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OCR for page 114
114 A Manual for Design of Hot Mix Asphalt with Commentary
Step 6. Calculate Target Binder Content
Once the target VMA is selected, calculation of the design binder content is straightforward.
The target value for VBE (effective binder content by volume) is calculated by subtracting the
design air void content--normally 4%--from the target VMA value. For example, a standard
12.5-mm mixture with a target VMA value of 15% would have a target VBE value of 15 - 4 = 11%.
The total binder content must also include the amount absorbed by the aggregate. This can be
estimated in several ways. A quick approximate estimate, suitable for developing trial batches,
is to simply add 1% to the target VBE value. In the example above, this would result in a total
target binder content of 12%. A more accurate estimate would be to calculate the volume of
water absorbed by the aggregate, divide this by two, and add it to the target VBE value:
VMA Gsb Pwa
Vb = VBE + 1 - (8-1)
100 2
where
Vb = total asphalt content by volume %
VBE = effective asphalt content by volume %
VMA = voids in the mineral aggregate
= Vbe + air void content
Gsb = aggregate bulk specific gravity
Pwa = water absorption of the aggregate, weight %
The best approach to estimating absorbed binder and the resulting total binder content is to use
past experience. However, this may not always be possible. In any case, it should be remembered
that the mixture proportions being determined at this point in the mix design process are only
for one or more trial mixtures and that further adjustments will almost always have to be made
prior to finalizing the mix design. Therefore, use of estimates in determining total binder content
from the target VMA will usually work quite well. In HMA Tools, Equation 8-1 is used to estimate
the binder content. As additional trial mixtures are made, the amount of binder absorbed by the
aggregate is adjusted according to the values measured during the previous trial mixtures.
The final proportions for the mix design must be given by both percent by volume and weight,
so the binder contents calculated above must be converted to percentages by total mix weight.
However, this conversion cannot be done until the next two steps in the mix design process are
completed and the aggregate proportions determined.
Step 7. Calculate Aggregate Content
The total aggregate content by volume is directly calculated as 100% minus the VMA content.
In the example above, the total aggregate volume would be 100 - 15 = 85%. Determination of
the total aggregate content by weight will depend on the aggregate specific gravity values and
the specific blend of aggregates used in each of the trial mixtures, as determined in Step 8 as
explained below. As with most other calculations needed during the mix design process, HMA Tools
automatically calculates the aggregate content.
Step 8. Proportion Aggregates for Trial Mixtures
Proportioning aggregates for trial mixtures is one of the most important steps in the HMA
mix design process. It can also be one of the most complicated. The procedure recommended
here sets the binder content at a value that will provide the proper VMA once the design air void
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Design of Dense-Graded HMA Mixtures 115
content is met. Therefore, proportioning aggregates can be thought of as determining the blend
of aggregates that will provide the proper air void content for the mixture. However, because
many HMA mix designs can make use of four or more aggregates, determining the right aggregate
blend can be difficult and is largely a trial-and-error process.
Engineers and technicians responsible for HMA mix designs should understand that several
systems for blending aggregates are very effective. The Asphalt Institute Manuals on mix design--
MS-2 and SP-2--provide detailed descriptions of aggregate proportioning methods typically
used with the Marshall, Hveem, and Superpave mix design methods. More recently, the Bailey
method of aggregate proportioning has become popular among some technicians and engineers.
This procedure is based on theoretical principles of particle packing and, although relatively
complicated, is unique in that it provides many quantitative rules for modifying aggregate blends
to achieve a desired change in VMA. An excellent reference for the Bailey method is Vavrik et al.,
"Bailey Method for Gradation Selection in HMA Mixture Design," Transportation Research
Board Circular E-C044. In any case, engineers and technicians who are comfortable with the
methods they are using for proportioning aggregates should continue to use them. HMA mix
design--particularly determining appropriate aggregate blends--involves science and math, but
is also largely an art based on experience and judgment.
The method described below is largely a graphical one and is intentionally simple and flexible,
so that it is potentially compatible with the widest possible range of mix design procedures and
combination of circumstances. Unfortunately, this also means that applying this procedure
efficiently requires some experience--both with the mix design process and with a wide range
of materials.
