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OCR for page 124
124 A Manual for Design of Hot Mix Asphalt with Commentary
final mix design should avoid borderline values for aggregate specification properties that, when
the blend is actually tested, might fail to meet requirements.
Aggregate Blending: Summary
One of the most important and complicated parts of the HMA mix design process is deter-
mining the appropriate aggregate blend to use for a given application. Various procedures are
available, including the Bailey method, and several techniques described in mix design manuals
published by the Asphalt Institute. Engineers and technicians comfortable with the methods they
are currently using for proportioning aggregates for HMA mix designs should continue to use
these methods. The procedure given in this manual is based on a few simple concepts relating
aggregate blends to VMA.
In most cases, HMA mix designs are not designed from scratch. Instead, existing mix designs
are modified by replacing aggregates or the binder or by changing the binder content and VMA.
In these cases, the best guide for adjusting the aggregate proportions is the experience of the
engineer or technician with the materials being used. When modifying existing mix designs, one
or two aggregate blends are developed by modifying the blend used in the existing mix. A trial-
and-error approach is then used to refine the aggregate blend until the desired mix properties
are achieved. In situations where an entirely new HMA mix design is to be developed, three initial
trial blends are developed using dense/coarse, dense/dense, and dense/fine aggregate gradations.
The design closest to meeting all requirements is then further refined by making additional trial
blends, evaluating their properties, and modifying the aggregate gradation as needed.
An important part of the mix design process is determining the specification properties of the
aggregate blends. For initial trial batches, specification properties can be estimated by using
mathematical equations and the specification property values for the individual aggregates. This
is done automatically in HMA Tools (and many similar spreadsheets and computer programs).
However, the specification properties for the final mix design should be verified by actual
measurements on the aggregate blend.
Step 9. Calculate Trial Mix Proportions by Weight
and Check Dust/Binder Ratio
At this point in the HMA mix design, the amount of air voids, binder, and aggregate has been
determined on a volume basis, and up to three different aggregate blends have been developed--
on a proportion-by-weight basis. Now, the overall mixture composition in percent by weight
must be calculated and the dust/binder ratio checked to make sure it is within the specified values.
If desired, the mixture composition by volume can also be determined. The following procedure
and equations can be used to calculate mix proportions by weight and related mix properties.
First, calculate the overall aggregate bulk specific gravity:
Ps1 A + Ps 2 A + Ps 3 A + . . .
Gsb = (8-4)
Ps1 A Ps 2 A Ps 3 A . . .
+ + +
Gsb1 Gsb 2 Gsb 3
where
Gsb = overall bulk specific gravity for aggregate blend
Ps1/A = volume % of aggregate 1 in aggregate blend
Gsb1 = bulk specific gravity for aggregate 1
Ps2/A = volume % of aggregate 2 in aggregate blend
OCR for page 125
Design of Dense-Graded HMA Mixtures 125
Gsb2 = bulk specific gravity for aggregate 2
Ps3/A = volume % of aggregate 3 in aggregate blend
Gsb3 = bulk specific gravity for aggregate 3
As discussed in Step 7, the volume percentage of the aggregate is simply 100% minus the target
VMA. The weight percentage of binder and aggregate are then calculated using the following
equations:
VbGb
Pb = × 100% (8-5)
VsbGsb + VbGb
VsbGsb
Ps = × 100% (8-6)
VsbGsb + VbGb
where
Pb = total binder content, % by total mix weight
Vb = total binder content, % by total mix volume
Gb = binder specific gravity
Vsb = aggregate content, % by total mix volume
Gsb = overall bulk specific gravity of aggregate (Equation 8-4)
Ps = total aggregate content, % by total mix weight
Then, calculate the effective asphalt binder content by weight:
Vbe Gb
Pbe = × 100% (8-7)
VsbGsb + VbGb
where
Pbe = effective binder content, % by total mix weight
Vbe = effective binder content, % by total mix volume
Gb = binder specific gravity
Vsb = aggregate content, % by total mix volume
Gsb = overall bulk specific gravity of aggregate (Equation 8-4)
Calculate the percent by weight of each aggregate:
Ps1 A
Ps1 = Ps (8-8)
100
where
Ps1 = weight percent (by total mix) of aggregate 1 (or aggregate 2, 3, etc.)
Ps = weight percent (by total mix) of combined aggregate, from Equation 8-6
Ps1/A = weight percent (in aggregate blend) of aggregate 1 (or aggregate 2, 3, etc.)
If desired, the volume percent of the aggregates can also be calculated, but the equation is more
complicated:
Ps1 (100 - Pb )
Vsb1 = (8-9)
Pb Ps1 Ps 2 Ps 3 . . .
+ + + +
Gb Gsb1 Gsb 2 Gsb 3
OCR for page 126
126 A Manual for Design of Hot Mix Asphalt with Commentary
where
Vsb1 = volume % of aggregate 1 in total mix
Ps1 = weight % of aggregate 1 in total mix
Pb = weight % binder in total mix
Gb = binder specific gravity
Gsb1 = bulk specific gravity for aggregate 1
Ps2 = volume % of aggregate 2 in aggregate blend
Gsb2 = bulk specific gravity for aggregate 2
Ps2 = volume % of aggregate 3 in aggregate blend
Gsb3 = bulk specific gravity for aggregate 3
Calculate the percent of mineral dust (material finer than 0.075 mm) in the total mixture:
P0.075 s1 Ps1 + P0.075 s 2 Ps 2 + P0.075 s 3 Ps 3 + . . .
