Superpave Gyratory Compactor: Settings, Gyrations, Results

A superpave gyratory compactor, usually shortened to SGC, is the lab machine that squeezes and tilts a sample of hot mix asphalt inside a steel cylinder until it reaches the density a road would see after years of traffic. Almost every Superpave mix design, and most plant quality control testing, starts with specimens made in one. If you have ever read a mix design sheet that lists air voids, VMA and a number like N_design = 75, those numbers came from a gyratory compactor. This guide explains what the machine does, the settings it runs at, how gyration levels are chosen, how to read the height data it records, and how its results connect to the tons of asphalt you order.

The short answer

What it is: a lab compactor that holds loose asphalt mix in a 150 mm steel mold, pushes down on it with a constant vertical pressure and wobbles the mold at a small angle, one gyration at a time.
Standard settings: AASHTO T 312 calls for 600 kPa of vertical pressure, an average internal angle of 1.16 degrees and 30 gyrations per minute.
How long it runs: until it reaches a set number of gyrations. The design level, N_design, depends on expected traffic and ranges from 50 to 125 gyrations in the AASHTO R 35 table.
What it produces: a compacted puck that is weighed and measured to find bulk specific gravity, air voids, VMA and VFA.
Why it matters to you: those volumetrics decide the binder content of the mix, and the compacted density feeds directly into how many tons it takes to pave a given area.

What the gyratory compactor actually does

Rollers on a paving job do not just push straight down. The mix shifts and kneads under the drum as it passes, and traffic keeps working it for years afterward. The gyratory compactor was developed under the Strategic Highway Research Program to copy that kneading action in a lab, so the specimens it makes have an aggregate structure closer to a real pavement than older impact methods produce.

Inside the machine, a ram presses down on the top of the sample while the base of the mold is tipped slightly off vertical. The mold then travels around in a circle, so the tilt keeps changing direction. Each full circle counts as one gyration. Every gyration packs the stone a little tighter, and the machine records how much the specimen height drops as it goes.

The method sits inside the broader Superpave system, which ties together binder grading, aggregate requirements and volumetric mix design. The SGC is the piece that turns a blend of stone and binder into a specimen you can test.

Superpave gyratory compactor settings

AASHTO T 312 is the test method most agencies use. Its core machine settings are short and fixed:

Setting	AASHTO T 312 value	What it controls
Vertical pressure	600 ± 18 kPa (about 87 psi)	The constant squeeze on the sample; works out to roughly 10,600 N on a 150 mm specimen
Internal angle	1.16 ± 0.02 degrees (20.2 mrad)	How far the mold tips; measured with the procedure in AASHTO T 344
Gyration rate	30.0 ± 0.5 per minute	Speed of compaction
Mold inside diameter	149.90 to 150.00 mm new, no more than 150.2 mm in service	Specimen size; molds must be at least 250 mm long
Height reading	To the nearest 0.1 mm, once per gyration	The density curve used for N_initial and N_maximum checks

The angle is the setting people argue about most. Older machines were calibrated to an external angle, and two compactors set to what looked like the same tilt could pack mix differently. NCHRP Report 573 found the gyrations needed to match field density differed by about 20 between two machines in its study, a gap the researchers traced to differences in the dynamic internal angle. Their recommendation was to set every compactor to an internal angle of 1.16 degrees, which is what the current method requires.

How many gyrations: N_initial, N_design and N_maximum

Superpave mix design looks at density at three points on the compaction curve:

N_initial is a low gyration count. A mix that is already very dense this early tends to be tender during construction, so specifications put a ceiling on density at this point.
N_design is the main level. The design binder content is chosen so the specimens hit the target air voids, generally 4.0 percent, at this gyration count.
N_maximum is a high gyration count that stands in for heavy future traffic. Density must stay under a limit here so the mix does not over-compact in service. NCHRP 573 notes that a pavement that ends up above about 98 percent of maximum density tends to bleed or rut.

The design level comes from the expected 20-year traffic in equivalent single axle loads (ESALs). The AASHTO R 35 table, as reproduced in FHWA guidance, looks like this:

20-year design traffic (million ESALs)	N_design (gyrations)
Under 0.3	50
0.3 to under 3	75
3 to under 30	100
30 or more	125

Many state agencies adjust these levels for their own materials and climate, so the specification for your project is the number that counts. Lower gyration levels generally push the design toward a little more binder, which is one reason some agencies have lowered N_design for low-volume roads and driveway-type mixes.

How a gyratory specimen is made, step by step

This outline helps you follow a lab report. The written test method and the agency specification govern real testing.

Batch or sample the mix. For a design, the lab blends aggregate and binder at several trial binder contents. For quality control, it takes loose mix from the plant or the truck.
Condition and heat it. Lab-mixed samples are aged in an oven for a set time to mimic plant mixing, then brought to the compaction temperature for that binder grade.
Heat the mold and plates so the mix does not chill against cold steel.
Load the mold in one lift with a paper disk below and above the mix. The method warns against letting the mix segregate as it goes in.
Apply pressure and angle, then gyrate. The machine runs until it reaches the target gyration count and stops on its own.
Extrude and cool the specimen, then let it reach room temperature before testing.
Measure density. The lab finds the specimen’s bulk specific gravity (G_mb) and compares it with the loose mix’s theoretical maximum specific gravity (G_mm).

