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Table 4. Minimum flow number acterize the fatigue resistance of a mixture using a limited
requirements. amount of testing (14). The same geometry specimen as used
for the dynamic modulus and flow number can be used in
Traffic Level, Minimum Flow
Million ESALs Number the direct tension-compression fatigue testing. With appro-
<3 --- priate tension grips, the test can be performed with the
3 to < 10 53
10 to < 30 190
AMPT.
30 740 · Thermal Cracking. The recommended method for analysis
of thermal cracking in flexible pavements requires measure-
ment of compliance and strength properties of the mixture
mixture for rutting resistance using the flow number test devel- at low temperatures. These properties are then used in a
oped in NCHRP Project 09-19 (12). This test is conducted thermo-viscoelastic stress analysis to estimate the thermal
using the Asphalt Mixture Performance Tester (AMPT) on stresses induced in the pavement during cooling cycles.
specimens that have been conditioned according to the volu- Mixture compliance and strength properties are obtained
metric design procedure, tentatively 2 h at the compaction by testing specimens in the indirect tensile (IDT) mode in
temperature. The AMPT was formerly called the Simple Per- accordance with AASHTO T 322, Determining the Creep
formance Test (SPT) system. The flow number test is con- Compliance and Strength of Hot Mix Asphalt (HMA) Using
ducted in accordance with AASHTO TP 79, Determining the the Indirect Tensile Device. Two software programs are
Dynamic Modulus and Flow Number for Hot Mix Asphalt available to perform the thermo-viscoelastic stress analysis.
(HMA) Using the Asphalt Mixture Performance Tester The first is the MEPDG, which includes a model to predict
(AMPT). The test is conducted unconfined with a repeated the extent of thermal cracking in a flexible pavement consid-
deviatoric stress of 87 psi (600 kPa) and a contact deviatoric ering environmental conditions at the project site and the
stress of 4.4 psi (30 kPa). The test temperature is the design thickness and properties of the asphalt concrete used in the
high pavement temperature at 50-percent reliability as deter- pavement. This model has been calibrated using data from
mined using LTPPBind Version 3.1 (13). The temperature is several in-service pavements (1). The second is an Excel
computed at a depth of 0.79 in. (20 mm) for surface courses, Workbook called "LTSTRESS.xls," which was developed at
and the top of the pavement layer for intermediate and base the Northeast Center for Excellence in Pavement Technol-
courses. Flow number criteria for various traffic levels are given ogy to reduce data from AASHTO T 322 and perform a sim-
in Table 4. These are the same criteria being recommended plified thermal cracking analysis (15). The output of this
for HMA in the mix design manual being developed under analysis is a critical cracking temperature, the temperature
NCHRP Project 09-33 (6). where the computed thermal stresses for a specified cooling
rate exceed the tensile strength of the mixture. LTSTRESS.xls
has not been calibrated to observed cracking and should be
2.3.7 Optional Mixture Analysis Tests
used for comparative evaluation of mixtures.
The preliminary mixture design and analysis procedure for
WMA included optional performance tests to evaluate the
2.4 Phase I Laboratory Studies
dynamic modulus, resistance to fatigue cracking, and resis-
tance to thermal cracking. Performance tests are not included In developing the preliminary mixture design and analysis
in AASHTO R 35 for HMA. The optional performance tests procedure for WMA, several areas were identified where lab-
can be used with the Mechanistic-Empirical Pavement Design oratory testing and analysis was needed to develop criteria for
Guide (MEPDG) to predict the performance of pavements the procedure. This section describes the laboratory studies
incorporating WMA (1). The following performance tests and that were conducted and analyzed during Phase I of the proj-
equipment were selected for the preliminary procedure: ect. Detailed results and analyses for each study are presented
in Appendix E and summarized in Chapter 3. The preliminary
· Dynamic Modulus. Dynamic modulus master curves for procedure was then revised based on the results of these
use in pavement structural design and performance analysis studies, and the revised preliminary procedure was used in the
using the MEPDG (1) can be developed using the AMPT in mix design, field validation, and fatigue studies. The resulting
accordance with AASHTO PP 61, Developing Dynamic draft standards for WMA mixture design and analysis are dis-
Modulus Master Curves for Hot-Mix Asphalt (HMA) Using cussed in Chapter 3.
the Asphalt Mixture Performance Tester.
