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

Chapter: Commentary to the Mix Design Manual for Hot Mix Asphalt

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Suggested Citation:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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:"Commentary to the Mix Design Manual for Hot Mix Asphalt." 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 Commentary, a companion to the Manual compiled during NCHRP Project 9-33, in- cludes large amounts of supporting technical information and detailed references that, although important, would distract the user’s attention from the essential information in the Manual. The information presented herein will also help engineers and researchers in the future to edit, up- date, and refine the Manual. The Commentary is organized in chapters, with each chapter in the Commentary correspon- ding exactly with a chapter in the Manual. Many of the chapters in the Commentary contain only general introductory information that can be found in many different texts and references on construction materials and pavements; in such cases, the chapters are brief and do not list spe- cific technical supporting information. Furthermore, information given in the Manual that is well supported, clearly referenced, or both, is generally not included in the Commentary. Tech- nical information in the Commentary has been presented as concisely as possible, its primary purpose being to provide a record for those wishing to thoroughly evaluate and revise the Man- ual in the future. It is not necessary to read the Commentary in order to understand or use the information in the Manual. The Commentary is intended for researchers and engineers seeking a deeper understanding of some of the more complex technical underpinnings of the Manual. 225 Commentary to the Mix Design Manual for Hot Mix Asphalt

This chapter serves only as an introduction to the Manual and contains no critical information that requires additional supporting details or justification. 226 C H A P T E R 1 Introduction

This chapter of the Manual presents general background information on construction materials and flexible pavements. It is intended for engineers and technicians with little background in these subjects. The information presented is not controversial and can be found in many other references, including introductory texts on construction materials and pavements. Therefore, further sup- porting details and justification are not needed for the information given in this chapter. 227 C H A P T E R 2 Background

This chapter of the Manual presents information on asphalt binders. This includes background information of a more detailed nature than that provided in Chapter 2, such as brief discussions of asphalt refining, temperature sensitivity, and age hardening. Performance grading of asphalt binders is described in some detail, including discussion of the various test methods involved and presentation of some of the pertinent specifications in tabular format. The chapter concludes with a few paragraphs giving practical guidelines for the selection of binders during the mix design process. Figure 3-1 is a plot of dynamic shear modulus (⎟G*⎟) as a function of temperature at 10 rad/s for a typical asphalt binder. This figure represents actual data for a PG 64-22 binder. Table 3-1 presents details of the current standard for performance-graded asphalt binders, as described in AASHTO M320. Table 3-1 includes low temperature grading solely based on the bending beam rheometer (BBR); the direct tension test is not used for this purpose. AASHTO M320 contains different tables for low temperature grading with the BBR and the direct tension test; the direct tension grading is omitted in this chapter in the interest of brevity and because grading using the BBR is much more common. 228 C H A P T E R 3 Asphalt Binders

This chapter of the Manual is an introduction to construction aggregates and includes detailed information on particle size analysis, definition of nominal maximum aggregate size, and a description of how to perform a sieve analysis. This chapter of the Manual discusses the different types of aggregate gradation, such as dense-graded and gap-graded aggregate blends, and includes a table giving specifications for the various AASHTO aggregate gradations. This chapter also presents information on aggregate specific gravity and absorption and what were formerly called the Superpave “consensus” properties: coarse aggregate fractured faces, fine aggregate angularity, flat and elongated particles, and the sand equivalent test. However, this chapter points out that because these tests are now generally accepted by the pavement engineering community and are supported by substantial experience, these properties no longer represent the “consensus” of an expert panel and so should be referred to simply as “specification” properties rather than “consensus” properties. Chapter 4 of the Manual concludes with discussions of aggregate toughness as measured by the Los Angeles Abrasion test, aggregate soundness tests, and tests for deleterious materials. All of the critical tables given in Chapter 4 are based on those found in existing AASHTO standards, as listed in Table 1. In two cases—requirements for coarse aggregate fractured faces (CAFF) and fine aggregate angularity (FAA)—the requirements have been modified slightly from those given in existing standards, as described in the notes to the table. These modifications are based in part on the recommendations of NCHRP Report 539 (1). In this report, it is suggested that there is no need for minimum CAFF values exceeding 95%. However, the minimum value in the Manual for the highest design traffic level is 98%, with the option of a further reduction to 95% if experience with local conditions and materials warrant such a reduction. This approach represents a compromise between the recommendations of NCHRP Report 539 and the reluctance of many engineers to reduce minimum CAFF values to 95% without further experience with HMA mixtures produced with coarse aggregates exhibiting lower values for fractured faces. The equations given in Chapter 4 are also taken directly from various AASHTO standards. Equations 4-1 through 4-3, dealing with an example calculation of aggregate gradation, are based on AASHTO T 27. Equations 4-5 through 4-7, dealing with aggregate specific gravity and absorption are based on AASHTO T 84 (fine aggregate) and T 85 (coarse aggregate). 229 C H A P T E R 4 Aggregates

230 A Manual for Design of Hot Mix Asphalt with Commentary Table No. Source Standard Table. 4-1. Minimum Test Sample Size for Sieve Analysis of Aggregate as a Function of Nominal Maximum Aggregate Size. AASHTO T 2 Table 4-3. Standard Sizes of Coarse Aggregates for Road and Bridge Construction as Adapted from AASHTO M 43. AASHTO M 43 Table 4-4. Standard Sizes of Fine Aggregates for Bituminous Paving Mixtures as Adapted from AASHTO M 29. AASHTO M 29 Table 4-6. Coarse Aggregate Fractured Faces Requirements. AASHTO M 323a Table 4-7. Fine Aggregate Angularity Requirements. AASHTO M 323b Table 4-8. Criteria for Flat and Elongated Particles. AASHTO M 323 Table 4-9. Clay Content Requirements. AASHTO M 323 aMinimum required values for coarse aggregate fractured faces given in Table 4-6 differ slightly from those in M 323; for design traffic levels of 30 million ESALs or more, the minimum required value is 98% for particles with both one and two fractured faces, rather than 100% as given in M323. Furthermore, this value may be further reduced to 95% if experience with local conditions and materials suggests that the lower value would provide mixtures with adequate rut resistance under very heavy traffic. These changes are largely based on recommendations made in NCHRP Report 539 (1). bMinimum required values for fine aggregate angularity given in Table 4-7 differ slightly from those in M 323; for mixtures placed within 100 mm of the pavement surface subject to design traffic levels of 3 million ESALs or higher, or for mixtures placed 100 mm or deeper from the pavement surface subject to design traffic levels of 30 million ESALs or more, the required FAA value may be reduced from 45% to 43% if experience with local conditions and materials suggests that this will produce mixtures with adequate rut resistance. These changes are largely based on recommendations made in NCHRP Report 539 (1). Table 1. Sources for critical tables in chapter 4 of the mix design manual.

Chapter 5 of the Manual discusses the volumetric composition of HMA mixtures. The chapter includes a significant amount of introductory material, including the definitions of many terms related to HMA composition and various relationships between HMA compositional factors such as voids in mineral aggregate (VMA) and air voids and pavement performance. Much of the second half of the chapter is devoted to a detailed description of volumetric analysis of HMA mixtures, including numerous equations and example calculations. The primary source for the terminology and equations given in this chapter is AASHTO R 35, Superpave Volumetric Design for Hot-Mix Asphalt. In some cases, the Asphalt Institute’s MS-2 and SP-2 manuals were also used as references since these manuals are also referenced in AASHTO R 35 (2, 3). The critical information in Chapter 5 is the various equations presented for calculating various factors of HMA volumetric composition, such as air void content, VMA, and effective binder content. There are many different ways of calculating these factors, and many different forms of what are, in many cases, identical mathematical relationships. Furthermore, all of the equations given in Chapter 5 can be derived from fundamental physical relationships among volume fraction, mass fraction, specific gravity, density, and absorption. These relationships—and the resulting mathematical equations—are often represented through the use of a phase diagram. Although a very useful concept, the phase diagram approach to volumetric analysis has not been included in the Manual because it was believed that its interpretation would be too challenging for many technicians and some engineers. Table 2 lists sources for the various equations presented in Chapter 5; again, it should be noted that all of these equations can be derived from the physical relationships involved, but it is useful to show other references using the same or similar equations in discussion of HMA volumetric composition and analysis. 231 C H A P T E R 5 Mixture Volumetric Composition Equation No. For Calculation of Source 5-1 Bulk specific gravity of compacted specimen AASHTO T 166 5-2 Maximum specific gravity of loose mixture AASHTO T 209 5-3 Bulk specific gravity of aggregate blend AASHTO R 35; TAI SP-2, MS-2 5-4 Air void content of compacted specimen,% by mixture volume AASHTO R 35, T 269 5-5 Total asphalt binder content of mixture,% by mixture mass By definition 5-6 Total asphalt binder content of mixture,% by mixture volume By definition 5-7 Absorbed asphalt binder content,% by mixture volume By Definition 5-8 Effective asphalt binder content,% by mixture volume By Definition 5-9 Effective asphalt binder content,% by mixture mass By Definition 5-10 Absorbed asphalt binder content,% by mixture mass By Definition 5-11 Voids in mineral aggregate,% by mixture volume AASHTO R 35 5-12 Voids filled with asphalt,% by volume AASHTO R 35 5-13 Apparent film thickness NCHRP Report 567 (4) 5-14 Aggregate specific surface (method 1) NCHRP Report 567 (4) 5-15 Aggregate specific surface (method 2) NCHRP Report 567 (4) Table 2. Sources for equations in chapter 5.

Chapter 6 discusses various means of evaluating the potential performance of HMA mixtures. It includes discussions of the relationships between mixture composition and performance and binder test properties and performance. The chapter also gives detailed practical information on various test methods being used to characterize HMA mixture performance. The last part of the chapter discusses how the Mechanistic-Empirical Pavement Design Guide (MEPDG) may be used to develop predictions of HMA pavement performance and includes specific guidelines for the number and types of tests required as input when using the higher MEPDG design levels. Much of the most important information provided in Chapter 6 is not in the form of tables, figures, and equations, but is descriptive information on various tests for characterizing HMA mixture performance. Table 3 lists references for the various performance tests discussed in the Manual. The references listed include those mentioned in the Manual and one or two additional references that will provide interested readers with more detailed information on the procedures. References for Table 3 5. Bonaquist, R., D. W. Christensen, and W. Stump, NCHRP Report 513: Simple Performance Tester for Super- pave Mix Design: First Article Development and Evaluation, Washington, DC: Transportation Research Board, 2003, 54 pp. 6. Bonaquist, R., NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester, Washington, DC: Transportation Research Board, 2008, 39 pp. 232 C H A P T E R 6 Evaluating the Performance of Asphalt Concrete Mixtures Performance Test Used to Evaluate References Asphalt Mixture Performance Test System (AMPT) Rut resistance, dynamic modulus 5, 6 Superpave shear tester, repeated shear at constant height test Rut resistance AASHTO T 320, 7, 8 High-temperature IDT strength test Rut resistance 9, 10, 11 Asphalt pavement analyzer (APA) Rut resistance AASHTO TP 63, 12 Hamburg wheel-tracking test Rut resistance and/or moisture resistance AASHTO T 324 Flexural fatigue test Fatigue resistance AASHTO T 321, 13 Low temperature IDT creep and strength tests Resistance to thermal/low temperature cracking AASHTO T 322, 14, 15 Modified Lottman procedure Resistance to moisture- induced damage AASHTO T 283 Short- and long-term oven conditioning Age-hardening AASHTO R 30 Table 3. References for performance tests discussed in chapter 6 of the mix design manual.

7. Harvey, J., C. Monismith and J. Sousa, “An Investigation of Field- and Laboratory-Compacted Asphalt- Rubber, SMA, Recycled and Conventional Asphalt-Concrete Mixes Using SHRP Project A-003A Equipment,” Journal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 511–548. 8. Sousa, J., “Asphalt-Aggregate Mix Design Using the Repetitive Simple Shear Test (Constant Height),” Jour- nal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 298–333. 9. Christensen, D. W., R. Bonaquist, and D. P. Jack, Evaluation of Triaxial Strength as a Simple Test for Asphalt Con- crete Rut Resistance, Final Report to the Pennsylvania Department of Transportation, Report No. FHWA-PA- 2000-010+97-04 (19), PTI Report 2K26, University Park: The Pennsylvania Transportation Institute, August 2000, 80 pp. 10. Christensen, D. W., et al., “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, Washington, D.C.: Transportation Research Board, 2004, pp. 44–57. 11. Zaniewski, J. P., and G. Srinivasan, Evaluation of Indirect Tensile Strength to Identify Asphalt Concrete Rutting Potential, Morgantown, WV: West Virginia University, Department of Civil and Environmental Engineer- ing, May 2004, 65 pp. 12. Kandhal, P. S., and L. A. Cooley, NCHRP Report 508: Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer, Washington, DC: Transportation Research Board, 2003, 73 pp. 13. Tayebali, A. et al., “Mix and Mode-of-Loading Effects on Fatigue Response of Asphalt-Aggregate Mixes,” Journal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 118–143. 14. Hiltunen, D. R. and R. Roque, “A Mechanics-Based Prediction Model for Thermal Cracking of Asphaltic Concrete Pavements,” Journal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 81–113. 15. Roque, R., D. R. Hiltunen and W. G. Buttlar, “Thermal Cracking Performance and Design of Mixtures Using Superpave,” Journal of the Association of Asphalt Paving Technologists, Vol. 64, 1995, pp. 718–733. Table 6-1 of the Manual summarizes the effects of various HMA characteristics on performance, and is reproduced here as Table 4 for the convenience of the reader. In the Manual, two NCHRP re- ports are cited as the major basis for this table: NCHRP Report 539 and NCHRP Report 567 (1, 4). Commentary to the Mix Design Manual for Hot Mix Asphalt 233 Typical Effects of Increasing Given Factor within Normal Specification Limits While Other Factors Are Held Constant within Normal Specification Limits “↑ ” indicates improved performance; “↓ ” indicates reduced performance Component Factor Resistance to Rutting and Permanent Deformation Resistance to Fatigue Cracking Resistance to Low Temperature Cracking Durability/ Resistance to Penetration by Water and Air Resistance to Moisture Damage Increasing High Temperature Binder Grade ↑↑↑ Increasing Low Temperature Binder Grade ↓↓↓Asphalt Binder Increasing Intermediate Temperature Binder Stiffness ↑↓ Increasing Aggregate Angularity ↑↑ Increasing Proportion of Flat and Elongated Particles Increasing Nominal Maximum Aggregate Size ↓ ↓ ↓ Increasing Mineral Filler Content and/or Dust/Binder Ratio ↑↑ ↑ Aggregates Increasing Clay Content ↓ Increasing Design Compaction Level ↑↑ ↑↑ Increasing Design Air Voids ↑↑ Increasing Design VMA and/or Design Binder Content ↓↓ ↑ ↓ Volumetric Properties Increasing Field Air Voids ↓↓ ↓↓ ↓ ↓↓↓ ↓↓ Table 4. Effect of mixture composition of performance—table 6-1 in the mix design manual.

