A paving project in Maryland involved the removal and replacement of a failed concrete pavement at an intersection, at left shows a roller at work and, at right, a paving operation. The new pavement is a Superpave mixture. Photos: NAPA
Although the practice of recycling hot-mix asphalt (HMA) pavement has been in widespread use since the early 1980s, the industry has became comfortable with using only a nominal amount of recycled material in asphalt mixtures. It has been common to find 10% to 20% reclaimed asphalt pavement (RAP) in new, dense-graded HMA. Now that energy prices are rising, with infrastructure budgets not yet adjusted to keep up with the increases, it is time look for ways to amplify our reuse of asphalt pavements to between 30% and 40% where possible.
The economic and environmental benefits of using recycled asphalt were thoroughly documented when the practice was begun more than 25 years ago. At that time, some field trials were constructed with mixtures containing up to 80% RAP. Subsequent limitations on usage of the recycled pavement were due to the emission controls in place then, the cracking of some projects containing a high percentage of reclaimed asphalt, and the lack of a performance test to indicate the behavior of the mix. But with current plant designs, emissions can be minimized at high RAP contents, and with proper mix design and assessment, performance problems can be addressed.
In high-RAP-content mixtures, it is important to understand the characteristics of the recycled materials as well as those of the virgin materials. With this knowledge, we can develop strategies for percentages of RAP to be used in different kinds of HMA, devise stockpiling plans to optimize recycled usage, and develop testing programs for materials and mixtures.
The location of the mixture containing RAP is important, too, since lower layers in the pavement are not subject to as much temperature change or rutting as the surface course; these factors will affect the performance grade of the binder needed. The asphalt content of the recycled pavement must be determined by ignition oven or solvent extraction to properly assess the grade and amount of virgin binder needed.
As the percentage of recycled asphalt in the mixture is increased, the aggregate gradation and other aggregate properties become increasingly important. The proper characterization of RAP aggregate requires that the aged asphalt binder be removed via ignition oven or solvent extraction prior to grading and other types of aggregate testing. The properties of the total combined aggregate (reclaimed pavement aggregate mixed with virgin) must be considered when compared to criteria for aggregate in the final mix.
The mix design process is slightly different from that required for hot-mix composed of virgin materials. During sample preparation, a predetermined percentage of RAP is added to the virgin aggregate after the virgin aggregate has been heated. The virgin asphalt is added, and then the process is similar to the volumetric design of any pavement made with hot-mix asphalt.
The National Cooperative Highway Research Program's Report 452 provides useful step-by-step RAP mix design procedures and examples (see Additional Information". The use of higher recycled content in mixtures comes at a time when agencies and contractors are moving toward mix design methods that include performance tests. These types of accelerated laboratory tests focus on the mixture behavior needed to resist rutting, cracking, and environmental degradation rather than mix designs based strictly on volumetric heuristics.