Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 28
II. Commentary on Special Mixture
Design Considerations and Methods
for Warm Mix Asphalt (WMA)
Part I of this report describes recommended procedures for designing dense-graded, asphalt
concrete mixtures that will be produced using any one of several currently available WMA
processes. These WMA mix design recommendations are based on research conducted in
National Cooperative Highway Research Program (NCHRP) Project 9-43, "Mix Design Practices
for Warm Mix Asphalt," which concluded that only minor modification of current mix design
practice is needed to address WMA. Although the procedures described have been specifically
selected for use in designing dense-graded mixtures, most can be applied to the design of other
mix types with little or no modification.
The following sections of this Part II of the report are a commentary that presents supporting
information from the NCHRP 9-43 research report for the recommendations included in Part I.
Many of these sections also include recommended additional research, because NCHRP Project
9-43 was the first major study addressing WMA mixture design, and some of the findings require
further validation through additional research. Both Parts I and II are organized around the eleven
steps described in Chapter 8 of NCHRP Report 673: A Manual for the Design of Hot Mix Asphalt
with Commentary for the design of dense-graded HMA. Table 1 summarizes the differences
between WMA and HMA design for each of the eleven steps.
Step 1. Gather Information
For the design of WMA, additional information must be collected on the WMA process that
will be used, additive rates, and planned production and compaction temperatures. The reason
that this information is needed is the design of WMA uses process-specific specimen fabrication
procedures. These specimen fabrication procedures were designed to simulate, in an approxi-
mate manner, the WMA process in the field. For the purposes of mixture design, the various
WMA processes can be 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.
Specimen fabrication techniques are somewhat different for each of these categories. Given that
viscosity-based mixing and compaction temperatures are not applicable to many WMA processes,
the planned production and compaction temperatures are used in the WMA mixture design process
to evaluate coating and the compactability/workability of the WMA. It should be emphasized that
the optimal production and compaction temperatures are different for the various WMA processes
and should be carefully considered when selecting production and compaction temperatures to be
used in the WMA design process.
C-1
