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OCR for page 151
Reclaimed Asphalt Pavement 151 When RAP is used in HMA, the first step in the mixture design process is to obtain samples of the RAP. Samples should be taken from random locations in each RAP pile. A portion of each sample is then used to determine the average and standard deviation of the binder content and gradation. This information is used to estimate feasible RAP contents that will satisfy gradation and vari- ability requirements. The remaining portion of the RAP samples for each stockpile are combined to provide (1) a representative sample for determining RAP binder and aggregate properties and (2) material for preparing specimens for the volumetric design and performance analysis. If the desired RAP content exceeds 15% or a limit specified by the agency, the properties of the RAP binder are analyzed and used to create a blending chart for the new and RAP binder. This blending chart can be used to determine the maximum RAP content of a specified new binder to ensure the blended binder meets the specified grade. Or it can be used to determine the grade of new binder needed for the desired RAP content to ensure that the blended binder meets the specified grade. If more than one RAP stockpile will be used in a mixture, the binder analysis is performed using a blended sample of the RAP stockpiles. The results of the variability and RAP binder analyses are used to select the final RAP content and performance grade of the new binder that will be used in the mixture. The volumetric design and performance analysis then proceed in the same manner as for a mixture produced with all new materials. The only difference is the binder content in the RAP stockpiles must be accounted for when computing binder contents and preparing specimens. A Note on General Methods of Handling RAP This manual assumes that RAP will be collected and stored in discrete stockpiles, with each stockpile representing a single RAP source or several very similar and carefully handled RAP sources. There are several good reasons for handling RAP in this way. Perhaps most impor- tantly, it ensures that each stockpile is relatively uniform in character and will not vary too much as the stockpile is depleted during production. It makes characterizing the RAP simpler and more accurate, since the variability in the RAP source is minimized. Some agencies require that RAP be handled in this way and place restrictions on the size of RAP stockpiles that can be used at HMA plants. However, there are other ways to handle RAP in HMA plants. One approach gaining popularity is to fully process the RAP--to sieve and blend it into carefully controlled size fractions which are then stored and handled in much the same way as aggregates. This approach has many advantages, but because it is relatively new it is not directly addressed in this manual. Many aspects of the mix design process, such as determining binder grades for mix designs containing RAP, will be essentially the same regardless of how the RAP is handled. One important aspect of the mix design process that will depend on how RAP materials are handled at the plant is the effect of RAP variability on the allowable RAP content. Producers using highly processed RAP and other alternative approaches to handling RAP should rely on their experience and judgment in determining the maximum amount of RAP that can be used in HMA mix designs without unacceptably increasing production variability. Using HMA Tools to Design HMA Mixes with RAP Many of the equations used in the design of HMA mixes containing RAP are complex, and the calculations required to properly complete such designs can be tedious and prone to error. The HMA Tools spreadsheet has been constructed to simplify the incorporation of RAP into dense-graded HMA mix designs. Technicians and engineers need not be overly concerned with the mathematics involved in incorporating RAP into HMA mix designs, but technicians and engineers