Fibre Reinforced Facade Encyclopedia: Properties
Toughness is the energy a material absorbs before it fractures, measured as the area under its load deflection or stress strain curve. It is not strength. A material can be very strong and still shatter, which is exactly what unreinforced cement does. The single most important thing fibre reinforcement does to a cement matrix is not to raise its strength but to raise its toughness by orders of magnitude, converting a brittle material that fails without warning into one that cracks, holds together and keeps carrying load. For a thin facade panel that is being demoulded, stacked, shipped, craned and bolted to a building, toughness is what makes the material usable at all.
Drop a 15 mm slab of unreinforced mortar and it breaks into pieces. Drop a 15 mm glass fibre reinforced concrete panel and it chips a corner, because the fibres bridge the crack and hold the piece together.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Unreinforced cement mortar | Effectively brittle; very low fracture energy | Fails at first crack with no reserve |
| Sprayed glass fibre reinforced concrete | Energy absorption one to two orders of magnitude higher | Typical, relative to unreinforced mortar |
| Strain to failure, unreinforced mortar | about 0.02% | Typical |
| Strain to failure, sprayed cementitious composite | 0.5 to 1.5% | Typical; post-cracking, fibre bridging regime |
| FRP laminate, Charpy impact | Substantially above unreinforced thermoset | Resin alone is brittle; the laminate is not |
| Governing mechanism | Fibre bridging and fibre pull-out across the crack | Energy is consumed pulling fibres out of the matrix |
| Reserve between LOP and MOR | Roughly 2 to 3 times for sprayed material | The visible measure of toughness in a bending test |
| Test methods | BS EN 1170-5 (load deflection curve), ASTM C1018, ASTM D256, ISO 179 | Area under the curve, or impact energy |
Strength, stiffness and toughness are three different things
The three do not track together. Glass is very strong and very brittle. Cement paste is strong in compression and extremely brittle. A fibre composite trades a little stiffness for a great deal of deformation capacity after first crack, and that is the trade a facade panel wants. In a bending test on sprayed glass fibre reinforced concrete (GRC, GFRC), the matrix cracks at the limit of proportionality, typically 7 to 11 MPa, and the panel then continues to carry increasing load up to the modulus of rupture at 20 to 30 MPa while deflecting several times further. Everything between those two points is toughness.
How fibre creates it
When a crack opens in a fibre reinforced matrix, the fibres crossing that crack do not break immediately. They bridge it, and as the crack widens they are progressively pulled out of the matrix against friction. Pulling a filament out of cement consumes energy along its whole embedded length, and there are millions of filaments across the crack plane. That pull-out work is where the toughness comes from.
This explains several things that otherwise look odd. It explains why fibre length matters as much as fibre content, because a longer embedded length means more pull-out work. It explains why the fibre matrix interface is the critical variable rather than fibre strength, because a fibre that is bonded too tightly breaks instead of pulling out, which is strong but brittle. And it explains why toughness in a cementitious composite reduces with age: as the matrix densifies around the filament, the interface stiffens, more fibres break rather than pull out, and the material becomes stronger at first crack but less tough overall. That is the real reason design uses aged property values rather than 28 day results.
Why it matters on a real project
Toughness is a manufacturing and handling property before it is a structural one. Almost everything that damages a facade panel is an energy event rather than a sustained load.
- Demoulding, where a green panel is separated from the mould and has to survive suction, peel and handling at a fraction of its final strength
- Stacking, stillaging and transport, where panels move against each other and against restraints over long distances
- Craneage and installation, where a panel is lifted, swung, landed and adjusted, and where an edge or corner can catch
- Impact in service: hail, windborne debris, trolleys and vehicles at ground level, and maintenance access equipment
- Fixing: drilling, bolting and torquing into a thin panel, which is a local energy input
- Post-cracking behaviour: a tough panel that is cracked still holds together and still carries wind load, so a crack is a maintenance item rather than a safety event
That last point is the one that matters most for safety at height. A brittle thin panel that cracks can shed pieces. A tough one does not, because the fibres hold the fragments in the plane of the panel. This is why fibre content, fibre length and fibre distribution are specified and tested rather than left to the manufacturer, and why a low fibre reading on a batch coupon is treated seriously even when the strength result passes.
How it is measured
For cementitious composites, toughness is taken from the full load deflection curve in a four point bending test to BS EN 1170-5 or ASTM C947, by integrating the area under the curve. ASTM C1018 defines toughness indices for fibre reinforced concrete based on the deflection at first crack and multiples of it, which gives a dimensionless number that can be specified and compared.
For polymer composites, impact energy is measured by Charpy or Izod pendulum tests to ISO 179 or ASTM D256, and by instrumented falling weight impact to ISO 6603 for panels, which gives the energy absorbed and shows whether failure is ductile or brittle. Cured polyester and vinyl ester resins on their own are brittle; the laminate is not, for the same fibre bridging reason. Gel coat is the exception: it is unreinforced resin, it is brittle, and surface crazing of a gel coat under impact is a finish issue rather than a structural one.
Standards and references
- BS EN 1170-5 Test method for glass-fibre reinforced cement, Measuring bending strength, complete bending test method, from which the load deflection curve is taken
- ASTM C1018 Standard Test Method for Flexural Toughness and First-Crack Strength of Fiber-Reinforced Concrete
- ASTM C947 Standard Test Method for Flexural Properties of Thin-Section Glass-Fiber-Reinforced Concrete
- ASTM D256 and ISO 179, Izod and Charpy impact resistance of plastics and composites
- ISO 6603 and ASTM D7136, instrumented falling weight impact of rigid plastics and composite laminates
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products, including fibre content and aged design values
- BS EN 1170-1, measuring the consistency and the glass content of the mix
Related terms
Questions specifiers ask
Is toughness the same as strength?
No. Strength is the stress a material reaches; toughness is the energy it absorbs before fracture. A material can be strong and brittle, which is what unreinforced cement is. Fibre reinforcement adds far more toughness than strength.
Why does fibre make cement tough?
Because fibres crossing a crack are pulled out of the matrix rather than breaking, and pull-out consumes energy along the whole embedded length of every filament. That pull-out work is the toughness.
Why does toughness reduce with age in GRC?
As the matrix densifies around the filaments, the interface stiffens and more fibres break instead of pulling out. The material becomes stronger at first crack and less tough, which is why design uses aged values.
How is toughness checked in production?
Formally from the area under the load deflection curve in a bending test. In practice the ratio between limit of proportionality and modulus of rupture is used as a working indicator: a large reserve between them means a tough composite.
Does a cracked panel have to be replaced?
Not necessarily. A tough fibre reinforced panel that has cracked still holds together and still carries load, because the fibres bridge the crack. The crack should be assessed for cause, but it is not the same event as a brittle panel breaking.
ShapeShell-RC is batch tested for modulus of rupture and limit of proportionality to the GRCA specification, which is where post-cracking reserve shows up. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.