Reclaimed Asphalt Pavement in Asphalt Mixtures: State of The Practice
CHAPTER 4. CONCLUSIONS AND RECOMMENDATIONS
Based on the information in this report, the following conclusions can be made regarding the state of the practice for RAP:
RAP is a valuable, high-quality material that can replace more expensive virgin aggregates and binders. The most economical use of RAP is in asphalt mixtures.
The use of RAP is primarily driven by the costs of virgin materials and transportation. Usage is optional and depends on the contractor to propose its use based on economic considerations, availability of materials, plant site, and production capabilities.
State transportation departments and contractors are reassessing the economic and environmental benefits of allowing higher percentages of RAP in premium pavements and asphalt surfaces while also maintaining a high-quality, well-performing pavement infrastructure. However, many States are not currently tracking the amount of RAP used or the cost savings associated with the use of RAP.
More widespread use of higher amounts of RAP in asphalt mixtures requires support from State transportation departments and contractors. State transportation departments have expressed concern over the lack of guidance and information on the performance of high RAP mixtures. Furthermore, some State transportation departments have previously had poor experience with RAP in asphalt mixtures, necessitating contractors to consistently demonstrate the ability to produce high-quality RAP mixes. There is a need for national guidance on best practices when using RAP and documented information about long-term performance of high RAP pavements.
It is estimated that the average use of RAP across the United States is 12 percent. However, according to State transportation department specifications, there is the potential to use up to 30 percent RAP in the intermediate and surface layers of pavements. As a result, the overall amount of RAP used in asphalt mixtures can be increased.
RAP mixtures must meet the same mix design specifications required for virgin mixtures. The most common method of mix design for RAP mixtures, including high RAP, is the Superpave® mix design process. For QC purposes, most State transportation departments do not have additional means of determining acceptability of high RAP mixtures.
The most common challenges to increasing the use of RAP are State transportation department specification limits, lack of processing (i.e., variability of RAP), lack of RAP availability, and past experiences. Furthermore, in regards to performance, the two most common concerns are the quality of the blended virgin and RAP binders, especially for high RAP mixes, as well as stiffening of the mix from high RAP quantities and resulting cracking performance.
The performance and life of pavement containing up to 30 percent RAP is similar to virgin pavements with no RAP. A survey of LTPP sections containing at least 30 percent RAP showed similar performance to virgin sections. The LTPP pavement sections were located throughout the United States and Canada. An analysis of Florida pavements show similar pavement life for pavements containing no RAP and pavements containing 30 percent RAP.
The following summary of current recommendations is provided to increase the use of RAP and ensure asphalt mixture quality. However, it should be noted that ongoing and future research may lead to refinements of these best practices.
Proper techniques should be used for obtaining, stockpiling, and processing RAP to maintain its quality. For high RAP mixtures, fractionation of the RAP material should be considered.
Sampling and testing of the RAP material should be performed. Random samples should be taken to identify the variability of the RAP material properties. Test results, including composition and variability, should be provided to the State transportation department or owner.(7)
The RAP material should be properly characterized for mix design purposes. The laboratory mixture design should be established using RAP as a component. This is especially important for State transportation departments considering permitting up to 20 percent RAP in mixtures without changing to a softer grade asphalt binder.
With RAP contents greater than 25 percent, careful consideration should be given to the selection of the grade of asphalt binder added to the recycled asphalt mixture according to State transportation department specifications.
Production sampling and testing programs should be implemented to verify mixture design assumptions including the asphalt binder blend properties, especially for high RAP mixtures.(7)
Evaluating mixture performance of the designed asphalt mixture containing RAP, especially high RAP, is recommended. There are a variety of performance tests available for evaluating the probable permanent deformation, fatigue, and thermal cracking performance of compacted asphalt mixtures.
Plant production best practices used in the production and placement strategies during the construction of HMA will address concerns when using high RAP. The plant production best practices should regularly monitor and adjust for moisture content and scalping screens. High-quality high RAP mixtures are achieved with processing and production best practices, which result in cost and energy savings and reduced emissions.
Further documentation of the production, construction, and long-term performance of high RAP mixtures is needed.
Consideration should be given to including documenting RAP use in a pavement management system with details concerning RAP quantities used, sources, and placement details.
Page Owner: Office of Research, Development, and Technology, Office of Infrastructure, RDT