Bulk Specific Gravity of Asphalt (Gmb): T 166 Test and Math

Bulk specific gravity of asphalt, shortened to Gmb, is how heavy a piece of compacted asphalt mix is compared with the same volume of water, with its air pockets still inside. It is the number a lab gets when it weighs a core cut from the road or a puck made in the gyratory compactor. On its own it looks like a dry three-decimal figure, but nearly every density and volume check on a paving job leans on it: air voids, percent compaction, VMA and the unit weight that turns square yards into tons. Below we cover what Gmb means, how the standard AASHTO T 166 test works, the math, and the mistakes that throw it off.

The short answer

What it is: the specific gravity of a compacted asphalt sample, counting the stone, the binder and the air voids sealed inside it. Labs report it to the nearest 0.001, for example 2.448.
How it is measured: weigh the sample dry, weigh it hanging under water, then weigh it again right after blotting the surface. Those three numbers give its volume and its Gmb.
Standard methods: AASHTO T 166 (saturated surface-dry specimens) and ASTM D2726. AASHTO T 275 (paraffin) and AASHTO T 331 (vacuum sealing) cover samples that soak up too much water.
Typical values: about 2.200 to 2.500, according to Pavement Interactive, depending on the aggregate, the binder content and how hard the sample was compacted.
Why it matters: divide Gmb by the Rice value (Gmm) and you get percent compaction; the gap between the two is the air voids.

Gmb, Gmm and Gsb: three gravities that get mixed up

Specific gravity is a ratio, so it has no units. A Gmb of 2.448 means the compacted sample weighs 2.448 times as much as water of the same volume. The trouble is that asphalt reports carry several of these ratios with similar names:

Gmb, bulk specific gravity of the mix: a compacted sample with its air voids. Measured on cores and lab specimens.
Gmm, theoretical maximum specific gravity: the same mix with every bit of air removed, measured on loose material in the Rice test. Our guide to theoretical maximum specific gravity explains that test.
Gsb, bulk specific gravity of the aggregate: the stone by itself, before any binder is added. It is used in the VMA calculation below.

Because the compacted sample takes up extra room for its air, Gmb is always lower than Gmm for the same mix. A report that shows a Gmb above the Gmm has an error in one of the two.

How the T 166 test is run

The steps below summarize Method A, the suspension method, from the WAQTC field operating procedure for AASHTO T 166 as printed in Washington State DOT’s Materials Manual (M 46-01.48, January 2026). It is an overview for readers, not a substitute for the procedure your agency uses.

Dry the sample if it needs it. Fresh lab specimens that never got wet can skip this. Field cores usually arrive damp, so they go in an oven at 52 ± 3 °C (126 ± 5 °F) overnight and are reweighed after each further 2 hours or more until the mass changes by no more than 0.05 percent.
Record the dry mass (A). Let the sample cool to 25 ± 5 °C (77 ± 9 °F) and weigh it to 0.1 g.
Set up the water bath. Fill it to the overflow with water at 25 ± 1 °C (77 ± 2 °F). The basket or holder hangs from the scale and must be fully under water without touching the tank. Zero the scale with it in place.
Weigh it under water (C). Lower the sample in, shake off clinging bubbles, lay it on its side in the holder and leave it there for 4 ± 1 minutes before recording the submerged weight.
Blot it and weigh it again (B). Lift the sample out, wipe the surface with a damp cloth towel within about 5 seconds and put it straight on the zeroed scale. That is the saturated surface-dry (SSD) mass. Water that drips out on the scale counts as part of the sample. The blot-and-weigh steps should take no more than 15 seconds altogether.

The same procedure also includes a volumeter method (Method B), which is not to be used on samples with more than 6 percent air voids, and a rapid method (Method C) that does the wet weighings first and then breaks the sample apart in a 110 °C oven to get its dry mass at the end. Pavement Interactive estimates that each test takes about 7 minutes, not counting drying or cleaning the sample.

The formula, with a worked example

With A as the dry mass, B as the SSD mass and C as the weight in water:

Gmb = A ÷ (B − C)
Water absorbed, percent by volume = (B − A) ÷ (B − C) × 100

B − C is the weight of water the sample pushes aside, which stands in for its volume. Here is an example with made-up readings for a lab-compacted puck:

A = 4,850.0 g, B = 4,861.5 g, C = 2,880.2 g, so B − C = 1,981.3 g.
Gmb = 4,850.0 ÷ 1,981.3 ≈ 2.448.
Absorption = 11.5 ÷ 1,981.3 × 100 ≈ 0.58 percent.

The absorption figure is the built-in quality check. If a sample takes on more than 2 percent water by volume, the SSD method is the wrong tool and the lab should switch to T 275 or T 331.