· Fatigue Cracking. Fatigue characteristics of WMA are eval-
2.4.1 Phase I Data Sources
uated using simplified continuum damage analysis of cyclic
direct tension-compression tests. This procedure was devel- Data for the Phase I studies were collected from four sources:
oped in NCHRP Projects 09-25 and 09-31 to quickly char- the FHWA Mobile Asphalt Laboratory, FHWA Turner-
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Table 5. Phase I project mix design data. Table 6. Summary of the sample reheating study.
Property I-70 Mixture Immediate Delayed Reheated
Colorado HMA Control X X X
Sieve Size Control Aspha-min X X X
HMA Evotherm ET X X X
1/2 in 100.0 LEA X X
3/8 in 95.0 Sasobit X X X
Gradation #4 73.0
(% passing) #8 54.0
#16 40.0
#30 29.0 additional stiffening of the binder in the mixture. The effect of
#50 18.0 sample reheating was evaluated for a control HMA and four
#100 11.0
#200 6.7
WMA processes: Aspha-min, Evotherm ET, LEA, and Sasobit.
Asphalt Content, % 6.2 The data for the control HMA, Aspha-min, Evotherm ET, and
Ndesign 75.0 Sasobit mixtures were provided by the FHWA Mobile Asphalt
Design Air Voids, % 3.9
Design VMA, % 16.9
Laboratory. The FHWA provided data for a WMA project
Design VFA, % 77.0 constructed in St. Louis, Missouri, where modulus tests were
Fines to Effective Asphalt Ratio 1.0 performed for three conditions: (1) samples prepared at the
Fractured Faces (one face/two faces), % 100/99
Fine Aggregate Angularity (FAA) 48.6 time of construction and immediately tested, (2) samples pre-
Aggregate Water Absorption, % 0.8 pared at the time of construction, but tested weeks later, and
Dry Tensile Strength, psi 64.0 (3) reheated samples. McConnaughay Technologies prepared
Conditioned Tensile Strength, psi 58.0
Tensile Strength Ratio, % 91.0 dynamic modulus specimens for the LEA process during con-
Binder Grade PG 58-28 struction and the research team prepared an additional set of
dynamic modulus specimens after reheating. Both sets of LEA
specimens were tested by the research team. All of the dynamic
Fairbank Highway Research Center, McConnaughay Tech- modulus tests were conducted in accordance with AASHTO
nologies, and a WMA project on I-70 in Colorado that was PP 61. Table 6 summarizes the sample reheating study. The
sampled by the research team. The FHWA Mobile Asphalt data analysis consisted of comparing dynamic modulus mas-
Laboratory and McConnaughay Technologies provided mix- ter curves for the various sample preparation and testing
ture modulus data that were used to evaluate the effect of conditions.
