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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 9
Table 10. Maximum clay content requirements.
Design ESALs (million) Minimum Sand Equivalency Value
< 0.30 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.
estimated from the specification property values for the individual aggregates using HMA Tools.
These estimates should then be verified by measurements on the final design blend.
Step 9. Calculate Trial Mix Proportions By Weight and
Check Dust-to-Binder Ratio
This step in the design of WMA mixtures is identical to that described in Chapter 8 for HMA
mixtures. This step involves calculating the following for each trial blend of aggregates:
1. Bulk specific gravity of the aggregate blend,
2. Weight percentage of binder in the mixture,
3. Weight percentage of total aggregate in the mixture,
4. Effective binder content by weight,
5. Weight percentage of each aggregate in the mixture,
6. Weight percentage of mineral dust in the mixture, and
7. Dust to effective binder ratio.
These calculations are performed automatically by HMA Tools and when RAP is included in
the mixture, binder from the RAP is properly accounted for in the calculations. WMA mixtures
should meet the requirements for the ratio of dust to effective binder content given in Chapter
8 for HMA. These requirements are reproduced in Table 11.
Step 10. Evaluate and Refine Trial Mixtures
This step involves the preparation and evaluation of laboratory specimens of WMA. The pro-
cedure follows that described in Chapter 8 for HMA with slight modification. Table 12 summa-
rizes the steps for WMA and HMA design. The modifications required for WMA design are
1. For some processes, the WMA additive must be calculated.
2. Viscosity-based mixing temperatures are not used with WMA. Laboratory mixing is done at
the planned production temperature.
Table 11. 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 13-6.
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10 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Table 12. Comparison of trial specimen fabrication procedures
for WMA and HMA design.
Step Description HMA WMA Comment
1 Calculate batch weights X X Must calculate WMA additive content for
some processes
2 Batch aggregates X X Must batch WMA additive for some
processes
3 Heat aggregates and X X Use planned production temperature for
asphalt binder WMA
4 Mix aggregates and X X Procedure is WMA process specific
binder
5 Mixture conditioning X X WMA uses lower temperature
procedures
6 Compact laboratory X X WMA uses lower temperature
specimens
7 Calculate volumetric X X
composition of
laboratory specimens
8 Adjust aggregate X X
proportions to meet
volumetric requirements
9 Evaluate coating and NA X Used in WMA design in place of viscosity-
compactability based mixing and compaction temperatures
10 Conduct performance X X Moisture sensitivity for all mixtures, rutting
testing resistance for design traffic levels of 3 m
ESALs or greater
3. The short-term conditioning temperature for WMA is the planned compaction temperature.
4. Viscosity-based compaction temperatures are not used with WMA. Laboratory compaction
is done at the planned compaction temperature.
5. WMA design includes an evaluation of coating and compactability using the planned pro-
duction and compaction temperatures.
These modifications are discussed in the sections that follow.
Calculate Batch Weights
Some WMA processes require an additive to be added either to the binder or to the mixture. The
amount of additive needed may be specified by the WMA process supplier as percent by weight of
binder or total mixture. The "Additive" sheet in HMA Tools allows the user to specify the dosage
rate for up to three additives and whether the dosage rate is based on binder or total mixture weight.
Batch Aggregates
For most WMA processes, aggregate batching is identical to that for HMA. HMA Tools pro-
vides a convenient tool for calculating batch weights for various specimens and degrees of aggre-
gate processing. In one WMA process, water is added to a portion of the fine aggregate, then this
wet, fine aggregate is added cold to the mixture during the mixing process. For this process, treat
the wet portion of the fine aggregate as a separate fine aggregate in HMA Tools. Compute the
dry aggregate batch weight for this aggregate, then add the required weight of water to the dry
aggregate, mix, cover, and let stand 2 hours before using it in the mixing process.
Heat Aggregates and Asphalt Binder
The most notable differences between the design of WMA and HMA occur in the specimen
fabrication process, which begins with this step and continues through the next three steps.
Viscosity-based mixing and compaction criteria cannot be used with the wide range of WMA
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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 11
processes available. In fact, research in progress suggests that enhanced lubrication, not viscosity
reduction, is the primary mechanism governing the success of WMA processes.
