Dynamic Shear Rheometer: DSR Test for Asphalt Binder

A dynamic shear rheometer (DSR) is the lab instrument that decides much of the PG grade printed on an asphalt binder bill of lading. It squeezes a thin disk of binder between two metal plates, twists one plate back and forth, and measures how stiff the binder is and how much of its response is springy versus gooey. Those two numbers are then checked against Superpave limits for rutting and fatigue cracking. This guide explains what the DSR measures, how AASHTO T 315 runs, what the pass/fail values are, and how to read a result, with a worked example.

The short answer

What it is: a rheometer that applies an oscillating shear to a small binder sample and reports the complex shear modulus (G*) and the phase angle (δ).
Standard: AASHTO T 315, used to grade binders under the Superpave performance grade (PG) specification, AASHTO M 320.
Speed: specification testing runs at 10 radians per second, about 1.59 cycles per second.
Plates: 25 mm plates with a 1 mm gap for high-temperature tests, and 8 mm plates with a 2 mm gap for the stiffer, intermediate-temperature test.
Limits: G*/sin δ of at least 1.00 kPa for unaged binder and 2.20 kPa after RTFO aging (rutting), and G*·sin δ of no more than 5,000 kPa after PAV aging (fatigue).

Why binder needs a shear test

Asphalt binder is viscoelastic. Load it and part of the deformation springs back like rubber, while part of it flows and stays put like thick syrup. How the binder splits between those two behaviors depends on temperature and on how fast the load comes and goes. Older grading systems leaned on single-point tests such as penetration or viscosity, which say little about that split. The DSR was borrowed from the plastics industry because it measures both parts at once, at temperatures a pavement actually sees in service.

What the DSR measures

The instrument reports two values for each test temperature:

Complex shear modulus, G*. Think of it as overall stiffness, or total resistance to being sheared over and over. It is the peak stress divided by the peak strain in a loading cycle. Higher G* means a stiffer binder.
Phase angle, δ. The delay between the peak stress and the peak strain, expressed as an angle. A perfectly elastic material responds instantly, δ = 0°. A purely viscous liquid lags fully, δ = 90°. Asphalt sits in between, and the closer δ is to 90°, the more fluid it acts.

Pavement Interactive notes that the 10 rad/s loading rate was chosen to represent the shearing from traffic at roughly 55 mph. It also lists typical phase angles of about 50° to 90°, and observes that polymer-modified binders usually show a higher G* and a lower δ than straight-run asphalt, meaning they are a bit stiffer and more elastic.

The two Superpave parameters

Rutting: G*/sin δ

Rutting happens early in a pavement’s life, when hot weather and heavy loads push the mix out of shape. To resist it, the binder should be stiff and should spring back rather than flow. Dividing G* by sin δ rewards both: the value rises as stiffness goes up and as the phase angle drops. Superpave sets a minimum on this parameter, checked twice:

On original (unaged) binder: at least 1.00 kPa.
On residue from the rolling thin-film oven (RTFO), which simulates aging during mixing and paving: at least 2.20 kPa.

Fatigue: G*·sin δ

Fatigue cracking shows up later, after years of oxidation have hardened the binder. Now the danger is a binder so stiff it cracks instead of flexing. Multiplying G* by sin δ gives a measure of the energy lost in each load cycle, and Superpave caps it: no more than 5,000 kPa on binder aged first in the RTFO and then in the pressure aging vessel (PAV), which simulates years in service. Pavement Interactive points out that the link between this parameter and field fatigue is weaker than the rutting link, which is one reason many agencies add other tests.

Binder condition	Parameter	Limit	Distress it guards against
Original (unaged)	G*/sin δ	≥ 1.00 kPa	Rutting
RTFO residue	G*/sin δ	≥ 2.20 kPa	Rutting
PAV residue	G*·sin δ	≤ 5,000 kPa	Fatigue cracking

How the AASHTO T 315 test runs

The steps below are an overview for understanding results, not a substitute for the standard.

Heat the sample until it pours. Cold binder stiffens over time through a reversible effect called steric hardening, which can inflate G* if it is not erased by heating.
Pick the plate size. Original binder and RTFO residue are tested at high temperatures (above about 46°C) as a disk 1 mm thick and 25 mm across. PAV residue is tested at intermediate temperatures (about 4°C to 40°C) as a disk 2 mm thick and 8 mm across, because a large thin sample would be too stiff to give a measurable phase angle.
Load and trim. The binder goes between preheated plates, the plates close to the test gap plus 0.05 mm, the excess is trimmed with a hot tool, and the plates close to the final gap so the edge bulges slightly. Trimming matters a great deal: the calculated G* depends on the fourth power of the specimen radius, so a ragged edge skews the result.
Hold temperature. The specimen is held at the test temperature, controlled to within 0.1°C, for at least 10 minutes before testing.
Oscillate and measure. The upper plate twists back and forth at 10 rad/s with a small amplitude chosen to keep the binder in its linear range. The software conditions the specimen for 10 cycles, records the next 10, and calculates G* and δ.
Work quickly. Because steric hardening continues in the instrument, testing at multiple temperatures should be finished within about four hours.