sample reheating on the mechanical properties of WMA. The
FHWA Turner-Fairbank Highway Research Center provided
2.4.3 Binder Grade Study
data from an experiment that used the Rolling Thin Film
Oven Test (RTFOT) to evaluate the effect of temperature on The lower production temperatures used with WMA pro-
the short-term aging of asphalt binders. These data were used duce less aging of the binder during construction. This reduced
to develop preliminary recommendations for binder grade aging may result in increased rutting of pavements produced
selection for WMA as a function of production temperature. using WMA processes and it may also result in improved resis-
Samples of loose mix and component materials from the Col- tance to fatigue and low-temperature cracking. NCHRP
orado I-70 WMA project were used to evaluate short-term Project 09-43 included analysis of an experiment conducted by
oven conditioning and the mixing of RAP at WMA tempera- the FHWA where the effects of WMA production tempera-
tures. The Colorado I-70 project included an HMA control tures were simulated using the RTFOT (AASHTO T 240). In
mix and three WMA processes: Advera, Evotherm, and Saso- this experiment, binders were short-term aged in the RTFOT,
bit. Table 5 presents the approved mixture design for the Col- at temperatures of 325°F, 266°F, and 230°F (163°C, 130°C, and
orado I-70 HMA. The three WMA processes used this same 110°C). The high-temperature properties of the binders were
mix design without modification. then measured in accordance with AASHTO T 315 at multiple
temperatures to determine the continuous RTFOT high-
temperature grade of the binder. Low-temperature properties
2.4.2 Sample Reheating Study
for several of the binders were measured during NCHRP Proj-
Since some of the planned experiments involved mechani- ect 09-43 for RTFOT temperatures of 325°F and 230°F (163°C
cal property tests on specimens prepared from loose mix, a and 110°C). Low-temperature properties were measured in
study was conducted to determine if sample reheating sig- accordance with AASHTO T 313 at two temperatures to deter-
nificantly affected the mechanical properties of WMA. The mine the continuous low-temperature grade of the binder. The
response variable used in this study was the mixture dynamic RTFOT aged binders were further aged in the pressure aging
modulus because it is very sensitive to changes in binder stiff- vessel (PAV) in accordance with AASHTO R 28 at a tem-
ness, and it was expected that sample reheating might result in perature of 100°C prior to bending beam rheometer testing.
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Table 7. Binders used in FHWA RTFOT temperature experiment.
High Low
Binder Source
Temperature Temperature
B-6354 Missouri WMA PG 70-22 X X
B-6348 Hawaii PG 70-16 X X
B-6328 Venezuelan PG 64-22 X X
AAM-1 SHRP MRL X X
AAM-2 SHRP MRL X X
AAG-1 SHRP MRL X Not Tested
AAD-1 SHRP MRL X X
B6272 ALF PG 70-22 Control X X
B6272+1.5% Sasobit ALF PG 70-22 Control + Sasobit X Not Tested
B6272+3.0% Sasobit ALF PG 70-22 Control + Sasobit X Not Tested
Table 7 summarizes the RTFOT temperature experiment. that was used was to compare the maximum specific gravity,
The continuous high-temperature grade data for these binders dynamic modulus, and tensile strength of laboratory-prepared
were used to develop preliminary production temperature lim- mixtures with those from field mixtures. For convenience and
its below which consideration should be given to increasing the to properly assess the effect of WMA process temperature, the
high-temperature grade of the binder. The continuous low- short-term conditioning temperature was selected to be equal
temperature grade data were used to develop preliminary rec- to the compaction temperature. Conditioning times of 2 h and
ommendations for low-temperature binder grade selection 4 h were included in the experiment. The short-term oven con-
based on production temperature. ditioning experiment was completed for the Colorado I-70
mixtures, and a tentative short-term conditioning time was
2.4.4 Short-Term Oven Conditioning Study selected. This tentative conditioning time was then verified in
the Phase II field validation study.
An important step in mixture design and analysis is short-
term oven conditioning of laboratory-prepared loose mix
2.4.5 RAP Study
prior to compaction. Short-term oven conditioning simulates
the binder absorption and aging that occurs during construc- The primary concern when using RAP in WMA is whether
tion. The short-term oven conditioning recommended for the RAP and new binders mix at the lower temperatures used
HMA at the end of the Strategic Highway Research Program in WMA. In the preliminary mixture design procedure, it was
(SHRP) was 4 h at 275°F (135°C) for both volumetric design hypothesized that the allowable RAP content of WMA mix-
and performance testing (16). This was included in AASHTO tures would decrease as the production temperature decreased.
