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Milling can be carried out while a road or highway is kept open to traffic.
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The hot-mix asphalt (HMA) industry has been recycling on a large scale since the 1970s. Research on mix properties and modifications to plant equipment were quickly done in response to the asphalt shortages experienced at that time. Within a few years, recycling became a commonplace practice—so commonplace that today, asphalt pavement is America's most recycled material.

Over the years, contractors generally stuck with having one stockpile of reclaimed asphalt pavement (RAP) and feeding anywhere from 10% to 25% RAP into the mix. This produced significant cost savings, and the industry was content to remain at this level of recycling. It may be time to consider means for increasing RAP content of asphalt pavement mixes even further, especially in light of the increasing oil prices that directly affect the cost of asphalt.

RAP is a resource rich in asphalt and aggregate, and just as we test and process virgin materials, so too should we judge the quality of RAP and process it. The sizing of RAP will help to refine its use in HMA. For an HMA producer to maintain two or even three different size stockpiles of recycled material means greater flexibility in designing mixes for specific applications. For instance, in finer surface mixes, an increased amount of fine RAP can be employed, whereas a greater fraction of coarse-sized RAP can be used in large stone mixes.

Like any other material in HMA, RAP should be engineered into the mix, not simply dumped. Understanding the asphalt content and gradation will help the mix designer integrate the recycled material in the right proportion and adjust the virgin materials. If RAP is being used in large quantities, it may be advisable to extract and test the binder to see if it is highly oxidized and brittle. If it is, then the virgin asphalt may be decreased by one grade to ensure that the resulting mix is not brittle.

If a mix using all virgin materials has a required binder content of 5.5%, then a 30% RAP mix, where the RAP contains 4% asphalt, will reduce the amount of liquid asphalt cement by 1.2%. This means the quantity of liquid asphalt can be reduced by 22%. On 10,000 tons of HMA, this would mean a reduction of 120 tons of liquid asphalt. If the cost of asphalt cement is $250 per ton, this works out to a savings of $30,000. Obviously, the savings with RAP go beyond just the reduction in consumption of liquid asphalt. The aggregate in RAP is equally, if not even more, valuable.

Mixes with a high RAP content may pose special problems in terms of workability and compactability. While this may be alleviated somewhat by the use of a lower grade of virgin binder, consideration also might be given to the use of additives or processes that improve workability at high temperatures.

RAP has always been a valuable commodity, and today its benefits are more evident than ever before. By increasing the percentage of RAP in asphalt mixes, agencies can save taxpayers money, free up landfill space, and conserve precious natural resources.

–David Newcomb, P.E., PhD, is vice president–research and technology at the National Asphalt Pavement Association.