The design of WMA mixtures is done using the planned field production and compaction
temperatures. The aggregates and binder that will be used are heated in an oven to approximately
27°F (15°C) above the planned production temperature. Aggregates may be heated overnight.
The asphalt binder and RAP, if used, should be heated the minimum time necessary to reach this
target temperature.
Mix Aggregates and Binder
For mixture design purposes, the various WMA processes can be grouped into the following
generic categories:
1. Additives blended into the binder,
2. Additives added to the mixture,
3. Wet aggregate mixtures, and
4. Foamed asphalt.
This section describes laboratory procedures for preparing each of these types of WMA
mixtures. Some WMA processes may include elements from two or more of these processes.
The laboratory equipment needed to produce the mixtures is generally the same as that
required for HMA. A mechanical mixer capable of mixing 10 to 45 lb (5 to 20 kg) batches is
needed for all WMA processes. The mixing times presented later are based on a planetary mixer
with a wire whip. Bucket mixers are less efficient than planetary mixers; therefore, the mixing
times may need to be increased. For WMA processes requiring the additive to be blended in the
binder, a low-shear mechanical stirrer with appropriate size impeller is needed to homoge-
neously blend the additive in the binder. Finally, for foamed asphalt mixtures, a laboratory-scale
foamed asphalt plant capable of producing consistent foamed asphalt at the water content used
in field production is needed. An example of such a device is shown in Figure 1. The device
should be capable of producing foamed asphalt for laboratory batches ranging in size from
approximately 20 to 45 lb (10 to 20 kg). Note that laboratory foaming plants designed for cold
mix applications will require a more precise flow controller to allow foamed asphalt production
Figure 1. A foaming device for preparing WMA
in the laboratory.
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12 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
at the lower water contents used in WMA. Also, because these machines are designed to produce
large quantities of foamed asphalt, it will be necessary to produce larger batches of WMA and
then split the material needed for the various tests required.
Additives Blended in the Binder
For WMA processes that require the WMA additive to be blended in the binder, the additive
must be blended into the binder before the WMA mixture can be produced. The required dosage
rate will be provided by the WMA process supplier who usually will also provide instructions for
blending the additive in the binder. HMA Tools will compute the mass of additive to add for a given
batch size. If instructions for blending the additive are not provided, use the following procedure:
1. Follow the manufacturer's instruction for storage of the additive (e.g., temperature and
humidity) particularly after opening the manufacturer's packaging.
2. Weigh the required amount of the additive into a small container. The additive is typically
specified as a percent by weight of binder. For mixtures containing RAP, determine the weight
of additive based on the total binder content of the mixture.
3. Heat the asphalt binder in a covered container in an oven set at 135°C until the binder is
sufficiently fluid to pour. During heating, occasionally stir the binder manually to ensure
homogeneity.
4. Add the required amount of additive to the binder and stir with a mechanical stirrer until the
additive is totally dispersed in the binder.
5. Store the binder with WMA additive at room temperature in a covered container until needed
for use in the mixture design.
Some binders are being supplied with the WMA additive pre-blended into the binder. For
these binders, it is not necessary to blend the additive, and the preparation of the WMA mixture
proceeds as outlined below.
Once the WMA additive has been added to the binder, the preparation of the WMA mixture
proceeds in a similar manner as that for HMA. The following steps summarize the mixture
preparation process:
1. Heat the aggregate, RAP, binder, and mixing tools to approximately 27°F (15°C) above the
planned production temperature. Aggregates may be heated overnight. The asphalt binder and
RAP should be heated the minimum time necessary to reach this target temperature.
2. If a liquid antistrip is required, add it to the binder per the manufacturer's instructions.
3. Place the hot mixing bowl on a scale and zero the scale.
4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly.
5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into
the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored
for an extended period of time in a humid environment, then it may be necessary to adjust
the weight of binder based on the oven dry weight of the aggregates and RAP as follows:
a. Record the oven dry weight of the aggregates and RAP, wi
b. Determine the target total weight of the mixture
wi
wt =
p
1 - bnew
100
where
wt = target total weight
wi = oven dry weight from Step a
Pbnew = % by weight of total mix of new binder in the mixture
c. Add new binder to the bowl to reach wt
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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 13
6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec.
7. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for
short-term conditioning.
