Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 34


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 33
Aggregates 33 100 fine-graded Weight % Passing 80 dense-graded 60 maximum coarse-graded 40 density gap-graded 20 0 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Figure 4-4. Types of HMA aggregate gradations; heavy black line represents maximum density gradation as calculated using equation 4-4. 12.5 mm. Coarse-graded and gap-graded aggregates for HMA mixes can be very similar; the main difference is that gap-graded aggregate blends tend to contain more large aggregate particles compared to coarse gradations, but they also contain larger amounts of very fine material, especially mineral filler. Methods for blending aggregates in HMA mix design are discussed in detail in Chapter 8 of this manual. Specifications for Aggregate Gradation There are two types of gradation specifications or requirements used in HMA technology: (1) specifications for the gradation of processed aggregate, as supplied by aggregate producers and (2) requirements for blends of aggregate developed as part of the HMA mix design process and used in the production of HMA. Requirements for aggregate blends are listed in the chapters of this manual specifically dealing with HMA mix design: Chapter 8 for dense-graded mixtures, Chapter 10 for gap-graded mixtures, and Chapter 11 for open-graded friction course mixtures. AASHTO M 43, Standard Specification for Sizes of Aggregate for Road and Bridge Construction, gives the gradation requirements for a wide range of coarse aggregate sizes used in the development of HMA mix designs. Table 4-3 is a shortened version of these requirements, listing the most commonly used aggregate gradations. Table 4-4 lists AASHTO specifications for fine aggregates for bituminous paving mixtures, as given in AASHTO M 29, Fine Aggregate for Bituminous Paving Mixtures. It should be noted that although many agencies may follow the AASHTO grading requirements listed in Tables 4-3 and 4-4, some agencies may use different specifications. In some cases, there may be minor modifications to the AASHTO requirements; in other cases, the specifications may be significantly different. Engineers and technicians responsible for developing HMA mix designs should obtain current specifications for aggregate gradations from the applicable state highway department (or other agency as appropriate). Aggregate Specific Gravity and Absorption When designing HMA, both the mass and volume of the aggregates and asphalt binder going into the mixture must be known. The mass and volume of a material are related through the values of density or specific gravity. Density refers to the mass of a material per unit volume. Density values for most construction materials, including aggregates, are usually reported in

OCR for page 33
34 A Manual for Design of Hot Mix Asphalt with Commentary Table 4-3. Standard sizes of coarse aggregates for road and bridge construction as adapted from AASHTO M 43. AASHTO % Passing (mass %) for Sieve Size: Size No. 50 mm 37.5 mm 25.0 mm 19.0 mm 12.5 mm 9.5 mm 4.75 mm 2.36 mm 1.18 mm 0.300 mm 4 100 90 to 100 20 to 55 0 to 15 --- 0 to 5 --- --- --- --- 467 100 90 to 100 --- 35 to 70 --- 10 to 30 0 to 5 --- --- --- 5 --- 100 90 to 100 20 to 55 0 to 10 0 to 5 --- --- --- --- 56 --- 100 95 to 100 40 to 85 10 to 40 0 to 15 0 to 5 --- --- --- 57 --- 100 100 --- 25 to 60 --- 0 to 10 0 to 5 --- --- 6 --- --- 100 90 to 100 20 to 55 0 to 15 0 to 5 --- --- --- 67 --- --- 100 90 to 100 --- 20 to 55 0 to 10 0 to 5 --- --- 68 --- --- --- 90 to 100 --- 30 to 65 5 to 25 0 to 10 0 to 5 --- 7 --- --- --- 100 90 to 100 40 to 70 0 to 15 0 to 5 --- --- 78 --- --- --- 100 90 to 100 40 to 75 5 to 25 0 to 10 0 to 5 --- 8 --- --- --- --- 100 85 to 100 10 to 30 0 to 10 0 to 5 --- 89 --- --- --- --- 100 90 to 100 20 to 55 5 to 30 0 to 10 0 to 5 9 --- --- --- --- --- 100 85 to 100 10 to 40 0 to 10 0 to 5 units of g/cm3; density values for HMA and other types of concrete are often reported in units of kg/m3. High-density materials feel heavy for their size. Water has a density of about 1.0 g/cm3, while many construction aggregates have density values between 2.5 and 3.0 g/cm3. Steel has a density of about 7.8 g/cm3. Table 4-5 lists density values for various materials, including common construction aggregates. The term specific gravity is often used interchangeably with density, but has a different meaning. Specific gravity is defined as the ratio of the mass of a material to the mass of an equal volume of water. It can also be defined as the ratio of the density of a material to the density of water. Because water has a density of 1.0 g/cm3 at room temperature, the values of density in units of g/cm3 are equal to specific gravity values. However, these terms should be used carefully. It is especially important to make sure that the units are included when reporting density values. Because specific gravity values are ratios of two numbers with the same units, specific gravity is dimensionless. The typical density of granite is 2.65 g/cm3, while the typical value for the specific gravity of granite is 2.65. Table 4-4. Standard sizes of fine aggregates for bituminous paving mixtures as adapted from AASHTO M 29. AASHTO % Passing (mass %) for Sieve Size: Grading No. 9.5 mm 4.75 mm 2.36 mm 1.18 mm 0.60 mm 0.30 mm 0.150 mm 0.075 mm 1 100 95 to 100 70 to 100 40 to 80 20 to 65 7 to 40 2 to 20 0 to 10 2 --- 100 75 to 100 50 to 74 28 to 52 8 to 30 0 to 12 0 to 5 3 --- 100 95 to 100 85 to 100 65 to 90 30 to 60 5 to 25 0 to 5 4 100 80 to 100 65 to 100 40 to 80 20 to 65 7 to 40 2 to 20 0 to 10 5 100 80 to 100 65 to 100 40 to 80 20 to 65 7 to 46 2 to 30 ---

