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

Chapter: Chapter 10 - Design of Gap-Graded HMA Mixtures

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Suggested Citation:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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:"Chapter 10 - Design of Gap-Graded HMA Mixtures." 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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174 Gap-graded HMA (GGHMA) consists of two parts: a coarse aggregate skeleton and a mortar. The coarse aggregate skeleton consists of crushed coarse aggregate particles; these make up about 70 to 80% of the total aggregate blend. The mortar consists of asphalt binder, fine aggregate, and mineral filler and fills the voids in the coarse aggregate skeleton. Stone matrix asphalt (SMA), one particular type of GGHMA, is widely known for its durability and rut resistance. SMA has been used in Europe for over 30 years. The first U.S. project that used this high-performance HMA was constructed in 1991. Since then, the use of SMA has steadily increased within the United States. The GGHMA discussed in this chapter is similar to SMA in many ways, but there are some differences, so to avoid confusion or arguments over whether or not the mix design presented in this chapter is truly an “SMA,” the term GGHMA is used instead. In Europe, SMA mixtures have primarily been designed by recipes. It was not until 1994 that a formalized mix design procedure was available in the United States This mix design procedure was developed by a Technical Working Group established by the FHWA. This procedure was based on the Marshall mix design method, since this was the method used to design SMA in Europe. In 1994, the National Center for Asphalt Technology (NCAT) began a 4-year study to develop a mix design system to design SMA using the concepts and methods of the Superpave mix design system. Results from this research project were published in 1998, and, along with more recent experience and research, they are the basis for the GGHMA mix design system described in this chapter. The philosophy of GGHMA mix design is not complicated. The first principle is that a gap-graded blend of aggregate is needed so that the coarse particles will have stone-on-stone contact. The second principle is that the voids within the coarse aggregate skeleton must be filled with fine aggregate, mineral filler, and asphalt binder. In order to provide increased durability, GGHMA has a relatively high asphalt binder content. This leads to the third principle of GGHMA mix design, which is that the aggregate must have a high VMA value—typically 18 to 20%. This relatively high asphalt binder content can result in an increased potential for draindown if not properly taken into account. Draindown can be a common problem in GGHMA; it occurs when the asphalt binder and fine aggregate separate from the coarse aggregate during storage, transport, or placement. The fourth and final principle of GGHMA mix design is that small amounts of stabilizing additives, such as mineral fibers or cellulose fibers, are usually needed to prevent draindown. The sections below describe in detail how to design GGHMA to achieve the unique properties and excellent performance for which this mix type is known. Overview of GGHMA Mix Design Procedure The mix design procedure for GGHMA contains five primary steps (Figure 10-1). First, suitable materials must be selected to compose the GGHMA. Materials needed include coarse aggregates, fine aggregates, mineral fillers, asphalt binder and stabilizing additives. The second step is to blend C H A P T E R 1 0 Design of Gap-Graded HMA Mixtures

Design of Gap-Graded HMA Mixtures 175 Identify Materials Select Materials Stabilizer Aggregates Asphalt Binder Select Trial Gradations Determine VCA of coarse aggregate in dry - rodded condition Medium % passing Break Point Sieve Low % passing Break Point Sieve High % passing Break Point Sieve Add 6.0% - 6.5% asphalt binder and compact Analyze data and select optimum gradation Fix gradation and vary asphalt binder content Adjust asphalt binder content as needed Within Specifications Step 1 Step 5 Step 4 Step 3 Step 2 Meet all specifications ? End No Yes Design of GGHMA Mixtures Conduct draindown, moisture susceptibility, and performance testing. Figure 10-1. Flow diagram illustrating GGHMA mix design methodology. three trial gradations. For each trial gradation, asphalt binder is added and the mixture compacted. After each trial gradation has been compacted, mixture volumetric data for each trial mixture is evaluated as the third step in order to select the best gradation. The fourth step is to fix the selected gradation and compact mixtures with varying asphalt binder contents. The asphalt binder content that produces 4% air voids is selected as optimum asphalt binder content. The final step in the mix design procedure is to evaluate the moisture susceptibility, draindown sensitivity, and rut resistance of the designed mixture.

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.

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 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 - aAggregates 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. bFlat and elongated criteria apply to the design aggregate blend. cThe 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. Table 10-1. Coarse aggregate quality requirements. Design of Gap-Graded HMA Mixtures 177

Figure 10-3. Typical mineral fillers used in GGHMA. 178 A Manual for Design of Hot Mix Asphalt with Commentary 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 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 - aThe 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. Table 10-2. Fine aggregate quality requirements.

