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A Manual for Design of Hot-Mix Asphalt with Commentary (2011)

Chapter: Chapter 8 - Design of Dense-Graded HMA Mixtures

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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 8 - Design of Dense-Graded HMA Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

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 C H A P T E R 8 Design of Dense-Graded HMA Mixtures

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. 102 A Manual for Design of Hot Mix Asphalt with Commentary Figure 8-1. Marshall compaction hammer.

• 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). Design of Dense-Graded HMA Mixtures 103

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. 104 A Manual for Design of Hot Mix Asphalt with Commentary

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 Design of Dense-Graded HMA Mixtures 105 Figure 8-2. Superpave gyratory compactor (courtesy of Pine Equipment Company).

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 106 A Manual for Design of Hot Mix Asphalt with Commentary

9. Calculate Trial Mix Proportions by Weight and Check Dust/Binder Ratio 10. Evaluate and Refine Trial Mixtures 11. Compile Mix Design Report Most of these steps are straightforward and easily accomplished. However, Steps 8 through 10 are more complicated and require some experience in order to perform them proficiently. The suggested procedures for performing each of these steps are described below, followed by an example. Step 1. Gather Information At the beginning of the mix design process, the technician must gather as much information as possible bearing on the mix design, including the design traffic level, the climate at the place of construction, information on available aggregates and binders, anticipated lift thickness, pavement type (that is, surface, intermediate, or base course), and any special issues pertaining to the mix design or pavement construction. Unfortunately, this information is often incomplete, and the technician must use her or his judgment to fill in the gaps. Frequently, the organization requesting the mix design will provide specific information concerning the aggregates and binders to be used, eliminating these steps from the mix design process and making the process of gathering information somewhat simpler. Generally, the information below is often useful during the mix design process: • Site Information – Geographic Location – Climate Relating to Binder Grade – Design Traffic Level • Construction Information – Lift Thickness • Pavement Information – Mix type (dense-graded, SMA, or OGFC) – Distance from Pavement Surface to Top of Layer • Materials Information – Information on Available or Recommended Aggregates  Nominal maximum aggregate size  Gradation  Specific gravity and absorption  Pertinent specification properties – Information on Available or Recommended Asphalt Binders  Performance grade (PG)  Specification test data, including mixing and compaction temperatures  Type of modification, if applicable  Other pertinent specification properties – Information on Other Mix Materials  Anti-strip additives  Properties of recycled asphalt pavement (RAP)  Properties of other additives such as ground rubber or fibers • Special Issues Pertaining To Mix Design – Unusual Specification Requirements – Use of Special Additives or Recycled Materials – Unusual Construction Constraints – Unusual Performance Constraints Design of Dense-Graded HMA Mixtures 107

In HMA Tools, the worksheet “General” should be filled out with information about the mix design. This includes the date, the name of the engineer or technician performing the mix design, the project name, the aggregate size (NMAS), the required binder grade, the target VMA, and the target air void content. Completion of this worksheet is essential, because much of this information is used in other parts of HMA Tools. Step 2. Select Asphalt Binder Details concerning the properties and specification of asphalt binders are presented in Chapter 3 of this manual; what is presented here is a concise description of the grade selection process. Details of the performance grading system are given in AASHTO M 320, Performance-Graded Asphalt Binder. Engineers and technicians unfamiliar with the performance grading system should review Chapter 3 and AASHTO M 320 before attempting to understand HMA mix design as presented in this manual. Selection of the asphalt binder grade in an HMA mix design will generally depend on five factors: 1. The base performance grade dictated by the climate 2. The grade adjustment required for traffic level and speed 3. The performance grade specified by the state highway agency or other authority 4. The effect of any RAP in the mix on the final effective performance grade of the binder 5. The available asphalt binders. In most cases, the performance grade will be specified by the state highway agency or other owner/agency and will not be selected during the mix design process. However, private clients will often leave binder selection up to the contractor or HMA supplier. Engineers and technicians involved in the design of HMA mixes should keep in mind that in many cases the available binder grades in a given geographic area are limited, given that state highway agencies tend to specify a limited number of binder grades, often referred to as a “binder slate.” For example, a state might specify only PG 58-28, PG 64-22, and PG 76-22 binder grades, even though some areas could use a PG 70-22 binder grade—in these locations, a PG 76-22 binder grade would be used. This is done to simplify the mix design process, and to limit the number of binders that must be produced by refineries or delivered to local terminals. In those special cases demanding a binder grade outside those normally available, the binder can be ordered, but the cost might be higher compared to binders within the standard slate. In those cases where the binder grade must be determined as part of the mix design process, the required final performance grade will depend on five factors: 1. Local climate, dictating the base performance grade 2. Design traffic level 3. Average traffic speed 4. The amount of RAP added to the mix 5. The performance grade of the RAP binder Details on handling RAP in HMA mix designs are presented in Chapter 9 of this manual; what follows is a brief discussion of the effect of RAP on binder performance grade selection. The essential concept that must be understood is that RAP contains significant amounts of asphalt binder, and this binder has usually undergone significant age hardening. Therefore, adding RAP to an HMA mixture tends to increase the effective grade of the binder in the final mix. For example, an HMA mix made with a PG 64-22 binder and 25% RAP might have an effective binder grade of PG 70-16. If the required grade for the mix is a PG 64-22, a softer binder grade must then be added to this mix—say, a PG 58-28 rather than a PG 64-22, so that the blend of the new binder and RAP binder meets the requirements of a PG 64-22. 108 A Manual for Design of Hot Mix Asphalt with Commentary

The effect of RAP on the final effective binder grade can be handled in three different ways. In the simplest case, if the RAP content is 15% or less, no adjustment is made to the performance grade of the new binder added to the HMA. At higher RAP contents, two approaches can be used in making sure the final effective binder grade meets the given requirements. In the first approach, the desired amount of RAP is selected, and the performance grade of the new binder to be added to the mix is determined so that the final effective performance grade meets the established performance grade requirements. The second approach is to select the performance grade of the new binder and then determine the minimum and maximum amounts of RAP that can be added while still meeting the established performance grade requirements for the final effective binder grade. Either approach can be used with HMA Tools, which gives both the final effective binder grade for a given RAP content and the new binder performance grade, and also the minimum and maximum allowable RAP content for a given new binder performance grade and RAP stockpile or blend of RAP stockpiles. A minimum RAP content requirement can occur when the selected new binder performance grade is softer than the required grade, so that the addition of RAP is needed to effectively stiffen the asphalt binder. For example, a given location might require a PG 70-22 binder, but a PG 64-28 binder is selected because it is expected that the addition of RAP to the mixture will stiffen the final effective binder grade. However, in such a situation, the technician must make sure that enough RAP is actually added to the mix so that the effective binder meets the final grade requirement of PG 70-22. An important issue related to the effect of RAP on binder grade is the effect of RAP on the variability of various mix properties. In some cases, the maximum amount of RAP that can be added to an HMA mix will be limited by variability, rather than by the final binder performance grade. This issue is discussed in detail in Chapter 9. The FHWA’s software program, LTPPBind, provides information on climate, base performance grade, and performance grade adjustments for traffic and speed. As of September 2008, information on LTPPBind, including a free download of the program, could be found at http://www.fhwa.dot. gov/pavement/ltpp/ltppbind.cfm. One limitation to the current LTPPBind, Version 3.1, is that only fast and slow traffic speeds are addressed, and the specific speeds in kph corresponding to these categories are not given, although it appears that fast traffic corresponds to an average speed of about 70 kph, and slow traffic to a speed of about 35 kph. Performance grade adjustments for very slow traffic are not addressed. Fortunately, recent research published in NCHRP Report 567: Volumetric Requirements for Superpave Mix Design has provided a model relating binder perfor- mance grade, compaction level (Ndesign), traffic level, traffic speed, and mix composition to rutting. This model was used to develop high-temperature performance grade adjustments, which are given in Table 8-1. These grade adjustments differ only slightly from those given in LTPPBind Design of Dense-Graded HMA Mixtures 109 Grade Adjustment for Average Vehicle Speed in kph (mph): Very Slow Slow Fast Design Traffic (MESALs) < 25 (< 15) 25 to < 70 (15 to < 45) ≥ 70 (≥ 45) < 0.3 --- --- --- 0.3 to < 3 12 6 --- 3 to < 10 18* 13 6 10 to < 30 22* 16* 10 ≥ 30 --- 21* 15* * Consider use of polymer-modified binder. If a polymer- modified binder is used, high-temperature grade may be reduced one grade (6 °C), provided rut resistance is verified using suitable performance testing. Table 8-1. High temperature binder grade adjustments for traffic level and speed.

for fast traffic. However, the assumed slow traffic speed in Table 8-1 is somewhat slower than that apparently used in LTPPBind and the adjustments are correspondingly larger. Also, as mentioned above, Table 8-1 includes adjustments for very slow traffic—a speed category not included in LTPPBind v. 3.1. Theoretically, high-temperature performance grades could be reduced for faster traffic at the lowest design traffic level (< 0.3 MESALs). However, construction of such pavements will often be poorly controlled and the use of softer binder grades could result in rutting, shoving, and flushing in many cases. No grade adjustments for very slow speed are given for the highest traffic level because such a combination of traffic speed and volume cannot occur— if the traffic on a road is very slow, the volume cannot be extremely high because the vehicles are not moving at a fast enough speed. This brings up another important point in applying Table 8-1 and similar grade adjustments, such as those given in LTPPBind: the traffic speeds are average speeds for the road, not minimum speeds. Average speeds should be those determined from traffic studies or other objective procedures, not personal judgment. A final difference between Table 8-1 and the traffic and speed adjustments given in LTPPBind is that the adjustments given in Table 8-1 are for binders that are not polymer modified; as explained in the note for the table, the high-temperature performance grade requirement can be reduced by one grade (6°C) if a polymer- modified asphalt binder is used, and if the mix successfully meets appropriate performance testing requirements, as discussed later in this chapter. A second important high-temperature grade adjustment must be made for temporary con- struction. This is necessary for two reasons: (1) lack of age hardening and (2) greater potential for extremely hot weather. A pavement carrying 20 million ESALs over 20 years will experience 1 million ESALs per year. During the first 2 to 3 years, such a pavement will undergo significant age hardening, which will greatly reduce the rate of rutting after this initial loading period. A pavement designed to carry 20 million ESALs over 2 years, on the other hand, will carry 10 million ESALs before significant age hardening occurs, and the final 10 million ESALs will be applied after only a modest amount of age hardening. The result will usually be substantial, even catastrophic failure, unless the high-temperature performance grade is adjusted upward. The other factor that must be considered is variability in climate. Occasional extremely hot summers over a 20-year design life are to be expected, but the damage will usually not be severe, given that the amount of traffic traveling over the pavement over a few such summers is limited. Using the same example of a pavement designed for very heavy traffic over 2 years, a single very hot summer would mean that about half of the design traffic would travel over the pavement while in a very soft condition; again, the results could be catastrophic rutting. For this reason, as in the Superpave system, all HMA mixtures should be designed for the projected 20-year traffic level. For example, if a temporary road is to carry 1,500,000 ESALs over 2 years, the mix should be designed not for 1,500,000 ESALs but for 15 million ESALs. Another question that must sometimes be addressed during binder selection is whether or not to select a polymer-modified asphalt binder. A common rule is that when the two numbers in a performance grade add to a number more than 90, the resulting binder must be modified. For example, most PG 64-22 binders are not modified (64 + 22 = 86), whereas most PG 76-22 binder are (76 + 22 = 98). However, refineries and chemical manufacturers are constantly developing new types of binder modification, and the performance of modified binders is not completely reflected in the current performance grading system. For example, the rut resistance and fatigue resistance exhibited by many commercially available polymer-modified binders is significantly better than non-modified binders of the same grade. For this reason, some states require that polymer-modified asphalt binder be used in certain critical situations. Such specifications often include one or two additional tests, such as elastic recovery, meant to ensure that only certain types of modification be used for these critical applications. Applicable state specifications must be reviewed to determine if such requirements apply. As noted in Table 8-1, consideration should be given to using polymer-modified binders in cases where 110 A Manual for Design of Hot Mix Asphalt with Commentary

the grade adjustment exceeds two grades. Furthermore, provided the mixture meets rut resistance requirements as discussed later in this chapter, the high-temperature performance grade can be reduced one level if a modified binder is used. This is because the current performance binder specification does not always adequately address the superior performance of many modified binders. Furthermore, without this adjustment in performance grade requirements, many areas in the southern United States would find it difficult or impossible to obtain suitable binders for HMA mixes intended for pavements subject to very heavy traffic. When selecting modified binders, optimum performance will be ensured if the binder selected is one that has been successfully used in the past under similar conditions or one approved by the state high- way agency. Step 3. Determine Compaction Level The design compaction level—Ndesign—is a function only of design traffic level. Suggested values for Ndesign as a function of design traffic in million ESALs are listed in Table 8-2. These values are identical to those used in the Superpave method. However, recommended design compaction levels for HMA mixtures are under review and could be modified soon; it is possible that slightly lower Ndesign values could be adopted, to aid in designing mixtures that have higher VMA and are easier to compact in the field. Step 4. Select Nominal Maximum Aggregate Size The nominal maximum aggregate size of the aggregate blend for an HMA mixture is most often specified by the owner/agency for a given project. In cases where the aggregate size is not specified, it is determined by the lift thickness during construction. Lift thickness and aggregate size can significantly affect the ease with which a mixture can be compacted in the field and the permeability of the resulting pavement. Brown and associates at the National Center for Asphalt Technology (NCAT) in 2004 published the results of research on this topic in NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot Mix Asphalt Pavements. The guidelines given here are based on their conclusions and recommendations. The nominal maximum aggregate size should be no more than one-third the lift thickness for fine mixtures, and one-fourth the lift thickness for coarse mixtures. Coarse mixtures are defined as those for which the percent passing is less than the control point for the primary control sieve as listed in Table 8-3; all other mixtures are considered fine graded. All else being equal, smaller aggregate sizes should be preferred for wearing course mixtures and where extra durability is desired; this will help provide a mix that compacts easily, has low permeability, and resists fatigue cracking. Table 8-4 lists recommended NMAS values for different applications of dense-graded HMA. Unless otherwise specified, the smallest possible NMAS from those listed in Table 8-4 should be selected for use in a given mix design. Design of Dense-Graded HMA Mixtures 111 Design Traffic (MESALs) Ndesign < 0.3 50 0.3 to < 3 75 3 to < 10 100 10 to < 30 100 ≥ 30 125 Table 8-2. Recommended design compaction levels for dense-graded HMA mixtures. Aggregate NMAS (mm) Primary Control Sieve (mm) PCS Control Point (% Passing) 4.75 1.18 42 9.5 2.36 47 12.5 2.36 39 19.0 4.75 47 25.0 4.75 40 37.5 9.5 47 Table 8-3. Primary control sieve sizes.

