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suppliers, agencies should have a procedure to ensure that the 2.3 Preliminary WMA Mixture
recommended dosage rate is appropriate. Design and Analysis Procedure
2.3.1 Overview
2.2.2 Design Aggregate Structure
Based on a review of available literature for the various
The design of the aggregate structure may also require some WMA processes and discussions with WMA process develop-
modifications for WMA. Since the goal of WMA is to produce ers, a preliminary mixture design and analysis procedure was
mixtures with strength and performance characteristics simi- developed. The preliminary procedure served two purposes.
lar to those of HMA, the volumetric criteria used in design First, the preliminary procedure provided a starting point for
should not differ from those used for HMA. However, the pro- the WMA mixture design and analysis procedure for review
cedures used to fabricate and condition specimens may require and comment by the project panel and WMA process develop-
some modification. Most WMA process developers have ers. Second, the preliminary procedure focused the Phase I
prepared laboratory procedures for specimen fabrication. testing and analysis effort on the areas of mixture design and
Additionally, mixture coating, workability, and compactabil- analysis that required additional development to properly
ity must be evaluated directly instead of using viscosity- address WMA. The preliminary procedure was revised based
based mixing and compaction temperatures. In many WMA on the findings of the Phase I testing and analysis and the com-
processes, it is impossible to directly measure the viscosity of ments received on the preliminary procedure. The revised pre-
the binder. Additionally, there is increasing evidence that the liminary procedure was further modified based on the findings
temperature reductions associated with many WMA processes of the Phase II laboratory mix design study, field validation
are not related to the change in viscosity of the binder (3, 7). study, and fatigue study to produce the draft standards that
were the primary products of NCHRP Project 09-43.
2.2.3 Design Binder Content Selection The preliminary WMA mixture design and analysis proce-
dure was based on AASHTO R 35, Standard Practice for Super-
The selection of the design binder content should not require pave Volumetric Design for Hot Mix Asphalt (HMA). The
substantial modification other than specimen-fabrication as preliminary procedure referred to AASHTO M 323, Stan-
discussed above. In NCHRP Project 09-25, "Requirements dard Specification for Superpave Volumetric Mix Design, and
for Voids in Mineral Aggregate for Superpave Mixtures" and AASHTO M 320, Standard Specification for Performance-
NCHRP Project 09-31, "Air Void Requirements for Superpave Graded Asphalt Binder, for criteria for materials selection, vol-
Mix Design," relationships between mixture volumetric prop- umetric design, and moisture sensitivity evaluation. Table 3
erties and pavement performance were developed (8). These summarizes the areas where the preliminary procedure dif-
relationships confirm the importance of many of the volumet- fered from AASHTO R 35. The differences are discussed below.
ric criteria included in the Superpave mixture design method.
An important step in achieving WMA with performance char-
2.3.2 WMA Process Selection
acteristics comparable to HMA is to use the same volumetric
criteria in the design of both mixtures. A section in the preliminary mixture design and analysis
procedure for WMA addressed WMA process selection. It
advised that WMA process selection should be done in consul-
2.2.4 Evaluate Moisture Sensitivity
tation with the specifying agency and technical assistance per-
and Performance Analysis
sonnel from WMA process suppliers. This section alerts users
Evaluation of the mixture for moisture sensitivity and per- that when selecting a WMA process, consideration should be
formance also will not require substantial modification other given to a number of factors including (1) available perfor-
than specimen fabrication. Although there is concern that some mance data, (2) the cost of any warm mix additives, (3) planned
WMA may exhibit greater moisture sensitivity than HMA plant mixing and field compaction temperatures, (4) planned
(9, 10, 11), AASHTO T 283, Resistance of Compacted Hot Mix production rates, (5) plant capabilities, and (6) modifications
Asphalt (HMA) to Moisture-Induced Damage, is a fairly reli- required to successfully use the WMA process with available
able indicator of moisture-induced adhesive failure, which field and laboratory equipment.
is the mechanism of greatest concern for WMA. The major
consideration in the preparation of moisture sensitivity and
2.3.3 Binder Grade Selection and RAP
performance specimens will be replicating the mechanical
properties of field-mixed material in laboratory-prepared spec- For the preliminary procedure, it was hypothesized that the
imens. The same tests and criteria that are used for performance WMA production temperature would be a consideration in the
evaluation of HMA should be used with WMA. selection of the high-temperature binder grade and the allow-
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Table 3. Major new elements included in the preliminary
WMA mixture design and analysis procedure.
