Recycled Pavement or Black Rocks

I have been working with recycled asphalt since presenting an AAPT paper in 1975 on 100% asphalt recycling. I have always considered all of the asphalt was acting as binder. I am not so sure anymore. As more and more RAP is being used the question comes up whether all of the asphalt in the mix should be considered truly asphalt or actually be part of the aggregate.  We have noticed that mixes that require, say, 5.1% asphalt  with no RAP may require 5.5-5.7% asphalt. The next question is whether the formation of the “black rocks” is actually the true condition of the RAP, or whether coking of the asphalt happened during the mix design phase because the technician added the RAP too soon in the mix cycle, such as leaving it in the aggregate in the oven overnight. This would not be happening in cold-in-place recycling. If part of the asphalt is not working as binder, perhaps we should change the mix design criteria as to how we calculate VMA etc.

How asphalt works

Prior to the deveopment of the Strategic Highway Research Program (SHRP) there was a meeting in Dallas to discuss the concepts. At that meeting it was stated that the goal was to make an asphalt that could correct any flaws that might be found in pavements. Unfortunately there is still a remnant of that opinion. Asphalt , even when modified, is a liquid. Its function is to waterproof and glue stuff together. (An additional function is to tar and feather,  however that generally is not considered a good construction practice unless things are really going bad.) This is an understanding I learned many years ago sharing an office at Shell Development with Ray Griffith, the inventor of the sliding plate microviscometer. The solution to rutting and tenderness is not asphalt modification but aggregate gradation, although modifying the asphalt can slow down rutting. The importance of gradation is something I learned from Vaughn Marker when he was the Asphalt Institute engineer in California.  The prime reason to polymer modify asphalt is to change the temperature susceptibility to combat non-load associated cracking. It is vital that the binder remains a liquid so that it can relax thermal stresses faster than they can build up. Thus trying to modify asphalt to combat load associated cracking is blowing in the wind. Load associated damage is caused by lack of adequate structural strength (or studded tires if you live where I do!). Lack of protection of the mix from water damage accelerates damage. I will discuss water damage later.