By Mike Anderson, P.E.
I’m a fan of “The Big Bang Theory” — mainly because I can identify all too well with the personae of the characters.
Engineer…check. Star Wars/Star Trek fan…check. Plays Dungeons and Dragons (D&D)…check, although not any more. When I was a teenager I subscribed to “Dragon” magazine. I’ll pause for the chuckling to die down. It was full of great ideas (for D&D, not life) and I used to read it cover to cover as soon as it arrived in my mailbox.
One day after reading, I became inspired. I created a battle point system — the purpose of which was to assign points to each character in an adventure based on their performance in an encounter and use those points to more fairly divide treasure. Battle with orcs? Fighters got points for winning their one-on-one battles. Clerics got points for healing wounds. Magic users got points for casting a light spell on their staff so they could see what was going on in the battle. It was the D&D version of a performance-based review system.
It was hopelessly complex.
The problem was that I had ignored the KISS Principle. KISS was an acronym coined in the military in the 1960s for “Keep It Simple Stupid.” It is based on the premise that systems work best if they are simple and not unnecessarily complicated. It is a principle that we would do well to keep in mind when establishing specifications for asphalt materials.
The PG asphalt binder specification (AASHTO M320) was established as an outcome of the Strategic Highway Research Program (SHRP) to minimize the contribution of the asphalt binder to any pavement distresses. In its original version it did a great job of adhering to the KISS principle. The asphalt binder’s contribution to rutting was addressed by high temperature Dynamic Shear Rheometer (DSR) testing in the original and short-term aged state (when rutting is a concern) using a parameter, G*/sin δ, that captures the magnitude of the binder stiffness and the proportion of viscous to elastic behavior.
Later in the pavement’s life, when thermal cracking is a concern, the asphalt binder is subjected to long-term conditioning in the Pressure Aging Vessel (PAV) before being tested using the Bending Beam Rheometer (BBR) to measure low temperature stiffness and relaxation. Also later in the pavement’s life, at average temperatures, fatigue and durability cracking become more of a concern. The PG asphalt binder specification attempted to address those particular distress concerns in a manner similar to the high temperature testing – by using a slightly different parameter, G*sin δ, that captures the magnitude of the binder stiffness and the proportion of viscous to elastic behavior.
While the specification worked well for unmodified asphalt binders, it soon became clear that all the anticipated benefits of using a polymer modified asphalt binder were not identified by the PG specification. So many user agencies elected to add one or more “PG Plus” tests to ensure that they continued to get the premium asphalt binders that they expected. In doing so, they complicated a simple system because the parameters weren’t quite right.
Today, the asphalt industry has a parameter that is more related to rutting in the non-recoverable creep compliance, Jnr, from the Multiple Stress Creep Recovery (MSCR) test. It can be substituted in the PG specification to replace G*/sin δ after RTFO aging making it a simple modification.
The next evolutionary step for asphalt technologists is to fix the intermediate binder specification parameter that is related to fatigue and durability cracking distress. Lots of new tests and parameters have been proposed – Linear Amplitude Sweep (LAS), R-Value, Glover-Rowe, ΔTc, and Double Edge Notched Tension (DENT) to name a few – but asphalt technologists have yet to agree on one that they believe will better relate to intermediate temperature cracking.
Research is currently underway in several institutions through the National Cooperative Highway Research Program to investigate the next intermediate binder parameter and how it fits in the specification. As research progresses, users and producers alike are hopeful that the research teams keep in mind the KISS principle. Because wherever asphalt binder specifications will be used in the future on Earth, or Middle Earth, keeping it simple is far from stupid.
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute. Email him at manderson@asphaltinstitute.org.