By Mike Anderson, P.E.
If you’re a frequent reader of “Lab Corner”, you know that previous entries have hinted at my love for all types of music. Although I may like the music in a song, the lyrics have always been crucial. Great lyrics can make a song. Poor lyrics, even just a word or phrase, will make me cringe.
It shouldn’t surprise you to learn that Bruce Springsteen is one of my favorite artists. Great music and lyrics…except, IMHO, for one word in a popular song off of his best-selling album. At the start of “Glory Days,” Springsteen starts telling the story of seeing an old friend who was a great high school baseball player. The song is intended to be a wistful look back on the passage of time, while generally rocking along. Love it, mostly, but I am bothered by the statement that his friend could “throw that speedball by ya.” Really? Speedball?
I’ve been a baseball fan my whole life and I don’t think I have ever heard a fastball referred to as a “speedball.” Fastball would’ve worked. Same number of syllables. Or curveball. Different pitch but you still could throw it by a hitter. Same for slider. All two syllables; all real terms for pitches. I don’t know what The Boss was thinking, and I feel unworthy even saying it, but the use of “speedball” for me is a missed opportunity that makes the song just a little off. Like seeing a scene in a movie that uses a “555” phone number.
The PG Asphalt Binder Specification feels the same to me – great in general, but just a little off.
The Performance Graded (PG) Asphalt Binder Specification was developed based on the concept of minimizing the contribution of the asphalt binder properties to mixture distresses like rutting and cracking. High-temperature properties were selected to address rutting; low-temperature properties were selected for thermal cracking. The properties at intermediate temperature were intended to address the most challenging of the distresses – fatigue cracking. The problem with characterizing cracking at intermediate temperatures is that it is sometimes difficult to know what kind of cracking you have.
Why was the current parameter, G*sin δ selected? Part of the answer can be found in SHRP Report A-367:
“…in light of an evaluation of the fatigue performance in field trials such as Zaca-Wigmore, the fatigue criterion was changed to reflect the energy dissipated per load cycle. Dissipated energy in a dynamic shear test is appropriately calculated as G*sin δ.”
Makes sense. But if we go back to the basis for that choice, the Zaca-Wigmore test road, we read the following in an AAPT paper from 1969 – more than 50 years ago:
“Two main types of failure during service life were encountered on the project. The most prevalent was fatigue cracking as displayed by wheel track ‘alligator’ type cracking. The other was large block type cracking together with pitting and raveling. This was most prevalent in the passing lane. The amount of fatigue type cracking appears to be related to the consistency of the recovered asphalt as measured by penetration and viscosity. The other form of cracking appears to be related to the gain in shear susceptibility during weathering. This is also indicated by a marked drop in ductility during service life.”
There was a hint then that we needed to look at multiple forms of cracking that might be impacted by different intermediate temperature properties. Maybe we reasoned that G*sin δ would serve as an adequate property for both fatigue and durability cracking. The asphalt binders used at the time may have even confirmed that. Maybe the choice of an additional test or property was deemed impractical to implement. Whatever the reason, it appears that we missed an opportunity in the original PG specification to fully consider durability cracking.
The NCHRP 09-60 research project was initiated with the understanding that asphalt binders today are being made in different ways and with different properties than 25+ years earlier when SHRP was being conducted. Their initial findings identified that ΔTc can be used as an indicator of relaxation and expected resistance to durability cracking – whether caused by weathering alone or incompatibility in the asphalt binder as it ages. The NCHRP 09-59 research also suggests using a new term, R value, calculated from the same BBR data as is used for determining ΔTc, as an additional parameter for asphalt binder cracking performance. The two parameters, ΔTc and R, appear to be related.
Whatever parameter we end up using, it now appears that we have an opportunity in the near future to correct that slight misstep in the original PG Asphalt Binder Specification. I wish that we had the same opportunity with “Glory Days.”
Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.
Petersen, J.C., R.E. Robertson, J.F. Branthaver, P.M. Harnsberger, J.J. Duvall, S.S. Kim, D.A. Anderson, D.W. Christensen, and H.U. Bahia. SHRP-A-367, Binder Characterization and Evaluation: Volume 1, Strategic Highway Research Program, National Research Council, Washington, DC, 1994.
Zube, E. and J. Skog. “Final Report on the Zaca-Wigmore Test Road,” Proceedings of the Association of Asphalt Paving Technologists, Volume 38, Ann Arbor, MI, 1969.