Featured Articles: Keys to Managing RAP Variability
The majority of contractors crush all RAP to a single maximum size, such as minus 1/2-inch, or minus 5/8-inch, so that the crushed RAP can be used in a wide range of mixes from black base to surface mixes. The price paid for this convenience is that as larger aggregate particles in the RAP get crushed, more dust is generated. The excess dust will often limit how much of the RAP can be used in a new mix design.
There has been a lot of discussion lately about fractionating RAP. This practice is simply the screening of RAP into two or more sizes. A common two-size fractionating system splits the RAP into a plus 1/2-inch fraction and a minus 1/2- inch fraction; three-size fractionation units typically produce a plus 3/4-inch, 3/4-inch to 1/4-inch, and minus 1/4-inch sized RAP stockpile. Other screen splits may work just as well. There are a couple of benefits to fractionating RAP, and there are also additional costs with the practice. The primary benefit of fractionating RAP is that it provides much greater flexibility in designing mixes. Mix designers can use different percentages of the coarse and fine RAP (or coarse, intermediate, and fine RAP) with virgin aggregates to meet gradation and volumetric requirements for practically any mix. In general, it is easier to use more total RAP in a mix when it is fractionated compared to a crusher-run RAP.
Stop Processing RAP When it Rains:
There are not a lot of data to substantiate this, but our experience with fractionating RAP for the NCAT Test Track in summer 2009 indicated that the RAP stockpiles that had been produced during a heavy afternoon thunderstorm changed in gradation affected the mixes so much that we had to remove a couple of test sections. It makes sense that RAP will not screen as efficiently when it is wet because the material sticks together more and the screens tend to clog up or blind.
Blend Again When Moving:
Usually, after processing RAP through a crushing system and/or fractionation unit, the new stockpile(s) will have to be moved from the processing location to another location closer to the asphalt plant’s cold feed bins. Contractors want to avoid moving materials any more than needed because it adds cost to the materials, but moving the processed RAP materials is an opportunity to further improve consistency. Moving processed RAP should be done so as to further mix and blend the material as it is being loaded and unloaded. However, sloppy moving practices can also lead to segregation, which will have the opposite effect on consistency.
Cover, Slope, and Pave:
No, it is not a late-night order at the local diner; it is the recipe for minimizing moisture in RAP stockpiles. RAP stockpiles tend to retain a lot of moisture and that will increase the drying and heating cost for superheating the virgin aggregate and limit the mix production rate. Relatively few contractors currently cover any of their stockpiles, but those who do know that covering their RAP stockpiles gives the best payback. As with virgin aggregates, paving under stockpiles provides an even foundation to minimize yard loss and contamination underlying materials. Sloping the surface under the stockpile away from the side where the front-end loader picks up will allow rainwater to drain away, so that drier materials go into the plant.
Testing and Analyzing:
The best time to test RAP is as the stockpile is being built at its final location. The recommended frequency for sampling the RAP for QC testing is at least one test per 1000 tons. At least five samples of each RAP stockpile should be obtained and tested before starting the mix designs. Further details on RAP testing methods and material properties that are needed are the subject of a future article. In QC testing and mix design, technicians often focus on average values of key properties such as asphalt content and gradation of the RAP aggregate. However, it is also important to understand the variability of a product. The most common measure of variability is the standard deviation, which is easily calculated for a set of test results in Excel or by a statistical package v
Randy West is the director for the National Center for
Asphalt Technology (NCAT) in Auburn, Ala.
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