PP 2, Practice of Short and Long Term Aging of Hot Mix Two experiments were conducted in Phase I of NCHRP Proj-
Asphalt (HMA), which later became AASHTO R 30, Mixture ect 09-43 in an attempt to determine production temperatures
Conditioning of Hot-Mix Asphalt (HMA). To expedite the below which it may be necessary to limit the RAP content of
mixture design process and reduce the number of ovens WMA to some amount less than the amount allowed in HMA.
required for mixture design, the FHWA Mixtures and Aggre- The first experiment included measurements of interfacial
gates Expert Task Group (ETG) reviewed data concerning the mixing to determine whether thin films of new binder on RAP
effect of conditioning time and temperature on the volumetric binder actually mix and measurements of binder compatibil-
properties of asphalt mixtures. The ETG ultimately recom- ity to determine the effect of mixing on the properties of the
mended that the short-term oven conditioning time for mix- combined binder. The interfacial mixing measurements used
ture design be changed to 2 h at the compaction temperature atomic force microscope imaging of "film-on-film" interface
for aggregates with water absorption less than 4.0 percent. For contact lines. Asphalt binders including Advera and Sasobit
aggregates with greater water absorption and for performance WMA additives were used in the interfacial mixing measure-
testing, the short-term oven conditioning time remained 4 h at ments. Thin films of these WMA binders were cast onto a film
275°F (135°C). AASHTO R 30 was eventually modified to of binder that was previously aged in the PAV to simulate
reflect the ETG's recommendation. an aged RAP binder. The specific procedures for the "film-
Short-term conditioning of 2 h at the compaction tempera- on-film" imaging were developed by the Western Research
ture has been recommended by some WMA process develop- Institute during Phase I of the project. The compatibility
ers for mixture design. No recommendations have been made measurements were performed in accordance with ASTM
for short-term conditioning of WMA for performance testing. D6703, Standard Test Method for Automated Heithaus
In Phase I of NCHRP Project 09-43, an experiment was under- Titrimetry. As the compatibility of an asphalt binder changes,
taken to establish short-term oven conditioning times for both the physical properties change. Less compatible binders tend
volumetric design and performance testing. The approach to have more structure and more elastic properties. Compati-
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Table 8. Summary of compatibility testing.
RAP AAB-1 AAG-1 Yellowstone National Park
Content Neat 1.5 % 5% Neat 1.5 % 5% Neat 1.5 % 5%
(%) Sasobit Advera Sasobit Advera Sasobit Advera
0 X X X X X X X X
5 X X
15 X X
25 X X X X X
50 X X X X X
Blank cells were not tested.
bility measurements were made for three neat asphalt binders, term oven aged at the compaction temperature listed in
two WMA additives (Advera and Sasobit), one RAP binder, Table 9 prior to compaction. Duplicate dynamic modulus spec-
and four RAP percentages. Table 8 summarizes the compati- imens were prepared and tested for each mixture in accordance
bility testing. with AASHTO PP 61. The binder from one of the specimens
The second experiment in the RAP study was a laboratory was recovered in accordance with ASTM D5404. Dynamic
mixing experiment designed to assess the degree of mixing shear rheometer (DSR) frequency sweep tests were performed
between RAP and new binders at WMA process tempera- on the recovered binders in accordance with AASHTO T 315 to
tures. This experiment used an approach that was developed by determine binder modulus input values for the Hirsch model.
Advanced Asphalt Technologies, LLC, for the Maryland State
Highway Administration and the Pennsylvania Department of 2.4.6 Workability Study
Transportation to evaluate the acceptability of plant mixing of
mixtures containing RAP and recycled asphalt shingles (RAS) Phase I also included a screening study to select an appro-
(17). The approach involves comparing dynamic moduli mea- priate workability device for use in WMA mixture design. To
sured on mixture samples with dynamic moduli estimated using accommodate the wide range of WMA processes currently
the properties of the binder recovered from the mixture sam- available and expected in the future, the preliminary procedure
ples. The dynamic modulus test is very sensitive to the stiffness proposed evaluating coating, workability, and compactability
of the binder in the mixture, and adding RAP will increase the directly during the evaluation of trial blends and during the
dynamic modulus significantly when the RAP is properly mixed optimum binder content selection. Six potential workability
with the new materials. The dynamic modulus for the as-mixed tests were identified by the research team. Table 10 presents a
condition was measured in accordance with AASHTO PP 61. summary of key elements of these devices. After careful review
The dynamic modulus for the fully blended condition was esti- of the workability devices in Table 10, the following devices
mated using the Hirsch model (18) from the shear modulus of were selected for the Phase I screening test:
binder recovered from the dynamic modulus specimens.
