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CHAPTER 1 Introduction This manual includes chapters on all the primary aspects of hot-mix asphalt (HMA) mix design: General background on HMA mixtures and pavements Asphalt binders Aggregates Volumetric composition of HMA mixtures Performance Mix type selection Design of dense-graded HMA Recycled asphalt pavement Gap-graded HMA mixtures Open-graded friction courses Field production and quality control of HMA Engineers and technicians already experienced in HMA technology need not read many of these chapters in detail, though they may find them useful as references. The most important chapters for those already experienced in HMA mix design are Chapter 8, Design of Dense-Graded HMA Mixtures, and Chapter 9, Reclaimed Asphalt Pavement. Chapter 8 contains all the essential information needed to develop HMA mix designs. Because of the complexity of incorporating recycled or reclaimed asphalt pavement (RAP) into HMA mix designs, this topic is discussed in a separate chapter in this manual. Using RAP is becoming increasingly important in the HMA industry, and engineers and technicians responsible for HMA mix design should read Chapter 9 of this manual carefully. Laboratory personnel involved in developing designs for other mix types may also find Chapters 10 and 11 useful, which discuss the design of gap-graded HMA mixtures and open-graded friction course mixtures, respectively. Readers who have not yet gained significant experience in developing HMA mix designs may find a careful reading of most or all of the chapters in the manual useful. The manual may also be incorporated into technician training and certification programs; training materials that instructors will find useful in presenting the information contained in this manual are available on the TRB website by searching for "NCHRP Report 673." These training materials focus on Chapter 8, since the design of dense-graded mixtures is the most important part of the manual and this chapter also discusses many of the most important parts of the other chapters. Furthermore, many technician training programs emphasize the design of dense-graded HMA. Although this manual was developed for use by practicing engineers and technicians, its format lends itself for use in college courses on construction materials. The chapters included in such a course will depend on the format selected by the instructor. In such a situation it is likely that information contained in some chapters would either be excluded from the course or covered 1

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2 A Manual for Design of Hot Mix Asphalt with Commentary using other text books. An introductory course on construction materials might include the following chapters from the manual: Chapter 3, Asphalt Binders Chapter 5, Mixture Volumetric Composition Chapter 8, Design of Dense-Graded HMA Mixtures An upper level course on construction materials or HMA technology might include all of the chapters except for Chapter 4 (Aggregates), since this is likely covered by a more general text on construction materials. Such an upper level course might either omit or simply quickly review material covered in the prerequisite courses. This manual is supported by a comprehensive spreadsheet (HMA Tools) for performing dense- graded HMA mix designs. This spreadsheet is among the materials available on the TRB website and can be found by search under "NCHRP Report 673." HMA Tools includes all calculation tools needed for performing a dense-graded mix design, including those required for mix designs containing RAP. Such calculations include determining (1) the effective binder grade of the mixture or (2) the new binder grade that should be added to a RAP-containing mixture in order to meet the requirements of the specified binder grade. The spreadsheet includes utilities for blending aggregates, performing volumetric calculations, determining weights for laboratory batches, and estimating specification properties for aggregate mixes from test data on individual aggregate stockpiles. In many cases, this manual refers directly to the HMA Tools spreadsheet and describes how HMA Tools is used to perform various steps in the mix design process. This is especially true in Chapter 9, given that the mathematics involved in proper design of HMA containing RAP can be exceedingly complex. Engineers and technicians already satisfied with their current spreadsheet or computer program can certainly continue using it to perform mix designs following the procedure given in this manual, although some of the specification values may need to be modified. This manual also includes a Commentary, which is designed to provide technical background information supporting specific parts of this manual. Such commentary is needed because this manual itself was intended for use by technicians and engineers, including those with little knowledge or experience with HMA mixtures. Detailed technical references and equations would make this manual difficult to read and use as a practical reference by laboratory personnel. On the other hand, excluding such information would make it difficult for engineers to review the mix design