Maximum Density Aggregate Gradation
and Fundamentals of Aggregate Blending
For many years, use of the maximum density aggregate gradation has been emphasized in
proportioning aggregates for HMA mix design. The maximum density gradation is that which
provides the smallest possible volume of space among the aggregate particles--that is, it is the
blend providing the lowest possible VMA for a given set of aggregates. Using the maximum
density gradation to produce an HMA mix was for many years considered desirable because it
would result in a mix with the minimum asphalt binder content and because asphalt binder is
much more expensive than aggregate, the resulting mixture would be relatively economical.
However, it has become clear that a certain minimum amount of asphalt binder is required in
order for an HMA mixture to be workable--easy to place and compact--and also to resist moisture
damage, age hardening, and fatigue cracking. Therefore, most HMA designs today use aggregate
gradations that vary significantly from maximum density. In fact, achieving the minimum required
VMA values can be a problem with some aggregates.
Even though most HMA mixtures do not precisely follow a maximum density gradation, it is
often used as a reference when proportioning aggregates. A good estimate of the maximum density
aggregate gradation for a given aggregate size can be estimated using the 0.45 power gradation:
0.45
d
% PMD × 100% (8-2)
D
where
% PMD = percent passing for maximum density gradation
d = sieve size, mm
D = maximum sieve size for gradation, mm
OCR for page 116
116 A Manual for Design of Hot Mix Asphalt with Commentary
100
Fine
80
Dense
Percent Passing
Coarse
60
Max. Density
40
20
0
0.010 0.100 1.000 10.000 100.000
Sieve Size, mm
Figure 8-3. Fine, dense, and coarse aggregate gradations
for HMA compared to the maximum density gradation.
Figure 8-3 is a plot of three different 12.5-mm NMAS aggregate gradations, with the maximum
density gradation calculated using Equation 8-2 included for reference. As shown in this figure,
HMA gradations are usually classified as coarse, fine, or dense, depending on where they pass
relative to the maximum density gradation. Those falling below the maximum density gradation
are called coarse gradations, those passing above are called fine gradations, and those passing
near the maximum density gradation are called dense. However, it should be emphasized that,
in fact, all three of these gradations are used in producing dense-graded HMA--therefore, a more
accurate classification of these gradations would be dense/coarse, dense/fine, and dense/dense,
accurately reflecting that all three gradations are close to dense gradations, but some are slightly
coarser and some slightly finer than the maximum density gradation.
A concept related to the maximum density gradation, and also useful in analyzing aggregate
gradation, is the continuous maximum density gradation. This is calculated using an equation
similar to Equation 8-2:
0.45
d
PCMD ( d2 ) 2 × P ( d1 ) (8-3)
d1
where
PCMD(d2) = percent passing, continuous maximum density gradation, for sieve size d2
d1 = one sieve size larger than d2
P(d1) = percent passing sieve d1
For example, in a selected aggregate gradation the percent passing the 4.75-mm sieve is 84%.
The PCMD for the 2.36-mm sieve would be calculated as (2.36/4.75)0.45 × 84 = 73%. The usefulness
of the continuous maximum density gradation is that it allows a more realistic evaluation of
how closely a given aggregate gradation follows a maximum density gradation compared to the
traditional maximum density gradation as calculated using Equation 8-2. The top graph in
Figure 8-4 shows a 9.5-mm gradation compared to the standard maximum density gradation as
calculated using Equation 8-2. The bottom graph shows the deviation from the continuous
maximum density gradation (calculated using Equation 8-3) for this same aggregate. Both figures
suggest that the gradation deviates significantly from the maximum density gradation, but the
lower graph is much clearer in the way in which this deviation occurs. For example, it is not
clear in the top graph that the aggregate gradation in fact follows a maximum density gradation
below the 1.18-mm sieve size; this is very clear in the lower graph. Furthermore, the lower plot
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Design of Dense-Graded HMA Mixtures 117
100
80
Percent Passing
60
40
maximum density
aggregate
20 gradation
gradation
0
0.010 0.100 1.000 10.000 100.000
Sieve Size, mm
15
% Deviation from Max. Density
0
-15
0.010 0.100 1.000 10.000 100.000
Sieve Size, mm
Figure 8-4. Top: 9.5-mm aggregate gradation compared
to the maximum density gradation; bottom: deviation from
the continuous maximum density gradation for the same
9.5-mm gradation.
exaggerates the deviation from maximum density, so comparing several similar aggregate blends
is much easier.