Table 5 gives a more complete list of references supporting the information given in Table 6-1 of the Manual, including several notes explaining complex relationships between HMA char- acteristics and performance. For convenience, references for Table 4 appear below the table and are given again at the end of the Commentary. In many cases, the relative strength of these relationships—indicated by the number of arrows in Table 6-1—is to some degree a matter of engineering judgment. Table 6-2 in the Manual, which lists performance-related test properties used in specifying asphalt binders in AASHTO M 320, is directly based on M 320 itself. However, as noted above, the use of⎟G*⎟ sin δ to limit susceptibility to fatigue damage is controversial and at this writing much effort is underway to develop a more effective means of specifying the fatigue-related properties of asphalt binders. 234 A Manual for Design of Hot Mix Asphalt with Commentary Component Factor Resistance to Rutting and Permanent Deformation Resistance to Fatigue Cracking Resistance to Low Temperature Cracking Durability/ Resistance to Penetration by Water and Air Resistance to Moisture Damage Increasing High Temperature Binder Grade M 320, 16, 17, 18 Increasing Low Temperature Binder Grade M 320, 19 Asphalt Binder Increasing Intermediate Temperature Binder Stiffness M 320, Note 1 Increasing Aggregate Angularity M 323, 1, 20, 21 Increasing Proportion of Flat and Elongated Particles Increasing Nominal Maximum Aggregate Size Notes 2, 3 Notes 2, 3 Note 4 Increasing Mineral Filler Content and/or Dust/Binder Ratio 4, 16 4 Aggregates Increasing Clay Content M 323, 20 Increasing Design Compaction Level M 323, 16 M 323, 16 Increasing Design Air Voids 16 Increasing Design VMA and/or Design Binder Content 16, 17, 18 16, 22, 13, 24 20 Volumetric Properties Increasing Field Air Voids 16, 17, 18 17, 23, 24 20 16, 25, 26 Note 5 1Most research suggests that the fatigue resistance of an HMA mixture shows a complex relationship with its stiffness and indirectlywith binder stiffness; for thin pavements, fatigue resistance decreases with increasing modulus, whereas for thick pavements, fatigue resistance increases with increasing modulus (see references 7 and 8). 2In current and previous HMA mix design systems, VMA and binder content typically decrease with increasing aggregate NMAS. In the interest of clarity, these two factors have been separated, and the effect of design VMA and binder content on mixture performance are listed separately in this table. However, it should be kept in mind that increasing aggregate NMAS will usually decrease VMA and binder content, which in turn will affect mixture performance in various ways. Decreasing aggregate NMAS will, in general, increase VMA and binder content, which will also affect mixture performance. 3Although there is currently little research linking HMA fatigue properties and resistance or low temperature cracking to aggregate NMAS, the strength properties and fatigue resistance of most particulate composites like HMA increase with decreasing particle size because the size of flaws and magnitude of internal stress concentrations tend to decrease with decreasing particle size. This is the reason for the thoroughly documented increase in compressive strength with decreasing aggregate size in portland cement concrete mixtures. It is highly likely that similar relationships exist for HMA mixtures. 4As discussed in Note 2 above, increasing aggregate NMAS will tend to result in an overall increase in the number of large flaws in an HMA mixture. This is partly a result of occasional poor bonding at the asphalt-aggregate interface. Such large flaws will tend to result in a significant increase in permeability to both air and water, reducing the durability of HMA mixtures made with large-sized aggregates. 5The current test procedure used widely to evaluate the resistance of HMA to moisture damage, AASHTO T 283, is performed at a constant air void content of 7 ± 1%. Therefore, little information concerning the effect of air void content on moisture resistance is available. However significant research shows permeability increases with increasing air voids, so it should be expected that as air voids and permeability increase, resistance to moisture damage will decrease. Table 5. References for table 6-1 in the manual, on the effect of mixture composition of performance.

Table 6-3 in the Manual lists recommended high-temperature performance grade adjust- ments to account for traffic volume and speed. These grade adjustments should be applied to the base high-temperature binder grade in order to ensure that the resulting HMA mixture will have adequate resistance to rutting for the given traffic conditions. In AASHTO M 320, these adjustments are given without explanation. In LTPPBind Version 3.1, the adjustments given follow directly from the rational approach taken in the development of the software— that is, the adjustments are based on predicted damage under different traffic levels and traffic speeds. However, the problem encountered with LTPPBind Version 3.1 is that the traf- fic speeds are limited to fast and slow—there is no adjustment for very slow traffic. Further- more, it is not clear what range in average traffic speeds were assumed in the calculation of grade adjustments. Table 6-3 in the Manual (reproduced as Table 6) is presented to provide grade adjustments for the full range of traffic speeds, and with a full rational derivation, as given below. References for Tables 4 and 5 1. Prowell, B. D., J. Zhang and E. R. Brown, NCHRP Report 539: Aggregate Properties and the Perfor- mance of Superpave-Designed Hot Mix Asphalt, Washington DC: Transportation Research Board, 2005, 90 pp. 16. Christensen, D. W., and R. F. Bonaquist, “Rut Resistance and Volumetric Composition of Asphalt Concrete Mixtures,” Journal of the Association of Asphalt Paving Technologists, Vol. 74, 2005. 17. Leahy, R. B., and M. W. Witczak, “The Influence of Test Conditions and Asphalt Concrete Mix Parameters on Permanent Deformation Coefficients Alpha and Mu,” Journal of the Association of Asphalt Paving Tech- nologists, Vol. 60, 1991, p. 333. 18. Kaloush, K. E., and M. W. Witczak, Development of a Permanent to Elastic Strain Ratio Model for Asphalt Mixtures, NCHRP 1-37 A Inter-Team Technical Report, College Park, MD: University of Maryland, March 1999, 106 pp. 19. Anderson, D. A., et al. Asphalt Behavior at Low Service Temperatures, Final Report to the Federal Highway Administration. PTI Report No. 8802. University Park, PA: The Pennsylvania Transportation Institute, March 1990, 337 pp. 20. Kandhal, P. S. and F. Parker, Jr., NCHRP Report 405: Aggregate Tests Related to Asphalt Concrete Performance in Pavements, Washington, DC: Transportation Research Board, 1998, 103 pp. 21. White, T. D., J. E. Haddock and E. Rismantojo, NCHRP Report 557: Aggregate Tests for Hot-Mix Asphalt Mixtures Used in Pavements, Washington, DC: Transportation Research Board, 2006, 38 pp. Commentary to the Mix Design Manual for Hot Mix Asphalt 235 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 6. Recommended changes to high temperature performance grade to account for traffic volume and speed—table 6-3 in the mix design manual.

22. Bonnaure, F. P., A. H. J. J. Huibers, and A. Boonders, “A Laboratory Investigation of the Influence of Rest Periods on the Fatigue Characteristics of Bituminous Mixes,” Proceedings, the Association of Asphalt Paving Technologists, Vol. 51, 1980, p. 104. 23. Shook, J. F., et al., “Thickness Design of Asphalt Pavements—The Asphalt Institute Method,” Proceedings, Fifth International Conference on the Structural Design of Asphalt Pavements, Vol. 1, The University of Michi- gan and The Delft University of Technology, August 1982. 24. Tayebali, A. A., J. A. Deacon, and C. L. Monismith, “Development and Evaluation of Surrogate Fatigue Models for SHRP A-003A Abridged Mix Design Procedure,” Journal of the Association of Asphalt Paving Technologists, Vol. 64, 1995, pp. 340–364. 25. Choubane, B., G. Page, and J. Musselman, “Investigation of Water Permeability of Coarse Graded Superpave Pavements,” Journal of the Association of Asphalt Paving Technologists, Vol. 67, 1998, p. 254. 26. Huang, B., et al., “Fundamentals of Permeability in Asphalt Mixtures,” Journal of the Association of Asphalt Paving Technologists, Vol. 68, 1999, pp. 479–496. Estimating grade adjustments such as those shown in Table 6 is somewhat complicated, given that three different factors must be considered: traffic volume, traffic speed, and design compaction. It must be remembered that when designing HMA mixtures following this method (or the Superpave system), the design compaction level changes along with traffic level, so the properties of the mix, including rut resistance, will change significantly. This will affect the required binder grade. One other piece of information is needed to develop binder grade adjustments: the typical change in binder⎟G*⎟ /sin δ values with temperature. Recent research performed for the Airfield Asphalt Pavement Technology Program (AAPTP) Project 4-2 described the calculation of high-temperature binder grade adjustments in detail; the development given here closely follows that given in the Final Report for AAPTP Project 4-2 (27). The effect on rut resistance of differences in mixture properties can be estimated using the resistivity-rutting model initially developed during NCHRP Projects 9-25 and 9-31, and further refined as part of NCHRP Project 9-33 and AAPTP Project 4-2 (4, 16, 27). The most recent ver- sion of the resistivity/rutting equation gives allowable traffic as a function of mixture composi- tion, compaction, and air voids: where TR = million ESALs to an average rut depth of 7.2 mm (50% confidence level) = million ESALs to a maximum rut depth of 12 mm (95% confidence level) Ρ = resistivity, s/nm = ⎟ G*⎟ /sin δ = Estimated aged performance grading parameter at high temperatures, deter- mined at 10 rad/s and at the yearly, 7-day average maximum pavement temper- ature at 20 mm below the pavement surface, as determined using LTPPBind, Ver- sion 3.1 (units of Pa); aged value can be estimated by multiplying the RRTFOT value by 4.0 for long-term projects (10 to 20 year design life), and by 2.5 for short- term projects of 1 to 2 years. Sa = specific surface of aggregate in mixture, m2/kg ≅ the sum of the percent passing the 75, 150, and 300 micron sieves, divided by 5.0 ≅ 2.05 + (0.623 × percent passing the 75 micron sieve) G S G VMA a a* sinδ( ) 2 2 349 TR NK V V Ms QC IP= × ( )− −9 85 10 5 1 373 1 5185 1 4727. . . .P ( )1 236 A Manual for Design of Hot Mix Asphalt with Commentary

Ga = the bulk specific gravity of the aggregate blend VMA = voids in the mineral aggregate for the mixture, volume%, as determined during QA testing N = design gyrations Ks = speed correction = (v/70)0.8, where v is the average traffic speed in km/hr VQC = air void content, volume%, determined during QA testing at design gyrations VIP = air void content, volume%, in-place M = 7.13 for mixtures containing typical polymer-modified binders, 1.00 otherwise The equation for resistivity can be inserted into Equation 1 and the results simplified to give an alternate form for allowable traffic: In order to develop high-temperature binder grade adjustments, Equation 2 must be manip- ulated into a form that allows the direct calculation of the temperature adjustment needed to off- set a specified change in a given property or combination of properties: In Equation 3, subscripts 1 and 2 refer to two different sets of conditions—binder⎟G*⎟ /sin δ, aggregate surface area, mix VMA, design gyrations, and so forth. Because we are only interested in changes in three of the properties included in Equation 3 (⎟G*⎟ /sin δ, N, and v), Equation 3 can be simplified by removing the other variables: An analysis was performed on a set of nine different binders from various accelerated pave- ment tests. Eight of the binders were from projects included in development of the AMPT: the FHWA ALF rutting test; MnRoad; and Westrack (28, 29, 30). One binder tested was a PG 64-22 used in NCHRP Projects 9-25 and 9-31 (4). These binders were chosen for this analysis because they have been included in well-known studies, and their flow properties have been thoroughly documented. As shown in Figure 1, the relationship between temperature and modulus (⎟ G*⎟/sin δ in this case) is exponential: The value of constant A in Equation 5 varies somewhat among the binders included in Fig- ure 1, but is typically very close to −0.135, as shown in Figure 9. Equation 5 can be substituted G G A T T * sin * sin exp ( ) δ δ ( ) ( ) = −( )[ ] 1 2 1 2 5 TR TR G G N N 2 1 2 1 1 373 2 1 = ( ) ( ) ⎡ ⎣⎢ ⎤ ⎦⎥ ⎛⎝⎜ * sin * sin .δ δ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟ 1 373 2 1 1 098 4 . . ( ) v v s s TR TR G G S S a a 2 1 2 1 1 373 2 1 = ( ) ( ) ⎡ ⎣⎢ ⎤ ⎦⎥ ⎛* sin * sin .δ δ ⎝⎜ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟ −2 746 2 1 2 746 2 1 . . G G VMA VMA a a 4 199 2 1 1 373 2 1 1 098 2 . . . N N v v V V s s QC ⎛⎝⎜ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟ QC IP IP V V M M1 1 5158 2 1 1 4727 2 1 ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛⎝⎜ ⎞⎠⎟ ⎛⎝ − . . ⎜ ⎞⎠⎟ ( )3 TR G S G VMAa= × ( )− −4 71 10 7 1 373. * sin .δ 2.746 a2.746 4 119 1 373 1 373 1 5185 1 4727 2. . . . . (N K V V Meq s QC IP− ) Commentary to the Mix Design Manual for Hot Mix Asphalt 237

