If you have followed my blog, you probably know that I am a believer in preservation and recycling. We have a huge investment in our pavements, and it makes no sense to ignore their value. With time and traffic, our pavements are gradually wearing out. We are struggling to maintain the levels of service of our roads.
Recycling has gained a place in the overall strategy. We commonly think of recycling as including RAP (recycled asphalt pavement) and RAS (recycled asphalt shingles) in our hot mixes. And that’s a huge piece of the effort. But there are other types of recycling. In this entry, I’m going to focus on in-place recycling.
For those of you wanting more information on in-place recycling, the Asphalt Recycling and Reclaiming Association (ARRA) have an excellent publication, the “Basic Asphalt Recycling Manual.” This manual is available through ARRA members; you cannot purchase it directly. You can learn more at www.arra.org.
Asphalt recycling is not new, it dates back to the early 1900s. A hot recycling project was documented in the 1930s. Recycling methods and technologically have improved significantly in the last 25 years. Some of the benefits ARRA attributes to recycling include:
• reusing and conserving natural resources
• preserving the environment and reducing land-fill impact
• preserving roadway geometry and clearances
• correcting pavement profile and cross-slope
• removing potholes, raveling and rutting
• improving pavement smoothness, and
• reducing or eliminating reflective cracks.
In-place recycling has two major variations – cold in-place recycling (CIR) and hot in-place recycling (HIR). CIR is done, as it self describes, in place, on the old pavement, as contrasted to hauling to a mixing plant. CIR can include a number of pieces of equipment, including tanker trucks, milling machines, crushing and screening units, mixers, pavers and rollers. Some of the equipment may accomplish multiple functions. All together, the equipment is commonly referred to as a “train.”
When an asphalt emulsion or a recycling agent is used to bind the milled material, the treatment depth is typically 2 to 4 inches. When chemical additives, such as Portland cement or lime, are used, the cutting depth can reach 5 to 6 inches. Compaction of the processed material is critical and requires considerable effort. Large pneumatic (rubber-tire) rollers and vibrating steel-drum rollers are recommended.
HIR involves heating and softening the existing pavement and scarifying or milling to a depth of ¾ to 2 inches (3 inches with some equipment). The loosened material is mixed, placed and compacted with conventional paving equipment. New aggregate, asphalt mix, binder or recycling agents can be introduced, as needed. These new components are determined by a mix design process. There are three sub-categories of HIR – Surface Recycling, Remixing and Repaving. These processes require specific equipment, are suited for particular conditions, and have differing final surface approaches.
For more specific information or to locate a qualified in-place recycling contractor, contact ARRA. In-place recycling is a viable treatment, but experience and knowledge of the process is critical. Like any other asphalt work, it pays to do it right.
To contact Dwight, email him at talkingasphalt@asphaltinstitute.org.