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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
