According to the 1993 AASHTO Guide, the option of recycling the existing pavement or material from other sources may be a reason to select a particular type of pavement. Future recycling opportunities may also factor into the decision.
ASPHALT IS AMERICA’S MOST RECYCLED MATERIAL
Asphalt is unique in composite construction materials in that when it is recycled back into 8a new asphalt mix, the binder is reused as well as the aggregate. Reclaimed asphalt pavement (RAP) saves on the amount of new binder needed in an asphalt mixture because the RAP binder still functions to coat the RAP aggregate and when it bonds to the new asphalt, it helps create the needed cohesion. This saves the need for both new asphalt and new aggregate, creating a more sustainable product.
About 90 percent of the asphalt pavement that is removed is recycled back into pavement. This amounts to about 100 million tons of material annually that is saved from landfills. Recycling saves resources in terms of virgin asphalt and aggregate, resulting in simultaneous savings in energy and cost. The “mill and fill” operation, frequently used in the surface renewal process, allows properly sized recycled material to be taken from the roadway and placed in a stockpile ready for use.
OTHER REUSED MATERIALS CAN ENHANCE ASPHALT PERFORMANCE
There is a variety of by-products from other industries that can serve useful functions in asphalt pavements. For instance, roofing shingles provide asphalt, fine aggregate, and fibers. The asphalt binder available in roofing shingles has been especially crucial in certain areas in reducing the demand for new asphalt. Adding as little as 5 percent waste roofing shingles to an asphalt mixture can save as much as 20 percent of the total binder needed in the mix. Also, steel slag has been used for many years as a hard, durable aggregate in asphalt pavements. Rubber from waste tires is being successfully used in asphalt mixtures in a number of states, most notably Arizona, California, Florida, and Texas. Sand from foundry castings can be used as a portion of the fine aggregate in the mix.
Simply viewing asphalt mixes as a depository for waste materials is not the goal the industry is striving to achieve. As long as waste materials provide improved economy, environmental friendliness, future opportunities for asphalt recycling, and engineering performance, their use in asphalt mixes should be encouraged. Each of these considerations should be weighed before introducing them.