Step 5. Determine Target VMA and Design Air Void Content One of the unique features of the design method described in this manual for dense-graded mixtures is that the target VMA and air void content—and the resulting target binder content— are determined early on and maintained throughout the mix design procedure. In this way, the proper binder content is ensured, and effort is not wasted evaluating mixtures that do not have the proper VMA and binder content. This also reduces the chances that an error in volumetric calculations or laboratory testing will result in a mix design that does not meet the specified requirements. The allowable VMA range depends only on the aggregate NMAS; as in the Superpave system, minimum and maximum VMA values increase with decreasing aggregate NMAS— 1% for each decrease in standard aggregate size. Limits for VMA are given in Table 8-5. A 2% range is specified for allowable VMA. When selecting VMA for a mix design, the target value is in the center of this allowable range. These target values should be used for the initial devel- opment of a mix design—for determining the composition of trial mixtures to be evaluated and refined in the laboratory during the mix design process. The design VMA value can be adjusted during the later stages of the mix design process or during construction, in order to further refine the mix or to adjust for field production. Using a target VMA value in the 112 A Manual for Design of Hot Mix Asphalt with Commentary Recommended Lift Thickness, mm Application Recommended NMAS, mm Fine-Graded Mixtures Coarse-Graded Mixtures 4.75 15 to 25 20 to 25 Leveling course mixtures 9.5 30 to 50 40 to 50 4.75 15 to 25 20 to 25 9.5 30 to 50 40 to 50 Wearing course mixtures 12.5 40 to 65 50 to 65 19.0 60 to 100 75 to 100 Intermediate course mixtures 25.0 75 to 125 100 to 125 19.0 60 to 100 75 to 100 25.0 75 to 125 100 to 125 Base course mixtures 37.5 115 to 150 150 9.5 30 to 50 40 to 50 Rich base course mixtures 12.5 40 to 65 50 to 65 Table 8-4. Recommended aggregate nominal maximum aggregate sizes for dense-graded HMA mixtures. Aggregate NMAS (mm) Minimum VMAa (%) Maximum VMAa (%) 16 18 15 17 14 16 13 15 12 14 4.75 9.5 12.5 19.0 25.0 37.5 11 13 aThe specifying agency may increase the minimum and maximum values for VMA by up to 1% to obtain mixtures with increased asphalt binder content, which can improve field compaction, fatigue resistance, and general durability. Care should be taken to ensure that the resulting HMA mixtures maintain adequate rut resistance for their intended application. Table 8-5. VMA requirements for standard dense-graded mixtures.

center of the allowable range ensures that such adjustments can be made. If a mix design is started at the minimum allowable VMA, adjustments needed later in the mix design process or during field production can be difficult or impossible without lowering the VMA below the specified minimum. As noted in Table 8-5, the specifying agency can increase the minimum and maximum (and resulting target) VMA values by up to 1% if desired. This will provide additional binder content in the resulting HMA mixtures, which can have several desirable effects—it will tend to produce a mixture that is easier to compact in the field, more fatigue resistant, and, in general, more durable. However, increasing VMA can also decrease rut resistance, so care is needed when increasing minimum VMA requirements. As discussed below, the required dust/binder ratio of 0.8 to 1.6 should not be lowered if VMA requirements are increased beyond those given in Table 8-5, or the resulting mixtures might at times exhibit poor rut resistance. Agencies should in general be wary of simultaneously changing mix design requirements that all tend to reduce rut resistance—these include increasing VMA, decreasing dust/binder ratio, decreasing Ndesign, reducing requirements for FAA, or lower requirements for CAFF. Agencies should also be aware that the only foolproof way of increasing binder content in HMA mixtures is to increase minimum VMA requirements. Reducing Ndesign values will make it easier to design mixtures with higher VMA, but producers will find it easy to adjust their aggregate proportions after such a change in order to maintain binder content at the lowest possible level when economic incentives make such an approach desirable. When considering increasing VMA requirements, it should be remembered that many HMA performance problems are the result of construction problems, especially poor field compaction, rather than improper mix design. If high in-place air void content is the cause of poor durability—raveling and surface cracking—increasing VMA or decreasing Ndesign will not improve field performance unless these changes result in significant improvement in field compaction. For most surface course and intermediate (binder) course mixes, a design air void content of 4.0% is recommended. However, the design air void content for these mixtures is allowed to vary from 3.5% to 4.5%. Specifying a lower design air void content of 3.5% will result in an increase in binder content of a few tenths of a percent and a mixture that is slightly easier to compact. It will, however, also tend to decrease rut resistance. Increasing the design air void content by 0.5% will have the opposite effect—it will slightly decrease the design binder content and produce a mix that is more difficult to compact, while increasing rut resistance. Rich bottom or base course mixes, as now sometimes used in the design and construction of perpetual pavements, should be designed at a slightly lower air void content of 3.0 to 4.0%. This helps ensure that these mixes have the binder needed for exceptional fatigue life and are also easy to compact to a very low air void content in the field. Because base course mixtures are located deep within the pavement structure, the decrease in rut resistance caused by a lower design air void content is not normally a major concern for these applications. In HMA Tools, the initial target values for VMA and air void content are selected in the worksheet “General.” This worksheet lists the minimum and maximum values for VMA along with the suggested target—the midpoint between the VMA limits. Target VMA and air void content can also be refined in the worksheet “Trial_Blends.” As discussed above, it is recommended that (1) such adjustments be made only after evaluating several trial batches and (2) they be kept small—about 0.5% or less. Changing target VMA and air void content during the initial stages of the mix design process can make it difficult to evaluate the effect that changes in aggre- gate gradation have on these values, making the mix design process longer and more complicated than it needs to be. As discussed above, designing a mix near the minimum or maximum allowable VMA and/or air void content can also make adjustments during field production more difficult. Design of Dense-Graded HMA Mixtures 113

Step 6. Calculate Target Binder Content Once the target VMA is selected, calculation of the design binder content is straightforward. The target value for VBE (effective binder content by volume) is calculated by subtracting the design air void content—normally 4%—from the target VMA value. For example, a standard 12.5-mm mixture with a target VMA value of 15% would have a target VBE value of 15 − 4 = 11%. The total binder content must also include the amount absorbed by the aggregate. This can be estimated in several ways. A quick approximate estimate, suitable for developing trial batches, is to simply add 1% to the target VBE value. In the example above, this would result in a total target binder content of 12%. A more accurate estimate would be to calculate the volume of water absorbed by the aggregate, divide this by two, and add it to the target VBE value: where Vb = total asphalt content by volume % VBE = effective asphalt content by volume % VMA = voids in the mineral aggregate = Vbe + air void content Gsb = aggregate bulk specific gravity Pwa = water absorption of the aggregate, weight % The best approach to estimating absorbed binder and the resulting total binder content is to use past experience. However, this may not always be possible. In any case, it should be remembered that the mixture proportions being determined at this point in the mix design process are only for one or more trial mixtures and that further adjustments will almost always have to be made prior to finalizing the mix design. Therefore, use of estimates in determining total binder content from the target VMA will usually work quite well. In HMA Tools, Equation 8-1 is used to estimate the binder content. As additional trial mixtures are made, the amount of binder absorbed by the aggregate is adjusted according to the values measured during the previous trial mixtures. The final proportions for the mix design must be given by both percent by volume and weight, so the binder contents calculated above must be converted to percentages by total mix weight. However, this conversion cannot be done until the next two steps in the mix design process are completed and the aggregate proportions determined. Step 7. Calculate Aggregate Content The total aggregate content by volume is directly calculated as 100% minus the VMA content. In the example above, the total aggregate volume would be 100 − 15 = 85%. Determination of the total aggregate content by weight will depend on the aggregate specific gravity values and the specific blend of aggregates used in each of the trial mixtures, as determined in Step 8 as explained below. As with most other calculations needed during the mix design process, HMA Tools automatically calculates the aggregate content. Step 8. Proportion Aggregates for Trial Mixtures Proportioning aggregates for trial mixtures is one of the most important steps in the HMA mix design process. It can also be one of the most complicated. The procedure recommended here sets the binder content at a value that will provide the proper VMA once the design air void V VBE VMA G P b sb wa = + − ⎛⎝⎜ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟1 100 2 8 1( )- 114 A Manual for Design of Hot Mix Asphalt with Commentary

content is met. Therefore, proportioning aggregates can be thought of as determining the blend of aggregates that will provide the proper air void content for the mixture. However, because many HMA mix designs can make use of four or more aggregates, determining the right aggregate blend can be difficult and is largely a trial-and-error process. Engineers and technicians responsible for HMA mix designs should understand that several systems for blending aggregates are very effective. The Asphalt Institute Manuals on mix design— MS-2 and SP-2—provide detailed descriptions of aggregate proportioning methods typically used with the Marshall, Hveem, and Superpave mix design methods. More recently, the Bailey method of aggregate proportioning has become popular among some technicians and engineers. This procedure is based on theoretical principles of particle packing and, although relatively complicated, is unique in that it provides many quantitative rules for modifying aggregate blends to achieve a desired change in VMA. An excellent reference for the Bailey method is Vavrik et al., “Bailey Method for Gradation Selection in HMA Mixture Design,” Transportation Research Board Circular E-C044. In any case, engineers and technicians who are comfortable with the methods they are using for proportioning aggregates should continue to use them. HMA mix design—particularly determining appropriate aggregate blends—involves science and math, but is also largely an art based on experience and judgment. The method described below is largely a graphical one and is intentionally simple and flexible, so that it is potentially compatible with the widest possible range of mix design procedures and combination of circumstances. Unfortunately, this also means that applying this procedure efficiently requires some experience—both with the mix design process and with a wide range of materials. Maximum Density Aggregate Gradation and Fundamentals of Aggregate Blending For many years, use of the maximum density aggregate gradation has been emphasized in proportioning aggregates for HMA mix design. The maximum density gradation is that which provides the smallest possible volume of space among the aggregate particles—that is, it is the blend providing the lowest possible VMA for a given set of aggregates. Using the maximum density gradation to produce an HMA mix was for many years considered desirable because it would result in a mix with the minimum asphalt binder content and because asphalt binder is much more expensive than aggregate, the resulting mixture would be relatively economical. However, it has become clear that a certain minimum amount of asphalt binder is required in order for an HMA mixture to be workable—easy to place and compact—and also to resist moisture damage, age hardening, and fatigue cracking. Therefore, most HMA designs today use aggregate gradations that vary significantly from maximum density. In fact, achieving the minimum required VMA values can be a problem with some aggregates. Even though most HMA mixtures do not precisely follow a maximum density gradation, it is often used as a reference when proportioning aggregates. A good estimate of the maximum density aggregate gradation for a given aggregate size can be estimated using the 0.45 power gradation: where % PMD = percent passing for maximum density gradation d = sieve size, mm D = maximum sieve size for gradation, mm % % ( ) . PMD d D ≈ ⎛⎝⎜ ⎞⎠⎟ × 0 45 100 8 2- Design of Dense-Graded HMA Mixtures 115

