Use of recycled asphalt pavement makes sense

By Jeff Winke

It makes total economic and environmental sense to use recycled asphalt pavement (RAP) in an asphalt mix design.

According to the Federal Highway Administration, there are approximately 2.8 million miles of paved roadways in the U.S. which meant that approximately 18 billion tons of asphalt were used to pave them. And since nearly all asphalt pavement material that is removed from existing roads can be recycled this provides a huge opportunity.

The management of RAP is important because it holds many resources with high value such as oil and sometimes scarce aggregate.

Cold milling is an integral part of the construction cycle of many roads. Asphalt milling removes an old and worn wearing course. It can also be used to improve the surface friction or remove ruts on a wearing course that is otherwise in good condition. Those asphalt millings can become recycled material that can be put back into future roads.

Best practices for milling should always be considered in order to produce a consistent, quality product. The speed of the milling machine can make or break both the quantity and the quality of the material milled. In order to achieve a quality milled material, start and stop slowly. Starting too fast will make a dip. When starting, move forward at a slow pace so the rear tracks can walk into the cut which allows the hydraulics for all legs to adjust accordingly. When stopping, slow down before coming to a complete stop. Also, be sure to maintain a consistent speed when milling. Milling at high speeds puts unneeded wear on the machine and will create a coarse pattern. This can result in larger RAP material, which means the material may have to be run through a crusher before it is useable.

Newer milling machines will have different drum speeds, which, of course, is different than machine speed. Running a milling machine at the highest drum speed at a lower machine speed will help achieve a fine quality texture. This also can make for better sizing of RAP material.
It is also recommended to mill surface and intermediate layers separately to help control quality.

Once asphalt millings are removed from the road surface, the contractor needs to find a place to store them until they can be reused.
The transportation, storage and utilization of RAP is always a logistical challenge, but the single most important best practice is to treat RAP the same way you treat virgin materials. This means always keeping the RAP clean and dry before it is reprocessed and put back on the roadways.
According to the National Asphalt Pavement Associations (NAPA) “Best Practices for RAP and RAS Management,” poor management of RAP stockpiles is commonly cited as a reason agencies are reluctant to increase allowable RAP contents in asphalt mixtures. For production of quality mixes with high RAP contents, excellent material management practices are essential.

When possible, store processed RAP stockpiles under cover, or at least in an area with a sloped surface and good drainage. NAPA recommends a paved surface underneath stockpiles to allow for proper drainage.

It is best to use RAP material within two to three months after producing it. Longer storage periods are acceptable when the material is under cover and protected. Also, avoid driving loaders on RAP stockpiles, which can lead to premature compaction of the processed RAP.

A producer should separate the material into two or more stockpiles, based on the size of the RAP particles. A common practice is to separate RAP into a coarse pile, with particles larger ½- or 3/8-inch in size, and a fine pile with particles of less than ½-inch or so. That lets contractors use fine RAP in finer asphalt mixtures and the larger-size RAP in coarse mixtures.

The most widely used sizes are ½- to 5/8-inch, however some states also allow producers to use smaller than 3/4-inch RAP in subbase and binder.

Another best practice is to produce only what you need for a two- to three-month period since stockpiles of crushed and classified RAP can have a shortened lifespan because the RAP can begin to gather moisture and re-bond to itself, which will result in large chunks and further processing would be needed before it can be used.

Once milled asphalt is crushed and reclassified by size, the asphalt producer can use the recycled material in its asphalt mixes. This helps save money on new raw aggregate products.

Once a high-quality RAP and virgin material mix is produced, it is up to the paving contractor to lay it down. If a RAP mixture is produced correctly, contractors should not have to change their paving process in any way. What they will see is a mix that works well, compacts well and doesn’t give them any difficulty.

State RAP usage rates vary from about 10% to 35%, with the average calling for a mix involving four parts virgin material and one part recycled material. If we double the amount of recycled materials in the mix, it can double the associated cost savings as well. When the ideal balance is achieved, a contractor can not only reduce its environmental impact, but can also save money.

Jeff Winke is a business and construction writer based in Milwaukee, Wisconsin. He can be reached through jeff_winke@yahoo.com