238 A Manual for Design of Hot Mix Asphalt with Commentary Property Recommended Test Design Traffic Levels for Which Property Should be Evaluated Moisture Sensitivity AASHTO T 283 All Permanent Deformation Flow Number or Dynamic Modulus, NCHRP 9-29 PT 01 3 Million ESAL and greater Fatigue Cracking None NA Thermal Cracking None NA Table 7. Recommended performance tests for HMA mixtures made with conventional materials including most modified binders— table 6-4 in the mix design manual. into Equation 4 and rearranged, giving the following relationship between binder grade adjust- ment ΔT, traffic level, design gyrations, and traffic speed: Equation 6 can then be used to estimate the high-temperature binder grade adjustments given in Table 6, keeping in mind the design gyration levels for various traffic levels: 50 gyrations for less than 0.3 million ESALs, 75 gyrations for 0.3 million to less than 10 million ESALs, 100 gyrations for 10 million to less than 30 million ESALs, and 125 gyrations for traffic levels of 30 million ESALs or more. The traffic level used to calculate the grade adjustments was the highest in the given range, and 100 million for traffic levels of 30 million ESALs or more. Traffic speeds used in the calcula- tions were 70 kph for fast traffic, 25 kph for slow traffic, and 10 kph for very slow traffic. Table 6-4, in the Manual (included here as Table 7) summarizes the performance testing rec- ommended for routine, dense-graded HMA mix designs. As in the Superpave system, moisture resistance testing is required for all mix designs. The only other performance testing normally required for any routine mix design is rut-resistance testing. This is because of the high level of com- plexity and cost for performing tests to characterize resistance to fatigue cracking and thermal crack- ing. This table is largely based on engineering judgment. It is assumed that the reliability for HMA mixtures intended for pavements at lower traffic levels—below 3 million ESALs—does not need to be as high as that for mixtures intended for higher traffic levels, and so rut resistance testing is not warranted for these cases. Table 6-5 in the Manual summarizes mixture properties used as input in the MEPDG. This table is straightforward and is based directly on the MEPDG User Manual (31). Table 6-6 in the Manual, reproduced as Table 8 below, summarizes the effect of changes in mixture composition and high-temperature binder grade on MEPDG performance predictions. Like Table 6-5, this is based on information contained in the MEPDG User Manual (31). ΔT T T TR TR N N v = − = ⎛⎝⎜ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟2 1 21 0 728 1 2 7 41. ln . s sN 1 2 0 800 6 ⎛⎝⎜ ⎞⎠⎟ ⎡ ⎣⎢ ⎤ ⎦⎥ . ( ) y = 3356e-0.1349x2 R = 98 % 100 1,000 10,000 100,000 -30.0 -20.0 -10.0 0.0 10.0 Relative Temperature G * /s in d el ta , Pa ALF AC-5 ALF AC-10 ALF AC-20 ALF SBS ALF PE PG 64-22 MN/Road 120 Pen MN//Road AC-20 WesTrack PG 64-22 Fit Figure 1. Temperature dependence of nine asphalt binders relative to the performance grading temperature.

Commentary to the Mix Design Manual for Hot Mix Asphalt 239 HMA Property Rutting Thermal Cracking Alligator Cracking HMA ≥ 5 in Alligator Cracking HMA < 3 in Longitudinal Cracking High Temperature Binder Grade Increase to improve Increase to improve Decrease to improve Decrease to improve Low Temperature Binder Grade Decrease to improve Design VMA Decrease to improve Increase to improve Increase to improve Increase to improve Design VFA Increase to improve Filler Content Increase to improve In-Place Air Voids Decrease to improve Decrease to improve Decrease to improve Decrease to improve Decrease to improve Table 8. Summary of effect of mixture composition on performance predictions—table 6-6 in the mix design manual.

Chapter 7 provides engineers and technicians with guidance on the selection of HMA mix types for different applications. In most cases, when an engineer or technician is performing a mix design, the mix type will be specified by the owner or agency requesting the mix design. However, this may not always be the case—especially for private paving work, where the owner may not have any idea what type of mix is best suited for her or his particular application. Fur- thermore, because selection of mix type is a direct function of where within a pavement the mix is located, Chapter 7 also discusses the topic of pavement structure in some detail. The primary reference for Chapter 7 is a publication of the National Asphalt Pavement Asso- ciation (NAPA): HMA Pavement Mix Type Selection Guide (33). Some additional information is given on lift thickness, based on NCHRP Report 531 (32), and on pavement structure and mix type selection for perpetual pavements, based on TRB Circular 503 (34). The figures and tables presented in Chapter 7 are not highly technical in nature, instead pre- senting, for the most part, general knowledge concerning pavement types and pavement struc- ture. Figure 7-1 presents the different types of pavement structures incorporating HMA in new construction. Figure 7-2 is similar, but presents different types of pavement structures using HMA that result from pavement maintenance operations. The traffic levels listed in Table 7-1 are defined in both AASHTO M 323 and R 35. However, the descriptions of typical traffic and road types for the different traffic levels occurs only in AASHTO R 35. Table 7-2, giving recommended lift thicknesses for different mix types and NMAS, is taken directly from NCHRP Report 531 (32), while Table 7-3 is a summary of infor- mation taken from NAPA’s publication HMA Pavement Mix Type Selection Guide (33). 240 C H A P T E R 7 Selection of Asphalt Concrete Mix Type

Chapter 8 is probably the most important chapter in the Manual. It describes in detail the rec- ommended procedure for designing dense-graded HMA mixtures. Much of the material pre- sented here also appears in other chapters—it is repeated in Chapter 8 for the convenience of the reader, and so that Chapter 8 can be used as a stand-alone document for designing dense-graded HMA mixtures. For example, the tables on aggregate specifications also appear in Chapter 4. Many of the tables that appear in Chapter 8 are either identical or nearly identical to tables used in the Superpave system, as described in AASHTO standards M 323 and R 35 and the Asphalt Institute’s SP-2 manual. Many of these tables have been only slightly modified, based upon the results of various recent research projects. Some of the tables, such as those providing guidelines for interpreting various performance tests, do not appear in standards and publications deal- ing with Superpave. As described below, in some cases development of these tables was simply a matter of presenting typical current practice. However, for some of the performance tests devel- oping meaningful guidelines for interpreting results was a complex task. Chapter 8 presents a brief history of HMA mix design methods, in order to provide inexperi- enced technicians and engineers with some background. Of particular interest is the information provided on the Superpave system, which is quite similar to the mix design method presented in the Manual. After the background section, Chapter 8 presents a short summary of the proposed mix design procedure, followed by a detailed, step-by-step description. This includes numerous example problems with solutions. An important feature in Chapter 8 of the Manual is the frequent references to HMA Tools, which is a Microsoft Excel spreadsheet application designed to accompany the Manual. HMA Tools is a powerful spreadsheet which can perform virtually all of the calculations needed when performing an HMA mix design. The examples given in Chapter 8 generally refer to HMA Tools. As pointed out in the Manual, it is not necessary to use HMA Tools to perform mix designs according to the recommended method, but if other software is used, it will usually be necessary to update the various specifications and limits to reflect those given in the Manual. Table 8-1 in the Manual shows recommended grade adjustments for traffic level and speed. This table is identical to Table 6-3, presented and discussed in detail previously in the Commentary as Table 6. Readers should refer to this discussion for the derivation of the grade adjustments shown in Table 8-1. Table 8-2 lists compaction effort as a function of design traffic level. The values for design gyrations—Ndesign—are identical to what appears in AASHTO R 35. However, values for Ninitial and Nmax have been eliminated from the table. Ninitial and Nmax requirements have been eliminated on the basis of work done during NCHRP Project 9-9(1)(35); as documented in NCHRP Report 573, two of the major conclusions of this project were that neither Ninitial nor Nmaximum correlated with 241 C H A P T E R 8 Design of Dense-Graded HMA Mixtures

rutting observed in an extensive field experiment and are not required in designing HMA mix- tures (35). NCHRP Report 573 also suggested new gyration levels, as listed in Table 9 below. NCHRP Report 573 recommends two sets of compaction levels—one for binders with a high temperature grade of 76 and greater (or for binders used in mixes placed more than 100 mm from the pavement surface), and one set for other binders. Furthermore, the overall level of com- paction is lower than is currently suggested in R 35 (35). The recommended compaction levels are based on matching densification as it occurs in the gyratory compactor and as it occurs under traffic loading. It should be pointed out that the correlations reported in NCHRP Report 573 between densification during laboratory compaction and under traffic loading are not strong— 34 and 37% for the two different compactors used in the study (35). Furthermore, this study does not address the strong effect laboratory compaction has on rut resistance, as noted in NCHRP Report 567 (4). In the procedure given in the Manual, the current compaction levels are main- tained for several reasons. First, HMA mixtures made with binders of grade PG 76-XX and higher are in general polymer modified and usually intended for pavements subjected to very heavy traf- fic loading—major urban highways which demand the highest levels of reliability against rut- ting. The significant reduction in Ndesign recommended in NCHRP Report 573 could reduce the rut resistance of such HMA designs to an unacceptable level. A second factor to consider when evaluating the use of a different set of compaction levels for what, in effect, are mostly polymer- modified binders is the probable adoption of the MSCR test to grade asphalt binders at high temperatures. Use of this test might result in changes in binder grade selection that in combi- nation with a change in compaction levels could yield inadequate performance for mixtures that should exhibit outstanding levels of performance. A third consideration is that there has of yet been little time to validate the findings of NCHRP Report 573. Additional time is needed for industry input and further independent evaluation of the proposed compaction levels prior to implementation. Table 8-3 lists the primary control sieve (PCS) size for different NMAS, along with the PZC control point, which is the % passing above which an aggregate gradation is considered a “coarse” gradation and below which it is considered a “fine” gradation. Table 8-3 is nearly iden- tical to Table 4 from AASHTO M 323-5, with the exception that Table 8-3 includes information on 4.75 mm NMAS gradations, while Table 4 in M 323-5 does not. Table 8-4 in the Manual (Table 10 below) lists recommended NMAS for different types of dense-graded HMA mixtures, along with recommended lift thicknesses. The values for NMAS follow directly from the recommendations of Chapter 7 on mix type selection—specifically, Table 7-3, which lists recommended mix types and NMAS values for different traffic levels. Table 7-3 in turn was based on the recommendations given in NAPA’s publication HMA Pavement Mix Type Selection Guide, IS 128 (33). Lift thickness values are based on recommendations given 242 A Manual for Design of Hot Mix Asphalt with Commentary From NCHRP Report 573: 20-Year Design Traffic, Million ESALs Ndesign for Binders < PG 76-XX Ndesign for Binders ≥ PG 76- XX or for Binders Used in Mixes Placed > 100 mm from Pavement Surface Ndesign in AASHTO R 35 and in the Mix Design Manual < 0.30 50 NA 50 0.30 to < 3.0 65 50 75 3.0 to < 10 80 65 100 10 to < 30 80 65 100 > 30 100 80 125 Table 9. Compaction effort as a function of design traffic level as recommended in NCHRP Report 573 and as given in AASHTO R 35 and the mix design manual (35).

in NCHRP Report 531: lift thickness values 3 to 5 times NMAS for fine-graded mixtures, and 4 to 5 times NMAS for coarse-graded mixtures. Table 8-5 in the Manual lists maximum and minimum VMA values as a function of aggre- gate NMAS. Minimum VMA values are the same as those specified in AASHTO M 323-04. However, there is a note to the table allowing agencies to increase minimum VMA values by up to 1.0%, in order to improve field compaction, fatigue resistance, and durability. The note also contains a caution that if VMA is increased, care should be taken to ensure that the result- ing mix maintains adequate rut resistance. This note has been included to address the concern of many agencies that HMA mixtures designed according to existing Superpave methods often exhibit durability problems—raveling, surface cracking, and moisture damage. Many agen- cies have already increased minimum VMA values for Superpave mixes in order to address these perceived problems. Allowing an increase of up to 1.0% in minimum VMA addresses the concerns of agencies that have experienced durability problems in Superpave mixes, but allows those agencies that have not seen such problems to maintain minimum VMA values at the cur- rent levels specified in M 323-04. Maximum VMA values in the Manual are held to 2% above the minimum values. VFA is no longer specified. One of the reasons for eliminating the requirements for minimum and maxi- mum VFA is that the relationship among VMA, VFA, and design air voids is complex and makes simultaneous control of all three difficult and confusing. The requirements currently given in Table 6 of M 323 in fact require some effort to interpret precisely; specification of minimum and maximum VFA, in combination with the specified design air void content of 4.0%, establish an alternate set of minimum and maximum VMA values, since VMA = 4.0/(1-VFA/100). The implied VMA values calculated in this way are either equal to or less than the stated VMA values (allow- ing for rounding errors), and so the stated minimum VMA values are not ambiguous. However, specifying a minimum VMA and a design air void content also implies a minimum VFA, since VFA = (VMA-4.0)/VMA × 100%. In this case, the implied minimum VFA values are often greater than those specifically listed in Table 6 of M 323-04. The conservative interpretation would be that the highest of the two alternate sets of minimum VFA values applies, but the standard as writ- ten is somewhat ambiguous. Table 11 lists the minimum VMA values and minimum and max- imum VFA values specified in Table 6 of M 323-04, along with the minimum VFA calculated from the specified minimum VMA, and the calculated maximum VMA values calculated from the maximum VFA. The approach used in the Manual—simply specifying a minimum and max- imum VMA—is simpler and avoids the ambiguity inherent in trying to simultaneously control Commentary to the Mix Design Manual for Hot Mix Asphalt 243 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 10. Recommended aggregate nominal maximum aggregate sizes (NMAS) for dense-graded HMA mixtures—table 8-4 in the mix design manual.