P0.075 = (8-10)
100
where
P0.075 = mineral dust content (material finer than 0.075 mm), percent by total mix weight
P0.075/s1 = % passing the 0.075-mm sieve for aggregate 1
Ps1 = weight percent (by total mix) of aggregate 1
P0.075/s2 = % passing the 0.075-mm sieve for aggregate 2
Ps2 = weight percent (by total mix) of aggregate 2
P0.075/s3 = % passing the 0.075-mm sieve for aggregate 3
Ps3 = weight percent (by total mix) of aggregate 3
Calculate the dust/binder ratio, using the effective asphalt binder content:
P0.075
D B= (8-11)
Pbe
where
D/B = dust/binder ratio, calculated using effective binder content
P0.075 = mineral dust content, % by total mix weight (Equation 8-10)
Pbe = effective binder content, % by total mix weight (Equation 8-7)
The required range for dust/binder ratio is 0.8 to 1.6 for all mixtures larger than 4.75-mm
NMAS. However, the specifying agency may reduce the requirements to a range of 0.6 to 1.2 if
local materials and conditions warrant this change. For 4.75-mm NMAS mixtures, the required
dust/binder ratio is 0.9 to 1.2, and this should not be modified. These requirements are similar
to those given in the Superpave method, but the required and optional ranges are reversed; in the
Superpave method, the required range is 0.6 to 1.2, but agencies can increase the requirement to
0.8 to 1.6. Higher dust/binder ratios are desirable for several reasons. Perhaps most importantly,
they help provide stiffness and rut resistance to the HMA. Higher dust/binder ratios also
will tend to reduce the permeability of an HMA mixture, improving durability. However,
it is possible that in some locations obtaining high dust/binder ratios might be prohibitively
expensive, and the nature of the local materials might allow the design of HMA with good
performance at lower dust/binder ratios. Because of the beneficial effects of high dust/binder
ratios on rut resistance, if VMA requirements are increased above those given in Table 8-5,
the dust/binder ratio requirement should not be reduced. Otherwise, the rut resistance of the
resulting mixtures might, in some cases, be marginal. Table 8-12 summarizes the requirements
for dust/binder ratio.
OCR for page 127
Design of Dense-Graded HMA Mixtures 127
Table 8-12. Requirements for
dust/binder ratio.
Allowable Range for
Mix Aggregate Dust/Binder Ratio, by
NMAS, mm Weight
> 4.75 0.8 to 1.6a
4.75 0.9 to 2.0
a
The specifying agency may lower the allowable range
for dust/binder ratio to 0.6 to 1.2 if warranted by local
conditions and materials. The dust/binder ratio should,
however, not be lowered if VMA requirements are
increased above the standard values as listed in Table 8-5.
When including RAP in a mixture, the same principles described above are applied. RAP is
composed of both binder and aggregate. The weight and volume of binder in the RAP must be
added to the weight and volume of new binder added to a mixture. Similarly, the weight and vol-
ume of aggregate must be added to the weight and volume of new aggregate added to the mix.
As will be discussed in Chapter 9, HMA Tools automatically performs the needed calculations
when including RAP in an HMA mix design.
Example Problem 8-1. Calculation of Mix Composition
Table 8-13 presents the results of an example calculation of mixture composition
for a trial batch. The mixture is a 12.5-mm NMAS design, with a target air void
content of 4% and a target VMA value of 15%. Column 1 describes the various mix
components; this includes total binder, absorbed binder, and effective binder--
this makes the relationship among these values clear. Column 2 gives the mix
composition in percentage by volume, which is determined using the procedure
described above. Column 3 lists the bulk specific gravity for the various components,
while Column 4 lists apparent specific gravity values for the aggregates. Column 5
lists the aggregate contents as a percentage by weight of the aggregate blend.
Table 8-13. Example calculation of HMA mix composition
by weight percentage from volume percentage and specific
gravity values.
(1) (2) (3) (4) (5) (6)
Percent by Bulk Apparent Percent by Percent by
Total Mix Specific Specific Aggregate Total Mix
Mix Component Volume Gravity Gravity Weight Weight
Air 4.00 --- --- --- 0.0
Total Asphalt Binder 11.38 1.025 --- --- 4.60
Absorbed Asphalt Binder -0.40 1.025 --- --- (0.16)
Effective Asphalt Binder 10.98 1.025 --- --- 4.44
No. 7 Traprock 19.54 2.971 2.992 24 22.90
Traprock screenings 24.47 2.867 2.893 29 27.67
Manufactured sand 24.46 2.868 2.891 29 27.67
Natural sand 13.74 2.642 2.676 15 14.31
Mineral filler 2.80 2.588 2.629 3 2.86
Note: Calculations may not agree exactly because of rounding.
(continued on next page)
