Fibre Reinforced Facade Encyclopedia: Manufacturing
Stay in place formwork is formwork that is not stripped. A thin shell of glass fibre reinforced concrete (GRC, GFRC) or fibre reinforced polymer is made in the factory, delivered to site, propped and braced, filled with structural concrete, and then left in position permanently as the finished face of the element. It replaces two separate operations, forming and finishing, with one component, and it is used where the geometry is complicated enough that building a strippable mould on site would be slow, expensive or impossible.
A tunnel portal with a curved, tapering mouth can be built as eight permanent shells filled in sequence. Built in timber and plywood, the same geometry is a set of one-use moulds that are cut on site and thrown away.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Shell material | Sprayed GRC 12 to 20 mm, or FRP laminate 4 to 10 mm | Thickness set by concrete pressure, not by the finished look |
| Shell weight | 30 to 50 kg/m² GRC; 5 to 32 kg/m² FRP | ShapeShell-RC and ShapeShell-RT published ranges |
| Design concrete pressure | Up to about 60 kPa for a normal pour rate | Falls with slower pours and lower lifts; calculate per AS 3610 |
| Typical pour lift | 1.0 to 1.5 m per hour | Slower pours reduce pressure on the shell; typical |
| Propping and bracing | External frames or internal ties at 600 to 1,200 mm centres | Sized by the temporary works engineer; typical range |
| Shell to concrete bond | Mechanical keys, cast-in ties or roughened back face | Design either fully composite or as a non-composite liner |
| Cover to reinforcement | Measured to the structural concrete face, not the shell face | The shell is not counted as cover unless demonstrated |
| Finished face | Off-form, acid etched, sandblasted or exposed aggregate | The mould face is the building face |
| Fire | Cementitious shell can be tested to AS/NZS 1530.1; polymer shell cannot | ShapeShell-RC: Not Deemed Combustible, Ignis Labs |
What the shell has to survive
The design case for a permanent shell is not the finished building. It is the twenty minutes during the pour when the shell is a pressure vessel full of wet concrete. Fresh concrete behaves as a fluid with a density around 2,400 kg per cubic metre until it stiffens, so lateral pressure builds with the height of concrete placed and the rate at which it is placed. AS 3610 gives the method for calculating it. A 1.5 m lift at a normal pour rate produces something of the order of 30 to 40 kPa, an order of magnitude more than the wind load the same panel would see in service.
A 15 mm glass fibre reinforced concrete skin cannot take that on its own over any useful span. The shell is therefore designed as one part of a temporary works system: the shell spans between stiffening ribs cast into it, and the ribs bear on an external soldier frame, internal ties, or both. The temporary works engineer sizes the propping; the panel manufacturer sizes the shell and the ribs. Neither can be done without the other, which is why permanent formwork is specified as a designed system rather than as a supply item.
Composite or non-composite
Two design philosophies exist and the specification has to say which one applies.
- Non-composite liner: the shell is treated as sacrificial formwork and a finish. The structural concrete carries everything and is designed and detailed as if the shell were not there. Cover to reinforcement is measured to the concrete face. This is the simpler and more common route.
- Composite: the shell is bonded to the structural concrete through mechanical keys, cast-in ties or a deliberately roughened back face, and is allowed to contribute to the section. This needs interface shear to be designed and demonstrated, and it needs the differential shrinkage and moisture movement between a thin shell and a thick core to be assessed.
Where it is used
- Bridge parapets, edge beams and pier heads, where a moulded profile runs for hundreds of metres and a site-built mould would be rebuilt continuously
- Tunnel portals and headwalls, where the geometry is curved and tapered and the finished face is the architectural statement
- Columns, particularly tapered, fluted or elliptical columns where a strippable mould is a one-use item anyway
- Waffle and coffered slab soffits, where large numbers of identical formers are set out and filled and the soffit is left exposed
- Architectural in situ walls with deep relief, where the relief would make stripping destructive
In each case the shell is doing three jobs at once: it forms the concrete, it is the finished surface, and it becomes a durable protective layer over the structural concrete. That last point is worth care. A dense cementitious shell with low water absorption adds a sacrificial skin over the structure. A polymer shell does not carbonate but can debond if the interface is not detailed for differential movement.
Fire, and why the material choice is not neutral
A permanent shell becomes part of the permanent building fabric, so it is assessed under the same rules as any other cladding or lining. In Australia, external walls of Type A and Type B construction must be non-combustible under the National Construction Code Deemed-to-Satisfy provisions (check the edition adopted in your jurisdiction). A cementitious shell can be tested to AS/NZS 1530.1 and classified accordingly; ShapeShell-RC carries a Not Deemed Combustible result from Ignis Labs.
Buildability and tolerance
The tolerance stack is the part that catches projects out. A factory-made shell holds tight tolerances. The site slab, the starter bars and the setting out do not. Every joint between shells, every junction with the in situ concrete, and every interface with a bearing or a bracket has to absorb the difference. Shells are normally detailed with an open or recessed joint, a returned edge, and a packing allowance at the base.
Standards and references
- AS 3610 Formwork for concrete, including the calculation of concrete pressure on formwork and surface finish classes
- AS 3600 Concrete structures (cover, durability exposure classifications and in situ concrete design)
- AS 5100 Bridge design series, for parapets, edge beams and civil structures
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products; GRCA guidance on permanent formwork
- BS EN 1169 and BS EN 1170 series (factory production control and test methods for glass-fibre reinforced cement)
- AS/NZS 1530.1 combustibility; AS 5113 and BS 8414 full-scale facade tests; NCC 2022 Volume One C2D10 (check the edition adopted in your jurisdiction)
- CIRIA C766 Guide to formwork for in-situ concrete (temporary works design)
Where this matters on a ShapeShift project
Spanda: A large freeform public artwork, and an example of the same problem permanent formwork solves: complex three dimensional geometry that has to be manufactured accurately off site because it cannot be built accurately on site.
Related terms
Questions specifiers ask
What is the difference between stay in place formwork and lost formwork?
They describe the same idea from different angles and are often used interchangeably. Stay in place is the engineering term for permanent formwork designed as part of the structure. Lost formwork is the older and looser term, and it also covers single-use moulds that are destroyed to release a casting. See the lost formwork page for the terminology and the selection case.
How thick does a permanent GRC shell need to be?
Thickness is governed by the pressure of wet concrete during the pour, not by the appearance. Sprayed shells are typically 12 to 20 mm with integral ribs, spanning between props or soldiers set by the temporary works engineer.
Does the shell count towards concrete cover?
Not unless it has been specifically designed and demonstrated to do so. The normal approach is to measure cover to the structural concrete face and treat the shell as a separate protective and architectural layer.
Can permanent formwork be used on an external wall of a Type A building?
A cementitious shell with an AS 1530.1 Not Deemed Combustible result satisfies the Deemed-to-Satisfy non-combustibility requirement. An FRP shell is combustible and requires a Performance Solution supported by full-scale testing such as AS 5113.
Who designs the propping?
The temporary works engineer, using the concrete pressure calculated to AS 3610 and the shell and rib capacities provided by the manufacturer. The two designs have to be issued together.
Considering permanent formwork for a curved parapet, portal or column? ShapeShift manufactures GRC and FRP shells to project geometry with the rib and propping interface designed in. Talk to the ShapeShift team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.