OCR for page 33
Aggregates 35 Table 4-5. Typical density values for various materials, including common construction aggregates. Material Density, g/cm3 Aluminum 2.71 Asphalt binder 1.03 Basalt 2.86 Concrete 2.40 Diabase 2.96 Dolomite 2.70 Glass 2.50 Gneiss 2.74 Granite 2.65 Iron 7.87 Lead 11.35 Limestone 2.66 Marble 2.63 Nylon 1.14 Portland cement 3.15 Quartz 2.65 Quartzite 2.69 Sandstone 2.54 Shale 1.85-2.50 Steel 7.80 Teflon 2.17 Wood 0.50 Aggregate specific gravity is determined using different techniques for coarse and fine aggregate. Obtaining accurate specific gravity values for aggregates prior to performing an HMA mix design is essential, and engineers and technicians responsible for mix designs must develop proper laboratory techniques for these procedures. For coarse aggregate, specific gravity is determined using the weight-in-water method. In this procedure, coarse aggregate is weighed in air, and then in water, in a mesh basket suspended from a balance. A sketch of a weight-in-water apparatus is shown in Figure 4-5. The bulk specific gravity of a sample of coarse aggregate is calculated using the following equation: Bulk Sp. Gr . = A ( B - C ) (4-5) Electronic balance Wire hook Mesh basket Tub filled with distilled water Aggregate Figure 4-5. Weight-in-water apparatus for determining specific gravity of coarse aggregate.

OCR for page 33
36 A Manual for Design of Hot Mix Asphalt with Commentary where Bulk Sp. Gr. = bulk specific gravity A = weight of dry aggregate in air, g B = weight of saturated, surface-dry aggregate in air, g C = weight of saturated aggregate in water, g The term saturated, surface-dry (SSD) as used in Equation 4-5 means that the aggregate specimen has been saturated with water (usually by overnight soaking in water) and then quickly dried with a clean cloth or towel just until the aggregate surfaces are no longer visibly wet and shiny. Aggregate particles contain a large number of microscopic voids or pores, both within the aggregate particle and on and near the aggregate surface. Voids that connect to the surface become filled with water when saturated, while internal voids do not. 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. Sometimes, two other specific gravity values are reported: bulk specific gravity, SSD basis, and apparent specific gravity. These two specific gravity values are sometimes used in the design and analysis of portland cement concrete mixtures, but are rarely used in HMA technology, where the term "specific gravity," when applied to aggregates, should be taken to mean bulk specific gravity. Because fine aggregate would fall through the wire mesh basket used in determining weight- in-water, this approach cannot be used in specific gravity measurements. Instead, the pycnometer method is used. This technique requires the use of a pycnometer, which is simply a container that can be repeatedly filled with the same--or nearly the same--volume of water. Usually, a volumetric flask is used for performing fine aggregate specific gravity measurements. The flask is partially filled with distilled water and then about 500 g of saturated sand is placed in the flask. The flask is rolled and gently shaken to remove all air bubbles and then more water is added until the flask is filled just to the calibration mark. The flask is weighed, and the contents carefully poured into a metal pan which is then dried in an oven. The bulk specific gravity of the fine aggregate is then calculated using the following equation: Bulk Sp. Gr . = A ( B + W - C ) (4-6) Impermeable voids contain no water Aggregate surface dry Permeable voids filled with water Figure 4-6. Sketch showing saturated, surface-dry condition in an aggregate particle.

OCR for page 33
Aggregates 37 where Bulk Sp. Gr. = bulk specific gravity A = weight of oven-dry aggregate in air, g B = weight of pycnometer filled with water to calibration mark, g W = weight of saturated, surface-dry aggregate in air, g C = weight of pycnometer with aggregate and filled with water to calibration mark, g Figure 4-7 is a diagram of a volumetric flask as used in determining the specific gravity of fine aggregate. Just as is done for coarse aggregate, part of the specific gravity test for fine aggregate involves weighing the aggregate in air in the SSD condition. This is much more difficult for fine aggregate than for coarse aggregate--the surfaces of fine aggregate particles cannot be quickly dried with a cloth or towel because the particles are too fine. Traditionally, the cone test has been used to determine if a fine aggregate is in the SSD condition. In this procedure, saturated fine aggregate is dried using a blow dryer. Every few minutes, a small conical metal mold is filled with fine aggregate and tamped down. The mold is then removed; when the sand first slumps when the mold is removed, it is assumed to be in the SSD condition. Unfortunately, the cone test is not very accurate. Engineers have been working on developing alternative procedures, but at this time, none are yet being implemented. For both coarse aggregate and fine aggregate, absorption is an important property. It is calculated using the following equation: Absorption = ( A - B ) B 100% (4-7) where Absorption = water absorption in weight, % A = weight of saturated, surface-dry aggregate in air, g B = weight of dry (air-dry or oven-dry) aggregate in air, g The absorption calculated using Equation 4-7 is the amount of water absorbed into the permeable voids of the aggregate. When aggregate is mixed with asphalt binder to produce HMA, asphalt will be absorbed into the permeable voids in the same way that water is. However, the amount of asphalt binder absorbed by the aggregate will in general not be as great as the water Calibration mark Water Pycnometer (volumetric flask) Fine aggregate Figure 4-7. Pycnometer method of determining fine aggregate specific gravity.