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CHAPTER 4 Aggregates Because about 85% of the volume of dense-graded HMA is made up of aggregates, HMA pave- ment performance is greatly influenced by the characteristics of the aggregates. Aggregates in HMA can be divided into three types according to their size: coarse aggregates, fine aggregates, and mineral filler. Coarse aggregates are generally defined as those retained on the 2.36-mm sieve. Fine aggregates are those that pass through the 2.36-mm sieve and are retained on the 0.075-mm sieve. Mineral filler is defined as that portion of the aggregate passing the 0.075-mm sieve. Mineral filler is a very fine material with the consistency of flour and is also referred to as mineral dust or rock dust. Gravel refers to a coarse aggregate made up mostly of rounded particles. Gravels are often dredged from rivers and are sometimes mined from deposits. Because of the rounded particle size, gravels are not suitable for use in HMA mixtures unless they are well crushed. Poorly crushed gravels will not interlock when used in HMA, and the resulting mixture will have poor strength and rut resistance. Crushed stone is coarse aggregate that is mined and processed by mechanical crushing. It tends to be a very angular material and, depending on its other properties, can be well suited for use in HMA pavements. One potential problem with crushed stone is that the particles sometimes will tend to be flat, elongated, or both, which can cause problems in HMA mixtures. Ideally, the particles in crushed stone aggregate should be cubicle and highly angular. The fine aggregate, or sand, used in HMA can be natural sand, manufactured sand, or a mixture of both types. Natural sand is dredged from rivers or mined from deposits and is then processed by sieving to produce a fine aggregate having the desired particle size distribution. Manufactured sand is produced by crushing quarried stone and, like natural sand, sieving to produce the desired grada- tion. The particles in manufactured sands tend to be more angular than those in natural sand and often will produce HMA mixtures having greater strength and rut resistance compared to those made with natural sand. However, this is not always true, and care is needed when selecting fine aggregate for use in HMA mixtures. The fine aggregate angularity test described later in this chapter, although not always reliable, can help to evaluate the angularity of both natural and manufactured sands. Pavement engineers have worked for many years to relate specific aggregate properties to HMA performance. Rutting, raveling, fatigue cracking, skid resistance, and moisture resistance have all been related to aggregate properties. It is essential that engineers and technicians responsible for HMA mix design thoroughly understand aggregate properties, how they relate to HMA pave- ment 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. Although only indirectly related to HMA performance, controlling particle size distribution, also called aggregate gradation, is critical to developing an effective mix design. The maximum aggregate 28

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Aggregates 29 size in an aggregate must be matched to the lift thickness used during construction, otherwise the pavement will be difficult to place and compact properly. The distribution of particle sizes in an aggregate must have just the right density so that the resulting HMA will contain the optimum amount of asphalt binder and air voids. Because the shape and texture of aggregate particles vary significantly depending on the aggregate type and the way it is mined and processed, specification limits for aggregate gradation tend to be very broad. This breadth helps technicians and engineers achieve the right blend of aggregates for different applications. The section below describes general terminology used when discussing aggregate particle size distribution and the relationship between aggregate gradation and different HMA mix types. Because suggested limits for aggregate gradation vary depending on the type of mix design being developed, only a few examples are given here-- a complete listing of aggregate gradation requirements are given in the various chapters discussing mix design procedures for the various HMA mix types: dense-graded HMA, gap-graded HMA, and open-graded friction courses. Nominal Maximum Aggregate Size Nominal maximum aggregate size (NMAS) is a way of specifying the largest aggregate size in an aggregate. In the mix design procedure described in this manual, as in the Superpave system, NMAS is defined as one sieve-size larger than the first sieve size to retain 10% or more of the total aggregate by mass. Aggregate Sieve Analysis The particle size distribution of construction aggregates is usually determined and specified by performing a sieve analysis. In this test, an aggregate is passed through a stack of sieves of decreasing size. The amount of aggregate on each sieve is weighed, and the percent passing each sieve size is calculated as a percent by weight. 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. Other sieve sizes are sometimes used for special purposes or in other aggregate test procedures. A schematic of a simplified sieve analysis is shown in Figure 4-1. An aggregate sample is collected and placed through a stack of sieves. The sieves are usually shaken mechanically, until the aggregate has been separated completely on the various sieves. At the bottom of the sieve stack is a pan, in which material is collected that has completely passed through the stack of sieves. The aggregate on each sieve is then weighed, and calculations are performed to determine the percent passing for each sieve. In performing a sieve analysis, there are several important considerations: The weight of the test sample must be large enough to produce reliable test results. Larger aggregate sizes will require larger sample sizes. Table 4-1 lists minimum weights for test samples for sieve analysis for different NMAS values. The sieve opening sizes selected should be appropriate for the aggregate being tested. Past data or the gradation specification for the aggregate being tested can be used to determine the sieves needed for a given aggregate. The physical size of the sieves--the diameter or area--increases with increasing aggregate size. For fine aggregate, 203-mm-diameter sieves are often used. The maximum amount of aggregate retained on sieves of this size should be limited to about 194 g; larger amounts may result in inaccurate sieve analyses because the aggregate can longer flow freely through the stack of sieves.

