Fibre Reinforced Facade Encyclopedia: Properties
Modulus of rupture is the maximum flexural stress a material sustains before failing in a bending test. It is the headline strength figure for glass reinforced concrete and the property that separates a well-made sprayed GRC (20 to 30 MPa) from premix GRC (10 to 14 MPa) and from unreinforced mortar (3 to 5 MPa). For a thin panel under wind load, bending is the critical action, so MOR is the number that matters.
A 10 mm thick GRC test coupon is supported near its ends and loaded at two points in the middle until it breaks. The stress in the outer fibre at the moment of failure is the modulus of rupture.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Sprayed GRC, 28 days | 20 to 30 MPa | GRCA Grade 18 and Grade 21 are common specifications |
| Premix GRC, 28 days | 10 to 14 MPa | Lower fibre content and shorter fibres |
| Unreinforced cement mortar | 3 to 5 MPa | For comparison |
| ShapeShell-RT (FRP) | 221 to 241 MPa flexural strength | Different material, same test principle |
| ShapeShell-RG (GRG) | 24 MPa flexural strength | |
| Limit of proportionality (LOP) | 7 to 11 MPa for sprayed GRC | First crack; the design basis for serviceability |
| Test methods | BS EN 1170-5 (GRC), ASTM C947 (GRC), ASTM D790 and ISO 14125 (FRP) | Four-point bending |
| Ageing | MOR reduces with age; design uses aged values | LOP is largely unchanged by ageing |
MOR and LOP
A GRC beam behaves elastically until the cement matrix cracks. The stress at that point is the limit of proportionality, LOP. Beyond it the glass fibres bridge the cracks and carry increasing load until they pull out or break, and the stress at that peak is the MOR. Between LOP and MOR the panel is cracked but not broken. Design for serviceability, meaning no visible cracking under normal loads, is therefore based on LOP with a safety factor; design for ultimate capacity uses MOR. The gap between the two is what fibre reinforcement buys.
How it is tested
Coupons are cut from test boards sprayed alongside the production panels, in both directions to check for anisotropy. Under BS EN 1170-5 or ASTM C947 the coupon sits on two supports and is loaded at two points within the span until failure while load and deflection are recorded. MOR, LOP and modulus are calculated from the curve. Every production batch is tested; the results form the quality record that a GRCA member or an ISO 9001 manufacturer keeps for each project.
Why MOR changes with age
Early GRC lost strength over time as the alkaline matrix attacked the glass. Alkali resistant fibre and modern mixes slowed that to a rate that is negligible over a building’s life, but MOR still falls from its 28-day value as the matrix densifies around the fibre and reduces its ability to pull out under load. The GRCA and the European standards address this by requiring designs to use long-term aged properties, typically established by accelerated hot-water ageing, rather than 28-day results. A supplier quoting a 28-day MOR without an aged design value is quoting a number that will not be available at year 20.
MOR for other composites
The same bending test applies to FRP and GRG, though the numbers are on different scales. An architectural FRP laminate reaches 150 to 250 MPa in flexure because the fibres are continuous and the resin matrix is ductile compared with cement; ShapeShell-RT is published at 221 to 241 MPa. GRG sits near GRC at 20 to 30 MPa. Comparing MOR across materials is only meaningful alongside density and modulus, which is why specifiers compare panels by stiffness and weight per square metre rather than by strength alone.
Standards and references
- BS EN 1170-5 Precast concrete products, Test method for glass-fibre reinforced cement, Measuring bending strength (complete bending test method)
- ASTM C947 Standard Test Method for Flexural Properties of Thin-Section Glass-Fiber-Reinforced Concrete
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (grades and aged design values)
- ASTM D790, ISO 14125 (flexural properties of FRP)
- ASTM C1355 (GFRG composites)
Related terms
Questions specifiers ask
What is a good MOR for GRC?
20 to 30 MPa at 28 days for sprayed GRC. The GRCA grades commonly specified are Grade 18 and Grade 21, referring to minimum characteristic MOR in MPa.
What is the difference between MOR and LOP?
LOP is the stress at which the matrix first cracks and the load-deflection curve leaves the straight line. MOR is the peak stress at failure. Fibre reinforcement carries the load between the two.
Why do designers use aged MOR?
Because GRC strength changes over time as the matrix and fibre interact. Long-term aged values, established by accelerated ageing, represent the strength available for the life of the panel.
How is MOR tested?
In four-point bending on coupons cut from test boards sprayed with each production batch, to BS EN 1170-5 or ASTM C947.
Is MOR the same as flexural strength?
Yes. Modulus of rupture is the term used for cementitious materials; flexural strength is the general term and the one used for FRP.
ShapeShell-RC is batch tested for MOR and LOP to the GRCA specification. Test certificates are available for every project. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.