One of the most important--and often the most difficult--parts of the mix design process is
adjusting aggregate blends to produce the desired level of air void content and VMA. The lower
graph in Figure 8-4, called a continuous maximum density (CMD) plot, is very helpful in this
process because, in general, for a given set of aggregate blends the more this plot deviates from
zero (the horizontal line through the center of the plot), the greater will be the VMA and air void
content. Using the CMD plot to blend aggregates has many advantages:
· It is soundly based in packing theory.
· It is completely flexible--it can be applied to any number of aggregates, any gradation size,
and any type of gradation or HMA mix.
· Once set up in a spreadsheet, as in HMA Tools, it is simple to apply; there is no long set of rules
and definitions to remember.
· Because of its simplicity and flexibility, the CMD approach can be used along with other
procedures.
Figure 8-5 shows how the CMD plot relates to changes in aggregate gradation for a series of
12.5-mm aggregate blends: an SMA aggregate blend; a dense/coarse blend; a dense/dense blend;
and a dense/fine blend. The top portion of the figure shows a traditional gradation plot, including
the maximum density gradation. The bottom chart in Figure 8-5 shows the CMD plots for these
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118 A Manual for Design of Hot Mix Asphalt with Commentary
100
Dense/fine
80 Dense/dense
Percent Passing
Dense/coarse
60 SMA
Max. Density
40
20
0
0.010 0.100 1.000 10.000 100.000
Sieve Size, mm
25 SMA
% Deviation from Max. Density
Dense/coarse
Dense/dense
Dense/fine
0
-25
0.010 0.100 1.000 10.000 100.000
Sieve Size, mm
Figure 8-5. Top: four different 12.5-mm gradations;
bottom: % deviation from continuous maximum density
gradation for these same blends.
same four aggregate blends. One of the most striking features of the CMD plot is that it shows
that despite the large differences in the four gradations, the fine aggregate portions of these blends
all are fairly close to a maximum density gradation. This does not mean that the fine aggregate
portion of these mixtures closely follows the gradation for the traditional maximum density
gradation for a 12.5-mm aggregate (the dashed line in the top chart in Figure 8-5). What it
means is that the fine aggregate portions of all four blends--considered separately from the
coarse portion--follow a maximum density gradation fairly closely. This is a very important
concept when adjusting aggregate blends to meet VMA and/or air void requirements. Consider
the dense/coarse gradation in Figure 8-5. In the top chart it appears that the fine aggregate
portion of this aggregate deviates significantly from the maximum density gradation, and
changing this portion of the blend might therefore tend to reduce VMA. However, it is clear from
the lower chart that attempting to reduce VMA by changing the fine aggregate portion of this
gradation will probably be counter-productive, since it already closely follows a maximum
density gradation. If a reduction in VMA is needed for this aggregate, the amount of material
between the 2.36- and 9.5-mm sieves must be reduced. In general, the greater the deviation
from the zero line on the CMD plot, the greater will be the VMA (and air void content) for the
resulting mixture.
In Figure 8-5, it appears that the largest differences in these blends are in the coarse aggregate.
This is in fact typical for aggregate blends used in HMA. However, even though the deviations
from the maximum density gradation for the fine aggregate portion of many aggregate blends
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Design of Dense-Graded HMA Mixtures 119
may seem small, such differences can have a significant effect on the air void content and VMA
of the resulting HMA mixture.
The specific interpretation of CMD plots such as those shown in Figures 8-4 and 8-5 is as
follows. The value on the vertical axis for a given sieve size shows the difference in the percent
of material between that sieve size and one sieve size larger for the actual gradation and the
continuous maximum density gradation. For example, in Figure 8-5, for the 4.75-mm sieve,
the SMA gradation has about 22% more material between the 4.75- and 9.5-mm sieves than
the maximum density gradation. This is typical for SMA gradations, which contain very large
proportions of coarse aggregates. The dense/coarse gradation, on the other hand, has about 14%
more material than the continuous maximum density gradation in this size range. When using
CMD plots to blend aggregates during a mix design, the technician should look not only at the
amount of the deviation in different size ranges, but also the effect of these deviations on the air
void content and VMA of the resulting mixture. The air void content and VMA for some mixtures
might be most sensitive to changes in the coarse fraction of the aggregate blend, while other
mixtures might be more sensitive to changes in the fine or intermediate portions of the aggregate
blend. For this reason, HMA Tools includes, as part of the CMD plot, values for air void content
and VMA for each aggregate blend (once they have been determined in laboratory testing). This
makes it easy for the technician to determine what changes in the aggregate blends are most
important in determining volumetric composition.
