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CHAPTER 8 Design of Dense-Graded HMA Mixtures This chapter presents a comprehensive procedure for the design of dense-graded HMA mixtures. Although the procedures described have been specifically selected for use in designing dense-graded mixtures, most can be applied to the design of other mix types with little or no modification. Before reading this chapter, engineers and technicians should make certain they understand the information presented in earlier chapters of this manual. Many steps in the procedure are covered in other chapters of the manual. For example, binder tests and grading are discussed in Chapter 3, aggregate properties and specifications in Chapter 4; Chapter 5 is a detailed discussion of the volumetric composition of HMA; and Chapter 6 deals with the performance of HMA, including tests for evaluating rut resistance and fatigue resistance. To keep this chapter as simple and direct as possible, the details of handling RAP in an HMA mix design are not addressed here but are covered in Chapter 9. Engineers and technicians using RAP in their mix designs should make sure they read Chapter 9 carefully and understand the proper procedures for incorporating RAP into HMA mixtures. Chapter 6 presents an in-depth discussion of performance testing of HMA mixtures, including background information important in understanding how and why many of these procedures were developed. In Chapter 8, specific tests recommended for use in HMA mix design are summarized and the specific conditions suggested for running these tests are described. Where appropriate, standard test methods are given so that technicians and engineers can refer to them for details on each procedure. This chapter begins with a short discussion of the history of HMA mix design, including com- ments on the Marshall, Hveem, and Superpave procedures. The suggested mix design procedure is then summarized, followed by a detailed, step-by-step presentation. Specific examples are included at several points in the discussion. Frequent reference is also made to the HMA Tools spreadsheet, which includes provisions for performing most of the calculations needed in the mix design process. Other Mix Design Methods Three HMA mix design methods have been widely used in the United States and Canada during the past 60 years: the Marshall method of mix design, the Hveem method, and the Superpave method of mix design and analysis. The Marshall and Hveem methods were largely developed in the 1940s and were the first systematic and widely used methods of HMA mix design. The Superpave system, developed during the late 1980s and early 1990s, was intended to improve on the Marshall and Hveem procedures. (Leahy and McGennis wrote an excellent article summarizing the evolu- tion of HMA mix design systems, "Asphalt Mixes: Materials, Design and Characterization," which appears in Volume 68A (1999) of the Journal of the Association of Asphalt Paving Technologists.) The sections below provide a brief background on these other mix design methods, in order to help provide perspective to the procedure described later in this chapter. Of special significance 101

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102 A Manual for Design of Hot Mix Asphalt with Commentary is the Superpave system, which is the basis for the mix design method presented in this manual. Because of this close relationship, the Superpave system is described in greater detail than the other two mix design methods. Marshall Mix Design The Marshall method of HMA mix design was originally developed by Bruce Marshall in the 1940s, while he was working for the Mississippi State Highway Department. The procedure was later adopted and further refined by the U.S. Army Corps of Engineers (USACE). A wide range of engineers and organizations have proposed improvements and variations to this design procedure; publications of the Asphalt Institute are considered by many to be the best references for this and many other mix design methods. There are four primary features of the Marshall method: 1. Asphalt binders and aggregates should be selected to meet all applicable project specifications. 2. Evaluation of trial mixtures is done using laboratory-compacted specimens 100 mm in diameter by approximately 70 mm thick, compacted using a standardized drop hammer (see Figure 8-1). 3. Laboratory-compacted specimens must meet requirements for air void content and VMA, and, in some cases, VFA. 4. Laboratory-compacted specimens must also meet requirements for stability and flow-- properties related to strength and flexibility that are determined in a quick and simple mechanical test. The specific requirements for air void content, VMA, VFA, and stability and flow varied over time and from agency to agency. In the Asphalt Institute's publication Mix Design Methods for Asphalt Concrete and Other Mix Types (MS-2), the requirements are as follows: Compaction level (number of "blows") varies with traffic level. For light traffic, the specified compaction level is 35 blows; for medium traffic, 50 blows; and for heavy traffic, 75 blows. Figure 8-1. Marshall compaction hammer.

