Fibre Reinforced Facade Encyclopedia: Manufacturing
Curing is the controlled management of moisture and temperature after casting, while a panel develops the strength, stiffness and dimensional stability it was designed for. Two unrelated chemistries share the one word on a composites factory floor. Cementitious curing is hydration: Portland cement needs water present to react, and a thin section loses that water fast. Thermoset resin cure is cross-linking: a liquid resin and its catalyst react into a solid and generate their own heat while doing it. Both are irreversible, and neither can be repaired after the fact.
A sprayed glass fibre reinforced concrete panel demoulded at 18 hours has roughly a quarter of the flexural strength it will carry at 28 days. Most of the difference is made in the week after it leaves the mould.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Cementitious moist cure | 7 days at 95% relative humidity or higher | GRCA requirement unless the mix is polymer modified |
| Demoulding age, cementitious | 16 to 24 hours | Typical for sprayed panels; strength at demould governs |
| Cure temperature, cementitious | 15 to 25 °C | Below about 10 °C hydration slows sharply; typical range |
| Polymer solids for air cure | 5 to 7% by weight of cement | Acrylic dispersion; industry typical range |
| Strength reference age, cementitious | 28 days | MOR and LOP are quoted at this age |
| Resin gel time | 10 to 45 minutes at 20 °C | Set by catalyst and accelerator dose; typical |
| Resin exotherm peak | 60 to 150 °C within the laminate | Rises with laminate thickness and resin mass; typical |
| Post-cure, thermoset | 2 to 16 hours at 50 to 80 °C | Raises degree of cure and heat distortion temperature |
| Shop conditions for resin work | 18 to 25 °C, relative humidity below 70% | Outside this the cure window moves |
| Alpha gypsum set time | 20 to 45 minutes | Interior gypsum composites; hydration without a long moist cure |
Two families under one word
A facade manufacturer running glass fibre reinforced concrete (GRC, GFRC), fibre reinforced polymer and glass fibre reinforced gypsum lines is running three different cure regimes in the same building. Confusing them is a common source of specification errors, because a clause written for concrete curing is meaningless applied to a resin laminate, and a resin post-cure schedule says nothing about a cement panel.
The distinction that matters for a specifier is simple. Cementitious materials need water kept in. Thermoset resins need heat managed and moisture kept out. Gypsum composites sit closest to the cementitious family but reach working strength in hours rather than days.
Cementitious cure: seven days of water
Cement hydration consumes water. A 10 to 20 mm skin has an enormous surface area relative to its mass, so if it is left in a dry factory it loses water to evaporation faster than the cement can use it. Hydration stalls, the surface dusts, shrinkage cracking appears, and the panel never reaches the modulus of rupture quoted in the batch test. The GRCA specification therefore calls for about seven days of moist cure after demoulding, held at high humidity and at a stable temperature.
In practice this is done by stacking demoulded panels in a curing chamber or a polythene tent, by covering them in wet hessian and plastic, or by fogging. What matters is that the surface stays continuously damp; wetting and drying cycles are worse than a steady damp condition because they drive early moisture movement in a panel that has no strength to resist it.
Polymer modified mixes and air cure
Adding an acrylic polymer dispersion at 5 to 7 percent solids by weight of cement changes the picture. The polymer forms a continuous film through the matrix as the water leaves, which both holds water in long enough for hydration to proceed and contributes its own strength. A polymer modified mix can therefore be air cured in the factory without the moist cure step, which removes a week of chamber time, removes the handling that goes with it, and lifts throughput.
The cost is a matrix that is no longer purely inorganic. That has a fire consequence. A non-combustibility claim belongs to the tested product at its tested formulation, not to the material category, so a polymer modified panel needs its own AS 1530.1 result rather than an inherited one. ShapeShell-RC is classified Not Deemed Combustible under AS/NZS 1530.1 by Ignis Labs; any variation to the mix is a separate question for the fire engineer.
Thermoset resin cure: gel, exotherm and post-cure
A thermoset resin laminate cures in three phases. Gel time is the working window between catalysing the resin and the point where it stops flowing, typically 10 to 45 minutes at 20 °C depending on catalyst and accelerator dose. Everything the laminator needs to do, wet out the reinforcement, consolidate the laminate and remove air, has to happen inside that window.
Exotherm follows. Cross-linking is an exothermic reaction, and the heat cannot escape from the middle of a thick laminate, so peak temperatures of 60 to 150 °C are normal inside the section. Too much exotherm in a section that is locally thick, at a rib, a boss or a fixing pad, causes resin shrinkage, print-through on the visible face, distortion of the panel and in bad cases cracking or scorching. The control is laminate schedule and lay-up sequence, not a curing chamber.
Post-cure is the final step: holding the demoulded laminate at 50 to 80 °C for a period set by the resin supplier to complete conversion. An under-cured laminate looks finished but has a lower heat distortion temperature, a softer surface, poorer chemical resistance and residual styrene. For an external facade panel that will see surface temperatures well above ambient on a dark colour, post-cure is not optional.
Why cure time drives programme
Cure time occupies mould and floor space, and both are finite. Seven days of moist cure on a cementitious line means panels are handled twice and stored wet, and it sets a floor under the time from casting to despatch. An air cured polymer modified mix removes that constraint. On the resin side, gel time sets how much laminate one team can lay in one shift, and post-cure oven capacity sets how many panels leave per day.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (cure regime, polymer modified mixes, test ages)
- BS EN 1169 Precast concrete products, General rules for factory production control of glass-fibre reinforced cement
- BS EN 1170-5 (bending strength) and BS EN 1170-8 (cyclic weathering), which define the specimen cure before testing
- AS 3600 Concrete structures, Section 17, curing and protection of concrete
- ASTM D2471 Standard Test Method for Gel Time and Peak Exothermic Temperature of Reacting Thermosetting Resins
- ISO 11357 and ASTM D3418, differential scanning calorimetry for degree of cure in thermosets
- Resin manufacturer technical data sheet for catalyst dose, gel time, exotherm and post-cure schedule
- AS/NZS 1530.1 combustibility, where a polymer modified mix is used and a non-combustibility claim is required
Related terms
Questions specifiers ask
How long does GRC need to be cured?
About seven days of moist cure after demoulding, at 95 percent relative humidity or better and a stable temperature, unless the mix is polymer modified and designed for air cure.
Why does a polymer modified mix not need a moist cure?
The acrylic polymer forms a film through the matrix that retains enough water for hydration to continue, so the panel reaches its design strength in normal factory air. The trade-off is an organic component in an otherwise inorganic matrix, which has to be considered separately for fire.
What is post-cure and can it be skipped?
Post-cure is holding a cured resin laminate at 50 to 80 °C to complete cross-linking. Skipping it leaves a panel with a lower heat distortion temperature, a softer surface and residual monomer, which matters on an external facade that reaches high surface temperatures.
What happens if a resin laminate exotherms too hard?
The heat cannot escape from thick sections, so the resin shrinks locally. The result is print-through on the visible face, distortion and, at worst, cracking. It is controlled by the laminate schedule rather than by cure conditions.
Does curing affect the finish?
Yes. Cementitious panels that dry unevenly craze or show patchy colour, and resin panels that exotherm unevenly show ribs and fixing pads through the gel coat. Both defects are set during cure and cannot be polished out later.
ShapeShell-RC is batch tested for modulus of rupture and limit of proportionality to the GRCA specification, with the cure regime recorded for every panel. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.