air voids, VMA, and VFA. For the smaller NMAS values and higher traffic levels, the approach in the Manual results in similar ranges for VMA. For the larger NMAS value, the approach in the Manual is somewhat more restrictive, since M 323-04 in effect specifies maximum VMA values 3 to 4% higher than the minimum for these aggregates. However, in reality, because of the high cost of asphalt binders most mix designers select VMA values very close to the minimum values specified in M 323-04. Therefore, the difference in the two approaches is probably negligible in practice. Tables 8-6 and 8-7 in the Manual list aggregate control points for aggregate blends of differ- ent NMAS values. These tables contain values identical to those given in Table 3 of AASHTO M 323-04. However, in the Manual, the aggregate control points are given as suggested limits, and not specified limits as is done in M 323-04. This change was made because it provides the mix designer with much greater flexibility in obtaining specified VMA values, and virtually all evidence relating HMA performance to composition suggests that it is much more important to control VMA than to control details of aggregate gradation. For instance, none of the models discussed in Chapter 6 relating HMA composition to rut resistance and fatigue resistance con- tain factors related to aggregate gradation as predictor variables (16, 17, 18, 22, 23, 24). Although NCHRP Report 405 states that aggregate gradation effects rut resistance and fatigue resistance, these statements are made without support—and in fact made with the admission that eval- uating the effect of aggregate gradation on HMA performance was outside the scope of the project (20). Aggregate specification properties—coarse aggregate fractured faces, flat and elongated par- ticles, fine aggregate angularity, and clay content—are given in Tables 8-8 through 8-11. These tables are identical to Tables 4-6 through 4-9 given in Chapter 4. As discussed in the Commen- tary section dealing with Chapter 4, these tables are very similar to the corresponding tables in M 323-04. The reader should refer to the Commentary section on Chapter 4 for a discussion of these tables. Table 8-12 in the Manual lists requirements for dust/binder ratio; it is reproduced here as Table 12. The requirements given for 4.75 mm NMAS mixes are identical to those given in AASHTO M 323-04. The requirements for other mixes—allowable dust/binder ratios in the range of 0.8 to 1.6, with an option of lowering this range to 0.6 to 1.2—are slightly higher than those in M 323-04. In M 323-04, the specified range for mixes other than 4.75 mm NMAS is from 0.6 to 1.2, with an option of raising this range to 0.8 to 1.6. The requirements in the Manual are therefore similar, but encourage slightly higher dust/binder ratios. There are two reasons for this increase. The first is that research performed during NCHRP Projects 9-25 and 9-31 showed a 244 A Manual for Design of Hot Mix Asphalt with Commentary NMAS, mm Design Traffic Level, Million ESALs Minimum VMA, % Minimum VFA, % Maximum VFA, % Calculated Minimum VFA, % Calculated Maximum VMA, % 4.75 < 3.0 16.0 70 80 75.0 20.0 4.75 ≥ 3.0 16.0 75 78 75.0 18.2 9.5 < 3.0 15.0 65 78 73.3 18.2 9.5 ≥ 3.0 15.0 73 76 73.3 16.7 12.5 All 14.0 65 75 71.4 16.0 19 All 13.0 65 75 69.2 16.0 25 ≥ 0.3 12.0 65 75 66.7 16.0 25 < 0.3 12.0 67 75 66.7 16.0 37.5 All 11.0 64 75 63.6 16.0 Table 11. Specified minimum VMA values and implied minimum and maximum VMA values calculated from VFA values specified in table 6 of AASHTO M 323-04.

strong relationship between permeability and aggregate surface area—as aggregate surface area increases, permeability tends to decrease, all else being equal (13). Therefore, specifying slightly higher dust/binder ratios should result in mixes with lower permeability to air and water and improved durability. The second reason for encouraging slightly higher dust/binder ratios in HMA mixes is that the design method given in the Manual attempts to encourage slightly higher VMA and asphalt binder contents in order to improve the durability of the resulting mixtures. One example of how this is done is the VMA requirements described above, which include an option of increasing the minimum VMA values by up to 1% to improve field compaction, fatigue resistance, and durability. It was found in NCHRP Projects 9-25 and 9-31 that rut resistance of HMA mixes tends to increase as aggregate specific surface increases relative to VMA. Since the Manual encourages higher VMA values, higher dust/binder values are also encouraged in order to maintain or improve rut resistance compared to mixes designed according to the Superpave system. The extremely premature rutting of many of the mixtures placed at the WesTrack facil- ity was attributed in part to high VMA and relatively low dust/binder ratios (36). Promoting an increase in dust/binder ratio will help to prevent such failures in the future. Table 8-20 in the Manual (reproduced here as Table 13) lists recommended minimum values for flow numbers as a function of design traffic level. The values in this table are based on a relationship between flow number values and maximum allowable traffic level estimated using the resistivity/rutting model given earlier as Equation 1. Data used in developing this relationship was collected by the FHWA in one of their field trailers, for nine different projects in New England, New York State, Nebraska, North Carolina, Minnesota, and Wisconsin. The mix composition, binder⎟ G*⎟ /sin δ values, flow number values, and related data appear in Table 14 of this report. Data forwarded by the FHWA included tests of both laboratory-prepared mixes having differ- ent binder contents and field produced mix. A meaningful relationship between flow number and calculated maximum traffic could only be developed using the laboratory mixes. The rea- son this relationship did not hold up for the field-produced mixes is not clear, but it is possibly due to differences in age hardening during production, transport, and sample storage. The design air void content was known precisely only for mixes which used the design binder content. For the other two mixes for each project—one above and one below the design binder content— the design air void content was estimated using the following equation (3): where Vadb = Estimated design air voids at some binder content Pb Vad = Design air voids at design binder content Pbd V V P Padb ad b bd= − −( )2 5 7. ( ) Commentary to the Mix Design Manual for Hot Mix Asphalt 245 Table 12. Requirements for dust/binder ratio—table 8-12 in the mix design manual. 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.

Another complication in calculating the allowable traffic values shown in Figure 2 and Table 14 is that, in most cases, it is not clear whether the binders used in the nine mixes was polymer mod- ified or not—as seen in Equation 1, there is a factor M that is applied to mixes made using poly- mer modified binders to account for the superior rut resistance of these materials compared to non-modified binders of the same high-temperature grade. Recent surveys of asphalt binder pro- ducers suggests that, at the time these mixes were produced (mostly in 2004) approximately 90% of PG 64-28 binders and PG 70-22 binders were modified (37, 38). The PG 70-28 binder used for the MN0465 project was clearly modified, based on the rheological behavior of the binder. From this information and the observed relationship between binder flow properties and mix- ture flow number, it was assumed that five of the binders used in the nine projects were polymer modified. The four projects in which non-modified binders were assumed to be used in the mixes were NC0360, WI0357, NE0569, and WA0463. Analysis of the data using this assumption provided good results, but it appeared that the value of M used in Equation 1 (7.13) was some- what too high. The best correlation between estimated maximum traffic and flow number was found when the value of M was assumed to be 3.0. The lower value of M might be because the modified binders in the nine projects contained only relatively small amounts of polymer, or possibly because much of the testing was performed at intermediate temperatures (mostly 38 or 45 °C), where the effect of polymer modification on rut resistance may not be as large as it is at higher temperatures. The values of flow number appearing in Table 13 were calculated from the regression equa- tion given in Figure 2 using traffic levels at the midpoint of the given design traffic range. For a range of 3 to 10 million ESALs, a value of 6.5 million ESALs was used to estimate a minimum flow number of 53. For the 10 to 30 million ESAL range, a value of 20 million ESALs was used to calculate the minimum flow number of 190. For traffic above 30 million ESALs, a traffic level of 65 million ESALs was used to calculate the required flow number value of 740. Selecting these values, rather than the maximum for each range was done to provide some insurance against excessive rutting, while avoiding being too restrictive, which might result in having a large num- ber of mixes fail the performance test and needing to be redesigned. Table 15 lists recommended minimum flow time values as given in the Manual (Table 8-21 in the Manual). These flow time values were calculated by developing a regression equation relating flow time and flow number, as shown in Figure 3. This plot includes data from five projects included in the NCHRP Project 9-19 database (39). The data used in the plot are shown in Table 16. The flow time values in Table 15 and the flow number values given in Table 13 are intended to be, for all practical purposes, equivalent. 246 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 13. Recommended minimum flow number require- ments (Table 8-20 in the Mix Design Manual). y = 6.222x1.145 R2 = 0.894 10 100 1,000 10,000 100,000 1.0 10.0 100.0 1000.0 Estimated Maximum MESALs Fl ow N u m be r MA0467 ME0359 ME0570 MN0465 NC0360 NE0569 NY0466 WA0463 WI0357 Fit Figure 2. Relationship between flow number and estimated maximum traffic from FHWA field data on six projects.

Commentary to the Mix Design Manual for Hot Mix Asphalt 247 y = 0.362x1.009 R2 = 0.707 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 Flow Number Fl ow T im e, s MN/Road NCAT ALF Indiana Nevada Fit Figure 3. Plot of flow time as a function of flow number for five projects from the NCHRP project 9-19 database (39). Project ID Binder Grade PG- Pb Voids as Tested Design Voids Ndesign Design VMA Agg. Spec. Surface Agg. Gs Temp. |G*|/sin δ M Allowable Traffic Flow No. Wt.% Vol.% Vol.% Gyr. Vol.% M2/kg °C Pa MESALs MA0467 4.6-1 64-28 4.6 6.9 5.8 100 17.0 3.96 2.681 45.2 38,073 3.00 48.4 346 MA0467 5.1-1 64-28 5.1 7.2 4.5 100 16.9 3.96 2.681 45.1 38,073 3.00 31.4 216 MA0467 5.6-1 64-28 5.6 7.2 3.3 100 16.9 3.96 2.681 45.0 38,073 3.00 19.1 171 ME0570 5.4-1 64-28 5.4 4.9 5.8 75 15.3 5.28 2.560 54.3 10,265 3.00 26.6 541 ME0570 5.9-1 64-28 5.9 5.0 4.5 75 15.3 5.28 2.560 54.1 10,265 3.00 17.6 351 ME0570 6.4-1 64-28 6.4 5.0 3.3 75 15.6 5.28 2.560 54.0 10,265 3.00 10.2 148 NC0360 4.5-1 70-22 4.5 8.0 3.9 100 13.9 6.32 2.599 45.0 73,343 1.00 131.1 1121 NC0360 5.0-1 70-22 5.0 7.9 2.6 100 14.0 6.32 2.599 45.1 73,343 1.00 70.5 801 NC0360 5.5-1 70-22 5.5 8.2 1.4 100 14.3 6.32 2.599 45.2 73,343 1.00 22.4 411 NY0466 4.5-1 64-28 4.5 7.1 5.5 100 13.5 4.24 2.618 44.9 44,891 3.00 153.2 1911 NY0466 5.0-1 64-28 5.0 7.0 4.2 100 13.5 4.24 2.618 44.9 44,891 3.00 106.4 841 NY0466 5.5-1 64-28 5.5 7.6 3.0 100 13.2 4.24 2.618 44.9 44,891 3.00 61.1 666 WI0357 4.4-1 64-22 4.4 7.1 6.7 100 15.2 3.64 2.725 31.3 326,035 1.00 478.5 9416 WI0357 4.9-1 64-22 4.9 7.0 5.4 100 15.1 3.64 2.725 31.8 326,035 1.00 364.2 7896 WI0357 5.4-2 64-22 5.4 6.9 4.2 100 15.0 3.64 2.725 31.6 326,035 1.00 255.8 5653 ME0359 5.3 - 1 64-28 5.3 4.7 5.8 75 14.5 3.78 2.599 37.3 96,540 3.00 322.1 5472 ME0359 5.8-2 64-28 5.8 4.8 4.5 75 14.3 3.78 2.599 37.6 96,540 3.00 224.7 4177 ME0359 6.3 - 2 64-28 6.3 4.8 3.3 75 14.4 3.78 2.599 37.5 96,540 3.00 135.1 2356 MN0465 4.8-1 70-28 4.8 7.9 6.0 100 15.5 3.78 2.686 44.7 38,036 3.00 53.7 283 MN0465 5.3-1 70-28 5.3 8.0 4.7 100 15.4 3.78 2.686 44.8 38,036 3.00 37.4 251 MN0465 5.8-1 70-28 5.8 8.0 3.5 100 15.4 3.78 2.686 44.8 38,036 3.00 23.1 186 NE0569 5.0-1 64-28 5.0 7.5 5.0 96 15.8 6.06 2.596 37.8 92,875 1.00 145.0 1083 NE0569 5.5-4 64-28 5.5 7.9 3.7 96 15.6 6.06 2.596 38.0 92,875 1.00 91.3 1294 NE0569 6.0-5 64-28 6.0 7.1 2.5 96 15.8 6.06 2.596 37.7 92,875 1.00 53.7 374 WA0463 5.5-1 64-22 5.5 7.9 4.9 100 14.8 5.26 2.717 44.8 41,895 1.00 45.7 378 WA0463 6.0-2 64-22 6.0 8.2 3.6 100 14.7 5.26 2.717 44.9 41,895 1.00 28.8 239 WA0463 6.5-1 64-22 6.5 7.9 2.4 100 14.5 5.26 2.717 45.0 41,895 1.00 16.5 156 Table 14. Results of AMPT tests and related properties used in calculation of flow number limits (39). Traffic Level Million ESALs Minimum Flow Time s < 3 --- 3 to < 10 20 10 to < 30 72 ≥ 30 280 Table 15. Recommended minimum flow time requirements— table 8-21 in the mix design manual.

