Bitumen performance grading explained

What a performance grade describes

Penetration grading characterises a bitumen with a single needle penetration measurement at 25 °C. It is a useful consistency check and it has served the industry for a century, but it says nothing about how the binder behaves at the temperature a pavement actually reaches in high summer, and nothing about how it behaves after several years of oxidation in service.

Performance grading, introduced through the Strategic Highway Research Program and codified in AASHTO M320, takes a different approach. The binder is measured at both ends of its intended service range and after two ageing simulations that stand in for the life of the road. The specification therefore relates directly to the three failure modes engineers design against: rutting in hot weather, fatigue cracking under repeated loading, and thermal cracking in cold weather.

A designation such as PG 64-22 states that the binder is certified for pavements whose average seven-day maximum design temperature is 64 °C, measured 20 mm below the surface, and whose minimum surface design temperature is −22 °C. Neither figure is an air temperature. Pavement temperature is derived from air temperature records together with latitude and depth, which is why two sites recording the same air maximum can require different grades.

Selecting the grade for a project

The site’s air temperature record is converted to pavement design temperatures, and the grade is then selected at a stated reliability. At 50 per cent reliability the grade matches the average year. At 98 per cent it covers all but the most extreme year in a long record. Most highway authorities specify 98 per cent for trunk routes, and that choice alone commonly moves the requirement one grade above what average conditions would suggest.

Adjusting for traffic

AASHTO M323 requires the upper grade to be raised where traffic is slow or stationary, because rutting is driven by load duration as much as by temperature. The lower grade is never adjusted.

A common procurement gap. A tender that names a grade for a climate but does not state the traffic category is incomplete. A standing-traffic site specified at its unadjusted climate grade will rut regardless of how well the delivered binder meets its certificate.

The test sequence and what each stage simulates

A performance grade is not one test. The binder is measured in its original state, then after two ageing simulations representing different points in the pavement’s life.

Deriving the intermediate test temperatures

The two intermediate temperatures reported on a certificate are not chosen by the laboratory or the supplier. They follow arithmetically from the grade designation, which means any buyer can verify a certificate without reference to another document.

The bending beam offset exists because of time–temperature superposition: a sixty-second loading at ten degrees above the design temperature produces the same stiffness response the binder would show at the true design temperature over a two-hour period. Testing at the design temperature directly would take impractically long.

The eight standard grades

Intermediate temperatures below are derived from each grade using the formulas above, not selected by the supplier.

Relationship to penetration and viscosity grades

Buyers migrating from older specifications frequently ask for a conversion table. The correlations shown in the table above are widely used as a starting point for discussion, but they are not equivalences and should never be treated as substitutions.

The reason is straightforward. A binder certified to penetration grade 60/70 has been tested for consistency at 25 °C. It has not been tested for dynamic shear at 64 °C, for fatigue behaviour after pressure ageing, or for creep stiffness and m-value on the bending beam. It may satisfy those criteria; it may not. Nothing on its certificate establishes either way.

For tender writers. A specification reading “PG 64-22 or penetration 60/70” asks for either of two different certifications and will produce inconsistent offers. Choose one system and state it unambiguously.

Handling and storage temperatures

Drummed material should be reheated in a controlled oven or hot room rather than by direct flame. Localised overheating hardens the binder at the drum wall and can take a compliant parcel out of specification before it reaches the mixer.

Packing and shipment

Verifying a certificate of analysis

Five checks that take under a minute and catch most documentation problems before a cargo is accepted.

• The bending beam and fatigue dynamic shear temperatures match the arithmetic derivation for the stated grade.
• Creep stiffness is at or below 300 MPa and m-value at or above 0.300. A stiffness figure in the thousands indicates a transcription error carried over from the dynamic shear row.
• Both original and RTFOT dynamic shear results are quoted at the upper design temperature, not at a single convenient temperature for all grades.
• Method references are the AASHTO series (T48, T240, T313, T315, R28), not a mixture of ASTM penetration-grade methods.
• A batch or parcel reference is present and can be tied to the bill of lading.

Frequently asked questions