A Manual for Design of Hot-Mix Asphalt with Commentary (2011) / Chapter Skim
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Pages 28-45
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From page 28... ...
It is essential that engineers and technicians responsible for HMA mix design thoroughly understand aggregate properties, how they relate to HMA pavement performance, and how aggregate properties are specified and controlled as part of the mix design process. Aggregate Particle Size Distribution Perhaps the most widely specified aggregate property is particle size distribution.
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From page 29... ...
Sometimes, for aggregates made up of different minerals or rocks having widely different specific gravities, the results of the sieve analysis are given as percent passing by volume. For HMA mix design and analysis, an aggregate sieve analysis uses the following standard sieve sizes: 37.5 mm, 25.0 mm, 19.0 mm, 12.5 mm, 9.5 mm, 4.75 mm, 2.36 mm, 1.18 mm, 0.60 mm, 0.30 mm, 0.15 mm, and 0.075 mm.
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From page 30... ...
Calculation of percent passing from the results of a sieve analysis is straightforward and is best explained through an example. Table 4-2 gives the results of a sieve analysis of a fine aggregate, along with the calculations of percent retained, cumulative percent retained, and 30 A Manual for Design of Hot Mix Asphalt with Commentary Figure 4-1.
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(1) Sieve Size, mm (2)
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, can be approximately calculated using the following formula: where % PMD = % passing, maximum density gradation d = sieve size in question, mm D = maximum sieve size, mm Figure 4-4 illustrates the different types of HMA aggregate gradations and includes the maximum density gradation calculated using Equation 4-4 for a maximum aggregate size of % % ( )
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Density values for most construction materials, including aggregates, are usually reported in Aggregates 33 fine-graded maximum density gap-graded coarse-graded dense-graded 0 20 40 60 80 100 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm W ei gh t % P as si ng Figure 4-4. Types of HMA aggregate gradations; heavy black line represents maximum density gradation as calculated using equation 4-4.
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The typical density of granite is 2.65 g/cm3, while the typical value for the specific gravity of granite is 2.65. 34 A Manual for Design of Hot Mix Asphalt with Commentary % Passing (mass %)
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Weight-in-water apparatus for determining specific gravity of coarse aggregate.
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When a saturated aggregate particle is quickly dried with a cloth or towel, water remains in the permeable voids, while the surface is dry, as shown in Figure 4-6. The SSD condition is important in HMA mix design because this approximately represents the condition of the aggregate in HMA mixtures, except that the permeable voids are now filled with asphalt binder instead of water.
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Fine aggregate Figure 4-7. Pycnometer method of determining fine aggregate specific gravity.
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From page 38... ...
within HMA layers. Angular and rough-textured aggregates also help improve the strength of HMA mixtures, which can help prevent fatigue cracking.
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The group of experts developed the aggregate requirements for HMA without the benefit of a formalized research program. Since the early 1990s, when the group of experts met to develop the aggregate requirements for the Superpave mix design system, a significant amount of work has been conducted to evaluate various aggregate tests and their relationship with pavement performance.
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Furthermore, the mix design system described in this manual includes performance testing for HMA mixtures designed for traffic levels of 10 million ESALs and greater. This performance testing provides additional assurance that HMA mixtures will have adequate rut resistance.
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BAlthough there is no FAA requirement for design traffic levels below 0.30 million ESALS, consideration should be given to requiring a minimum uncompacted void content of 40 % for 4.75-mm nominal maximum aggregate size mixes. CThe 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.
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42 A Manual for Design of Hot Mix Asphalt with Commentary 5:1 pivot point swinging armfixed post (A) fixed post (B)
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Source Aggregate Properties Some aggregate properties were identified by the expert group as important, but about which a consensus could not be reached on specification limits. These aggregate properties were called "Source Properties." Test methods were recommended; however, development of specification limits was left to local agencies that had experience with area materials.
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Low values of soundness loss are desirable since this suggests that an aggregate is not susceptible to weathering. Soundness test results obtained using sodium sulfate and magnesium sulfate solutions are not interchangeable, since the expansive forces generated by these salt solutions are 44 A Manual for Design of Hot Mix Asphalt with Commentary Figure 4-11.
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From page 45... ...
Bibliography AASHTO Standards M 29, Fine Aggregate for Bituminous Paving Mixtures M 43, Standard Specification for Sizes of Aggregate for Road and Bridge Construction M 323, Superpave Volumetric Mix Design R 35, Superpave Volumetric Design for Hot-Mix Asphalt T 2, Sampling of Aggregates T 11, Materials Finer than 75-μm (No.
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