By Mike Anderson, P.E.
Doctors will tell you all you care to know about the evils of stress and what it can do to your mental and physical well-being. Stress increases cortisol which can lead to weight gain. It can lead to sleep deprivation and depression and hypertension. In our fast-paced society, work stress is a constant companion for many of us.
People combat stress by relaxing in various ways. What are my top three most relaxing activities? I’m glad you asked. I’d have to say:
1. Sitting on the beach on a bright, clear day listening to/watching the waves roll in;
2. Participating in an instructor-led basic yoga class – particularly the part where you get to nap quietly on your mat at the end after pretending to be a dog, cat, cow and cobra. It’s like kindergarten all over again; and
3. Watching my robot vacuum wander around sweeping up pet fur. At the end of its hard work, it returns to the docking station and plays a few happy notes to let me know it made it home safely. I’m surprised Tesla hasn’t thought of that for their cars.
No matter how you choose to relax, it’s clear that the ability to relax is important to carbon-based humans…so why shouldn’t it also be important to carbon-based asphalt?
The PG Asphalt Binder Specification, although an improvement over past specification systems, really doesn’t capture the ability of the asphalt binder to relax. There may have been some discussion when the system was first conceived about including a parameter that could relate to relaxation properties, but ultimately it didn’t make it in the spec.
What does the ability to relax mean for an asphalt binder? In the simplest terms it means that as stresses build, the asphalt binder has the ability to release those stresses through viscous flow. If it can’t relax sufficiently then the stresses may build to a point that cracking occurs.
Temperature matters because asphalt acts less like a viscous material as the temperature decreases. Using the right binder grade for the climate is important.
Composition and processing matter because the chemical make-up of the asphalt binder and how it is produced affects the balance of viscous and elastic properties at a given temperature.
Aging matters because aged asphalt becomes stiffer and loses its flexibility…much like humans do as we age.
Asphalt technologists are coming to have a renewed appreciation for the relaxation properties of asphalt binders and their role in durability cracking performance. Since the PG specification doesn’t directly capture the effects of relaxation in its current form what tests and parameters are being considered for inclusion in a future version of the spec?
Delta Tc – a parameter derived from BBR testing. It is currently being used and/or evaluated by at least ten agencies. AI has a very detailed state-of-the-knowledge document (IS-240) available if you want to know more.
Glover-Rowe – a parameter derived from DSR testing that better accounts for the viscoelastic response of the asphalt binder than the standard G*sin δ parameter typically used at intermediate temperature. Past research determined the value from a mastercurve constructed at a specified reference temperature from multiple temperature-frequency sweep tests. One national research project suggests using the parameter with the value determined at a specified temperature and frequency.
R-value – not a measure of insulation, but a parameter representing the shape of the mastercurve and how it transitions from elastic to viscous behavior. It is determined from a mastercurve at a specified reference temperature that is constructed from multiple temperature-frequency sweep tests. It can also be estimated using a simple equation at a single testing condition (ideally where the shear modulus is 10 MPa or higher). Thinking about the parameter does keep me warm at night.
Double-Edge Notched Tension (DENT) – a test used to determine the crack tip opening displacement (CTOD) at intermediate temperature using a ductilometer with load cell.
Asphalt Binder Cracking Device (ABCD) – a test used to determine the temperature at which the asphalt binder cracks at lower temperatures when subjected to a standard rate of decreasing temperature. It is not necessarily a test about relaxation but rather the failure strain of asphalt binders. There is no truth to the rumor that it is used in combination with the Three Rs – RTFO, Rheology and Recovery.
Several national research projects have included one or more of these parameters in their evaluation. The goal – which we hope will be realized sooner rather than later – is to have a revised specification that captures all the key properties to minimize the binder’s contribution to asphalt pavement distress. Once we have that, I think asphalt technologists everywhere will be able to sit back and relax…at least for a while.
For information on Asphalt Institute’s laboratory training, research, and testing services please contact Mike Anderson (manderson@asphaltinstitute.org) or Gary Irvine (girvine@asphaltinstitute.org). We’ll be happy to respond…as soon as one of us can roll out of our respective hammocks.
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.