By Mike Anderson, P.E.
Probably not a surprise given that I’m an engineer, but I have always loved science fiction. When I was younger, I remember reading a book by Robert Heinlein called “Time Enough for Love.” Despite it sounding more like a non-fiction relationship book instead of a science fiction book I dove in anyway. By contrast, “Men Are from Mars, Women Are from Venus” sounds like it could be a sci-fi book instead of a non-fiction relationship book. In the words of Captain Jack Sparrow, “Funny ole world, innit?
Anyway… “Time Enough for Love” was the tale of Lazarus Long, a person who was blessed with extraordinary longevity. His own natural genetics were then enhanced periodically by undergoing rejuvenation at a clinic where the doctors literally replaced almost every part of his body until he looked and felt like a person who was much younger. I was fascinated by the idea of rejuvenation when I was younger. I’m even more fascinated now…for obvious reasons. Being able to take my body back to a state that it once was before the ravages of time has its appeal.
In the asphalt world, the concept of rejuvenation is also appealing for older reclaimed asphalt products, providing an opportunity to recycle old asphalt materials with new, virgin asphalt mixtures. But what is real rejuvenation? For that, I turn to another love from my youth – french fries – and an analogy to asphalt rejuvenation which I call “The French Fry Analogy.”
Have you ever wanted to bring home leftovers from dinner and had to decide whether to include the remaining french fries? Why would you consider leaving them? For me, it’s because leftover, reheated fries never taste like fresh french fries. I’ll pause briefly for alliteration appreciation. [pause]. I can easily make them hot again, but what I really want is for them to be hot, crispy and salty – the way they were. Barbra Streisand fries, if you will.
We have the same issue with using reclaimed asphalt materials. We can add lower viscosity asphalt binders and additives or modifiers to get a softer blended asphalt binder in the mixture but are we restoring the viscous/elastic relationship that was present in the original asphalt binder before aging? For that, I turn to Black Space diagrams, which are a way for us to look at modulus and viscoelastic behavior on the same graph, and the figures below.
Shown in Figure 1 are two data points (represented by circles) representing equal testing conditions (temperature, applied strain and loading frequency) of an unmodified asphalt binder. As can be seen, the process of aging results in a movement from A to B up and to the right, increasing stiffness and decreasing phase angle. It may be an oversimplification but think of a decrease in phase angle with an increase in stiffness as increasing brittleness.* In Figure 2 we see two possible paths that can happen when we try to bring an older asphalt back to a newer state. Point C represents a softening. We have returned the asphalt binder to the same modulus (or stiffness or viscosity) as the original asphalt binder, but the phase angle remains lower – meaning its viscoelastic properties are not the same as the original. It will behave more like a brittle material as it further ages than the original asphalt binder. In other words, this is the result when we heat our fries in the microwave – hot, but not crispy. If we really want rejuvenation, we need the path to follow from B back to A, restoring the original modulus, but also restoring the phase angle. This is what we want from our leftover fries – hot and crispy.
In our testing, it is easy to see the rejuvenation path in Black Space diagrams, but other parameters can give us similar indications as long as they capture that balance between modulus and phase angle as an asphalt binder ages. The Glover-Rowe Parameter is one; Delta Tc is another. If you haven’t heard of these yet you probably will soon.
So, whenever you think about how you design your asphalt mixtures using recycled materials think about how you like your leftover french fries. Hot is better than cold, but hot and crispy are best.
For information on Asphalt Institute’s laboratory training, research, and testing services please contact Mike Anderson (firstname.lastname@example.org) or Gary Irvine (girvine@ asphaltinstitute.org). A new kind of testing. A new kind of fries.