Introduction
Bitumen and asphalt binders are classified using several different grading systems, each developed to describe particular characteristics of the material. Two of the most widely encountered systems are Penetration Grade and Performance Grade (PG).
Although both systems are used to specify asphalt binders, they answer fundamentally different questions.
A penetration grade primarily describes the consistency of a binder under a standardized test condition. Performance Grading, by contrast, is designed to relate binder properties to the temperature conditions and pavement stresses under which the material is expected to perform.
This distinction becomes particularly important when selecting binders for heavily trafficked highways, high-temperature environments, or pavements exposed to significant thermal cycling.
The penetration test remains an important traditional characterization method. ASTM D5 measures the penetration of a needle into a bituminous material under specified conditions, with higher penetration values indicating a softer consistency.
Modern Performance Grading systems take a broader approach by evaluating binder behavior at temperatures associated with common pavement distress mechanisms.
1. What Is Penetration Grade Bitumen?
Penetration grading is one of the traditional systems used to classify paving bitumen.
The classification is based primarily on the depth to which a standardized needle penetrates the binder under controlled conditions.
In the ASTM D5 penetration test, the penetration value represents the consistency of the bituminous material. A higher penetration value generally indicates a softer binder.
Penetration is commonly reported in units of 0.1 mm.
For example:
- 40/50 indicates a relatively hard penetration-grade binder.
- 60/70 represents a softer binder.
- 80/100 is softer still.
- 100/120 and higher grades represent progressively softer materials.
The exact grades available depend on the specification and market.
ASTM D946/D946M, for example, lists penetration grades including 40–50, 60–70, 85–100, 120–150 and 200–300. ASTM also notes that these penetration ranges are legacy paving classifications and that different penetration ranges are used in other parts of the world.
The basic principle
The fundamental relationship can be summarized simply:
Higher penetration number → softer binder
Lower penetration number → harder binder
However, this does not mean that a 60/70 binder will necessarily perform better or worse than an 80/100 binder under every pavement condition.
That is one of the major limitations of relying on penetration grade alone.
2. How Is the Penetration Test Performed?
The penetration test uses a standardized needle, load, temperature and testing duration.
The binder is conditioned to the specified test temperature, and a needle is allowed to penetrate the material under controlled conditions.
The penetration depth is then measured.
The resulting value provides a convenient way to compare the consistency of different asphalt binders.
For example, if one binder produces a penetration of approximately 65 and another produces approximately 45 under the same test conditions, the 65-penetration material is considered softer at the specified test condition.
The test is relatively simple, which is one reason penetration grading became widely adopted.
However, its simplicity is also part of its limitation.
A single penetration measurement does not fully describe how an asphalt binder behaves across the wide range of temperatures and loading conditions encountered in a pavement.
3. The Main Limitation of Penetration Grading
A pavement does not operate at one fixed temperature.
During its service life, an asphalt binder may experience:
- extremely high surface temperatures,
- moderate daytime temperatures,
- low winter temperatures,
- rapid temperature changes,
- repeated traffic loading,
- long-term aging,
- and millions of load cycles.
A penetration value measured under one standardized condition cannot directly describe all of these behaviors.
For example, two binders can have similar penetration values while exhibiting different rheological behavior at high and low temperatures.
This is particularly important because asphalt pavement distress is strongly influenced by temperature and loading.
At high temperatures, the binder must provide sufficient resistance to deformation.
At low temperatures, it must remain sufficiently flexible to reduce the risk of thermal cracking.
This need for a temperature- and performance-oriented classification contributed to the development of Performance Grading.
4. What Is Performance Grade (PG)?
Performance Grade, commonly abbreviated as PG, is a binder classification system designed to relate asphalt binder properties to pavement temperature conditions and performance requirements.
The PG designation uses temperature-based limits.
A typical designation may look like:
PG 64-22
The two numbers represent different temperature conditions:
- 64 relates to the high-temperature performance requirement.
- -22 relates to the low-temperature performance requirement.
In simplified terms, the designation identifies a binder intended to satisfy specified performance criteria over a particular temperature range.
ASTM’s Performance-Graded Asphalt Binder specification, D6373, describes PG classifications in relation to the calculated maximum and minimum pavement design temperatures.
This makes the system fundamentally different from a simple penetration classification.
