Polymer modified GRC

Fibre Reinforced Facade Encyclopedia: Composition

Polymer modified GRC is glass fibre reinforced concrete (GRC, GFRC) made with an acrylic polymer dispersion added to the mix, typically at 5 to 7 percent polymer solids by weight of cement. The polymer forms a continuous film through the matrix as the panel dries, holding water in for hydration and filling the pore structure. Its headline effect is that the panel no longer needs seven days of moist curing, which is why nearly all commercial production is polymer modified.

At a glance

What the polymer does

The dispersion is a suspension of submicron acrylic particles in water. As the panel loses water, those particles come together and coalesce into a continuous film through the capillary pore network of the cement matrix. Two things follow. The film obstructs the escape of the remaining water, so the cement keeps enough moisture to hydrate properly without an external moist cure. And the film occupies pore space that would otherwise be open, reducing permeability and water absorption.

There is a third effect that matters for durability. As explained on the cement matrix page, part of the long term change in glass fibre reinforced concrete is physical: calcium hydroxide crystallising into the spaces between filaments within a fibre bundle, stiffening the bundle and reducing the strain capacity of the composite. Polymer occupying those interstitial spaces limits that process and cushions the filaments against each other, so aged flexural strain capacity is better retained. This is the reason polymer modification appears in durability arguments and not just in production arguments.

The 5 to 7 percent rule

Dosage is quoted as polymer solids by weight of cement, and 5 to 7 percent is the accepted working range. Below about 5 percent the film is not continuous enough to guarantee internal water retention, and air curing becomes a risk rather than a method. Above about 7 percent the returns fall away while cost, air entrainment and organic content all keep rising.

The arithmetic catches people out. A dispersion supplied at 46 percent solids has to be dosed at roughly twice the target by liquid weight, so 6 percent solids means about 13 percent of the cement weight as supplied liquid. That liquid is mostly water and has to be counted in the water to cement ratio, or the mix ends up far wetter than intended. Getting this wrong is the most common polymer dosing error.

What is gained

• No moist cure: no cure shed, no seven day hold, less double handling, shorter lead time
• Lower water absorption and permeability, which improves freeze thaw resistance and reduces efflorescence
• Better retention of aged strain capacity and flexural performance
• Improved bond between the facing mix and the backing mix in a sprayed panel, and better adhesion of applied finishes
• Reduced drying shrinkage and less shrinkage cracking in thin sections
• Better green strength, so complex shapes survive demoulding more reliably

For a manufacturer the first item alone usually justifies the cost. For a specifier the durability items are the ones worth verifying.

The trade-offs

Polymer modification introduces organic material into a matrix whose principal advantage over fibre reinforced polymer is that it contains none. At 5 to 7 percent of cement weight the polymer is roughly 1 to 2 percent of the panel by weight, which is low, and polymer modified glass fibre reinforced concrete products are routinely tested and classified as non-combustible. ShapeShell-RC holds an AS/NZS 1530.1 result of Not Deemed Combustible from Ignis Labs. What cannot be done is to assume the result: combustibility is determined by testing the actual product at its actual composition, and a change of polymer dosage is a change of product.

The mechanical trade-offs are modest but real. Elastic modulus is slightly lower and creep slightly higher than an equivalent unmodified matrix, which matters for long span and cantilevered elements. Early strength develops more slowly. Air entrainment from the dispersion produces surface pinholing unless a defoamer is used and the mix is properly deaerated. Cure depends on temperature, since the polymer needs to be above its minimum film forming temperature to coalesce, so cold weather production needs heated curing space. And the polymer must be an alkali stable grade; polyvinyl acetate dispersions hydrolyse in cement and must not be used.

Finally, verification is harder. Loss on ignition, the usual check for fibre content, burns off the polymer as well as the fibre sizing, so the polymer content has to be accounted for when interpreting the result rather than ignored.

Specifying it

State the polymer type as an alkali stable acrylic or styrene acrylic dispersion, the dosage as percent solids by weight of cement, and the curing regime that dosage permits. Require batch records showing the dosage used, and require modulus of rupture and limit of proportionality testing on boards from production batches to the GRCA specification, since that is what confirms the mix is performing as designed. Where non-combustibility is required, require the AS 1530.1 report for the product at that composition and name the testing body.

Standards and references

• GRCA Specification for the Manufacture, Curing and Testing of GRC Products
• BS EN 1169 (factory production control) and BS EN 1170 parts 1 to 8 (test methods including MOR and LOP)
• AS 1478.1 Chemical admixtures for concrete, mortar and grout
• ASTM C1228 and ASTM C1229 (GFRC test methods, US) and PCI MNL-128
• AS 1530.1 (combustibility) and NCC 2022 Volume One C2D10 (check the edition adopted in your jurisdiction)
• ASTM C1585 or BS EN 1170-6 (water absorption of the cured composite)

Related terms

• Glass fibre reinforced concrete (GRC / GFRC)
• Curing
• Cement matrix
• Alkali resistant glass fibre (AR glass)
• Modulus of rupture (MOR)
• Limit of proportionality (LOP)
• Water absorption
• Sprayed GRC

Questions specifiers ask

Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.