How GRC Panels Are Made: A Complete Technical Guide to the Manufacturing Process

If you’ve ever stood in front of a modern architectural façade – perhaps a rippled, pale-limestone-textured skin wrapped around a high-rise tower, or an intricately moulded cornice on a heritage restoration project – there’s a good chance you were looking at GFRC. Glass Fibre Reinforced Concrete panels have become one of the defining cladding materials of contemporary construction. They’re lighter than traditional precast concrete, far more shapeable, fire-safe, and when made correctly, extraordinarily durable.

But understanding how GFRC panels are made is essential for everyone in the supply chain – architects specifying them, structural engineers designing their fixings, contractors receiving them on site, and manufacturers producing them day after day. This guide walks through the entire process from raw materials to quality-tested finished panels, drawing on the GRCA’s own specification and design standards alongside the most current technical research.

What GRC Actually Is

GFRC (Glass Fibre Reinforced Concrete) and GRC (Glassfibre Reinforced Concrete) refer to exactly the same material – the former term is used predominantly in North America, the latter everywhere else. At its core, it is a composite made from Portland cement, fine silica sand, alkali-resistant (AR) glass fibres, water, and typically a small addition of acrylic polymer and superplasticiser. Each of these ingredients plays a specific role, and changing any one of them – the water-to-cement ratio, the fibre content, the polymer dosage – directly changes the mechanical performance of the hardened panel.

What makes GFRC genuinely unusual as a building material is that the reinforcement works differently from conventional reinforced concrete. In standard RC, steel bars are placed deliberately in the tensile stress zones and the cover concrete is largely just protection. In GFRC, short bundles of AR glass fibre strands are distributed randomly throughout the thin cement matrix, providing tensile and flexural capacity across the whole section rather than at a predetermined depth. This is why GFRC panels can be as thin as 10–15 mm and still carry wind loads, resist impact, and survive the thermal cycling of exterior façade life.

It is worth to mention that GRC is A1 non-combustible material as per EN 13501-1.

The Raw Materials: Getting the Mix Right

Every discussion of how GFRC panels are made has to start here, because poor material selection will undermine every subsequent step of production.

Alkali-Resistant Glass Fibres in GRC

Standard E-glass fibres – the type used in fibreglass boat hulls and GRP composites – are destroyed relatively quickly by the alkaline pore solution of Portland cement, which has a typical pH of around 12.9. AR glass fibres are formulated with a minimum zirconium dioxide (ZrO₂) content of 16% to resist this attack, and current commercial products contain between 16% and 19% ZrO₂. The fibres must conform to EN 15422 or EN 14649 Category B, and the GRCA Specification requires the manufacturer to provide certification confirming conformance.

Individual filaments are drawn to diameters of 13–20 microns and grouped into strands, which are either wound into rovings (cylindrical packages of approximately 18–20 kg for use in the spray process) or pre-cut into chopped strands of defined lengths for the premix process. A size coating applied at nanometre thickness immediately after drawing protects the pristine filament surfaces from abrasion, controls the bonding between fibre and matrix, and is specifically formulated for either spray or premix production.

Single filament tensile strength can reach 3.0–3.5 GN/m², with a Young’s Modulus of 72–74 GN/m² and a breaking strain of 2.0–2.5% for the strand. These figures represent the material’s potential – maintaining them through careful production is the manufacturer’s core challenge.

Cement

Ordinary Portland Cement (OPC), Rapid Hardening Portland Cement (RHPC), and White Portland Cement are the most widely used binders in GFRC production. White cement is specified when a light or bright colour is required, since it is made from iron-free raw materials and provides a more neutral base for pigments. Cement must be stored correctly – bulk cement in silos remains usable for up to three months, but bagged cement stored in even good conditions can lose around 20% of its strength after just four to six weeks.

Sand

Fine aggregate – almost universally silica sand – is the second-largest ingredient by mass. For the spray process, the maximum particle size is limited to 1.2 mm; for premix production it may be up to 2.4 mm. The fine fraction passing a 150-micron sieve must remain below 10% of the total sand weight, which specifically excludes most standard building sands. Acceptable silica sand specifications typically require a SiO₂ content above 96%, soluble salts below 1%, and a loss on ignition below 0.5%.

Polymer, Admixtures, and Pigments

A small addition of acrylic thermoplastic co-polymer dispersion (typically 4–7% polymer solids by weight of cement) is added to virtually all architectural GFRC mixes today. The polymer serves a critical dual purpose: during the first hours of curing, it forms a film within the matrix that dramatically reduces water evaporation, making dry curing in normal factory conditions possible without a humid fog room. Over the long term, it reduces surface crazing and enhances durability. The polymer must have a minimum film-formation temperature (MFFT) of no higher than 7°C and good alkali resistance.

