Fibre Reinforced Facade Encyclopedia: Properties
The limit of proportionality is the stress at which a glass fibre reinforced concrete (GRC, GFRC) section first cracks and its load deflection curve stops being a straight line. It is typically 7 to 11 MPa for sprayed GRC, and it is the value that governs serviceability design, because a panel that has cracked has failed visually and has opened a path for water long before it comes anywhere near failing structurally.
In a bending test the GRC curve rises straight, bends over at the limit of proportionality as the matrix cracks, then climbs again as the fibres take over until it peaks at the modulus of rupture. Design sits at the first bend, not the peak.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Sprayed GRC, LOP at 28 days | 7 to 11 MPa | Four point bending to BS EN 1170-5 |
| Premix GRC, LOP at 28 days | 5 to 8 MPa | Typical; lower fibre content |
| Sprayed GRC, MOR at 28 days | 20 to 30 MPa | For comparison; see modulus of rupture |
| Ratio of MOR to LOP, sprayed | about 2.5 to 3.5 at 28 days | Falls towards 1.2 to 1.5 when fully aged |
| Aged LOP | equal to or slightly above the 28 day value | Matrix densification raises LOP while it lowers MOR |
| Matrix cracking strain | about 0.02 percent | Why the cement cracks long before the fibre is stressed |
| Serviceability design basis | a factored proportion of the aged LOP | GRCA Practical Design Guide; |
| ShapeShell-RC | Batch tested for MOR and LOP to the GRCA specification | Certificates issued per project |
What happens at first crack
A cement matrix reaches its tensile strain capacity at roughly 0.02 percent, which is a very small movement. Up to that point the section behaves as a single elastic material and the glass fibre does almost nothing. At the limit of proportionality the matrix cracks and the fibres crossing the crack take the load it sheds. Because the fibres are millions of short filaments rather than a few bars, the first crack does not run; further fine cracks form elsewhere as load increases, which is the multiple cracking that gives GRC its pseudo-ductile behaviour.
That behaviour is why GRC can be made 10 to 20 mm thick when a reinforced concrete section could not, and why the first crack rather than the collapse load is the design event.
Why design is to LOP and not to MOR
Between LOP and the modulus of rupture the panel is cracked but not broken. Structurally it still works; architecturally and functionally it does not. A cracked skin admits water, carbonates faster along the crack, can show efflorescence and a dirt line on the face, and in a cold climate gives freezing water somewhere to sit. None of that is reversible on a facade that has to look right for decades.
So the serviceability case, panels under service wind pressure plus self weight and any restrained movement, is designed to keep the peak tensile stress below a factored proportion of the aged LOP, while the ultimate case is checked against MOR with its own factors. In practice the serviceability check governs the skin thickness, the rib spacing and the position of the anchors.
How LOP is measured
LOP comes from the same test as MOR, on the same coupon. Under BS EN 1170-5 the coupon is loaded in four point bending while load and deflection are recorded continuously, and LOP is read from the point at which the curve departs from its initial straight line. ASTM C947 describes the same quantity as the proportional elastic limit. Because that departure is a judgement on a curve, the standard defines how it is taken.
Coupons are cut from test boards sprayed alongside production panels, in both directions to check anisotropy, and tested wet and dry as the specification requires. Every batch is tested, and those results are the quality record supporting the design values the engineer uses.
Ageing moves LOP and MOR in opposite directions
The matrix continues to hydrate and densify around the glass filaments for years. A denser matrix cracks at a higher stress, so LOP rises slightly or holds. The same densification grips the filaments harder and reduces their ability to pull out and absorb energy, so MOR falls. A fully aged sprayed GRC can show a MOR only 1.2 to 1.5 times its LOP rather than the factor of three seen at 28 days.
What it means for the panel
Stresses that approach LOP rarely come only from wind. Demoulding, handling, transport and lifting can put more stress into a thin skin than the building ever will, which is why ribs, lifting points and temporary stiffeners are designed alongside the panel. Restraint is the other source: a panel fixed so that it cannot expand with temperature or shrink as it dries will crack at the fixings whatever its wind capacity.
Modulus of rupture covers the peak strength side of the same test, and sprayed GRC covers how spraying, curing and fibre content set both values.
Standards and references
- BS EN 1170-5 Test method for glass-fibre reinforced cement, Measuring bending strength, complete bending test method
- ASTM C947 Flexural Properties of Thin-Section Glass-Fiber-Reinforced Concrete, proportional elastic limit
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- GRCA Practical Design Guide for Glassfibre Reinforced Concrete
- BS EN 1169 Precast concrete products, General rules for factory production control of glass-fibre reinforced cement
- BS EN 1170-8 Test method for glass-fibre reinforced cement, Cyclic weathering type test
Related terms
Questions specifiers ask
What is a typical LOP for GRC?
7 to 11 MPa at 28 days for sprayed GRC, and 5 to 8 MPa for premix. Both are reported with the corresponding modulus of rupture from the same test.
Why design to LOP instead of the failure stress?
Because cracking is the failure that matters on a facade. Past the limit of proportionality the panel still carries load, but it is cracked, and a cracked skin admits water and shows on the face.
Does LOP get worse with age?
No. The matrix densifies with age, so LOP holds or rises slightly. It is the modulus of rupture that falls, which is why the two converge in aged material.
How is LOP obtained?
From the same four point bending test as MOR, by reading the point at which the load deflection curve departs from its initial straight line, to BS EN 1170-5 or ASTM C947.
ShapeShell-RC is batch tested for MOR and LOP to the GRCA specification, with aged design values available for the engineer of record. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.