Additives Added to the Mixture
Some WMA processes specify that the additive be added to the mixture during plant mixing.
The additive dosage rate may be given as a percent of the total mixture mass or a percentage of
the binder in the mixture. If the mixture contains RAP and the dosage rate is as a percentage
of the binder, remember to include the RAP binder contribution when computing the amount
of additive. HMA Tools will compute the mass of additive to add for a given batch size. For these
processes, the following mixing procedure should be followed:
1. Follow the manufacturer's instruction for storage of the additive (e.g., temperature and
humidity) particularly after opening the manufacturer's packaging.
2. Weigh the required amount of the additive into a small container.
3. Heat the aggregate, RAP, binder, and mixing tools to approximately 27°F (15°C) above the
planned production temperature. Aggregates may be heated overnight. The asphalt binder
and RAP should be heated the minimum time necessary to reach this target temperature.
4. If a liquid antistrip is required, add it to the binder per the manufacturer's instructions.
5. Place the hot mixing bowl on a scale and zero the scale.
6. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly.
7. Form a crater in the blended aggregate and weigh the required amount of asphalt binder
into the mixture to achieve the desired batch weight. If the aggregates and RAP have been
stored for an extended period of time in a humid environment, then it may be necessary
to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as
follows:
a. Record the oven dry weight of the aggregates and RAP, wi
b. Determine the target total weight of the mixture
wi
wt =
p
1 - bnew
100
where
wt = target total weight
wi = oven dry weight from Step a
Pbnew = % by weight of total mix of new binder in the mixture
c. Add new binder to the bowl to reach wt
8. Pour the WMA additive into the pool of new asphalt binder.
9. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec.
10. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for
short-term conditioning.
Figure 2 shows a WMA additive being added to a mixture in the laboratory.
Wet Aggregate Mixtures
One WMA process uses cold, wet fine aggregate to produce asphalt concrete at significantly
lower discharge temperatures. In this process, a portion of the total aggregate is added wet. The
coarse aggregate and dry portion of the fine aggregate are mixed with the binder at normal HMA
production temperatures. The percentage of the fine aggregate that will be added wet, the mois-
ture content of that portion of the fine aggregate, and the initial mixing temperature are recom-
mended by the WMA process supplier. An additive is also added to the binder following the steps
described above for Additives Blended in the Binder. In HMA Tools, treat the portion of the fine
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14 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Figure 2. Adding a WMA additive to a mixture
in the laboratory.
aggregate that will be added wet as a separate aggregate. Compute the dry aggregate batch weight
for this aggregate, then the weight of water to add (based on the recommended moisture content),
and then proceed as follows:
1. Add the required moisture to the wet fraction of the aggregate, mix thoroughly, then cover
and let stand for at least 2 hours before mixing with the heated fraction.
2. Heat the aggregate, RAP, binder, and mixing tools to approximately 27°F (15°C) above the
initial mixing temperature. Aggregates may be heated overnight. The asphalt binder and RAP
should be heated the minimum time necessary to reach this target temperature.
3. Place the hot mixing bowl on a scale and zero the scale.
4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly.
5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder
into the mixture to achieve the desired batch weight. If the aggregates and RAP have been
stored for an extended period of time in a humid environment, then it may be necessary
to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as
follows:
a. Record the oven dry weight of the heated aggregates and RAP, wi
b. Determine the target total weight of the mixture:
(w i + w dwf )
wt =
p
1 - bnew
100
where
wt = target total weight
wi = oven dry weight from Step a
wdwf = oven dry weight of the wet fraction from the batch sheet
Pbnew = percent by weight of total mix of new binder in the mixture
c. Determine the target weight of the heated mixture:
w thm = w t - w dwf
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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 15
where
wthm = target weight of the heated mixture
wt = target total weight
wdwf = oven dry weight of the wet fraction from the batch sheet
d. Add new binder to the bowl to reach wthm
6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 30 sec.
7. Stop the mixer and immediately add the wet fraction.
8. Restart the mixer and continue to mix for 60 sec.
9. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for
short-term conditioning.
10. Check the temperature of the mixture in the pan. It should be between 90 and 100°C.
Foamed Asphalt
The preparation of foamed asphalt mixtures requires special asphalt binder foaming equip-
ment that can produce foamed asphalt using the amount of moisture that will be used in field
production. The procedure for preparing foamed asphalt mixtures is as follows:
1. Prepare the asphalt binder foaming equipment and load it with binder per the manufacturer's
instructions.