Superplasticisers are routinely included to achieve the workability needed for efficient spraying without raising the water-to-cement ratio and thereby weakening the matrix. Pigments – typically iron-oxide based powders or dispersions – can be incorporated to produce coloured GFRC, though some colour variation between batches must be anticipated and an acceptable range agreed between manufacturer and client before production begins.

GRC Grade Selection: What the Numbers Mean

Before production starts, the GRCA Specification defines three material grades based on characteristic 28-day flexural strength (Modulus of Rupture, MOR):

P designates polymer-modified grades. Source: +1

The distinction between LOP (Limit of Proportionality – the stress at which behaviour ceases to be linear) and MOR (the ultimate flexural stress at failure) is fundamental to GFRC design. Grade 18 sprayed GRC will typically achieve a MOR of 18–30 N/mm² and an LOP of 5–10 N/mm² at 28 days. The GRCA Design Guide bases all structural design on limit state theory using these characteristic values, with partial factors of safety applied both to loads and to material strengths to account for variations in production quality, panel thickness, and the mode of structural collapse.

GRC Mix Design: Ratios That Matter

The GRCA Specification gives clear guidance on mix proportions, which can be summarised as follows:

Grade 18 / 18P (Spray)

• Aggregate-to-cement ratio: 0.5–1.5
• Water-to-cement ratio: 0.30–0.38
• AR glass fibre content: 4.0–5.5% by weight of total mix
• Polymer solids (18P only): 4–7% by weight of cement

Grade 10 / 10P (Sprayed premix)

• Water-to-cement ratio: 0.30–0.38
• AR glass fibre content: 2.0–3.5%

Grade 8 / 8P (Cast premix)

• Water-to-cement ratio: 0.30–0.40
• AR glass fibre content: 2.0–3.0%

All dry ingredients must be batched by weight using calibrated equipment accurate to ±2% of the stated batch weight, and liquids must also achieve ±2% accuracy, whether weighed, volume-batched, or automatically dispensed. Workability of the freshly mixed slurry is assessed using the mini-slump test: a 57 mm internal diameter Perspex cylinder is filled with slurry and lifted, and the spread is measured against a plate engraved with concentric rings numbered 0–8. A spread between rings 2 and 5 typically confirms the slurry is suitable for spraying. This must be checked and recorded before each production run.

The Two Main Production Processes of GRC panels

Understanding how GFRC panels are made means understanding how these two fundamentally different manufacturing methods work – and why they produce composite panels with different mechanical properties.

1. Simultaneous Spray-Up

This is the predominant method for architectural cladding panels, and it is responsible for producing Grade 18 material with the highest achievable MOR. The process uses a concentric spray gun – a nozzle that simultaneously delivers chopped AR glass fibre and atomised cement slurry, mixing them in mid-air as they hit the mould surface.

Step-by-step:

1. Batching and mixing. Dry ingredients are weighed and loaded into a high-speed, high-shear mixer – not a standard site concrete mixer, which is explicitly prohibited because it cannot produce the lump-free, consistent slurry required. The slurry is pumped to the spray gun nozzle.
2. Equipment calibration. Before every shift begins, the spray equipment must be calibrated using the Bag and Bucket Tests. In the Bag Test, fibre is chopped for 15 seconds into a bag, weighed, and multiplied by four to calculate glass output in g/minute. In the Bucket Test, slurry is sprayed for 30 seconds, weighed, and multiplied by two to confirm slurry output in kg/minute. For a target fibre content of 5%, a slurry output of 12 kg/minute requires a glass depositor output of approximately 630 g/minute. Calibration must be repeated after any equipment adjustment, any change in mix, and whenever a washout test result is unsatisfactory.
3. Mist coat. As thin a layer as possible – targeting around 1 mm – of pure cementitious slurry without fibre is sprayed first onto the prepared mould surface. This closes the mould face surface and prevents the coarser fibre bundles from creating a textured read-through on the exposed panel face. Where a facing mix is used for architectural colour or aggregate exposure, this is applied now – either sprayed or poured – and is typically 3–5 mm thick. The facing coat may be allowed to stiffen slightly, but the first GRC layer must be applied before initial set occurs.
4. Building up the GRC layers. The main body of the panel is built up in passes, with each complete pass depositing a layer approximately 3–4 mm thick. A typical 12 mm thick panel therefore requires three to four passes. Each pass is compacted with a serrated or spring hand roller before the next layer is applied. Rolling is not optional – it ensures the panel surface conforms precisely to the mould, expels entrapped air, and bonds successive layers together. Particular attention must be paid to corners and deep profiles, where a wall effect reduces natural fibre coverage and where compaction is most easily neglected.
5. Thickness checking. After the final pass, the thickness of the panel is checked across its entire area using a depth gauge or template. No flat area may exceed the specified design thickness by more than 4 mm, nor may the total weight of the panel exceed the maximum weight assumed in the structural design. Any under-thickness area must be re-sprayed; any over-thickness material in flat areas must be removed and discarded.
6. Back face finishing. The exposed back face is finished using a float or roller to the required specification.

