Fibre Reinforced Facade Encyclopedia: Properties
Flexural strength is the stress in the outer fibre of a specimen at the moment it fails in bending, reported in megapascals. It is the governing strength property for thin facade panels, which are loaded almost entirely in bending by wind. For cementitious materials the same quantity is called the modulus of rupture; for fibre reinforced polymer, flexural strength is the usual term, and ShapeShell-RT is published at 221 to 241 MPa.
A 3 mm FRP laminate and a 15 mm GRC skin can carry similar wind loads over similar spans. The FRP reaches a far higher stress in a much thinner section; the GRC gets its capacity from depth.
At a glance
| Property | Typical value | Note |
|---|---|---|
| ShapeShell-RT (FRP) | 221 to 241 MPa | Published flexural strength |
| Architectural FRP laminate, typical | 150 to 250 MPa | Glass reinforced polyester or vinyl ester, hand laid or infused |
| Sprayed GRC, 28 days | 20 to 30 MPa | Reported as modulus of rupture |
| Premix GRC, 28 days | 10 to 14 MPa | Lower fibre content and shorter fibres |
| ShapeShell-RG (GRG) | 24 MPa | Published flexural strength, interior use |
| Flexural modulus, FRP laminate | 7 to 15 GPa typical | Stiffness, not strength; this is what sizes the panel |
| Test methods | ISO 14125 and ASTM D790 for FRP; BS EN 1170-5 and ASTM C947 for GRC | Three point or four point bending |
| Span to depth ratio | 16:1 or 32:1 depending on the method | Shorter spans introduce shear and inflate the result |
What the test measures
A coupon of known width and thickness is supported near its ends and loaded until it breaks. The maximum bending moment is calculated from the failure load and the span, then divided by the section modulus of the coupon to give the stress in the extreme fibre. That stress is the flexural strength. The calculation assumes the material is homogeneous, elastic to failure and equally strong in tension and compression, which no composite is, so the result is a comparative index.
Bending is also the real action under wind load, and a flexural coupon is far easier to make and grip than a tensile coupon cut from a 10 mm cement skin or a 3 mm laminate.
Three point and four point bending
In three point bending the load is applied at a single point at midspan. The bending moment peaks under the loading nose and falls away linearly, so failure is forced to the one section under the load, where the nose also adds a local stress concentration. The test is quick and needs only a short specimen.
In four point bending the load is split between two points inside the span, producing a region of constant moment with no shear between them. Failure then occurs at the weakest section anywhere in that region, which samples the material better and gives a more honest, usually slightly lower, result. Four point loading is specified for glass fibre reinforced concrete (GRC, GFRC) in BS EN 1170-5 and ASTM C947. A value quoted without its loading configuration and span to depth ratio cannot be reproduced.
Why it is not a material constant
- Test configuration: three point values run higher than four point values on the same material
- Span to depth ratio: standards fix 16:1 or 32:1, because shorter spans bring shear into the failure
- Thickness and scale: thin coupons of a brittle material show a higher apparent strength, since there is less volume in which a critical flaw can sit
- Fibre orientation: sprayed and laid up composites are anisotropic, so coupons are cut in both directions and the lower value governs
- Age, moisture, rate and temperature: GRC changes with age and is tested wet and dry, and a polymer matrix stiffens when loaded quickly and softens near its heat distortion temperature
This is why a panel is never designed by taking a published flexural strength and dividing it by a factor. The design value is a characteristic strength derived from batch testing, reduced for long-term effects, and applied to the real section, which for a moulded panel is a ribbed or folded profile rather than a flat plate.
Flexural strength in architectural FRP
Glass reinforced polymer laminates reach 150 to 250 MPa in flexure because continuous or long fibres carry the tension and a comparatively ductile thermoset resin transfers load between them without the brittle matrix cracking that limits cement. ShapeShell-RT is published at 221 to 241 MPa. The value is sensitive to how the laminate is built: fibre volume fraction, reinforcement type and the number and orientation of plies matter more than the choice of resin.
Two qualifications apply on a facade. A gel coat contributes nothing structurally and is not counted in the section. And strength is rarely what sizes an FRP panel; stiffness is, because a laminate at a small fraction of the modulus of steel reaches its deflection limit long before its strength limit.
How the number reaches the panel
Capacity in bending is flexural strength multiplied by section modulus, and section modulus grows with the square of the depth. The fastest way to make a thin panel stronger is therefore not a stronger material but a deeper section: ribs cast into the back of a GRC skin, a returned edge on an FRP panel, or a stud frame behind the skin.
For cementitious panels the design conversation moves past flexural strength quickly, because a GRC panel that has cracked has failed in service even though it still carries load. The peak value is covered under modulus of rupture, and the first crack value that governs design under limit of proportionality.
Standards and references
- ISO 14125 Fibre-reinforced plastic composites, Determination of flexural properties
- ASTM D790 Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
- BS EN 1170-5 Test method for glass-fibre reinforced cement, Measuring bending strength
- ASTM C947 Flexural Properties of Thin-Section Glass-Fiber-Reinforced Concrete
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- AS/NZS 1170.2 Structural design actions, Wind actions, the load case that makes bending govern
Related terms
Questions specifiers ask
Is flexural strength the same as modulus of rupture?
Yes. Modulus of rupture is the term used for cementitious materials such as GRC. Flexural strength is the general term and the one used for FRP and for plastics.
Why does three point bending give a different answer to four point?
Three point loading concentrates the peak moment under a single nose and usually returns a higher value. Four point loading gives a constant moment region and samples more material, so it is preferred for facade composites.
What flexural strength does an architectural FRP panel have?
Typically 150 to 250 MPa for a glass reinforced laminate. ShapeShell-RT is published at 221 to 241 MPa.
What should a specification ask for?
A characteristic value with the test method, the loading configuration, the span to depth ratio, the specimen age and condition, and the direction the coupons were cut.
ShapeShell-RT is published at 221 to 241 MPa, and ShapeShell-RC is batch tested in bending to the GRCA specification. Test data is available for every project. See ShapeShell-RT data
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.