Fibre Reinforced Facade Encyclopedia: Composition
E glass fibre is a low alkali calcium aluminoborosilicate glass drawn into continuous filaments, and it is the reinforcement in the overwhelming majority of fibre reinforced polymer laminates. The E originally stood for electrical, because the glass was developed for its insulating properties. It is inexpensive, strong in tension, dimensionally stable and chemically compatible with polyester, vinyl ester, epoxy and phenolic resins. It is not compatible with cement, and that distinction is the single most consequential thing on this page.
A single E glass filament is 9 to 24 microns in diameter, thinner than a human hair, and carries a tensile strength of about 3,400 MPa, higher than most structural steels.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Filament tensile strength | 3,100 to 3,800 MPa | Virgin filament; strand values are lower |
| Elastic modulus | 72 to 76 GPa | Similar to aluminium, about a third of steel |
| Density | 2,540 to 2,600 kg/m³ | Used to convert weight to volume fraction |
| Filament diameter | 9 to 24 microns | Coded by letter, for example K is 13 microns |
| Elongation at break | 4.5 to 4.9% | Higher than the resins it reinforces |
| Alkali content of the glass | less than 1% by weight as Na₂O | Low alkali IN the glass, not alkali resistant |
| Zirconia content | 0% | Alkali resistant glass carries 16% or more |
| Softening point | about 840 to 860 °C | Inorganic; the resin is the combustible part |
| Loss on ignition of sizing | 0.3 to 1.5% by weight | Sizing is organic and burns off in ASTM D2584 |
How it is made and why sizing matters
Molten glass is drawn through a platinum bushing with hundreds or thousands of holes, pulled to filament diameter, coated immediately with a sizing, and gathered into a strand. Strands are wound as continuous roving, chopped, woven or needled into the reinforcement formats used in the workshop: chopped strand mat, woven roving, non-crimp fabric, continuous filament mat and spray roving.
The sizing is not incidental. It protects the filaments from abrasion against each other, holds the strand together, and carries a coupling agent, normally a silane, that forms the chemical bridge between the glass surface and the cured resin. A fibre sized for polyester will underperform badly in epoxy, and an emulsion bound mat will not dissolve properly in an epoxy laminate. Sizing compatibility is a specification item, not a detail; mismatched sizing shows up as low interlaminar shear strength and premature delamination.
Grades beyond plain E glass
- E glass: the general purpose grade, used unless there is a reason not to
- ECR glass: boron free and with improved acid and corrosion resistance, used in chemical plant, piping and some infrastructure
- S glass and R glass: higher strength and modulus at substantially higher cost, used where stiffness governs
- C glass: chemical grade, used as surface veil rather than as structural reinforcement
- AR glass: a completely different formulation containing zirconia, for cement matrices only
In architecture the working choice is between E glass in a polymer laminate and alkali resistant glass in a cement matrix. The other grades appear rarely and for specific chemical or stiffness reasons.
The cement problem
Fresh Portland cement pore solution has a pH of 12.5 to 13.5. Silica dissolves in strong alkali, and E glass is essentially a silicate with no chemical defence against it. Placed in a cement matrix, E glass filaments are etched, notched and progressively dissolved. Published accelerated ageing work shows E glass losing the majority of its strength in cement within months, and the composite then behaves as unreinforced mortar: brittle, low in flexural capacity, liable to crack under wind load.
Alkali resistant glass fibre carries 16 percent or more zirconium dioxide, which forms a protective zirconium rich layer at the fibre surface and slows the attack to a rate that no longer matters over the design life of a panel. That 16 percent is the number the GRCA specification turns on, and it is covered on the zirconia content page.
Fibre content, and what it does not tell you
The proportion of glass in a laminate drives its stiffness and strength, and is measured by burning the resin away to ASTM D2584 or ISO 1172 and weighing what is left. Because glass is roughly twice the density of cured resin, a laminate at 50 percent glass by weight is only about 32 percent glass by volume; the conversion matters and is set out on the fibre volume fraction page.
A burn-off test confirms how much glass is present. It does not confirm which glass it is. Chemical analysis of the glass, by X-ray fluorescence or equivalent, is what distinguishes E glass from alkali resistant glass, and that is the test to call for when the fibre type is in question.
Fire behaviour
E glass is inorganic. It does not burn, and it softens at around 840 to 860 °C, above the temperature at which the resin around it has already decomposed. In a fibre reinforced polymer laminate the glass is the part that survives and the resin is the fuel, which is why raising the fibre volume fraction reduces the organic content and improves reaction to fire results.
This does not make the laminate non-combustible. A glass fibre reinforced polymer panel contains a cured thermoset and is classified by reaction to fire testing and, on external walls, by full scale facade testing. ShapeShell-RT holds an AS 5637 Group 1 classification from AWTA Product Testing and passed the full scale BS 8414 and AS 5113 test at BRE Global.
Standards and references
- ISO 2078 (textile glass: yarns, designation) and ISO 1889 (linear density)
- ASTM D578 Standard Specification for Glass Fiber Strands
- ASTM D2584 and ISO 1172 (ignition loss, used for fibre content)
- ASTM C1666 (specification for alkali resistant glass fibre, for contrast with E glass)
- BS EN 15422 (specification of glass fibres for reinforcement of mortars and concretes)
- AS 5637.1 and EN 13501-1 (reaction to fire classification of the finished laminate)
Related terms
Questions specifiers ask
Is E glass the same as fibreglass?
In everyday use, yes. When someone says fibreglass they almost always mean an E glass reinforced polyester laminate. E glass is the specific glass formulation; fibreglass is the common name for the composite.
Why is E glass called low alkali if it fails in cement?
The low alkali refers to the sodium and potassium content of the glass itself, which is below about 1 percent and is why the glass is a good electrical insulator. It says nothing about resisting alkali attack from outside, which is a different property requiring zirconia.
Can E glass be coated to make it suitable for concrete?
Coated E glass has been marketed for cement use and has consistently performed worse than alkali resistant glass in independent accelerated ageing. Coatings are damaged in mixing and spraying, and any breach gives the alkali a path to the filament.
What is the difference between E glass and ECR glass?
ECR is boron free and formulated for better acid and corrosion resistance in chemical service. It is still not suitable for a cement matrix; only alkali resistant glass with 16 percent or more zirconia is.
Does E glass make a laminate fire resistant?
It is inorganic and does not burn, and a higher glass fraction means less resin to burn, so it helps. It does not make the laminate non-combustible, because the cured resin remains an organic polymer.
Specifying a glass fibre composite and need to compare polymer and cement systems side by side? Both of our material systems are documented in full. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.