Theoretical Maximum Specific Gravity: Gmm and the Rice Test

Theoretical maximum specific gravity, written Gmm, tells you how heavy a given asphalt mix would be if every last bubble of air were squeezed out of it. No road ever gets there, and that is the point: Gmm is the ceiling that every density test is measured against. When a nuclear gauge reading or a core result says a mat is at “93 percent,” it means 93 percent of Gmm. Labs find it with a quick vacuum test on loose mix that most people call the Rice test. Below we explain what the number means, how the test is run, how to do the math and how it ties straight back to the tons on your ticket.

The short answer

What it is: the specific gravity of an asphalt mixture with zero air voids, meaning the combined stone and binder only. It is reported to three decimals, such as 2.481.
How it is measured: a loose, broken-up sample is put under water in a sealed container, a vacuum pulls out the trapped air, and the sample’s volume is found from the water it pushes aside.
Standard methods: AASHTO T 209 and ASTM D2041.
Typical values: about 2.400 to 2.700 for most hot mix, according to Pavement Interactive, depending on the stone and the binder content.
Why it matters: air voids, percent compaction and the maximum possible unit weight of the mix all come from it.

What Gmm means, in plain terms

Specific gravity compares how heavy something is with the same volume of water. A Gmm of 2.500 means a solid, air-free chunk of that mix would weigh two and a half times as much as an equal volume of water.

Asphalt mix has a few specific gravities that are easy to mix up. The two that matter most on a paving job are:

Gmm, theoretical maximum specific gravity: the mix with no air at all. It is measured on loose material.
Gmb, bulk specific gravity: a compacted sample, air pockets included. It is measured on a lab-compacted puck or a core cut from the road.

Both use the same mass of material; only the volume differs, and the extra volume in the compacted sample is air. So Gmb can never be higher than Gmm. The specific gravities of the aggregate by itself sit above both, because binder is much lighter than rock. If you ever see a Gmb above the Gmm on a report, one of the two numbers is wrong.

Why it is called the Rice test

The method is named for James Rice, who developed it. You will see “Rice,” “Rice density,” “Rice value” and “max theoretical” used for the same idea on plant tickets and lab reports. The density version, called theoretical maximum density or TMD, is just Gmm multiplied by the unit weight of water.

How the Rice test is run

What follows is a summary based on the field operating procedure for AASHTO T 209-25 written by WAQTC, the Western Alliance for Quality Transportation Construction, and printed in Washington State DOT’s current materials manual. It is not a full procedure, and your agency’s version controls the details.

Get a big enough sample. Minimum dry mass depends on the mix’s nominal maximum aggregate size: 1,500 g for 12.5 mm (1/2 in.) and smaller, 2,500 g for 19 to 25 mm (3/4 to 1 in.), and 4,000 g for 37.5 mm (1½ in.) and larger. A sample that will not fit can be run in two or more parts.
Break it apart. Separate the particles by hand so the fine portion is in pieces no bigger than about 1/4 inch, without crushing any stone. If the mix is too stiff, warm it in an oven, but not above the compaction temperature of the job mix formula.
Cool and weigh. Let the sample reach room temperature, place it in the standardized bowl or flask and record its dry mass. That mass is “A.”
Cover with water. Add water at about 77 °F (25 °C) until it sits roughly an inch above the sample.
Pull the vacuum. Seal the lid and hold the residual pressure at 4.0 ± 0.6 kPa (30 ± 5 mm Hg) for 15 ± 1 minutes. Shake the container hard every two minutes, or use a mechanical shaker, so the air bubbles break free.
Release slowly. Bring the pressure back to normal over 10 to 15 seconds.
Find the volume. In the bowl method, hang the bowl and sample in a 77 °F water bath for about 10 minutes and record the weight under water. In the flask or pycnometer method, top the container off with 77 °F water, cover it, dry the outside and weigh the whole thing, all within about 10 minutes of releasing the vacuum.

The bowl or flask has to be standardized ahead of time. For a bowl, the procedure takes three underwater weighings of the empty bowl and accepts them only if they fall within 0.3 g of each other. Pavement Interactive puts the hands-on time at about 45 minutes per test once samples are ready, and labs usually run two.

The formula, with an example

For the bowl method:

Gmm = A ÷ (A + B − C)

where A is the dry sample mass in air, B is the standardized weight of the empty bowl under water and C is the weight of the bowl plus sample under water. For the flask method the form is the same, Gmm = A ÷ (A + D − E), where D is the flask filled with water and E is the flask with the sample and water.

Here is a made-up example to show the math. Say a sample weighs 2,000.0 g dry. The empty bowl weighs 1,200.0 g under water, and the bowl with the sample weighs 2,394.0 g under water. Then A + B − C = 2,000.0 + 1,200.0 − 2,394.0 = 806.0, and Gmm = 2,000.0 ÷ 806.0 = 2.481.

