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OCR for page 37
C-10 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Step 9. Calculate Trial Mixture Proportions by Weight
and Check Dust-to-Binder Ratio
These calculations are identical to those for HMA.
Step 10. Evaluate and Refine Trial Mixtures
This step involves the preparation and evaluation of laboratory specimens of WMA. The pro-
cedure follows that for HMA with slight modification. Table 7 summarizes 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.
3. Process-specific specimen fabrication procedures are used to prepare laboratory mixtures.
4. The short-term conditioning temperature for WMA is the planned compaction temperature.
5. Viscosity-based compaction temperatures are not used with WMA. Laboratory compaction
is done at the planned compaction temperature.
6. WMA design includes an evaluation of coating and compactability using the planned pro-
duction and compaction temperatures.
Supporting data from NCHRP Project 9-43 for these modifications are discussed in the
sections that follow.
Additive Dosage
The computation of WMA additive dosage rates is straightforward. The amount of additive
needed may be specified by the WMA process supplier as percent by weight of binder or total
Table 7. 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 Short-term oven X X WMA uses lower temperature.
conditioning
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
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-11
mixture. For wet aggregate processes, water is added to a portion of the fine aggregate, and then
this wet, fine aggregate is added cold to the mixture during the mixing process. The proportion
of the aggregate that is added wet and the moisture content are provided by the WMA technol-
ogy provider.
Mixing Temperatures
Viscosity-based mixing temperatures cannot be used with the wide range of WMA processes
currently available. Laboratory specimens are mixed at the planned production temperature, and
coating is evaluated to determine the acceptability of the WMA process.
Process-Specific Specimen Fabrication Procedures
For mixture design, the various WMA processes were grouped into four generic categories:
1. Additives blended into the binder,
2. Additives added to the mixture,
3. Wet aggregate mixtures, and
4. Foamed asphalt.
The procedures in the report address laboratory mixing. These were developed based on
recommendations from various WMA technology providers and verified during the mix design
experiment completed in NCHRP Project 9-43. Once mixing is complete, specimen fabrication
for all processes continues with short-term conditioning and specimen compaction. These steps
are the same for all processes and the same as done with HMA.
WMA mixture designs will require additional equipment. Since coating is used in lieu of
viscosity-based mixing and compaction temperatures, a mechanical mixer is required. During
NCHRP Project 9-43, it was observed that planetary mixers and bucket mixers do not have the
same mixing efficiency. Planetary mixers are more efficient. The specimen fabrication proce-
dures were developed in NCHRP Project 9-43 using a planetary mixer. For WMA processes
where the additive is blended in the binder, a mechanical stirrer is needed. For designing mixtures
for plant foaming processes, a laboratory foamed asphalt plant that can produce foamed asphalt
at the moisture content used by the field equipment is also needed. NCHRP Project 9-43 demon-
strated that it is feasible to perform foamed asphalt WMA mixture designs in the laboratory. In
NCHRP Project 9-43, a modified Wirtgen WLB-10 laboratory foaming plant was used to simu-
late the Gencore Ultrafoam GX process using 1.25% water by weight of binder and the Astec
Double Barrel Green process using 2.0% water by weight of binder. The modification that was
required was to the replace the flow controller with a smaller, more precise flow controller to
accommodate the water contents used in WMA mixtures.
Short-Term Conditioning
Short-term conditioning for WMA was set at 2 hours at the planned compaction temper-
ature to represent the absorption and binder stiffening that occurs during construction. This
level of conditioning is used for the volumetric design and for the moisture sensitivity and
rutting evaluation. These conditions were selected based on comparisons of properties of
laboratory-mixed, laboratory-compacted specimens with those from field-mixed, laboratory-
compacted specimens.
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C-12 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Figures 5 and 6 summarize the results of comparisons of maximum specific gravity and
indirect tensile strength for the field sections tested in NCHRP Project 9-43. The error bars shown
in Figure 5 are the single operator D2s precision from AASHTO T 209. These data show that
the maximum specific gravity of the lab and field mixtures are the same, indicating that the
binder absorption is the same for the lab and field mixtures. The aggregate water absorption
ranged from 0.5% for the Pennsylvania SR2007 mixtures to 2.5% of the Yellowstone National
Park mixtures.