Table 17 (Table 8-22 in the Manual) lists recommended minimum rut depths for the Accel- erated Pavement Analyzer (APA) test. At the time of the writing of the Manual and Commen- tary, there was not a large amount of data on the use of the APA as a performance test. In NCHRP Report 508, the results of research evaluating the APA were reported (40). It was found that although there were reasonable correlations between rut depths determined with the APA and those observed in the field on a project-by-project basis, an overall relationship for multiple proj- ects could not be developed. As a result, NCHRP Report 508 does not provide specific guidelines for interpreting the results of the APA test (40). For the purposes of providing such guidelines in the Manual, test procedures and requirements for several states were reviewed (41, 42, 43). The most common conditions, as reported by Mr. Chad Hawkins at the 2006 APA User Group Meeting, for running the APA have been reported as a 100 lbf load applied through the hose inflated to a pressure of 100 lb/in2; the rut depth is measured after 8,000 loading cycles (43). The most common test temperature is 64°C. These are the conditions used in Oklahoma—the values in 248 A Manual for Design of Hot Mix Asphalt with Commentary Project Phase Section Con. Stress Dev. Stress Temp. Voids Flow No. Flow Time lb/in2 lb/in2 °C Vol.% s MN/Road 1 Cell 16 0 207 37.8 7.7 2,041 730 MN/Road 1 Cell 17 0 207 37.8 8.0 2,482 360 MN/Road 1 Cell 18 0 207 37.8 5.9 2,991 935 MN/Road 1 Cell 20 0 207 37.8 6.1 659 236 MN/Road 1 Cell 22 0 207 37.8 6.9 1,511 770 MN/Road 2 Cell 01 0 173 37.8 6.5 683 226 MN/Road 2 Cell 01 0 173 54.4 6.8 27 6 MN/Road 2 Cell 03 0 173 37.8 6.3 416 267 MN/Road 2 Cell 03 0 173 54.4 6.6 49 6 MN/Road 2 Cell 04 0 173 37.8 6.5 753 371 MN/Road 2 Cell 04 0 173 54.4 6.8 24 21 NCAT 2 E06 0 173 37.8 6.9 12,289 1,807 NCAT 2 E06 0 173 54.4 7.1 1,926 455 NCAT 2 N02 0 173 37.8 5.3 23,181 31,953 NCAT 2 N02 0 173 54.4 5.5 6,860 3,024 NCAT 2 N03 0 173 37.8 5.5 22,563 324,161 NCAT 2 N03 0 173 54.4 6.6 1,211 102 NCAT 2 N05 0 173 54.4 6.1 3,682 17,009 NCAT 2 N07 0 173 54.4 5.8 22,203 26,677 NCAT 2 N11 0 173 54.4 6.2 39,000 2,576 NCAT 2 N12 0 173 37.8 5.2 29,605 43,763 NCAT 2 N12 0 173 54.4 5.6 39,000 18,230 ALF 2 Cell 05 0 138 54.4 9.2 241 104 ALF 2 Cell 07 0 69 54.4 10.6 7,761 13,208 ALF 2 Cell 07 0 138 54.4 10.4 6,028 2,611 ALF 2 Cell 08 0 138 54.4 9.3 6,651 3,542 ALF 2 Cell 09 0 138 54.4 7.3 376 202 Indiana 2 4-A 64-28 0 173 54.4 7.7 165,931 2,251 Indiana 2 4-B 64-28 0 173 54.4 3.7 15,060 25,151 Indiana 2 6-B 64-16 I 0 173 54.4 6.4 3,159 589 Nevada 2 HV 64-22b 0 173 37.8 5.8 29,497 47,851 Nevada 2 HV 64-22b 0 173 `54.4 5.9 1,494 1,676 Nevada 2 HV 64-22T 0 173 54.4 6.8 541 205 Nevada 2 HV AC 20-P B 0 173 54.4 7.7 1,009 61 Nevada 2 HV AC 20-P T 0 173 37.8 6.2 3,071 5,603 Nevada 2 HV AC 20-P T 0 173 54.4 6.2 1,858 144 Nevada 2 SP 64-22 B 0 173 54.4 1.8 4,531 5,649 Nevada 2 SP 64-22 T 0 173 37.8 5.8 22,050 1,210 Nevada 2 SP 64-22 T 0 173 54.4 5.8 149 47 Nevada 2 SP AC-20P B 0 173 54.4 1.8 67,000 3,228 Nevada 2 SP AC-20P T 0 173 54.4 5.7 1,087 159 Table 16. Data from NCHRP project 9-19 used to correlate flow number and flow time (39). Traffic Level Million ESALs Maximum Rut Depth Mm < 3 --- 3 to < 10 5 10 to < 30 4 ≥ 30 3 Table 17. Recom- mended maximum rut depths for the APA test— table 8-22 in the mix design manual.

Table 16 are, in fact, the same as those used in Oklahoma’s APA specification, although Okla- homa includes requirements at lower traffic levels (41). North Carolina’s specification uses somewhat higher maximum rut depth values, but its standard specifies a load of 120 lbf and a hose pressure of 120 lb/in2 (42). According to LTTPBind Version 3.1, the typical high-temperature PG binder grade in Oklahoma is very close to 64.0, so the test temperature in their APA standard corresponds to the high temperature binder grade at 98% reliability. This is the basis for the sug- gested test temperature for the APA equal to the high-temperature binder performance grade specified by the local state highway agency for traffic levels of 3 million ESALs or less. Establishing guidelines for interpreting the results of the Hamburg wheel tracking test is dif- ficult because the test is not widely used (43). The Manual gives test conditions and minimum passes to a half-inch rut depth as specified by the State of Texas (44). Test conditions for the Hamburg test as specified by the Texas Department of Transportation are as follows (44): • 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 Test requirements used in Texas—in terms of minimum passes to a 0.5-inch rut depth—are given in Table 18 (Table 8-23 in the Manual). The Manual states that agencies wishing to use the Hamburg device as a performance test should consider doing an engineering study to develop appropriate requirements for their local conditions and materials. Suggested test values for both maximum permanent shear strain (MPSS) determined using the repeated shear at constant height (RSCH) test and strength measured using the high-temperature indirect tension (HT/IDT) test were calculated in a manner similar to that used to develop suggested requirements for the flow number and flow time test. Data from NCHRP Projects 9-25/9-31 were used in combination with the rutting-resistivity model (Equation 1) to estimate maximum allow- able traffic, which was then compared to test values for MPSS and IDT strength (4, 27). For both sets of test data, the air void content varied from about 2 to about 6%, with an average of 4.0%. The protocol for the HT/IDT test is to compact specimens using the design gyrations (Ndesign) which is what was done for the 9-25/9-31 tests. However, the protocol for RSCH/MPSS (AASHTO T 320) is to prepare specimens at an air void content of 3.0 ± 0.5%. This requires an adjustment to the MPSS values reported in 9-25/9-31, which was made by developing a relationship between MPSS and al- lowable traffic at the design voids, calculating the allowable traffic at 3% voids, and then adjusting the MPSS value according to the difference between the estimated allowable traffic values at the de- sign voids and at 3% air voids. As discussed in the analysis of flow number values, in order to avoid rejection of a large number of mixes by performance testing, the traffic level used in estimating the values for both HT/IDT strength and MPSS was the midpoint of the traffic levels for the 3 to 10 mil- lion ESALs and 10 to 30 million ESAL ranges; for traffic levels above 30 million ESALs, a value of 65 million ESALs was used to estimate the minimum IDT and maximum MPSS. Commentary to the Mix Design Manual for Hot Mix Asphalt 249 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 18. Texas requirements for Hamburg wheel tracking test—table 8-23 in the mix design manual (44).

A final adjustment to HT/IDT strength is needed because the suggested protocol—testing at a loading rate of 50 mm/min at 10°C below the average 7-day maximum pavement temperature, rather than at 3.75 mm/min at 20°C below the critical high pavement temperature—gives IDT strength values about 10% higher than the original protocol, as used in 9-25/9-31. The relationship between IDT strength using the two protocols is shown in Figure 4, reproduced from a letter report prepared for PennDOT as part of a small research project investigating the IDT strength test (45). Figures 5 and 6, respectively, show the relationships between MPSS and IDT strength and allowable ESALs determined from the 9-25/9-31 data. Tables 19 and 20 show recommended values for max- imum MPSS and minimum IDT strength as determined from the relationships shown in the two figures. The data on which this analysis is based are summarized in Table 21 (4). 250 A Manual for Design of Hot Mix Asphalt with Commentary y = 1.10x R2 = 0.99 0 20 40 60 80 100 120 0 20 40 60 80 100 120 IDT Str. at 30 C and 3.75 mm/min ID T St r. at 4 0 C an d 50 m m /m in equality Figure 4. Comparison of IDT strengths from the two procedures (45). y = -1.13Ln(x) + 5.5 R2 = 76 % 0.0 2.0 4.0 6.0 8.0 0 20 40 60 80 100 120 140 Design Traffic, Million ESALs M PS S, % at 3 % Vo id s Figure 5. Plot of MPSS vs. allowable million ESALs for NCHRP 9-25/9-31 data. Traffic Level Million ESALs Maximum Value for MPSS % < 3 --- 3 to < 10 3.4 10 to < 30 2.1 ≥ 30 0.8 Table 19. Recommended maximum values for MPSS determined using the SST/RSCH test—table 8-24 in the mix design manual. Traffic Level Million ESALs Minimum HT/IDT Strength kPa < 3 --- 3 to < 10 270 10 to < 30 380 ≥ 30 500 Table 20. Recommended minimum high- temperature indirect tensile strength require- ments—table 8-25 in the mix design manual. y = 92.6Ln(x) + 68 R2 = 89 % 0 200 400 600 800 0 50 100 150 Design Traffic, Million ESALs ID T St re n gt h, k Pa Figure 6. Plot of IDT strength vs. estimated allow- able million ESALs for NCHRP 9-25/9-31 data.

Commentary to the Mix Design Manual for Hot Mix Asphalt 251 Aggregate, NMAS, Gradation Binder Grade Temp. °C Ndesign Gyrations Agg. Gs Agg. Spec. Surface m2/kg Binder |G*|/sin δ Pa VTM Vol.% VMA Vol.% MPSS @ 3% Voids % HT/IDT Strength lb/in2 Traffic @ 7% Voids MESALs KY limestone, 19 mm, coarse PG 64-22 54 100 2660 4.32 14,500 4.40 15.10 149.6 4.75 5.59 KY limestone, 19 mm, coarse PG 64-22 54 50 2660 4.32 14,500 3.30 17.40 114.1 7.97 0.78 KY limestone, 19 mm, dense PG 64-22 54 100 2648 4.75 14,500 3.60 12.10 226.8 3.92 13.14 KY limestone, 19 mm, dense PG 64-22 54 50 2648 4.75 14,500 4.00 13.60 182.0 4.83 3.68 Crushed gravel, 19 mm, coarse PG 64-22 54 100 2566 3.76 14,500 3.20 14.00 202.0 3.62 2.91 Crushed gravel, 19 mm, coarse PG 64-22 54 75 2566 3.76 14,500 4.20 14.90 192.1 3.71 2.29 Crushed gravel, 19 mm, dense PG 64-22 54 100 2575 4.32 14,500 4.40 12.80 316.9 1.83 10.09 Crushed gravel, 19 mm, dense PG 64-22 54 75 2575 4.32 14,500 3.40 13.00 273.5 2.64 4.31 VA limestone, 9.5 mm, coarse PG 64-22 54 100 2671 4.40 14,500 4.30 15.80 160.0 4.71 4.76 VA limestone, 9.5 mm, coarse PG 64-22 54 50 2671 4.40 14,500 3.50 18.20 6.46 0.75 VA limestone, 9.5 mm, coarse PG 76-16 54 100 2671 4.40 66,200 4.30 15.80 314.8 1.61 38.29 VA limestone, 9.5 mm, coarse PG 58-28 54 100 2671 4.40 4,880 4.30 15.80 108.6 6.21 1.07 VA limestone, 9.5 mm, fine PG 64-22 54 100 2659 6.21 14,500 4.50 18.70 199.6 4.19 6.48 VA limestone, 9.5 mm, fine PG 64-22 54 50 2659 6.21 14,500 3.90 20.30 7.39 1.44 VA limestone, 9.5 mm, fine PG 76-16 54 100 2659 6.21 66,200 4.50 18.70 416.8 2.12 52.14 Granite, 12.5 mm, dense PG 64-22 54 125 2632 4.79 14,500 4.20 13.10 2.33 16.37 Granite, 12.5 mm, dense PG 76-16 54 125 2632 4.79 66,200 4.20 13.10 589.6 0.24 131.71 Granite, 12.5 mm, dense PG 58-28 54 125 2632 4.79 4,880 4.20 13.10 164.8 2.54 3.67 Granite, 12.5 mm, fine PG 64-22 54 125 2631 6.15 14,500 3.50 14.90 2.20 14.49 Granite, 12.5 mm, fine PG 76-16 54 125 2631 6.15 66,200 3.50 14.90 550.6 0.33 116.59 Granite, 12.5 mm, fine PG 58-28 54 125 2631 6.15 4,880 3.50 14.90 171.4 3.00 3.25 VA limestone, 9.5 mm, coarse PG 64-22 60 100 2671 4.40 6,920 4.30 15.80 116.2 4.90 1.72 VA limestone, 9.5 mm, coarse PG 76-16 60 100 2671 4.40 33,100 4.30 15.80 235.5 2.39 14.78 VA limestone, 9.5 mm, fine PG 64-22 60 100 2659 6.21 6,920 4.50 18.70 133.1 5.42 2.35 VA limestone, 9.5 mm, fine PG 76-16 60 100 2659 6.21 33,100 4.50 18.70 300.6 2.73 20.13 Granite, 12.5 mm, dense PG 64-22 60 125 2632 4.79 6,920 4.20 13.10 198.2 2.36 5.93 Granite, 12.5 mm, dense PG 76-16 60 125 2632 4.79 33,100 4.20 13.10 436.1 0.65 50.85 Granite, 12.5 mm, fine PG 64-22 60 125 2631 6.15 6,920 3.50 14.90 247.2 2.51 5.25 Granite, 12.5 mm, fine PG 76-16 60 125 2631 6.15 33,100 3.50 14.90 460.8 1.30 45.01 Table 21. Data used in developing correlations between maximum permanent shear strain, high-temperature IDT strength and estimated allowable traffic (4).

