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Design of Dense-Graded HMA Mixtures 111 the grade adjustment exceeds two grades. Furthermore, provided the mixture meets rut resistance requirements as discussed later in this chapter, the high-temperature performance grade can be reduced one level if a modified binder is used. This is because the current performance binder specification does not always adequately address the superior performance of many modified binders. Furthermore, without this adjustment in performance grade requirements, many areas in the southern United States would find it difficult or impossible to obtain suitable binders for HMA mixes intended for pavements subject to very heavy traffic. When selecting modified binders, optimum performance will be ensured if the binder selected is one that has been successfully used in the past under similar conditions or one approved by the state high- way agency. Step 3. Determine Compaction Level Table 8-2. Recommended design The design compaction level--Ndesign--is a function only of design traffic level. Suggested values compaction levels for for Ndesign as a function of design traffic in million ESALs are listed in Table 8-2. These values are dense-graded HMA identical to those used in the Superpave method. However, recommended design compaction mixtures. levels for HMA mixtures are under review and could be modified soon; it is possible that slightly lower Ndesign values could be adopted, to aid in designing mixtures that have higher VMA and are Design easier to compact in the field. Traffic Ndesign (MESALs) < 0.3 50 0.3 to < 3 75 Step 4. Select Nominal Maximum Aggregate Size 3 to < 10 100 10 to < 30 100 The nominal maximum aggregate size of the aggregate blend for an HMA mixture is most often 30 125 specified by the owner/agency for a given project. In cases where the aggregate size is not specified, it is determined by the lift thickness during construction. Lift thickness and aggregate size can significantly affect the ease with which a mixture can be compacted in the field and the permeability of the resulting pavement. Brown and associates at the National Center for Asphalt Technology (NCAT) in 2004 published the results of research on this topic in NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot Mix Asphalt Pavements. The guidelines given here are based on their conclusions and recommendations. The nominal maximum aggregate size should be no more than one-third the lift thickness for fine mixtures, and one-fourth the lift thickness for coarse mixtures. Coarse mixtures are defined as those for which the percent passing is less than the control point for the primary control sieve as listed in Table 8-3; all other mixtures are considered fine graded. All else being equal, smaller aggregate sizes should be preferred for wearing course mixtures and where extra durability is desired; this will help provide a mix that compacts easily, has low permeability, and resists fatigue cracking. Table 8-4 lists recommended NMAS values for different applications of dense-graded HMA. Unless otherwise specified, the smallest possible NMAS from those listed in Table 8-4 should be selected for use in a given mix design. Table 8-3. Primary control sieve sizes. Aggregate Primary PCS Control NMAS Control Sieve Point (mm) (mm) (% Passing) 4.75 1.18 42 9.5 2.36 47 12.5 2.36 39 19.0 4.75 47 25.0 4.75 40 37.5 9.5 47