angularity by measuring uncompacted voids. However, the Rigden voids test is smaller in scale, and the mineral filler is compacted with a small drop hammer prior to determining the void content. Mineral fillers with very high Rigden voids can sometimes cause excessive stiffening in SMA mixtures. The equipment and test method for conducting the Dry Compaction Test can be found in the National Asphalt Pavement Association’s Information Series 127, “Evaluation of Baghouse Fines for Hot Mix Asphalt.” Other requirements for mineral fillers can be found in AASHTO M-17, “Mineral Fillers for Bituminous Paving Mixtures.” However, the gradation requirements stated in AASHTO M-17 should only be used for guidance. The important gradation is that of the designed GGHMA and not the mineral filler. Stabilizing Additives Stabilizing additives are needed in GGHMA to prevent the draining of mortar from the coarse aggregate skeleton during storage, transportation, and placement. Stabilizing additives such as cellulose fiber, mineral fiber, and polymers have been used with success to minimize draindown potential. Other types of fibers have been used with success; however, the most common types are cellulose and mineral fibers. When using a polymer as a stabilizer, the amount of polymer added should be that amount necessary to meet the performance grade of the asphalt binder. Step 2—Trial Gradations As with any HMA, specified aggregate gradations should be based on aggregate volume and not aggregate mass. However, for most conventional HMA mixtures (dense-graded), the specific gravities of the different aggregate stockpiles are close enough to make a gradation based on mass percentages similar to that based on volumetric percentages. With GGHMA, the specific gravities of the different aggregate components are not always similar. This is especially true when commercial fillers are used in GGHMA. Therefore, the gradation bands presented in Table 10-3 are based on % passing by volume. Similar to those for dense-graded mixes, GGHMA gradation bands are described by the nominal maximum aggregate size (NMAS) of the gradation. The following section provides guidance in the form of an example problem on how to blend aggregate components based on volumes to meet the gradation bands in Table 10-3. However, if the bulk specific gravities of the different stockpiles (including mineral filler) used to compose the aggregate blend vary by 0.02 or less, gradations based on mass percentages can be used. 19-mm NMAS 12.5-mm NMAS 9.5-mm NMAS Siev e Size, mm Min. Max. Min. Max. Min. Max. 25.0 100 100 19.0 90 100 100 100 12.5 50 88 90 100 100 100 9.5 25 60 26 88 70 95 4.75 20 28 20 35 30 50 2.36 16 24 16 24 20 30 1.18 -- -- -- -- -- 21 0.6 -- -- -- -- -- 18 0.3 -- -- -- -- -- 15 0.075 8.0 11.0 8.0 11.0 8.0 12.0 Note: NMAS – Nominal Maximum Aggregate Size – one sieve size larger than the first sieve that retains more than 10 %. Table 10-3. Stone matrix asphalt gradation specification bands (Percent Passing by Volume). Design of Gap-Graded HMA Mixtures 179

180 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 10-1. Blending Aggregates to Meet GGHMA Gradation Requirements Percent Mass Retained per Sieve Sieve, mm Aggregate A Aggregate B Aggregate C Mineral Filler 25.0 0.0 0.0 0.0 0.0 19.0 5.0 0.0 0.0 0.0 12.5 29.0 30.0 2.6 0.0 9.5 23.0 24.0 12.8 0.0 4.75 34.0 40.0 35.7 0.0 2.36 4.0 2.0 21.1 0.0 1.18 3.0 0.0 11.2 0.0 0.60 0.0 1.0 5.9 0.0 0.30 0.0 0.0 3.1 0.0 0.075 1.0 1.5 3.0 27.5 -0.075 1.0 1.5 4.6 27.5 Total, 100 100 100 100 Table 10-5. Percent by mass retained on each sieve. Stockpile and Percent Passing Based on Mass, % Sieve, mm Aggregate A Aggregate B Aggregate C Mineral Filler 25.0 100.0 100.0 100.0 100.0 19.0 95.0 100.0 100.0 100.0 12.5 66.0 71.0 97.4 100.0 9.5 43.0 46.0 84.6 100.0 4.75 9.0 6.0 48.9 100.0 2.36 5.0 4.0 27.8 100.0 1.18 2.0 4.0 16.6 100.0 0.60 2.0 3.0 10.7 100.0 0.30 2.0 3.0 7.6 100.0 0.075 1.0 1.5 4.6 72.5 Gsb 2.616 2.734 2.736 2.401 Table 10-4. Results of gradation and specific gravity tests for stockpiles to be used in example GGHMA mix design. Washed Sieve Analyses As with any blending problem, the first step is to perform washed sieve analyses based on mass for the various stockpiles to be used in the GGHMA mixture, following procedures described in AASHTO T 27. For this example, a 19.0-mm GGHMA mixture is to be blended. Table 10-4 provides the results of washed gradation tests performed on four stockpiles, which are to be blended for this example problem. Also needed to determine aggregate gradations based on volume are the bulk specific gravities (Gsb) of the different stockpiles. Table 10-4 also provides the Gsb values for each stockpile. Notice that the Gsb values differ by more than 0.02 in Table 10-4. Determine Percent Mass Retained After performing the washed sieve analyses, the percent mass retained on each sieve for the different stockpiles is determined. For a given sieve, this is done by subtracting the percent passing the given sieve from the percent passing the next larger sieve. For example, using Aggregate C in Table 10-4, the % mass retained on the 4.75-mm sieve would be calculated as %Retained on 4.75-mm Sieve = 84.6 48.9 = 35.7%−