The final critical part of the mix design procedure as described in the Manual is adjustment of the performance test temperature to account for slow traffic speeds. This is necessary because as traffic speed decreases, permanent deformation can significantly increase. Three approaches are possible to account for this effect: (1) increase or decrease required performance test value as traffic speed decreases; (2) decrease test loading rate as traffic speed decreases; or (3) increase test temperature as traffic speed decreases. There is not enough data at this time to use the first approach and it would also be a relatively complicated approach. The second approach is not feasible, since the loading rate on some of the proposed test methods is fixed and is not normally varied. Therefore, the third approach is suggested. The Manual suggests increasing the test tem- perature 6°C for slow traffic and 12°C for very slow traffic. These adjustments were calculated directly from Equation 6 given above, using a traffic speed of 70 kph for fast traffic, 25 kph for slow traffic and 10 kph for very slow traffic. 252 A Manual for Design of Hot Mix Asphalt with Commentary

Chapter 9 of the Manual deals with incorporation of reclaimed asphalt pavement (RAP) into HMA mix designs. This is a complicated topic potentially involving numerous calculations: 1. Calculation of the blended aggregate gradation, including contribution from the RAP. 2. Calculation of the binder content, again including contribution from the RAP. 3. Calculation of the blended binder grade, based upon both the new binder added and the binder contributed from the RAP. 4. Calculation of the required new binder grade needed to achieve a specified binder grade, given a certain RAP content and a binder grade for the RAP binder. 5. Calculation of the minimum and maximum RAP that can be used in a mix, given a new binder grade and a grade for the RAP binder. 6. Estimation of the variability in aggregate gradation and binder content, given a job mix formula (JMF) containing a certain amount of RAP. 7. Estimation of the maximum amount of RAP that can be used in a mix without exceeding typ- ical limits on variability of production, given variability in the RAP stockpiles being used. The mathematics of some of these calculations—in particular the variability calculations— can be challenging. For this reason, HMA Tools has been designed to perform these calculations, and the Manual in general simply discusses how to use this spreadsheet to perform the needed calculations during the mix design process. This avoids having to show and document some involved equations. The Commentary for Chapter 9 therefore involves showing and describing the equations used in HMA Tools to perform these RAP calculations, along with any associated assumptions and/or simplifications. Chapter 9 is structured for the most part as a series of example problems of increasing com- plexity. The section below, on critical tables, figures, and equations therefore is mostly organized on the basis of these examples, presenting and describing the critical information used in solving each example problem. Calculations involving RAP binder properties are based on Appendix A of AASHTO M 323 and are not documented in detail here. Much of what is contained in Chapter 9 of the Manual has been based on NCHRP Report 452 (46). This is an excellent reference for technicians and laboratory engineers responsible for the design and/or analysis of HMA mix designs containing RAP. Example 1. Gradation and Binder Content Analysis for an HMA Mixture Containing RAP The computation of blends for mixtures incorporating RAP is a little different than that for mixtures made with all new stockpiles. When RAP is used, the RAP material that is added includes both the RAP aggregate and the RAP binder. Since gradation data are based on the weight of 253 C H A P T E R 9 Reclaimed Asphalt Pavement

254 A Manual for Design of Hot Mix Asphalt with Commentary aggregate, and binder contents are based on the total weight, the stockpile percentages must be adjusted for combined gradation analysis based on the amount of binder contained in the RAP. The binder included in the RAP is computed from Equation 8: where wbRAPi = weight of RAP binder from RAP stockpile i, wt% psRAPi = percentage of RAP stockpile i in the total blend,% PbRAPi = binder content of RAP stockpile i, wt% The total weight of binder contributed by all RAP stockpiles is the sum of the weight con- tributed by each RAP stockpile and is computed from Equation 9: where wbRAPTotal = total weight of RAP binder from all RAP stockpiles, weight% wbRAPi = weight of RAP binder from RAP stockpile i, wt% j = total number of RAP stockpiles The total binder content for the mix is simply calculated by adding the weight of binder con- tributed by all RAP stockpiles to the weight of new binder added. For gradation analysis, the percentage of each stockpile based on the total weight of aggregate is needed. The percentage of each new aggregate stockpile based on the total weight of aggregate is given by Equation 10: where Pnewk = percentage of new aggregate k, weight% of total aggregate psnewk = percentage of new aggregate stockpile k in the total blend, weight% wbRAPTotal = total weight of RAP binder from all RAP stockpiles, weight% The percentage of each RAP aggregate based on the total weight of aggregate is given by Equation 11: where pRAPi = percentage of RAP aggregate i, weight% of total aggregate psRAPi = percentage of RAP stockpile i in the total blend, weight% wbRAPi = weight of RAP binder from RAP stockpile i, weight% wbRAPTotal = total weight of RAP binder from all RAP stockpiles, weight% For each sieve, the gradation of the blend of the stockpiles is then computed using the per- centage of each stockpile based on the total weight of aggregate using Equation 12: tpp p pp p ppnewk k k n RAPi i= × ⎛ ⎝⎜ ⎞⎠⎟ + × ⎛ ⎝⎜ = ∑ 100 1001 ⎞⎠⎟ = ∑ i j 1 12( ) p ps wb wb RAPi RAPi RAPi RAPTotal = − −( ) ⎡ ⎣⎢ ⎤ ⎦⎥ ×100 100 11% ( ) p ps wb newk newk RAPTotal = −( ) ×100 100 10% ( ) wb wbRAPTotal RAPi i j = = ∑ 1 9( ) wb ps Pb RAPi RAPi RAPi = × 100 8( )

where tpp = total percent passing a given sieve, weight% of total aggregate Pnewk = percentage of new aggregate k, weight% of total aggregate pRAPi = percentage of RAP aggregate i, weight% of total aggregate ppk = percent passing a given sieve for new aggregate k, weight% ppi = percent passing a given sieve for RAP aggregate i, weight% j = number of RAP stockpiles n = number of new stockpiles Example 2. Calculation of Mean, Standard Deviation, and Maximum Allowable RAP Content for a Single RAP Stockpile The mean is calculated using Equation 13 (47): where X _ = stockpile average Xi = result for location i n = total number of locations tested The standard deviation is calculated using Equation 14 (47): where s = standard deviation X _ = stockpile average Xi = result for location i n = total number of locations tested Derivation of the procedure for calculation of the maximum allowable RAP content is as fol- lows. ASTM D 4460 gives equations for calculating standard deviation values for quantities deter- mined from calculations involving two other values. From these equations, the following formula for calculating the standard deviation of a blend of two materials can be derived: where σm = standard deviation of the mixture σa = standard deviation of component “a” σb = standard deviation of component “b” α = proportion of component “a” in the mixture X _ a = mean value for component “a” X _ b = mean value for component “b” σα = standard deviation of the proportions σ α σ α σ σm a b a b aX X= + −( ) + +( )2 2 2 2 2 2 21 15( ) s X X n i i n = −( ) − = ∑ 2 1 1 14( ) X X n i i n = = ∑ 1 13( ) Commentary to the Mix Design Manual for Hot Mix Asphalt 255

256 A Manual for Design of Hot Mix Asphalt with Commentary We can rewrite this for percent passing for a selected sieve for HMA mixtures consisting of a blend of new HMA materials with RAP: where σPM = standard deviation of percent passing for a selected sieve for the mixture with RAP wR = weight fraction of RAP in the mixture σPR = standard deviation of percent passing for the selected sieve for the RAP wN = weight fraction of new materials (new HMA) in the mixture = (1 − wR) σPN = standard deviation of percent passing for the selected sieve for the new HMA P _ R = mean value for RAP% passing for the selected sieve P _ N = mean value for new HMA% passing for the selected sieve σw = standard deviation of the weight fractions, also called “batching variability,” Equation 17 can be solved for the maximum amount of RAP that can be added to new material without increasing the standard deviation for percent passing on the selected sieve above a selected maximum value by application of the quadratic equation where Max. RAP% = maximum amount of RAP that can be added to the mix, weight% a = σ2PR + σ2PN b = − 2 σ2PN c = σ2PN + (P _ 2 PN + P _ 2 N) σ2w − σ2PM/Max and σPM/Max is the maximum allowable standard deviation for percent passing for the selected sieve. Equations 1 and 2 can be rewritten for asphalt content rather than for aggregate percent pass- ing, calculated using the following equation: where σBM = standard deviation of binder content (weight%) for the mixture with RAP wR = weight fraction of RAP in the mixture σBR = standard deviation of binder content (weight%) for the RAP wN = weight fraction of new materials (new HMA) in the mixture = (1 − wR) σBN = standard deviation of binder content (weight%) for the new HMA B _ R = mean value for the RAP binder content, weight% B _ N = mean value for new HMA binder content, weight% σw = standard deviation of the weight fractions or “batching variability” As for Equation 1, Equation 3 can also be solved for the maximum amount of RAP that can be added to new material without increasing the variability above a selected maximum by appli- cation of the quadratic equation: Max RAP b b ac a . % % ( )= − + − × 2 4 2 100 19 σ σ σ σBM R BR N BN R N ww w B B= + + +( )2 2 2 2 2 2 2 18( ) Max RAP b b ac a . % % ( )= − + − × 2 4 2 100 17 σ σ σ σPM R PR N PN R N ww w P P= + + +( )2 2 2 2 2 2 2 16( )

where Max. RAP% = maximum amount of RAP that can be added to a mix without increasing the production variability, weight% a = σ2BR + σ2BN b = −2σ2BN c = σ2BN + (B _ 2 R + B _ 2 N)σ2w − σ2BM/Max and σBM/Max is the maximum allowable standard deviation for binder content for the entire mix, including RAP. The approach used in the Manual in applying Equations 17 and 19 to calculate the maximum allowable RAP in an HMA mix design is to assume that the maximum allowable standard devia- tion for the final HMA should be no larger than the standard deviation for the new materials in the mix. This is equivalent to saying that the amount of RAP should be limited so that the overall variability in HMA production is not increased above what it would normally be without the addition of any RAP. This approach simplifies some of the calculations, and as discussed below, if typical variability in aggregate% passing and binder contents are assumed, the mix designer can determine the maximum allowable RAP without knowing the existing HMA variability or the maximum allowable variability desired by the producer. The maximum amount of RAP that can be added to a mix simply becomes a function of the RAP variability. A critical issue in this approach is what values to use for typical standard deviations for HMA production when applying Equations 17 and 19. The simplest and most conservative approach is to base the standard deviation values on information given in ASTM D 3515: Standard Speci- fication for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures. Although this document does not specify standard deviations for aggregates and asphalt binder, it does give tolerances; these are shown in Table 22. The question becomes what is the ratio between these tolerances and typical standard deviations? The standard deviation values corresponding to those given in ASTM D 3515 must be much smaller than the tolerances, otherwise, plants would often violate these specified tolerances. For example, if typical standard deviation values were one-fourth the specified toler- ance, HMA plants would exceed these limits one time in 20. This is probably too frequent; a somewhat smaller standard deviation is more likely—one-fifth of the specified tolerance range appears to provide reasonable estimates of typical standard deviation values in well-run HMA plants (typical variability of HMA production is discussed in more detail in Chapter 12 of the Manual and the Commentary). The resulting estimated typical standard deviations are also shown in Table 22. A second complication arises from the uncertainty in estimating standard deviations. When sev- eral samples of RAP are taken and used to estimate the standard deviation, the resulting value is an estimate. In fact, unless the number of replicate samples is about 30 or higher, the uncertainty Sieve Size Tolerance Typical Standard Deviation > 12. 5 mm ± 8.0% 3.2% 4.75 and 9.5 mm ± 7.0% 2.8% 1.18 and 2.36 mm ± 6.0% 2.4% 0.300 and 0.600 mm ± 5.0% 2.0% 0.150 mm ± 4.0% 1.6% 0.075 mm ± 3.0% 1.2% Asphalt binder ± 0.5% 0.2% Table 22. Production Tolerances for Hot Mix Plants as Given in ASTM D 3515. Commentary to the Mix Design Manual for Hot Mix Asphalt 257