Key Element Included in Preliminary Procedure
Key considerations for WMA process
WMA Process Selection
selection.
Concept of high-temperature grade bumping
Binder Grade Selection
based on WMA production temperature.
Concept of limiting RAP content based on
Recycled Asphalt Pavement
production temperature and compatibility of
Materials
new and recycled binders.
Process-specific fabrication procedures
Specimen-Fabrication Procedures
provided for major WMA processes.
Direct evaluation of coating, workability, and
Process Temperature
compactability.
Flow number in addition to moisture
Required Performance Testing
sensitivity testing in mixture design.
Recommended methods for measuring
dynamic modulus, resistance to fatigue
Optional Performance Testing
cracking, and resistance to thermal cracking
included in mixture analysis.
able RAP content of the mixture. Because the lower produc- 2.3.5 Process Temperature
tion temperatures in WMA result in reduced binder aging,
the preliminary procedure provided a conceptual table for Since binder viscosity-temperature relationships cannot be
bumping the high-temperature binder grade based on the developed for many WMA processes, mixing and compaction
planned production temperature. Similarly, since the degree temperatures cannot be used to control coating, workability,
of mixing of RAP and new binders in mixtures containing and compactability of WMA. The preliminary procedure
RAP is likely to be temperature dependent, the preliminary proposed evaluating coating, workability, and compactability
procedure provided a second conceptual table for limiting the directly during the evaluation of trial blends. This is accom-
RAP content of mixtures based on the production tempera- plished by preparing trial blends using the planned production
ture and the compatibility of the RAP and new binder. An temperature and compacting the trial blends using the planned
appendix was added to provide procedures for measuring the field compaction temperature. Coating is evaluated using
compatibility of two binders with ASTM D6703, Standard AASHTO T 195, Determining Degree of Particle Coating of
Test Method for Automated Heithaus Titrimetry. Experi- Bituminous-Aggregate Mixtures. A standard procedure for
ments to flesh out the conceptual tables for binder grade evaluating workability is not available; therefore, as part of the
bumping and RAP mixing were included in the Phase I test- Phase I testing and analysis several possible workability tests
ing and analysis. were evaluated. In the preliminary procedure for WMA, it was
In addition to the above, the preliminary procedure pro- envisioned that the density at Ninitial in the gyratory compactor
vided more detailed information on how to characterize RAP would serve as a measure of compactability. The use of the den-
materials for mixture design. This information was provided sity at Ninitial as a measure of compactability was evaluated dur-
in an appendix and was consistent with the recommendations ing the Phase I testing and analysis.
for RAP analysis that were included in the mix design manual
for HMA being developed under NCHRP Project 09-33 (6).
2.3.6 Required Performance Testing
Evaluation of the moisture sensitivity of the design mixture
2.3.4 Specimen-Fabrication Procedures
in the preliminary mixture design and analysis procedure
The preliminary design procedure documented specimen- for WMA is the same as that for HMA in AASHTO R 35.
fabrication procedures for several WMA processes. These pro- AASHTO T 283, Resistance of Compacted Hot Mix Asphalt
cedures identify the equipment and methods that are needed (HMA) to Moisture-Induced Damage, is used except that the
to prepare WMA specimens in the laboratory. These specimen- mixture conditioning procedure is the same as that used in the
fabrication procedures were included in an appendix to the volumetric design, tentatively 2 h at the compaction temper-
preliminary mixture design and analysis procedure for WMA. ature. The minimum tensile strength ratio is 0.80 as specified
Short-term aging of WMA was tentatively set at 2 h at the in AASHTO M 323 for HMA.
planned field compaction temperature based on limited The preliminary procedure for WMA mixture design and
research performed by some WMA process developers. analysis also included a mandatory evaluation of the design
