Performance Grade (PG) bitumen, developed under the Superpave (Superior Performing Asphalt Pavements) research program in the United States, is a method of classifying asphalt binders based directly on their real-world performance rather than empirical properties like penetration or viscosity alone. Instead of testing at fixed, arbitrary temperatures, the PG system evaluates binders under the actual temperature extremes and traffic conditions they'll face in service — making it a fundamentally more scientific, engineering-based approach.
How the PG System Works?
Every PG binder is tested using the same battery of tests, but the temperatures at which those tests must be passed are tied to the specific climate and traffic conditions of the project location. This is the core innovation of PG grading: physical properties are measured and directly linked to expected field performance through engineering principles, rather than relying on indirect, empirical correlations like the old penetration or viscosity systems.
Grade Naming: PG XX-YY
A PG grade is written as two numbers, such as PG 64-22:
- The first number (positive) is the high-temperature grade — the highest 7-day average pavement temperature the binder can resist without excessive rutting.
- The second number (negative) is the low-temperature grade — the lowest pavement temperature the binder can withstand without cracking.
| High Temperature of Performance | Low Temperature of Performance |
|---|---|
| PG 46 | -34, -40, -46 |
| PG 52 | -10, -16, -22, -28, -34, -40, -46 |
| PG 58 | -10, -16, -22, -28, -34, -40 |
| PG 64 | -10, -16, -22, -28, -34, -40 |
| PG 70 | -10, -16, -22, -28, -34, -40 |
Why It's More Reliable Than Older Systems?
Penetration and viscosity grading are limited because they test consistency at only one or two fixed temperatures, which doesn't capture how differently binders behave across the full range of conditions a pavement actually experiences. Two binders can show nearly identical penetration at 25°C yet perform very differently at high and low extremes — one might be graded PG 64-10 and the other PG 76-22, despite looking similar under older test methods.
Traffic-Based Adjustments
Beyond climate, the PG system also accounts for traffic loading. Under advancements like the Multiple Stress Creep Recovery (MSCR) test, PG grades can carry an additional letter suffix — S, H, V, or E — indicating standard, high, very high, or extremely high traffic loading, which further refines binder selection for high-stress locations like intersections or heavy freight corridors.
Standards
The PG system is governed primarily by:
- AASHTO M320 (Superpave PG binder specification)
- AASHTO M332 (MSCR-based PG specification)
- SHRP (Strategic Highway Research Program) — the original research initiative that developed Superpave
PG vs. Penetration Grade vs. Viscosity Grade
The table below highlights the key differences between Penetration Grade, Viscosity Grade, and Performance Grade bitumen.
| System | Basis | Strength |
|---|---|---|
| Penetration Grade | Needle penetration at 25°C | Simple, widely used globally |
| Viscosity Grade (VG) | Viscosity at 60°C and 135°C | Better temperature-range insight |
| Performance Grade (PG) | Engineering properties tied directly to field climate/traffic | Most accurate; performance-based, not empirical |
PG grading is generally considered the most advanced classification system, since it directly links laboratory test results to predicted pavement performance — rather than relying on a rough empirical correlation, as penetration and viscosity grading do.