Figure 8-3 is a plot of three different 12.5-mm NMAS aggregate gradations, with the maximum density gradation calculated using Equation 8-2 included for reference. As shown in this figure, HMA gradations are usually classified as coarse, fine, or dense, depending on where they pass relative to the maximum density gradation. Those falling below the maximum density gradation are called coarse gradations, those passing above are called fine gradations, and those passing near the maximum density gradation are called dense. However, it should be emphasized that, in fact, all three of these gradations are used in producing dense-graded HMA—therefore, a more accurate classification of these gradations would be dense/coarse, dense/fine, and dense/dense, accurately reflecting that all three gradations are close to dense gradations, but some are slightly coarser and some slightly finer than the maximum density gradation. A concept related to the maximum density gradation, and also useful in analyzing aggregate gradation, is the continuous maximum density gradation. This is calculated using an equation similar to Equation 8-2: where PCMD(d2) = percent passing, continuous maximum density gradation, for sieve size d2 d1 = one sieve size larger than d2 P(d1) = percent passing sieve d1 For example, in a selected aggregate gradation the percent passing the 4.75-mm sieve is 84%. The PCMD for the 2.36-mm sieve would be calculated as (2.36/4.75)0.45 × 84 = 73%. The usefulness of the continuous maximum density gradation is that it allows a more realistic evaluation of how closely a given aggregate gradation follows a maximum density gradation compared to the traditional maximum density gradation as calculated using Equation 8-2. The top graph in Figure 8-4 shows a 9.5-mm gradation compared to the standard maximum density gradation as calculated using Equation 8-2. The bottom graph shows the deviation from the continuous maximum density gradation (calculated using Equation 8-3) for this same aggregate. Both figures suggest that the gradation deviates significantly from the maximum density gradation, but the lower graph is much clearer in the way in which this deviation occurs. For example, it is not clear in the top graph that the aggregate gradation in fact follows a maximum density gradation below the 1.18-mm sieve size; this is very clear in the lower graph. Furthermore, the lower plot P d d d P dCMD 2 2 1 0 45 1 8 3( ) ≈ ⎛⎝⎜ ⎞⎠⎟ × ( ) . ( )- 116 A Manual for Design of Hot Mix Asphalt with Commentary 0 20 40 60 80 100 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Pe rc en t P as s in g Fine Dense Coarse Max. Density Figure 8-3. Fine, dense, and coarse aggregate gradations for HMA compared to the maximum density gradation.

exaggerates the deviation from maximum density, so comparing several similar aggregate blends is much easier. One of the most important—and often the most difficult—parts of the mix design process is adjusting aggregate blends to produce the desired level of air void content and VMA. The lower graph in Figure 8-4, called a continuous maximum density (CMD) plot, is very helpful in this process because, in general, for a given set of aggregate blends the more this plot deviates from zero (the horizontal line through the center of the plot), the greater will be the VMA and air void content. Using the CMD plot to blend aggregates has many advantages: • It is soundly based in packing theory. • It is completely flexible—it can be applied to any number of aggregates, any gradation size, and any type of gradation or HMA mix. • Once set up in a spreadsheet, as in HMA Tools, it is simple to apply; there is no long set of rules and definitions to remember. • Because of its simplicity and flexibility, the CMD approach can be used along with other procedures. Figure 8-5 shows how the CMD plot relates to changes in aggregate gradation for a series of 12.5-mm aggregate blends: an SMA aggregate blend; a dense/coarse blend; a dense/dense blend; and a dense/fine blend. The top portion of the figure shows a traditional gradation plot, including the maximum density gradation. The bottom chart in Figure 8-5 shows the CMD plots for these Design of Dense-Graded HMA Mixtures 117 0 20 40 60 80 100 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Pe rc en t P as si ng maximum density gradation aggregategradation -15 0 15 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm % D ev ia tio n fro m M ax . D en si ty Figure 8-4. Top: 9.5-mm aggregate gradation compared to the maximum density gradation; bottom: deviation from the continuous maximum density gradation for the same 9.5-mm gradation.

same four aggregate blends. One of the most striking features of the CMD plot is that it shows that despite the large differences in the four gradations, the fine aggregate portions of these blends all are fairly close to a maximum density gradation. This does not mean that the fine aggregate portion of these mixtures closely follows the gradation for the traditional maximum density gradation for a 12.5-mm aggregate (the dashed line in the top chart in Figure 8-5). What it means is that the fine aggregate portions of all four blends—considered separately from the coarse portion—follow a maximum density gradation fairly closely. This is a very important concept when adjusting aggregate blends to meet VMA and/or air void requirements. Consider the dense/coarse gradation in Figure 8-5. In the top chart it appears that the fine aggregate portion of this aggregate deviates significantly from the maximum density gradation, and changing this portion of the blend might therefore tend to reduce VMA. However, it is clear from the lower chart that attempting to reduce VMA by changing the fine aggregate portion of this gradation will probably be counter-productive, since it already closely follows a maximum density gradation. If a reduction in VMA is needed for this aggregate, the amount of material between the 2.36- and 9.5-mm sieves must be reduced. In general, the greater the deviation from the zero line on the CMD plot, the greater will be the VMA (and air void content) for the resulting mixture. In Figure 8-5, it appears that the largest differences in these blends are in the coarse aggregate. This is in fact typical for aggregate blends used in HMA. However, even though the deviations from the maximum density gradation for the fine aggregate portion of many aggregate blends 118 A Manual for Design of Hot Mix Asphalt with Commentary 0 20 40 60 80 100 Pe rc en t P as si ng Dense/fine Dense/dense Dense/coarse SMA Max. Density -25 0 25 % D ev ia tio n fro m M ax . D en si ty SMA Dense/coarse Dense/dense Dense/fine 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Figure 8-5. Top: four different 12.5-mm gradations; bottom: % deviation from continuous maximum density gradation for these same blends.

may seem small, such differences can have a significant effect on the air void content and VMA of the resulting HMA mixture. The specific interpretation of CMD plots such as those shown in Figures 8-4 and 8-5 is as follows. The value on the vertical axis for a given sieve size shows the difference in the percent of material between that sieve size and one sieve size larger for the actual gradation and the continuous maximum density gradation. For example, in Figure 8-5, for the 4.75-mm sieve, the SMA gradation has about 22% more material between the 4.75- and 9.5-mm sieves than the maximum density gradation. This is typical for SMA gradations, which contain very large proportions of coarse aggregates. The dense/coarse gradation, on the other hand, has about 14% more material than the continuous maximum density gradation in this size range. When using CMD plots to blend aggregates during a mix design, the technician should look not only at the amount of the deviation in different size ranges, but also the effect of these deviations on the air void content and VMA of the resulting mixture. The air void content and VMA for some mixtures might be most sensitive to changes in the coarse fraction of the aggregate blend, while other mixtures might be more sensitive to changes in the fine or intermediate portions of the aggregate blend. For this reason, HMA Tools includes, as part of the CMD plot, values for air void content and VMA for each aggregate blend (once they have been determined in laboratory testing). This makes it easy for the technician to determine what changes in the aggregate blends are most important in determining volumetric composition. Trial Blends for New Mix Designs When developing an HMA mix design with a new set of aggregates, the general procedure recommended here is very similar to that used in the Superpave method, but with the inclusion of the CMD plot as an additional tool in analyzing the aggregate gradations and resulting volumetric compositions. After performing the initial steps of the mix design as outlined above (including determination of the design VMA, air void content, and binder content), three trial aggregate gradations are prepared: a dense/coarse gradation, a dense/dense gradation, and a dense/fine gradation. As explained later in this chapter, trial batches based on these gradations are prepared in the laboratory, specimens compacted and the VMA, air void content, and effective binder content determined for each. Usually none of the three initial batches will precisely meet all requirements, and one is selected for further refinement. Additional trial batches are prepared and evaluated until all essential mix design criteria are met. It is strongly suggested that during the initial trial batches only the aggregate gradation should be modified, while the asphalt binder content is kept constant. This will make the way changes in the aggregate gradation are affecting VMA and air void content much clearer. Once a trial batch is close to the required composition, the binder content may be slightly adjusted if desired to fine-tune the design. However, engineers and technicians should remember that mix designs will usually be adjusted significantly during the initial stages of field production, so effort spent in unnecessary, minor adjustments to a laboratory mix design will often be wasted. The HMA Tools spreadsheet has been designed to allow blending of up to eight different aggregates, up to four of which may be recycled asphalt pavement (RAP) material. Gradation data and other properties are entered in the worksheet “Aggregates” for new materials and “RAP_Aggregates” for RAP materials. In the worksheet “Trial_Blends,” the technician enters various proportions for each aggregate or RAP, and the gradation is calculated and plotted on a standard plot and on a CMD plot. Once volumetric test data are available for the trial batches, these plots include VMA and air void values to aid the technician in determining which gradation is most suitable and what sort of adjustments in the gradation (if any) are needed to produce a mixture with the desired properties. Design of Dense-Graded HMA Mixtures 119

Adjusting Aggregate Gradations When Modifying Existing HMA Designs In practice, most HMA mix design work involves modifying existing mixtures to meet some new requirement or improve some aspect of the design (such as workability). Therefore, the approach discussed above will usually be the exception, rather than the rule. In modifying an existing mix design, the procedure suggested here is similar to that described above, but somewhat abbreviated. First, the goal of the modification must be clearly understood, primarily in terms of the needed change in VMA. Increased air void content or increased binder content both require increases in VMA. Similarly, if a specification requires an increase in VMA, and the design air void content does not change, an increase in binder content will be needed. Remember, VMA consists of the volume taken up by both asphalt binder and air voids. Once the required change in VMA has been determined, the aggregate gradation for the existing mix design is plotted, using a traditional gradation plot and the CMD plot. If an increase in VMA is needed, in general, the aggregate gradation must be modified to increase the difference between the CMD plot and the zero line. If a decrease in VMA is needed, the gradation should be modified to decrease this difference. This is shown in Figure 8-6. The heavy, dark line in this example is the aggregate gradation for an existing mix design. Looking at the coarse aggregate portion of the gradation, the lighter lines above this are gradations that would likely increase the VMA for this mixture, since they are further from the zero line. The lighter lines below the existing gradation would probably decrease the VMA, since they are, in general, closer to the zero line on the CMD plot. However, engineers and technicians using this approach should remember that VMA for a given mixture will exhibit different sensitivities to changes in different portions of the aggregate gradation. Some mixtures might be more sensitive to changes in the fine aggregate portion of the gradation, while some might be more sensitive to the coarse aggregate portion of the gradation. Some might be very sensitive to changes in mineral filler content. It should also be noted that the zero line on the CMD plot is only an approximate indicator of the maximum density gradation—the actual position on the CMD plot of the maximum density gradation might vary somewhat from the zero line. This should become apparent when plotting gradations and VMA values for trial mixtures. It is especially important when modifying existing mix designs to make use of experience with the given aggregates. Often, an engineer or technician who has done previous mix design work with the aggregates at hand will know what changes in the gradation are needed 120 A Manual for Design of Hot Mix Asphalt with Commentary -25 0 25 % D ev ia tio n fro m M ax . D en si ty higher VMA lower VMA 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Figure 8-6. Effect of changes in aggregate gradation on VMA as shown on a CMD plot. Aggregates plotting closer to the zero line will usually have lower VMA values.

to produce a specific change in VMA or other mix properties. In such cases, the CMD plot can be a useful tool. When modifying existing mix designs, the HMA Tools spreadsheet is used in a manner very similar to that described above for new mix designs. As before, the needed aggregate and RAP data (if used) are entered in the worksheets “Aggregates,” and “RAP_Aggregates.” Then, aggregate blend for the existing mix is entered in the worksheet “Trial_Blends,” followed by one or two modified aggregate blends. If no information is available concerning the aggregates being used, the general rule described above should be used in developing the new trial aggregate blends— gradations closer to the zero line on the CMD plot will have lower VMA, while those further away will have higher VMA. After this, the design proceeds as before, determining the air void content and VMA for the trial batches and then making further refinements in the aggregate gradation as needed until the desired mix properties are met. Guidelines for Aggregate Gradations As with previous HMA mix design methods, there are limits for aggregate gradation for each NMAS; suggested control points for aggregate gradations for dense-graded HMA mixtures are listed in Tables 8-6 and 8-7. It is important to note that, in the system described in this manual, Design of Dense-Graded HMA Mixtures 121 Percent Passing for Nominal Maximum Aggregate Size: 37.5 mm 25.0 mm 19.0 mm Sieve Size (mm) Min. Max. Min. Max. Min. Max. 50.0 100 37.5 90 100 100 25.0 90 90 100 100 19.0 90 90 100 12.5 90 9.5 4.75 2.36 15 41 19 45 23 49 1.18 0.600 0.075 0 6 1 7 2 8 Table 8-6. Control points for 19.0-mm through 37.5-mm aggregate gradations for dense-graded HMA mixtures. Percent Passing for Nominal Maximum Aggregate Size: 12.5 mm 9.5 mm 4.75 mm Sieve Size (mm) Min. Max. Min. Max. Min. Max. 50.0 37.5 25.0 19.0 100 12.5 90 100 100 100 9.5 90 90 100 95 100 4.75 90 90 100 2.36 28 58 32 67 1.18 30 60 0.600 0.075 2 10 2 10 6 12 Table 8-7. Control points for 4.75-mm through 12.5-mm aggregate gradations for dense-graded HMA mixtures.