A full grading run usually takes one to two hours, depending on how many temperatures are needed. T 315 covers moduli from about 100 Pa to 10 MPa, which at 10 rad/s typically corresponds to roughly 6°C to 88°C, depending on the binder and its aging.

Worked example: grading the high end

The high-temperature part of a PG grade comes in 6°C steps (58, 64, 70 and so on). The lab tests at a step and keeps going up until the binder fails. Here is how that looks with made-up but realistic readings:

Test	G*	δ	Result	Pass?
Original, 64°C	1.45 kPa	86.0°	G*/sin δ = 1.45 ÷ 0.9976 = 1.45 kPa	Yes (≥ 1.00)
RTFO, 64°C	3.10 kPa	82.0°	G*/sin δ = 3.10 ÷ 0.9903 = 3.13 kPa	Yes (≥ 2.20)
Original, 70°C	0.68 kPa	87.5°	G*/sin δ = 0.68 kPa	No
RTFO, 70°C	1.42 kPa	84.5°	G*/sin δ = 1.43 kPa	No

The binder passes both rutting checks at 64°C and fails at 70°C, so its high-temperature grade is 64. If its PAV residue then tests at 25°C with G* = 4,200 kPa and δ = 48°, the fatigue parameter is 4,200 × 0.7431 = about 3,120 kPa, comfortably under the 5,000 kPa cap. The low-temperature number (the “-22” in PG 64-22) comes from a different instrument, the bending beam rheometer, so the DSR alone never sets the whole grade. Our guide to what PG 64-22 means explains how the two numbers fit together.

Reading a DSR report

Check the conditioning. A value labeled “original,” “RTFO” or “PAV” tells you which limit applies. Comparing an RTFO number with the 1.00 kPa limit is a common slip.
Look at the margin. A binder that barely clears 1.00 kPa at its grade temperature has little cushion for variability between shipments.
Watch the phase angle. Two binders can share a G* but differ in δ. The lower δ usually signals more elasticity, often from polymer modification.
Know the “true grade.” Many labs report the exact temperature where the binder would hit the limit, such as 66.8°C, which says more than the rounded grade.

Beyond the basic test

The same instrument runs other procedures. The multiple stress creep recovery test (AASHTO T 350) loads the binder in repeated creep-and-rest cycles and is used by many agencies to grade for traffic level and to judge polymer-modified binders. Researchers also run frequency and temperature sweeps on a DSR to build master curves. For the mix side of rutting, agencies often add a performance test such as the Hamburg wheel tracking test.

Why it matters on a paving job

Most driveway and parking-lot owners never see a DSR printout, but they live with its results. A binder graded too soft for the climate can rut under truck traffic in summer, and one that ages too stiff can crack early. When a supplier quotes a mix with a specific PG binder, the DSR is part of how that grade was verified. If you are estimating material for a project, our driveway asphalt calculator handles the tonnage, and the spec sheet tells you which binder grade goes into it.

Questions people ask

What does a dynamic shear rheometer measure?

It measures the complex shear modulus (G*), a measure of overall stiffness, and the phase angle (δ), which shows how much of the binder's response is elastic versus viscous. Both are measured while a thin binder disk is twisted back and forth between two plates.

What is the DSR test used for?

It is used to grade asphalt binders under the Superpave PG system (AASHTO M 320). The results help set the high-temperature grade and check fatigue resistance after aging.

What are the Superpave DSR limits?

G*/sin δ must be at least 1.00 kPa on unaged binder and 2.20 kPa on RTFO residue. G*·sin δ must be no more than 5,000 kPa on PAV residue.

What frequency is the DSR test run at?

Specification testing runs at 10 radians per second, about 1.59 Hz. That rate is meant to represent traffic loading at roughly 55 mph.

Why are there two plate sizes?

High-temperature tests on unaged and RTFO binder use 25 mm plates with a 1 mm gap. PAV residue is tested at lower, intermediate temperatures where it is much stiffer, so an 8 mm plate with a 2 mm gap is used to get a measurable phase angle.

Does the DSR set the low-temperature part of a PG grade?

No. The low-temperature number comes from the bending beam rheometer and, in some specs, the direct tension test. The DSR handles the high and intermediate temperatures.

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