HMA Notes:
Is Your RAP Pile Growing?
By Bob Nady

The answer to that, if it is “yes”, is clear: Use more RAP in HMA. First off, I say a resounding “no” to using the RAP in aggregate-only applications, i.e., shouldering, culvert fill, and any application that does not make full use of the binder contained in the RAP. Why? Let’s say you use 100 tons of RAP which contains 5% binder. That is 5 tons of binder. At a price of $300 per ton, you have lost the value of the binder. In this case you won’t need to do the math; I have done it for you. Does anyone here like to lose $1,500, even in a craps game? I thought not. Your turn to do the math is coming up.

The next thing you need to do is to find the most efficient ratio of RAP to Virgin aggregate that your drying/heating equipment can handle. Start with 30% RAP and 70% aggregate. How much fuel per hour does it take to bring that mix up to temperature? Write this number down; now try 40/60 RAP to aggregate. Write down that fuel usage. And so on through 50/50, 60/40, etc. Somewhere along the line your fuel usage will bottom out. That is your best RAP/Virgin ratio. (I am assuming drum plant with RAP collar). Now it is time to try out your ratio. Set it on your materials feeds and run awhile. Do the math and make sure your ratio gives you the best (least) fuel consumption.

You are making progress. You have determined the most efficient amount of RAP that you can put in your mix. But we are not done yet.

Do you know how much binder your RAP contains? Do you know the true gradation of the RAP aggregate? This must be determined by sampling the RAP burning the binder, and analyzing the remaining aggregate. You will then know the blend of the aggregate, which with the RAP will give you a total aggregate blend, which satisfies the project requirements.

One thing you may notice: Here we are considering RAP material generated by milling. Milled RAP has about ten percent passing the 200 sieve. Because of the milling operation, the amount of RAP may, therefore, be limited. Don’t sweat it. We can do something about that. We can screen (fractionate) the RAP to reduce the fines content. Caution: do not throw away the fine, it contains more binder percentage-wise than the original RAP. It will be necessary to determine the binder content and gradation of each of the RAP fractions, coarse and fine. Now you can probably use more of the coarse screened RAP, without violating the gradation requirements. You can develop a mix using the fine-screened RAP probably with clean virgin aggregate. Now you have two mixes with higher RAP contents than you are now using. It is important to know more about the materials you have been using so that you can try various amounts and come up with economical mixes that your plant can handle. You can run trial lab sized batches to find the amount of binder your mix needs for good performance. Just accounting for the binder in the RAP and add new binder to make up the difference needed to reach the total binder content of the mix. Isn’t it worth the price of the binder in the RAP of which you are now making full use?

So far we have been talking about milled RAP because of its wider distribution and high fines content. Let us now consider what has been called Random RAP. Several stationary plant operators invite local contractors to bring in removed asphalt materials from their local projects – city streets, county roads, parking lots, and which, for whatever reasons, was removed from a project. The material piled (no PC concrete, please) to be crushed during the winter months. A typical pile will have slabs of various sizes and thicknesses, chunks, sections of asphalt curb, any HMA materials. The pile is ugly, but the pile is HMA. There was an opportunity to sample the crusher run product, daily or twice daily, and to test the samples for composition, binder content and aggregate gradation. Despite the appearance and the random nature of the contents of the pile, the end product was uniform and consistent, over one season product and over seasons. This was reported in the Journal of AAPT,

Vol. 66. One salient feature the fact that the P200 content was about 5%, significantly less than the milled RAP. The basic gradation of the HMA material was not upset by the crushing operation. This indicates that high percentages of Random RAP can be used in a mix without violating the filler/bitumen ratio or the P200 specifications. So, it is important to keep them in separated piles, milled in one pile, Random in another pile. But again, it is important to have each pile analyzed, preferably when the pile is built and the composition is known. It is much easier, and more accurate to test the RAP as it is processed.

A word about binder: asphalt ages in service. It gets stiffer, but is still a good binder material. When using small amounts of RAP, the virgin binder can be the typical asphalt grade in every day use, such as in this area. However, as we are discussing much higher proportions of RAP, using a blending chart, the correct grade of virgin binder may be selected so that the total binder in the mix is of the proper grade.

In summary, it is not rocket science:

1. Know your materials, Virgin and RAP,
   
2. Keep your piles separate,
   
3. High percentages of RAP can be successfully used,
   
4. A history of 15 years is behind these remarks,
   
5. When in doubt, call your friendly testing lab.