Example Problem 10-1. (Continued) where 84.6 = percent mass passing 9.5-mm sieve (Table 10-4) 48.9 = percent mass passing 4.75-mm sieve (Table 10-4) 35.7 = percent mass retained on 4.75-mm sieve Table 10-5 presents the values of percent mass retained for all sieves and stockpiles. Note that a row has been added to reflect that material finer than the 0.075-mm (−0.075) sieve is included. Calculate Percent Mass Retained In this calculation a simple assumption is made, “Assume the Mass of Each Aggregate Stockpile is 100 grams.” Using this assumption allows for the mass that would be retained of each size fraction for each stockpile to be determined and can be shown to be equal to the numbers shown in Table 10-5. Convert Percent Mass Retained to Volume per Sieve In this step of developing an SMA gradation, the values for percent mass retained determined previously are converted to volumes per sieve. To make this conversion, the bulk specific gravity of the individual stockpiles is needed. The volume of aggregate retained on each sieve for each stockpile can be determined from the following equation: where Vagg = volume of aggregate retained on a given sieve, cm3 Magg = mass of aggregate retained on a given sieve, g γw = unit weight of water (1.0 g/cm3) The following calculation demonstrates how the volume is calculated for the aggregate retained on the 4.75-mm sieve of Aggregate C. where 35.7 g = mass of Aggregate C retained on 4.75-mm sieve (Table 10-5) 2.736 = bulk specific gravity of Aggregate C (Table 10-4) 1.0 g/cm3 = unit weight of water (γw) 13.05 cm3 = volume of Aggregate C retained on 4.75-mm sieve The volumes retained on all sieves for each of the four stockpiles are provided in Table 10-6. Volume g g cm cm= × 35 7 2 736 1 0 13 05 3 3. . . . V = M Gagg agg sb wγ Design of Gap-Graded HMA Mixtures 181 (continued on next page)

182 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 10-1. (Continued) Blend Stockpiles The values provided in Table 10-7 are used to blend the different stockpiles to meet the desired gradation based on volumes. This process is identical to blending stockpiles by mass and is a trial and error process. To perform the blending, select the estimated percentages of the different stockpiles to be used. For this example, the following percentages will be evaluated first: The percent of each stockpile in the blend is multiplied by the volume retained on a given sieve for each stockpile to determine the total volume retained on Stockpile % Blend Aggregate A 30 Aggregate B 30 Aggregate C 30 Mineral filler 10 Volume of Aggregate Retained per Sieve, cm3Sieve, mm Aggregate A Aggregate B Aggregate C Mineral Filler 25.0 0.00 0.00 0.00 0.00 19.0 1.91 0.00 0.00 0.00 12.5 11.09 10.97 0.95 0.00 9.5 8.79 8.78 4.68 0.00 4.75 13.00 14.63 13.05 0.00 2.36 1.53 0.73 7.71 0.00 1.18 1.15 0.00 4.09 0.00 0.60 0.00 0.37 2.16 0.00 0.30 0.00 0.00 1.13 0.00 0.075 0.38 0.55 1.10 11.45 -0.075 0.38 0.55 1.68 30.20 Table 10-6. Volumes of aggregate retained on each sieve. Sieve, mm Volume Retained per Sieve, cm3 25.0 0.00 19.0 0.57 12.5 6.90 9.5 6.67 4.75 12.20 2.36 2.99 1.18 1.57 0.60 0.76 0.30 0.34 0.075 1.75 -0.075 3.80 Total Volume, 37.55 Table 10-7. Total volumes retained per sieve.