in the estimate can be quite large. To be conservative and to make certain that the variability in plant production will not be increased to an unacceptable level by the addition of RAP, an upper confidence limit for the standard deviation should be used, rather than the actual calculated value. This upper confidence limit is calculated using the following equation (47): where U = upper confidence limit for the standard deviation at 1−α confidence level; use in place of σPR and/or σBR in Equations 2 and 4. n = number of measurements used in estimating the standard deviation s χ2 = chi-squared distribution with confidence level 1−α and n−1 degrees of freedom A value for α of 0.20 (an 80% confidence level) appears to provide reasonable values for U and the resulting calculated maximum allowable RAP contents. One advantage to using an upper confidence limit U for the standard deviation, rather than the actual estimate, is the effect of sam- ple size on the resulting value of U—the larger the number of RAP samples used to estimate the standard deviation, the lower will be the value of U and the greater the resulting maximum allow- able RAP content. Producers who take large numbers of RAP samples to characterize their stock- pile will be rewarded, while those using only a few samples will be severely limited in how much RAP they can use in their mixtures. For example, for the case where σPN = σPM/max = 2.0, and σPR is estimated to be 4.0, if the RAP standard deviation is based on n = 5 samples, the maximum allowable RAP content is 17%. However, if the RAP standard deviation is based on n = 10 sam- ples, the maximum allowable RAP content increases to 27%. A third important question in applying the equations for analyzing the effect of RAP on HMA production variability is what is a typical value for the standard deviation of the proportions, also called “batching variability.” Information is given in ASTM D 995: Standard Specification for Mixing Plants for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures on the required accuracy of aggregate scales, control of automated plants, and related information that can be used to estab- lish typical blending standard deviations for HMA plants: • The accuracy of metering of asphalt binder must be within 1.0% when compared to another metering device or to within 0.5% when compared to test weights. Assuming a ±2 s precision applies to the comparison with test weights, this implies a blending standard deviation for asphalt binders of 0.25% or 0.0025. • Aggregate scales for batch plants must also be accurate to within 0.5% when compared to test weights, again, implying a blending standard deviation of 0.25% or 0.0025. • Automatic proportioning systems, for both batch plants and drum plants, must batch aggre- gate (other than mineral filler) to within ±1.5% of the total batch weight, or for continuous drum plants, to within ±1.5% of the mix production per drum rotation/unit time. Assuming that this tolerance refers to comparison with test weights or similarly accurate reference, and that a ±2 s precision is implied, this translates to a blending standard deviation of 0.75% or 0.0075. The required tolerance for mineral filler is ±0.5%, implying a blending standard deviation of 0.0025. For asphalt binder, the required tolerance is ±0.1%, implying a blending standard deviation of 0.0005. Assuming that the tolerance for adding RAP to a mix is similar to that for aggregate, and con- servatively applying the standard deviation calculated from the required tolerance for automated plants, the blending standard deviation for analyzing RAP variability should be 0.0075. It should be noted that this is based on the maximum permitted tolerance in automated plants and is thus U n s n = −( ) −( ) 1 1 20 2 2χ α; ( ) 258 A Manual for Design of Hot Mix Asphalt with Commentary

Commentary to the Mix Design Manual for Hot Mix Asphalt 259 a conservative assumption. The actual blending variability in many plants will probably be lower than this. HMA Tools calculates mean, standard deviation, the upper confidence limit for standard devi- ation, and the maximum allowable RAP content based on variability for up to four different RAP stockpiles. This analysis uses Equations 13 through 20 along with the assumptions described con- cerning typical variability in HMA production and batching variability. Example 3. Determination of Maximum Allowable RAP Content Based on Variability Analysis Using the Graphical Approach This example problem is essentially identical to the previous one, but in this case a simplified graphical approach is used in determining the maximum allowable RAP. This approach involves the use of four charts—Figures 9-3 through 9-6 in the Manual—to determine the maximum allow- able RAP content. These charts are reproduced here for the convenience of the reader. The charts have been developed based on the analysis described above, with a sample size of N = 5; the sam- ple size is small because it has been assumed that those producers wishing to use this simplified approach would probably not want to use large sample sizes of RAP. Figures 7 and 8 (9-3 and 10 20 30 40 50 Standard Deviation for RAP Aggregate % Passing, % M ax . R A P Co nt en t, W t. % 0 1 2 3 4 5 6 7 8 Sieve size, mm: 0.075 0.150 0.300 1.18 4.75 > 9.5 to 0.600 to 2.36 to 9.5 Figure 7. Maximum RAP content as a function of RAP aggregate sieve size and standard deviation (Figure 9-3 in the Mix Design Manual). 15 20 25 30 35 40 45 50 0.2 0.3 0.4 0.5 0.6 0.7 Binder Standard Deviation M ax . R A P Co n te n t, W t. % Figure 8. Maximum RAP content as a function of standard deviation for asphalt binder content (Figure 9-4 in the Mix Design Manual). For n = 5 Samples from a single RAP stockpile.

9-4 in the Manual) are for the case where only a single RAP stockpile is used in a mix design. The development of these charts was straightforward, involving a direct application of the equations given above. As an example of using these charts, if a given RAP stockpile has a standard devia- tion of 4.0% for aggregate passing the 0.60 mm sieve, the maximum allowable RAP content can be found from Figure 7 to be 30%. It must be emphasized that Figure 7 is based on standard devi- ations calculated using at least 5 samples—it should not be applied to standard deviations calcu- lated using a lower number of samples. It can be used for standard deviations calculated using larger samples, but the results will be overly conservative. Also, in developing these figures it is assumed that the difference in percent passing between the RAP aggregate and the new aggre- gate will not exceed these limits: 30% for mineral filler; 40% for passing the 0.150 mm sieve, and 50% for all other aggregate sizes. If the differences exceed these limits, the difference term in Equations 15 through 18 starts to become significant and must be considered in the calculation. In using Figure 7 in an actual mix design, all aggregate sizes would be evaluated, and the overall maximum allowable RAP content would be the lowest value calculated for all sieve sizes. The maximum RAP content based on asphalt binder standard deviation should also be evaluated (Figure 8, or Figure 9-4 in the Manual)—again, the lowest calculated of these maximum RAP contents should be applied. Figures 7 and 8 will in general not be accurate when more than one RAP stockpile is used in a mix design. This is because as RAP stockpiles are blended, the variability of the resulting stockpile will be reduced. The more stockpiles in a blend, the lower will be its variability when compared to the average variability for the materials in the blend. The standard deviation for % passing and binder content for a blend of three or more RAP stockpiles can be accurately estimated by sequen- tial application of Equations 15 and 17, respectively. This calculation is done as follows. The stan- dard deviation for % passing for a blend of Stockpiles 1 and 2 is first calculated. Then, the standard deviation is calculated for a second blend, made up of stockpile 3 and the blend of Stockpiles 1 and 2. If a fourth stockpile is used, the standard deviation for a blend of Stockpile 4 and the blend of Stockpiles 1, 2, and 3 is calculated. This is done for both aggregate % passing and binder content. This approach is very accurate and is used in HMA Tools in calculating the standard deviation values for RAP blends involving more than two stockpiles. Unfortunately, constructing graphs similar to Figures 7 and 8 for this situation is somewhat complicated. The approached used for the Manual was to develop an empirical relationship between the average standard deviation of a blend of stockpiles and the standard deviation calculated using Equations 15 and 17. The data was generated using a Monte Carlo approach. A total of 500 data points were generated, with simulated RAP stockpile blends composed of from two to four separate stockpiles, having a wide range of standard deviations along with a wide range of blend compositions. The relationships between average standard deviation and cal- culated standard deviation for % passing is shown in Figure 9, and for binder content in Fig- ure 10. These plots include the regression function for predicted standard deviation, along with the 80% upper prediction limit. In order to provide a conservative estimate of standard deviation, the upper prediction limits were used in generating the charts for determining maximum RAP content in HMA designs using more than one stockpile. The equation for estimating the 80% upper prediction limit for standard deviation of a blend of RAP stock- piles for aggregate% passing is as follows: where SD(PP, 80% UPL) = Estimated standard deviation for % passing for the overall RAP stockpile blend, 80% upper prediction limit SD — = average standard deviation for % passing for the RAP stockpile blend SD PP UPL SD, % . ( ).80 0 70 211 023( ) = × −− 260 A Manual for Design of Hot Mix Asphalt with Commentary

Commentary to the Mix Design Manual for Hot Mix Asphalt 261 0.1 1.0 10.0 1.0 10.0 Average RAP Standard Deviation for % Passing Ov e ra ll R A P St d. De v . Data Predicted Std. Dev. 80 % Upper Prediction Limit Figure 9. Relationship between average RAP standard deviation for % passing and calculated overall RAP standard deviation for % passing. 0.0 0.1 1.0 0.1 1.0 Average RAP Standard Deviation for Binder Content O v e ra ll R A P St d. D e v . Data Predicted Std. Dev. 80 % Upper Prediction Limit Figure 10. Relationship between average RAP standard deviation for binder content and calculated overall RAP standard deviation for binder content. The 80% upper prediction limit for standard deviation of the binder content for a blend of RAP stockpiles can be estimated using the following equation: where SD(BC, 80% UPL) = Estimated standard deviation for binder content (weight%) for the over- all RAP stockpile blend, 80% upper prediction limit SD — = average standard deviation for binder content (weight %) for the RAP stockpile blend Using Equations 21 and 22, Figures 11 and 12 were developed, respectively, showing maxi- mum allowable RAP content as a function of average RAP standard deviation values. It should be noted that these charts are conservative and like Figures 7 and 8 are based on standard devi- ation values calculated using N = 5 independent samples of RAP. HMA Tools uses the pertinent equations directly, without any assumptions or simplifications. HMA Tools will therefore pro- vide more accurate estimates of maximum allowable RAP contents. Furthermore, in general, the estimated maximum RAP contents found using HMA Tools will be somewhat larger than those found with the charts. This is especially true when more than five samples of RAP are used in esti- mating standard deviation values, when more than one RAP is used in a mix design, or both. SD BC UPL SD, % . –– ( ).80 0 69 220 973( ) = ×

Calculation of Aggregate Specific Gravity Values for RAP Stockpiles. The bulk specific gravity of each aggregate stockpile used in an HMA mixture is needed for the computation of the voids in the mineral aggregate (VMA). Two methods can be used to deter- mine the bulk specific gravity of the RAP aggregate (46). The first is to estimate the bulk specific gravity of the RAP aggregate from the RAP binder content, the maximum specific gravity of the RAP, and estimates of the binder absorption in the RAP and the specific gravity of the RAP binder. The second is to measure the bulk specific gravity of the coarse and fine fraction of the RAP aggregate after removing the binder with the ignition oven or solvent extraction. Details of these approaches are discussed below (46). Estimating RAP Aggregate Bulk Specific Gravity In this approach, the maximum specific gravity of the RAP is measured in accordance with AASHTO T 209. The maximum specific gravity is measured on a sample split from the repre- sentative sample formed for the RAP aggregate and binder analysis. The measured maximum specific gravity, the average RAP binder content from the variability analysis, and an estimate of the RAP binder specific gravity are then used to calculate the effective specific gravity of the RAP aggregate using Equation 23 (Equation 9-1 in the Manual) (46): 262 A Manual for Design of Hot Mix Asphalt with Commentary Average Standard Deviation for RAP Aggregate % Passing 0 1 2 3 4 5 6 7 8 9 10 15 20 25 30 35 40 45 50 M ax . R A P Co nt en t, W t. % Sieve size, mm: 0.075 0.150 0.300 1.18 4.75 > 9.5 & 0.600 & 2.36 & 9.5 Figure 11. Maximum RAP content as a function of average standard deviation for aggregate % passing (Figure 9-5 in the Mix Design Manual). For n = 5 Samples from a blend of RAP stockpiles, and no stockpile making up more than 70% of the RAP blend. 15 20 25 30 35 40 45 50 0.2 0.3 0.4 0.5 0.6 0.7 Average Binder Standard Deviation M ax . R A P Co n te n t, W t. % Figure 12. Maximum RAP content as a function of average standard deviation for asphalt binder content (Figure 9-6 in the Mix Design Manual). For n = 5 Samples from a blend of RAP stockpiles, and no stockpile making up more than 70% of the RAP blend.

where Gse = effective specific gravity of the RAP aggregate Gmm = maximum specific gravity of the RAP measured by AASHTO T 209 Pb = RAP binder content, wt % Gb = estimated specific gravity of the RAP binder The bulk specific gravity of the RAP aggregate can then be estimated from Equation 24 (Equa- tion 9-2 in the Manual), which is a rearranged version of the equation used in volumetric analy- sis to compute asphalt absorption. where Gsb = estimated bulk specific gravity of the RAP aggregate Gse = effective specific gravity of the RAP aggregate from Equation 23 Pba = estimated binder absorption for the RAP, wt% of aggregate Gb = estimated specific gravity of the RAP binder The overall error associated with this analysis is difficult to quantify. It depends on the preci- sion of the maximum specific gravity measurement, the accuracy of the RAP binder content measurement, and the estimated RAP binder absorption and specific gravity. As shown in the analysis below, the accuracy of the RAP binder content in turn depends on the accuracy of the correction factor that was used to analyze the ignition oven data. The single-operator precision of the maximum specific gravity test, AASHTO T 209, is 0.011 when the dry-back procedure is not required and 0.018 when it is. These are somewhat better than the single-operator precision of the aggregate bulk specific gravity tests which are 0.032 for fine aggregate (AASHTO T 84) and 0.025 for coarse aggregate (AASHTO T 85). The potential error associated with estimating the bulk specific gravity of the RAP binder is small. For a typi- cal mixture it is only ±0.002 for a ±0.010 error in the bulk specific gravity of the binder. Poten- tial errors associated with errors in the RAP binder content or the RAP binder absorption are significantly larger. These errors are shown in Figure 13 for RAP having a maximum specific gravity of 2.500, a total binder content of 4.0%, and binder absorption of 0.5%. In this case underestimating the absorbed binder by 0.3% results in an overestimation of the bulk specific gravity of the RAP aggregate of 0.020. Underestimating the total binder content of the RAP by 0.5% results in an underestimation of the bulk specific gravity of the RAP aggregate of 0.021. Thus the accuracy of estimating the RAP aggregate specific gravity from the maximum spe- cific gravity and binder content of the RAP depends mostly on the accuracy of the estimated cor- rection factor used to determine binder content with the ignition oven and the accuracy of the assumed binder absorption. The correction factor for the ignition oven should not be in error by more than 0.3% and the assumed binder content should not be in error by more than 0.2% to obtain estimated RAP aggregate specific gravity values with similar accuracy as those mea- sured in AASHTO T 84 and T 85. As discussed earlier, correction factors for the ignition oven can be established by performing both the ignition oven and solvent extraction analyses on split sam- ples from at least three locations in the RAP stockpile. G G P G G sb se ba se b = × ⎛⎝⎜ ⎞⎠⎟ +100 1 24( ) G P G P G se b mm b b = −( ) − ⎛⎝⎜ ⎞⎠⎟ 100 100 23( ) Commentary to the Mix Design Manual for Hot Mix Asphalt 263