The simultaneous spray process naturally distributes fibres in a predominantly two-dimensional random pattern – largely parallel to the plane of the mould. This 2-D orientation improves in-plane flexural strength but produces very low transverse tensile and interlaminar shear resistance. This is why sprayed GRC panels should never be assessed for performance by tests that assume isotropic behaviour.

2. Premix and Cast Premix

In premix production, the slurry is prepared first in a high-shear stage, and the pre-cut AR glass fibres are then blended in during a second, slower mixing stage. The two-stage process is essential: the high-shear first stage produces a thoroughly wetted, lump-free slurry, and the gentler second stage disperses the fibres without damaging their integrity. Chopped strands for premix production are typically 6 mm, 12 mm, or 18 mm long; longer fibres improve composite performance but become more difficult to blend uniformly.

Fibre content in premix is lower – typically 2.0–3.5% – partly because the three-dimensional dispersion of fibres in a cast mix is less efficient at generating flexural strength than the 2-D orientation of the spray process, and partly because mixing workability limits how much fibre can be incorporated without damage or balling.

The resulting mix can be:

• Poured (cast premix): pumped or carried in a holding vessel to the mould and poured or pumped in. Compaction is by internal vibration, external vibration of the whole mould assembly, or by using a self-compacting mix formulated with high-range superplasticisers.
• Sprayed premix: the premixed GRC material is delivered through a specialist pump and spray gun, deposited in 3–4 mm layers and rolled after each pass, identical to the spray-up back coat sequence. This method is well-suited to high-volume production of standard shapes, with modern plants capable of producing around 2 tonnes of fresh mix per hour.

Premix is particularly effective for small, complex architectural items – column casings, pilasters, sills, cornices – where multiple identical units can be cast in rapid succession using a bank of moulds. It is also useful for production of Grade 10 material for sprayed architectural features where the full MOR of Grade 18 is not structurally required.

Moulds and Surface Finishes for GRC production

The mould determines almost everything about a panel’s appearance, dimensional accuracy, and whether it can be produced economically. This is why mould design and material selection is taken so seriously in professional GFRC production.

GRC Mould Materials

Rigid timber or steel moulds were standard in early GRC production. Today, the most common approach is a two-component mould – a rigid substrate (often steel or timber) lined with a flexible polyurethane rubber or silicone elastomer. Polyurethane rubber liners are particularly popular for complex architectural profiles because their flexibility allows panels with deep textures, fine details, and even modest undercuts to be stripped cleanly without damage. These liners can be reused many times, making them economical for medium-to-large production runs.

For very large or complex curved elements, FRP (fibreglass) moulds are often used, cast off a full-scale model of the element. It typically takes 3–7 days for the mould material to cure and stabilise dimensionally before production can begin. Mould release agents appropriate for concrete technology are used; they must not interact chemically with either the mould surface or the GRC matrix.

The mould design must allow for the dimensional movement of the panel during curing – primarily drying shrinkage – and must not restrain that movement in a way that induces cracking. Corners should be slightly chamfered or rounded; sharp internal angles concentrate stress and can cause cracking during demoulding or in service.

