Intelligent design

Creation of the PG binder specification

By Mike Anderson

“The path to our destination is not always a straight one. We go down the wrong road, we get lost, we turn back. Maybe it doesn’t matter which road we embark on. Maybe what matters is that we embark.”
– Northern Exposure, 1993

I read a lot of Stephen King books, which probably explains why I chose to start this article in a manner similar to the way he starts sections in his various books — with a relevant quotation. However, I did resist the urge to quote song lyrics from The Ramones (although “my brain is hanging upside down” might describe your feelings by the end of this article.)

Regardless, I think the quote from “Northern Exposure,” a popular television show in the 1990s, is relevant to this article for two reasons: (1) it came from 1993, the year the Strategic Highway Research Program (SHRP) ended, and (2) it really describes the state of mind of those charged with implementing the SHRP products — what matters is that we get started.

In the beginning

In the beginning there was the goal of developing a performance-based specification for asphalt binders that will “…allow the engineer to select an asphalt binder on the basis of the performance level required of the pavement under the present and predicted traffic and environmental conditions.” The performance-based asphalt binder specification was intended to work in cooperation with a performance-based mixture specification that would establish acceptable ranges to control fatigue cracking, permanent deformation (rutting), low temperature cracking, aging, adhesion and water sensitivity. The first draft of the specification was released in 1989 and was intended to provide focus for the various SHRP researchers on the goal and to encourage participation by both users and producers in the evolution, refinement and ultimate implementation.

In the first draft, asphalt binder grade was principally established by the high and low temperature rheological properties of the aged asphalt binder. A low-temperature, high-oxygen pressure aging test was envisioned as the means to produce an asphalt binder that had been aged the equivalent of approximately five years. Thus an AB 30-20 would have a rheology index of 3000 (with a 10% tolerance) at 0¿C and a rheology index of 2000 (with a 10% tolerance) at 80ºC. Changing the low temperature rheology index value and leaving the high temperature rheology index value the same would change the temperature dependency of the asphalt binder. Leaving the low temperature rheology index value the same and changing the high temperature rheology index value would simply ensure a softer grade without affecting the temperature dependency of the asphalt binder.

Although the high temperature rheology index was expected to relate to rutting and the low temperature rheology index to low temperature cracking, the specification also included characteristics to address other distresses. The healing factor was intended to control fatigue cracking by specifying a minimum value of an index that would measure “…the ability of the asphalt binder to chemically bridge and repair fatigue and low temperature cracks.” The nitrogen factor and acid factor were intended to address water sensitivity and adhesion. The high temperature viscosity was intended to address constructability. The flash index, not necessarily the flash point test, was intended to address safety.

It is important to note that one of the SHRP goals was to “definitively explain the relationship between asphalt composition and pavement performance.” The initial draft specification addressed that goal by combining physical specification properties (high and low temperature rheology indices, Healing Factor) with chemical specification properties (Nitrogen and Acid Factors).

Draft 1 begat draft 2, and draft 2 begat?

No draft is perfect from the start and the PG Binder Specification was no exception. Draft 1 of the specification led eventually to Draft 5 that represented a significant advancement in the development of a performance-based asphalt binder specification.

How so? Well first and foremost, the grading system was new in Draft 5 compared to earlier versions. In earlier versions, grading was based on the rheology indices at high (80ºC) and low (0ºC) temperatures. In this, the early versions were most similar to the pre-SHRP grading systems which were based on achieving a certain range of penetration values at 25ºC or a certain range of viscosity values at 60ºC. In Draft 5, the values for the specification properties remain the same for all grades, but the temperatures at which the required values change based on the expected temperatures of the environment in which the asphalt binder will be used. For example, in Draft 5 an asphalt binder having an AB 3-2 grade would be expected to have acceptable properties in a climate where the highest mean monthly temperature was between 90 and 100ºF and the lowest anticipated temperature was between -10 and -20ºF. To ensure the asphalt binder would have acceptable properties to warrant against permanent deformation, the viscous component of stiffness would need to be a minimum of 15 psi at a test temperature of 100ºF. Similarly, to ensure the asphalt binder would have acceptable properties to warrant against low temperature cracking, the stiffness would need to be a maximum of 30 ksi after two hours of loading at a test temperature of -20ºF.

(In case you were wondering, in the United States an air temperature of 100ºF would correspond roughly to a pavement temperature of 58¿C at a depth of 20 millimeters. On the low side, an air temperature of -20ºF would correspond roughly to a pavement temperature of -28ºC.)

Note that in Draft 5, four principal specification properties are used to address three pavement distresses — permanent deformation, fatigue cracking and low temperature cracking. Adhesion and moisture sensitivity were pulled from the binder specification as research indicated that they are unique characteristics of asphalt-aggregate combinations that are best addressed within a mixture specification. As a result, the nitrogen factor and acid factor were removed from the specification framework, leaving a specification without any chemical properties.

By abandoning the two-temperature mastercurve approach used in earlier versions and directly measuring properties at critical air (pavement) temperatures, it was possible to develop a specification for both unmodified and modified asphalt binders. This was an important decision in the path towards the final PG asphalt binder specification.

And so it came to pass?

By involving industry in the evaluation of the draft specification, the process of evolution was somewhat accelerated. Draft 5 was presented in 1991. Draft 7G (now adding letters to the draft version…maybe Draft 14 sounded too clunky?) was presented in 1992.

Draft 7G looks a little more like the current PG Binder Specification, although some of the specification values are different. The property offered to address fatigue cracking distress changed from “m” – a measurement of the flatness of the stiffness-temperature dependency of the asphalt binder — to G*sin ∂, a property related to dissipated energy. In Draft 7G, though, the maximum value of G*sin ∂ was 3000 kPa. To address low temperature cracking, the maximum Stiffness from the Bending Beam Rheometer (BBR) was 200 MPa and the minimum m-value was 0.35.

The grading nomenclature in Draft 7G also changed slightly with the term “Performance Grade” or “PG” being used instead of “AB.”

Along the way to the final specification (now Table 1 in AASHTO M320) there were many other revisions and considerations. To address low temperature ph

SHRP Draft 1