Fibre Reinforced Facade Encyclopedia: Manufacturing
Sprayed GRC is the manufacturing process in which cement slurry and chopped alkali resistant glass roving are sprayed simultaneously into a mould and built up in thin layers, each one roller compacted before the next is applied. It is called spray-up GFRC in North America. Because the fibre is chopped at the gun rather than mixed into the slurry beforehand, sprayed GRC carries about 5 per cent fibre by weight at full length, roughly double the reinforcement of premix, and reaches the highest strength of any GRC process. It is the standard process for facade panels.
A typical facade skin is built up in 6 to 10 sprayed passes to a finished thickness of 12 to 18 mm, with the whole panel sprayed wet-on-wet in a single session so the layers bond as one section.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Fibre content | about 5% by weight | Compared with 3 to 3.5% for premix |
| Fibre length | 25 to 40 mm, chopped at the gun | Longer than premix fibre at 6 to 13 mm |
| Skin thickness | 12 to 18 mm typical, 10 to 20 mm range | Built in 6 to 10 passes |
| Modulus of rupture (MOR) | 20 to 30 MPa at 28 days | Premix reaches 10 to 14 MPa |
| Limit of proportionality (LOP) | 7 to 11 MPa | First crack; the serviceability design value |
| Sand to cement ratio | typically 1:1 by weight | Fine silica sand, graded |
| Water to cement ratio | 0.30 to 0.35 | Low; superplasticiser makes it sprayable |
| Demoulding | 16 to 24 hours after spraying | Panel handled with care until 7 days |
| Cure | 7 days moist cure, or air cure if polymer modified | 5% polymer solids by volume of cement |
| Quality control | Test board per mix batch or per shift | MOR, LOP and fibre content to EN 1170 methods |
How the spray-up process works
Two streams meet in mid-air at the gun. A pump delivers cement slurry, a mix of Portland cement, graded fine silica sand, water, superplasticiser and usually a polymer, through one nozzle. A chopper unit on the same head feeds continuous alkali resistant glass roving through a set of blades and blows the chopped strands out through a concentric air stream.
The critical variable is the ratio between the two streams, which sets the fibre content. It is checked at the start of every shift by spraying into a bag for a timed period, separating and weighing the fibre and the slurry, and adjusting the chopper feed or the pump until the ratio is right. This is a physical measurement, not a machine setting, and the single most important control on the strength of the finished panel. The gun is hand-held for architectural work, because an operator has the reach for deep returns and complex mould geometry; gantry or robot spray gives more consistent thickness on large flat panels. See robotic-manufacturing.
Layer build-up and roller compaction
Sprayed GRC is built in passes of roughly 3 to 4 mm rather than one thick application, and after each pass, or every second pass, the operator works the surface with a serrated roller. Compaction does three things the spray alone cannot: it drives out entrapped air, it beds the fibre into the slurry so each filament is fully encased in cement, and it consolidates the layer against the mould face so the panel picks up the mould detail. Under-compaction is the commonest cause of low strength and of pinholing and fibre bloom; over-rolling a thin layer tears the fresh material or drags fibre to the surface.
Thickness is checked wet with a pin gauge at a grid of points across the mould. Because the skin is thin, a 2 mm shortfall is a 15 per cent loss of section, so thickness control is a structural check rather than a finish matter. Returns, edges and rib roots are compacted with the same discipline as the face, and integral ribs or bonding pads for a stud frame are formed while the material is green. See stud-frame-grc and flex-anchor.
Why sprayed reaches higher strength than premix
The difference is fibre length and fibre content, and both follow from where the fibre is introduced. In premix GRC the fibre goes into the mixer with the slurry, so it must be short enough not to ball up and the mix must stay workable, capping the loading at about 3 to 3.5 per cent by weight with fibres of 6 to 13 mm. In spray-up the fibre is chopped at the moment of deposition and never passes through a mixer, so it can be 25 to 40 mm long and loaded to about 5 per cent.
