651 Test method for metallic fibre concrete: measuring the flexural tensile strength
- fib Model Code 2010, design provisions for fibre reinforced concrete
- ASTM C1609 Flexural Performance of Fiber-Reinforced Concrete
- ASTM C1116 Standard Specification for Fiber-Reinforced Concrete
- EN 14889-1 and EN 14889-2 (steel and polymer fibres for concrete)
- AS 3600 Concrete structures; AS 1379 Specification and supply of concrete
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
Related terms
Questions specifiers ask
Does fibre replace reinforcing steel?
Rarely, and never by assumption. Fibre can replace light crack control mesh in a slab on ground and can carry defined residual tensile stresses where the design code allows it, but main bending reinforcement in a structural element remains a bar design question.
Is GRC a type of fibre reinforced concrete?
Technically yes, practically no. GRC has no coarse aggregate, a fibre content of 3 to 5 percent by weight, a section of 10 to 20 mm and no steel at all, so it is designed and tested against its own standards rather than against concrete codes.
How much fibre is needed?
It depends on the residual strength class required, not on a rule of thumb. Steel fibre typically runs 20 to 60 kg per cubic metre and macro synthetic 2 to 9 kg per cubic metre, but the correct specification states fR1 and fR3, and the supplier selects the dosage that achieves them.
Do steel fibres rust?
Fibres at an exposed wet surface will corrode and stain. Fibres inside the section do not, because the concrete around them stays alkaline and there is no continuous corrosion path. Where appearance or chloride exposure matters, macro synthetic fibre avoids the issue.
Does fibre improve fire performance?
Concrete is already non-combustible where tested to AS 1530.1. What polypropylene microfibre does is relieve internal pore pressure during a fire by melting and leaving channels, which reduces explosive spalling in dense mixes. That is a fire resistance benefit, not a combustibility one.
Working with a thin section cementitious facade rather than a structural slab? ShapeShell-RC is our glass fibre reinforced concrete system, batch tested for MOR and LOP. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.