By Mike Anderson, P.E.
I’m sure by the time this publishes – whether actual print or just the electronic version – the premise in which I’m engaging for this article will be old hat for most of you, having read it in one form or another for the past six weeks. But I can’t help myself; please forgive me.
It’s 2020; now inspiring year-long themes of vision and hindsight and prompting a look back. I thought about doing a “20 for 20” for asphalt – like ESPN’s “30 for 30” on sports stories except not nearly as good. “What if I told you…” is the start of the famous tag line uttered by a deep-throated voiceover actor to set up the premise for the “30 for 30” topic. Keeping in that vein, read the following in your deepest voice:
“What if I told you that the PG System has lasted longer in the United States without being replaced by an improved system than either the Penetration or Viscosity Graded Systems?”
Seriously. We need to pause a minute in appreciation for the work conducted as part of the Strategic Highway Research Program (SHRP). Take your time; I’ll wait.
The Penetration Grading System made its way into ASTM as D946 in 1947. The Viscosity Grading System made it in 1970. That’s approximately 23 years of use for Pen Grading before most user agencies started moving to an improved system.
The PG System made its way to user agencies in 1994 (allowing for one-year post-SHRP). Although not everyone made the transition right away, it’s safe to say that the transition had started, ushering out 24 years of Viscosity Grading. Even assuming a 1994 start rather than a 1993 start at the end of SHRP, that means that the Performance Graded (PG) Asphalt Binder Specification has been around for over 25 years. I’m sure we can quibble over the correctness of the statement that the PG system has been around longer, but the larger point is that the PG system, with minor tweaks, has stood the test of time.
It can be instructive to look back at the past so that we learn what was done and why to guide our future actions. It’s why research projects always start with a literature review and Mondays in the NFL mean film study for the coaches and players.
So, hindsight being what it is, what are some lessons we have learned with the PG system?
• The addition of long-term aging to the specification using the PAV was a big advancement…but maybe it didn’t go far enough in simulating longer-term aging. Preliminary indications point to more severe aging in the PAV (e.g. 40 hours) to come closer to the original intent. How to get that level of aging in a more reasonable length of time is the subject of the work in the NCHRP 09-61 project.
• The high-temperature parameter, G*/sin δ, worked great for unmodified asphalt binders but the slight benefit gained by modified asphalt binders from a lower phase angle didn’t separate their rutting performance enough. The non-recoverable creep compliance parameter, Jnr, from the MSCR test fixes that shortcoming for both unmodified and modified asphalt binders.
• Evaluating asphalt binder properties related to cracking wasn’t easy because cracking is a complicated distress. G*sin δ was okay for relating to fatigue cracking but was more for classic bottom-up fatigue cracking distress. BBR stiffness and m-value worked well for low temperature cracking – as long as the asphalt binder wasn’t overly susceptible to physical hardening. The Direct Tension Test was important as it truly was a failure test – the only one in the PG Specification – but was hampered by difficulties in specimen preparation and consistent testing. What was missing was a durability cracking parameter to account for non-load related cracking at intermediate temperatures. Lots of work has since been accomplished on this issue in recent years leading to parameters like Delta Tc, Glover-Rowe, R-value, DENT, and others. The NCHRP 09-59 and 09-60 projects have considered these issues and are expected to provide the industry with some answers.
• The switch to testing asphalt binders at environmental temperatures was a much-needed improvement, making the specification rational to the user.
• The use of new testing equipment and procedures modernized the testing of asphalt binders, providing greater flexibility in characterizing asphalt binder properties.
• Last, but not least…doing great research is important, but getting the research findings implemented is critical to the system’s success.
Hindsight is important but only if the look back honestly assesses not only what went right or wrong, but why it did. Then that 20-20 vision can guide you clearly on the path forward.
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.