A recent survey by the National Asphalt Pavement Association (NAPA) showed that state governments and agencies are increasing the adoption of sustainable technologies such as reclaimed asphalt pavement (RAP) nationwide. The survey stated that 66.7 million tons of RAP were collected in the United States during 2011 for use in new pavements.
The amount of RAP used in a given mix design is usually spelled out in state specifications, yet there are some asphalt projects in the state of Missouri that are pushing the envelope and using high RAP mixes (up to 50% and more).
West Contracting, headquartered in St. Louis, with asphalt plants in Sullivan, House Springs and Pacific, MO, has had the opportunity to provide mix for several projects using high RAP content.
Steve Jackson, PE, technical manager for West Contracting, feels we are just seeing the beginning of high RAP mixes.
“The asphalt industry has been using recycled products for decades to improve our roads,” he says. “In the short term, with decreasing highway funding to the states, using more RAP in asphalt pavements will provide a more economical way to meet the growing demands for infrastructure rehabilitation and improvement. In the long term, I think that a 100% recycled asphalt pavement that does not require heat is achievable.”
There are several benefits to high RAP mixes, including decreased demand for virgin aggregates and new asphalt binder. But there are many considerations if you’re planning on running high RAP mixes at your plant.
Producing the RAP
The best way to get a quality product is to start at the beginning, so if your milling crews are producing the RAP, emphasize the importance of maintaining a constant quality when milling.
“A consistent feed of millings is also going to make it easier to minimize the dust while screening or crushing the millings,” says Jackson.
If you can, screen your millings to minimize the amount of dust generated compared to crushing the millings. “We are lucky to work with Doug Hill at Aggrecon Crushing and Recycling and his experience crushing millings allows us to use our oversized clean millings to produce a consistent crushed RAP that matches our screened RAP.”
When crushing millings, be aware of felt fabric, says Jackson. Felt fabric that is placed between layers to aid with crack propagation can become a problem when you start to crush your millings. “It can build up on the crushing unit and have to be cleaned off,” he says. “The fabric can also become wrapped around some of the moving parts on the crusher and cause them to lock up.”
Handling the RAP
When you use high percentage of RAP in an asphalt pavement, it becomes critical to handle the RAP in a way that won’t lead to segregation, says Jackson.
“Fractionating the RAP into different sizes will give you better control over the volumetric properties of the mix,” he explains. “When we started using Superpave in the mid 1990s, we started using more fractions of aggregate to give us better control of the mix and to optimize our mix designs. The same principles apply to RAP.”
If a contractor uses millings or a single, unfractionated RAP as 50% of the aggregate in a mix, then there will be a large amount of variation in the field volumetrics. “Many contractors have the skill to handle this variation, but it takes a lot of work,” says Jackson. “Fractionating the RAP shifted that workload to the off season when we can characterize the stockpiles better instead of reacting to problems on the job.”
The screens that you choose to fractionate your RAP on will be determined by the work demand in your area. If your local market is focused on thin lift overlays then you do not need a lot of 1” clean RAP.