GRC Surface Finishes

The surface facing the mould is reproduced faithfully in the finished panel – which is why mould surfaces must be defect-free and have no visible joints. The range of achievable finishes is one of GFRC’s most significant architectural advantages:–

• Smooth/polished ex-mould finish: produced directly from smooth mould liners. Beautiful but unforgiving – hairline crazing and any surface blemishes are visible.
• Textured ex-mould finish: the mould liner carries a texture, which is faithfully reproduced on the panel face. Texture also reduces the visual impact of minor surface imperfections.
• Exposed aggregate finish: a retarder is applied to the mould surface before the face mix is cast. After demoulding, the thin surface cement paste is water-washed away to reveal the decorative aggregate beneath. Acid etching with dilute hydrochloric acid can deepen the exposure or enhance colour.–
• Mechanical finishes: wire brushing, grit blasting, or grinding/polishing can be applied once the panel reaches at least 35 MPa compressive strength. These simulate natural stone textures effectively.–
• Pigmented GRC: iron oxide pigments incorporated into the face mix or full-body mix allow a wide range of earthy colours. Consistent colour requires strict batching discipline; some variation between batches should always be anticipated.
• Photocatalytic coating: self-cleaning coating applied to GRC surface

Architectural facing mixes use specially selected decorative aggregates – crushed granite, limestone, marble, calcite – graded to 0–3 mm for sprayed applications and up to 10 mm for poured face layers. The mix design of the face layer must be considered carefully relative to the GRC backing coat, because differential cement content can generate differential shrinkage, causing delamination or surface distortion.

GRC Panel Types and Structural Forms

Not all GFRC panels are made the same way from a structural standpoint. How a panel is stiffened – and how it will be fixed to the building – is decided at design stage and has a direct impact on how it is manufactured.

Single-Skin Panels

The simplest form is a plain single skin, typically 10–15 mm thick. Flat panels of any appreciable size are stiffened by incorporating edge returns – upstanding flanges formed by spraying additional GRC over the panel edges and back – and by changing the section geometry to a corrugated, flanged, or profiled form. For larger single-skin panels, integral stiffening ribs are formed by spraying over polystyrene or preformed GRC rib formers that remain cast into the panel.

Stud Frame Panels

For large cladding panels – typically anything above 3–4 m² – the preferred structural solution is the stud frame system. A prefabricated lightweight steel frame (usually thin-walled hollow sections in heavily galvanised or stainless steel) is manufactured separately, and the GRC skin is attached to it via flex anchors (which provide lateral restraint against wind loads while allowing free in-plane shrinkage movement) and gravity anchors (which carry the self-weight of the panel).

From a manufacturing perspective, the key point is that the frame is introduced into production while the GRC skin is still fresh and uncured. Flex anchors are attached to the back of the freshly sprayed GRC skin using bonding pads – small, hand-packed blocks of premix GRC pressed over the anchor bar and then compacted with a serrated roller. These bonding pads must be applied as quickly as possible after the final roller compaction of the skin, to ensure monolithic bonding and avoid subsequent de-bonding. Ghosting – the appearance of the bonding pad outline on the face of the panel – can occur if the anchor or pad is allowed to press into the fresh GRC skin.

Stud frame panels can achieve very large sizes – up to approximately 25 m² per unit – with the space between the GRC skin and the inner face of the frame typically filled with rockwool or rigid foam for thermal insulation and fire performance.

GRC Sandwich Panels

Sandwich construction places two GRC skins either side of a lightweight insulating core – expanded polystyrene, polyurethane foam, or light foamed concrete – with the GRC wrapping around the edges to completely encapsulate the core. The structural efficiency of the deep section is offset by the risk of differential thermal and moisture movement between the outer and inner faces causing bowing. The GRCA Design Guide recommends flat sandwich panels only, with a maximum area of 6.5 m².

When considering such a layered construction designers, specifiers and manufacturers need to consider fire rating of the entire assembly. If combustible insulation is used, such GRC sandwich panel may not be installed everywhere due to local codes and standards and increased risk of combustibility. 

GRC Curing: The Step That Determines Long-Term Performance

Curing is where much of the long-term performance of a GFRC panel is determined, and it is an area where shortcuts cause lasting damage. The requirements are specific:

For non-polymer grades: curing must maintain a controlled environment of approximately 20°C and 95% relative humidity for seven days. Freshly placed panels must be covered with polythene sheet immediately after initial set and stored on a level surface without movement. This regime ensures complete cement hydration and minimises early drying shrinkage.

For polymer-modified grades (the P grades, used for the majority of architectural production today): dry curing is possible and is the normal factory method. The acrylic polymer film formed within the matrix during the first hours retains internal moisture and allows hydration to continue without a humid curing room. However, panels must be loosely covered overnight and must not be exposed to drying winds or excessive heat for a minimum of two days. The ambient temperature during this period must stay between 5°C and 35°C – temperatures outside this range compromise the polymer film formation and can produce panels with inadequate long-term strength.

Demoulding must not occur until the GRC has gained sufficient strength to be handled without over-stressing. The required time is temperature-dependent; in cold conditions it can be significantly longer than the standard overnight schedule.

