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176 A Manual for Design of Hot Mix Asphalt with Commentary The standards and overall procedure given in this chapter closely follow those given in two AASHTO standards: M 325 and R 46. These in turn are based on research on SMA mix designs performed by Brown and Cooley as described in NCHRP Report 425. Although developed for SMA mixtures, these guidelines can be effectively applied to GGHMA. The following sections describe each step in the design of a GGHMA mixture in detail. Step 1--Materials Selection As with most HMA mixes, suitable coarse aggregate, fine aggregate, and asphalt binder must be selected for GGHMA. However, two additional materials are also typically needed for GGHMA designs: commercial mineral filler and stabilizing additives. Aggregates used for GGHMA should be angular, cubical, and roughly textured. These properties help ensure that the aggregate particles composing the stone skeleton cannot slide past one another. Angular, cubical, and textured aggregate particles will lock together, providing a rut-resistant pavement layer. Figure 10-2 illustrates desirable aggregates for GGHMA. Asphalt binders used in GGHMA mixtures should perform at high, intermediate, and low temperatures. Chapter 3 provided a detailed discussion on asphalt binders. It should be stated, however, that most GGHMA mixes have been designed with polymer-modified binders. Polymer- modified binders are not required, but are generally used to (1) reduce the potential for draindown and (2) improve durability. Mineral fillers help fill the voids within the coarse aggregate skeleton. Many types of materials have been used as mineral filler, including marble dust, limestone dust, agricultural lime, and fly ash. For agencies that require the use of hydrated lime to reduce the potential of moisture damage, hydrated lime can be considered a portion of the mineral filler. Because of the large percentage of coarse aggregate in GGHMA blends, natural crushed aggregate stockpiles do not generally have sufficient materials passing the 0.075-mm sieve to help fill the voids of the stone skeleton, hence the need for mineral fillers. Without the use of mineral fillers to fill the voids, GGHMA mixes would be very permeable. Figure 10-2. Illustration of desirable aggregate shape and angularity.
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Design of Gap-Graded HMA Mixtures 177 The primary purpose for stabilizing additives is to reduce the potential for draindown. When added to stiffen an asphalt binder, polymer modifiers can be considered a stabilizing additive. Likewise, mineral fillers can also be considered a stabilizing additive, since these small particles help "soak up" the asphalt binder. However, the most effective stabilizing additive is a fiber. Several types of fiber have been used in GGHMA with cellulose and mineral fiber being the most common. Generally, cellulose fibers are added at 0.3% of the total mix mass and mineral fibers are added at 0.4%. The following sections provide requirements for the various materials used to fabricate GGHMA. These requirements are provided for guidance to agencies not having experience with these types of mixtures. Some agencies have used other test methods and criteria with success. Coarse Aggregates As described previously, the success of a GGHMA pavement depends heavily on the existence of stone-on-stone contact. Therefore, in addition to angularity, shape, and texture, the toughness and durability of the coarse aggregates must be such that they will not degrade during production, construction, and service. Table 10-1 presents coarse aggregate requirements for GGHMA mixtures. The Los Angeles Abrasion and Soundness tests should be required for individual stockpiles while the Flat or Elongated and Uncompacted Voids tests should be required for the total aggregate blend. Fine Aggregates The role of fine aggregates in GGHMA is to help fill the voids between coarse aggregate particles. Therefore, the primary requirements for fine aggregates in GGHMA are to ensure a durable and angular material. Requirements for fine aggregates in GGHMA are provided in Table 10-2. The Uncompacted Voids and Sand Equivalency tests should be required for the total aggregate blend, while the soundness test should be applied to individual fine aggregate stockpiles. Asphalt Binder Asphalt binders should be performance graded, in accordance with the requirements of AASHTO M320-04, to satisfy the climate and traffic loading conditions at the site of the GGHMA project. Guidelines described in Chapter 8 for selecting binder performance grades for dense-graded HMA also apply to GGHMA, with the exception that the high-temperature performance grade Table 10-1. Coarse aggregate quality requirements. Test Method Minimum Maximum Los Angeles Abrasion, % Loss AASHTO T96 - 30a Flat and Elongated, 5 to 1 Ratio, % ASTM D 4791 - 10b Soundness (5 Cycles), % AASHTO T104 Sodium Sulfate - 15 Magnesium Sulfate - 20 Fractured Faces, % AASHTO D 5821 One face 98c - Two faces 98c - a Aggregates with L.A. Abrasion loss values up to 50 have been successfully used to produce GGHMA mixtures. However, when the L.A. Abrasion exceeds approximately 30, excessive breakdown may occur in the laboratory compaction process or during in-place compaction. b Flat and elongated criteria apply to the design aggregate blend. c The CAFF requirement for design traffic levels of 30 million ESALs or more may be reduced to 95/95 if experience with local conditions and materials indicate that this would provide HMA mixtures with adequate rut resistance under very heavy traffic.
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178 A Manual for Design of Hot Mix Asphalt with Commentary Table 10-2. Fine aggregate quality requirements. Test Method Minimum Maximum Soundness (5 Cycles), % AASHTO T 104 Sodium Sulfate - 15 Magnesium Sulfate - 20 Uncompacted Voids AASHTO T 304, Method A 45a - Sand Equivalency AASHTO T176 50 - a The FAA requirement of 45 may be reduced to 43 if experience with local conditions and materials indicate that this would produce HMA mixtures with adequate rut resistance under the given design traffic level. should be no less than 76 and the binder should be polymer modified. This ensures that GGHMA mixes will exhibit the exceptional rut resistance that pavement engineers expect from this mix type. Note that, for some applications, GGHMA mixes might require binders with high-temperature performance grades exceeding PG 76. The required binder grade for a GGHMA mix should be determined following the procedure given in Chapter 8; if the high-temperature performance grade is a PG 76 or less, then a PG 76 binder should be used. If the resulting high-temperature PG grade is above a PG 76, then the higher PG grade should be used in the GGHMA mix design. Mineral Fillers Mineral fillers used for GGHMA should be finely divided mineral matter such as crushed fines, agricultural limes, or fly ash. Figure 10-3 illustrates some typically used mineral fillers. The mineral filler should be free from organic impurities. It is recommended that mineral fillers with modified Rigden voids (sometimes called Dry Compaction Test) higher than 50% not be used in GGHMA. Rigden voids is in some ways a similar test to that used to evaluate fine aggregate Figure 10-3. Typical mineral fillers used in GGHMA.