Fibre Reinforced Facade Encyclopedia: Composition
Alkali resistant glass fibre is a glass fibre formulated with a high zirconium dioxide content, 16 percent or more by weight, so that it retains its strength in the highly alkaline environment of Portland cement. It is the reinforcement that makes durable glass reinforced concrete possible; ordinary E glass loses most of its strength in cement within a few years.
Fresh cement paste has a pH of 12 to 13. E glass fibre is attacked at that pH and can lose more than half its strength within months. AR glass with 16 percent zirconia retains most of its strength for the design life of the panel.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Zirconia content | 16% or more by weight | GRCA minimum; some grades 19% or more |
| Filament diameter | 13 to 19 microns | Bundled into strands and rovings |
| Strand tensile strength | about 1,500 to 1,700 MPa | Single filaments are higher |
| Elastic modulus | about 72 GPa | Similar to other glass fibres |
| Density | about 2,680 kg/m³ | |
| Softening point | about 860 °C | Fibre survives the AS 1530.1 furnace |
| Chopped length | 25 to 38 mm sprayed; 12 to 25 mm premix | Longer fibre, higher MOR |
| Dosage in GRC | 5% by weight sprayed; 3 to 3.5% premix |
Why zirconia
Glass is silica, and silica dissolves slowly in strong alkali. Zirconium dioxide in the glass network forms a protective layer on the fibre surface as the cement attacks it, slowing the reaction to a rate that no longer matters over a building’s life. The 16 percent figure comes from decades of accelerated ageing tests and is written into the GRCA specification and the European GRC standards; it is the single most important line in any GRC material specification.
E glass, ECR glass and AR glass
E glass is the general purpose fibre for FRP laminates and has a low alkali content of its own, which is not the same thing as resisting the alkali in cement. ECR glass adds corrosion resistance for chemical environments but is still not suited to cement. AR glass is the only fibre engineered for a cement matrix. Coated E glass and basalt fibre have been marketed as substitutes; both degrade in cement faster than AR glass in independent testing.
How it is supplied and used
AR glass comes as continuous roving for the spray process, where a chopper gun cuts it to 25 to 38 mm at the nozzle, and as pre-chopped strand for premix. Sizing on the fibre controls how it wets out in the slurry and how strongly it bonds to the matrix. Fibre content is checked by wash-out tests on production samples, and MOR test boards from every batch confirm the fibre is doing its job.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (minimum 16% zirconia)
- BS EN 15422 Precast concrete products: specification of glass fibres for reinforcement of mortars and concretes
- ASTM C1666 Standard Specification for Alkali Resistant (AR) Glass Fiber for GFRC and Fiber-Reinforced Concrete and Cement
- BS EN 1170 series (GRC test methods including fibre content)
Related terms
Questions specifiers ask
Why can’t ordinary fibreglass be used in concrete?
E glass, the standard fibreglass, is attacked by the alkali in cement and loses most of its strength within a few years. AR glass contains at least 16 percent zirconia, which protects the fibre in that environment.
What does AR stand for?
Alkali resistant.
How much AR glass is in GRC?
About 5 percent by weight in sprayed GRC and 3 to 3.5 percent in premix GRC.
Does AR glass make GRC non-combustible?
AR glass is an inorganic fibre with a softening point around 860 °C, so it does not compromise the non-combustibility of a cement matrix. The classification still has to be earned by testing the finished product.
ShapeShell-RC is produced with alkali resistant glass to the GRCA specification and batch tested for fibre content and MOR. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.