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I. Special Mixture Design
Considerations and Methods for
Warm Mix Asphalt (WMA)
This report presents special mixture design considerations and methods used with warm mix
asphalt (WMA) and is a supplement to NCHRP Report 673: A Manual for Design of Hot Mix
Asphalt. In this report, all references to chapters refer to the corresponding chapters in NCHRP
Report 673. 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. Before reading this report, engineers and technicians should make certain they have
a thorough understanding of the dense-graded mix design process presented in Chapter 8 and the
procedures for incorporating RAP into hot mix asphalt (HMA) discussed in Chapter 9 of NCHRP
Report 673. NCHRP Report 714 is based on research conducted in NCHRP Project 9-43, "Mix
Design Practices for Warm Mix Asphalt," which concluded that only minor modifications of cur-
rent mix design practice are needed to address WMA. These modifications are discussed in detail
herein. Part II provides a commentary to support the proposed design considerations and methods.
What is WMA?
WMA refers to asphalt concrete mixtures produced at temperatures approximately 50°F (28°C)
or more cooler than typically used in the production of HMA. The goal with WMA is to produce
mixtures with similar strength, durability, and performance characteristics as HMA while using sub-
stantially reduced production temperatures. There are important environmental and health bene-
fits associated with reduced production temperatures including lower greenhouse gas emissions,
lower fuel consumption, and reduced exposure of workers to asphalt fumes. Lower production tem-
peratures can also improve pavement performance by (1) reducing binder aging, (2) providing
added time for mixture compaction, and (3) allowing improved compaction during cold weather
paving. For these reasons, many WMA technologies may also be incorporated in the production of
HMA at typical production temperatures during cold weather paving.
WMA technologies were first introduced in Europe in the late 1990s as one measure to reduce
greenhouse gas emissions. Since then, many WMA processes have been developed in Europe and
the United States. At the time this report was completed (2011), approximately 20 WMA
processes were marketed in the United States. These processes included chemical, wax, and syn-
thetic zeolite additives that can be blended with the binder or added to the mixture during pro-
duction; plant foaming systems; and sequential mixing processes. The National Asphalt Pavement
Association (NAPA) publication, Quality Improvement Series 125, "Warm-Mix Asphalt: Best
Practices," presents more detailed information on many of these processes including the types of
plant modifications needed with each.
Overview of WMA Design
The design of WMA is very similar to the design of HMA, following the 11 steps described in
Chapter 8 for the design of dense-graded HMA. Table 1 summarizes the differences between
WMA and HMA for each of the 11 steps.
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