Fibre Reinforced Facade Encyclopedia: Composition
Glass fibre is a reinforcement made by drawing molten glass through a heated bushing into continuous filaments 9 to 24 microns across, then bundling, sizing and chopping them for use in composites. It is spelled glass fiber in North America. The filaments are strong in tension and stiff for their weight, and they do the structural work in both fibre reinforced polymer and glass fibre reinforced concrete; the resin or cement around them holds their alignment and transfers load between them.
A single glass filament is about a fifth the width of a human hair and carries a tensile stress of roughly 3,400 MPa, higher than most structural steels. It is also brittle, which is why the fibre is only useful once it is embedded in a matrix.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Filament diameter | 9 to 24 microns | 13 to 19 microns for most construction grades |
| Virgin filament tensile strength | 3,100 to 3,800 MPa | E glass; strand values are lower after handling |
| Elastic modulus | 72 to 76 GPa | About a third of steel at a quarter of the density |
| Density | 2,540 to 2,700 kg/m³ | E glass about 2,580; AR glass about 2,680 |
| Softening point | 800 to 860 °C | The fibre survives furnace test temperatures |
| Moisture absorption | Less than 0.1% by weight | The fibre itself does not rot or swell |
| Content in a hand laid FRP laminate | 25 to 45% by weight | Higher with infusion or woven reinforcement |
| Content in sprayed GRC | about 5% by weight | 3 to 3.5% in premix GRC |
| Thermal expansion | about 5 x 10⁻⁶ per °C | Lower than the resin or cement around it |
How glass fibre is made
Batch materials, principally silica sand with alumina, lime, boron and other oxides depending on the grade, are melted at about 1,400 °C and drawn through a platinum alloy bushing with several hundred to several thousand holes. The filaments are attenuated by a winder pulling at high speed, cooled with a water spray, coated with a sizing and gathered into a strand. Everything that matters about the finished fibre is set in those few metres: diameter is a function of bushing hole size and draw speed, and the sizing decides how the fibre will behave in the matrix.
Sizing is a thin film of film former, coupling agent and lubricant. The coupling agent, usually a silane, chemically bridges the glass surface and the resin or cement. A fibre with the wrong sizing for the matrix will not wet out, will not bond and will leave a laminate that looks correct and performs badly, so the sizing designation on the data sheet is not a detail to skip.
The grades used in construction
- E glass: the general purpose electrical grade, the default fibre for polymer composites and the fibre people mean by fibreglass
- ECR glass: boron free and more resistant to acid and to corrosion, used where chemical exposure or long wet service is expected
- AR glass: alkali resistant, with 16 percent or more zirconium dioxide, the only grade suitable for a Portland cement matrix
- S glass and R glass: higher strength and modulus magnesium aluminosilicate grades, used in aerospace and rarely in architecture on cost
- C glass: chemical grade, largely restricted to surface veils
The distinction that catches specifiers out is between E and AR. E glass is fine in a resin matrix and is destroyed by the alkalinity of cement; AR glass carries the zirconia that resists it. The alkali resistant glass fibre entry covers that chemistry and the test evidence in full.
Forms supplied
Continuous roving is the input to spray-up, filament winding and pultrusion, and to the chopper gun used in sprayed GRC. Chopped strand is pre-cut for premix mortars and for compounding. Chopped strand mat is a random-orientation felt held with a binder, the workhorse of hand lay-up. Woven roving, biaxial and triaxial stitched fabrics place fibre along known directions, which is how a laminate is given stiffness where the load is. Surface veil is a thin tissue that holds resin at the face and keeps fibre from printing through a gel coat. Milled fibre is a filler, not a reinforcement.
Fibre architecture, not just fibre quantity, sets laminate properties. A random mat laminate is close to isotropic in plane and modest in strength; the same fibre weight as unidirectional reinforcement is several times stiffer along the fibres and weak across them.
What the fibre contributes
In a polymer laminate the fibre carries tension and the resin carries shear, holds alignment and protects the fibre from abrasion and moisture. Strength and stiffness scale with fibre volume fraction and with how well the fibre is aligned to the load, so quality control focuses on glass content by burn-off and on laminate thickness rather than on resin alone.
In a cement matrix the fibre does something different. It does not raise the first crack stress much, which is the limit of proportionality; it takes over after the matrix cracks and gives the section a post-crack load path, which is what raises the modulus of rupture and lets a 10 to 20 mm skin be used as a panel at all.
Handling and health
Continuous filament glass fibre is classified by IARC in Group 3, not classifiable as to its carcinogenicity to humans. The practical hazards are mechanical: dust and fragments irritate skin, eyes and the upper airway, so gloves, eye protection and long sleeves are normal practice, along with local extraction at chopping and cutting stations.
Standards and references
- ISO 2078 Textile glass: yarns, designation
- ASTM D578 Standard Specification for Glass Fiber Strands
- ISO 1889 and ISO 1887 (linear density and combustible matter content of glass fibre)
- BS EN 15422 Precast concrete products: specification of glass fibres for reinforcement of mortars and concretes
- ASTM C1666 Standard Specification for Alkali Resistant (AR) Glass Fiber for GFRC
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- IARC Monographs Volume 81, man-made vitreous fibres (continuous glass filament, Group 3)
Related terms
Questions specifiers ask
Is glass fibre the same as fibreglass?
In everyday use the words are interchangeable, but fibreglass is also used for the finished composite and for glass wool insulation. Glass fibre means the reinforcement itself. The fibreglass entry sets out the three different things the trade term covers.
Which glass fibre goes into GRC?
Alkali resistant glass with at least 16 percent zirconia. E glass loses most of its strength in a cement matrix within a few years and must not be substituted.
How much glass fibre is in a laminate?
Typically 25 to 45 percent by weight for hand lay-up with chopped strand mat, and higher for vacuum infusion or woven reinforcement. Glass content is verified by burn-off testing of production samples.
Does glass fibre burn?
No. The glass softens somewhere near 800 to 860 °C and does not contribute fuel. In an FRP panel the combustible component is the resin, which is why reaction to fire is tested on the finished laminate rather than on either constituent.
Does glass fibre corrode?
The fibre does not rust. E glass is slowly attacked by strong alkali and by some acids, ECR glass improves acid resistance, and AR glass is the grade formulated for cement. Moisture at the fibre to matrix interface, not the glass itself, is usually what limits wet service life.
Specifying a glass fibre composite and need the fibre grade, content and test data on the record? ShapeShell-RC and ShapeShell-RT are batch tested for fibre content. Browse the materials knowledge base
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.