Voids Filled With Asphalt (VFA): Formula, Limits, Example
Voids filled with asphalt, or VFA, tells you how much of the open space between aggregate particles is filled by binder. It is one of three volume numbers a lab reports for every compacted asphalt specimen, alongside air voids and voids in the mineral aggregate. Unlike the other two it is not measured directly; it is calculated from them. This guide covers the formula, the Superpave limits, what a high or low value means on a mix design and a worked example.
Asphalt tonnage calculator →Where VFA fits among the volumetrics
Picture a compacted core. The stone takes up most of the volume. What is left over between the particles is the VMA. Part of that space is coated and filled with effective binder (binder that was not absorbed into the stone), and the rest is air. VFA is simply the filled fraction of the VMA, written as a percent.
- VMA: total void space in the aggregate skeleton, percent of mix volume.
- Va (air voids): the empty part of that space.
- VFA: the part of VMA filled with effective binder, as a share of VMA.
See voids in mineral aggregate and air voids in asphalt for how those two are found from specific gravities.
The VFA formula
VFA = 100 × (VMA − Va) ÷ VMA
Both inputs are in percent. Because Va is usually held near 4 percent at the design gyration count, VFA moves mostly with VMA: more VMA at the same air voids means a higher VFA and a thicker binder film.
Superpave VFA limits
AASHTO M 323 ties the allowed range to design traffic in 20-year ESALs:
| Design traffic (million ESALs) | VFA range, percent |
|---|---|
| --- | --- |
| Under 0.3 | 70 to 80 |
| 0.3 to under 3 | 65 to 78 |
| 3 and over | 65 to 75 |
The standard adds a few footnotes, for example tighter upper bounds on fine 9.5 mm mixes at high traffic and a lower floor for coarse 25 and 37.5 mm mixes on low-volume roads. Your agency's spec may tweak these values, so read the project special provisions before you design to a table.
Worked example (made-up numbers)
A 12.5 mm mix at Ndesign shows VMA of 14.6 percent and air voids of 4.1 percent.
- Filled space: 14.6 − 4.1 = 10.5 percent of the mix volume.
- VFA: 100 × 10.5 ÷ 14.6 = 71.9 percent.
- For a road designed for 8 million ESALs, the 65 to 75 percent window is met.
If the same mix were pushed to 16.0 percent VMA at 4.0 percent air voids, VFA would climb to 75.0 percent, right at the edge. That is a hint the extra space is being filled with binder that traffic could later squeeze to the surface.
What a VFA result is telling you
- High VFA: thick films and little room left for further densification under traffic. Rutting, flushing and bleeding become more likely, especially on slow or heavy lanes.
- Low VFA: thin films. The mix can be harder to compact and more prone to aging, raveling and moisture damage.
- Fixing it: VFA is controlled through VMA and binder content. Changing gradation, dust content or binder grade usually moves it more than any single adjustment.
Why VFA matters to buyers of mix
A homeowner ordering driveway asphalt will never see a VFA number, but it shapes how the mat behaves. Mixes with sensible VFA compact well and hold up; the tonnage you order still comes from area and thickness, which the asphalt tonnage calculator handles.
Questions people ask
What does VFA mean in asphalt?
VFA stands for voids filled with asphalt. It is the percentage of the voids in the mineral aggregate (VMA) that is occupied by effective binder rather than air.
What is the formula for VFA?
VFA = 100 × (VMA − Va) ÷ VMA, where VMA is voids in the mineral aggregate and Va is air voids, both in percent of the compacted mix volume.
What is a normal VFA range?
Superpave criteria in AASHTO M 323 call for roughly 65 to 75 percent on high-traffic projects, 65 to 78 percent for medium traffic and 70 to 80 percent for light traffic, with a few size-specific exceptions.
Is high VFA good or bad?
Too high means the mix carries more binder than its aggregate skeleton can hold, which raises the risk of rutting and bleeding. Too low points to a dry mix that may crack, ravel or let water in.
Is VFA the same as voids filled with bitumen?
Yes. Outside North America the binder is usually called bitumen, so the same number is reported as VFB. The math is identical.
Can VFA be checked in the field?
Yes. Plant-produced mix is compacted in the gyratory and run for bulk and maximum specific gravity, which gives Va and VMA, and VFA follows from those two numbers.
Related guides
- Voids in mineral aggregate (VMA)
- Air voids in asphalt
- Superpave mix design
- Theoretical maximum specific gravity
- Bulk specific gravity of asphalt
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