2. If a liquid antistrip is required, add it to the binder in the foaming equipment per the manu-
facturer's instructions.
3. Heat the aggregate, RAP, and mixing tools to approximately 27°F (15°C) above the planned
production temperature. Aggregates may be heated overnight. The asphalt binder and RAP
should be heated the minimum time necessary to reach this target temperature.
4. Prepare the foamed asphalt binder per the instructions for the foaming equipment.
5. Place the hot mixing bowl on a scale and zero the scale.
6. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly.
7. Form a crater in the blended aggregate and add the required amount of foamed asphalt
into the mixture to achieve the desired batch weight. If the aggregates and RAP have been
stored for an extended period of time in a humid environment, then it may be necessary
to adjust the weight of foamed binder based on the oven dry weight of the aggregates and
RAP as follows:
a. Record the oven dry weight of the aggregates and RAP, wi
b. Determine the target total weight of the mixture
wi
wt =
p
1 - bnew
100
where
wt = target total weight
wi = oven dry weight from Step a
Pbnew = percent by weight of total mix of new binder in the mixture
c. Add foamed binder to the bowl to reach wt
The laboratory foaming equipment uses a timer to control the amount of foamed binder
provided. Make sure the batch size is large enough that the required amount of foamed binder
is within the calibrated range of the foaming device. This may require producing one batch
for the two gyratory specimens and the maximum specific gravity specimen at a given asphalt
content and then splitting the individual samples.
8. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec.
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16 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
9. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for
short-term conditioning.
Mixture Conditioning Procedures
AASHTO R 30 describes three different procedures for mixture conditioning in a forced draft
oven: (1) mixture conditioning for volumetric mix design; (2) short-term conditioning for
mixture mechanical property testing; and (3) long-term conditioning for mixture mechanical
property testing. The last procedure, long-term conditioning for mixture mechanical property
testing, is generally not used in the design and testing of WMA mixtures and is not addressed
here. WMA mixtures for both volumetric mixture design and mechanical property testing
(performance evaluation) should be conditioned for 2 hours at the planned compaction
temperature. The conditioning should follow AASHTO R 30, using a preheated forced-draft
oven, a mixture thickness of 1 to 2 in (25 to 50 mm), and stirring of the mixture after 1 hour.
The conditioning procedure for volumetric mix design is essentially identical for WMA and
HMA, the only difference being in the definition of compaction temperature for the two types
of mixes. Short-term conditioning for performance testing differs in that only 2 hours is required
for WMA mixtures, while 4 hours is required when conditioning HMA. Also, WMA mixtures
for performance testing are conditioned at the planned compaction temperature, whereas
AASHTO R 30 specifies a conditioning temperature of 135°C for HMA mixtures for mechanical
testing.
Compact Laboratory Specimens
WMA specimens are compacted in the same manner as described in Chapter 8 for HMA using
a properly calibrated and maintained Superpave gyratory compactor (see Figure 3). Compact
duplicate specimens in accordance with AASHTO T 312.
Calculate Volumetric Composition of Laboratory Specimens
Volumetric analysis of compacted WMA specimens is the same as described in Chapter 8
for HMA. Recall that the procedure used in this manual sets the binder content at a value that
Figure 3. Compacting a WMA specimen in a
Superpave gyratory compactor.
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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 17
will provide the proper VMA once the design air void content is met, and the gradation was
selected to provide an acceptable ratio of dust to effective binder content by mass. Thus the
air void content is the primary volumetric factor used to determine the acceptability of the
trial mixture. Given that the binder content was initially set using an assumed binder absorp-
tion, the effective binder content and VMA of the trial mixture should also be analyzed. The
following equations (which were derived in Chapter 5) are used to perform the volumetric
analysis.
