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CHAPTER 4
Conclusions and Recommendations
4.1 Conclusions to aid in implementing the research conducted in NCHRP
Project 09-43.
The objective of NCHRP Project 09-43 was to develop mix- The draft appendix to AASHTO R 35 addresses the most
ture design and analysis procedures that can be used with the widely used WMA processes, including (1) additives that
wide range of WMA processes that are currently available or are added to the binder, (2) additives that are added to the
are likely to become available in the future. The research con- mixture during production, (3) wet aggregate mixtures, and
ducted under NCHRP Project 09-43 included the following: (4) plant foaming systems. The following unique aspects of
WMA mixture design are addressed by the draft appendix to
1. Development of a preliminary procedure based on a review AASHTO R 35:
of the literature and research in progress.
2. A first phase of testing and analysis to investigate critical · Process-specific specimen-fabrication procedures,
aspects of the preliminary procedure, including (1) the effect · An evaluation of coating at the planned production
of sample reheating, (2) binder grade selection, (3) mixing temperature,
of RAP and new binders at WMA process temperatures, · An evaluation of compactability at the planned field com-
(4) appropriate short-term oven conditioning for WMA, paction temperature and lower using the Superpave gyra-
and (5) evaluation of devices to measure workability. tory compactor, and
3. Revisions to the preliminary procedure based on the find- · A check on rutting resistance using the flow number test.
ings of the first phase of testing and analysis.
4. A second phase of testing and analysis to evaluate the The standard practice for measuring performance proper-
revised preliminary procedure. This phase included (1) a ties for WMA describes how to prepare WMA performance
mix design study to test the engineering reasonableness, test specimens and conduct dynamic modulus and low-
sensitivity, and practicality of the revised preliminary pro- temperature creep compliance and strength tests to obtain
cedure; (2) a field validation study that used properties of material properties for analysis using the MEPDG (1). The
laboratory- and field-produced WMA to validate the pro- sections that follow describe specific conclusions from the
cedure; and (3) a fatigue study to investigate whether lower research completed in NCHRP Project 09-43.
WMA temperatures improve mixture fatigue properties.
5. Final revision of the preliminary procedure based on the
4.1.1 Volumetric Properties
findings of the second phase of testing and analysis.
A major conclusion drawn from the research conducted
The primary products of NCHRP Project 09-43 are (1) a under NCHRP Project 09-43 was that the volumetric prop-
draft appendix to AASHTO R 35 titled Special Mixture Design erties of properly designed WMA and HMA mixtures are very
Considerations and Methods for Warm Mix Asphalt (WMA) similar. For HMA mixtures with 1.0-percent binder absorp-
and (2) a draft standard practice titled Standard Practice for tion or less, the volumetric properties of WMA designed with
Measuring Properties of Warm Mix Asphalt (WMA) for Per- the procedures developed in NCHRP Project 09-43 were
formance Analysis Using the Mechanistic-Empirical Pavement essentially the same as those obtained from an HMA design.
Design Guide Software. Training materials and a commen- This conclusion supports the current practice of substitut-
tary for the draft appendix to AASHTO R 35 were developed ing a WMA process into an approved HMA mixture design.
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However, the compactability, moisture sensitivity, and rut- however, is less than that obtained using the AASHTO R 30
ting resistance of the WMA may be significantly different conditioning for performance testing--4 h at 275°F (135°C).
than those of the HMA. Each of these is evaluated directly in
the recommended WMA mixture design method.
4.1.5 Coating, Workability,
and Compactability
4.1.2 Binder Grade Selection
For the wide range of WMA processes available, viscosity-
The same grade of binder should be used in WMA and HMA based mixing and compaction temperatures cannot be used
mixtures designed for the same project location. Although the to control coating, workability, and compactability. The draft
RTFOT experiment that was conducted in Phase I of NCHRP appendix to AASHTO R 35 uses direct measures of coating and
Project 09-43 showed a significant effect of temperature on the compactability on laboratory-prepared mixtures. The degree
high-temperature grade of the binder, recovered binder test of coating obtained in the laboratory depends on the type of
data from projects sampled and tested in Phase II of the proj- mixer that is used. The mixing times included in the draft
ect indicated that only extremely low production tempera- appendix to AASHTO R 35 were developed using a planetary
tures resulted in a significant decrease in the stiffness of the mixer with a wire whip. If bucket mixers are used, appropriate
recovered binder from the mixture. WMA production tem- WMA mixing times should be established by evaluating the
perature showed a minor improvement in the low-tempera- coating of HMA mixtures prepared for various mixing times at
ture grade of binders in both the RTFOT experiment and the the appropriate viscosity-based mixing temperature specified
recovered binder testing. The draft appendix to AASHTO R in Section 8.2.1 of AASHTO T 312.
35, therefore, recommends that the same grade of binder be Several workability devices were evaluated under NCHRP
used in both WMA and HMA mixtures. High-temperature Project 09-43. These devices, which measure the torque or
grade bumping may be necessary for WMA processes with force required to move an auger or blade through the mix-
extremely low production temperatures to meet the flow ture, were able to measure differences between HMA and
number rutting resistance requirements included in the draft WMA mixtures, but only when temperatures dropped to the
appendix. compaction range of WMA. At these temperatures, differ-
ences in air voids also were evident in gyratory-compacted
specimens. The draft appendix to AASHTO R 35 uses the
4.1.3 RAP in WMA
change in the number of gyrations to 92-percent relative
RAP and new binders do mix at WMA process tempera- density when the compaction temperature is decreased 54°F
tures provided the mixture is held at elevated temperatures (30°C) to characterize the compaction temperature sensitiv-
for a sufficient length of time. Because the mixing is time ity of the WMA processes. Increases that exceed 25 percent
dependent, it appears that the new binder added to the mix- indicate that the WMA is more temperature sensitive than
ture coats the virgin aggregate and RAP; then, during storage HMA. This measure of compactability is sensitive to the com-
at elevated temperature, the two binders continue to mix. paction temperature, the WMA process, and the presence of
In the laboratory mixing studies that were conducted, 2 h RAP in the mixture. The combination of RAP and low WMA
of conditioning at the compaction temperature resulted in production and compaction temperatures may lead to WMA
substantial mixing of RAP and new binders when the com- mixtures that are more sensitive to changes in temperature
paction temperature exceeded the high-temperature grade of than similar HMA mixtures.
the "as recovered" RAP binder. To ensure good mixing of
RAP and new binders, the draft appendix to AASHTO R 35
4.1.6 Moisture Sensitivity
recommends that the planned field compaction tempera-
ture for WMA exceed the high-temperature grade of the "as Moisture sensitivity as measured by AASHTO T 283 will
recovered" RAP binder. likely be different for WMA and HMA mixtures designed
using the same aggregates and binder. WMA processes that
included anti-strip additives improved the tensile strength
4.1.4 Short-Term Oven Conditioning
ratio of some of the mixtures included in the NCHRP Project
Short-term oven conditioning is included in mixture design 09-43 testing and analysis. Of the nine WMA mixtures that
to simulate the absorption and aging of the binder that occurs used a WMA process that included an anti-strip additive, the
during construction. For WMA, it is appropriate to use 2 h of tensile strength ratio remained the same or improved in 67 per-
oven conditioning at the planned field compaction tempera- cent of the mixtures. For WMA mixtures produced using
ture, the same short-term conditioning that is used for design processes that did not include anti-strip additives, the tensile
of HMA mixtures. The degree of binder aging that occurs, strength ratio never improved and decreased in 79 percent of