5. Penetration Grade vs. Performance Grade
The difference can be summarized as follows:
| Characteristic | Penetration Grade | Performance Grade |
|---|---|---|
| Primary basis | Needle penetration | Rheological / performance properties |
| Typical reference | ASTM D5 / penetration specifications | AASHTO M 320 / ASTM D6373 |
| Main concept | Binder consistency | Binder performance |
| Temperature range | Primarily represented through standardized test conditions | Explicitly considers high and low pavement temperatures |
| Rheological behavior | Not directly characterized across temperatures | Directly evaluated through specified rheological tests |
| Typical designation | 40/50, 60/70, 80/100 | PG 64-22, PG 70-10, etc. |
| Climate relationship | Indirect | Direct |
| Traffic effects | Limited representation | Incorporated through specification/testing framework |
| Best suited to | Traditional specifications and established markets | Performance-based pavement design |
The two systems therefore should not be treated as direct equivalents.
A penetration grade cannot simply be converted into a PG grade by using a universal conversion table.
6. Why PG Grading Uses Rheology
Asphalt binder is a viscoelastic material.
This means its response to loading can contain both viscous and elastic components.
Its behavior changes significantly with:
- temperature,
- loading time,
- frequency,
- aging,
- and material composition.
At high temperatures, asphalt becomes softer and more susceptible to deformation.
At low temperatures, it becomes stiffer and more susceptible to thermal cracking.
Performance-based binder testing therefore attempts to characterize material behavior under conditions that are more closely related to actual pavement distress mechanisms.
This is a major conceptual difference between the two grading systems.
7. High-Temperature Performance
One of the primary concerns in asphalt pavement is permanent deformation, commonly associated with rutting.
Rutting occurs when repeated traffic loading causes the pavement structure or asphalt layer to accumulate permanent deformation.
High pavement temperatures can significantly reduce binder stiffness and increase susceptibility to deformation.
A PG specification therefore establishes requirements for binder behavior at elevated temperatures.
For example, a binder designated PG 70-22 is specified for a higher high-temperature performance level than a binder designated PG 64-22, assuming the same grading framework.
The important point is that the number 70 is not a penetration value.
It represents a temperature-related performance classification.
8. Low-Temperature Performance
Low temperatures create a different problem.
As asphalt binder cools, it becomes increasingly stiff.
If thermal stresses accumulate faster than the binder can relax them, cracking can occur.
This can contribute to thermal cracking, particularly in regions experiencing severe winter temperatures.
The second number in a PG designation addresses the low-temperature side of the binder’s performance envelope.
For example:
PG 64-22
indicates a binder classified for a low-temperature performance level of approximately -22°C under the applicable grading framework.
A binder with a designation of:
PG 64-28
has a lower specified temperature limit than PG 64-22.
Again, these numbers should be interpreted as specification temperatures rather than as a guarantee that pavement distress will never occur at those temperatures.
Actual pavement performance also depends on the mixture, layer structure, aging, traffic, construction quality and environmental conditions.
9. What Does AASHTO M 320 Add?
AASHTO M 320 established a widely used framework for Performance-Graded Asphalt Binder specifications.
The broader Superpave approach moved binder selection toward pavement temperature and performance considerations rather than relying solely on empirical consistency measurements.
AASHTO’s related practice R 29 describes procedures for determining or verifying the PG of an asphalt binder according to M 320 requirements.
The PG approach therefore provides a framework in which binder selection can be connected more directly to the environmental conditions of the pavement.
10. Why the Same Penetration Grade Can Behave Differently
A common misconception is that two binders with the same penetration grade should perform essentially identically.
That assumption can be misleading.
Penetration is one measured characteristic.
It does not provide a complete description of:
- temperature susceptibility,
- elastic response,
- viscous response,
- aging behavior,
- stress relaxation,
- or deformation resistance.
Two materials may therefore satisfy a particular penetration range while having different performance characteristics when subjected to different temperatures or loading conditions.
This becomes increasingly important when comparing binders from different crude sources, refining processes, modification technologies or production environments.
11. Modified Binders and PG Classification
Polymer-modified asphalt binders provide an important example of why performance-based characterization can be useful.
Polymer modification can alter the rheological behavior of the binder, potentially improving properties such as:
- elastic response,
- resistance to high-temperature deformation,
- fatigue behavior,
- and low-temperature flexibility.
The effect of modification cannot always be adequately represented by a single penetration value.
A modified binder may therefore be specified using a PG designation that communicates its expected performance characteristics more directly.
The appropriate specification depends on the project requirements and governing standard.
12. Is Penetration Grade Obsolete?
No.
Penetration grading remains relevant in many markets and specifications.
ASTM continues to publish penetration-graded asphalt binder specifications, including D946/D946M. The current ASTM specification lists several penetration grades and recognizes the distinction between penetration-, viscosity-, and performance-graded binders.
Penetration data can also remain useful for:
- quality control,
- product identification,
- historical comparison,
- procurement specifications,
- laboratory characterization,
- and markets where penetration grading remains the established commercial language.
The important distinction is that useful does not mean universally sufficient.
For high-performance pavement applications, additional binder characterization may be required.