122 A Manual for Design of Hot Mix Asphalt with Commentary aggregate control points—with the exception of the maximum aggregate size—are considered guidelines, and not specification requirements. This provides engineers and technicians with additional flexibility in modifying aggregate gradations in order to meet VMA requirements. Differences in aggregate particle shape, angularity, and texture make it impossible to specify one particular gradation that will provide the best performance for all HMA mixtures of a given NMAS. Treating gradation control points with some flexibility helps ensure that engineers and technicians can adequately address these differences and provide HMA mixtures with the proper binder content and air void content needed for good durability. The gradation plots included in the worksheet “Trial_Blends” in HMA Tools include boundaries showing the control limits for a given mixture. In order for the proper limits to be included in the plot, the proper value for the aggregate NMAS must be entered in the worksheet “General.” Check Aggregate Specification Properties As in the Superpave method, there are four aggregate specification properties: (1) coarse aggregate fractured faces (CAFF); (2) flat and elongated particles in the coarse aggregate; (3) fine aggregate angularity (FAA); and (4) clay content of the fine aggregate (sand equivalent). A detailed description of these specification properties and the tests used to determine them is given in Chapter 4 of this manual. For the convenience of the readers of this manual, four tables listing the aggregate specification properties as given in Chapter 4 are reproduced below. Table 8-8 lists requirements for coarse aggregate fractured faces; Table 8-9 lists requirements for flat and elongated coarse aggregate particles; Table 8-10 lists requirements for fine aggregate angularity; and Table 8-11 lists requirements for fine aggregate clay content. The values in these tables are very similar to those used in the Superpave method; there are some slight Percentage of Particles with at Least One/Two Fractured Faces, for Depth of Pavement Layera, mm Design ESALs (million) 0 to 100 Below 100 < 0.3 55 / --- --- / --- 0.3 to < 3 75 / --- 50 / --- 3 to < 10 85 / 80 60 / --- 10 to < 30 95 / 90 80 / 75 30 or more 98 / 98b 98/ 98b aDepth of pavement layer is measured from pavement surface to surface of pavement layer. bThe CAFF requirement for design traffic levels of 30 million ESALs or more may be reduced to 95/95 if experience with local conditions and materials indicate that this would provide HMA mixtures with adequate rut resistance under very heavy traffic. Table 8-8. Coarse aggregate fractured faces requirements. Design ESALs (million) Maximum Percentage of Flat and Elongated Particles at 5:1 < 0.3 --- 0.3 to < 3 10 3 to < 10 10 10 to < 30 10 30 or more 10 Criteria are presented as percent flat and elongated particles by mass. Table 8-9. Criteria for flat and elongated particles.

differences in the requirements for coarse aggregate fractured faces and fine aggregate angularity, intended to make these requirements easier to meet without making any significant sacrifice in performance. As in the Superpave method, it is intended that aggregate specification properties be applied to the aggregate blend, and not to individual aggregates. An important step in the mix design process is to determine specification property values for aggregate blends, to ensure that the blends will likely meet specification property requirements. For the initial trial batches, the specification properties of the aggregate blends are normally estimated mathematically, by calculating a weighted average for each property. When calculating these weighted averages, care must be taken to consider only that portion of the aggregate tested in a given procedure. For example, CAFF is determined on only that portion of the aggregate retained on the 4.75-mm sieve, so the weighted average CAFF is based on the proportions of this fraction for each aggregate in the blend—not on the overall proportions for each aggregate. An additional complication occurs when RAP is included in the mix design; then, the specification properties of the aggregate in the RAP must also be considered (with the exception of sand equivalent, which is only applied to new aggregates). Estimation of specification properties can be tedious and prone to errors. Fortunately, HMA Tools performs this calculation for the technician. In the worksheets “Aggregates” and “RAP_Aggregates,” specification properties are entered for each aggregate. Up to two user-defined properties can also be entered here—these would be aggregate properties specified by the local highway agency, in addition to those given in this manual. In the worksheet “Trial_Blends” the estimated values for all specification properties are then shown. Aggregate properties for the final mix design should be determined by actual measurement. This is done by preparing the aggregate blend and then sieving out the fraction needed for the particular test and performing the test. Because this can be a time-consuming procedure, the Design of Dense-Graded HMA Mixtures 123 Depth of Pavement Layer from Surface, mm Design ESALs (million) 0 to 100 Below 100 < 0.3 ---a --- 0.3 to < 3 40 --- 3 to < 10 45b 40 10 to < 30 45b 45b 30 or more 45b 45b Criteria are presented as percent air voids in loosely compacted fine aggregate. aAlthough there is no FAA requirement for design traffic levels below 0.30 million ESALs, consideration should be given to requiring a minimum uncompacted void content of 40 percent for 4.75-mm nominal maximum aggregate size mixes. bThe FAA requirement of 45 may be reduced to 43 if experience with local conditions and materials indicate that this would produce HMA mixtures with adequate rut resistance under the given design traffic level. Table 8-10. Fine aggregate angularity requirements. Design ESALs (million) Minimum Sand Equivalency Value < 0.3 40 0.3 to < 3 40 3 to < 10 45 10 to < 30 45 30 or more 50 Criteria are presented as Sand Equivalent Value. Table 8-11. Maximum clay content requirements.

final mix design should avoid borderline values for aggregate specification properties that, when the blend is actually tested, might fail to meet requirements. Aggregate Blending: Summary One of the most important and complicated parts of the HMA mix design process is deter- mining the appropriate aggregate blend to use for a given application. Various procedures are available, including the Bailey method, and several techniques described in mix design manuals published by the Asphalt Institute. Engineers and technicians comfortable with the methods they are currently using for proportioning aggregates for HMA mix designs should continue to use these methods. The procedure given in this manual is based on a few simple concepts relating aggregate blends to VMA. In most cases, HMA mix designs are not designed from scratch. Instead, existing mix designs are modified by replacing aggregates or the binder or by changing the binder content and VMA. In these cases, the best guide for adjusting the aggregate proportions is the experience of the engineer or technician with the materials being used. When modifying existing mix designs, one or two aggregate blends are developed by modifying the blend used in the existing mix. A trial- and-error approach is then used to refine the aggregate blend until the desired mix properties are achieved. In situations where an entirely new HMA mix design is to be developed, three initial trial blends are developed using dense/coarse, dense/dense, and dense/fine aggregate gradations. The design closest to meeting all requirements is then further refined by making additional trial blends, evaluating their properties, and modifying the aggregate gradation as needed. An important part of the mix design process is determining the specification properties of the aggregate blends. For initial trial batches, specification properties can be estimated by using mathematical equations and the specification property values for the individual aggregates. This is done automatically in HMA Tools (and many similar spreadsheets and computer programs). However, the specification properties for the final mix design should be verified by actual measurements on the aggregate blend. Step 9. Calculate Trial Mix Proportions by Weight and Check Dust/Binder Ratio At this point in the HMA mix design, the amount of air voids, binder, and aggregate has been determined on a volume basis, and up to three different aggregate blends have been developed— on a proportion-by-weight basis. Now, the overall mixture composition in percent by weight must be calculated and the dust/binder ratio checked to make sure it is within the specified values. If desired, the mixture composition by volume can also be determined. The following procedure and equations can be used to calculate mix proportions by weight and related mix properties. First, calculate the overall aggregate bulk specific gravity: where Gsb = overall bulk specific gravity for aggregate blend Ps1/A = volume % of aggregate 1 in aggregate blend Gsb1 = bulk specific gravity for aggregate 1 Ps2/A = volume % of aggregate 2 in aggregate blend G P P P P G P G sb s A s A s A s A sb s A s = + + + ⎛ ⎝⎜ ⎞ ⎠⎟ + 1 2 3 1 1 2 . . . b s A sb P G2 3 3 8 4⎛ ⎝⎜ ⎞ ⎠⎟ + ⎛ ⎝⎜ ⎞ ⎠⎟ + . . . ( )- 124 A Manual for Design of Hot Mix Asphalt with Commentary

Gsb2 = bulk specific gravity for aggregate 2 Ps3/A = volume % of aggregate 3 in aggregate blend Gsb3 = bulk specific gravity for aggregate 3 As discussed in Step 7, the volume percentage of the aggregate is simply 100% minus the target VMA. The weight percentage of binder and aggregate are then calculated using the following equations: where Pb = total binder content, % by total mix weight Vb = total binder content, % by total mix volume Gb = binder specific gravity Vsb = aggregate content, % by total mix volume Gsb = overall bulk specific gravity of aggregate (Equation 8-4) Ps = total aggregate content, % by total mix weight Then, calculate the effective asphalt binder content by weight: where Pbe = effective binder content, % by total mix weight Vbe = effective binder content, % by total mix volume Gb = binder specific gravity Vsb = aggregate content, % by total mix volume Gsb = overall bulk specific gravity of aggregate (Equation 8-4) Calculate the percent by weight of each aggregate: where Ps1 = weight percent (by total mix) of aggregate 1 (or aggregate 2, 3, etc.) Ps = weight percent (by total mix) of combined aggregate, from Equation 8-6 Ps1/A = weight percent (in aggregate blend) of aggregate 1 (or aggregate 2, 3, etc.) If desired, the volume percent of the aggregates can also be calculated, but the equation is more complicated: V P P P G P G P sb s b b b s sb s 1 1 1 1 100 = −( ) ⎛⎝⎜ ⎞⎠⎟ + ⎛⎝⎜ ⎞⎠⎟ + 22 3 3 8 9 G P Gsb s sb ⎛⎝⎜ ⎞⎠⎟ + ⎛⎝⎜ ⎞⎠⎟ + . . . ( )- P P P s s s A 1 1 100 8 8= ⎛ ⎝⎜ ⎞ ⎠⎟ ( )- P V G V G V G be be b sb sb b b = + ×100 8 7% ( )- P V G V G V G s sb sb sb sb b b = + ×100 8 6% ( )- P V G V G V G b b b sb sb b b = + ×100 8 5% ( )- Design of Dense-Graded HMA Mixtures 125

where Vsb1 = volume % of aggregate 1 in total mix Ps1 = weight % of aggregate 1 in total mix Pb = weight % binder in total mix Gb = binder specific gravity Gsb1 = bulk specific gravity for aggregate 1 Ps2 = volume % of aggregate 2 in aggregate blend Gsb2 = bulk specific gravity for aggregate 2 Ps2 = volume % of aggregate 3 in aggregate blend Gsb3 = bulk specific gravity for aggregate 3 Calculate the percent of mineral dust (material finer than 0.075 mm) in the total mixture: where P0.075 = mineral dust content (material finer than 0.075 mm), percent by total mix weight P0.075/s1 = % passing the 0.075-mm sieve for aggregate 1 Ps1 = weight percent (by total mix) of aggregate 1 P0.075/s2 = % passing the 0.075-mm sieve for aggregate 2 Ps2 = weight percent (by total mix) of aggregate 2 P0.075/s3 = % passing the 0.075-mm sieve for aggregate 3 Ps3 = weight percent (by total mix) of aggregate 3 Calculate the dust/binder ratio, using the effective asphalt binder content: where D/B = dust/binder ratio, calculated using effective binder content P0.075 = mineral dust content, % by total mix weight (Equation 8-10) Pbe = effective binder content, % by total mix weight (Equation 8-7) The required range for dust/binder ratio is 0.8 to 1.6 for all mixtures larger than 4.75-mm NMAS. However, the specifying agency may reduce the requirements to a range of 0.6 to 1.2 if local materials and conditions warrant this change. For 4.75-mm NMAS mixtures, the required dust/binder ratio is 0.9 to 1.2, and this should not be modified. These requirements are similar to those given in the Superpave method, but the required and optional ranges are reversed; in the Superpave method, the required range is 0.6 to 1.2, but agencies can increase the requirement to 0.8 to 1.6. Higher dust/binder ratios are desirable for several reasons. Perhaps most importantly, they help provide stiffness and rut resistance to the HMA. Higher dust/binder ratios also will tend to reduce the permeability of an HMA mixture, improving durability. However, it is possible that in some locations obtaining high dust/binder ratios might be prohibitively expensive, and the nature of the local materials might allow the design of HMA with good performance at lower dust/binder ratios. Because of the beneficial effects of high dust/binder ratios on rut resistance, if VMA requirements are increased above those given in Table 8-5, the dust/binder ratio requirement should not be reduced. Otherwise, the rut resistance of the resulting mixtures might, in some cases, be marginal. Table 8-12 summarizes the requirements for dust/binder ratio. D B P Pbe = 0 075 8 11. ( )- P P P P P P Ps s s s s s 0 075 0 075 1 1 0 075 2 2 0 075 3 3 . . . . = + + + . . . ( ) 100 8 10- 126 A Manual for Design of Hot Mix Asphalt with Commentary

When including RAP in a mixture, the same principles described above are applied. RAP is composed of both binder and aggregate. The weight and volume of binder in the RAP must be added to the weight and volume of new binder added to a mixture. Similarly, the weight and vol- ume of aggregate must be added to the weight and volume of new aggregate added to the mix. As will be discussed in Chapter 9, HMA Tools automatically performs the needed calculations when including RAP in an HMA mix design. Design of Dense-Graded HMA Mixtures 127 Mix Aggregate NMAS, mm Allowable Range for Dust/Binder Ratio, by Weight > 4.75 0.8 to 1.6a 4.75 0.9 to 2.0 aThe specifying agency may lower the allowable range for dust/binder ratio to 0.6 to 1.2 if warranted by local conditions and materials. The dust/binder ratio should, however, not be lowered if VMA requirements are increased above the standard values as listed in Table 8-5. Table 8-12. Requirements for dust/binder ratio. Example Problem 8-1. Calculation of Mix Composition Table 8-13 presents the results of an example calculation of mixture composition for a trial batch. The mixture is a 12.5-mm NMAS design, with a target air void content of 4% and a target VMA value of 15%. Column 1 describes the various mix components; this includes total binder, absorbed binder, and effective binder— this makes the relationship among these values clear. Column 2 gives the mix composition in percentage by volume, which is determined using the procedure described above. Column 3 lists the bulk specific gravity for the various components, while Column 4 lists apparent specific gravity values for the aggregates. Column 5 lists the aggregate contents as a percentage by weight of the aggregate blend. (1) Mix Component (2) Percent by Total Mix Volume (3) Bulk Specific Gravity (4) Apparent Specific Gravity (5) Percent by Aggregate Weight (6) Percent by Total Mix Weight Air 4.00 --- --- --- 0.0 Total Asphalt Binder 11.38 1.025 --- --- 4.60 Absorbed Asphalt Binder -0.40 1.025 --- --- (0.16) Effective Asphalt Binder 10.98 1.025 --- --- 4.44 No. 7 Traprock 19.54 2.971 2.992 24 22.90 Traprock screenings 24.47 2.867 2.893 29 27.67 Manufactured sand 24.46 2.868 2.891 29 27.67 Natural sand 13.74 2.642 2.676 15 14.31 Mineral filler 2.80 2.588 2.629 3 2.86 Note: Calculations may not agree exactly because of rounding. Table 8-13. Example calculation of HMA mix composition by weight percentage from volume percentage and specific gravity values. (continued on next page)