Example Problem 10-1. (Continued) that sieve. Using the 4.75-mm sieve as an example, the total volume retained on the 4.75-mm sieve would be calculated as follows: where 0.30, 0.30, 0.30 and 0.10 are the percentages by mass of each aggregate in blend expressed as decimals; and 13.00, 14.63, 13.05, and 0.00 are the % volume retained on 4.75-mm sieve for each stockpile (Table 10-6). This calculation is performed for each of the sieves in the gradation. Table 10-7 presents the total volume retained for each of the sieves in the gradation. Now, based on the total volume retained per sieve and the summed total volume of the blended aggregates, the percent retained per sieve by volume can be determined for the blend. This is accomplished for a given sieve by dividing the volume retained on that sieve by the total volume of the blend. The following equation illustrates this calculation for the 4.75-mm sieve. where 12.20 = volume retained on 4.75-mm sieve (Table 10-7) 37.55 = summed total volume of blend (Table 10-7) 32.50 = percent volume of blend retained on 4.75-mm sieve Table 10-8 provides the percents retained based on volumes for each of the sieves and converts this to percent volume passing. Using the % retained per sieve based on volume, the % passing by volume for the gradation can be determined similar to the method used with gradations based on mass. Determine the cumulative % retained for each sieve and then subtract from 100. Now, the blended gradation is compared to the required gradation band %Volume Retained on 4.7-mm Sieve =12.20 100 37.55× = 32.50% Total Volume Retained on 4.7 - mm sieve = 0 30 13 00. .×( ) + ×( ) + ×( ) + ×( ) = 0 30 14 63 0 30 13 05 0 10 0 00 1 . . . . . . 2 20 3. cm Sieve, mm Percent Retained Per Sieve Cumulative Percent Retained Percent Passing by Volume 25.0 0.0 0.0 100.0 19.0 1.5 1.5 98.5 12.5 18.4 19.9 80.1 9.5 17.8 37.7 62.3 4.75 32.5 70.1 29.9 2.36 8.0 78.1 21.9 1.18 4.2 82.3 17.7 0.60 2.0 84.3 15.7 0.30 0.9 85.2 14.8 0.075 4.7 89.9 10.1 -0.075 10.1 100.0 --- Table 10-8. Percent passing based on volumes. Design of Gap-Graded HMA Mixtures 183 (continued on next page)

184 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 10-1. (Continued) (also based on volume) provided in Table 10-3. Table 10-9 compares the gradation band for a 19.0-mm NMAS GGHMA to the gradation shown in Table 10-8. Based on Table 10-9, the blended gradation did not meet the specified gradation band for a 19.0-mm nominal maximum aggregate size GGHMA. Therefore, different blending percentages for the various stockpiles are needed. Below are the percentages of the four stockpiles used for the second trial. Table 10-10 presents the blended gradation of the four aggregates for the second trial. The second trial blend percentages were used along with the values of Table 10-6 to determine the percent passing by volume for the blend. Stockpile % Blend Aggregate A 40 Aggregate B 41 Aggregate C 10 Mineral Filler 9 Sieve, mm Gradation Band Requirements Blend Percent Passing 25.0 100 100 19.0 90-100 98.5 12.5 50-88 80.1 9.5 25-60 62.3* 4.75 20-28 29.9* 2.36 16-24 21.9 1.18 --- 17.7 0.60 --- 15.7 0.30 --- 14.8 0.075 8-10 10.1 * Does not meet requirements Table 10-9. Comparison of gradation blend based on volume to specified gradation band. Sieve, mm Percent Retained Per Sieve by Volume Cumulative Percent Retained by Volume Percent Passing by Volume Percent Passing by Mass (For Comparison) Gradation Band by Volume 25.0 0.0 0.0 100.0 100.0 100 19.0 2.0 2.0 98.0 98.0 90-100 12.5 24.0 26.0 74.0 74.3 50-88 9.5 20.1 46.1 53.9 53.5 25-60 4.75 33.2 79.3 21.7 20.0 20-28 2.36 4.5 83.7 16.3 15.4 16-24 1.18 2.3 86.0 14.0 13.1 --- 0.60 1.0 87.0 13.0 12.1 --- 0.30 0.3 87.3 12.7 11.8 --- 0.075 4.0 91.3 8.7 8.0 8-11 -0.075 8.7 100.0 --- --- --- Table 10-10. Percents passing based on volumes.

Selection of Trial Gradations When designing GGHMA mixtures, the initial trial gradations should be selected to be within the master specification range shown in Table 10-3. To design a GGHMA mixture it is recommended that at least three trial gradations be initially evaluated. It is suggested that the three trial grada- tions fall along and in the middle of the coarse and fine limits of the gradation range. These trial gradations are obtained by adjusting the amount of fine and coarse aggregates in each blend. The percent passing the 0.075-mm sieve should be approximately 10 percent for each trial gradation. Determination of VCA in the Coarse Aggregate Fraction For best performance, the GGHMA mixtures must have a coarse aggregate skeleton with stone-on-stone contact. The coarse aggregate fraction is not defined by a particular sieve size but rather is that portion of the total aggregate blend retained on the breakpoint sieve. The breakpoint sieve is defined as the finest (smallest) sieve to retain at least 10% of the aggregate gradation (Figure 10-4). Example Problem 10-1. (Continued) Based on Table 10-10, the following percentages produce a gradation based on volume, which meets the 19.0-mm nominal maximum aggregate size gradation band for GGHMA. Stockpile % Blend by Mass Aggregate A 40 Aggregate B 41 Aggregate C 10 Mineral Filler 9 100.0100.0 87.0 59.2 35.5 20.6 13.512.812.310.5 7.5 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 Sieve Size (mm) Raised to 0.45 Power Pe rc en t P as si ng Blend 2 . 07 5 2.36 1. 18 . 60 . 30 . 15 4.75 9.5 12.5 19.0 25.0 Break Point Sieve Finest Sieve to Have at Least 10 Percent Retained Figure 10-4. Definition of breakpoint sieve. Design of Gap-Graded HMA Mixtures 185