From those two numbers come the volumetric results: air voids, voids in mineral aggregate, voids filled with asphalt and the dust to binder ratio. Minimum VMA depends on the nominal maximum aggregate size of the blend.

Reading the height data

Because the compactor logs height every gyration, one specimen gives a whole density curve rather than a single number. The lab only measures G_mb on the finished puck, then back-calculates density at earlier gyrations from the height ratio, since the mass and the mold diameter do not change:

%G_mm at gyration N = 100 × G_mb (final) × h_final ÷ (G_mm × h_N)

Here h_final is the specimen height at the end of compaction and h_N is the height recorded at gyration N. Many labs also apply a correction factor based on the measured density, which this simplified form leaves out.

Worked example (made-up numbers for illustration): a specimen compacted to 75 gyrations ends at 115.0 mm tall and has a measured G_mb of 2.395. The mix’s G_mm is 2.495.

Density at N_design: 100 × 2.395 ÷ 2.495 = 96.0 percent of G_mm, so air voids are 4.0 percent.
At the first few gyrations the recorded height was 126.5 mm. Density there: 100 × 2.395 × 115.0 ÷ (2.495 × 126.5) = about 87.3 percent of G_mm.

The lab compares each of those results with the limits in the project specification. A steep curve means the mix compacts easily; a flat one means it fights the roller.

From gyratory density to asphalt tons

The SGC result is a lab density, but it is also a sanity check on the unit weight you use for ordering. Specific gravity times the weight of water (62.4 pounds per cubic foot) gives a density in pounds per cubic foot.

Continuing the made-up example: a G_mm of 2.495 means voidless mix would weigh about 155.7 pcf. A mat compacted in the field to 93 percent of G_mm would weigh about 0.93 × 155.7 = 144.8 pcf. One square yard at one inch thick is 0.75 cubic foot, so that mat weighs about 109 pounds per square yard per inch. A 2-inch lift over 1,000 square yards would then need about 1,000 × 2 × 109 ÷ 2,000 = 109 tons, before waste.

That is why the mix design density and the plant’s reported unit weight are worth a look before you trust a rule-of-thumb number. See our guide to asphalt density in pounds per cubic foot, and plug your own figures into the asphalt tonnage calculator or the driveway asphalt calculator. The asphalt compaction factor post explains why loose depth is greater than compacted depth.

Gyratory compactor vs Marshall hammer

Before Superpave, most mix designs used the Marshall method, which compacts a smaller specimen by dropping a hammer on it a set number of times per side. The two approaches differ in a few practical ways:

	Superpave gyratory compactor	Marshall hammer
Compaction action	Constant pressure plus a tilting, kneading motion	Repeated impact
Usual specimen diameter	150 mm	4 inches (about 102 mm)
Effort set by	Number of gyrations tied to design traffic	Number of blows per face
Data during compaction	Height every gyration, so a full density curve	Final density only

Some agencies and airports still use Marshall designs, and a few labs run both, so check which method your specification names.

Common reasons gyratory results drift

Angle out of calibration. A small change in internal angle changes how hard the mix is worked. Labs verify it on a schedule.
Temperature. Mix that cools before compaction ends up less dense at the same gyration count.
Specimen mass. Too much or too little mix changes the final height and the back-calculated densities.
Segregation in the mold. Coarse pockets give misleading air voids.
Different machines. Comparing results from two compactors is only fair when both are calibrated to the same internal angle.

In the field, density is checked a different way, usually with cores or a gauge. Our nuclear density test guide covers that side. For pricing a job once you know the tons, the asphalt cost per ton page is a good next step.

The bottom line

The superpave gyratory compactor makes lab specimens by pressing loose asphalt mix at 600 kPa while tilting the mold 1.16 degrees and gyrating it 30 times a minute until it reaches a traffic-based gyration count. The densities it produces set the design binder content and air voids, and they give you a solid basis for the unit weight behind your tonnage estimate. Use the project specification for the exact gyration levels and limits, and treat the numbers in this guide as a way to understand the report, not to replace it.

Questions people ask

What is a superpave gyratory compactor used for?

It compacts loose hot mix asphalt into lab specimens that mimic the density a pavement reaches under traffic. The specimens are used to find air voids, VMA and the design binder content in Superpave mix design and to check plant mix during production.

What are the settings for a superpave gyratory compactor?

AASHTO T 312 calls for a vertical pressure of 600 kPa, an average internal angle of 1.16 degrees and a rate of 30 gyrations per minute, using a mold with an inside diameter of about 150 mm.

How many gyrations are used in Superpave mix design?

The design level depends on 20-year traffic. In the AASHTO R 35 table it is 50 gyrations for under 0.3 million ESALs, 75 for 0.3 to under 3 million, 100 for 3 to under 30 million and 125 for 30 million or more. Many states adjust these values.

What air voids should a gyratory specimen have at Ndesign?

Superpave designs generally target 4.0 percent air voids, which is 96 percent of theoretical maximum density, at the design gyration level. Your project specification sets the exact target and tolerance.

What is the difference between a gyratory compactor and a Marshall hammer?

A gyratory compactor squeezes the mix under constant pressure while tilting and rotating the mold, and records height every gyration. A Marshall hammer compacts a smaller specimen by repeated drops of a weight and gives only the final density.

Why does the internal angle matter?

The tilt controls how hard each gyration works the mix. Research for NCHRP Report 573 found that differences in internal angle between machines changed the gyrations needed to reach the same density, which is why compactors are calibrated to 1.16 degrees.

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