Trial Blends for New Mix Designs
When developing an HMA mix design with a new set of aggregates, the general procedure
recommended here is very similar to that used in the Superpave method, but with the inclusion
of the CMD plot as an additional tool in analyzing the aggregate gradations and resulting
volumetric compositions. After performing the initial steps of the mix design as outlined above
(including determination of the design VMA, air void content, and binder content), three trial
aggregate gradations are prepared: a dense/coarse gradation, a dense/dense gradation, and a
dense/fine gradation. As explained later in this chapter, trial batches based on these gradations
are prepared in the laboratory, specimens compacted and the VMA, air void content, and effective
binder content determined for each. Usually none of the three initial batches will precisely meet
all requirements, and one is selected for further refinement. Additional trial batches are prepared
and evaluated until all essential mix design criteria are met. It is strongly suggested that during
the initial trial batches only the aggregate gradation should be modified, while the asphalt binder
content is kept constant. This will make the way changes in the aggregate gradation are affecting
VMA and air void content much clearer. Once a trial batch is close to the required composition,
the binder content may be slightly adjusted if desired to fine-tune the design. However, engineers
and technicians should remember that mix designs will usually be adjusted significantly during
the initial stages of field production, so effort spent in unnecessary, minor adjustments to a
laboratory mix design will often be wasted.
The HMA Tools spreadsheet has been designed to allow blending of up to eight different
aggregates, up to four of which may be recycled asphalt pavement (RAP) material. Gradation
data and other properties are entered in the worksheet "Aggregates" for new materials and
"RAP_Aggregates" for RAP materials. In the worksheet "Trial_Blends," the technician enters
various proportions for each aggregate or RAP, and the gradation is calculated and plotted on a
standard plot and on a CMD plot. Once volumetric test data are available for the trial batches,
these plots include VMA and air void values to aid the technician in determining which gradation
is most suitable and what sort of adjustments in the gradation (if any) are needed to produce a
mixture with the desired properties.
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120 A Manual for Design of Hot Mix Asphalt with Commentary
Adjusting Aggregate Gradations When Modifying
Existing HMA Designs
In practice, most HMA mix design work involves modifying existing mixtures to meet some
new requirement or improve some aspect of the design (such as workability). Therefore, the
approach discussed above will usually be the exception, rather than the rule. In modifying an
existing mix design, the procedure suggested here is similar to that described above, but somewhat
abbreviated. First, the goal of the modification must be clearly understood, primarily in terms
of the needed change in VMA. Increased air void content or increased binder content both require
increases in VMA. Similarly, if a specification requires an increase in VMA, and the design air void
content does not change, an increase in binder content will be needed. Remember, VMA consists
of the volume taken up by both asphalt binder and air voids.
Once the required change in VMA has been determined, the aggregate gradation for the
existing mix design is plotted, using a traditional gradation plot and the CMD plot. If an increase in
VMA is needed, in general, the aggregate gradation must be modified to increase the difference
between the CMD plot and the zero line. If a decrease in VMA is needed, the gradation should
be modified to decrease this difference. This is shown in Figure 8-6. The heavy, dark line in this
example is the aggregate gradation for an existing mix design. Looking at the coarse aggregate
portion of the gradation, the lighter lines above this are gradations that would likely increase the
VMA for this mixture, since they are further from the zero line. The lighter lines below the existing
gradation would probably decrease the VMA, since they are, in general, closer to the zero line on
the CMD plot. However, engineers and technicians using this approach should remember that
VMA for a given mixture will exhibit different sensitivities to changes in different portions of the
aggregate gradation. Some mixtures might be more sensitive to changes in the fine aggregate
portion of the gradation, while some might be more sensitive to the coarse aggregate portion of
the gradation. Some might be very sensitive to changes in mineral filler content. It should also
be noted that the zero line on the CMD plot is only an approximate indicator of the maximum
density gradation--the actual position on the CMD plot of the maximum density gradation
might vary somewhat from the zero line. This should become apparent when plotting gradations
and VMA values for trial mixtures.