Step 10. Evaluate and Refine Trial Mixtures As discussed previously, when developing mix designs with new aggregates, three initial trial mixes are prepared, representing dense/fine, dense/dense, and dense/coarse aggregate gradations. When modifying an existing mix design, one or two trial batches might be prepared by making slight adjustments in the existing aggregate blend. In either case, the next step in the mix design process is the same: the trial mixtures must be evaluated to determine if any meet the given specifications or, if none meet all criteria, the mix closest to the specifications must be identified and the way it must be modified to produce an acceptable mix must be determined. Table 8-13 showed the results of an initial estimate of volumetric composition for a trial batch. But as described at the end of Step 9, the values in this table are only estimates—the actual composition for this and any other trial mixtures must be determined in the laboratory, by batching, mix- ing, compacting, and testing laboratory specimens for each of these trial mixtures. The steps involved in this process are described below, using the same trial mix design summarized in Table 8-13. Calculate Batch Weights Weights for trial batches are easily calculated from the mix composition by weight percent, as calculated in the example above and shown in Table 8-14. First, the number and size of gyratory specimens must be determined; for normal volumetric analysis, two specimens 150 mm in diameter by 115 mm high are normally required. For moisture resistance testing, as described later in this chapter, six specimens 150 mm in diameter by 95 mm high are needed. Some performance tests require compacted specimens 150 mm in diameter by 165 mm high. The total weight of material needed for a batch is then calculated using the following equation: W G V Nmix mb spec spec= ( )8 12- 128 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 8-1. (Continued) Column 6 lists the composition of the mix by weight percentage, calculated using Equations 8-4 through 8-11. The procedure described above is tedious and prone to error if done by hand, so it is normally done with the aid of a spreadsheet designed to perform the calculations needed during the mix design process. It should be emphasized that when developing trial mix designs, calculations such as those given in Table 8-13 are only estimates of the actual mix composition. That is because the actual air void content and the amount of absorbed binder can only be accurately determined by making and testing HMA specimens in the laboratory. The air void content in this example was assumed to be the target value of 4%, even though it would be mostly luck if any of the trial mixtures produced exactly 4% air voids. As described above, the amount of absorbed asphalt is usually only estimated when designing initial trial batches; the actual amount of absorbed asphalt binder might vary significantly from this estimated value. These differences will usually mean that the actual mix composition will differ significantly from the initial estimate as calculated in this example.

where Wmix = total weight of mix in batch, g Gmb = estimated bulk specific gravity of mix Vspec = Volume of specimen, cm3 = 2,439 for 150-mm diameter by 115-mm high (including 20% extra) = 2,015 for 150-mm diameter by 95-mm high (including 20% extra) = 3,499 for 150-mm diameter by 165-mm high (including 20% extra) Nspec = number of specimens, normally two Batch Aggregates Although the batch weights given in Table 8-14 could be used to weigh out material for the trial batch directly, this is not recommended because many aggregates tend to segregate during stockpiling, sampling, and handling in the laboratory, so that direct batching of aggregates will often produce specimens with aggregate gradations deviating significantly from the desired target gradation. For this reason, when handling and batching aggregates in the laboratory, they are often broken down and weighed in a number of size fractions. This helps ensure that the aggregate gradation actually used in the specimen is close to the target gradation. The way in which an aggregate is broken down is a matter of judgment and experience. Some engineers or Design of Dense-Graded HMA Mixtures 129 Example Problem 8-2. Calculating Trial Mix Batch Weights For example, for the trial mix described in Table 8-13, the bulk specific gravity is estimated to be 2.536. If two 150-mm-diameter by 115-mm-high cylinders are to be prepared, the amount of mixture needed is calculated as 2.536 × 2,439 × 2 = 12,369 grams. The weight of each component is then calculated by multiplying the total required by weight by the weight percentage of the mix component and dividing by 100%. Continuing with the same example, a table is easily constructed showing the weight percentages and the batch weights for each of the mix components. This is shown in Table 8-14; the weight percentages are the same as those listed in Table 8-13, although carried out to an extra decimal place for greater accuracy in calculating batch weights. Mix Component Percent by Total Mix Weight (P) Batch Weight, grams (12,369 × P/100) Air --- --- Total Asphalt Binder 4.60 569 Absorbed Asphalt Binder --- --- No. 7 Traprock 22.90 2,832 Traprock screenings 27.67 3,422 Manufactured sand 27.67 3,422 Natural sand 14.31 1,770 Mineral filler 2.86 354 Total 100.00 12,369 Note: Calculations do not agree exactly because of rounding Table 8-14. Example calculation of batch weights for mixture listed in table 8-13.

technicians may choose to completely break down aggregates, while some may break down aggre- gates into only a few size fractions. A typical and fairly conservative approach is to break down aggregates into the following size fractions: • 37.5 to 50.0 mm • 25.0 to 37.5 mm • 19.0 to 25.0 mm • 12.5 to 19.0 mm • 9.5 to 4.75 mm • 2.36 to 4.75 mm • Passing 2.36 mm If there is less than 5% within one of these size fractions, it can be combined with an adjacent fraction. Mineral filler is not normally broken down prior to batching. HMA Tools includes the worksheet, “Batch,” which calculates batch weights for different levels of aggregate processing. The technician enters which trial batch (out of up to seven) the batching sheet is being prepared for, the number and dimensions of laboratory specimens, and the desired percentage of extra material. The worksheet then provides the appropriate batch weights, including the binder weight and batch weights of each aggregate; coarse aggregates are broken down completely, while fine aggregates are broken down in three different ways—completely, partially (by groups of two sieves), and with no breakdown (that is, a single weight for each fine aggregate). The technician performing the mix design can select among the three different ways of breaking down the fine aggregate in the batching process. 130 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 8-3. Breaking Down Aggregates and Calculating Aggregate Batch Weights An example of aggregate breakdown and batching is shown in Tables 8-15 and 8-16, using the same example problem described in Tables 8-13 and 8-14. Table 8-15 lists the gradations for the coarse and fine aggregates for the example problem. Table 8-16 describes the breakdown and lists the batch weights for each of the four aggregates. The No. 7 traprock is broken down into five size fractions, ranging from the 12.5- to 19.0-mm fraction to the passing 2.36-mm fraction. The screenings are broken down into three fractions: 4.75 to 9.5 mm, 2.36 to 4.75 mm and passing 2.36 mm. The two sands are both broken down into two Weight Percent Passing for Aggregate: Sieve Size (mm) No. 7 Traprock Traprock Screenings Manufactured Sand Natural Sand Mineral Filler 19.0 100 100 100 100 100 12.5 93 100 100 100 100 9.5 53 100 100 100 100 4.75 32 90 97 99 100 2.36 9 57 78 91 100 1.18 2 34 51 70 100 0.600 1 25 32 53 100 0.300 1 19 14 29 100 0.150 1 14 11 17 92 0.075 1 8 7 9 79 Table 8-15. Aggregate gradations for example batching problem.

Heat Aggregates and Asphalt Binder After batching out the aggregates, they are combined in a suitable metal container and heated in an oven to the desired compaction temperature. Asphalt binder is also heated to the same temperature. The binder will be weighed out into the aggregate after it has been thoroughly heated; the amount placed in the oven should be more than enough to provide for the given batch weight (569 g in this example). It is essential that the aggregates and binders are thoroughly heated to the proper temperature before mixing. For non-modified binders, the mixing and compaction temperatures are calculated on the basis of binder viscosity. The mixing temperature range is that providing a binder viscosity of from 150 to 190 Pa-s, while the compaction temperature range is that providing a binder viscosity of from 250 to 310 Pa-s. For non-modified binders, mixing and compaction temperatures can be estimated using a viscosity-temperature chart, as shown in Figure 8-7. Log viscosity is plotted against temperature, and a curve fit through the data points. The mixing and compaction temperature ranges can then be estimated from the chart as shown. For modified binders, the manufacturer should supply information concerning the mixing and compaction temperature for HMA made with their product. Mixing and compaction temperatures are usually provided by suppliers as part of the specification data provided on the bill of lading for a given binder. Design of Dense-Graded HMA Mixtures 131 Example Problem 8-3. (Continued) fractions: 2.36 to 9.5 mm and passing 2.36 mm. For both sands, the 4.75 to 9.5 fraction contained less than 5%, and this fraction was combined with the 2.36- to 4.75-mm fraction to create a 2.36- to 9.5-mm fraction. The mineral filler does not need to be further broken down, and the batch weight is as given in Table 8-14. Size Fraction Wt. % Batch Weight (g) No. 7 Traprock 12.5 to 19.0 mm 7 198 9.5 to 12. 5 mm 40 1,133 4.75 to 9. 5mm 21 595 2.36 to 4.75 mm 23 651 Passing 2.36 mm 9 255 Total 100 2,832 Traprock Screenings 4.75 to 9. 5 mm 10 342 2.36 to 4.75 mm 33 1,129 Passing 2.36 mm 57 1,950 Total 100 3,421 Manufactured Sand 2.36 to 9.5 mm 22 753 Passing 2.36 mm 78 2,669 Total 100 3,422 Natural Sand 2.36 to 9.5 mm 9 160 Passing 2.36 mm 91 1,611 Total 100 1,771 Mineral Filler Passing 0.075 mm 100 354 Table 8-16. Aggregate breakdown and batch weight for example 3.

A major research project on mixing and compaction temperatures for HMA was completed as this manual was being completed; the results have been compiled in NCHRP Report 648: Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Two new promising procedures for determining mixing and compaction temperatures were recommended for further evaluation. The phase angle method involves developing a high-temperature master curve for binder phase angle, determining the frequency where the phase angle is 86 degrees and then applying empirical equations to determine mixing and compaction temperatures. In the steady shear method, viscosity is determined at high shear stresses over a range of temperatures. Viscosity values at a shear stress of 500 MPa are then plotted on a log viscosity versus log temperature chart to determine mixing and compaction temperatures. At the time this manual was being completed, neither method had been accepted as an AASHTO standard, but it is possible than one or both methods could be adopted in the future. Heating materials prior to compaction will typically take from 2 to 4 hours, but the actual time required to heat aggregates and binders to reach the specified mixing temperature will vary considerably depending on the size and type of oven used, the amount of material being heated, and the properties of the aggregate. The oven should be set to a temperature about 15°C above the mixing temperature range. The actual temperature of the aggregates and binder should be checked prior to mixing and compaction with a properly calibrated electronic thermometer. Mixing bowls, mixing paddles/stirrers, and gyratory compaction molds must also be heated to the same compaction temperature range prior to compacting specimens. Mix Aggregate and Asphalt Binder Because the use of 150-mm-diameter specimens requires very large batch sizes, laboratory mixing should be done with a large, heavy-duty mechanical mixer. Mixing should be done quickly and efficiently, so that the materials do not cool significantly before mixing is completed. If the mix is the first one to be prepared that day, the mixer should be “buttered” first. This is important because significant amounts of binder and fine aggregate will stick to the bowl and stirrer during mixing. If the mixer is not buttered first, binder and fines will be removed from the batch, and its composition will not be as designed. The mixer can be buttered either by mixing a batch of HMA or sand asphalt that is then discarded. The composition of these materials is 132 A Manual for Design of Hot Mix Asphalt with Commentary 10 100 1,000 10,000 100 110 120 130 140 150 160 170 180 Temperature, OC Vi sc os ity , m Pa -s 250 to 310 mPa-s 150 to 190 mPa-s compaction temperature 138 to 144 °C mixing temperature 150 to 156 °C Figure 8-7. Example viscosity-temperature chart showing determination of mixing and compaction temperature ranges for a non-modified binder.

not important—their only purpose is to coat the mixing bowl and stirrer with binder and fine aggregate. The mixer need only be buttered prior to the first batch of the day. After that, the mixing bowl and stirrer should remain well coated as additional batches are prepared, so that additional buttering is not needed. The procedure for actual mixing of HMA in the laboratory is as follows. Place the heated bowl on an electronic balance, and zero the balance. Form a depression in the center of the aggregate, and weight out the appropriate amount of hot binder into the aggregate. Place the bowl with aggregate and asphalt on the mixer stand, attach the heated stirring attachment, and begin mixing. Mix just until the aggregate is thoroughly coated with binder—too much mixing can cause the aggregate to break down, changing the aggregate gradation in the specimen. The mix is now ready for short-term oven conditioning, as described below. Figure 8-8 shows a typical mixer used in preparing HMA specimens in the laboratory. When HMA is produced in a plant, it is not immediately placed and compacted. Often it is held in a silo, placed in a truck, hauled to the site, and then placed and compacted. During this time period the hot aggregate in the mix may absorb significant amounts of asphalt binder, potentially changing the composition and properties of the mix. Short-term oven conditioning of HMA in the laboratory, as described below, is designed to imitate the absorption of binder that occurs during actual production. Short-Term Oven Conditioning The procedure for performing short-term oven conditioning is described in AASHTO R 30, Mixture Conditioning of Hot-Mix Asphalt. Immediately after mixing the aggregate and asphalt, place it in a shallow metal pan, spreading it out evenly until the depth is between 25 and 50 mm. Place the mix in a forced-draft oven, pre-heated to within 3°C of the midpoint of the compaction Design of Dense-Graded HMA Mixtures 133 Figure 8-8. Typical mixer used in preparing HMA specimens in the laboratory.