13. Can 60/70 Bitumen Be Converted Directly to PG 64-22?
There is no universal one-to-one conversion.
A 60/70 penetration grade and PG 64-22 describe the material using fundamentally different classification principles.
The first describes penetration behavior under a standardized test.
The second represents compliance with performance-based requirements over specified temperature conditions.
Therefore, a 60/70 binder should not automatically be marketed or specified as PG 64-22 without the appropriate testing and verification.
Similarly, two 60/70 products from different sources should not automatically be assumed to have identical PG classifications.
The correct approach is to test the material according to the applicable PG specification when a performance grade is required.
14. Choosing the Appropriate Grading System
The appropriate grading system depends on the project, specification, climate, traffic and procurement environment.
Penetration grading may be appropriate when:
- the project specification explicitly requires penetration grade;
- the local market is based on penetration classifications;
- historical specifications must be maintained;
- the application is relatively conventional;
- or penetration is being used as one component of a broader quality-control program.
Performance grading becomes particularly relevant when:
- pavement temperatures are extreme;
- traffic loading is severe;
- rutting resistance is a major concern;
- low-temperature cracking is a significant risk;
- polymer-modified binders are being considered;
- or the project specification is performance-based.
The governing project specification should always take precedence over assumptions based solely on the binder’s commercial designation.
15. A Practical Example
Consider two hypothetical asphalt binders:
Binder A: 60/70 penetration grade
Binder B: PG 64-22
It would be incorrect to conclude that Binder B is simply a “better version” of Binder A.
They are classified using different systems.
Binder A tells the purchaser something about its penetration consistency.
Binder B communicates a performance-based temperature classification.
If a highway project requires a PG binder, the appropriate question is not:
“Which penetration grade is closest to PG 64-22?”
Instead, the appropriate question is:
“Does the proposed binder meet the required PG specification under the applicable test methods?”
This distinction prevents a common procurement and specification error.
16. Key Differences for Buyers and Technical Teams
For procurement teams, the distinction between grading systems has practical consequences.
A purchase specification should clearly identify:
- The grading system
- The required standard
- The applicable test methods
- The required temperature range
- Any modification requirements
- Quality-control requirements
- Sampling and certification procedures
Simply specifying “60/70 bitumen” may be sufficient in one market but inadequate for a project whose specification is based on Performance Grade.
Likewise, requesting “PG 64-22” without identifying the applicable standard and testing requirements may leave important details unresolved.
17. Penetration Grade vs. PG: The Core Concept
The simplest way to remember the distinction is:
Penetration grading describes consistency. Performance grading describes performance across specified temperature conditions.
Penetration asks, in effect:
How far does a standardized needle penetrate the binder under defined test conditions?
Performance grading asks a broader question:
Does this binder satisfy specified performance-related properties at the temperatures relevant to the pavement?
Neither question is inherently meaningless.
They simply serve different purposes.
Conclusion
Penetration Grade and Performance Grade represent two different generations of asphalt-binder classification.
The penetration system provides a straightforward empirical measure of binder consistency and remains an established specification method in many markets. ASTM’s penetration specification continues to define conventional grades such as 40–50, 60–70 and 85–100.
Performance Grading takes a different approach by relating binder properties to pavement temperature conditions and performance requirements. ASTM D6373, for example, defines PG classifications in relation to maximum and minimum pavement design temperatures.
For engineers, suppliers and buyers working on modern high-stress pavements, understanding this distinction is essential.
A penetration number should not be interpreted as a direct substitute for a PG designation, and a PG designation should not be treated as a simple conversion of penetration grade.
The correct binder specification ultimately depends on the governing standard, climate, traffic conditions, pavement design and project requirements.
Quick Reference
| Term | Meaning |
|---|---|
| Penetration Grade | Empirical classification based primarily on needle penetration |
| ASTM D5 | Standard test method for penetration of bituminous materials |
| 60/70 | Traditional penetration-grade designation |
| Performance Grade (PG) | Performance-oriented binder classification |
| PG 64-22 | PG designation with specified high- and low-temperature limits |
| AASHTO M 320 | Performance-Graded Asphalt Binder specification framework |
| ASTM D6373 | ASTM specification for Performance-Graded Asphalt Binder |
| Rutting | Permanent deformation associated with repeated loading |
| Thermal Cracking | Cracking associated with low-temperature thermal stresses |
Technical note: Standards and specifications can be revised. For contractual procurement, laboratory testing, or compliance decisions, the current edition specified by the project or authority should be consulted rather than relying solely on a general article. ASTM currently lists D946/D946M-25a as its active penetration-graded asphalt binder specification and D6373-23 as an active Performance-Graded Asphalt Binder specification.