GRC Quality Control Testing: How Compliance Is Demonstrated

Knowing how GFRC panels are made is only part of the picture – demonstrating that they meet the specified grade requires a continuous programme of sampling and testing throughout production.

Test Boards

The standard approach is to manufacture a test board – a flat sheet of GRC, minimum 500 × 800 mm, made at the same time and in the same manner as the product it represents. Test coupons are cut from this board for mechanical testing. The GRCA Specification sets minimum frequencies:

• Spray process: at least one test board per day per mixer/pump; flexural testing at least twice per week per spray station, or every 10 tonnes produced, whichever comes first.
• Premix process: at least one test board per day per mixer; flexural testing at least once per week per mixer, or every 10 tonnes produced.

GRC Key Tests

Wash-out test (fibre content): Fresh GRC from the uncured test board is washed in water to separate the fibres, which are then weighed to determine the actual percentage fibre content of the mix. For the spray process, this is performed to calibrate the equipment, in accordance with GRCA Methods of Testing Part 1 or EN 1170-2.

Four-point bending test (LOP and MOR): Cured coupons are tested in four-point bending at 7 and/or 28 days, per GRCA Methods of Testing Part 3 or EN 1170-5. The LOP (the stress at first deviation from linear behaviour) and MOR (peak stress at failure) are both recorded. For Grade 18 material, minimum compliance values for the mean of four consecutive test board means are 8.0 MPa for LOP and 21.0 MPa for MOR; the minimum for any individual test board mean is 6.0 MPa and 15.0 MPa respectively.

Bulk density, water absorption, and apparent porosity: Tested per EN 1170-6 or GRCA Methods of Testing Part 2, at a minimum of once per month. Minimum bulk dry density is 1800 kg/m³ and minimum bulk wet density 2000 kg/m³ for all grades. Density is more than just a weight figure – a well-compacted, correctly proportioned panel will naturally achieve the theoretical density; a low result is a direct indicator of poor compaction or excess water in the mix.

Dimensional variation (shrinkage/moisture movement): Tested to EN 1170-7 when a new mix design is established. Total ultimate shrinkage for a standard 1:1 sand-cement ratio GRC mix is approximately 0.12%, with the irreversible component representing roughly a quarter to a third of that total. Allowance for moisture movement of 1.0–1.5 mm per metre of panel dimension must be built into the joint design and fixing system.

If any test board fails to meet compliance requirements, the GRCA Specification requires the producer to identify which GRC is “at risk” – defined as all material produced between the previous complying board and the next complying board – and to determine, in consultation with the structural engineer, whether the sub-standard material must be rejected or whether it remains adequate given the safety factors built into the structural design.

Storage, Handling, and Transport of GRC

Even a perfectly made GFRC panel can be damaged or rendered non-compliant before it reaches the site if it is stored or transported incorrectly. The GRCA Specification requires that GRC components are stored, handled, and transported so that no part of the component is over-stressed, bowing or twisting is not induced, no damage occurs (particularly at edges and corners), and no permanent staining or discolouration results from contact with storage materials. Large panels must have their method of handling, storage, and transport agreed with the structural engineer.

The manufacturer’s quality assurance system – whether certified to ISO 9001, the GRCA Full Member grade (which involves annual independent assessment), or an equivalent scheme – must document all of the above and demonstrate consistent compliance across the production programme.

Why the GRC Manufacturing Process Determines Everything

One theme runs through every aspect of how GFRC panels are made: the interdependence of material selection, production method, and structural performance. Spray and premix processes produce genuinely different materials with different mechanical properties and different structural capabilities, even when the same cement and fibres are used. The 2-D fibre orientation of sprayed GRC generates higher in-plane flexural strength but negligible transverse tensile resistance; the more 3-D distribution of premix fibres produces more isotropic but generally lower-strength material.

This means the design engineer’s choice of grade, the architect’s choice of panel form, and the manufacturer’s choice of production method must be made together, not sequentially. And it means that an GFRC panel is not merely a piece of precast concrete – it is a highly engineered composite product whose performance is defined as much by how it was made as by what it is made from.

Technical references: GRCA Specification for the Manufacture, Curing and Testing of GRC Products (February 2021 Rev.); GRCA Practical Design Guide for GRC, Version 1.1 (March 2018); GRCA Methods of Testing GRC Material (October 2017); Bartos, P.J.M., Glassfibre Reinforced Concrete: Principles, Production, Properties and Applications, Whittles Publishing (2017).GRCA-Methods-of-Testing-GRC.pdf+3