186 A Manual for Design of Hot Mix Asphalt with Commentary The method of measuring the existence of stone-on-stone contact is called the voids in coarse aggregate (VCA) method. The concept is quite simple and practical. The first step is to determine the VCA of the coarse aggregate fraction only (material larger than breakpoint sieve) in a dry- rodded condition (VCADRC) (Figure 10-5). AASHTO T 19, “Bulk Density (“Unit Weight”) and Voids in Aggregate,” is used to compact the aggregate. Then, using Equation 10-1, the VCADRC can be calculated. The VCADRC is nothing more than the volume between the coarse aggregate parti- cles after compaction in accordance with AASHTO T 19. When asphalt binder is added and the GGHMA is compacted, the VCA will again be calculated (VCAMIX). This calculation for VCAMIX also calculates the volume between the coarse aggregate particles. As long as the volume between the coarse aggregate particles is less in the compacted GGHMA (VCAMIX) than the coarse aggregate only (VCADRC), then the GGHMA is deemed to have stone-on-stone contact and the aggregate structure is acceptable. This means that the GGHMA mixture has been compacted more than the dry-rodded condition of the aggregate; therefore, the coarse aggregate particles within the compacted mixture must be compacted closer than the dry-rodded condition and stone-on-stone contact exists. where VCADRC = voids in coarse aggregate in dry-rodded condition γs = unit weight of the coarse aggregate fraction in the dry-rodded condition (kg/m3), γw = unit weight of water (998 kg/m3), and Gca = bulk specific gravity of the coarse aggregate fraction VCA G G DRC ca w s ca w = −γ γ γ 100 10 1( )- Figure 10-5. Method of determining VCA in dry-rodded condition.

Selection of Target Asphalt Content The minimum desired asphalt binder content for GGHMA mixtures is presented in Table 10-11. This table illustrates that the minimum asphalt binder content is based on the combined bulk specific gravity of the aggregates used in the mix. These minimum asphalt binder contents are provided to ensure enough volume of asphalt binder exists in the GGHMA mix to provide a desirable mortar and, thus, a durable mixture. It is recommended that the mixture be designed at 0.3% above the minimum values given in Table 10-11 to allow for adjustments during plant production without falling below the minimum requirement. For example, for a GGHMA mixture to be made with an aggregate blend having a combined bulk specific gravity of 2.75, the minimum asphalt content is 6.0% by mass, and the target asphalt content would be 6.0 + 0.3 = 6.3% by mass. The minimum binder content values given in Table 10-11 have been calculated so that, in most cases, the resulting mixes will meet the suggested minimum VMA of 17.0% at 4.0% air voids for GGHMA mixtures. Sample Preparation As with the laboratory design of any HMA, the aggregates to be used in GGHMA should be dried to a constant mass and separated by dry-sieving into individual size fractions. The following size fractions are recommended: • 37.5 mm to 25.0 mm • 25.0 mm to 19.0 mm • 19.0 mm to 12.5 mm • 12.5 mm to 9.5 mm • 9.5 mm to 4.75 mm • 4.75 mm to 2.36 mm • Passing 2.36 mm (for 25.0-, 19.0-, 12.5-, and 9.5-mm NMAS gradations) • 2.36 mm to 1.18 mm (for 4.75-mm NMAS gradations) • Passing 1.18 mm (for 4.75-mm NMAS gradations) After separating the aggregates into individual size fractions, they should be recombined at the proper percentages based on the gradation blend being evaluated. The mixing and compaction temperatures are determined in accordance with AASHTO T 245, Section 3.3.1. Mixing temperature will be the temperature needed to produce an asphalt binder Combined Aggregate Bulk Specific Gravity Minimum Asphalt Content Based on Mass, % 2.40 6.8 2.45 6.7 2.50 6.6 2.55 6.5 2.60 6.3 2.65 6.2 2.70 6.1 2.75 6.0 2.80 5.9 2.85 5.8 2.90 5.7 2.95 5.6 3.00 5.5 Table 10-11. Minimum asphalt binder content requirements for aggregates with varying bulk specific gravities. Design of Gap-Graded HMA Mixtures 187

