🛣️ Calculators

Voids in the Mineral Aggregate (VMA) of an Asphalt Mix

Computes the voids in the mineral aggregate of a compacted asphalt mix, the volumetric parameter that decides whether a Marshall or Superpave mix design is approved. VMA is the space between aggregate grains inside the compacted specimen, that is, the air plus the effective binder: VMA = 100 − (bulk specific gravity of the mix × percentage of aggregate) ÷ bulk specific gravity of the aggregate. The number drives pavement durability: a VMA that is too low leaves no room for a thick binder film and the mix ages and cracks, while a VMA that is too high leaves the mix unstable and prone to rutting — normative ranges sit around 13 to 15% depending on the nominal maximum aggregate size. The critical convention is which aggregate gravity goes in the denominator: here it is the bulk dry gravity (Gsb), as required by Asphalt Institute MS-2; using the effective gravity (Gse), which is larger and sits in the denominator, overestimates VMA by more than a percentage point and may wrongly approve a failing mix. Enter the bulk specific gravity of the compacted mix, the aggregate percentage by mass and the bulk dry specific gravity of the aggregate.

Bulk specific gravity of the compacted mix Gmb (g/cm³) Aggregate in the mix, by mass (%) Bulk dry specific gravity of the aggregate Gsb (g/cm³)

Result

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VMA: the gap between grains that holds the binder

An asphalt mix design passes on volumetrics, never on strength alone. Once specimens come out of the Marshall mould or the gyratory compactor, the lab measures bulk gravity, works out air voids and lands on VMA, the space between aggregate grains inside the compacted sample — air plus effective binder. That space is what allows a binder film thick enough for the pavement to last. A mix short on VMA ages fast, oxidises and cracks by its third year; a mix with too much of it bleeds, and rutting shows up even sooner.

The calculation reads VMA = 100 − (Gmb × Ps) ÷ Gsb. Gmb is the bulk specific gravity of the compacted specimen, Ps the percentage of aggregate in the total mass of the mix, Gsb the bulk dry specific gravity of the combined aggregate. With the on-screen defaults — Gmb 2.41, Ps 94.5% and Gsb 2.68 — VMA works out at 15.02%. Superpave minima track nominal maximum aggregate size: 13% at 19 mm, 14% at 12.5 mm, 15% at 9.5 mm, so the very same 15.02% has comfortable room in one gradation and scrapes through in another. The denominator is Gsb, the bulk dry value, as Asphalt Institute MS-2 requires: it stands for the real volume the grains occupy, surface pores included.

Swapping Gsb for Gse is the mistake that passes bad mixes. Effective gravity always runs higher — 2.72 against 2.68 in this example — and it lifts VMA from 15.02 to 16.27%, better than a full percentage point of room that was never there. Ps misleads as well: the field wants aggregate, never binder, so 5.5% asphalt goes in as 94.5. Type 5.5 and the tool answers 95.05% without complaint. VMA on its own approves nothing either; it travels with design air voids near 4% and with voids filled with asphalt between 65 and 75%, a band that tightens as traffic grows.

Frequently asked questions

Does the default 15.02% pass a mix design?

Depends on the nominal maximum aggregate size. Under Superpave criteria the minimum is 13% for a 19 mm NMAS, 14% for 12.5 mm and 15% for 9.5 mm. The 15.02% on screen clears a 19 mm gradation easily, sits well inside a 12.5 mm one, and stops two hundredths above the limit on a 9.5 mm mix — a margin any laboratory scatter will swallow. Keep the practical ceiling in mind too, roughly two points above the minimum, beyond which the mix runs short of stone skeleton and turns unstable under heavy traffic.

Where does binder content enter this calculation?

Indirectly, through the aggregate field. Ps is the percentage of aggregate in the total mass of the mix, so binder content is simply the complement: the default 94.5% means 5.5% asphalt by total mass, a typical figure for dense-graded asphalt concrete. If your test report quotes binder on the mass of aggregate instead of on total mass, convert before typing — 5.8% on aggregate becomes 5.48% on the mix, and Ps lands at 94.52. Small as it looks, that shift moves the second decimal of VMA.

Can I use effective gravity Gse instead of Gsb?

No. Gse describes grain volume net of the pores the binder soaked into, so it always exceeds Gsb, and a bigger denominator hands back a bigger VMA. In the on-screen example, replacing 2.68 with 2.72 moves the result from 15.02% to 16.27%: over a full point of invented room, enough to pass a mix that ought to fail. MS-2 and the design specifications call for Gsb precisely because VMA has to count the volume that absorbed binder occupies inside aggregate pores — that asphalt plays no part in the effective film.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.