temperature range for the mixture. Condition the mix for a total time of 2 hours ± 5 minutes, stirring after 1 hour ± 5 minutes. For mixtures having a water absorption value over 2%, the conditioning time should be extended to 4 hours ± 5 minutes, and the mixture should be stirred every hour. Also, if the mix is to be used to prepare specimens for performance testing, the con- ditioning time should be 4 hours ± 5 minutes at 135°C, regardless of the aggregate absorption. Specimens are compacted immediately after completion of short-term oven conditioning. Figure 8-9 shows HMA mixture spread out in a pan for short-term oven conditioning. Compact Laboratory Specimens In this mix design method, specimens are compacted using the Superpave gyratory compactor (SGC), using the standard angle of gyration of 1.25°, and a compaction pressure of 600 kPa, as described in AASHTO T 312, Preparing and Determining the Density of Hot-Mix Asphalt Specimens by Means of the Superpave Gyratory Compactor. It is essential that the SGC is properly maintained and calibrated; engineers and technicians should refer to appropriate specifications and the manufacturer’s instructions for information on maintaining and calibrating their device. Prior to compacting specimens, make sure that the mold, top plate, and base plate have been heated to the compaction temperature for the mix. This generally takes about an hour in an oven set to the compaction temperature. The HMA mix must be short-term conditioned, as described above, prior to compaction. Remove the molds and plates from the oven, place the base plate inside the mold and place a paper disk on the base plate. Then, place the hot mixture in the mold, level, and cover with another paper disk. Place the top plate over the paper disk, and place the mold in the compactor. Set the compactor to the appropriate level of Ndesign (see Table 8-2) and compact the specimen. After compaction is complete, remove the mold from the SGC, and then remove the specimen from the mold. SGCs are normally equipped with a sample press for extruding compacted specimens from molds. Removing the specimens should be done slowly to avoid distorting or even breaking the specimen. Waiting a few minutes after completing compaction to allow the specimen to cool can help prevent damage to the specimen during de-molding. Specimens compacted to high air void levels—about 6% or more—can be even more prone to damage during de-molding and may require additional cooling before removal from the SGC mold. 134 A Manual for Design of Hot Mix Asphalt with Commentary Figure 8-9. Short-term oven conditioning of HMA mixture in the laboratory.

Remove the paper disks from the top and bottom of the specimen and allow the specimen to cool at room temperature. Handle freshly compacted specimens carefully to avoid damaging them. Specimens must be completely cool prior to performing bulk specific gravity tests, as required for volumetric analysis. Figure 8-10 shows an SGC mold, top and base plates, and the paper disks used in compacting specimens. Calculate Volumetric Composition of Laboratory Specimens Chapter 5 of this manual described in detail volumetric analysis of HMA in the laboratory, including the primary tests involved—bulk and maximum theoretical specific gravity of HMA mixtures. Interested readers or those unsure of the details of these tests and the calculations used in volumetric analysis may wish to review Chapter 5. Some of the calculations are similar to those used in estimating the composition of trial batches as presented previously in Step 9. The information presented here is meant only to be a brief review of the major features of volumetric analysis. Volumetric analysis of compacted HMA mixtures involves two laboratory tests: bulk specific gravity of the compacted HMA mixture and theoretical maximum specific gravity of the loose HMA mixture. As discussed in Chapter 5 of this manual, there are two procedures for determining bulk specific gravity of HMA mixtures: • AASHTO T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens, and • AASHTO T 275, Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin- Coated Specimens. AASHTO T 166 can be used for most HMA mixtures; however, if the absorption of the specimens during AASHTO T 166 is greater than 2.0%, AASHTO T 275 should be used. The procedure for theoretical maximum specific gravity is given in AASHTO T 209, Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures. Bulk specific gravity gives the specific gravity of the compacted specimen, including air voids within the mixture. The theoretical maximum specific gravity is the specific gravity of the Design of Dense-Graded HMA Mixtures 135 Figure 8-10. Mold, top and base plates, and paper disks used in compacting specimens with the superpave gyratory compactor.

mixture at zero air voids; if a laboratory specimen could be compacted to zero air voids, the bulk and theoretical maximum specific gravity values would be equal. One of the most important equations used in volumetric analysis of HMA is Equation 8-13, which relates air void content, bulk specific gravity of the compacted mixture, and maximum theoretical specific gravity: where VA = air void content, volume % Gmb = bulk specific gravity of compacted mixture Gmm = maximum theoretical specific gravity of loose mixture Various other equations given in Chapter 5 are used to calculate other important properties or volumetric factors of the mixture: • Total asphalt content by weight (Pb) • Effective asphalt content by weight (Pbe) • Absorbed asphalt content by weight (Pba) • Total asphalt content by volume (VB) • Effective asphalt content by volume (VBE) • Absorbed asphalt content by volume (VBA) • Voids in the mineral aggregate (VMA) • Voids filled with asphalt (VFA) • Dust/binder ratio (D/B) • Apparent film thickness (AFT) The normal practice in HMA mix design is to determine the bulk specific gravity of two com- pacted specimens, and then heat these specimens and break them up and use the resulting loose mixture to determine the maximum theoretical specific gravity of the mixture. Alternately, extra loose mixture can be prepared when the specimens are compacted and used in the determina- tion of maximum specific gravity. Actual calculation of air void content, VMA, and other volu- metric factors is usually done using a spreadsheet such as HMA Tools. Most SGCs also include spreadsheets for performing these calculations. Values for specified volumetric factors are then compared to the requirements for the mixture. In a complete mix design for new materials, this comparison is made for three trial mixtures made at widely different coarse aggregate contents, to determine the aggregate blend that will provide the proper volumetric composition. In many cases, an existing mix design is being slightly modified, and only one or two trial mixtures will be evaluated. VA G G mb mm = − ⎛⎝⎜ ⎞⎠⎟⎡⎣⎢ ⎤ ⎦⎥100 1 8 13( )- 136 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 8-4. Volumetric Analysis of an HMA Mixture Table 8-17 summarizes a typical volumetric analysis as performed in the evaluation of three trial mixes. The dense/ fine mixture in Table 8-17 is the same trial mix used in the examples given in Tables 8-13 through 8-16. The other two trial mixes—the dense/dense and dense/coarse—have been developed using the same binder and same aggregates, but blended in different proportions, as listed in Table 8-18; this table also includes proportions for a fourth trial mix, discussed below. Table 8-17 shows the specific gravity test data and calculations and the results of volumetric analysis for all three trial mixtures. Table 8-17 includes specification limits, and also lists equations that can be used for calculating the various volumetric factors, such as air void content and VMA.

Design of Dense-Graded HMA Mixtures 137 Example Problem 8-4. (Continued) Most often, calculations such as those used in compiling Table 8-17 are done using a spreadsheet developed for this purpose, such as HMA Tools or spreadsheets included with many SGCs. In HMA Tools, specific gravity data for the trial batches are entered in the worksheet “Specific_Gravity”; the necessary calculations are performed and the resulting volumetric composition is summarized in the worksheet “Trial_Blends.” Three specified volumetric factors are shown in Table 8-17 and included in most volumetric analyses: • Air void content • VMA • Dust/binder ratio The air void content in this example has an allowable range of 3.5 to 4.5%. This range has been established for practical purposes, since it is very difficult to match the target air void content of 4.0% exactly. Also, it must be realized that laboratory mix designs almost always must be adjusted during field production, so attempting to exactly meet air void requirements in a laboratory mix design is usually pointless. VMA requirements for dense-graded HMA mixtures are given in Table 8-5; the allowable range for VMA for a 12.5-mm NMAS mixture is from 15.0 to 17.0%. The specified range for dust/binder ratio is 0.8 to 1.6, as given in Table 8-12 previously. Specification Property Equation Min. Max. Trial Mix 1: Dense/Fine Mix Trial Mix 2: Dense/Dense Mix Trial Mix 3: Dense/Coarse Mix Bulk Specific Gravity of Compacted Mixture Dry mass in air, g --- --- --- 5,221.0 5,190.3 5,135.7 5,392.8 5,321.5 5,175.7 Saturated, surface-dry mass in air --- --- --- 5,241.1 5,211.4 5,153.0 5,414.4 5,343.6 5,212.3 Mass in water --- --- --- 3,160.9 3,140.3 3,172.9 3,324.2 3,228.7 3,137.2 Bulk specific gravity, dry basis 5-1 --- --- 2.510 2.506 2.594 2.580 2.516 2.494 Theoretical Maximum Specific Gravity of Loose Mixture Dry mass in air, g --- --- --- 2,109.7 2,245.5 2,225.8 2,156.9 2,076.4 2,332.7 Mass in water --- --- --- 1,312.3 1,394.3 1,394.0 1,352.9 1,312.0 1,471.5 Theoretical maximum specific gravity 5-2 --- --- 2.646 2.638 2.676 2.683 2.716 2.709 Average --- --- --- 2.642 2.679 2.713 Volumetric Analysis Aggregate bulk specific gravity, dry basis 5-3 --- --- 2.846 2.879 2.915 Air void content, Vol. % 5-4 3.5 4.5 5.1 3.5 7.6 VMA, Vol. % 5-11 14.0 16.0 15.9 14.2 17.9 Asphalt content, Wt. % 5-5 --- --- 4.60 4.55 4.53 Effective asphalt content, Wt. % 5-9 --- --- 4.44 4.27 4.22 Effective asphalt content, Vol. % 5-8 --- --- 10.9 10.8 10.3 VFA, Vol. % 5-12 --- --- 68.2 75.7 57.4 Mineral filler (dust) content, Wt. % 8-10 --- --- 7.9 6.6 5.1 Dust/binder ratio 8-11 0.8 1.6 1.79 1.54 1.20 Table 8-17. Summary of volumetric analysis for example HMA mix design. Aggregate Trial Mix No. 1: Dense/Fine Trial Mix No. 2: Dense/Dense Trial Mix No. 3: Dense/Coarse Trial Mix No. 4: Dense/Fine No. 7 Traprock 24 45 68 58 Screenings 29 21 12 16 Manufactured sand 29 21 12 16 Natural sand 15 10 5 8 Mineral dust 3 3 3 2 Table 8-18. Aggregate proportions for trial mixes listed in Table 8-17.

138 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 8-5. Adjusting an HMA Trial Mixture Looking at the three specified volumetric factors in Table 8-17, the dense/dense trial mixture appears to meet all specification requirements—the average air void content of 3.5% is acceptable, as is the average VMA of 14.2%; and the dust/binder ratio of 1.54% is also within the specified range. As long as the workability of this mix is acceptable, it would be an acceptable final mix design. However, the air void content, VMA, and dust/binder ratio values are all close to various limits. It is therefore desirable to adjust this mixture to obtain values closer to the midpoint for air voids, VMA, and dust/binder ratio. In deciding how to adjust the composition of the fourth trial mix, it is helpful to present the data in Tables 8-17 and 8-18 graphically. Figure 8-11 shows the gradation plot (top) and CMD plot (bottom) for the three initial trial mixes. This figure also includes the fourth trial mix, shown as the dashed line. The plots are 0 20 40 60 80 100 0.01 0.10 1.00 10.00 100.00 1000.00 Particle Size, mm % P as si ng b y W ei gh t Trial, voids/VMA No. 1, 5.1/15.9 No. 2, 3.5/14.2 No. 3, 7.6/17.9 Trial No. 4 Min., 3.5 / 14.0 Max., 4.5 /16.0 Max. Dens. -15 0 15 0.01 0.10 1.00 10.00 100.00 Particle Size, mm D ev ia tio n fro m M ax im um D en si ty G ra da tio n densecoarse fine Figure 8-11. Top: gradation plot for example mix design, including fourth trial mix; bottom: CMD plot for example mix design.

