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CHAPTER 10 Design of Gap-Graded HMA Mixtures 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