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Design of Dense-Graded HMA Mixtures 103 Design air void content ranges from 3 to 5%. Minimum values for VMA depend upon nominal maximum aggregate size; for a 9.5-mm mix, the minimum VMA is 14% for an air void content of 3%, 15% for an air void content of 4%, and 16% for an air void content of 5%. For a 12.5-mm mix, minimum VMA values are 1% lower; for a 19-mm mix, minimum VMA values are 2% lower. As aggregate size increases, minimum VMA decreases. The allowable range for VFA depends on traffic level (light, medium or heavy). For light traffic, allowable VFA ranges from 70 to 80%; for medium traffic, the allowable range is from 65 to 78%; for heavy traffic, the allowable range is from 65 to 75%. Stability and flow values also depend on traffic level. Minimum stability values are 3340 N for light traffic, 5340 N for medium traffic, and 8010 N for heavy traffic. The allowable range for flow (in flow units of 0.25 mm) is 8 to 18 for light traffic, 8 to 16 for medium traffic, and 8 to 14 for heavy traffic. An important aspect of the Marshall design method is compaction of laboratory specimens over a range of asphalt binder contents and evaluation of the mixture volumetrics over this entire range in order to determine the optimum binder content. Originally, Bruce Marshall recommended producing HMA mixtures at the lowest possible VMA, since this produced the densest, most stable mixtures and required the lowest asphalt contents. However, engineers eventually realized that such mixtures often exhibited durability problems, and minimum VMA values such as those given by the Asphalt Institute were established. Although the current Asphalt Institute version of the Marshall method does not explicitly specify maximum values for VMA, the combination of specifying air void content and VFA in fact indirectly establishes such maximums. For example, at 4% air voids, a maximum VFA value of 75% implies a maximum VMA value of 16%. The Marshall design method was widely used in the United States and Canada through the early 1990s, at which point the Superpave Method of Mix Design and Analysis (the Superpave system) began replacing it. At the writing of this manual, the FAA and most United States military organi- zations were changing their mix design methods from the Marshall method to the Superpave system. For example, Airfield Asphalt Pavement Technology Program (AAPTP) Project 04-03, started in 2007, funded by the FAA, involves developing an implementation plan for the Superpave mix design system for airfield pavements. Hveem Mix Design Method The Hveem method of mix design was developed at about the same time as the Marshall method, by Francis Hveem, who was at the time a materials and research engineer for the then California Department of Highways. The Hveem method was not as widely used as the Marshall method, but was used by many highway departments in the Western United States until the Superpave method became the generally accepted method for HMA mix design. There are several important features of the Hveem method: 1. Like the Marshall method, asphalt binder and aggregates should meet all applicable project specifications. 2. Evaluation of trial mixtures is done using the same size specimens as those used in the Marshall method (100 mm diameter by 70 mm thick), but the specimens are compacted using a kneading compactor, rather than a Marshall drop hammer. 3. The asphalt binder content for trial mixtures is estimated using the centrifuge kerosene equivalent (CKE) test and a series of charts. This is a somewhat complicated procedure; a detailed description can be found in the Asphalt Institute's Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types (MS-2).

