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OCR for page 112
112 A Manual for Design of Hot Mix Asphalt with Commentary
Table 8-4. Recommended aggregate nominal maximum aggregate
sizes for dense-graded HMA mixtures.
Application Recommended Recommended Lift Thickness, mm
NMAS, mm Fine-Graded Coarse-Graded
Mixtures Mixtures
4.75 15 to 25 20 to 25
Leveling course mixtures
9.5 30 to 50 40 to 50
4.75 15 to 25 20 to 25
Wearing course mixtures 9.5 30 to 50 40 to 50
12.5 40 to 65 50 to 65
19.0 60 to 100 75 to 100
Intermediate course mixtures
25.0 75 to 125 100 to 125
19.0 60 to 100 75 to 100
Base course mixtures 25.0 75 to 125 100 to 125
37.5 115 to 150 150
9.5 30 to 50 40 to 50
Rich base course mixtures
12.5 40 to 65 50 to 65
Step 5. Determine Target VMA
and Design Air Void Content
One of the unique features of the design method described in this manual for dense-graded
mixtures is that the target VMA and air void content--and the resulting target binder content--
are determined early on and maintained throughout the mix design procedure. In this way, the
proper binder content is ensured, and effort is not wasted evaluating mixtures that do not have
the proper VMA and binder content. This also reduces the chances that an error in volumetric
calculations or laboratory testing will result in a mix design that does not meet the specified
requirements. The allowable VMA range depends only on the aggregate NMAS; as in the Superpave
system, minimum and maximum VMA values increase with decreasing aggregate NMAS--
1% for each decrease in standard aggregate size. Limits for VMA are given in Table 8-5. A 2%
range is specified for allowable VMA. When selecting VMA for a mix design, the target value
is in the center of this allowable range. These target values should be used for the initial devel-
opment of a mix design--for determining the composition of trial mixtures to be evaluated
and refined in the laboratory during the mix design process. The design VMA value can be
adjusted during the later stages of the mix design process or during construction, in order
to further refine the mix or to adjust for field production. Using a target VMA value in the
Table 8-5. VMA requirements for
standard dense-graded mixtures.
Aggregate Minimum Maximum
NMAS VMAa VMAa
(mm) (%) (%)
4.75 16 18
9.5 15 17
12.5 14 16
19.0 13 15
25.0 12 14
37.5 11 13
a
The specifying agency may increase the minimum and
maximum values for VMA by up to 1% to obtain
mixtures with increased asphalt binder content, which
can improve field compaction, fatigue resistance, and
general durability. Care should be taken to ensure that
the resulting HMA mixtures maintain adequate rut
resistance for their intended application.
OCR for page 113
Design of Dense-Graded HMA Mixtures 113
center of the allowable range ensures that such adjustments can be made. If a mix design is
started at the minimum allowable VMA, adjustments needed later in the mix design process
or during field production can be difficult or impossible without lowering the VMA below
the specified minimum.
As noted in Table 8-5, the specifying agency can increase the minimum and maximum (and
resulting target) VMA values by up to 1% if desired. This will provide additional binder content
in the resulting HMA mixtures, which can have several desirable effects--it will tend to produce
a mixture that is easier to compact in the field, more fatigue resistant, and, in general, more
durable. However, increasing VMA can also decrease rut resistance, so care is needed when
increasing minimum VMA requirements. As discussed below, the required dust/binder ratio
of 0.8 to 1.6 should not be lowered if VMA requirements are increased beyond those given in
Table 8-5, or the resulting mixtures might at times exhibit poor rut resistance. Agencies should
in general be wary of simultaneously changing mix design requirements that all tend to reduce
rut resistance--these include increasing VMA, decreasing dust/binder ratio, decreasing Ndesign,
reducing requirements for FAA, or lower requirements for CAFF. Agencies should also be aware
that the only foolproof way of increasing binder content in HMA mixtures is to increase minimum
VMA requirements. Reducing Ndesign values will make it easier to design mixtures with higher
VMA, but producers will find it easy to adjust their aggregate proportions after such a change in
order to maintain binder content at the lowest possible level when economic incentives make such
an approach desirable. When considering increasing VMA requirements, it should be remembered
that many HMA performance problems are the result of construction problems, especially poor
field compaction, rather than improper mix design. If high in-place air void content is the cause
of poor durability--raveling and surface cracking--increasing VMA or decreasing Ndesign will
not improve field performance unless these changes result in significant improvement in field
compaction.
For most surface course and intermediate (binder) course mixes, a design air void content of
4.0% is recommended. However, the design air void content for these mixtures is allowed to vary
from 3.5% to 4.5%. Specifying a lower design air void content of 3.5% will result in an increase
in binder content of a few tenths of a percent and a mixture that is slightly easier to compact. It will,
however, also tend to decrease rut resistance. Increasing the design air void content by 0.5% will
have the opposite effect--it will slightly decrease the design binder content and produce a mix
that is more difficult to compact, while increasing rut resistance. Rich bottom or base course
mixes, as now sometimes used in the design and construction of perpetual pavements, should
be designed at a slightly lower air void content of 3.0 to 4.0%. This helps ensure that these mixes
have the binder needed for exceptional fatigue life and are also easy to compact to a very low air
void content in the field. Because base course mixtures are located deep within the pavement
structure, the decrease in rut resistance caused by a lower design air void content is not normally
a major concern for these applications.
In HMA Tools, the initial target values for VMA and air void content are selected in the
worksheet "General." This worksheet lists the minimum and maximum values for VMA along
with the suggested target--the midpoint between the VMA limits. Target VMA and air void
content can also be refined in the worksheet "Trial_Blends." As discussed above, it is recommended
that (1) such adjustments be made only after evaluating several trial batches and (2) they be
kept small--about 0.5% or less. Changing target VMA and air void content during the initial
stages of the mix design process can make it difficult to evaluate the effect that changes in aggre-
gate gradation have on these values, making the mix design process longer and more complicated
than it needs to be. As discussed above, designing a mix near the minimum or maximum
allowable VMA and/or air void content can also make adjustments during field production
more difficult.