procedure and specifications to determine if such procedures and specifications might need to be adjusted to account for unusual local conditions or special projects. Information in the Commentary will also make it much easier to evaluate and revise this manual when such revisions become necessary. Engineers and technicians primarily concerned with developing or evaluating HMA mix designs need not read the Commentary. The information presented herein, for the most part, is meant to follow as accurately as possible current practice as well as recommendations made by researchers and reviewed and approved by industry panels. This manual reflects research and engineering work performed by a wide range of individuals and organizations and is not simply a statement of the authors' opinions and judgments concerning HMA mix design. When possible, current AASHTO standards have been followed and included as references at the end of each chapter. In some cases, such as the chapters on RAP and HMA performance, AASHTO standards have not yet caught up with practice in the industry, and the information reflects research reports and, in some cases, state agency standards, rather than AASHTO standards. Differences in the procedures given in this manual compared to previous practice are significant but should not be too difficult to implement: Although this mix design procedure closely follows many of the procedures and standards used in the Superpave system of mix design and analysis, it is not meant to be the "next generation"

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Introduction 3 of Superpave. For this reason, the term Superpave is not used to describe the technology contained herein. There are some minor differences in terminology compared to previous usage. In the Superpave system, aggregate specification properties are called "consensus" properties, since they rep- resented the consensus of a panel of experts. These properties have now been studied and reviewed by a wide range of engineers, and no longer simply represent such a consensus. Therefore, they are simply referred to as aggregate specification properties. There are also a few small changes in these aggregate specification properties, making it easier to meet some requirements at the highest design traffic levels. In this manual, the control points that define aggregate blend gradations are meant to be guidelines, rather than strict specifications. This distinction provides engineers and technicians with additional flexibility in adjusting aggregate gradations to meet volumetric requirements when developing mix designs. Furthermore, there is little research suggesting that aggregate gradation in and of itself affects HMA performance. As discussed above, this manual contains a comprehensive treatment of RAP, including the relatively new issue of variability and how it may limit the amount of RAP that can be incorporated into a mix design. This manual includes a chapter on HMA performance, which describes specific tests to be used in evaluating the rut resistance of HMA mixtures. Guidelines are given for interpreting these tests, but they are just that--guidelines. These guidelines are likely to be modified through research, experience, and judgment by individual agencies to suit local materials and conditions. Perhaps the most significant change is in the overall philosophy of mixture design. Traditionally, when developing an HMA mix design, several trial mixtures with different aggregate gradations were evaluated, and then one was picked for further refinement by evaluating a range of asphalt binder contents. The final mix design was selected as the binder content that provided the proper volumetric composition and best met other pertinent requirements. This manual proposes a somewhat different approach in which the proper asphalt binder content is determined early in the mix design process and maintained throughout the various trial mixtures. Volumetric properties are adjusted in these trial mixes not by adjusting binder content, but by adjusting aggregate gradation. This approach is suggested primarily because it ensures that the final mix design will contain the proper amount of asphalt binder. It is also a simpler approach, requiring fewer trial mixtures to finalize a mix design. However, this approach to mix design is only a suggestion--engineers and technicians are free to use whatever procedures they deem appropriate to develop HMA mix designs, as long as the final results meet the requirements given in this manual. This manual is designed to be a complete, up-to-date reference that will complement other manuals available to engineers and technicians for mix design. Such other resources include the Asphalt Institute's Superpave Mix Design (SP-2) and the National Asphalt Pavement Association's (NAPA's) Hot Mix Asphalt Materials, Mixture Design and Construction. Bibliography The Asphalt Institute (2001) Superpave Mix Design (SP-2), 128 pp. The Asphalt Institute (1997) Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types (MS-2), 6th Ed., 141 pp. National Center for Asphalt Technology (NCAT) (1996) Hot-Mix Asphalt Materials, Mixture Design and Construction, NAPA, Lanham, MD, 585 pp.