Fibre Reinforced Facade Encyclopedia: Manufacturing
Premix GRC is glass fibre reinforced concrete in which short alkali resistant fibres are blended into the cement slurry before the material reaches the mould, so the mix arrives as a homogeneous fibre filled slurry that is poured, pumped or sprayed. It is the counterpart to sprayed GRC, where slurry and continuous roving are delivered separately to the mould and the fibre is chopped at the gun. The difference in when the fibre meets the slurry produces a different fibre length, a different fibre orientation and a materially lower strength, and it is the reason the two processes are used for different things.
Premix GRC carries about 3 to 3.5 percent fibre by weight and reaches 10 to 14 MPa modulus of rupture. Sprayed GRC carries about 5 percent and reaches 20 to 30 MPa.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Fibre content | 2.5 to 3.5 percent by weight | Against about 5 percent for sprayed GRC |
| Fibre length | 6 to 13 mm | Against 25 to 40 mm chopped at the spray gun |
| Fibre orientation | Three-dimensional random | Sprayed GRC is two-dimensional random, which is more efficient |
| Modulus of rupture (MOR) | 10 to 14 MPa at 28 days | Sprayed GRC reaches 20 to 30 MPa |
| Limit of proportionality (LOP) | 5 to 8 MPa | First crack; the serviceability design value |
| Typical section thickness | 10 to 25 mm, thicker where cast solid | Not limited to a sprayed skin thickness |
| Density | 1,900 to 2,100 kg/m3 | Comparable to sprayed GRC |
| Demoulding | 16 to 24 hours | Then moist cured about 7 days unless polymer modified |
| Fire | Cementitious; combustibility established by test to AS 1530.1 | ShapeShell-RC: Not Deemed Combustible, Ignis Labs |
How premix is made
The slurry is mixed first, usually in a high shear mixer that disperses cement, fine silica sand, water, superplasticiser and polymer admixture into a smooth paste. The mixer is then switched to a low shear paddle action and the chopped fibre is added, because high shear blades will break and ball the glass. The finished mix is a workable, fibre filled slurry with a short working life.
From there it goes into the mould by whatever method suits the geometry: poured and vibrated, poured as a self-compacting mix, pumped into a closed mould, or sprayed through a premix gun that atomises the fibre filled slurry without a chopper. Sections can be built up in layers, and inserts, mesh, ribs and fixing anchors are placed as the mould is filled.
Why the strength is lower
Two things cost premix its strength. The first is fibre content. A spray gun deposits slurry and fibre together at the mould face, so fibre can be added without regard to the workability of a mix in a bucket. Premix has to remain pourable after the fibre is in it, and glass fibre thickens a slurry rapidly, so the practical ceiling is around 3.5 percent by weight against about 5 percent for sprayed.
The second is orientation. Sprayed fibre lands flat against the mould in a plane, so almost all of it is aligned to resist bending. Premix fibre is randomly oriented in three dimensions, so a substantial fraction of it points through the thickness, where it contributes very little to flexural capacity. Shorter fibres also develop less bond length before pulling out. The combined effect is a modulus of rupture roughly half that of sprayed GRC and a limit of proportionality proportionally lower, which is why premix is designed to different values and not simply treated as thinner sprayed GRC.
Where premix is the right choice
- Small, intricate or deeply modelled elements where a spray gun cannot reach the mould face at a workable angle
- Deep undercuts, narrow ribs, mouldings, finials and cornice profiles that must be filled rather than laid up
- Sections cast solid rather than as a thin skin, such as bollards, planters, seating, bases and column casings
- Long runs of small repeated units where casting is faster than spraying
- Backfilling, bonding pads, rib filling and detail work on panels whose main skin is sprayed
- Elements where the back face is visible and a sprayed back face would be unacceptable
What premix is not for is a large facade panel carrying wind load. Those are sprayed, for the strength, and the facade panel case is covered on the GFRC panels and sprayed GRC pages. It is common for a single panel to use both: sprayed skin for capacity, premix for the bonding pads and the fine detail.
Casting against spraying a premix
A premix can be cast or it can be sprayed, and the choice is about the mould rather than the material. Casting fills the mould under gravity or vibration and reaches into recesses a gun cannot see, but it demands a mould that can be filled without trapping air, which usually means a closed or two-part mould for anything with an undercut. Vibration must be controlled, because over-vibration segregates the fibre from the slurry and leaves a fibre poor face.
Spraying a premix suits large open moulds where the value is speed and an even skin thickness rather than reaching into detail. It gives no fibre orientation benefit, because the fibre is already in the slurry and lands in whatever direction it arrives; the mechanical properties remain premix properties, not sprayed properties. Assuming otherwise is a common specification error.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- BS EN 1170 parts 1 to 8, test methods for glass fibre reinforced cement, including bending strength
- BS EN 1169, factory production control for glass fibre reinforced cement
- ASTM C1228, C1229 and C1230, GRC test methods used in North America
- PCI MNL-128 Recommended Practice for Glass Fiber Reinforced Concrete Panels
- AS 1530.1 combustibility, where the element forms part of an external wall (check the edition of the NCC adopted in your jurisdiction)
Related terms
Questions specifiers ask
What is the difference between premix and sprayed GRC?
In premix, short fibres are blended into the slurry before it reaches the mould and the material is cast or sprayed as one mix. In sprayed GRC, slurry and continuous roving are delivered separately and the fibre is chopped at the gun onto the mould face. Sprayed reaches about 5 percent fibre and 20 to 30 MPa MOR; premix about 3 percent and 10 to 14 MPa.
Why is premix weaker if it uses the same materials?
Because it carries less fibre and the fibre is randomly oriented in three dimensions rather than aligned in the plane of the panel. Shorter fibres also develop less bond. Roughly half the flexural capacity results.
Can premix be used for facade panels?
Not as the structural skin of a wind loaded facade panel; those are sprayed. Premix is used for smaller cast elements, for intricate detail, and for bonding pads and rib fill within a sprayed panel.
Does spraying a premix give sprayed GRC properties?
No. The fibre is already dispersed in the slurry, so spraying changes only how the material is placed. The design values remain premix values.
How is premix GRC verified?
By batch testing to the GRCA specification and BS EN 1170, measuring modulus of rupture and limit of proportionality on boards cast from production batches, with the results recorded against the panels they represent.
Detailed mouldings and cast elements alongside a sprayed facade? ShapeShell-RC is our GRC system, batch tested for MOR and LOP to the GRCA specification. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.