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OCR for page 179
Design of Gap-Graded HMA Mixtures 179
angularity by measuring uncompacted voids. However, the Rigden voids test is smaller in scale,
and the mineral filler is compacted with a small drop hammer prior to determining the void
content. Mineral fillers with very high Rigden voids can sometimes cause excessive stiffening in
SMA mixtures. The equipment and test method for conducting the Dry Compaction Test can
be found in the National Asphalt Pavement Association's Information Series 127, "Evaluation
of Baghouse Fines for Hot Mix Asphalt."
Other requirements for mineral fillers can be found in AASHTO M-17, "Mineral Fillers for
Bituminous Paving Mixtures." However, the gradation requirements stated in AASHTO M-17
should only be used for guidance. The important gradation is that of the designed GGHMA and
not the mineral filler.
Stabilizing Additives
Stabilizing additives are needed in GGHMA to prevent the draining of mortar from the coarse
aggregate skeleton during storage, transportation, and placement. Stabilizing additives such as
cellulose fiber, mineral fiber, and polymers have been used with success to minimize draindown
potential. Other types of fibers have been used with success; however, the most common types
are cellulose and mineral fibers. When using a polymer as a stabilizer, the amount of polymer added
should be that amount necessary to meet the performance grade of the asphalt binder.
Step 2--Trial Gradations
As with any HMA, specified aggregate gradations should be based on aggregate volume
and not aggregate mass. However, for most conventional HMA mixtures (dense-graded), the
specific gravities of the different aggregate stockpiles are close enough to make a gradation
based on mass percentages similar to that based on volumetric percentages. With GGHMA, the
specific gravities of the different aggregate components are not always similar. This is especially
true when commercial fillers are used in GGHMA. Therefore, the gradation bands presented
in Table 10-3 are based on % passing by volume. Similar to those for dense-graded mixes,
GGHMA gradation bands are described by the nominal maximum aggregate size (NMAS) of
the gradation. The following section provides guidance in the form of an example problem on
how to blend aggregate components based on volumes to meet the gradation bands in Table 10-3.
However, if the bulk specific gravities of the different stockpiles (including mineral filler)
used to compose the aggregate blend vary by 0.02 or less, gradations based on mass percentages
can be used.
Table 10-3. Stone matrix asphalt gradation
specification bands (Percent Passing by Volume).
Sieve 19-mm NMAS 12.5-mm NMAS 9.5-mm NMAS
Size,
mm Min. Max. Min. Max. Min. Max.
25.0 100 100
19.0 90 100 100 100
12.5 50 88 90 100 100 100
9.5 25 60 26 88 70 95
4.75 20 28 20 35 30 50
2.36 16 24 16 24 20 30
1.18 -- -- -- -- -- 21
0.6 -- -- -- -- -- 18
0.3 -- -- -- -- -- 15
0.075 8.0 11.0 8.0 11.0 8.0 12.0
Note: NMAS Nominal Maximum Aggregate Size one sieve size
larger than the first sieve that retains more than 10%.
OCR for page 180
180 A Manual for Design of Hot Mix Asphalt with Commentary
Example Problem 10-1. Blending Aggregates to Meet GGHMA Gradation
Requirements
Washed Sieve Analyses
As with any blending problem, the first step is to perform washed sieve analyses
based on mass for the various stockpiles to be used in the GGHMA mixture, following
procedures described in AASHTO T 27. For this example, a 19.0-mm GGHMA mixture
is to be blended. Table 10-4 provides the results of washed gradation tests performed
on four stockpiles, which are to be blended for this example problem. Also needed
to determine aggregate gradations based on volume are the bulk specific gravities
(Gsb) of the different stockpiles. Table 10-4 also provides the Gsb values for each
stockpile. Notice that the Gsb values differ by more than 0.02 in Table 10-4.
