Fibre Reinforced Facade Encyclopedia: Composition
Fibre volume fraction is the proportion of a composite laminate, by volume, that is reinforcing fibre rather than cured resin or void. It is written Vf and expressed as a percentage or a decimal. It is the single number that best predicts how stiff and how strong a laminate will be, because fibre carries load and resin mostly transfers it, and it is also the number that tells you how much organic, combustible material a panel contains.
Two laminates of the same thickness can differ by a factor of two in strength. A hand laid chopped strand mat laminate sits near 18 percent fibre by volume; an infused non-crimp laminate reaches 50 percent or more.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Hand lay-up, chopped strand mat | 15 to 22% by volume | 25 to 35% by weight |
| Spray-up with a chopper gun | 12 to 20% by volume | Depends heavily on operator consistency |
| Hand lay-up, woven roving and mat | 25 to 32% by volume | 40 to 50% by weight |
| Vacuum infusion | 45 to 55% by volume | Void content typically below 2% |
| Resin transfer moulding | 45 to 55% by volume | Closed mould, repeatable |
| Prepreg and autoclave | 55 to 65% by volume | The practical ceiling for laminated composites |
| Pultrusion | 55 to 70% by volume | Unidirectional, constant section only |
| E glass density | 2,540 to 2,600 kg/m³ | Needed for the weight to volume conversion |
| Cured resin density | 1,100 to 1,250 kg/m³ | Polyester and vinyl ester; epoxy similar |
| Acceptable void content | below 2% structural, below 5% hand laid | Measured to ASTM D2734 |
Volume fraction is not weight fraction
Glass is roughly twice as dense as cured resin, so the two figures are never the same and confusing them produces serious errors. A laminate at 50 percent glass by weight contains only about 31 percent glass by volume. The conversion is straightforward: divide the weight fraction of each phase by its density, then take the fibre term as a proportion of the total. Test reports normally quote weight fraction, because that is what a burn-off measures, while design calculations use volume fraction, because that is what the mechanics of the laminate depend on.
Why it predicts strength
In the fibre direction the laminate modulus follows a rule of mixtures: the fibre modulus multiplied by the fibre volume fraction, plus the resin modulus multiplied by the remainder. Because E glass is about 73 GPa and cured resin is about 3.5 GPa, the fibre term dominates and the relationship is close to linear. Doubling Vf roughly doubles stiffness in that direction.
Real laminates are not all aligned with the load, so an efficiency factor is applied. Fibre running parallel to the load is fully effective. A balanced 0 and 90 degree fabric such as woven roving is about half effective in either direction. Random in-plane reinforcement such as chopped strand mat is about three eighths effective. This is why a mat laminate at 18 percent Vf and a woven laminate at 30 percent Vf differ by much more than the ratio of their fibre contents.
Strength follows the same logic but is more sensitive to defects. Voids, dry bundles and poor fibre to matrix bonding all reduce strength without changing the measured fibre content at all, which is why void content and interlaminar shear strength are measured alongside Vf rather than inferred from it.
How it is measured
The standard method for a glass reinforced laminate is ignition loss, to ASTM D2584 or ISO 1172. A cut sample is weighed, placed in a crucible and heated in a muffle furnace at about 565 to 600 °C until the resin and the fibre sizing have burned away completely, then cooled in a desiccator and reweighed. The residue is the glass. Fibre weight fraction is the residue mass divided by the original mass, and volume fraction follows from the two densities. The test needs only a furnace and a balance, which is why it is the normal production quality control check.
Two cautions. The sizing on the fibre is organic and burns off with the resin, so ignition loss slightly overstates the resin content; on a laminate with 0.5 percent sizing this is within the noise, but it is why the method is called ignition loss rather than resin content. And fillers such as aluminium trihydrate do not burn off, so a filled fire retarded laminate cannot be assessed by burn-off without correcting for the filler.
Carbon fibre burns, so carbon laminates use matrix digestion in acid to ASTM D3171 instead. For any laminate, void content is measured separately to ASTM D2734 by comparing the theoretical density calculated from the constituents with the density actually measured to ASTM D792.
What sets the achievable value
- Reinforcement format: random strands trap resin, ordered fabrics pack tightly, unidirectional packs tightest
- Process: open hand lay-up relies on roller pressure alone, vacuum consolidation applies up to one atmosphere, autoclave and pultrusion apply far more
- Resin viscosity: a thick resin will not compact through a dense fabric without vacuum or heat
- Operator consistency: in hand lay-up and spray-up the number varies panel to panel, which is why the design value is conservative
- Geometry: tight radii, corners and returns are resin rich almost by definition
This is the practical reason a specification for an architectural panel states a laminate schedule and a minimum fibre content by weight, verified by burn-off on production samples, rather than only a thickness. Thickness is easy to achieve with resin.
The fire consequence
Fibre volume fraction also controls how much combustible material is in the panel. Glass is inorganic and does not burn; the cured resin is the fuel. A laminate at 20 percent fibre by volume is roughly 80 percent organic by volume, while one at 50 percent is half that. Raising fibre content therefore improves heat release and smoke results measurably, and it is one of the levers used alongside resin choice and fillers to reach a classification.
It does not change the category. A fibre reinforced polymer laminate at any fibre content contains a cured thermoset and is not non-combustible. Compliance on an external wall comes from reaction to fire classification and full scale 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
- ASTM D2584 Standard Test Method for Ignition Loss of Cured Reinforced Resins
- ISO 1172 (textile glass reinforced plastics: determination of textile glass and mineral filler content)
- ASTM D3171 (constituent content of composite materials, including matrix digestion)
- ASTM D2734 (void content of reinforced plastics) and ASTM D792 (density by displacement)
- ASTM D3039, ASTM D790 and ASTM D2344 (tensile, flexural and interlaminar shear properties)
- AS 5637.1 and EN 13501-1 (reaction to fire classification of the finished laminate)
Related terms
Questions specifiers ask
What is a good fibre volume fraction for an architectural FRP panel?
For hand laid panels, 25 to 32 percent by volume with a woven roving and mat schedule is a realistic target. Infused panels reach 45 to 55 percent. Anything below about 15 percent is a resin rich laminate that will be heavy for its strength.
How is fibre content actually tested?
By burning a cut sample in a muffle furnace at about 565 to 600 °C to ASTM D2584 or ISO 1172. The resin burns away, the glass is weighed, and the weight fraction is converted to volume fraction using the densities of the glass and the cured resin.
Why does a data sheet quote fibre content by weight?
Because the burn-off test measures mass. Volume fraction has to be calculated from it. Design calculations use volume fraction, so always confirm which basis a quoted figure uses before comparing two laminates.
Does higher fibre content always mean a better panel?
Up to a point. Beyond about 65 percent by volume there is not enough resin to wet the fibre fully, voids rise and interlaminar strength falls. A high Vf with poor consolidation is worse than a moderate Vf that is fully wet out.
Can fibre volume fraction be measured on a filled fire retardant laminate?
Not directly. Mineral fillers such as aluminium trihydrate survive the furnace and are weighed as though they were glass. The filler loading has to be known and corrected for, or a different method used.
Assessing a laminate specification for a facade package? Our fibre reinforced polymer system is documented with published mechanical and fire data. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.