Featured Articles: Keys to Managing RAP Variability
| October 01, 2009 |
Specifying agencies sometimes say that they limit the amount of reclaimed asphalt pavement (RAP) in asphalt mixes because these stockpiles are highly variable. Therefore, the agencies reason that allowing higher percentages of RAP could have a negative impact on the consistency of asphalt mixes and diminishes the performance of asphalt pavements. This perception of high variability may come from the notion that all RAP stockpiles are a heterogeneous agglomeration of reclaimed materials from a range of sources.
Many contractors maintain separate stockpiles of RAP obtained from different sources. When a large quantity of millings is obtained from a single project, for example 5000 tons or more, then it makes perfect sense to keep this RAP stockpiled separately from other RAP. Considering that the millings came from a given project that was originally built with under strict quality assurance specifications 15 to 20 years ago, it is fairly safe to assume the millings will be a consistent and high-quality material. In many areas of the country, this milled material is used directly in mix designs without further processing. An advantage of this practice is that the dust content of the RAP is kept as low as possible. High dust in RAP often limits how much RAP can be used and still meet mix design requirements.
On the other hand, for small quantities of millings, most contractors do not consider it worth the trouble of setting up a mix design for a small production run. Therefore, they typically combine small milling quantities with other sources of RAP to create the GOK stockpiles. It is common for the multiple-source RAP stockpiles to contain paving materials from small milling jobs, plant waste, rejected asphalt pavement material, and pavement rubble from complete demolition of roads or parking lots.
What contractors and agency personnel want to know is, what are the best ways to process multiple-source RAP stockpiles to achieve consistency in the material. The National Center for Asphalt Technology (NCAT) in Auburn, Ala., is working to find the best RAP management practices by surveying contractors across the U.S., examining quality control data, and identifying what techniques provide the most uniform RAP materials. NCAT has collected information on the current state of the practice for handling RAP and has gathered quality control (QC) data from contractors on their processed materials. This information has been used to draft preliminary best practices as outlined below.
Many contractors use independent companies to do their RAP crushing or have portable crushing units that move from one plant site to another. RAP crushing units differ by crusher type, screening capability, and other factors, so that there are numerous variations in processing operations.
However, the following general principles work well for achieving consistent RAP products that can be used successfully at higher percentages in new mixes:
NCAT tabulated standard deviations from more than 70 RAP stockpiles and from more than 60 crushed-stone products from quarries in six states. Analysis of the data has revealed some interesting results. First of all, contrary to the perception of many people, the analysis shows that RAP stockpiles are as consistent as virgin aggregates. Researchers compared the standard deviations of the percent passing the median sieve (the sieve with closest to 50 percent passing) for all RAP and all aggregate products. The mean of the standard deviations for RAP was statistically less than that of virgin aggregate for the median sieve. The variances of the standard deviations on the median sieve were the same for RAP and aggregate. The same analyses were conducted on the standard deviations for the percent passing the No. 200 sieve. In this case, the means and variances of the standard deviations for RAP and virgin fine aggregates were not statistically different.
A statistical analysis was also conducted to compare variability of fractionated RAP stockpiles to non-fractionated RAP. For this analysis, we compared standard deviations of asphalt content, percent passing the median sieve, and percent passing the No. 200 sieve. Surprisingly, the results do not show that fractionating RAP into different sizes will improve the consistency of the material. We expected that fractionating would further reduce variability in RAP stockpiles, but the data from contractors did not prove that out.
An important lesson from this analysis is that agencies should not use a method specification like requiring fractionation of RAP with the expectation that it will result in a more consistent RAP. A better approach is to set variability limits for RAP stockpiles and allow the contractor to do what is best and most economical to meet those limits. Based on the collection of RAP QC data from contractors we have so far, the following preliminary variability limits are suggested.
The above benchmark variability values may need to be revised in the future as more information is collected. However, using these kinds of limits for quality control of RAP stockpiles becomes more important when higher RAP contents are used in asphalt mixes.
After assessing their own RAP QC data, some contractors will find that they need to improve their processing operations. The eight basic principles above are a good place to start.
Although we joke about what might be in a GOK pile, it behooves the contractor to be diligent in keeping deleterious materials out of any RAP stockpile from the beginning. Contamination can occur from dumping general road debris with dirt and vegetation on the pile, including milled-up paving fabrics in the pile, or just because an uninformed or misinformed truck driver believes that the GOK pile is a place to dump construction trash.
Mix as You Feed:
A common practice is to use a backhoe, excavator, or front-end loader to dig into a GOK pile to feed into a RAP crushing unit. As the GOK material is being excavated, it is important to randomly dig into different areas of the pile so that the material going into the crusher at any time gets mixed up and is not just from one place in the pile.
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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