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12 A Manual for Design of Hot Mix Asphalt with Commentary Another type of HMA mixture is open-graded friction course (OGFC). OGFC mixtures contain very large amounts of coarse aggregate, with very little fine aggregate or mineral filler. The air void content is much higher than in conventional HMA, typically 15 to 20%. A large amount of high-performance binder is needed to provide adequate stability in these mixtures. OGFC mixtures are usually used as thin overlays, where they can help control noise and limit spray on wet pavements. OGFC mixtures are discussed in detail in Chapter 11 of this manual. HMA Mix Design Methods When HMA pavements first began to be constructed, mixture composition was determined based on the judgment and experience of the contractor or used proprietary mix designs. Some of these pavements performed well, others did not. In the 1930s, Bruce Marshall, an engineer working for the Mississippi Highway Department, developed a more rational system for designing HMA mixtures, which became known as the Marshall mix design method. This method of mix design became common by the 1950s and continued to be widely used through the 1980s. It was adopted for use by the U.S. Army Corps of Engineers (USACE) during World War II and was modified by that agency and by the many state highway departments that eventually chose the Marshall method of HMA mix design. Briefly, the Marshall mix design procedure relies on compacting specimens using a standard drop hammer over a range of asphalt binder contents. The binder content is selected to produce proper air void content and voids in the mineral aggregate (VMA). An essential part of the Marshall design method is the stability and flow test, which is an empirical procedure used to evaluate the strength and flexibility of the HMA mixture. Figure 2-7 shows a mechanical Marshall drop hammer and several Marshall specimens prepared in the laboratory. Francis Hveem, an engineer working for the California Division of Highways at about the same time Bruce Marshall was developing his procedure, developed an HMA mix design method adopted Figure 2-7. Marshall compactor and laboratory specimens for use in the Marshall mix design method.
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Background 13 by many agencies in the western United States. The Hveem method of mix design is unique in its use of the centrifuge kerosene equivalent (CKE) test to determine an initial estimate of the asphalt binder content for a given aggregate. Laboratory specimens are prepared using a kneading com- pactor and then evaluated using a stabilometer test and a swell test. At one time, a cohesiometer test was also used to evaluate HMA properties in the Hveem method. As with the Marshall mix design method, evaluation of mixture volumetrics was an important part of the Hveem procedure. In the late 1980s and early 1990s, the Strategic Highway Research Program (SHRP) was con- ducted by engineers and researchers at various universities and research organizations throughout the United States. The SHRP Asphalt Research Program was a 5-year, $50 million-dollar research program to develop improved test methods and specifications for asphalt binder and aggregates and an improved procedure for HMA mixture design and analysis. The performance grading system now used to specify binders in the United States and other countries was one of the products of SHRP. Another SHRP product was the Superpave system of mix design and analysis, often simply referred to as Superpave. This method is similar to the Marshall system in its use of volumetrics, but laboratory specimens are prepared using a gyratory compactor rather than a drop hammer. Superpave also includes a comprehensive set of requirements for aggregate gradations and property requirements. The Superpave method of mix design was meant to include mixture test methods and an associated computer program that would predict the performance of HMA pavements, as an aid in the design and analysis of mixtures. However, this computer program never produced reliably accurate predictions of rutting and fatigue cracking, and this aspect of Superpave was never implemented, although the Mechanistic-Empirical Pavement Design Guide (MEPDG) is in many ways a continuation of the SHRP efforts for flexible pavements. Figure 2-8 shows the Superpave gyratory compactor (SGC). In many ways, the Superpave system was a success. The performance grading system appears to do a better job of ensuring that asphalt binders have adequate stiffness at high temperature Figure 2-8. Superpave gyratory compactor (Courtesy of Pine Equipment Company).
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14 A Manual for Design of Hot Mix Asphalt with Commentary while remaining flexible at low temperatures for a wide range of applications, helping to provide resistance to both rutting and low-temperature cracking in HMA pavements. Very few pavements using mixtures designed using the Superpave system have exhibited excessive rutting. However, recently some highway agencies have expressed concern over surface cracking and high perme- ability in pavements using HMA designed using the Superpave system. Many highway agencies have modified the Superpave system to address these problems and others associated with local materials and conditions. A major research effort, continued since the implementation of SHRP, is under way to refine various aspects of this system, including volumetric requirements, com- paction levels, and specifications for aggregate properties and gradation and finally implement practical mixture performance tests and methods of analysis. This mix design manual is largely based on the Superpave system of mixture design and analysis, but an attempt has been made to address the perceived performance problems associated with some mixtures designed using this system. Also, the results and recommendations of some recent research projects have been adopted where such results have been published and presented to the paving technology community and favorably received. Because local conditions and materials vary significantly throughout the United States and Canada, an attempt has been made to provide more flexibility in the procedure used to design dense-graded HMA mixtures, compared to the Superpave sys- tem. This manual is also much more comprehensive than the procedures given in the original Su- perpave system; it includes design procedures not just for dense-graded HMA, but also for gap- graded or SMA-like mixtures and OGFC types. Because of these changes, the term "Superpave" is not used to describe the procedure, tests, and requirements used in the mix design methods presented in this manual. Bibliography Referenced AASHTO Standards T 283, Resistance of Compacted Asphalt Mixture to Moisture-Induced Damage Other Publications The Asphalt Institute (2007) The Asphalt Handbook (MS-4A), 7th Ed., 832 pp. The Asphalt Institute (2001) Introduction to Asphalt (MS-5), 8th Ed., 74 pp. The Asphalt Institute (2001) Superpave Mix Design (SP-2), 128 pp. The Asphalt Institute (1997) Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types (MS-2), 6th Ed., 141 pp. Brown, E. R., et al., (2004) NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot- Mix Asphalt Pavements, Washington, DC, Transportation Research Board, National Research Council, 37 pp. Hot-Mix Asphalt Paving Handbook (1991) AASHTO, FAA, FHWA, NAPA, USACE, American Public Works Association, National Association of County Engineers, James Sherocman (Consultant), USACE Publication UN-13 (CEMP-ET), July. McNichol, D. (2005) Paving the Way: Asphalt in America, NCAT, Auburn, AL, 304 pp. NCAT (1996) Hot-Mix Asphalt Materials, Mixture Design and Construction, NAPA, Lanham, MD, 585 pp.