Determine Percent Mass Retained
After performing the washed sieve analyses, the percent mass retained on each
sieve for the different stockpiles is determined. For a given sieve, this is done by
subtracting the percent passing the given sieve from the percent passing the next
larger sieve. For example, using Aggregate C in Table 10-4, the % mass retained
on the 4.75-mm sieve would be calculated as
% Retained on 4.75 -mm Sieve = 84.6 - 48.9 = 35.7%
Table 10-4. Results of gradation and specific gravity tests
for stockpiles to be used in example GGHMA mix design.
Stockpile and Percent Passing Based on Mass, %
Sieve, mm Aggregate A Aggregate B Aggregate C Mineral Filler
25.0 100.0 100.0 100.0 100.0
19.0 95.0 100.0 100.0 100.0
12.5 66.0 71.0 97.4 100.0
9.5 43.0 46.0 84.6 100.0
4.75 9.0 6.0 48.9 100.0
2.36 5.0 4.0 27.8 100.0
1.18 2.0 4.0 16.6 100.0
0.60 2.0 3.0 10.7 100.0
0.30 2.0 3.0 7.6 100.0
0.075 1.0 1.5 4.6 72.5
Gsb 2.616 2.734 2.736 2.401
Table 10-5. Percent by mass retained on each sieve.
Sieve, Percent Mass Retained per Sieve
mm Aggregate A Aggregate B Aggregate C Mineral Filler
25.0 0.0 0.0 0.0 0.0
19.0 5.0 0.0 0.0 0.0
12.5 29.0 30.0 2.6 0.0
9.5 23.0 24.0 12.8 0.0
4.75 34.0 40.0 35.7 0.0
2.36 4.0 2.0 21.1 0.0
1.18 3.0 0.0 11.2 0.0
0.60 0.0 1.0 5.9 0.0
0.30 0.0 0.0 3.1 0.0
0.075 1.0 1.5 3.0 27.5
-0.075 1.0 1.5 4.6 27.5
Total, 100 100 100 100
OCR for page 181
Design of Gap-Graded HMA Mixtures 181
Example Problem 10-1. (Continued)
where
84.6 = percent mass passing 9.5-mm sieve (Table 10-4)
48.9 = percent mass passing 4.75-mm sieve (Table 10-4)
35.7 = percent mass retained on 4.75-mm sieve
Table 10-5 presents the values of percent mass retained for all sieves and stockpiles.
Note that a row has been added to reflect that material finer than the 0.075-mm
(-0.075) sieve is included.
Calculate Percent Mass Retained
In this calculation a simple assumption is made, "Assume the Mass of Each
Aggregate Stockpile is 100 grams." Using this assumption allows for the mass
that would be retained of each size fraction for each stockpile to be determined
and can be shown to be equal to the numbers shown in Table 10-5.
Convert Percent Mass Retained to Volume per Sieve
In this step of developing an SMA gradation, the values for percent mass retained
determined previously are converted to volumes per sieve. To make this conversion,
the bulk specific gravity of the individual stockpiles is needed. The volume of
aggregate retained on each sieve for each stockpile can be determined from the
following equation:
Magg
Vagg =
Gsb w
where
Vagg = volume of aggregate retained on a given sieve, cm3
Magg = mass of aggregate retained on a given sieve, g
w = unit weight of water (1.0 g/cm3)
The following calculation demonstrates how the volume is calculated for the
aggregate retained on the 4.75-mm sieve of Aggregate C.
35.7g
Volume = 13.05 cm3
2.736 × 1.0 g cm3
where
35.7 g = mass of Aggregate C retained on 4.75-mm sieve (Table 10-5)
2.736 = bulk specific gravity of Aggregate C (Table 10-4)
1.0 g/cm3 = unit weight of water (w)
13.05 cm3 = volume of Aggregate C retained on 4.75-mm sieve
The volumes retained on all sieves for each of the four stockpiles are provided in
Table 10-6.
(continued on next page)
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182 A Manual for Design of Hot Mix Asphalt with Commentary
Example Problem 10-1. (Continued)
Table 10-6. Volumes of aggregate retained on each sieve.
