Fibre Reinforced Facade Encyclopedia: Manufacturing
Lost formwork is formwork that is never recovered. The term carries two distinct meanings on a construction site and they are worth separating before a specification is written. In the first, the form is left permanently in the finished structure and becomes the visible face, which engineers usually call stay in place or permanent formwork. In the second, the mould is deliberately sacrificed, cut, broken or dissolved away to release a casting whose geometry has no draft and cannot be demoulded any other way. Both are lost. Only one of them ends up in the building.
A closed, re-entrant sculptural form has no direction in which a rigid mould can be withdrawn. The mould has to be destroyed to get the casting out, and its cost is written off against a single piece.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Meaning one | Permanent form left in the structure | Also called stay in place formwork or participating formwork |
| Meaning two | Single-use mould destroyed at demoulding | Also called sacrificial or breakaway mould |
| Permanent shell materials | GRC 12 to 20 mm, FRP 4 to 10 mm, profiled steel, fibre cement | Thickness set by wet concrete pressure |
| Sacrificial mould materials | Polystyrene, plaster, wax, sand cores, low-cost timber | Chosen to be cheap to make and easy to destroy |
| Break-even against strip formwork | Roughly 1 to 3 uses of a complex shape | Indicative; depends on geometry and access |
| Reusable mould life | 20 to 100 casts for a well-made GRP or steel mould | Typical; the comparison point for any lost form |
| Cost shift | Form cost moves from site labour to factory tooling | Programme certainty improves, unit cost may not |
| Durability question | The permanent face becomes part of the durability envelope | Assess absorption, movement and the interface, not just looks |
Two meanings, one phrase
The ambiguity is real and it causes tender queries. When a civil engineer says lost formwork they usually mean permanent formwork: a shell that forms the concrete and stays there, such as a glass fibre reinforced concrete (GRC, GFRC) parapet liner or a profiled steel deck. When a mouldmaker says lost formwork they often mean the opposite end of the process: a mould that is destroyed at demoulding because the casting cannot be released from a rigid tool.
The practical test is where the material ends up. If it is in the finished structure, the correct term for a drawing or a specification is stay in place formwork, and that page covers the engineering. If it is in the skip, it is a sacrificial mould and belongs to the tooling cost of the panel, not to the permanent works.
Why sacrificial moulds exist
A mould has to release its casting. That requires draft, a slight taper on every face perpendicular to the direction of withdrawal, and a shape with no undercuts. Architectural geometry routinely fails both tests. Double curvature that folds back on itself, deep re-entrant relief, closed sections and negative draft all produce a form that a rigid mould will lock onto.
There are three standard answers. Split the mould into multiple pieces that come apart in different directions, which works until the number of split lines starts showing on the face. Use a flexible mould liner in silicone or rubber that stretches over the undercut, which works for shallow relief and short runs. Or accept that the mould is a one-use item, build it as cheaply as the accuracy allows, and destroy it. Expanded polystyrene cut on a hot wire or milled on a CNC router is the usual choice, because it is fast to shape, light to handle and can be broken or dissolved out.
When lost formwork is the economic answer
Formwork is the dominant cost in complex concrete work, and the decision is almost always about repetition.
- One-off geometry: a single element with no repeats. A reusable mould has nothing to amortise against, so the cheapest route is a sacrificial mould, or a permanent shell that removes the site forming operation entirely.
- High repetition of a simple shape: conventional strip formwork or a reusable steel or GRP mould wins comfortably. Lost formwork is the wrong answer here.
- Moderate repetition of a complex shape with bad access: permanent shells usually win, because the saving is not in the form itself but in the site labour, the scaffold time and the rework that a site-built mould generates.
- Elements that must be finished to an architectural standard: permanent shells win because the finish is made in a factory under controlled conditions rather than stripped and patched on site.
The durability question nobody asks early enough
A permanent form is a permanent material. Once the decision is made, that shell is in the durability envelope of the structure for its design life, and it should be assessed the way any other facade material is assessed rather than treated as leftover formwork.
Three checks matter. Water absorption, because a porous permanent face will hold water against the structural concrete and will stain. Moisture movement and thermal expansion, because a thin shell on a thick concrete core moves differently from the core and the interface has to tolerate it or the shell will craze, debond or spall at the edges. And the interface itself, because a debonded shell that is not mechanically restrained becomes a maintenance liability at height.
Fire is the fourth check where the element is part of an external wall. A cementitious shell can be tested to AS/NZS 1530.1 and classified; ShapeShell-RC holds a Not Deemed Combustible result from Ignis Labs. A fibre reinforced polymer shell is a polymer composite and is not non-combustible, so it follows a tested reaction-to-fire and full-scale pathway rather than a Deemed-to-Satisfy one under the National Construction Code (check the edition adopted in your jurisdiction).
Standards and references
- AS 3610 Formwork for concrete, including surface finish classes and formwork pressure
- AS 3600 Concrete structures (cover and durability exposure classification)
- CIRIA C766 Guide to formwork for in-situ concrete
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- BS EN 1169, factory production control for glass-fibre reinforced cement used as permanent formwork
- AS/NZS 1530.1 combustibility; NCC 2022 Volume One C2D10 where the element forms part of an external wall
Related terms
Questions specifiers ask
Is lost formwork the same as stay in place formwork?
Not quite. Stay in place formwork is always lost, but lost formwork also covers single-use moulds that are destroyed to release a casting and never reach the building. Use stay in place formwork on drawings when you mean a permanent form.
Why would a mould be destroyed deliberately?
Because the casting has undercuts or negative draft and cannot be withdrawn from a rigid mould in any direction. Splitting the mould or using a flexible liner solves some cases; the rest need a sacrificial mould.
What is a sacrificial mould usually made from?
Expanded polystyrene cut on a hot wire or milled on a CNC router is the most common, because it is cheap, accurate enough and easy to break or dissolve out. Plaster, wax and bonded sand cores are also used.
When does lost formwork cost less than conventional formwork?
Typically at one to three uses of a complex shape, and wherever site access, scaffold duration or finish quality dominate the cost. For simple shapes with high repetition, conventional strip formwork remains cheaper.
Does a permanent form need to be non-combustible?
If it forms part of an external wall of a Type A or Type B building in Australia, yes, or the wall needs a Performance Solution supported by full-scale testing. A cementitious shell tested to AS 1530.1 satisfies the Deemed-to-Satisfy route; a polymer shell does not.
Weighing a one-off mould against a permanent shell? ShapeShift builds both, and will tell you which one your geometry actually needs. Talk to the ShapeShift team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.