Utilization of Recycled Materials in Illinois Highway Construction
Reclaimed Asphalt Pavement
- Reclaimed Asphalt Pavement (RAP) is bituminous concrete material removed and/or reprocessed from pavements undergoing reconstruction or resurfacing. Reclaiming the bituminous concrete may involve either cold milling a portion of the existing bituminous concrete pavement or full depth removal and crushing (1, 3).
- Physical Properties:
- RAP properties largely depend on its existing in-place components. There can be significant variability among existing in-place mixes depending on type of mix, and in turn, aggregate quality and size, mix consistency, and asphalt content. Due to traffic loading and method of processing, RAP is finer than its original aggregate constituents are; it is finest when milled (1).
- Engineering Value:
- RAP is produced by crushing and screening the material to a 1/4- to 1/2-inch in size. It is tested to ensure that the proper applicable gradation and quality is satisfied, and if so, the RAP is mixed with virgin aggregate and asphalt as needed, then placed. Since millings from different projects will have different characteristics, contractors must maintain separate stockpiles of milled material, and the properties of particular stockpiles will change as it used and reused (1, 3).
- Present Applications:
- As of the new policy brought into effect January 2000, the Department allows incorporating RAP into Superpave mixes. The amount of RAP allowed for low volume roads increased from 25 percent to 30 percent. For some non-critical mixes, such as the shoulder, base, and subbase, up to 50 percent RAP is allowed. For high-type binder courses, up to 25 percent is allowed. For surface courses, the amount allowed ranges from 10 percent to 15 percent for all but the highest volume highways. RAP is not allowed in the Department's highest-class bituminous concrete surface or polymer-modified mixes to maintain acceptable friction requirements (3, 7).
The Department also allows RAP to be used in place of aggregate or earth in some non-structural backfill situations. Last year, RAP was used in approximately 40 to 60 percent of the Department's most common surface and base course mixes, and over 60 percent of total shoulder mix tonnage (3, 7).
- Quantity Used:
- 623,000 tons (2001 MISTIC estimate).
- Economic Impact:
- In 2001, the Department has spent approximately $19,940,000 using RAP as a viable aggregate substitute for scarce bituminous resources.