Design of Dense-Graded HMA Mixtures 139 Example Problem 8-5. (Continued) as produced in HMA Tools, though similar plots can be prepared using other spreadsheets or software packages. The legend, given in the top plot, includes the air void content and VMA values for each of the initial three trial mixes, as listed in Table 8-18. It also includes the minimum and maximum for air void content and VMA, given in the legend under “Min.” and “Max.” As noted above, the second trial mix—the dense/dense mix—meets all requirements for the mix design. However, both the measured air void content (3.5%) and the VMA value of 14.1% are at or very near to the minimum values of 3.5% and 14.0%, respectively. It appears that the air void content and VMA could be increased by either making the gradation slightly finer or slightly coarser. However, because the dust/binder ratio of the dense/fine mix is too high (and only marginal for the dense/dense trial mix), making the fourth trial mix slightly coarser will ensure that the dust/binder ratio remains within allowable limits. The aggregate blend for the fourth trial mix is therefore designed to be intermediate between the dense/dense gradation and the dense/coarse gradation, as is shown in Figure 8-11. It also has slightly less mineral dust (2% rather than 3%) in order to keep the dust/binder ratio near the center portion of the specification. The next step in the mix design process is to calculate mix proportions and batch weights for the fourth trial mix, in the same way as was done for the initial trial mixtures. Two specimens are then prepared and their bulk and theoretical maximum specific gravity values determined. A second volumetric analysis is performed to determine if this fourth trial mix better meets the specified requirements. The results of specific gravity tests and the resulting volumetric analysis are shown in Table 8-19. The specification properties—air void content, VMA and dust/binder ratio—are well within the specified range. The air void content and VMA are slightly high, but this is desirable since both will tend to drop during field production. Therefore, trial mix four is accepted as the final laboratory mix design. Specification Property Min. Max. Dense/Fine Mix Dry mass in air, g --- --- 5,221.3 5,182.9 Saturated, surface-dry mass in air --- --- 5,237.5 5,206.6 Mass in water --- --- 3,214.6 3,200.4 Bulk specific gravity, dry basis --- --- 2.581 2.583 Dry mass in air, g --- --- 2,115.7 2,268.2 Mass in water --- --- 1,328.6 1,422.7 Theoretical maximum specific gravity --- --- 2.651 2.658 Average --- --- 2.685 Aggregate bulk specific gravity, dry basis --- --- 2.900 Air void content, Vol. % 3.5 4.5 3.8 VMA, Vol. % 14.0 16.0 15.0 Asphalt content, Wt. % --- --- 4.59 Effective asphalt content, Wt. % --- --- 4.45 Effective asphalt content, Vol. % --- --- 11.2 VFA, Vol. % --- --- 74.5 Mineral filler (dust) content, Wt. % --- --- 5.0 Dust/binder ratio 0.8 1.6 1.13 Table 8-19. Summary of volumetric analysis trial mix four for example HMA mix design.

Adjusting Aggregate Proportions to Meet VMA and Other Volumetric Requirements The procedure described here for adjusting aggregate proportions to meet the given require- ments for VMA and air void content is straightforward. However, a short discussion may help inexperienced technicians and engineers better understand this important topic. First, it should be again emphasized that the procedure presented here is only one of many possible techniques for preparing aggregate blends for trial mixtures prepared during the mix design process. This manual is in large part intended as an instructional tool for the inexperienced; for this reason, the approach given here is relatively simple and flexible and relies on HMA Tools for performing cumbersome calculations. Technicians and engineers who have successfully used other procedures with consistently satisfactory results should continue to use them. Those who try the procedures given here, but think they need some additional tools should look into other procedures; as discussed earlier in this chapter, the Bailey method has recently become a very popular method for blending aggregates to meet volumetric requirements. The relationship between VMA, air void content, and effective asphalt content must be under- stood to fully appreciate the procedure given in this manual. VMA is composed of air voids and effective asphalt (a small amount of asphalt binder is absorbed into the aggregate surface). There- fore, if the target VMA is fixed, once the target air void content is met, the effective asphalt content is also met. Therefore, there is no need to simultaneously evaluate air void content and VMA—once the proper air void content is obtained, the VMA level will also meet requirements. HMA Tools makes this calculation for the user, so there is no need to calculate the binder content. However, there is some flexibility in selecting target values for VMA and air void content, which indirectly allows for adjustments in asphalt binder content. Lower VMA values will give less binder, higher VMA values will give more binder. Lower air void contents will provide additional binder at a given VMA value, while higher air void contents will provide less binder at a given VMA value. In the first trial batch in a series, HMA Tools assumes that the amount of absorbed binder is one-half the calculated water absorption (calculated from aggregate bulk and apparent specific gravity values). However, after this first trial batch, HMA Tools compares estimated absorption values with those actually measured in the laboratory and adjusts absorption values in subsequent batches accordingly. The general rule given here for adjusting aggregate blends to meet VMA requirements is that the closer an aggregate gradation is to a maximum density gradation, the lower will be its VMA. Technicians and engineers should remember that this is only an approximate rule. The maximum density gradation is only approximated by the 0.45 power law; the actual maximum density gradation for a given set of aggregates may deviate significantly from this. Furthermore, some aggregates have unique properties that will affect mixture VMA in unusual ways. For example, relatively soft aggregates can break down during compaction—especially at high gyration levels— making it difficult to reach high VMA values. Other aggregates, with unusually good texture or angular shape, will tend to increase VMA, even when their addition would seem likely to make the aggregate blend denser. The specifications given in this manual for dense-graded HMA may require some mix designs to be adjusted by increasing the mineral filler content. Although a certain amount of mineral filler is necessary for good rut resistance and durability, adding mineral filler to a mix design will normally tend to reduce VMA. Thus, adding mineral filler to a mix design will require adjusting the gradation to provide additional VMA to compensate for the effect of increasing mineral filler. Aggregate blending is one of the most critical aspects of the HMA mix design process, and proficiency requires practice, experience, and judgment. Conduct Performance Testing as Required The final stage of laboratory work in an HMA design involves evaluating the performance of the mixture. Chapter 6 of this manual presents a thorough discussion of various factors 140 A Manual for Design of Hot Mix Asphalt with Commentary

affecting HMA performance and ways of evaluating this performance through mixture testing and analysis. The following discussion is limited to the practical application of performance test- ing as part of the routine design of dense-graded HMA mixtures. This involves the evaluation of (1) moisture resistance for all mixtures and (2) evaluation of rut resistance for mixtures designed for traffic levels of 3 million ESALs and higher. As discussed in Chapter 6, more advanced types of performance testing, such as the IDT creep and strength test and fatigue testing, are in general not suitable for use in routine mix design, though they may be useful in research and in developing HMA mixtures for critical or special applications. Evaluate Moisture Resistance The moisture resistance of all dense-graded HMA mixtures should be evaluated using AASHTO T 283. Moisture resistance testing is normally performed after a mix design has been developed that meets all requirements for binder grade, mixture composition, and compaction. As also discussed in Chapter 6, in AASHTO T 283, specimens are prepared to an air void content of 7.0 ± 0.5%, then divided into two subsets with approximately equal average air void contents. The tensile strength of one subset is measured dry. The tensile strength of the second subset is measured after conditioning by vacuum saturation followed by a freeze-thaw cycle and a warm-water soak. The ratio of the average tensile strength of the conditioned to unconditioned subsets and a visual assessment of stripping is used to assess moisture sensitivity. A mixture is considered acceptable if the tensile strength ratio is equal to or greater than 80% and there is no visual evidence of stripping in the conditioned test specimens. There are several ways of improving the performance of mixtures initially failing these require- ments for moisture resistance. Small amounts of anti-strip additives can be added to the mixture. Anti-strip suppliers should be contacted concerning recommended products and concentrations for a given HMA mix design. Local hot-mix suppliers may be able to offer suggestions concerning the most effective anti-strip additives for local materials. In many cases, the binder supplier can blend an appropriate anti-strip additive at the necessary concentration directly into the binder. This is a simple but effective approach that requires no special equipment at the plant. One of the most effective and least expensive anti-strip additives is hydrated lime. When used to help prevent moisture damage, hydrated lime should be blended with water to form a slurry and applied directly to the aggregate prior to heating. The typical concentration is 1% hydrated lime by aggregate weight. If used in a laboratory mix design, a slurry composed of 50% hydrated lime and 50% water by weight is prepared and applied to the aggregate prior to heating. Besides anti-strip additives, the other ways of improving moisture damage are to change the binder, the aggregate, or both materials. Different binders can exhibit a wide range in susceptibility to moisture damage. Aggregates that are most susceptible to moisture damage are those that contain significant amounts of quartz, including many igneous rocks. Eliminating these aggregates from a mix design can, in some cases, significantly improve moisture resistance. Evaluate Rut Resistance Before discussing rut resistance testing in detail, it must be noted that the design procedure set forth in this manual has been structured to provide HMA mix designs that will exhibit a high level of rut resistance. The level of reliability against excessive rutting—even without performance testing—ranges from 90 to over 99%, with a typical level of about 95% reliability for design traffic levels of 3 million ESALs and higher. The purpose of rut resistance testing is to increase this level of reliability. For three of the rut resistance tests discussed below—the flow number from the asphalt mixture performance tester (AMPT), the repeated shear at constant height (RSCH) test, and the high-temperature indirect tension (HT-IDT) strength test—the suggested minimum or maximum test values were determined specifically to increase the level of reliability against excessive rutting from about 95 to 98% and higher. It must be emphasized that the reliability Design of Dense-Graded HMA Mixtures 141

achieved through the recommended performance tests is a result of applying both the suggested mix design procedure and the selected performance test together. If the given guidelines for performance test results are applied to mixtures designed following some other procedure, the resulting level of reliability will not necessarily be the same. It might be similar, or it might be lower or higher. It should also be noted that the specified test values have in most cases been selected so that if the procedures given in this manual are followed, most of the resulting HMA mixtures will pass the selected performance test. It is estimated that only about 10 to 20% of properly designed mixtures will fail. Thus, the suggested rutting performance tests not only increase reliability against excessive rutting to a very high level, they do so in a relatively efficient way. The suggested maximum rut depths for the asphalt pavement analyzer (APA) and the Hamburg Wheel-Track (HWT) tests were taken from specifications already in place in numerous states. In this case, implementation of these performance tests will certainly increase the reliability against excessive rutting, but the specific amount of improvement is unknown as is the percentage of mixes likely to fail the tests. However, because these tests with the stated maximum rut depths have been implemented in several states, it is likely that the increase in reliability and the rejection rate will both be reasonable. The various rut resistance tests and guidelines are summarized below. The stated minimum or maximum values for each test should be considered guidelines. Although based either on a careful analysis of laboratory and field data or on existing standards, it is quite possible that these values will need to be adjusted by the specifying agency for optimum results in the region. Factors that need to be considered when making such adjustments are climate, the types and grades of binders commonly used in a given locale, aggregates with unusual properties, and typical traffic mixes and traffic levels. For various reasons, some agencies may wish to alter the conditions a test is run under, which will significantly alter the resulting test values and the appropriate specification values. For details on the proper procedures for performing each test, laboratory engineers and technicians should refer to the appropriate standard test method as listed at the end of this chapter. Some additional background on performance testing in general and on these five tests in particular is provided in Chapter 6 of this manual. The Asphalt Mixture Performance Tester. The asphalt mixture performance tester (AMPT) was initially called the simple performance test system or SPT. Details of the latest equipment specification and test procedure are given in NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester. Tests are performed on specimens cored and trimmed from large gyratory specimens to final nominal dimensions of 100 mm diameter by 150 mm high. There are three different tests for rut resistance using the AMPT: the dynamic modulus (sometimes referred to as the E* test), the repeated load test (also called the flow number test) and the flow time test. To use the E* test to evaluate rut resistance the E* Implementation Program software must be used. This software was not yet commercially available at the time this manual was written, but should soon be available from AASHTO. In the flow number test, a 600-kPa load is applied to the specimen every second, until the flow point is reached. The flow point represents failure of the specimen, as evidenced by an increasing rate of total permanent strain during the test. Flow number tests are run at the average, 7-day maximum pavement temperature 20 mm below the surface, at 50% reliability as determined using LTPPBind, Version 3.1. Test specimens should be prepared at the expected in-place air void content, typically about 7%. Table 8-20 lists minimum values for flow number determined using the AMPT. The flow time test is similar to the flow number test, but a constant load is applied to the specimen and the total deformation is monitored. Test temperature and specimen preparation are identical to those used in the flow number test described previously. It is simply a static creep test and the flow time is the loading time required to initiate tertiary creep, which is the point at 142 A Manual for Design of Hot Mix Asphalt with Commentary Traffic Level Million ESALs Minimum Flow Number Cycles < 3 --- 3 to < 10 53 10 to < 30 190 ≥ 30 740 Table 8-20. Recommended minimum flow number requirements.

which the rate of deformation begins to increase. Recommended minimum flow times are given in Table 8-21. The Asphalt Pavement Analyzer. The asphalt pavement analyzer (APA) is growing in popularity among pavement agencies as a test for evaluating the rut resistance of HMA pavements. At the time this manual was written, nine states had specifications for performance testing of HMA pavements using the APA device. The APA test method is available as AASHTO TP 63, which was developed from the test procedure found in Appendix B of NCHRP Report 508: Accelerated Laboratory Rutting Tests—Evaluation of the Asphalt Pavement Analyzer. In the APA test, a pressurized hose is placed over a short cylindrical HMA specimen and a wheel is repeatedly passed over the hose. The rut depth is measured after several thousand cycles. Details of the APA test are given in Chapter 6 of this manual. Typical conditions for the APA test—and the ones suggested here for using this procedure as a performance test—are as follows: • Hose pressure: 100 lb/in2 • Wheel load: 100 lbf • Seating cycles: 50 • Test cycles: 8,000 • Specimen size: 75-mm deep by 150-mm diameter • Specimen air void content: 4.0 ± 1.0% • Rut depth calculated as the average of three tests of two specimens (six specimens total) The APA test is most frequently run at 64°C. However, to account for differences in local climate, it is suggested that the APA test be run at the temperature corresponding to the high-temperature binder performance grade specified for the project by the agency for traffic levels of 3 million ESALs or more. Suggested criteria for the APA in terms of maximum rut depth after 8,000 loading and 50 seating cycles are given in Table 8-22. These guidelines are based on values used by the Oklahoma Department of Transportation and are fairly typical for agencies using this test. However, suitable limits for the APA test will depend on the test conditions—if the test conditions are varied, the maximum rut depths may also need to be changed. Furthermore, as with the other performance test guidelines given in this manual, agencies using the APA as a performance test should modify the maximum allowable rut depths given in Table 8-22 if, in their judgment, such modifications are needed to account for unusual conditions or materials. The Hamburg Wheel-Track Test. The Hamburg wheel-track test (here termed the “Hamburg test”) is, like the APA test discussed above, a “torture” test for evaluating the rut resistance or moisture resistance of HMA mixtures, but the procedure given here is meant only to evaluate rut resistance. In the Hamburg test, a 204-mm (8-in)-diameter, 47-mm-wide steel wheel is passed over an HMA slab immersed in a heated water bath. The Hamburg test is not as widely used as the APA, so it is not possible to provide typical test conditions and guidelines. The following test conditions are used by the Texas Department of Transportation: • Specimen dimensions: 150 mm (6 in) in diameter, 62 ± 2 mm- (2.4 in) thick • Wheel load: 705 ± 2 N (158 ± 0.5 lb) • Air void content: 7 ± 1% • Test temperature: 50 ± 1°C The requirements for the Texas version of the Hamburg test are given in Table 8-23. As dis- cussed above, developing typical guidelines for the Hamburg test is difficult because the test is not widely used. A detailed procedure for the Hamburg wheel-track test is given in AASHTO T 324. Because this test is not as widely used as some others of this type, agencies wishing to use the Hamburg test as a performance test should consider performing an engineering study to develop appropriate requirements for their local conditions and materials. Design of Dense-Graded HMA Mixtures 143 Traffic Level Million ESALs Minimum Flow Time s < 3 --- 3 to < 10 20 10 to < 30 72 ≥ 30 280 Table 8-21. Recommended minimum flow time requirements. Traffic Level Million ESALs Maximum Rut Depth mm < 3 --- 3 to < 10 5 10 to < 30 4 ≥ 30 3 Table 8-22. Recommended maximum rut depths for the APA test.