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30 A Manual for Design of Hot Mix Asphalt with Commentary Figure 4-1. Schematic of an aggregate sieve analysis. Overloading sieves can sometimes be prevented by placing intermediate sieve sizes in a stack of sieves. For example, a specification may only require the determination of percent passing the 9.5- and 19-mm-diameter sieves, but using these sieves only might overload the 9.5-mm sieve. Inserting a 12.5-mm-diameter sieve between the 9.5- and 19-mm-diameter sieves will help prevent overloading of the 9.5-mm-diameter sieve. The amount of time a stack of sieves is shaken should be long enough to ensure that the aggre- gate particles have been completely sorted through the stack, but not so long that significant aggregate degradation might occur. Accurate determination of mineral filler--material finer than 0.075 mm--will in general require a washed sieve analysis. Figure 4-2 shows a stack of sieves for fine aggregate assembled in a mechanical sieve shaker. Engineers and technicians should refer to appropriate specifications for details on performing aggregate sieve analyses: AASHTO T 27, Sieve Analysis of Fine and Coarse Aggregate; AASHTO T 11, Materials Finer than 75-m Sieve in Mineral Aggregates by Washing; and AASHTO T 30, Mechan- ical Analysis of Extracted Aggregates. Calculations for Aggregate Sieve Analyses The results of an aggregate sieve analysis in HMA technology are usually presented as weight percent passing. Calculation of percent passing from the results of a sieve analysis is straight- forward 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 Table 4-1. Minimum test sample size for sieve analysis of aggregate as a function of nominal maximum aggregate size. Nominal Maximum Minimum Weight Aggregate Size, mm for Test Sample, kg 9.5 1 12.5 2 19.0 5 25.0 10 37.5 15

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Aggregates 31 Figure 4-2. Stack of sieves for fine aggregate assembled in a mechanical sieve shaker. percent passing. The weight retained, as shown in Column 2, is the weight in grams of the aggregate separated onto each sieve. The total of these values, 1143.6 g, is slightly less than the original sample weight of 1146.0 g. The difference is due to material lost, either as dust lost to the air, particles trapped within the mesh of the sieves, or particles fallen from the sieves without being weighed. The percent error is calculated as the difference between the total weight retained and the original sample weight, expressed as a percent of the original sample weight: 1146.0 - 1143.6 Error = 100% = 0.21% (4-1) 1146.0 The % retained is calculated by dividing the weight retained for each sieve by the original sample weight and, again, expressing the result as a weight percentage. For the 2.36-mm-diameter sieve 231.7 % retained 2.36 mm sieve = 100% = 20.2% (4-2) 1146.0 Table 4-2. Example sieve analysis. (1) (2) (3) (4) (5) Cumulative Sieve Size, Weight % Retained, % Retained, % Passing, mm Retained, g Wt. % Wt. % Wt. % 19.0 0.0 0.0 0.0 100.0 12.5 0.0 0.0 0.0 100.0 9.5 97.5 8.5 8.5 91.5 4.75 214.6 18.7 27.2 72.8 2.36 231.7 20.2 47.5 52.5 1.18 215.8 18.8 66.3 33.7 0.60 116.3 10.1 76.4 23.6 0.30 90.4 7.9 84.3 15.7 0.15 75.2 6.6 90.9 9.1 0.075 57.8 5.0 95.9 4.1 pan 44.3 3.9 99.8 --- Total: 1143.6 99.8 Original Sample Size: 1146.0 Error, Wt. %: 0.21

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32 A Manual for Design of Hot Mix Asphalt with Commentary 100 Weight % Passing 80 60 40 20 0 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Figure 4-3. % passing plotted against sieve size for example sieve analysis given in table 4-2. The cumulative % retained is calculated by summing all the values for % retained up to the given sieve size. For the 0.60-mm sieve: cumulative % retained 0.60 mm sieve = 8.5 + 18.7 + 20.2 + 18.8 + 10.1 = 76.4% (4-3) The % passing is calculated as 100%--the cumulative % retained. For the 0.60-mm-diameter sieve, for example, the % passing is calculated as 100 - 76.4 = 23.6%. It should be pointed out that there are slightly different ways of calculating these values for sieve analyses, and those responsible for HMA mix design and associated testing should follow the procedures as required by their state agencies. The results of aggregate sieve analyses are usually presented graphically, by plotting percent passing against sieve size in mm. Sieve size is often plotted on a logarithmic scale. Figure 4-3 is a plot of the results of the example sieve analysis given in Table 4-2. Aggregate Gradation The plot given in Figure 4-3 is an example of an aggregate gradation. For purposes of classifying HMA mix types, there are four types of aggregate gradation: dense-graded, fine-graded, coarse- graded, and open-graded. As explained in Chapter 8, when designing HMA, two, three, four, or even more aggregates are combined in specific proportions to create an aggregate blend to which asphalt binder is added forming an HMA mixture. Because fine and coarse aggregate gradations as used in HMA are really slight variations of dense gradations, a more accurate description of these would be dense/fine and dense/coarse aggregate gradations or blends. The densest possible aggregate gradation, called the maximum density gradation (or sometimes the Fuller maximum density curve), can be approximately calculated using the following formula: 0.45 d % PMD = 100% (4-4) D 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