The quality of any GRC element is inseparable from the quality of the mould used to make it. Unlike ordinary precast concrete — where formwork is a relatively crude, load-bearing container — a GRC mould is a precision engineering artefact. It must faithfully reproduce surface texture down to sub-millimetre detail, maintain tight dimensional tolerances through dozens or hundreds of production cycles, and release cleanly without damaging even deeply undercut profiles. Getting mould design and selection right is not optional; it is a fundamental prerequisite for producing GRC that meets specification.
Each GRC manufacturer has own approach to this topic. Some prefer to manufacture their own moulds whereas others sublet that element of works to a specialist producer. Both strategies have their pros and cons. Many aspects of the production need to be taken into consideration before choosing one or the other.
Why GRC Demands More from Its Moulds
GRC elements are typically thin-walled — often just 10–15 mm thick — and frequently feature complex, deeply profiled geometries that would be impractical in ordinary concrete. The pressures exerted on formwork by fresh GRC are far lower than those from a conventional reinforced concrete pour, which means GRC moulds can be larger, lighter and more intricate than their concrete counterparts. However, this apparent advantage is more than offset by a set of demanding requirements that ordinary formwork simply does not face.
The key performance requirements of a GRC mould are:
Dimensional accuracy and stability — the mould must hold its shape precisely through the full production run, accounting for volume changes in the GRC including drying shrinkage and thermal movement
Surface fidelity — every texture, profile and joint in the mould face is faithfully reproduced as a mirror image on the finished element; defects and visible joints in the mould become defects in the product
Release performance — the mould must allow the cured GRC to be stripped cleanly and without damage, especially critical for elements with deep undercuts, sharp returns or fine surface detail
Reuse life — the economics of GRC production depend heavily on how many pulls a mould can deliver before dimensional accuracy or surface quality degrades
The Evolution of GRC Mould Materials
Timber and Plywood
The earliest GRC moulds were fabricated from timber and plywood — materials borrowed directly from conventional concrete formwork practice. Timber is cheap, easy to work with hand tools, and perfectly adequate for simple, flat or singly-curved elements in short production runs. It holds nails and screws well, making it straightforward to assemble box moulds or ribbed formers.
The limitations of timber become apparent quickly in demanding applications. Moisture cycling causes swelling and warping, which compromises dimensional accuracy. The surface of plywood, even when sealed, absorbs moisture from fresh GRC and tends to degrade after a modest number of uses. Timber is unsuitable where deep profiling, undercuts or fine surface texture are required, because the rigidity of the material makes clean stripping difficult or impossible without damaging either the mould or the GRC element.
Steel Moulds
Welded steel moulds offer far greater dimensional stability and service life than timber. They are the material of choice for high-volume production of standard elements — drainage channels, cable ducts, lintels and simple cladding panels — where the geometry is straightforward, production runs are long and the investment in mould fabrication is easily amortised.
Steel moulds are heavy, which adds handling complexity in the factory, and their rigidity — an asset for dimensional stability — becomes a liability when stripping elements with any degree of undercut. Where a product profile changes direction by more than a few degrees without a generous draft angle, a rigid steel mould will either trap the cured GRC or require the use of collapsible or multi-part construction, significantly increasing fabrication cost and complexity. Surface finish on steel moulds is generally excellent when new, but scratches, corrosion and surface damage accumulate with use, and repair welding or grinding can introduce surface irregularities.
Polyurethane Rubber Moulds for GRC
Flexible polyurethane rubber moulds represent one of the most significant advances in GRC mould technology and are now particularly popular in current production. Cast from a master model, a polyurethane rubber mould can stretch and peel away from deeply undercut profiles, sharp internal angles and complex surface textures that would trap or tear a rigid mould beyond recovery. The elastic recovery of the material means the mould returns to its original shape after each stripping cycle.
The practical consequences for GRC production are substantial. Elements with deeply profiled classical mouldings, bold bas-relief decoration, complex gothic tracery or faceted geometric patterns — all of which would require expensive multi-part rigid moulds with many parting planes — can be produced from a single flexible mould. Damage to the GRC element during stripping is minimised, breakage rates fall, and surface quality of even the most intricate profiles is maintained consistently through many production cycles.
Two-component mould systems, in which a rigid GRP or timber shell provides the structural backing and a polyurethane rubber liner provides the working surface, combine the dimensional stability of a rigid backing with the release performance of a flexible face. These hybrid constructions are now common in current GRC production. The rigid shell prevents the flexible liner from distorting under the weight of fresh GRC and during compaction, while the rubber face ensures clean, damage-free release.
Silicone Rubber GRC Moulds
Silicone rubber moulds share many of the advantages of polyurethane but offer superior resistance to temperature extremes and better chemical compatibility with certain release agents and matrix formulations. Silicone is the preferred choice when the master model or pattern is itself sensitive to the chemistry of polyurethane systems — particularly for patterns made from certain waxes, clays or polymers that react adversely with isocyanate-based systems. Silicone moulds tend to be more expensive than polyurethane equivalents and are somewhat less abrasion-resistant, but their use in decorative and heritage reproduction work is well established.