It is especially important when modifying existing mix designs to make use of experience
with the given aggregates. Often, an engineer or technician who has done previous mix
design work with the aggregates at hand will know what changes in the gradation are needed
25
% Deviation from Max. Density
higher
VMA
0
lower
VMA
-25
0.010 0.100 1.000 10.000 100.000
Sieve Size, mm
Figure 8-6. Effect of changes in aggregate gradation on
VMA as shown on a CMD plot. Aggregates plotting closer
to the zero line will usually have lower VMA values.
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Design of Dense-Graded HMA Mixtures 121
Table 8-6. Control points for 19.0-mm through
37.5-mm aggregate gradations for dense-graded
HMA mixtures.
Sieve Size Percent Passing for Nominal Maximum Aggregate Size:
(mm) 37.5 mm 25.0 mm 19.0 mm
Min. Max. Min. Max. Min. Max.
50.0 100
37.5 90 100 100
25.0 90 90 100 100
19.0 90 90 100
12.5 90
9.5
4.75
2.36 15 41 19 45 23 49
1.18
0.600
0.075 0 6 1 7 2 8
to produce a specific change in VMA or other mix properties. In such cases, the CMD plot
can be a useful tool.
When modifying existing mix designs, the HMA Tools spreadsheet is used in a manner very
similar to that described above for new mix designs. As before, the needed aggregate and RAP
data (if used) are entered in the worksheets "Aggregates," and "RAP_Aggregates." Then, aggregate
blend for the existing mix is entered in the worksheet "Trial_Blends," followed by one or two
modified aggregate blends. If no information is available concerning the aggregates being used,
the general rule described above should be used in developing the new trial aggregate blends--
gradations closer to the zero line on the CMD plot will have lower VMA, while those further away
will have higher VMA. After this, the design proceeds as before, determining the air void content
and VMA for the trial batches and then making further refinements in the aggregate gradation
as needed until the desired mix properties are met.
Guidelines for Aggregate Gradations
As with previous HMA mix design methods, there are limits for aggregate gradation for each
NMAS; suggested control points for aggregate gradations for dense-graded HMA mixtures are
listed in Tables 8-6 and 8-7. It is important to note that, in the system described in this manual,
Table 8-7. Control points for 4.75-mm through
12.5-mm aggregate gradations for dense-graded
HMA mixtures.
Sieve Size Percent Passing for Nominal Maximum Aggregate Size:
(mm) 12.5 mm 9.5 mm 4.75 mm
Min. Max. Min. Max. Min. Max.
50.0
37.5
25.0
19.0 100
12.5 90 100 100 100
9.5 90 90 100 95 100
4.75 90 90 100
2.36 28 58 32 67
1.18 30 60
0.600
0.075 2 10 2 10 6 12
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122 A Manual for Design of Hot Mix Asphalt with Commentary
Table 8-8. Coarse aggregate fractured faces
requirements.
Percentage of Particles with at Least
One/Two Fractured Faces, for
Design ESALs (million)
Depth of Pavement Layera, mm
0 to 100 Below 100
< 0.3 55 / --- --- / ---
0.3 to < 3 75 / --- 50 / ---
3 to < 10 85 / 80 60 / ---
10 to < 30 95 / 90 80 / 75
30 or more 98 / 98b 98/ 98b
aDepth of pavement layer is measured from pavement surface to surface of
pavement layer.
bThe CAFF requirement for design traffic levels of 30 million ESALs or more may
be reduced to 95/95 if experience with local conditions and materials indicate that
this would provide HMA mixtures with adequate rut resistance under very heavy
traffic.
aggregate control points--with the exception of the maximum aggregate size--are considered
guidelines, and not specification requirements. This provides engineers and technicians with
additional flexibility in modifying aggregate gradations in order to meet VMA requirements.
Differences in aggregate particle shape, angularity, and texture make it impossible to specify one
particular gradation that will provide the best performance for all HMA mixtures of a given
NMAS. Treating gradation control points with some flexibility helps ensure that engineers and
technicians can adequately address these differences and provide HMA mixtures with the proper
binder content and air void content needed for good durability.