Measuring RAP Aggregate Specific Gravity If a reasonable estimate of the binder absorption for the RAP is not available, the specific grav- ity of the RAP aggregate can be measured after removing the RAP binder using an ignition oven or solvent extraction. The specific gravities of the coarse and fine fractions of the RAP aggregate are measured in accordance with AASHTO T 85 and AASHTO T 84, respectively. HMA Tools and RAP Aggregate Specific Gravity HMA Tools has been designed so that either approach can be used to estimate RAP aggregate specific gravity values. If the specific gravity values are to be estimated from maximum theoret- ical specific gravity, binder content, and related information, the data is entered in cells C6:F9 in worksheet “RAP_Aggregates.” If actual measured values for RAP aggregate specific gravity are used, these are entered in cells C11:F14. The calculated values for bulk and apparent specific grav- ity for each of up to four RAP stockpiles then appear in cells C16:F17. If data for both methods are entered in the worksheet, HMA Tools will use the measured aggregate specific gravity values in estimating the RAP specific gravity values. The estimated water absorption for each RAP stockpile appears in cells C18:F18. RAP Binder Properties The section in the Manual on RAP binder properties is based on information and equations given in Appendix A of AASHTO M 323. The various equations and calculations described herein have been implemented in HMA Tools. Using HMA Tools to perform calculations related to RAP binder properties should give results identical to manual calculations carried out following the instructions given in Appendix A of AASHTO M 323. 264 A Manual for Design of Hot Mix Asphalt with Commentary -0.040 -0.030 -0.020 -0.010 0.000 0.010 0.020 0.030 0.040 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 ERROR IN RAP BINDER CONTENT OR ABSORPTION, % ER R O R IN B UL K SP EC IF IC G RA VI TY O F R A P A G G RE G AT E RAP Binder Content RAP Binder Absorption Figure 13. Potential errors in bulk specific gravity of the RAP aggregate for errors in RAP binder content and binder absorption (Figure 9-7 in the Mix Design Manual).

Chapter 10 is based largely on the AASHTO Standards M 325-08: Standard Specification for Stone Matrix Asphalt and R 46-08: Standard Practice for Designing Stone Matrix Asphalt. How- ever, there are several significant differences between the information provided in Chapter 10 and the requirements of these two AASHTO standards: • In Table 10-1, giving requirements for coarse aggregate, the requirements for flat and elon- gated particles is given as a maximum of 10% at a 5 to 1 ratio, whereas M 325 has a maximum of 20% at a 3:1 ratio and 5% at a 5:1 ratio. The 10% maximum at a 5:1 ratio is used to provide consistency with the requirements for dense-graded HMA designed for traffic levels of 30 mil- lion ESALs or more (see Chapter 8 of the Manual). • Also in Table 10-1, there is no absorption requirement, again, to maintain consistency with the requirements for dense-graded HMA given in Chapter 8. • Also in Table 10-1, the requirements for fractured faces are 98/98 (one face/two faces), with the option of reducing this requirement to 95/95 if experience with local materials indicates that this will provide HMA with adequate rut resistance under very heavy traffic; this require- ment is used, again, to provide consistency with the requirements for dense-graded HMA as given in Chapter 8 (the fractured faces requirement in M 325 is 100/95). • In Table 10-2, giving requirements for fine aggregate, there are no requirements for liquid limit and plasticity as are listed in M 325. Instead, requirements for fine aggregate angularity and sand equivalency are given; these requirements are identical to those given in Chapter 8 for dense-graded HMA designed for traffic levels of 30 million ESALs or more. • Table 10-11 is used explicitly to establish minimum asphalt binder contents; it is identical to Table X2.1 given in R 46, but according to R 46, the requirements in X2.1 are only to be used when the “standard” minimum binder content of 6.0% cannot be met. 265 C H A P T E R 1 0 Design of Gap-Graded HMA Mixtures

The information given in this chapter comes directly from the following document (48): Mallick, R. B., P. S. Kandhal, L. A. Cooley, Jr. and D. E. Watson. Design Construction and Performance of New- Generation Open-Graded Friction Courses, NCAT Report 00-01. National Center for Asphalt Technology. Auburn University. Auburn, Alabama. 2001. The mix design procedure, tables, equations, and other critical information given in Chapter 11 are taken from this report. 266 C H A P T E R 1 1 Design of Open-Graded Mixtures

Chapter 12 of the Manual is made up of two distinct parts. The first part of this chapter deals with how a typical HMA mix design, as developed in the laboratory must be adjusted during field production. The most important of these adjustments involves accounting for the increase in mineral dust that typically occurs during field production. The second part of the chapter deals with quality assurance (QA) of HMA. Although not strictly part of the HMA mix design process, it was thought that QA is such an important part of a typical HMA laboratory’s work and is so closely related to the mix design process that the Manual would not be complete without ad- dressing this topic. Information for this part of the chapter comes from various sources, but by far the most important are Hot Mix Asphalt Construction, Instructor Manual, Part A: Lecture Notes and the NHI course manual Highway Materials Engineering, Module I: Materials Control and Acceptance—Quality Assurance (49, 50). Table 23 (Table 12-1 in the Manual) shows typical amounts of mineral dust generated during HMA plant production. This table is based in part on data from the NCAT test track; Figure 14 shows the increase in the passing 0.075-mm fraction of aggregate in QA data compared to the JMF gradation as a function of % passing the 2.36-mm sieve (51). For gradations having more than 35% passing the 2.36-mm sieve, the increase appears to mostly fall in the range of from 1.0 to 3.0%. However, as the % passing the 2.36-mm sieve decreases, the amount of dust generated during plant production appears to increase. A second factor affecting the amount of dust gen- erated during plant production is aggregate hardness. Unfortunately the nature of the NCAT data does not allow development of such a relationship. The values shown in Table 23 are there- fore based on engineering judgment. The L.A. abrasion values for different aggregate types are 267 C H A P T E R 1 2 Field Adjustments and Quality Assurance of HMA Mixtures % Retained on 2.36-mm sieve in theoretical aggregate blend: Abrasion Resistance Level L.A. Abrasion Loss Wt.% Examples < 25 25 to 35 > 35 Good < 20% Dense igneous rocks such as basalt, diabase and gabbro (“trap rock”) 2.5 1.5 1.0 Moderate 20 to 35% Good quality igneous rocks of moderate density such as granite, syenite, diorite; good quality dolomites, limestones and dolomitic limestones; most sandstones, graywackes, quartzites, slags, and crushed gravels 3.5 2.5 2.0 Poor > 35% Soft limestones, sandstones, graywackes and granite 4.5 3.5 3.0 Table 23. Typical amounts of mineral dust generated during HMA plant production for different aggregates and gradations (Table 12-1 in the Mix Design Manual).

based on a number of sources, including NCHRP Report 405 and NCHRP Report 557. It should be emphasized that Table 23 is meant to provide approximate guidelines concerning the amount of dust generated during HMA production. Although such values cannot be predicted exactly, using estimated values in the mix design process will provide more reliable HMA mix designs than ignoring the amount of dust likely to be generated during plant production. Table 24 (Table 12-2 in the Manual) gives precision values for commonly used tests on aggre- gates and HMA mixtures. These values are taken directly from AASHTO or ASTM standards for the various tests. Equations 25 and 26 (Equations 12-2 and 12-3, respectively, in the Manual) for calculating the upper and lower control limits for an X-bar control chart are taken from Hot Mix Asphalt Construc- tion, Instructor Manual, Part A: Lecture Notes (49). Similar equations appear in other documents dis- 268 A Manual for Design of Hot Mix Asphalt with Commentary 0 1 2 3 4 5 0 10 20 30 40 50 60 % Passing 2.36 mm In cr ea se in % P as si ng 0. 07 5 m m NCAT 12.5-mm SP NCAT 12.5-mm SMA NCAT 19-mm SP Figure 14. Increase in % passing the 0.075-mm Sieve during HMA Plant production as a function of % passing the 2.36-mm Sieve, for mixes placed in the first cycle of the NCAT test track. Single Operator Multi-laboratory Test Procedure Std. Dev. D2S Std. Dev. D2S Aggregate gradation, percent passing Coarse aggregate (CA)* 0.27 to 2.25 0.8 to 6.4 0.35 to 2.82 1.0 to 8.0 Fine aggregate (FA)* 0.14 to 0.83 0.4 o 2.4 0.23 to 1.41 0.6 to 4.0 Mineral Filler (in CA/ in FA) 0.10/0.15 0.28/0.43 0.22/0.29 0.62/0.82 Asphalt content, weight % Ignition oven 0.04 0.11 0.06 0.17 Quantitative extraction** 0.19 to 0.30 0.54 to 0.85 0.29 to 0.37 0.82 to 1.05 Maximum theoretical specific gravity 0.0040 0.011 0.0064 0.019 Bulk specific gravity, SSD 0.0124 0.035 0.0269 0.076 Bulk specific gravity, Paraffin-coated 0.028 0.079 0.034 0.095 Air void content, Vol. %*** 0.5 1.5 1.1 3.0 Effective asphalt content, Vol. %*** 0.3 0.9 0.6 1.6 Voids in mineral aggregate, Vol. %*** 0.5 1.5 1.1 3.1 Voids filled with asphalt, Vol. %*** 2.2 6.2 4.5 12.8 Dust/asphalt ratio, by weight*** 0.05 0.13 0.09 0.25 * Lower values are for very high, very low, or both percent passing; higher values are for percent passing values close to 50%. ** Value depends on method used. *** Typical values, estimated from data on aggregate gradation, aggregate and mixture specific gravity, and asphalt content using ignition oven. Values estimated using standard deviations for quantitative extraction vary slightly from these values. Table 24. Single-operator and multi-laboratory precision for test results commonly used in HMA quality assurance and acceptance plans (Table 12-2 in the Mix Design Manual).

cussing QA for HMA production and other industrial operations. These equations calculate con- trol limits for the average (X-bar or X – ) as a function of the overall range (R-bar or R – ): where UCL = upper control limit LCL = lower control limit X – = overall average or X-bar R – = overall range or R-bar A2 = a factor that depends on sample size n (given in Table 25) Table 25 (Table 12-4 in the Manual) gives factors for calculating control limits for both X-bar charts and R charts. These numbers were also taken from Hot Mix Asphalt Construction, Instruc- tor Manual, Part A: Lecture Notes (49), but can be found in other references on QA. Tables 26 and 27 (12-5 and 12-6, respectively, in the Manual) show ranges for typical overall standard deviation for aggregate% passing, asphalt content, air voids, voids in the mineral aggregate (VMA) and voids filled with asphalt (VFA). These values are taken from a National Highway Institute (NHI) course manual on QA (50). Equations 27 and 28 (Equations 12-4 and 12-5 in the Manual) are used to calculate lower and upper control limits for range charts (R charts) for quality assurance of HMA production: where LCL = lower control limit UCL = upper control limit D3, D4 = factors for computing control limits for standard deviation control charts; see Table 25 R – = overall range UCL D R= × = × =4 2 12 1 35 2 86 28. . . ( ) LCL D R= × = × =3 0 0 1 35 0 0 27. . . ( ) LCL X A R= − ×( )2 26( ) UCL X A R= + ×( )2 25( ) Commentary to the Mix Design Manual for Hot Mix Asphalt 269 Sample Size n A2 D3 D4 2 1.88 0.00 3.27 3 1.02 0.00 2.58 4 0.73 0.00 2.28 5 0.58 0.00 2.12 6 0.48 0.00 2.00 7 0.42 0.08 1.92 Table 25. Factors for computing control limits for control charts (Table 12-4 in the Mix Design Manual) (49). Sieve Size Typical Range for Overall Standard Deviation 19 mm 1.5 to 4.5% 12.5 mm 2.5 to 5.0% 9.5 mm 2.5 to 5.0% 4.75 mm 2.5 to 5.0% 2.36 mm 2.5 to 4.0% 1.18 mm 2.5 to 4.0% 0.60 mm 2.0 to 3.5% 0.30 mm 1.0 to 2.0% 0.15 mm 1.0 to 2.0% 0.075 mm 0.6 to 1.0% Table 26. Typical overall standard deviation values for aggregate gradation (Table 12-5 in the Mix Design Manual) (50). Property Typical Range of Value for Overall Standard Deviation Asphalt content 0.15 to 0.30% Air voids, from field cores 1.3 to 1.5% Laboratory air voids 0.9% VMA 0.9% VFA 4.0% Table 27. Typical overall standard deviation values for asphalt content, air voids, VMA, and VFA (Table 12-6 in the Mix Design Manual) (50).

These equations, like Equations 25 and 26 presented previously, are taken from Hot Mix Asphalt Construction Instructor Manual, Part A: Lecture Notes (49). As with much of the other information in Chapter 12, these equations can be found in numerous other references on QA. The rules for interpreting statistical control charts (page 12-21) are based on those in refer- ence 50. The plan for investigating possible production problems as indicated by a control chart is based on the authors’ engineering judgment and experience, as is much of the discussion for the last part of Chapter 12. The example of a stratified random sampling plan is based in part on the acceptance plan described in the Pennsylvania Department of Transportation’s specification “Section 409—Superpave Mixture Design, Standard and RPS Construction of Plant-Mixed HMA Courses,” as given in Publication 408 (52). The example QA plan given toward the end of Chapter 12 is based on this same specification. 270 A Manual for Design of Hot Mix Asphalt with Commentary

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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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