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CHAPTER 2
Research Approach
2.1 Overview acceptance by industry. Performance tests are available for
measuring mixture modulus, rutting resistance, and resis-
The general approach taken in NCHRP 09-43 to develop tance to fatigue cracking and thermal cracking. The new mix
mix design and analysis procedures for WMA was to adapt as design manual being developed under NCHRP Project 09-33
many of the current methods used with HMA as possible and includes performance testing to ensure that mixtures sub-
to concentrate development efforts on areas where WMA and jected to traffic levels greater than 3 million equivalent single
HMA differ substantially. Figure 1 presents a flow chart for the axle loads have adequate rutting resistance (6).
project. The project was divided into two phases. In Phase I, Several modifications to current HMA mix design proce-
a preliminary mixture design and analysis procedure was dures are needed to address the wide range of WMA processes
developed based on a review of the literature and research in currently available and likely to become available in the future.
progress. The preliminary procedure was then revised based The first step in NCHRP Project 09-43 was to identify poten-
on the results of several laboratory studies directed at elements tial areas of the HMA mixture design process requiring mod-
of the mixture design process that were expected to be differ- ification for WMA. These are summarized in Table 2 and
ent for WMA as compared to HMA. In Phase II, the revised discussed below for the major steps in the mixture design and
preliminary procedure was evaluated through a laboratory analysis process.
sensitivity study designed to test the engineering reasonable-
ness, sensitivity, and practicality of the mixture design proce-
dure and a field validation study using mixtures from paving 2.2.1 Materials Selection
projects. Phase II also included a study to evaluate fatigue Some elements of materials selection may require modifi-
characteristics of WMA, the development of draft standards cation for WMA. Aggregate requirements for warm mix will
for WMA, and the development of workshop materials for the not be different than requirements for hot mix, but it may
proposed WMA mixture design methods. be necessary to select different binder grades for WMA. The
lower temperatures used in WMA as compared to HMA prob-
ably result in less aging during plant mixing and construction;
2.2 Differences Between the Design
therefore, a stiffer high-temperature binder grade may be
of WMA and HMA
needed for satisfactory rutting performance. This effect, how-
HMA mixture design and analysis generally consists of five ever, may be offset by the addition of warm mix additives and
major steps: (1) materials selection, (2) design aggregate struc- the effect that these additives and water have on binder aging.
ture, (3) design binder content selection, (4) evaluate mois- The lower production temperatures may also limit the types
ture sensitivity, and (5) performance analysis. Criteria for and quantity of recycled asphalt materials that can be used in
Steps 1 through 4 for HMA are contained in AASHTO M 323, WMA. Design of HMA assumes substantial mixing of new
Standard Specification for Superpave Volumetric Mix Design. and recycled binders, which may not be possible at the lower
AASHTO R 35, Standard Practice for Superpave Volumetric production temperatures used in warm mix. Lower produc-
Design for Hot Mix Asphalt (HMA), provides procedures for tion temperatures may also limit the effectiveness of some
Steps 1 through 4. Although there is not a standard practice anti-strip additives. Finally, WMA design will require the selec-
addressing performance testing of HMA, several performance tion of an appropriate warm mix additive and dosage rate.
tests have been developed and have received some level of Although dosage rates may be provided by warm mix additive
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Review Literature and
Research in Progress
Develop Preliminary
Mixture Design and
Analysis Procedure
Design and Execute
Phase I Laboratory
Studies
Revise Preliminary
Mixture Design and
Phase I Analysis Procedure
Phase II
Design and Execute Design and Execute Design and Execute
Laboratory Mix Design Laboratory Fatigue Field Verification Study
Study Study
Prepare Draft Mixture
Design and Analysis
Procedures
Prepare Workshop
Materials
Prepare Final Report
Figure 1. Flow chart for NCHRP Project 09-43.
Table 2. Areas of HMA mixture design and analysis potentially requiring modification for WMA.
Step Item Special Warm Mix Considerations
· Potentially less aging during mixing and construction due to lower production temperatures.
Binder Selection
· Effect of any warm mix additives and warm mix processing on binder properties.
Aggregate Properties · None
Materials · Effect of production temperature on the degree of commingling of recycled and new binders.
Selection Recycled Asphalt Pavement · Effect of warm mix additives and warm mix processing on the degree of commingling of recycled
and new binders.
· Warm-mix additive selection.
Additives
· Effect of lower production temperatures and warm mix additives on anti-strip additives.
Nominal Maximum Aggregate
· None
Size
Trial Gradations · None
Batching · WMA process specific
· WMA process specific
Design Aggregate
Mixing · Method to determine appropriate mixing temperatures for warm mix processes.
Structure
· Method to assess workability of WMA.
Conditioning · Verify that short-term conditioning per AASHTO R 30 applies to WMA processes.
· Method to determine appropriate compaction temperatures for warm mix processes.
Compaction
· Verification of compaction levels.
Volumetric Analysis and Criteria · None
Design Binder Specimen Preparation · See considerations above for laboratory batching, mixing, conditioning, and compaction.
Content Selection Volumetric Analysis and Criteria · None
Evaluate Specimen Preparation · See considerations above for laboratory batching, mixing, conditioning, and compaction.
Moisture
Sensitivity
Testing and Analysis · None
Performance Specimen Preparation · See considerations above for laboratory batching, mixing, conditioning, and compaction.
Analysis Testing and Analysis · None