188 A Manual for Design of Hot Mix Asphalt with Commentary viscosity of 170±20 cSt. Compaction temperature will be the temperature required to provide an asphalt binder viscosity of 280±30 cSt. However, although these temperatures work for neat asphalt binders, the selected temperatures may need to be changed for polymer-modified asphalt binders. The asphalt binder supplier’s guidelines for mixing and compaction temperatures should be used. When preparing GGHMA in the laboratory, a mechanical mixing apparatus should be utilized. The gap-grading and use of stabilizing additives make GGHMA very difficult to hand-mix. Aggregate batches and asphalt binder are heated to a temperature not exceeding 28°C above the temperature established for mixing temperature. The heated aggregate batch is placed in the mechanical mixing container. Asphalt binder and any stabilizing additive are placed in the container at the required masses. Mix the aggregate, asphalt binder, and stabilizing additives rapidly until thoroughly coated. Mixing times for GGHMA should be slightly longer than for dense-graded HMA to ensure a good distribution of the stabilizing additives. After mixing, the GGHMA mixture should be short-term aged in accordance with AASHTO R 30, Mixture Conditioning of Hot Mix Asphalt. For aggregate blends having water absorption less than 2% the mixture should be aged for 2 hours. If the water absorption of the blend is 2% or higher, the mixture should be aged for 4 hours. Number of Samples Twelve samples are initially required—four samples for each of the three trial gradations. Each sample is mixed with the trial asphalt binder content and three of the four samples for each trial gradation are compacted. All 12 samples should be short-term aged. The remaining sample of each trial gradation is used to determine the theoretical maximum specific gravity (Gmm) according to AASHTO T 209. Sample Compaction Specimens should be compacted at the established compaction temperature after laboratory short-term aging in accordance with AASHTO R 30. Laboratory samples of GGHMA are com- pacted using 75 gyrations of the Superpave gyratory compactor (SGC). Some agencies have used 100 gyrations with success; however, GGHMA is relatively easy to compact in the laboratory and exceeding 100 gyrations can cause aggregate breakdown and may result in an unacceptably low asphalt content. Step 3—Selection of Optimum Gradation After the samples have been compacted and allowed to cool, they are tested to determine the bulk specific gravity, Gmb (AASHTO T166). Using Gmb, Gmm, and the bulk specific gravity of the coarse aggregate fraction (Gca), the percent air voids (VTM), voids in mineral aggregate (VMA), and VCAMIX are calculated. The VTM, VMA, and VCAMIX are calculated as shown below: VCA G P G MIX mb ca ca = − ⎛⎝⎜ ⎞⎠⎟100 10 4  ( )- VMA G P G mb s sb = − ⎛⎝⎜ ⎞⎠⎟100 10 3  ( )- VTM G G mb mm = −⎛⎝⎜ ⎞⎠⎟100 1 10 2 ( )-

where Ps = percent of aggregate in the mixture Pca = percent of coarse aggregate in the mixture Gsb = combined bulk specific gravity of the total aggregate Gca = bulk specific gravity of the coarse aggregate (coarser than break point sieve) Once the VTM, VMA, and VCAMIX are determined, each trial blend mixture is compared to the GGHMA mixture requirements. Table 10-12 presents the requirements for GGHMA designs. The trial blend mix that meets or exceeds the minimum VMA requirement, has an air void content between 3.5 and 4.5%, and has a VCAMIX less than VCADRC is selected as the design gradation. If none meet these requirements, additional aggregate blends should be evaluated. If one of the trial blends is very close to meeting these requirements, with the air void content and VMA just outside their acceptable ranges, an adjustment in the binder content might provide an acceptable mix design, as discussed below. Step 4—Refine Design Asphalt Binder Content Once the design gradation of the mixture is chosen, it may be necessary to raise or lower the asphalt binder content to obtain the proper amount of air voids in the mixture. In this case, additional samples are prepared using the selected gradation and varying the asphalt binder content. The optimum asphalt binder content is chosen to produce 4.0% air voids in the mixture; because of typical error in volumetric analysis, air void contents within ± 0.5% of this target are acceptable. The optimum asphalt binder content should meet the minimum asphalt content requirements in Table 10-11. The number of samples needed for this portion of the procedure is again twelve, with three compacted and one uncompacted sample at each of three asphalt binder contents. The mixture properties are again determined, and the optimum asphalt binder content is selected. The designed GGHMA mixture at optimum asphalt content selected should have properties meeting the criteria shown in Table 10-12. If these criteria are not met, the mixture must be modified so that all criteria are met. Step 5—Conduct Performance Testing Performance testing of GGHMA mixtures consists of three tests: (1) evaluation of moisture susceptibility; (2) evaluation of draindown sensitivity; and (3) evaluation of rut resistance. Evaluation of Moisture Susceptibility Moisture susceptibility of the selected mixture is determined using AASHTO T 283. One minor change to AASHTO T 283 is that GGHMA samples should be compacted to 6±1% air voids instead Property Requirement Asphalt Binder, % Table 10-12 Air Void Content, % 4.0 ± 0.5 VMA, % 17 min. VCAMIX, % Less than VCADRC Tensile Strength Ratio 0.80 min. Draindown at Production Temperature, % 0.30 max Table 10-12. GGHMA mixture specification for SGC compacted designs. Design of Gap-Graded HMA Mixtures 189

