My career in the asphalt industry started with my first interview. My future boss, the Bituminous Engineer for the Commonwealth of Kentucky, asked what I knew about “rutting” and then gave me a paper to read while he stepped out of the room briefly. Up to that point, the only “rutting” I had heard of was in reference to deer. I was unsure what the behavior of deer had to do with the performance of roads, but I needed a job, so I read and hoped I could answer his questions well enough.
It would not have been my preferred interview technique, but I later came to understand that Dwight was so distressed (pun unintended) over the state of rutting on our asphalt pavements that he was looking for someone who could share his angst. It was the late 1980s and I often heard him wistfully say, “I would love for our maintenance crews to have to fill a crack.”
Times have certainly changed. But as I also heard early on, “problems tend to be cyclical in the asphalt industry.” We would be wise not to forget the lessons of the past.
When we see rutting, channelized depressions caused by traffic loads, it could be caused by a weak subgrade or by a weak asphalt layer. As asphalt mixture technologists we worry about the latter. So, what affects rutting in an asphalt mixture layer? It basically comes down to the three main components in a mixture: aggregate, asphalt binder and air.
Aggregate provides the structure. Ideally, we would have an aggregate that is angular and durable so that it will not break down during compaction and traffic loading. The angular faces of the aggregate particles permit the particles to interlock with each other when subjected to loads so that the individual particles act like one big rock.
For Marvel fans, think of the Nova Corps ships in the final battle of “Guardians of the Galaxy.” Being without spaceships, we demonstrate the effects of angularity in our lab by having two beakers – one filled with single-sized crushed limestone chips and the other filled with single-sized glass beads – and asking students to try and stick their fingers into each. The rounded glass beads are much easier to displace. Good for fingers; not for roads.
Size distribution, or gradation, matters too. Larger rocks cannot interact with each other if they are separated by much finer particles. The distribution of aggregate sizes and angularity affects how the particles will pack when compacted, leaving valuable void spaces. In mix designs, we refer to this as VMA, or Voids in the Mineral Aggregate. This is the space that is available for the asphalt binder and air. The balance of those two components is important to the mix performance. The amount of dust in the mix, aggregate finer than 0.075 mm (#200 sieve), is another potential concern for performance.
The asphalt binder properties can significantly impact the rutting performance of the mix. The asphalt binder should be stiff enough at the expected high temperature of the project location to resist traffic loads. Polymer modification can help with stiffness and also provide needed elasticity so that some of the deformation caused by traffic loading is recovered.
The properties of our third component, air, are also important. Not really. Just seeing if you were still with me.
So, we know the properties of the aggregate and asphalt binder are important and the properties of air are not. The other part that is important is the volume of each of the components, specifically the volume of air and asphalt binder in the mix. I alluded to this a couple of paragraphs prior to this one.
The percentage of air voids in a mixture was first proposed by the U.S. Army Corps of Engineers as a design criterion for the Marshall mix design procedure in a paper published in 1949. In that paper, the authors noted that “It appears that the boundary between plastic and satisfactory mixes for these tests is, in general, between 3 and 3.5 percent.”
The concept of using the percentage of air voids for design continued into the Superpave mix design system and the current version of AASHTO M323. The use of a maximum VMA ensures that we are not just filling voids in the aggregate structure with asphalt binder, leading to potential bleeding, if not rutting.
Although there are many options today for using recycled materials and additives, the rutting potential of an asphalt mixture mostly comes down to: (1) the properties and size distribution of the aggregate; (2) the properties and volume of the asphalt binder; and (3) the volume of the air voids. If all three are selected correctly for the expected traffic and climate, your chances of successful resistance to permanent deformation should be high, leaving rutting behavior to your local deer.
Anderson is the Asphalt Institute Vice President of Research and Laboratory Services.
