Performance-Graded Binder Specifications Today

The Superpave performance grading system described in Superpave Performance Grading was published in the 1990s as a provisional specification, AASHTO MP 1. It became the full specification AASHTO M 320, which is still in use. Since then a second specification, AASHTO M 332, has replaced the high-temperature part of the grading with a better test, and many state DOTs have adopted it. This article explains what changed and why.

M 320: The PG System Today

AASHTO M 320 keeps the structure of the original system. A binder is named by two pavement temperatures, as in PG 64-22: it must meet stiffness and other limits at a high temperature of 64°C and a low temperature of −22°C. The same tests are run whatever the grade; the grade sets the temperatures they are run at:

• the Dynamic Shear Rheometer (DSR) on unaged and RTFO-aged binder for rutting, and on PAV-aged binder for fatigue;
• the Bending Beam Rheometer (BBR), and in some cases the Direct Tension Tester, on PAV-aged binder for low-temperature cracking; and
• the Rotational Viscometer and Flash Point for handling and safety.

Grading a binder, or verifying that it meets a grade, follows AASHTO R 29 (formerly PP 6). R 29 also describes how to find a binder's continuous grade, the exact temperatures at which it just meets each limit, for example PG 66.7-24.1 (see ΔTc and Continuous Grading).

The Problem M 332 Addresses

In M 320 the high-temperature requirement on RTFO residue is a minimum value of G*/sin δ, measured by the DSR at very small, oscillating strains. That measurement captures stiffness but not the delayed elastic response that polymer modifiers provide. A polymer network is barely engaged at such small strains, so M 320 tends to rate modified binders as little better than their stiffness alone suggests (Federal Highway Administration, 2011¹).

Agencies made up for this with "PG-plus" tests such as elastic recovery, force ductility, and toughness and tenacity, which check that polymer is present. Those tests are recipe checks rather than performance measures, and they vary from state to state.

Full-scale testing at FHWA's Accelerated Loading Facility showed the problem clearly. Across a set of neat and modified binders, G*/sin δ explained very little of the measured rutting, while the nonrecoverable creep compliance from the new MSCR test explained most of it (Federal Highway Administration, 2011¹).

M 332: Grading With the MSCR Test

AASHTO M 332 keeps the M 320 low- and intermediate-temperature requirements but replaces the RTFO G/sin δ requirement with the Multiple Stress Creep Recovery (MSCR) test (AASHTO T 350). See Multiple Stress Creep Recovery Test* for how the test works.

The biggest change is how heavy traffic is handled. Under M 320, a designer "bumps" the grade for slow or heavy traffic: a PG 64 climate becomes a PG 70 or PG 76 binder, tested at temperatures the pavement may never reach. Under M 332 the test temperature stays at the climate's high temperature, and traffic is handled by tightening the limit on Jnr, the nonrecoverable creep compliance at 3.2 kPa. A letter after the high temperature names the traffic level (Federal Highway Administration, 2011¹):

So a binder for a PG 64 climate with heavy traffic is specified as PG 64H-22 rather than PG 70-22, and all of its high-temperature testing is done at 64°C.

Recovery and Polymer Modification

The MSCR test also reports the percent recovery, how much of the strain the binder springs back from after each load. A well-dispersed elastomeric polymer gives high recovery for a given Jnr. Agencies that want to be sure a binder is polymer modified can require a minimum recovery that depends on Jnr, in place of the old PG-plus tests. The FHWA brief gives one such schedule, rising from 30 percent recovery for Jnr between 1.01 and 2.0 kPa⁻¹ to 50 percent for Jnr between 0.125 and 0.25 kPa⁻¹ (Federal Highway Administration, 2011¹). M 332 itself does not require a recovery value; agencies that use one set it in their own specifications.

Intermediate Temperature in M 332

M 332 retains the PAV-residue DSR fatigue parameter, G*·sin δ, at the intermediate temperature. In the version reproduced in the FHWA brief the limit is 5,000 kPa for S grades and 6,000 kPa for H, V and E grades, on the reasoning that the stiffer, more heavily modified binders used for heavy traffic are placed in thicker, stiffer pavements (Federal Highway Administration, 2011¹).

Choosing Between M 320 and M 332

Most state DOTs now specify binders under M 332, or under M 320 with MSCR-based requirements in place of PG-plus tests; a smaller number still use M 320 grade bumping. Because the two systems name grades differently, a PG 70-22 under M 320 and a PG 64H-22 under M 332 are not the same specification, even if the same binder may meet both. Contract documents should state which specification applies.

Standard Specifications and Practices

• AASHTO M 320: Performance-graded asphalt binder
• AASHTO M 332: Performance-graded asphalt binder using the MSCR test
• AASHTO R 29: Grading or verifying the performance grade of an asphalt binder
• AASHTO T 350: Multiple stress creep recovery test of asphalt binder using a DSR

@misc{pavint-performance-graded-binder-specifications-today,
  author = {{Pavement Interactive}},
  title = {Performance-Graded Binder Specifications Today},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/materials/asphalt/performance-graded-binder-specifications-today/}},
  note = {Accessed [date]}
}