ΔTc and Continuous Grading

A performance grade such as PG 64-22 says only that a binder passes at 64°C and −22°C. The binder's actual limits might be 67.3°C and −25.8°C. Those exact temperatures are its continuous grade, found by testing at two or more temperatures around each limit and interpolating to the point where the binder just meets the criterion (AASHTO R 29, ASTM D7643). Continuous grades show how much margin a binder has, and they make it possible to compare binders that share a grade. They are also the basis of ΔTc, a parameter many agencies now use to screen for binders that age poorly (Federal Highway Administration, 2021¹).

Two Low-Temperature Criteria

At low temperature the Bending Beam Rheometer (BBR) measures two properties of PAV-aged binder at 60 seconds of loading:

• the creep stiffness, S, which must not exceed 300 MPa; and
• the m-value, the slope of the stiffness curve on a log scale, which must be at least 0.300.

A binder's low-temperature grade is set by whichever criterion it fails first as the temperature drops. Stiffness describes how much thermal stress the binder builds up as it cools; m-value describes how quickly it relaxes that stress. A binder that fails on m-value first is said to be m-controlled; one that fails on stiffness first is S-controlled (Federal Highway Administration, 2021¹).

Finding the Critical Temperatures

The BBR is run at two test temperatures 6°C apart that bracket each limit. The temperature at which each criterion is exactly met is found by interpolation: on a log scale for stiffness, and on a linear scale for m-value. BBR test temperatures are 10°C warmer than the grade temperatures they represent, so 10°C is subtracted to report the critical temperature on the grade scale:

The binder's continuous low grade is the warmer of the two critical temperatures.

ΔTc

ΔTc is the difference between the two critical temperatures:

A positive ΔTc means the binder is S-controlled; a negative one means it is m-controlled. The more negative ΔTc becomes, the more the binder's ability to relax stress lags behind its stiffness. Binders generally become more negative as they age, and aging-prone binders, some additives such as re-refined engine oil bottoms, and high contents of recycled binder from RAP and shingles push ΔTc further negative (Federal Highway Administration, 2021¹).

ΔTc has been linked to non-load cracking such as block cracking and to age-related embrittlement. The FHWA technical brief notes that only block cracking has been directly correlated with it, and cautions that polymer-modified binders can show a more negative ΔTc and still perform well (Federal Highway Administration, 2021¹).

Warning and Limit Values

Researchers have suggested a warning value of −2.5°C and a limit of −5.0°C, after the standard 20-hour PAV aging. Some agencies apply limits after 40 hours of PAV aging instead, to represent longer service. These values are research suggestions and agency choices, not part of M 320 or M 332 (Federal Highway Administration, 2021¹).

A Worked Example

This example uses made-up results for a PAV-aged binder tested at −12°C and −18°C:

Stiffness:

m-value:

So:

• the continuous low grade is −22.3°C, the warmer of the two, which meets a −22 grade;
• the binder is m-controlled; and
• ΔTc = −25.1 − (−22.3) = −2.8°C, past the −2.5°C warning value.

This binder passes PG xx-22, but its ΔTc would draw attention under a specification that uses the warning value.

Practical Points

• ΔTc needs BBR results at two temperatures that bracket both criteria. Routine acceptance testing at a single temperature doesn't give enough to calculate it.
• Results are reported to 0.1°C. Because ΔTc is a difference of two interpolated values, its repeatability is limited; small differences between binders are not meaningful.
• For blends with RAP or shingle binder, the binder is extracted and recovered first, which fully blends the old and new binders and may not represent what happens in the plant (Federal Highway Administration, 2021¹).

Standard Practices

• AASHTO R 29: Grading or verifying the performance grade of an asphalt binder
• ASTM D7643: Determining the continuous grading temperatures and continuous grades of PG binders
• AASHTO T 313: Bending beam rheometer
• AASHTO R 28: Pressure aging vessel

@misc{pavint-tc-and-continuous-grading,
  author = {{Pavement Interactive}},
  title = {ΔTc and Continuous Grading},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/binder-tests/tc-and-continuous-grading/}},
  note = {Accessed [date]}
}