190 A Manual for Design of Hot Mix Asphalt with Commentary of 7±1%. This air void content approximates the recommended higher level of compaction in the field of 94 to 95% of Gmm. The mixture should have a minimum tensile strength ratio of 80%. Evaluation of Draindown Sensitivity Draindown sensitivity of the selected mixture is determined in accordance with AASHTO T 305. Draindown sensitivity is determined at the anticipated plant production temperature and should not exceed 0.3%. Evaluation of Rut Resistance The final step in the design of a GGHMA mixture is the evaluation of rut resistance, also called performance testing. Chapter 6 presents a general discussion of performance evaluation of HMA mixtures and discusses specific tests. Chapter 8 provides a more practical discussion of how, during the HMA mix design process, rut resistance is evaluated using one from among the following tests: (1) the flow number test; (2) the flow time test; (3) the asphalt pavement analyzer (APA) test; (4) the Hamburg wheel-track test; (5) the repeated shear at constant height (RSCH) test as performed using the Superpave shear tester (SST); or the high-temperature indirect tension (IDT) strength test. Detailed discussions of these tests can be found in Chapters 6 and 8. A summary of performance testing requirements and their application to GGHMA mixtures is given below. The design procedures set forth in this manual—including that for GGHMA—are structured to provide HMA mix designs that exhibit a high level of rut resistance. The level of reliability against excessive rutting—even without performance testing—ranges from 90 to over 99%, with a typical level of about 95% reliability for design traffic levels of 3 million ESALs and higher. The purpose of rut resistance testing is to increase this level of reliability. For three of the rut resistance tests recom- mended in this manual—the flow number from the asphalt mixture performance test (AMPT), the repeated shear at constant height (RSCH) test, and the high-temperature indirect tension (HT-IDT) strength test—the suggested minimum or maximum test values were determined specifically to increase the level of reliability against excessive rutting from about 95 to 98% and higher. It must be emphasized that the reliability achieved through the recommended performance tests is a result of applying both the suggested mix design procedure and the selected performance test together. If the given guidelines for performance test results are applied to mixtures designed following some other procedure, the resulting level of reliability will not necessarily be the same. It might be similar, or it might be lower or higher. It should also be noted that the specified test values have, in most cases, been selected so that if the procedures given in this manual are followed, most of the result- ing HMA mixtures will pass the selected performance test. It is estimated that only about 10 to 20% of properly designed mixtures will fail. Thus, the suggested rutting performance tests not only increase reliability against excessive rutting to a very high level, they do so in a relatively efficient way. The suggested maximum rut depths for the asphalt pavement analyzer (APA) and the Hamburg wheel-track (HWT) tests were taken from specifications already in place in a number of states using these tests. In this case, implementation of these performance tests will certainly increase the reliability against excessive rutting, but the specific amount of improvement is unknown as is the percentage of mixes likely to fail the tests. However, because these tests with the stated maximum rut depths have been implemented in several states, it is likely that the increase in reliability and the rejection rate will both be reasonable. Guidelines for interpreting the various rut resistance tests are given in Tables 8-21 through 8-25 in Chapter 8. As an example, Table 10-13 gives recommended minimum values for flow number as determined using the AMPT. This test was initially called the simple performance test or SPT. Details of the latest equipment specification and test procedure are given in AASHTO TP 79-09. Traffic Level Million ESALs Minimum Flow Number < 3 --- 3 to < 10 200 10 to < 30 320 ≥ 30 580 Table 10-13. Recommended minimum flow number requirements.

Tests are performed on specimens cored and trimmed from large gyratory specimens to final nominal dimensions of 100 mm diameter by 150 mm high. In the flow number test, a 600 kPa load is applied to the specimen every second, until the flow point is reached, representing failure of the specimen as seen in an increasing rate of total permanent strain during the test. Flow number tests are run at the average, 7-day maximum pavement temperature 20 mm below the surface, at 50% reliability as determined using LTPPBind version 3.1. Specimens should be prepared at the expected average air void content at the time of construction, typically 7.0±0.5%. Because GGHMA mixtures are high-performance materials, usually intended for very demanding applications, it is likely that the required test values used for these mixes will be those for the highest design traffic levels. For example, most GGHMA mixtures should probably meet or exceed a flow number of 580—representing the minimum flow number for design traffic levels of 30 million ESALs and higher—when tested using the flow number test as described in Chapter 8. As noted in Chapter 8, the minimum flow number values given in Table 10-13 are for fast traffic; it is suggested that, to account for the greater damage associated with slow traffic, the test temperature be increased for slower traffic speeds. The specifics of such adjustments are given in Chapter 8. Although the suggested required test values given in Table 10-13 and in the related tables presented in Chapter 8 are based either on a careful analysis of laboratory and field data, or on existing standards, it is quite possible that they will need to be adjusted by the specifying agency for optimum results for GGHMA mixtures in its region. Factors that need to be considered when making such adjustments are climate, the types and grades of binders commonly used in a given locale, aggregates with unusual properties, and typical traffic mixes and traffic levels. For various reasons, some agencies might wish to alter the conditions a test is run under, which will significantly alter the resulting test values and the appropriate specification values. For details on the proper procedures for performing each test, engineers and technicians should refer to the appropriate standard test method. Trouble Shooting GGHMA Mix Designs If the designer cannot produce a mixture that meets all requirements, remedial action will be necessary. Some suggestions to improve mixture properties are provided below. Air Voids The amount of air voids in the mixture can be controlled by the asphalt binder content. However, a problem occurs when low voids exist at the minimum asphalt binder content. Lowering the asphalt binder content below the minimum to achieve the proper amount of air voids violates the required minimum asphalt binder content (Table 10-11). Instead, the aggregate gradation should be modified to increase the VMA so that additional asphalt binder can be added without decreasing the voids below an acceptable level. Voids in the Mineral Aggregate The VMA may be raised by decreasing the percentage of aggregate passing the breakpoint sieve or by decreasing the percentage passing the 0.075-mm sieve. Changing aggregate sources or stockpiles may also be required to solve the problem. Voids in the Coarse Aggregate If the VCAMIX is higher than that in the dry-rodded condition (VCADRC) then the mixture gradation must be modified. This is typically accomplished by decreasing the percent passing the breakpoint sieve. Design of Gap-Graded HMA Mixtures 191

