Air Voids in Asphalt: Formula, Targets and Why They Matter

Air voids in asphalt are the tiny pockets of air trapped between the binder-coated stones after the mix is rolled. They sound like a lab detail, but they decide a lot about how long a pavement lasts. Too many voids and water and oxygen work their way in, so the surface ages, ravels and cracks early. Too few and the mix has nowhere to go on hot days, so it ruts and bleeds. Two numbers come up again and again: mixes are usually designed at about 4 percent air voids in the lab, and freshly rolled pavement is expected to come in at roughly 8 percent voids or less. Here is what the numbers mean, the formula behind them, what agencies require and what it all does to your tonnage.

The short answer

What they are: the share of a compacted asphalt sample’s volume that is air, given as a percent.
Lab design target: Superpave mixes are designed to hit 96 percent of maximum density at the design compaction level, which is 4.0 percent air voids.
On the road after rolling: most state specs set a minimum of about 91 to 93 percent of maximum density, so in-place voids of about 7 to 9 percent at most, with 92 percent the most common floor.
Why it matters: FHWA reports that each 1 percent of added density is estimated to stretch pavement life by about 10 percent, conservatively.
Formula: air voids = 100 × (Gmm − Gmb) ÷ Gmm, using two specific gravities measured in the lab.

What “air voids” actually means

Picture a cube cut from a finished road. Most of it is stone, a smaller share is asphalt binder, and the rest is empty space the rollers could not squeeze out. That empty share, as a percent of the cube’s total volume, is the air void content, written Va. AASHTO M 323, the Superpave mix design standard, describes it as the combined volume of all the little air pockets sitting between binder-coated stones, stated as a share of the whole compacted sample’s volume.

Air voids and density are two sides of one number. If a core is 93 percent as dense as the mix could possibly be with zero air, it has 7 percent air voids. Paving specs usually talk in “percent density” or “percent of Gmm,” and lab reports usually talk in “percent air voids,” but they say the same thing.

The air voids formula, with an example

Labs work out air voids from two measurements of the same mix:

Gmm, theoretical maximum specific gravity: how dense the loose mix would be with every bit of air removed. It is measured by breaking up a sample and pulling a vacuum on it under water (often called the Rice test).
Gmb, bulk specific gravity: how dense a compacted sample really is, found by weighing a lab-compacted specimen or a field core in air and in water.

Then:

Air voids (%) = 100 × (Gmm − Gmb) ÷ Gmm, and percent density = 100 × Gmb ÷ Gmm, which always equals 100 minus the air voids.

A worked example with made-up but realistic values: say the plant mix has a Gmm of 2.500. A lab specimen compacted to the design level comes out at a Gmb of 2.400. That gives 100 × (2.500 − 2.400) ÷ 2.500 = 4.0 percent air voids, right on a typical design target. A core drilled from the new road the next day has a Gmb of 2.310. That is 92.4 percent density, or 7.6 percent air voids, which would pass a 92 percent minimum with little room to spare.

Design voids vs in-place voids

The 4 percent figure and the 7 or 8 percent figure are not in conflict. They describe different moments in a pavement’s life.

In the lab, a mix designer compacts specimens in a gyratory compactor to a set number of gyrations meant to stand in for years of traffic, then picks the binder content that leaves 4.0 percent air voids at that point. AASHTO M 323 lists 96.0 percent of Gmm as the required density at the design gyration count for every traffic level. Out on the job, rollers cannot reach that density on day one, so the mat starts with more air. Traffic then kneads it tighter over the first years of service. Specs try to make the starting point dense enough that water cannot easily get in, without asking the impossible from the paving crew.

VMA and VFA: the other two void numbers

Air voids rarely travel alone on a mix design sheet. Two related numbers describe the space between the stones:

VMA, voids in the mineral aggregate: all the space between the aggregate particles, which is filled partly with binder and partly with air. More VMA leaves room for a thicker binder film, which helps durability.
VFA, voids filled with asphalt: the share of that space taken up by binder. It is found as VFA = 100 × (VMA − Va) ÷ VMA. With 15 percent VMA and 4 percent air voids, VFA is 100 × 11 ÷ 15, or about 73 percent.

Minimum VMA depends on the nominal maximum aggregate size (NMAS) of the mix. AASHTO M 323 sets these minimums at the design air void level:

4.75 mm mix: 16.0 percent
9.5 mm mix: 15.0 percent
12.5 mm mix: 14.0 percent
19 mm mix: 13.0 percent
25 mm mix: 12.0 percent
37.5 mm mix: 11.0 percent

More is not always better. The standard warns that mixes running more than 2 points above the VMA minimum may flush and rut, so designers aim a little above the floor rather than far over it. If those mix names are new to you, our Superpave guide explains what labels like 9.5 mm and 19 mm mean on a quote.