Air Void Content
G
VA = 100 1 - mb (2)
Gmm
where
VA = Air void content, volume %
Gmb = Bulk specific gravity of compacted mixture
Gmm = Maximum theoretical specific gravity of loose mixture
Total Binder Content by Volume
PbGmb
VB = (3)
Gb
where
VB = Total asphalt binder content, % by total mix volume
Pb = Total asphalt binder content, % by mix mass
Gmb = Bulk specific gravity of the mixture
Gb = Specific gravity of the asphalt binder
Absorbed Binder by Volume
P P 100
VBA = Gmb b + s - (4)
Gb Gsb Gmm
where
VBA = Absorbed asphalt content, % by total mixture volume
Gmb = Bulk specific gravity of the mixture
Pb = Total asphalt binder content, % by mix mass
Gb = Specific gravity of the asphalt binder
Ps = Total aggregate content, % by mix mass, equal to 100 - Pb
Gsb = Average bulk specific gravity for the aggregate blend
Gmm = Maximum specific gravity of the mixture
Effective Binder Content by Volume
VBE = VB - VBA (5)
where
VBE = Effective asphalt content, % by total mixture volume
VB = Asphalt binder content, % by mix volume
VBA = Absorbed asphalt content, % by total mixture volume
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18 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Effective Binder Content by Mass
VBE
Pbe = Pb
VB
(6)
where
Pbe = Effective asphalt binder content, % by total mass
Pb = Asphalt binder content, % by total mass
VBE = Effective asphalt binder content, % by total mixture volume
VB = Asphalt binder content, % by mix volume
Voids in Mineral Aggregate
VMA = VA + VBE (7)
where
VMA = Voids in the mineral aggregate, % by total mixture volume
VA = Air void content, % by total mix volume
VBE = Effective binder content, % by total mixture volume
Dust Proportion
P
D B = 0.075 (8)
Pbe
where
D/B = dust/binder ratio, calculated using effective binder content
P0.075 = mineral dust content, % by total mix weight
Pbe = effective binder content, % by total mix weight
HMA Tools performs the volumetric analysis using data on the bulk and maximum specific
gravity of the trial mixture. The design procedure in this manual provides acceptable ranges for
three volumetric factors: (1) air void content, (2) VMA, and (3) dust proportion. The acceptable
ranges are summarized in Table 13. These are the same ranges used for the design of HMA.
Adjust Aggregate Proportions to Meet Volumetric Requirements
Adjusting WMA mixtures to meet volumetric requirements is the same as discussed in
Chapter 8 for HMA. The procedure presented in this manual is designed to provide accept-
Table 13. Acceptable range for volumetric factors.
Aggregate VMAA Air Voids Dust ProportionB
NMAS Minimum Maximum Minimum Maximum Minimum Maximum
(mm) (%) (%) (%) (%)
4.75 16.0 18.0 3.5 4.5 0.9 2.0
9.5 15.0 17.0 3.5 4.5 0.8 1.6
12.5 14.0 16.0 3.5 4.5 0.8 1.6
19.0 13.0 15.0 3.5 4.5 0.8 1.6
25.0 12.0 14.0 3.5 4.5 0.8 1.6
37.5 11.0 13.0 3.5 4.5 0.8 1.6
A
The specifying agency may increase the minimum and maximum values for VMA by up to 1.0% 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.
B
The specifying agency may lower the allowable range for dust/binder ratio to 0.6 to 1.2 for NMAS mixtures of 9.5
mm and greater if warranted by local conditions and materials. The dust/binder ratio should, however, not be
lowered if VMA requirements are increased using Note A above.
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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 19
able volumetric properties when specimens of the trial mixture meet the design air void
content. After preparing trial specimens, it may be necessary to make minor adjustments
to the binder content to account for differences between the assumed and actual binder
absorption. If the air voids of the trial specimens are more than a few tenths of a percent out-
side the design range, then the aggregate gradation should be adjusted to change the VMA of
the mixture. The general rule for adjusting aggregate blends to meet VMA requirements is
that the closer an aggregate gradation is to a maximum density gradation, the lower will be
its VMA.
Evaluate Coating and Compactability
The viscosity-based mixing and compaction temperatures used in the design of HMA can-
not be used with the wide range of WMA processes currently available. For WMA, the design
procedure, therefore, includes an evaluation of coating at the planned production temperature
and an evaluation of compactability at the planned compaction temperature. The sections that
follow describe these evaluations.