192 A Manual for Design of Hot Mix Asphalt with Commentary Moisture Susceptibility If the mixture fails to meet the moisture susceptibility requirements, lime or liquid anti-strip additives can be used. If these measures prove ineffective, the aggregate source, asphalt binder source, or both can be changed to obtain better aggregate and asphalt binder compatibility. Draindown Sensitivity Problems with draindown sensitivity can be remedied by increasing the amount of stabilizing additive or by selecting a different stabilizing additive. Fibers have been shown to be very effec- tive in reducing draindown. Rut Resistance As mentioned earlier in this section, the rut resistance tests and recommended minimum or maximum values for test results have been selected so that most GGHMA designs developed following the procedures given in this manual will meet the requirements, and no additional laboratory work will be needed. However, some mix designs may fail to meet requirements for rut resistance. In such cases, the test results should first be checked to make sure there were no errors in either the procedures used or in the calculation of the test results. If no errors are found, and the test results are close to meeting the requirements, the test can be repeated. In this case, the results of both tests should be averaged and compared to the test criteria. If the mix still fails to meet the requirements for rut resistance testing, the mix design will have to be modified. The rut resistance of a GGHMA mix design can be improved in the following ways: • Increase the binder high-temperature grade. • If the binder is not modified, consider using a polymer-modified binder of the same grade or one high-temperature grade lower. • If the binder is polymer modified, try a different type of modified binder. • Increase the amount of mineral filler in the mix, adjusting the aggregate gradation if necessary to maintain adequate VMA. • Decrease the design VMA value, if possible, by adjusting the aggregate gradation. • Replace part or all of the aggregate (fine or coarse or both) with a material or materials having improved angularity. If a different asphalt binder is used in the mix, the volumetric composition should not change. However, if other aspects of the mix design are changed, the volumetric composition might change significantly, which will require further refinement of the mix prior to further rut resistance testing. Bibliography AASHTO Standards M 17, Mineral Filler for Bituminous Paving Mixtures M 320, Performance-Graded Asphalt Binder M 325, Standard Specification for Designing Stone Matrix Asphalt (SMA) R 46, Standard Practice for Designing Stone Matrix Asphalt (SMA) R 30, Mixture Conditioning of Hot-Mix Asphalt T 19, Bulk Density (“Unit Weight”) and Voids in Aggregate T 96, Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine. T 104, Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalency Test.

T 209, Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures. T 245, Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus. T 283, Resistance of Compacted Asphalt Mixtures to Moisture-Induced Damage. T 304, Uncompacted Void Content of Fine Aggregate T 305, Determination of Draindown Characteristics in Uncompacted Asphalt Mixtures. T 326, Uncompacted Void Content of Coarse Aggregate (As Influenced by Particle Shape, Surface Texture and Grading). TP 79-09, Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT) ASTM Standards C 612, Mineral Fiber Block and Board Insulation D 4791, Flat Particles, Elongated Particles, or Flat and Elongated Particles in Coarse Aggregate Other Publications Bonaquist, R. F. (2008) NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester, Transportation Research Board, National Research Council, Washington, DC, 130 pp. Brown, E. R., and L. A. Cooley, Jr. (1999) NCHRP Report 425: Designing Stone Matrix Asphalt Mixtures for Rut Resistant Pavements. Transportation Research Board, National Research Council, Washington, DC. NAPA (1999) Designing and Constructing SMA Mixtures—State of the Practice (QIP-122), Lanham, MD, 43 pp. NAPA (1999) Evaluation of Baghouse Fines for Hot Mix Asphalt (IS-127), Lanham, MD, 36 pp. Design of Gap-Graded HMA Mixtures 193

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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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