Expecting Binder Research to Solve the Problem
In a previous article “Fundamental Causes of Cracking, Potholes, Raveling, and Rutting in Asphalt Pavements” I touched on some of the causes of rutting. I wish to expand on this subject. However I wish to exclude rutting from studded tires as that problem has not been solved at this time.
Prior to the establishment of the Strategic Highway Research Program (SHRP) I attended a meeting in which it was stated that the goal was to develop an asphalt that could solve all of the problems that occur in pavements. The philosophy that problems reside primarily with the asphalt is still deeply encountered, however, in my opinion it is all “vanity and blowing into the wind”. That does not mean that there isn’t a place for asphalt research because great strides have been made in resolving pavement problems with modified asphalts. In fact, with rutting, I am sure many will state that with such and such binder, the rut tester shows an improvement in rut resistance. And I am sure that their data is correct. However I would suggest that those modifications only affect the rate at which rutting occurs not the basic cause. The misconception is that it is the properties of the asphalt that allows rutting. That is false. A properly performing asphalt is a liquid and is purposely designed to not resist rutting or any other stress that might prevent it from flowing. In fact one of the solutions to low temperature cracking is to modify the asphalt so that it can flow to relax thermal stresses before they reach the point where the asphalt fractures.
If one wishes a life time research project on rutting, concentrate only on the binder and work only with oversanded aggregate gradations. Do I mean that the aggregate gradation is part of the problem? Yes. In fact the aggregate gradation is the problem; and the present gradation specifications specifically allow oversanded mixes, thus, allowing the construction of tender and rut prone pavements to be built. I learned this from Vaughn Marker and Went Lovering of the Asphalt Institute back when I had more hair, and it was not so grey. (Went Lovering also had worked for CALTRANS and was a great source of knowledge and wisdom. He was instrumental in the development of the Hveem Design.)
How can we get oversanded mixes. First draw a straight line on the 0.45 power gradation chart from the % passing of first sieve that retains aggregate to that of the – #200. If you want an oversanded mix, make sure that the -# 4 gradation is above that line. If it is below the line, you can still meet that goal of an oversanded rut prone mix if the gradation in the -#30 range goes above that line. It is true that messing with the ability of the binder to flow will help reducing the rate of rutting, but, of course, non-load associated cracking is associated with the lack of ability for the binder to relax thermal stresses. In this manner research can be continually funded so that one can be an expert on how to not to stop rutting and tenderness.
You do want an oversanded mix for hydraulic structures, however.