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104 A Manual for Design of Hot Mix Asphalt with Commentary 4. Laboratory-compacted specimens must meet requirements for the stabilometer and swell tests, and sometimes the cohesiometer test. 5. Although volumetric analysis is recommended, the Hveem method does not include specific requirements for air void content, VMA, VFA, etc. The Superpave System The Superpave Method of Mix Design and Analysis was developed during the Strategic Highway Research Program (SHRP) and was intended to be an improvement over the Marshall and Hveem procedures. The Superpave method was meant to be based on engineering principles and performance-related properties. The Asphalt Institute publishes an excellent manual covering the Superpave system, Superpave Mix Design (SP-2). Several AASHTO standards deal with the Superpave system of mix design, the most important being AASHTO R 35, Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt, and AASHTO M 323, Standard Specification for Superpave Volumetric Mix Design. In many ways, the Superpave system can be viewed as an evolution of the Marshall method of mix design. Like the Marshall method, it places much emphasis on volumetric analysis and requirements for air void content, VMA, and VFA. In fact, the specific volumetric requirements for the Superpave system and for the current Marshall method are quite similar. However, as discussed below, the Superpave system is more comprehensive than either the Marshall or the Hveem method of mix design and includes important innovations. The five primary features of the Superpave system are as follows: 1. Selection of the asphalt binder grade is made on the basis of local climate and expected traffic level. Binder grading is done using a performance-based system of tests and specification requirements, commonly, but somewhat redundantly, referred to as "PG grading." 2. Aggregate gradations are given for each aggregate size (NMAS), ranging from 4.75 to 37.5 mm. In the early versions of the Superpave system, the aggregate gradations included a restricted zone--a region that should be avoided to ensure against tender mixes--but the most recent Superpave standard (AASHTO M 323-07) no longer includes this feature. 3. Evaluation of trial mixtures is done on laboratory-compacted specimens 150 mm in diameter by about 100 mm thick. These specimens are compacted using the Superpave gyratory compactor (see Figure 8-2). As in the Marshall method, the level of compaction varies with the expected traffic level. 4. Trial mixtures are evaluated on the basis of volumetric composition, with requirements for design air void content, VMA, and VFA. 5. All mixtures must be evaluated for moisture resistance using a standard test (AASHTO T 283). Unlike the Marshall method, there is no final test of stability, flexibility, or strength. The Superpave mix design process is generally described as consisting of four primary steps: 1. Selecting materials 2. Selecting the design aggregate structure 3. Selecting the design binder content 4. Evaluating moisture resistance The selection of the design aggregate structure is based on determining an estimated design binder content and preparing test specimens using three different aggregate gradations. The gradation giving a mixture composition closest to the given requirements is selected for continued refinement, the first step of which is the selection of the design asphalt content. This is done by preparing specimens with the design aggregate gradation, using a range of asphalt contents. The design asphalt content is that giving an air void content of 4%. Moisture resistance is evaluated using AASHTO T 283. This procedure is discussed both in Chapter 6 of this manual and toward the end of this chapter.

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Design of Dense-Graded HMA Mixtures 105 Figure 8-2. Superpave gyratory compactor (courtesy of Pine Equipment Company). Procedures for volumetric analysis and requirements for air void content, VMA, and VFA are similar to those used in the Marshall method. For example, for a 9.5-mm mix, the minimum VMA value is 15%, exactly the same as the minimum VMA for a 9.5-mm Marshall mix designed at 4% air voids. As in the current version of the Marshall method, VFA values depend on traffic level--at higher traffic levels, the allowable range for VFA in the Superpave system is 65 to 75%. Compaction of specimens in the laboratory and requirements for compaction properties are more complicated in the Superpave system than for the Marshall method. Currently, three compaction points are defined during the compaction process: Ninitial, Ndesign, and Nmax, although some engineers and researchers have recently questioned the need for Ninitial and Nmax, and these requirements might be eliminated in the near future. Within the original Superpave system a maximum value of density at Ninitial was specified in order to help ensure proper aggregate structure, since it was believed that mixtures that compacted too quickly had poor aggregate structures. Ndesign was (and still is) the actual point at which air void content, VMA, and VFA are specified-- the design compaction level. Nmax is the maximum number of gyrations applied to the specimen. A maximum density at Nmax was specified in the original Superpave system in order to ensure that the mix would remain stable at the expected maximum traffic level. From the mid 1990s to the mid 2000s, the Superpave system gradually replaced the Marshall and Hveem methods of mix design in most highway agencies. However, as it was being adopted, many engineers thought that the specific requirements given in the Superpave system were not providing HMA mixtures with the best performance for their local conditions and climates. For this reason, many state highway agencies have modified some of the requirements for HMA mixtures designed using the Superpave system. Many engineers have also criticized the Superpave system for its lack of a "proof" test, that is, a final test of strength or stability, similar to the Marshall stability and flow test. Since the initial implementation of Superpave, significant research has been done on various aspects of HMA mix design and analysis, suggesting that a number of changes