GFRC Mix Design: Materials, Fibre Content and Why AR Glass Is Non-Negotiable
Direct answer
Glass fibre reinforced concrete is a thin-section cementitious composite made from cement, finely graded sand, alkali-resistant glass fibre, water, a high-range water reducer and — in most façade and architectural work — an acrylic polymer used for curing. The fibre must be alkali-resistant glass; standard E-glass loses strength in the alkaline cement matrix over time. The other governing choices are the production method (spray-up or premix), the sand grading, and the water-to-cement ratio, which should be kept as low as the mix can be worked at.
Spray-up versus premix changes the whole material list
Spray-up feeds continuous AR-glass roving into a chopper at the spray gun and deposits fibre and slurry simultaneously onto the mould face. Fibres stay long and lie in the plane of the panel, which gives the highest strength for a given fibre content. It needs a spray rig, a trained operator and roller compaction.
Premix blends pre-chopped AR-glass strand into the mortar in a high-shear mixer, then pours, sprays or casts it. Fibres are shorter and randomly oriented, so strength is lower at the same fibre content, but the process is far more accessible and suits intricate moulds, garden and architectural elements, and small shops.
Decide this first. A premix shop buying spray roving, or a spray shop buying chopped strand, has bought the wrong product regardless of quality.
The material list
| Component | Function | Selection notes |
|---|---|---|
| Portland cement | Binder | White cement for pigmented and light architectural finishes; grey for structural and back-of-house work. Cement colour consistency between lots is a real risk on visible façades |
| Fine silica sand | Aggregate | Washed, dried, graded fine sand. Oversize particles block spray equipment and mark thin sections |
| AR glass fibre | Reinforcement | Roving for spray-up, chopped strand for premix. Both must be alkali-resistant grades formulated for cement |
| High-range water reducer | Workability at low water content | Naphthalene sulfonate or polycarboxylate types; the two behave very differently with respect to slump retention |
| Acrylic polymer curing admixture | Internal curing, reduces the need for extended wet curing | Widely used in architectural GFRC; check solids content, not just dose |
| Pozzolan (silica fume, metakaolin) | Reduces matrix alkalinity and refines pore structure | Commonly specified to improve long-term durability of the fibre-matrix interface |
| Pigment | Colour | Iron oxide types are the standard for cementitious systems; dose is limited by strength and cost |
Where a mix will be exposed to freeze-thaw, staining or de-icing salts, a sealer or water repellent applied after cure belongs on the same purchasing list.
Why alkali-resistant glass, specifically
Hydrating Portland cement produces a strongly alkaline pore solution. Conventional E-glass is progressively attacked in that environment; a panel can test well at 28 days and lose a significant fraction of its flexural capacity over years. AR glass is formulated with zirconia to resist that attack, and is the reason GFRC exists as a durable material rather than a short-lived one.
Two consequences for buyers:
- Do not substitute E-glass chopped strand mat or roving into a cementitious mix because it is cheaper and physically similar. The failure is slow, invisible at handover, and structural.
- Ask for the zirconia content and confirm the product is sold for cementitious use. "Alkali-resistant" on a label without a specification behind it is not evidence.
Pozzolan additions such as silica fume or metakaolin reduce free calcium hydroxide and are commonly used together with AR glass to further protect the interface. They are a complement to AR glass, not a substitute for it.
Proportioning framework
Published GFRC practice — including the specifier's guidance issued by the international GRC associations — is written as a framework rather than a single recipe, because the correct proportions depend on the process and the panel. Work in this order:
- Set the cement-to-sand ratio. Spray-up mixes conventionally sit near equal parts by weight; premix mixes often carry slightly more cement to keep the mortar mixable with fibre in it.
- Set the water-to-cement ratio as low as the process allows. This is the single largest lever on strength and on long-term durability. The water reducer exists to let you keep it low, not to make an over-watered mix flow.
- Set fibre content by process. Spray-up carries a higher effective fibre content than premix because fibre orientation is favourable and fibres remain long. Premix fibre content is limited by mixability — beyond a certain point the mortar balls up and the fibres clump.
- Add polymer and pozzolan to the durability requirement, not to habit.
- Verify by testing, not by arithmetic. Cast test boards from the production mix on the production equipment and test flexural performance. A mix that works in one shop's mixer at one temperature is not automatically transferable.
Because every one of those steps is equipment- and material-specific, treat any single set of numbers you find online as a starting point for trials with your own materials, and record the proportions that worked as a controlled specification.
Curing
Thin sections have a very high surface-to-volume ratio and lose water fast. Under-cured GFRC is weak, dusty and prone to crazing. Either cure wet under cover for the period your specification requires, or use an acrylic polymer at the dose that permits reduced wet curing. Choosing polymer curing and then also skipping the demould protection is how shops end up with panels that craze in the first summer.
Defects and their material cause
| Symptom | Likely cause |
|---|---|
| Fibre clumping and "balling" in premix | Fibre content above mixable limit, wrong strand length, or fibre added before the mortar was fluid |
| Fibre visible on the finished face | Insufficient face-coat mist before the fibre-laden layer, or spraying too coarse |
| Crazing and surface cracking | Excess water, inadequate curing, or over-rich cement content |
| Panel strength lower than test boards | Compaction differences; hand-rolled areas versus mould corners |
| Long-term strength loss | Non alkali-resistant glass in a cementitious matrix |
| Colour variation between panels | Cement lot change, pigment dosing error, or inconsistent water content |
| Efflorescence on the face | Free lime migrating with moisture; often addressed with pozzolan content and post-cure sealing |
What to specify when you buy
- Production method: spray-up or premix, and the equipment you run.
- Fibre form and length: continuous roving for a chopper, or chopped strand and its cut length.
- Cement type and colour: white or grey, and whether inter-lot colour consistency is contractual.
- Sand grading you can accept, and whether the sand is dried.
- Water reducer chemistry and required slump retention time in your shop temperature.
- Whether the panel is external, whether it faces freeze-thaw or de-icing salt, and what surface treatment follows.
- Panel size, thickness and annual volume, which determine packaging, lead time and freight.