The gradation plots included in the worksheet "Trial_Blends" in HMA Tools include boundaries
showing the control limits for a given mixture. In order for the proper limits to be included in
the plot, the proper value for the aggregate NMAS must be entered in the worksheet "General."
Check Aggregate Specification Properties
As in the Superpave method, there are four aggregate specification properties: (1) coarse
aggregate fractured faces (CAFF); (2) flat and elongated particles in the coarse aggregate; (3) fine
aggregate angularity (FAA); and (4) clay content of the fine aggregate (sand equivalent). A detailed
description of these specification properties and the tests used to determine them is given in
Chapter 4 of this manual. For the convenience of the readers of this manual, four tables listing
the aggregate specification properties as given in Chapter 4 are reproduced below. Table 8-8
lists requirements for coarse aggregate fractured faces; Table 8-9 lists requirements for flat
and elongated coarse aggregate particles; Table 8-10 lists requirements for fine aggregate
angularity; and Table 8-11 lists requirements for fine aggregate clay content. The values in
these tables are very similar to those used in the Superpave method; there are some slight
Table 8-9. Criteria for flat and elongated particles.
Maximum Percentage of Flat and
Design ESALs (million)
Elongated Particles at 5:1
< 0.3 ---
0.3 to < 3 10
3 to < 10 10
10 to < 30 10
30 or more 10
Criteria are presented as percent flat and elongated particles by mass.
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Design of Dense-Graded HMA Mixtures 123
Table 8-10. Fine aggregate angularity requirements.
Depth of Pavement Layer from
Design ESALs (million) Surface, mm
0 to 100 Below 100
< 0.3 ---a ---
0.3 to < 3 40 ---
3 to < 10 45b 40
10 to < 30 45b 45b
30 or more 45b 45b
Criteria are presented as percent air voids in loosely compacted fine aggregate.
a
Although there is no FAA requirement for design traffic levels below 0.30
million ESALs, consideration should be given to requiring a minim um
uncompacted void content of 40 percent for 4.75-mm nominal maximum
aggregate size mixes.
b
The FAA requirement of 45 may be reduced to 43 if experience with local
conditions and materials indicate that this would produce HMA mixtures with
adequate rut resistance under the given design traffic level.
differences in the requirements for coarse aggregate fractured faces and fine aggregate angularity,
intended to make these requirements easier to meet without making any significant sacrifice in
performance.
As in the Superpave method, it is intended that aggregate specification properties be applied
to the aggregate blend, and not to individual aggregates. An important step in the mix design
process is to determine specification property values for aggregate blends, to ensure that the
blends will likely meet specification property requirements. For the initial trial batches, the
specification properties of the aggregate blends are normally estimated mathematically, by
calculating a weighted average for each property. When calculating these weighted averages,
care must be taken to consider only that portion of the aggregate tested in a given procedure.
For example, CAFF is determined on only that portion of the aggregate retained on the
4.75-mm sieve, so the weighted average CAFF is based on the proportions of this fraction for
each aggregate in the blend--not on the overall proportions for each aggregate. An additional
complication occurs when RAP is included in the mix design; then, the specification properties
of the aggregate in the RAP must also be considered (with the exception of sand equivalent, which
is only applied to new aggregates). Estimation of specification properties can be tedious and prone
to errors. Fortunately, HMA Tools performs this calculation for the technician. In the worksheets
"Aggregates" and "RAP_Aggregates," specification properties are entered for each aggregate.
Up to two user-defined properties can also be entered here--these would be aggregate properties
specified by the local highway agency, in addition to those given in this manual. In the worksheet
"Trial_Blends" the estimated values for all specification properties are then shown.
Aggregate properties for the final mix design should be determined by actual measurement.
This is done by preparing the aggregate blend and then sieving out the fraction needed for the
particular test and performing the test. Because this can be a time-consuming procedure, the
Table 8-11. Maximum clay content requirements.
Design ESALs (million) Minimum Sand Equivalency Value
< 0.3 40
0.3 to < 3 40
3 to < 10 45
10 to < 30 45
30 or more 50
Criteria are presented as Sand Equivalent Value.