What happens when voids are too high or too low

Too high (under-compacted): FHWA lists three ways low density shortens pavement life: ruts form as traffic squeezes the extra voids shut, cracking follows when air and water reach the binder and age it, and moisture damage sets in as water gets inside the mix. Research for the Strategic Highway Research Program described a “pessimum” range of roughly 8 to 10 percent voids where water can get into a mat but cannot drain freely, so traffic builds pore pressure inside it. Above that range the voids link together and the pavement becomes permeable. Finer mixes stay watertight at lower density than coarse ones; one study cited by FHWA put the minimum density for acceptable permeability at 94.0 percent for 19 mm mixes, 92.8 percent for 12.5 mm and 90.5 percent for 9.5 mm.

Too low (over-dense): when traffic packs a mix down until almost no air is left, the binder has no spare room on hot days. It pushes up to the surface as shiny, sticky spots (bleeding or flushing) and the mix starts to shove and rut. That is one reason specs set an upper density limit as well as a lower one, and why designers watch VMA and VFA instead of just adding more binder.

What agencies require on the road

An FHWA demonstration report summarized a 2017 survey of state DOTs and other agencies: 89 percent of responding agencies had a lower density limit between 91.0 and 93.0 percent of Gmm, and 57 percent used exactly 92.0 percent. Most also had an upper limit, usually 97.0 to 98.0 percent. The same review noted broad agreement in recent research that a finished mat should be above about 92 percent density, with 93 to 94 percent preferred.

The payoff is large for a small change. According to FHWA, a 1 percent gain in in-place density, which is the same as 1 percent fewer air voids, was estimated to improve fatigue performance by 8 to 44 percent and rutting resistance by 7 to 66 percent, and to extend service life by about 10 percent as a conservative figure. A Colorado calibration study in the same report found pavements built at 93 to 95 percent density reached their full expected life, while those at 90 to 92 percent reached only about 65 percent of it.

How air voids are measured in the field

Cores: the referee method. Small cylinders are drilled from the mat, their Gmb is measured, and the result is compared with the Gmm of mix sampled from the same lot. FHWA found that every agency in its review used cores for acceptance.
Density gauges: nuclear and non-nuclear gauges give quick readings that help crews adjust their rolling, then get checked against cores. See our guide to the nuclear density test.
Lab specimens: plant mix compacted in the gyratory compactor confirms the mix itself still hits its design voids.

For the full list of checks a paving job goes through, see asphalt testing.

How crews keep air voids down

Roll while the mix is hot. Binder stiffens as it cools, and rolling a cold mat does little. Our guide to asphalt compaction temperature covers the working window.
Match lift thickness to stone size. FHWA’s demonstration report calls a lift thickness of at least 3 times the NMAS for fine-graded mixes and at least 4 times for coarse-graded mixes desirable. A 12.5 mm coarse mix, for example, wants a lift of roughly 2 inches or more. See why two lifts beat one thick lift.
Use enough roller. Several states in the FHWA project added rollers to raise density; our page on asphalt roller weight covers sizes.
Bond the layers. FHWA notes that an even, adequate tack coat helps compaction as well as bonding.
Watch the joints. Longitudinal joints are usually the least dense part of a mat, so many specs test them separately.

What air voids mean for asphalt tonnage

Density is weight per volume, so the same thickness of asphalt weighs more when it is packed tighter. A compacted mix weighs its Gmb times 62.4 pounds per cubic foot (the weight of water). In our example, the lab specimen at Gmb 2.400 works out to about 150 pounds per cubic foot, while the road core at 2.310 works out to about 144. One square yard at 1 inch thick is 0.75 cubic feet, so that is roughly 112 pounds versus 108 pounds per square yard per inch, a difference of about 4 percent.

That gap is why the common planning figure of about 110 pounds per square yard per inch is only an average. If your supplier’s mix is denser or lighter, or the crew rolls it tighter, actual tons will drift. Our 110-pound rule page and how density changes asphalt coverage show how to adjust, and you can try different densities in the asphalt tonnage calculator.

Questions people ask

What percent air voids should asphalt have?

Mixes are usually designed at 4 percent air voids in the lab. After rolling, most specs want the pavement at 92 percent of maximum density or higher, which means about 8 percent air voids or less.

How do you calculate air voids in asphalt?

Subtract the bulk specific gravity of the compacted sample (Gmb) from the theoretical maximum specific gravity (Gmm), divide by Gmm and multiply by 100. A Gmm of 2.500 and a Gmb of 2.400 gives 4.0 percent.

What happens if air voids are too high?

Water and air get into the mix more easily, so the binder ages faster and the pavement is more prone to raveling, cracking, moisture damage and early rutting as traffic squeezes the voids shut.

What happens if air voids are too low?

With little air left, the binder has no room to expand in hot weather. It can bleed to the surface and the mix can shove or rut, which is why specs also cap density at the top end.

What is the difference between air voids and VMA?

VMA is all the space between the stones, filled by both binder and air. Air voids are only the part of that space left empty. VFA is the share of VMA that binder fills.

Does compaction change how many tons of asphalt I need?

A little. Denser asphalt weighs more per square yard at the same thickness. Going from about 144 to 150 pounds per cubic foot changes tonnage by roughly 4 percent, so ask your supplier for the compacted density of the mix.

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