Figure 6 shows average differences in indirect tensile strength for the field mixtures minus
the laboratory mixtures. The error bars in this figure are 95% confidence intervals for a paired
t-test comparison. If the error bars do not capture zero, then the difference in the tensile strength
of the field- and laboratory-mixed specimens is different from zero. Figure 6 shows that several
mixtures have significantly different tensile strengths. The differences are not consistently in
one direction except for the Pennsylvania SR2006 project, where the field-mixed specimens
always have significantly higher tensile strengths compared to the laboratory-mixed specimens.
Given that one-third of the mixtures were from this project, this difference biased the results.
The average difference for all projects was 7 psi (48 kPa); not considering the Pennsylvania
SR2006 project, the average difference was essentially zero.
Short-term conditioning for performance evaluations, moisture sensitivity, and rutting was
one of the areas where additional research was recommended in NCHRP Project 9-43. This addi-
tional research was recommended because it appears that the current HMA short-term condition-
ing procedure for performance evaluation, 4 hours at 275°F (135°C), represents the stiffening that
occurs during construction and some short time in service.
Compaction Temp 2 Hours Field Mix
Monroe NC Astec
PA SR2006 Sasobit
PA SR2006 LEA
PA SR2006 Gencor
PA SR2006 Advera
PA SR2006 Control
Mixture/Process
PA SR2007 Evotherm
PA SR2007 Control
YNP Sasobit
YNP Advera
YNP Control
CO I-70 Sasobit
CO I-70 Evotherm
CO I-70 Advera
CO I-70 Control
2.300 2.400 2.500 2.600
Maximum Specific Gravity
Figure 5. Comparison of maximum specific gravity
between field mixes and laboratory mixes short-term
conditioned 2 hours at the compaction temperature.
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-13
55
45
IDT Strength Differences, psi
35
25
15
5
YNP Sasobit
PA SR2006 Sasobit
CO I-70 Sasobit
YNP Advera
YNP Control
Average
PA SR2007 Evotherm
PA SR2006 Gencor
PA SR2006 Advera
CO I-70 Evotherm
CO I-70 Advera
PA SR2007 Control
PA SR2006 Control
CO I-70 Control
PA SR2006 LEA
Monroe NC Astec
-5
-15
-25
Mixture/Process
Figure 6. Differences in indirect tensile strength between field mixes
and laboratory mixes short-term conditioned 2 hours at the compaction
temperature.
Compaction Temperatures
Viscosity-based compaction temperatures cannot be used with the wide range of WMA
processes currently available. Laboratory specimens are compacted at the planned compaction
temperature. Additionally a compactability evaluation is conducted to ensure that the mixture
is compactable at the planned compaction temperature.
WMA Evaluations
Four evaluations are conducted on WMA mixtures at the design binder content: (1) coating,
(2) compactability, (3) moisture sensitivity, and (4) rutting resistance. The sections below
describe the supporting information from NCHRP Project 9-43 for these evaluations.
Coating
Coating is one way to evaluate proposed WMA production temperatures that is relevant to all
WMA processes. In NCHRP Project 9-43, coating was evaluated on a number of HMA and WMA
mixtures using AASHTO T 195. AASHTO T 195 counts the percentage of the coarse aggregates in
the mixture that are fully coated. This is a strict criterion. When a planetary mixer was used, coat-
ing was always found to be nearly 100 percent for both WMA and HMA. When a bucket mixer was
used with a smaller number of WMA mixes, the coating was much lower. This indicates that the
bucket mixer is less efficient than the planetary mixer. The criterion of 95% was based on the plan-
etary mixer data. Though bucket mixers are less efficient than planetary mixers, they are signifi-
cantly less expensive and likely more readily available in mix design laboratories. Until additional
research is conducted to develop appropriate mixing times for bucket mixers, technicians and engi-
neers will have to develop mixing times for their WMA mixtures based on coating evaluations for
HMA mixtures produced using the traditional viscosity-based mixing temperatures.
