Fibre Reinforced Facade Encyclopedia: Composition
Zirconia content is the proportion of zirconium dioxide in the glass from which an alkali resistant fibre is drawn, and it is the property that makes the fibre survive in cement. The GRCA specification sets a minimum of 16 percent by weight. It is the single most important line in a glass fibre reinforced concrete material specification, and the one most often left out.
A 16 percent zirconia fibre and a zero percent zirconia E glass fibre look identical, weigh the same, and produce panels with the same 28 day test results. They diverge over the following years, and only one of them is still reinforced.
At a glance
| Property | Typical value | Note |
|---|---|---|
| GRCA minimum | 16% ZrO₂ by weight | Written into the GRCA specification |
| Common commercial grades | 16.0 to 19.5% ZrO₂ by weight | Higher grades for aggressive exposure |
| E glass zirconia content | 0% | Attacked by the cement pore solution |
| Cement pore solution pH | 12.5 to 13.5 | The environment the fibre must survive |
| Verification by chemical analysis | X-ray fluorescence on the glass | Distinguishes AR glass from E glass |
| Strand in cement ageing | 28 days at 80 °C, or 50 °C equivalents | Accelerated ageing per ASTM C1666 |
| Fibre density | about 2,680 kg/m³ | Higher than E glass, from the zirconia |
| Dosage in sprayed GRC | 5% by weight of the mix | 3 to 3.5% in premix |
Why zirconia works
Glass is a silicate network, and silica dissolves in strong alkali. Hydrating Portland cement gives a pore solution at pH 12.5 to 13.5, which attacks a plain silicate fibre continuously: hydroxide ions break silicon to oxygen bonds, the filament surface is etched, notches form, and the fibre loses strength progressively from the outside in.
Zirconium dioxide behaves differently in that environment. Zirconium is not appreciably soluble at high pH, so as the silica at the surface is leached away the zirconium stays behind and concentrates into a zirconium rich layer on the filament. That layer is a barrier: it slows further hydroxide attack by orders of magnitude and converts a reaction that would consume the fibre in months into one that no longer matters over the life of the panel. Below about 16 percent the zirconium is too dilute to form a coherent protective layer, which is where the threshold comes from. It was established through decades of accelerated ageing work and written into the GRCA specification, BS EN 15422 and ASTM C1666.
How it is verified
- The fibre manufacturer’s data sheet and batch certificate, stating the zirconia content by weight. This is the minimum a specifier should require on every submission
- Chemical analysis of the glass by X-ray fluorescence or an equivalent method, which is the only direct check and the test to call for where the fibre type is in any doubt
- Strand in cement accelerated ageing to ASTM C1666, in which strands are aged in cement at elevated temperature and their retained strength measured, confirming that the glass performs as well as its composition suggests
- Ongoing modulus of rupture and limit of proportionality testing on boards from production batches to the GRCA specification, which confirms the fibre is present and working at the time of manufacture
What happens if the minimum is not met
Nothing visible, for a while. A panel made with E glass or with an under-specified fibre demoulds correctly, passes its 28 day modulus of rupture test, arrives on site looking exactly like a compliant panel and installs normally. The reinforcement then degrades. Published accelerated ageing work shows E glass in cement losing the majority of its strength within months of equivalent exposure, and the composite drifts toward the behaviour of unreinforced mortar: brittle, low in flexural capacity, with the limit of proportionality and the modulus of rupture converging as there is no longer a fibre contribution beyond first crack.
In service that means a panel that cannot accommodate the wind load, the thermal movement and the handling stress it was designed for. Cracking appears at anchor points and around openings, and because the failure is progressive and internal there is no inspection that catches it early. Remediation means replacing the panels.
This is why the distinction between E glass and alkali resistant glass is a material failure rather than a substitution. The two fibres are not different grades of the same product for the same application; one is formulated for a polymer matrix and one for a cement matrix, and they are not interchangeable in either direction.
What to write in a specification
Require alkali resistant glass fibre with a minimum zirconia content of 16 percent by weight, to the GRCA Specification for the Manufacture, Curing and Testing of GRC Products, and require the fibre manufacturer’s certificate stating the composition for each batch supplied. Require modulus of rupture and limit of proportionality test boards from production batches, and state that the design values used are long term aged values rather than 28 day values. Where the exposure is aggressive, a higher zirconia grade can be specified, but the 16 percent figure is the floor and is not negotiable.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (minimum 16% zirconia)
- ASTM C1666 Standard Specification for Alkali Resistant (AR) Glass Fiber for GFRC and Fiber-Reinforced Concrete and Cement
- BS EN 15422 Precast concrete products: specification of glass fibres for reinforcement of mortars and concretes
- BS EN 1170 parts 1 to 8 (GRC test methods, including fibre content and flexural properties)
- BS EN 1169 Factory production control of glass fibre reinforced cement
Related terms
Questions specifiers ask
Why 16 percent and not some other figure?
Below roughly 16 percent by weight the zirconium is too dilute to form a coherent protective layer on the filament surface as the silica leaches away. The figure comes from decades of accelerated ageing testing and is written into the GRCA specification, BS EN 15422 and ASTM C1666.
How can a specifier check the zirconia content?
Require the fibre manufacturer’s batch certificate stating the composition, and where there is doubt require chemical analysis of the glass by X-ray fluorescence. A fibre content test tells you how much glass is present, not which glass it is.
What happens if E glass is used in GRC by mistake?
The panel looks and tests normally at 28 days, then loses its reinforcement over the following years as the cement attacks the fibre. It becomes brittle, cracks under wind and handling stress, and the only remedy is replacement.
Is a higher zirconia content better?
Grades above 16 percent, typically up to about 19.5 percent, offer additional margin and are used for aggressive exposure or where long design life has to be justified. The 16 percent minimum applies regardless.
Does zirconia affect the fire performance of GRC?
No. Both the glass and the cement matrix are inorganic, and the fibre softens around 860 °C. Non-combustibility still has to be earned by testing the finished product; ShapeShell-RC holds an AS/NZS 1530.1 result of Not Deemed Combustible from Ignis Labs.
Checking a GRC submission against the GRCA specification? ShapeShell-RC is produced with alkali resistant glass and batch tested for fibre content, MOR and LOP. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.