Sieve, Volume of Aggregate Retained per Sieve, cm3
mm Aggregate A Aggregate B Aggregate C Mineral Filler
25.0 0.00 0.00 0.00 0.00
19.0 1.91 0.00 0.00 0.00
12.5 11.09 10.97 0.95 0.00
9.5 8.79 8.78 4.68 0.00
4.75 13.00 14.63 13.05 0.00
2.36 1.53 0.73 7.71 0.00
1.18 1.15 0.00 4.09 0.00
0.60 0.00 0.37 2.16 0.00
0.30 0.00 0.00 1.13 0.00
0.075 0.38 0.55 1.10 11.45
-0.075 0.38 0.55 1.68 30.20
Blend Stockpiles
The values provided in Table 10-7 are used to blend the different stockpiles to
meet the desired gradation based on volumes. This process is identical to blending
stockpiles by mass and is a trial and error process. To perform the blending, select
the estimated percentages of the different stockpiles to be used. For this example,
the following percentages will be evaluated first:
Stockpile % Blend
Aggregate A 30
Aggregate B 30
Aggregate C 30
Mineral filler 10
The percent of each stockpile in the blend is multiplied by the volume retained
on a given sieve for each stockpile to determine the total volume retained on
Table 10-7. Total volumes retained
per sieve.
Volume Retained
Sieve, mm per Sieve, cm3
25.0 0.00
19.0 0.57
12.5 6.90
9.5 6.67
4.75 12.20
2.36 2.99
1.18 1.57
0.60 0.76
0.30 0.34
0.075 1.75
-0.075 3.80
Total Volume, 37.55
OCR for page 183
Design of Gap-Graded HMA Mixtures 183
Example Problem 10-1. (Continued)
that sieve. Using the 4.75-mm sieve as an example, the total volume retained on
the 4.75-mm sieve would be calculated as follows:
Total Volume Retained on 4.7 - mm sieve = ( 0.30 × 13.00 ) + ( 0.30 × 14.63) +
(0.30 × 13.05) + (0.10 × 0.00 ) = 12.20 cm3
where 0.30, 0.30, 0.30 and 0.10 are the percentages by mass of each aggregate in
blend expressed as decimals; and 13.00, 14.63, 13.05, and 0.00 are the % volume
retained on 4.75-mm sieve for each stockpile (Table 10-6).
This calculation is performed for each of the sieves in the gradation. Table 10-7
presents the total volume retained for each of the sieves in the gradation.
Now, based on the total volume retained per sieve and the summed total volume
of the blended aggregates, the percent retained per sieve by volume can be
determined for the blend. This is accomplished for a given sieve by dividing the
volume retained on that sieve by the total volume of the blend. The following
equation illustrates this calculation for the 4.75-mm sieve.
%Volume Retained on 4.7-mm Sieve = 12.20 × 100 37.55 = 32.50%
where
12.20 = volume retained on 4.75-mm sieve (Table 10-7)
37.55 = summed total volume of blend (Table 10-7)
32.50 = percent volume of blend retained on 4.75-mm sieve
Table 10-8 provides the percents retained based on volumes for each of the sieves
and converts this to percent volume passing.
Using the % retained per sieve based on volume, the % passing by volume for the
gradation can be determined similar to the method used with gradations based
on mass. Determine the cumulative % retained for each sieve and then subtract
from 100. Now, the blended gradation is compared to the required gradation band
Table 10-8. Percent passing based on volumes.
Percent Retained Cumulative Percent Passing by
Sieve, mm Per Sieve Percent Retained Volume
25.0 0.0 0.0 100.0
19.0 1.5 1.5 98.5
12.5 18.4 19.9 80.1
9.5 17.8 37.7 62.3
4.75 32.5 70.1 29.9
2.36 8.0 78.1 21.9
1.18 4.2 82.3 17.7
0.60 2.0 84.3 15.7
0.30 0.9 85.2 14.8
0.075 4.7 89.9 10.1
-0.075 10.1 100.0 ---
(continued on next page)
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184 A Manual for Design of Hot Mix Asphalt with Commentary
Example Problem 10-1. (Continued)
Table 10-9. Comparison of gradation blend based
on volume to specified gradation band.