Superpave Shear Tester/Repeated Shear at Constant Height. The Superpave Shear Tester, or SST, can also be used effectively to evaluate the rut resistance of HMA mixtures using the repeated shear at constant height (RSCH) test. Like the flow number test, the RSCH test is a repeated load test, however, the load is applied in shear rather than in compression as in the flow number test. The primary test result is the maximum permanent shear strain (MPSS), which is the total accumulated shear strain at 5,000 loading cycles. However, the SST is a complicated, expensive piece of equipment and the RSCH test can be difficult to run. Therefore, it is not recommended that commercial laboratories, hot-mix producers, and similar organizations purchase SST devices for use in routine mix design work. Either the AMPT or the IDT strength test are far better suited for routine use in HMA mix design and analysis. However, some laboratories in the United States and Canada have SST devices and use them regularly both for research purposes and in the design and analysis of critical or unusual HMA mix designs. As an aid to these laboratories, Table 8-24 gives the recommended maximum allowable values for MPSS. AASHTO has developed a standard procedure for the test, listed under Standard T 320, Determining the Permanent Shear Strain and Stiffness of Asphalt Mixtures Using the Superpave Shear Tester. The maximum values for MPSS given in Table 8-24 are based on specimens prepared at 3.0 ± 0.5% air void content, as recommended in AASHTO T 320. Indirect Tensile Strength at High Temperatures. Recently some studies have been done supporting the use of high-temperature indirect tensile (HT-IDT) strength to evaluate the rut resistance of HMA mixtures; a good description of this test (and other performance tests) can be found in “New Simple Performance Tests for Asphalt Mixes” in Transportation Research Circular E-C068. Although not as widely used as the other methods given here, it is a very simple inexpensive test that most engineers and technicians are already familiar with. The HT/IDT strength test is performed as described in AASHTO T 283 for unconditioned (dry) specimens, but at a test temperature that is 10°C below the average, 7-day maximum pavement temperature, 20 mm below the pavement surface at 50% reliability, as determined using LTPPBind, Version 3.1. Unlike AASHTO T 283, in this procedure, specimens should be compacted using the design gyrations, which should produce an air void content close to 4.0%. As described in AASHTO T 283, the specimens are conditioned at the test temperature by placing them in a water bath controlled to within ± 0.5°C of the test temperature for 2 hours ± 10 minutes. The specimens should be wrapped tightly in plastic or placed in a heavy-duty, leak-proof plastic bag prior to conditioning, to prevent them from getting wet. Table 8-25 lists recommended minimum values for HT/IDT strength determined following this protocol. Design Traffic Speed, Depth within the Pavement and Performance Test Requirements Earlier in this chapter the effect of traffic speed on binder grade was discussed—as design traffic speed decreases, the required high-temperature binder grade increases significantly (see Table 8-1). This is because loading at low speeds will cause much more rutting in a pavement than loading at fast speeds, all else being equal. For this reason, the test requirements given above for the various performance tests should be adjusted if the design traffic speed is slow (25 to < 70 kph or 15 to < 45 mph) or very slow (< 25 kph or < 15 mph). Perhaps the simplest approach to making 144 A Manual for Design of Hot Mix Asphalt with Commentary High Temperature Binder Grade Minimum Passes to 0.5-inch Rut Depth PG 64 or lower 10,000 PG 70 15,000 PG 76 or higher 20,000 Table 8-23. Texas requirements for Hamburg wheel tracking test. Traffic Level Million ESALs Maximum Value for MPSS % < 3 --- 3 to < 10 3.4 10 to < 30 2.1 ≥ 30 0.8 Table 8-24. Recommended maximum values for MPSS determined using the SST/RSCH test. Traffic Level Million ESALs Minimum HT/IDT Strength kPa < 3 --- 3 to < 10 270 10 to < 30 380 ≥ 30 500 Table 8-25. Recommended minimum high- temperature indirect tensile strength requirements.

these adjustments, and the one recommended in this manual, is to adjust the test temperature upwards as design traffic speed decreases. The recommended temperature adjustment is +6°C for slow traffic and +12°C for very slow traffic. However, as with other aspects of performance testing, agencies should use judgment and experience with local conditions and materials when establishing performance test requirements for slow and very slow traffic speeds. For many applications, performance testing is probably only necessary on material placed within 100 mm of the pavement surface. However, for critical projects, material placed 100 to 200 mm within the pavement surface might also be tested. In such cases, the test temperature should be adjusted to reflect the estimated temperature at the surface of the material as placed within the pavement structure, as determined using LTPPBind. For example, a base course placed 100 mm below the pavement surface will have an estimated critical high pavement temperature 7.7°C lower than material at the pavement surface; the test temperature for this material would then be reduced by 7.7°C. Adjusting Mix Designs to Improve Rut Resistance As mentioned earlier in this section, the rut resistance tests and recommended minimum and maximum values for test results have been selected so that most dense-graded HMA designs developed following the procedures given in this manual will meet the requirements, and no additional laboratory work will be needed. However, some mix designs will fail to meet require- ments for rut resistance. In such cases, the test results should first be checked to make sure there were no errors in either the procedures used or in the calculation of the test results. If no errors are found, and the test results are close to meeting the requirements, the test can be repeated. In this case, the results of both tests should be averaged and compared to the test criteria. If the mix still fails to meet the requirements for rut resistance testing, the mix design will have to be modified. Rut resistance of an HMA mix design can be improved as follows: • Increase the binder high-temperature grade. • If the binder is not modified, consider using a polymer-modified binder of the same grade or one high-temperature grade lower. • If the binder is polymer-modified, try a different type of modified binder. • Increase the amount of mineral filler in the mix, adjusting the aggregate gradation if necessary to maintain adequate VMA. • Decrease the design VMA value, if possible, by adjusting the aggregate gradation. • Replace part or all of the aggregate (fine or coarse or both) with a material or materials having improved angularity. If a different asphalt binder is used in the mix, the volumetric composition should not change. However, if other aspects of the mix design are changed, the volumetric composition might change significantly, which will require further refinement of the mix prior to further rut resistance testing. Step 11. Compile Mix Design Report The final step in preparing an HMA mix design is compiling a report documenting the mix design. In many states, standard forms must be filled out by hot-mix producers and submitted to the appropriate state agency or office for approval. In some cases, engineers or technicians may wish to develop their own mix design reports, for internal purposes or for use on private jobs. In such cases, the following information should be included in the report: • The organization that performed the mix design. • The name of the technician or engineer responsible for developing the mix design. Design of Dense-Graded HMA Mixtures 145

• The date the mix design was completed. • The name of the client for which the mix design was developed. • The name of the project for which the mix design was developed (if applicable). • General mix design information, including the type of mix (surface course, intermediate course, base course), the nominal maximum aggregate size, the design traffic level, the Ndesign value, and any special requirements. • Complete aggregate information, including for each aggregate the producer, the size designation of the aggregate, gradation, specific gravity, and all applicable specification properties. • Binder information, including the binder performance grade and the name of the supplier. • Composition of the mixture, including the design air void content, the design VMA, the design VBE, the mineral filler content, the target dust/binder ratio, and the estimated unit weight for the mix • Brief comments on the workability of the mix. • The results of moisture resistance testing. • The results of rut resistance testing, if applicable (generally for mixtures designed for traffic levels of 3 million ESALs and over). The spreadsheet, HMA Tools, can generate a comprehensive mix design report containing all of this information as well as additional information on the results of trial mixtures evaluated during the mix design process. This report might be useful to some engineers and technicians for internal purposes and might also serve as a template for those wishing to develop their own customized mix design report. Bibliography AASHTO Standards M 320, Performance-Graded Asphalt Binder M 323, Standard Specification for Superpave Volumetric Mix Design R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA) R 35, Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt (HMA) T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens T 209, Theoretical Maximum Specification Gravity and Density of Bituminous Paving Mixtures T 269, Percent Air Voids in Compacted Dense and Open Asphalt Mixtures T 275, Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens T 283, Resistance of Compacted Asphalt Mixture to Moisture-Induced Damage T 312, Preparing and Determining the Density of Hot-Mix Asphalt Specimens by Means of the Superpave Gyratory Compactor T 320, Determining the Permanent Shear Strain and Stiffness of Asphalt Mixtures Using the Superpave Shear Tester. T 324, Hamburg Wheel-Track Testing of Compacted Hot-Mix Asphalt (HMA) TP 63-09, Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA) Other Publications 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. Bonaquist, R. F. (2008) NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester, TRB, National Research Council, Washington, DC, 130 pp. Brown, E. R., et al. (2004) NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot-Mix Asphalt Pavements, TRB, National Research Council, Washington, DC, 37 pp. Burns, Cooley, Dennis, Inc., AAPTP Project 04-03: Implementation of Superpave Mix Design for Airfield Pavements, Quarterly Progress Report for the Period January 1, 2007 through March 31, 2007, available at www.aaptp.us/ Report.Interim.04-03.pdf. 146 A Manual for Design of Hot Mix Asphalt with Commentary

Christensen, D. W., and R. F. Bonaquist (2006) NCHRP Report 567: Volumetric Requirements for Superpave Mix Design, TRB, National Research Council, Washington, DC, 57 pp. Christensen, D. W., et al. (2004) “Indirect Tension Strength as a Simple Performance Test,” New Simple Performance Tests for Asphalt Mixes, Transportation Research Circular E-C068, http://gulliver.trb.org/publications/ circulars/ec068.pdf, TRB, National Research Council, Washington, DC, pp. 44-57. Kandhal, P. S., and L. A. Cooley (2003) NCHRP Report 508: Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer, TRB, National Research Council, Washington, DC, 73 pp. Leahy, R. B., and R. B. McGennis (1999) “Asphalt Mixes: Materials, Design and Characterization,” Journal of the Association of Asphalt Paving Technologists, Vol. 68A, pp. 70-127. Maupin, G. W., Jr. (2003) Final Report: Additional Asphalt to Increase the Durability of Virginia’s Superpave Surface Mixes, Report VTRC 03-R15, Charlottesville, VA: Virginia Transportation Research Council, June, 13 pp. Vavrik, W. R., et al. (2002) Bailey Method for Gradation Selection in HMA Mixture Design, Transportation Research Circular E-C044, http://gulliver.trb.org/publications/circulars/ec044.pdf, TRB, National Research Council, Washington, DC, October, 34 pp. West, R. C., et al. (2010) NCHRP Report 648: Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt, TRB, National Research Council, Washington, DC, 77 pp. Design of Dense-Graded HMA Mixtures 147

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary incorporates the many advances in materials characterization and hot-mix asphalt (HMA) mix design technology developed since the conclusion of the Strategic Highway Research Program (SHRP).

The final report on the project that developed NCHRP Report 673 and Appendixes C through F to NCHRP Report 673 were published as NCHRP Web-Only Document 159. The titles of the appendixes are as follows:

• Appendix C: Course Manual

• Appendix D: Draft Specification for Volumetric Mix Design of Dense-Graded HMA

• Appendix E: Draft Practice for Volumetric Mix Design of Dense-Graded HMA

• Appendix F: Tutorial

The companion Excel spreadsheet HMA tool and the training course materials described in NCHRP Report 673 are available for download from the Internet.

In January 2012, TRB released NCHRP Report 714: Special Mixture Design Considerations and Methods for Warm Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot Mix Asphalt with Commentary. The report presents special mixture design considerations and methods used with warm mix asphalt.

In January 2012, TRB released an errata to NCHRP Report 673: Page 41, Table 4-7, and page 123, Table 8-10, respectively, should be replaced with a new table.

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