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OCR for page 106
106 A Manual for Design of Hot Mix Asphalt with Commentary can be made in the Superpave system that would improve the performance of HMA mixtures. Perhaps most importantly, the experience of many engineers suggested that Superpave mixtures were difficult to compact and often exhibited only fair to poor durability. For example, the Virginia Transportation Research Council in Report VTRC 03-R15 concluded that Superpave mixes often lack sufficient binder content for adequate durability. After the 2003 paving season, New Jersey formed a task force on Superpave durability because of concern about raveling, segregation, and the generally dry appearance of Superpave mixes. Despite the possible shortcomings of the Superpave system, the mix design procedure presented here is not meant to replace the Superpave system but to build on it--correcting some of its shortcomings and incorporating the findings of recent research dealing with HMA mix design and performance. Many terms have been borrowed from the Superpave system and have identical or very similar meanings in this manual as they do within the Superpave system: Ndesign, binder performance grades, aggregate gradation control points, coarse and fine aggregate angularity. There are, however, several important differences between the Superpave method and the mix design method described in this manual. The Superpave method deals only with the design of dense-graded HMA mixes, while this manual addresses not only dense-graded HMA, but also stone-matrix asphalt (SMA) mixes (Chapter 10) and open-graded friction course (OGFC) mixes (Chapter 11). In the design of dense-graded HMA, there are three important differences between Superpave system and the mix design method described in the remainder of this chapter: 1. Developing mix designs containing RAP is addressed directly and thoroughly (see Chapter 9); the original Superpave system did not address RAP directly, although various modifications to address RAP usage were developed and implemented over the past few years. 2. In the Superpave method, one of the steps of the mix design process is selecting the design asphalt binder content for the mixture. This involves preparing three or four mixtures with the selected aggregate structure (gradation) over a range of binder contents and determining which best meets the mix specifications. In the method described below, the design asphalt content is determined early in the procedure and maintained throughout the design process. To meet the requirements for VMA and air void content, the aggregate gradation is varied, rather than the binder content. This emphasizes the importance of the design binder content and helps ensure that the final mix has the proper amount of binder. It also prevents unnecessary work in preparing and evaluating trial mixes that do not contain the proper amount of asphalt binder. 3. The Superpave method included no provisions for a final performance or "proof" test, such as the Marshall stability and flow. The method described below includes three different options for evaluating the rut resistance of dense-graded HMA mixtures as a final step in the mix design process. Overview of Design Method The method described below for designing HMA mixtures is similar to the Superpave method, but uses a larger number of simpler steps as follows: 1. Gather Information 2. Select Asphalt Binder 3. Determine Compaction Level 4. Select Nominal Maximum Aggregate Size 5. Determine Target VMA and Design Air Void Content 6. Calculate Target Binder Content 7. Calculate Aggregate Content 8. Proportion Aggregates for Trial Mixtures