· UMass Workability Device
Table 9 summarizes the experimental design for the labora-
· Nynäs Workability Device
tory mixing experiment. The experimental design included
· University of New Hampshire Workability Device
testing a control HMA and three WMA processes: Advera,
· Gyratory Compactor with Shear Stress Measurement
Evotherm, and Sasobit. Each of the four mixtures was tested
at two temperatures and three aging times. Each mixture was
The UMass, Nynäs, and University of New Hampshire work-
mixed at the mixing temperatures listed in Table 9, then short-
ability devices are shown in Figures 2, 3, and 4, respectively.
These devices measure either the torque (UMass and Univer-
Table 9. Experimental design for the sity of New Hampshire) or force (Nynäs) required to move a
laboratory RAP mixing experiment. blade through the mixture. The University of New Hampshire
device is very simple, consisting of a handheld drill with variable
Mixing/Compaction Conditioning Time
Process Temperatures (h) torque chuck clutch. The UMass and Nynäs devices are much
(°F) 0.5 1.0 2.0 more complex.
280/255 X X X Some gyratory compactors are equipped with devices that
Control
248/230 X X X
measure the force required to apply the gyratory compaction
248/230 X X X
Advera
230/212 X X X
angle. This measurement may be provided as a force or con-
248/230 X X X verted to stress based on the geometry of the equipment. The
Evotherm
230/212 X X X specific compactor used in the workability screening study was
Sasobit
248/230 X X X an Intensive Compaction Tester Model ICT 150R/RB manu-
230/212 X X X factured by Invelop Oy of Finland and shown in Figure 5.
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Table 10. Key elements of potential workability devices for WMA.
Modification of
Device Measurement Procedure Needed for Advantages Disadvantages
WMA
NCAT Prototype Torque to rotate paddle at None · Measure workability Requires new
Workability Device constant speed. during mixing. mixer.
· Previous research.
UMass Prototype Torque to rotate an auger at None · Measure workability Requires new
Workability Device constant speed. during mixing. mixer.
· Augur may better
represent field movement.
Modified Nynäs Force to push a blade into a Temperature control at · Simulates screed action. Requires new
Workability Device loose mix sample. WMA placement and · Relatively inexpensive. device.
compaction temperatures.
ASTM D6704 Force to push a blade into a Temperature control at · Simple and inexpensive. May not represent
loose mix sample. WMA placement and · Uses existing equipment. field conditions.
compaction temperatures.
Gyratory Shear Shear stress during None for gyratory · Measure workability Requires gyratory
Stress gyratory compaction. compactors with this during compaction. compactor with
capability. shear stress
measurement.
University of New Torque using blade None · Simple and inexpensive. Blade and drill
Hampshire attached to hand drill with · Can easily be performed torque settings need
adjustable torque settings. after mixing or prior to to be standardized.
compaction.
The primary concern in the initial screening study was the
effect of temperature and WMA additive on the workability of
the mixture. The Phase I screening experiment is summarized
in Table 11. It consisted of performing workability tests on a
single mixture produced with three binders: PG 64-28 control,
PG 64-28 with Sasobit, and PG 64-28 with Advera. Table 12
presents pertinent properties of the mixture used in the exper-
iment. Sasobit and Advera were selected as the warm mix addi-
tives because these additives are easy to use in the laboratory.
Duplicate workability tests were made with each device at three
temperatures. Analysis of variance was used to evaluate the
sensitivity of the test to changes in temperature and WMA
additive. The sensitivity of the test along with ease of integra-
tion into the WMA design procedure were the factors consid-
ered in the final selection.
Figure 2. UMass workability device. Figure 3. Nynäs workability device.