To turn that into theoretical maximum density, multiply by the unit weight of water at 77 °F, which the procedure gives as 62.245 lb/ft³ (or 997.1 kg/m³). For our example, 2.481 × 62.245 ≈ 154.4 lb/ft³, or about 2,474 kg/m³. No pavement made from that mix can weigh more than that per cubic foot.

When a large sample is run in parts, each part gets its own Gmm and the results are combined as an average weighted by dry mass. If the parts differ by more than 0.013, the test is rerun.

The dry-back step for porous stone

Some aggregates are porous enough that the binder does not fully seal them. Under vacuum, water can creep into those pores, which makes the sample look smaller than it is and throws off the result. The procedure includes a supplemental check for this, often called the dry-back:

Drain the sample through a towel so no fines are lost, then spread it in a pan in front of a fan.
Weigh it every 15 minutes until it loses less than 0.5 g between weighings. That surface-dry mass is called A_SSD.
Recalculate with A_SSD in place of A in the bottom half of the formula and compare the two results.

Pavement Interactive notes that the extra step is generally not needed when the aggregate absorbs less than about 1.5 percent water. Agencies also tell labs to short-term condition plant mix in the oven when absorptive stone is used, so the binder has soaked in the way it would during hauling. Skipping that step can give unrealistic void numbers later.

What Gmm is used for

Air voids and density. Air voids (%) = 100 × (Gmm − Gmb) ÷ Gmm, and percent density = 100 × Gmb ÷ Gmm. Our guide to air voids in asphalt covers the targets agencies set.
Field compaction checks. A nuclear density gauge or a core gives an in-place density, which is then compared with the most recent Gmm from the plant to get percent compaction.
Mix design. In Superpave design, lab specimens are compacted and their Gmb is compared with Gmm to pick the binder content that hits the design void level.
How much binder the stone soaks up. With the binder content and binder specific gravity, Gmm lets a lab back-calculate the aggregate’s effective specific gravity, and from that the binder lost into the stone and the effective binder left to coat it.

Because the binder content and the stone change a little from day to day, Gmm is not a one-time number. Pavement Interactive advises checking it at regular intervals during production. It also warns about a trap: adding binder fills voids and can make density look better without more rolling, but a richer mix can be more likely to rut or shove.

How Gmm connects to tonnage

Gmm sets the most a cubic foot of your mix can weigh. The real weight of the finished mat is that maximum times the density you actually reach. Here is a made-up example using the 154.4 lb/ft³ from above:

At 93 percent of Gmm, the compacted mat weighs about 154.4 × 0.93 ≈ 143.6 lb/ft³.
A square yard of pavement an inch deep holds 0.75 ft³ of mix, so that comes to about 108 lb per square yard per inch.
A 500-square-yard lot paved 2 inches thick would then need about 500 × 2 × 108 ÷ 2,000 ≈ 54 tons, before any waste allowance.

That is why the familiar planning figure of roughly 110 lb per square yard per inch works for many mixes, and why it can be off for a heavy or light one. Our explainer on the 110-pound rule and the guide to how density changes coverage go further. If your supplier can give you the mix’s Gmm or design unit weight, use it, then run your numbers through the asphalt tonnage calculator. After the job, checking yield against tons shows whether the mat came out as planned.

Common mistakes that skew a Rice result

Lumps left in the sample. Clumps hold air the vacuum cannot reach, so the volume reads high and Gmm reads low.
Crushed stone. Breaking aggregate while separating the sample exposes new uncoated faces that can soak up water.
Wrong water temperature. The calculation assumes 77 °F water. A bath that drifts changes the weights.
Weak or uneven vacuum and lazy shaking. Both leave bubbles behind.
An out-of-date bowl or flask standardization. Every result depends on that number.

Questions people ask

What is theoretical maximum specific gravity?

It is the specific gravity of an asphalt mixture with no air voids at all, only the stone and the binder. Labs call it Gmm and report it to three decimals. It is the reference that air voids and percent compaction are calculated from.

Why is it called the Rice test?

The test is named for James Rice, who developed the vacuum method used to measure it. Rice, Rice density and max theoretical all refer to the same measurement.

What is a typical Gmm for asphalt?

Most hot mix falls between about 2.400 and 2.700, according to Pavement Interactive. Unusually light or heavy aggregate can push it outside that range.

How do you calculate theoretical maximum density?

Multiply Gmm by the unit weight of water at 77 degrees F, which T 209 procedures give as 62.245 lb per cubic foot, or 997.1 kg per cubic meter. A Gmm of 2.481 works out to about 154.4 lb per cubic foot.

What is the difference between Gmm and Gmb?

Gmm is measured on loose mix with all the air removed. Gmb is measured on a compacted sample or core with its air pockets still inside. Air voids equal 100 times (Gmm minus Gmb) divided by Gmm.

What standard covers the Rice test?

AASHTO T 209 and ASTM D2041 both cover theoretical maximum specific gravity and density of asphalt mixtures. Many state agencies publish field procedures based on T 209.

Related guides

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