Longer fibres develop more bond length either side of a crack, so more load transfers before pull-out, and more fibre by weight means more filaments crossing any plane. Sprayed material is also close to two dimensionally random in the plane of the panel rather than three dimensionally random, so more of the fibre is usefully oriented for flexure. Together these give sprayed GRC an MOR of 20 to 30 MPa against 10 to 14 MPa, and a markedly higher strain to failure. The trade-off is access: spray-up needs an open mould the gun can reach, so deep, narrow or enclosed shapes are premix work. See premix-grc and gfrc.
Demoulding and curing
Panels are demoulded at 16 to 24 hours, once the material has enough green strength to be lifted without distortion. Earlier demoulding risks cracking at rib roots and edge returns; leaving a panel in longer ties up tooling, which on a project with few moulds sets the production rate. See demoulding and cnc-formwork.
Curing follows one of two routes. A conventional mix is moist cured for about seven days, in a chamber or under polythene, so the cement hydrates fully before the panel dries. A polymer modified mix, with an acrylic dispersion at about 5 per cent solids by volume of cement, forms a film as it dries that retains water in the matrix and is air cured without a moist cure period. Polymer modification is now usual practice for architectural GRC, because it removes the curing chamber from the production sequence and improves aged strength and resistance to moisture movement. Design uses long-term aged property values rather than 28-day figures. See curing and polymer-modified-grc.
Quality control and the test board
The control document for GRC is the GRCA Specification, supported by the EN 1170 test methods. Production practice is to spray a test board from the same mix and gun setting as the panels, at each mix batch or at least once per shift, and cut specimens from it. EN 1170-5 covers bending strength by the complete bending test, from which both the modulus of rupture and the limit of proportionality are taken; EN 1170-1 covers slurry consistency by slump, and EN 1170-2 the fibre content by wash-out. Specimens are tested at 7 and 28 days and results recorded against batch and panel numbers, under factory production control to BS EN 1169, so any panel traces back to mechanical test results. Ask for the batch records for your own project rather than a generic data sheet. ShapeShell-RC is batch tested for MOR and LOP to the GRCA specification.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- BS EN 1170-1 to 1170-8, test methods for GRC; EN 1170-5 complete bending test for MOR and LOP
- BS EN 1169 Factory production control of glass fibre reinforced cement
- BS EN 15422 Specification for glass fibres for reinforcement of mortars and concretes
- ASTM C1228 (preparing coupons), C1229 (fibre content by wash-out) and C1230 (early-age flexural)
- PCI MNL-128 Recommended Practice for Glass Fiber Reinforced Concrete Panels
- AS 1530.1 combustibility; ShapeShell-RC tested Not Deemed Combustible by Ignis Labs
Where this matters on a ShapeShift project
One Sydney Harbour, Barangaroo: A high rise residential tower, and an illustration of why the spray process suits facade work: a moulded, three-dimensional panel face held in a thin section, on a building whose external walls fall under the non-combustible provisions.
Related terms
Questions specifiers ask
What is the difference between sprayed and premix GRC?
Sprayed GRC chops the fibre at the gun and deposits it with the slurry, giving about 5 per cent fibre at 25 to 40 mm and an MOR of 20 to 30 MPa. Premix mixes shorter fibre into the slurry first, giving about 3 to 3.5 per cent and 10 to 14 MPa. Sprayed is the facade panel process; premix suits small intricate elements.
How thick is sprayed GRC?
A facade skin is typically 12 to 18 mm, built up in 6 to 10 sprayed passes with roller compaction between them. Stiffness comes from integral ribs or a steel stud frame rather than from thickness.
When is a sprayed GRC panel demoulded?
At 16 to 24 hours, once it has enough green strength to be lifted without distortion. It is then either moist cured for about seven days or, if the mix is polymer modified, air cured.
How is sprayed GRC quality controlled?
By spraying a test board from the same mix and gun setting as the panels, at each batch or shift, and cutting specimens for bending tests to EN 1170-5 for MOR and LOP plus fibre content by wash-out, recorded against batch and panel numbers under factory production control to BS EN 1169.
Specifying a sprayed GRC facade? ShapeShell-RC is batch tested for MOR and LOP to the GRCA specification and holds an AS/NZS 1530.1 Not Deemed Combustible result from Ignis Labs. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.