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C-14 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Compactability
The compactability evaluation is used in lieu of the viscosity-based mixing temperature used
for HMA. Compactability is evaluated by compacting specimens to Ndesign at the planned field
compaction temperature and again at 54°F (30°C) below the planned field compaction temper-
ature. The number of gyrations to reach 92% relative density is then calculated from the height
data. The ratio of the gyrations to 92% relative density at the lower temperature to the higher
temperature should be less than 1.25.
The methodology for the compactability evaluation resulted from a workability study conducted
in NCHRP Project 9-43. The workability study evaluated the feasibility of using various workabil-
ity devices and the gyratory compactor to measure WMA workability during the mixture design
process. The workability study demonstrated that it is possible to measure differences in the
workability and compactability of WMA compared to HMA. The differences, however, were only
significant at temperatures that are below typical WMA discharge temperatures. Figures 7 and 8
show the effect of WMA process and temperature on workability and compactability.
Given that the workability devices were not able to discriminate more precisely than compaction
data obtained from a standard Superpave gyratory compactor, the method for evaluating the tem-
perature sensitivity of the compactability of WMA was developed for assessing WMA workability
and compactability. The method involves determining the number of gyrations to 8% air voids at
the proposed compaction temperature and a second temperature that is approximately 54°F
(30°C) lower than the proposed compaction temperature. A tentative limit allowing a 25% increase
in the number of gyrations when the temperature is decreased was developed. This limit was inves-
tigated using data from nine WMA field mixture projects sampled in NCHRP 9-43. The increase
in gyrations for the WMA processes ranged from 0 to 20%. Workability and compactability was
not reported to be a problem on any of the projects.
Moisture Sensitivity
Moisture sensitivity is evaluated using AASHTO T 283, the same as HMA. The criterion for
AASHTO T 283 is the same as that for HMA.
Control Advera Sasobit
450
400
350
300
Torque, in-lb
250
200
150
100
50
0
300 250 190 150
Temperature, F
Figure 7. Effect of temperature and WMA additive on
torque measured in the UMass workability device.
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II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) C-15
Control Advera Sasobit
45
40
35
30
Gyrations
25
20
15
10
5
0
300 250 190
Temperature, F
Figure 8. Effect of temperature and WMA additive on gyrations
to 92% relative density.
Tests conducted during NCHRP Project 9-43 showed that the moisture sensitivity will likely be
different for WMA and HMA mixtures designed using the same aggregates and binder. WMA
processes that included antistrip additives improved the tensile strength ratio of some of the mix-
tures included in the NCHRP Project 9-43 testing and analysis. Of the nine WMA mixtures that used
a WMA process that included an antistrip additive, the tensile strength ratio remained the same or
improved in 67% of the mixtures. For WMA mixtures produced using processes that do not include
antistrip additives, the tensile strength ratio never improved and decreased in 79% of the mixtures.
Rutting Resistance
Rutting resistance is evaluated using the flow number test, AASHTO TP 79. The same testing
conditions that are used for HMA flow number testing are used with WMA:
· Air voids of 7.0 ± 0.5%
· 50% reliability high pavement temperature from LTPPBind 3.1 for the project location,
20 mm below the pavement surface, or 20 mm below the top of the sub-surface pavement layer
of interest
· Unconfined
· Repeated deviator stress of 87 psi (600 kPa), contact deviator stress of 4.4 psi (30 kPa),
Minimum flow numbers as a function of traffic level are provided and these are lower than
those for HMA. Table 8 compares the recommended flow numbers for WMA and HMA. The
Table 8. Flow number criteria for WMA
and HMA mixtures.
Traffic Level, Minimum Flow Number
Million ESALs WMA HMA
<3 NA NA
3 to < 10 30 50
10 to < 30 105 190
30 415 740