Gradation Band
Sieve, mm Requirements Blend Percent Passing
25.0 100 100
19.0 90-100 98.5
12.5 50-88 80.1
9.5 25-60 62.3*
4.75 20-28 29.9*
2.36 16-24 21.9
1.18 --- 17.7
0.60 --- 15.7
0.30 --- 14.8
0.075 8-10 10.1
* Does not meet requirements
(also based on volume) provided in Table 10-3. Table 10-9 compares the gradation
band for a 19.0-mm NMAS GGHMA to the gradation shown in Table 10-8.
Based on Table 10-9, the blended gradation did not meet the specified gradation
band for a 19.0-mm nominal maximum aggregate size GGHMA. Therefore,
different blending percentages for the various stockpiles are needed. Below
are the percentages of the four stockpiles used for the second trial.
Stockpile % Blend
Aggregate A 40
Aggregate B 41
Aggregate C 10
Mineral Filler 9
Table 10-10 presents the blended gradation of the four aggregates for the
second trial. The second trial blend percentages were used along with
the values of Table 10-6 to determine the percent passing by volume for
the blend.
Table 10-10. Percents passing based on volumes.
Percent Cumulative Percent
Sieve, Retained Percent Passing by Percent Passing by Gradation
mm Per Sieve Retained Volume Mass Band by
by Volume by Volume (For Comparison) Volume
25.0 0.0 0.0 100.0 100.0 100
19.0 2.0 2.0 98.0 98.0 90-100
12.5 24.0 26.0 74.0 74.3 50-88
9.5 20.1 46.1 53.9 53.5 25-60
4.75 33.2 79.3 21.7 20.0 20-28
2.36 4.5 83.7 16.3 15.4 16-24
1.18 2.3 86.0 14.0 13.1 ---
0.60 1.0 87.0 13.0 12.1 ---
0.30 0.3 87.3 12.7 11.8 ---
0.075 4.0 91.3 8.7 8.0 8-11
-0.075 8.7 100.0 --- --- ---
OCR for page 185
Design of Gap-Graded HMA Mixtures 185
Example Problem 10-1. (Continued)
Based on Table 10-10, the following percentages produce a gradation based on
volume, which meets the 19.0-mm nominal maximum aggregate size gradation
band for GGHMA.
Stockpile % Blend by Mass
Aggregate A 40
Aggregate B 41
Aggregate C 10
Mineral Filler 9
Selection of Trial Gradations
When designing GGHMA mixtures, the initial trial gradations should be selected to be within the
master specification range shown in Table 10-3. To design a GGHMA mixture it is recommended
that at least three trial gradations be initially evaluated. It is suggested that the three trial grada-
tions fall along and in the middle of the coarse and fine limits of the gradation range. These trial
gradations are obtained by adjusting the amount of fine and coarse aggregates in each blend. The
percent passing the 0.075-mm sieve should be approximately 10 percent for each trial gradation.
Determination of VCA in the Coarse Aggregate Fraction
For best performance, the GGHMA mixtures must have a coarse aggregate skeleton with
stone-on-stone contact. The coarse aggregate fraction is not defined by a particular sieve size but
rather is that portion of the total aggregate blend retained on the breakpoint sieve. The breakpoint
sieve is defined as the finest (smallest) sieve to retain at least 10% of the aggregate gradation
(Figure 10-4).
100.0 100.0 100.0
90.0
87.0
80.0
70.0
Percent Passing
60.0 59.2
50.0
40.0
35.5
30.0 Blend 2
Break Point Sieve
20.0 20.6
Finest Sieve to Have
12.3 12.8 13.5 at Least 10 Percent
10.0 10.5
7.5 Retained
0.0
.075
.15
.30
.60
1.18
2.36 4.75 9.5 12.5 19.0 25.0
Sieve Size (mm) Raised to 0.45 Power
Figure 10-4. Definition of breakpoint sieve.
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186 A Manual for Design of Hot Mix Asphalt with Commentary
Figure 10-5. Method of determining VCA in dry-rodded condition.
