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OCR for page 148
CHAPTER 9 Reclaimed Asphalt Pavement The asphalt paving industry is a leader in the use of recycled products. HMA pavement is the most recycled material in the world. In addition to recycled asphalt pavement (RAP), other recycled materials are routinely used in HMA mixtures including Foundry sand Glass Roofing shingles Slag Tire rubber Recommendations for designing HMA with recycled materials other than RAP are not included in this manual. In areas where they are used, HMA producers have worked closely with agencies to modify mixture design and production methods to effectively use recycled products. Engineers and technicians considering the use of such products in HMA should obtain specific guidance from agencies with substantial experience in their use. The references at the end of this chapter include several publications addressing various recycled products used regionally in HMA. RAP, on the other hand, is a recycled product that is used extensively in HMA throughout the United States. RAP is old asphalt pavement that has been removed from a road by milling or full depth removal. With appropriate mixture design and production considerations, RAP can be reused in HMA to produce mixtures meeting normal specification requirements. The use of RAP in HMA eliminates the need to dispose of old asphalt pavements and conserves asphalt binders and aggregates. This results in significant production cost savings and benefits to society. RAP has been effectively used in HMA since the mid 1970s. Advances in HMA plants and processing equipment make it possible to consider mixtures with RAP contents of 50% or more. Experience has shown that, when properly designed and constructed, HMA mixtures with RAP will perform as well as mixtures produced from all new materials. The remainder of this chapter presents recommended methods for designing HMA mixtures with RAP. These methods are based primarily on those contained in NCHRP Report 452: Recommended Use of Reclaimed Asphalt Pavement in the Superpave Mix Design Method: Technician's Manual. General Mixture Design Considerations for RAP The design of HMA containing RAP assumes that the mixture is produced using a process that results in complete mixing of the RAP with the new binder and aggregate. When complete mixing occurs, the total binder content of the mixture includes both the binder contained in the RAP and the amount of new binder added. In this situation, the properties of the binder in the mixture are the blended properties of the new binder and the RAP binder. Additionally, the 148

OCR for page 148
Reclaimed Asphalt Pavement 149 gradation and other properties of the aggregates are those for the combined blend of the new and RAP aggregates. An important consideration in the production of HMA containing RAP is ensuring that adequate mixing of the new and RAP materials occurs. The design of HMA containing RAP requires substantial testing to characterize the properties of the RAP--testing that is not needed in mix designs that do not contain RAP. To effectively design and produce any mixture with RAP, the following properties of the RAP must be determined: RAP binder content RAP aggregate gradation Specific gravity of the RAP aggregate Angularity of the coarse aggregate particles in the RAP Flat and elongated particles for the coarse aggregate in the RAP Angularity of the fine aggregate in the RAP To determine these properties, the binder must be removed from the RAP using either an ignition oven or solvent extraction. The ignition oven is preferred when correction factors can be reasonably estimated and it is known that the properties of local aggregates are not altered substantially by exposure to the high temperatures in the ignition oven. When the RAP is limited to a low percentage of the new mixture, 15% or less (as a percentage of the aggregate blend), it is not necessary to determine the properties of the RAP binder. Small amounts of the aged RAP binder have little effect on the properties of the mixture. When higher percentages of RAP are used, the stiffness of the RAP binder must be considered. This can be done in several ways. Some agencies simply require that mixtures containing RAP use a softer binder than normally used in their region. A more precise approach involves using a blending chart or a spreadsheet to estimate the grade of the binder formed by blending the RAP binder and the new binder. When a blending chart (or an appropriately designed spreadsheet such as HMA Tools) is used, samples of the RAP binder must be obtained for testing through the use of solvent extraction and subsequent recovery of binder. The recommended blending analysis considers the high, intermediate, and low pavement temperature properties of both the RAP binder and the new binder. The objective of the analysis is to obtain a blended binder meeting the performance grade requirements for the environment where the pavement will be constructed. The range of RAP that can be feasibly added to a mixture is an important consideration in the design of HMA with RAP. The minimum RAP content depends on the capability of plant equipment to accurately and consistently add the RAP. For most plants, this amount is on the order of 10 percent. The maximum amount of RAP that can be added to a mixture depends on several factors including Amount of RAP available Specification limits Capability of the hot-mix plant to dry, heat, and effectively mix the RAP material Gradation of the RAP aggregate, particularly the amount of material passing the 0.075-mm sieve Variability of the RAP Properties of the RAP binder and available new binders Often the maximum RAP content is governed by the amount of material in the RAP passing the 0.075-mm sieve and the variability of the RAP. In recent years, equipment to size (or fractionate) RAP has been developed to overcome these limitations. This equipment can separate RAP into various sizes for more effective use in HMA mixtures. Sizing the RAP reduces variability in the gradation and binder content and provides the flexibility to use specific components of the available RAP in different mixtures. A consequence of this practice is that the design of mixtures