Fibre Reinforced Facade Encyclopedia: Properties
Compressive strength is the stress a material carries before it crushes, measured on a cube or cylinder loaded in one axis. It is the defining number for structural concrete, where a 40 MPa specification means a great deal, and it is one of the least useful numbers for a thin facade panel, where the panel is never loaded in compression in any significant way. Understanding why it is quoted anyway, and what it actually tells a specifier, saves a good deal of confusion when a concrete specification is applied to a composite panel.
A facade panel spends its life carrying its own weight in hanging tension and shear at the anchors, and wind pressure and suction in bending. Nothing about that is a crushing load.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Sprayed glass fibre reinforced concrete | 50 to 80 MPa | Typical industry range at 28 days |
| Premix glass fibre reinforced concrete | 40 to 60 MPa | Typical |
| Normal structural concrete | 25 to 50 MPa | For comparison; the number a structural spec calls up |
| Ultra high performance concrete | 120 to 200 MPa | For comparison; typical |
| FRP laminate, in plane | 100 to 200 MPa | Typical; lower than its tensile strength |
| Glass fibre reinforced gypsum | 20 to 50 MPa | Alpha gypsum composites, interior use; typical |
| Governing action in a facade panel | Bending under wind load | Compression is not the design case |
| Where compression does govern | Bearing at gravity anchors and stacking loads | Local, not global |
| Test methods | BS EN 12390-3, AS 1012.9, ASTM C109, ASTM D695 | Cubes, cylinders or coupons depending on material |
Why the number is high and why it does not matter
Cementitious materials are strong in compression and weak in tension. The ratio is roughly ten to one, which is the whole reason reinforcement exists. A sprayed glass fibre reinforced concrete (GRC, GFRC) mix is a dense, low water to cement ratio, fine aggregate mortar, so it reaches 50 to 80 MPa in compression, comfortably above ordinary structural concrete, while its tensile capacity is only 5 to 10 MPa and its modulus of rupture 20 to 30 MPa.
None of that compressive capacity is used. A facade panel hangs from its fixings and is pushed and pulled by wind. It carries its own weight in tension and shear at the anchor points and bending in the field. The only compressive stresses in the panel are the compression face of the bending couple, which is an order of magnitude below capacity, and local bearing at gravity anchors and packing points.
Where compression does control
There are real compressive checks in panel design, and they are all local.
- Bearing at gravity anchors, where the full weight of the panel is transferred into a small area of the skin or into a rib. This is a bearing and punching check, not a crushing one, and it is usually resolved with a cast-in plate or a thickened boss
- Packing and shim points at installation, where the panel is set down on a small bearing area during setting out
- Stacking during storage and transport, where panels are stacked on stillages and the lower panels carry the load of those above through the bearing points
- Lifting inserts, where the pull-out cone in the material is governed by the tensile and shear capacity around the insert rather than by compression
- In stay in place formwork, where the shell bears on props and soldiers during the pour
In each case the calculation is a local bearing stress against a design value, with a generous margin, rather than a section utilisation. Failures at these locations are almost always caused by a missing or misplaced bearing detail, not by an inadequate material.
Polymer composites in compression
Fibre reinforced polymer behaves differently and the difference is worth knowing. A glass reinforced laminate is weaker in compression than in tension, typically 100 to 200 MPa against 170 to 270 MPa for a woven laminate, because compressive failure is governed by fibre microbuckling within the resin rather than by fibre strength. The resin has to hold the fibre straight, and when it cannot, the fibre buckles at a stress well below its tensile capacity.
The practical consequences for a panel are about local stability rather than material strength. A thin unsupported laminate under compression buckles as a panel long before the material crushes, which is why FRP panels use ribs, returns and sandwich construction with a foam core to provide stiffness rather than thicker solid laminate. Compressive strength is also temperature dependent in a thermoset, because the resin softens as it approaches its glass transition, so a compression-governed detail on a dark facade needs the design surface temperature applied.
Creep matters here too. A polymer composite under sustained compressive load deforms with time, so a bearing detail that relies on an FRP section to carry permanent dead load has to be checked against long-term rather than short-term values.
What to specify instead
For a moulded facade panel, the useful acceptance properties are the ones that relate to the actions the panel sees and to the manufacturing quality that produces them.
For cementitious panels: modulus of rupture and limit of proportionality at 28 days with aged design values, fibre content by weight, skin thickness and its tolerance, density, water absorption, and the factory production control regime under BS EN 1169. ShapeShell-RC is batch tested for modulus of rupture and limit of proportionality to the GRCA specification.
For polymer panels: flexural strength and modulus, fibre volume fraction, laminate schedule, glass transition or heat distortion temperature, and the reaction to fire classification. ShapeShell-RT publishes flexural strength of 221 to 241 MPa and holds an AS 5637 Group 1 classification from AWTA Product Testing.
Compressive strength belongs in a specification for structural concrete behind the panel, for a stay in place formwork core, or for a precast element that genuinely carries axial load. In a 12 mm facade skin it is a number that describes the mix, not the design.
Standards and references
- AS 1012.9 Methods of testing concrete, compressive strength of concrete specimens
- BS EN 12390-3, testing hardened concrete, compressive strength of test specimens
- ASTM C109, compressive strength of hydraulic cement mortars using 50 mm cube specimens
- ASTM D695, compressive properties of rigid plastics; ASTM D6641 for combined loading compression of composite laminates
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (the properties that should be specified instead)
- BS EN 1169 factory production control for glass-fibre reinforced cement
- AS 3600 Concrete structures, where a structural concrete grade is genuinely required
Related terms
Questions specifiers ask
What is the compressive strength of GRC?
Typically 50 to 80 MPa for sprayed material at 28 days, which is above ordinary structural concrete because the mix is a dense, fine aggregate, low water to cement ratio mortar.
Why is compressive strength not used to specify facade panels?
Because panels are never loaded in compression in any meaningful way. They hang from fixings and bend under wind. Modulus of rupture, limit of proportionality, thickness and fibre content are the properties that describe how the panel performs.
Is FRP weaker in compression than in tension?
Yes. Compressive failure in a glass reinforced laminate is governed by fibre microbuckling within the resin, so compressive strength is typically well below tensile strength for the same laminate.
Where does compression actually matter in a panel?
Locally: bearing at gravity anchors, packing and shim points at installation, stacking during storage and transport, and bearing on props where a shell is used as permanent formwork.
Should a specification call up a concrete grade for a GRC panel?
No. That is a structural concrete clause applied to a composite. Specify to the GRCA specification, with modulus of rupture, limit of proportionality, fibre content, thickness and aged design values.
Specifying moulded panels? ShapeShell-RC is batch tested for modulus of rupture and limit of proportionality to the GRCA specification. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.