Coating
Coating is evaluated at the planned production temperature by preparing loose mix of the
design mixture following the specimen fabrication procedures presented earlier and evaluating
the coating of the coarse aggregate particles using AASHTO T 195, Standard Method of Test for
Determining Degree of Particle Coating of Bituminous-Aggregate Mixtures. This test method
consists of separating out the coarse aggregates of the mixture and determining the percentage
of the coarse aggregate particles that are fully coated. The recommended criterion is 95% of the
coarse aggregates fully coated. It should be noted that this criterion and the mixing times given
earlier were developed using a planetary mixer with a wire whip. Bucket mixers are not as effi-
cient as planetary mixers; therefore, laboratory mixing times may need to be increased if a bucket
mixer is used.
Compactability
Compactability is evaluated by compacting two specimens at the planned compaction
temperature and two specimens at 30°C below the planned compaction temperature. The num-
ber of gyrations required to reach 8% air voids is determined for both sets of specimens. It is
recommended that the increase in gyrations to 8% air voids between the planned compaction
temperature and 30°C below the planned compaction temperature should be less than 25% of
the number of gyrations at the planned compaction temperature. The procedure is described in
detail below:
1. Prepare a sufficient quantity of the design mixture for four gyratory specimens and
one maximum specific gravity measurement using the appropriate WMA fabrication
procedure.
2. Determine the theoretical maximum specific gravity (Gmm) according to AASHTO T 209.
3. Compact duplicate specimens at the planned compaction temperature to Ndesign gyrations in
accordance with AASHTO T 312. Record the specimen height for each gyration.
4. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166.
5. Allow the mixture to cool to 30°C below the compaction temperature. Compact duplicate
specimens to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen
height for each gyration
6. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166.
7. For each specimen determine the height at a relative density of 92.0% using Equation 9.
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20 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
%Gmmd
h92 = hd
92
(9)
where
h92 = height at a relative density of 92%
hd = height at Ndesign, as measured by the gyratory compactor
%Gmmd = relative density at Ndesign
8. For each specimen, determine the number of gyrations to reach 92% relative density. This
can be done by looking at the output from the gyratory compactor giving specimen height
as a function of gyrations--simply find the number of gyrations at a height where the rela-
tive density is 92%, h92 as determined in Step 7 above.
9. Determine the gyration ratio using Equation 10.
( N 92 )T -30
Ratio = (10)
( N 92 )T
where
Ratio = gyration ratio
(N92)T-30 = gyrations to 92% relative density at 30°C below the planned compaction
temperature.
(N92)T = gyrations to 92% relative density at the planned compaction temperature
10. The compactability is acceptable if the gyration ratio is less than 1.25.
Conduct Performance Testing
Like HMA, the final stage of laboratory work in a WMA design is evaluating the performance
of the mixture. Performance evaluation for WMA is the same as HMA and includes evaluation
of the resistance to moisture damage for all mixtures and evaluation of rutting resistance for mix-
tures designed for traffic levels of 3 million ESALs and higher. As discussed above, the primary
difference between WMA and HMA is in mixture conditioning--HMA mixtures are condi-
tioned for 4 hours at 135°C, whereas WMA mixtures for performance testing are conditioned at
the planned compaction temperature for 2 hours. Otherwise, the procedure for short-term con-
ditioning for mixture mechanical testing as described in AASHTO R 30 should be followed when
preparing WMA specimens for performance testing.
The resistance to moisture damage is evaluated using AASHTO T 283, Resistance of Com-
pacted Asphalt Mixture to Moisture-Induced Damage. Like HMA, WMA mixtures have accept-
able resistance to moisture damage if the tensile strength ratio is equal to or greater than 80%
and there is no visual evidence of stripping in the conditioned test specimens. When redesign-
ing an existing HMA mixture using a WMA process that does not include an antistrip additive,
it is not uncommon to find that the WMA mixture is less resistant to moisture damage. For these
mixtures, the use of hydrated lime or an antistrip additive will usually provide acceptable results.
Many WMA processes include an antistrip additive. For these processes, consult the process sup-
plier for recommendations to improve resistance to moisture damage if unacceptable results are
obtained at normal WMA additive dosage rates.
The rutting resistance of WMA is evaluated for mixtures designed for 3 million ESALs and
higher using the flow number test, AASHTO TP 79, Determining the Dynamic Modulus and Flow
Number of Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT).
The same testing conditions used for HMA are used with WMA.