The Mould-Making Process: From Master Model to Production
The precision required of GRC moulds means that the master model — from which the mould is cast — is itself a critical artefact. For a complex architectural element, it is usually necessary to construct a full-scale physical model before mould-making begins. This model must be produced to the required dimensional tolerances, accounting for the expected shrinkage of the GRC element during curing and subsequent drying. It is typically treated with a release agent before the mould material is applied, to ensure clean separation of the finished mould.
Mould material curing times vary with the system used, but a period of three to seven days is typical before the mould can be safely stripped from the master and put into production. During this period the mould material develops its full dimensional stability, and any premature stripping risks distortion that will carry through into every element subsequently produced from that mould.
Critical design considerations at this stage include:
Draft angles — all vertical faces should be given sufficient draft (typically a minimum of 2–3°) to allow clean release; the more flexible the mould material, the less critical this becomes, but rigid moulds demand careful attention to draft angles at the design stage
Corner treatment — sharp internal corners in the mould create stress concentration points in the GRC element and should be avoided where possible; corners should be slightly chamfered or rounded, since sharp angles can also restrict compaction of fresh GRC during spraying and restrain natural shrinkage movements, generating internal tensile stress and potential cracking
Dimensional allowances — shrinkage of GRC between casting and final service dimensions must be accounted for in the master model, typically by scaling the model slightly larger than the target finished dimension
Surface condition — any defects, tool marks or parting lines on the master model will be faithfully reproduced in the mould and then reproduced again in every element; the surface of the master must be finished to a standard at least as high as that required on the finished GRC product
Adaptive Mould Technology for Double-Curved Surfaces
A significant challenge in contemporary architectural GRC production is the growing demand for free-form, double-curved façade elements driven by computational design tools. Conventional mould-making methods require a unique physical master model for each differently curved panel, which is prohibitively expensive when an entire façade consists of hundreds of panels each with a subtly different curvature.
Adaptive mould systems address this directly by integrating digital panel design with a reconfigurable mould-forming platform. The principle involves a bed of computer-controlled actuator pins whose heights are individually adjusted to define any desired surface profile within the system’s range. A flexible mould skin — typically a thin elastomeric sheet — is draped over the pin bed and takes up the required curvature. The GRC is then sprayed or cast onto this surface in the normal way.
The video below presents adaptive mould system by Adapa (recently aquired by Bespline).
The adaptive mould process substantially reduces the manual labour required to produce moulds for curved surfaces and eliminates the need for individual master models. It is particularly well-suited to façade systems where the panel geometry varies systematically across the building surface, as is common in parametrically designed architecture. The technology represents a direct integration of computational design and physical production — and it is one of the clearest examples of how GRC has positioned itself at the leading edge of construction technology.
Mould Release Agents
The choice and application of release agent is a surprisingly consequential decision in GRC mould management. Release agents used in standard concrete technology are generally suitable for GRC moulds, but the agent must satisfy two conditions: it must not interact chemically with the mould surface material in a way that degrades the mould, and it must not affect the surface quality or finish of the GRC product.
Oil-based release agents can stain or discolour GRC surfaces, which is particularly undesirable on exposed or pigmented facing mixes. Wax-based and water-emulsion release agents are generally preferred for smooth or polished finishes. For rubber moulds, specific polyurethane- or silicone-compatible release agents should be selected. Over-application of any release agent can cause blowholes, surface voids or a matt bloom on what should be a smooth finish — a consistent, thin film is always preferable to a heavy coat.
In some processes, a chemical retarder is applied to the mould surface before the facing mix is sprayed. This delays setting of the thin surface layer of cement paste, which can then be washed away with a water jet after demoulding to reveal the decorative fine aggregate beneath — the exposed aggregate finish. This technique requires the retarder to be fully compatible with the mould material and applied at a controlled, uniform thickness.
How the Mould Face Influences the GRC Element
The relationship between mould face and GRC performance extends beyond surface aesthetics. The mould face of a sprayed GRC panel — the face that was in contact with the mould during production — is consistently denser and smoother than the back face, which is finished by roller compaction or trowelling. This has a direct effect on mechanical test results: flexural strength tests are routinely conducted in both orientations (mould face up and mould face down) and the ratio of these results — the top/bottom ratio — is a standard quality indicator in GRC specification and compliance testing.
The proximity of the mould face also influences fibre distribution. The ‘wall effect’ caused by the mould face means that fibre content in the GRC is slightly lower immediately adjacent to the mould surface, as the physical boundary restricts the positioning of fibre strands perpendicular to the plane of the panel. This is a fundamental consequence of the spray-up process and is one reason why transverse tensile and inter-laminar shear strengths of flat GRC sheet are inherently lower than in-plane flexural strengths — a characteristic that designers must account for in the structural design of GRC panels.
Practical Guidance: Mould Selection by Application
The mould is not a passive container — it is an active participant in the quality of every GRC element produced from it. Investment in mould design and material selection pays dividends in reduced breakage during stripping, consistent surface quality, dimensional compliance, and production efficiency. Polyurethane rubber, flexible liners and adaptive systems have transformed what is achievable in complex architectural GRC, and the integration of digital design with adaptive mould technology is pushing the boundaries further still.
For any new production project, the starting point should always be the geometry and surface specification of the finished element. Working backwards from these requirements to select the optimal mould system — material, construction method, release strategy and dimensional allowances — is the most reliable path to achieving specification compliance and a product that does justice to the architectural intent it was designed to realise.