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OCR for page 229
CHAPTER 4 Aggregates 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generally accepted by the pavement engineering community and are supported by substantial experience, these properties no longer represent the "consensus" of an expert panel and so should be referred to simply as "specification" properties rather than "consensus" properties.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
OCR for page 230
230 A Manual for Design of Hot Mix Asphalt with Commentary Table 1. Sources for critical tables in chapter 4 of the mix design manual. Table No. Source Standard Table. 4-1. Minimum Test Sample Size for Sieve Analysis of AASHTO T 2 Aggregate as a Function of Nominal Maximum Aggregate Size. Table 4-3. Standard Sizes of Coarse Aggregates for Road and AASHTO M 43 Bridge Construction as Adapted from AASHTO M 43. Table 4-4. Standard Sizes of Fine Aggregates for Bituminous AASHTO M 29 Paving Mixtures as Adapted from 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 a Minimum 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). b Minimum 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).