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.


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