What Gmb is used for

Air voids and percent compaction

Pair the Gmb with the mix’s Gmm and two key numbers fall out:

Air voids (Va) = 100 × (Gmm − Gmb) ÷ Gmm
Percent compaction = Gmb ÷ Gmm × 100

Using our made-up puck and an assumed Gmm of 2.540, air voids come to 100 × 0.092 ÷ 2.540 ≈ 3.6 percent, which is the same as 96.4 percent of Gmm. On a road, the same math is run on cores to check the mat against the agency’s density spec. Our pages on air voids in asphalt and the nuclear density test cover how those field checks are judged; gauges are commonly checked against cores for this reason.

VMA in mix design

Voids in the mineral aggregate (VMA) is the room between the stone particles, filled partly with binder and partly with air. The usual formula is VMA = 100 − (Gmb × Ps ÷ Gsb), where Ps is the percent of stone by total mix weight. As an example with assumed numbers, a Gmb of 2.448, 94.5 percent stone and a Gsb of 2.680 give a VMA of about 13.7 percent. Pavement Interactive points out that a wrong Gmb throws off air voids, VMA and VFA together, which can make a sound mix look bad or a poor one look fine. See our Superpave guide for how those targets fit into design.

Unit weight and tons

Multiply Gmb by the unit weight of water at 77 °F, given in AASHTO procedures as 62.245 lb per cubic foot, and you have the density of the compacted mix. Here is an example for a finished mat:

A mat compacted to 93 percent of a 2.540 Gmm has a Gmb of about 2.362.
2.362 × 62.245 ≈ 147.0 lb per cubic foot.
A square yard one inch thick is 0.75 cubic feet, so that is about 110 lb per square yard per inch.

That lines up with the planning figure explained in our 110-pound rule post, and it shows why a heavier or lighter mix changes the order. Plug your own depth and area into the asphalt tonnage calculator, and see asphalt density in lbs per cubic foot for typical ranges.

Other ways to find Gmb

Paraffin coating (AASHTO T 275): seals the surface with wax so water cannot drain out. It is fussy to apply, and results can vary.
Vacuum sealing (AASHTO T 331): shrink-wraps the sample in a plastic bag inside a vacuum chamber, a method sold under the CoreLok name. It is suited to open or porous samples.
Dimensions: volume from measured height and diameter. It is simple but tends to overstate volume because it ignores the rough surface.
Gamma ray devices: read density from how radiation scatters through the sample, after calibration.

Mistakes that skew the result

Water draining from open voids. Pavement Interactive notes that dense-graded samples start to have connected voids at around 8 to 10 percent air. Water runs out as the sample leaves the bath, the SSD mass comes out light and the Gmb reads too high, making the mat look denser than it is.
Slow blotting or a dry towel. Either one pulls water out of surface pores. The towel should be damp, and the weighing should be quick.
Wrong water temperature. The math assumes a bath held at 25 °C (77 °F).
A running circulator. The procedure says to switch off any bath circulator before recording masses.
Damp or dirty cores. A field core weighed before it is truly dry, or with base material still stuck to its bottom, gives a false dry mass.

Questions people ask

What is bulk specific gravity of asphalt?

It is the specific gravity of a compacted asphalt sample, such as a road core or a lab puck, with its air voids included. Labs write it as Gmb, rounded to 0.001, and compare it with the Rice value (Gmm) to find air voids and percent compaction.

How do you calculate bulk specific gravity of asphalt?

Divide the dry mass by the difference between the saturated surface-dry mass and the weight under water: Gmb = A / (B - C). For example, made-up readings of 4,850.0 g, 4,861.5 g and 2,880.2 g give about 2.448.

What is a typical Gmb for asphalt?

Pavement Interactive gives a typical range of about 2.200 to 2.500. The exact value depends on the aggregate, the binder content and how much the sample was compacted.

What is the difference between Gmb and Gmm?

Gmb is measured on compacted mix with its air pockets inside. Gmm comes from the Rice test on uncompacted mix with the air pulled out by vacuum, so it is always the larger number. Percent air voids is the gap between the two as a share of Gmm: 100 x (Gmm - Gmb) / Gmm.

What test method is used for Gmb?

AASHTO T 166 and ASTM D2726 use the saturated surface-dry method. If a sample absorbs more than 2 percent water by volume, labs use AASHTO T 275 (paraffin) or AASHTO T 331 (vacuum sealing) instead.

Why would a Gmb result be too high?

A common cause is water draining out of connected voids as the sample is lifted from the bath. That makes the saturated surface-dry mass too low, so the calculated volume is too small and the Gmb reads high.

Related guides

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