The method of measuring the existence of stone-on-stone contact is called the voids in coarse
aggregate (VCA) method. The concept is quite simple and practical. The first step is to determine
the VCA of the coarse aggregate fraction only (material larger than breakpoint sieve) in a dry-
rodded condition (VCADRC) (Figure 10-5). AASHTO T 19, "Bulk Density ("Unit Weight") and
Voids in Aggregate," is used to compact the aggregate. Then, using Equation 10-1, the VCADRC can
be calculated. The VCADRC is nothing more than the volume between the coarse aggregate parti-
cles after compaction in accordance with AASHTO T 19. When asphalt binder is added and
the GGHMA is compacted, the VCA will again be calculated (VCAMIX). This calculation for
VCAMIX also calculates the volume between the coarse aggregate particles. As long as the volume
between the coarse aggregate particles is less in the compacted GGHMA (VCAMIX) than the coarse
aggregate only (VCADRC), then the GGHMA is deemed to have stone-on-stone contact and the
aggregate structure is acceptable. This means that the GGHMA mixture has been compacted
more than the dry-rodded condition of the aggregate; therefore, the coarse aggregate particles
within the compacted mixture must be compacted closer than the dry-rodded condition and
stone-on-stone contact exists.
Gca w - s
VCADRC = 100 (10-1)
Gca w
where
VCADRC = voids in coarse aggregate in dry-rodded condition
s = unit weight of the coarse aggregate fraction in the dry-rodded condition (kg/m3),
w = unit weight of water (998 kg/m3), and
Gca = bulk specific gravity of the coarse aggregate fraction
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Design of Gap-Graded HMA Mixtures 187
Table 10-11. Minimum asphalt binder
content requirements for aggregates with
varying bulk specific gravities.
Combined Aggregate Bulk Minimum Asphalt Content
Specific Gravity Based on Mass, %
2.40 6.8
2.45 6.7
2.50 6.6
2.55 6.5
2.60 6.3
2.65 6.2
2.70 6.1
2.75 6.0
2.80 5.9
2.85 5.8
2.90 5.7
2.95 5.6
3.00 5.5
Selection of Target Asphalt Content
The minimum desired asphalt binder content for GGHMA mixtures is presented in Table 10-11.
This table illustrates that the minimum asphalt binder content is based on the combined bulk
specific gravity of the aggregates used in the mix. These minimum asphalt binder contents are
provided to ensure enough volume of asphalt binder exists in the GGHMA mix to provide a
desirable mortar and, thus, a durable mixture. It is recommended that the mixture be designed
at 0.3% above the minimum values given in Table 10-11 to allow for adjustments during plant
production without falling below the minimum requirement. For example, for a GGHMA mixture
to be made with an aggregate blend having a combined bulk specific gravity of 2.75, the minimum
asphalt content is 6.0% by mass, and the target asphalt content would be 6.0 + 0.3 = 6.3% by mass.
The minimum binder content values given in Table 10-11 have been calculated so that, in most
cases, the resulting mixes will meet the suggested minimum VMA of 17.0% at 4.0% air voids for
GGHMA mixtures.
Sample Preparation
As with the laboratory design of any HMA, the aggregates to be used in GGHMA should be
dried to a constant mass and separated by dry-sieving into individual size fractions. The following
size fractions are recommended:
· 37.5 mm to 25.0 mm
· 25.0 mm to 19.0 mm
· 19.0 mm to 12.5 mm
· 12.5 mm to 9.5 mm
· 9.5 mm to 4.75 mm
· 4.75 mm to 2.36 mm
· Passing 2.36 mm (for 25.0-, 19.0-, 12.5-, and 9.5-mm NMAS gradations)
· 2.36 mm to 1.18 mm (for 4.75-mm NMAS gradations)
· Passing 1.18 mm (for 4.75-mm NMAS gradations)
After separating the aggregates into individual size fractions, they should be recombined at the
proper percentages based on the gradation blend being evaluated.
The mixing and compaction temperatures are determined in accordance with AASHTO T 245,
Section 3.3.1. Mixing temperature will be the temperature needed to produce an asphalt binder
