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OCR for page 174
CHAPTER 10
Design of Gap-Graded
HMA Mixtures
Gap-graded HMA (GGHMA) consists of two parts: a coarse aggregate skeleton and a mortar.
The coarse aggregate skeleton consists of crushed coarse aggregate particles; these make up about
70 to 80% of the total aggregate blend. The mortar consists of asphalt binder, fine aggregate, and
mineral filler and fills the voids in the coarse aggregate skeleton.
Stone matrix asphalt (SMA), one particular type of GGHMA, is widely known for its durability
and rut resistance. SMA has been used in Europe for over 30 years. The first U.S. project that used
this high-performance HMA was constructed in 1991. Since then, the use of SMA has steadily
increased within the United States. The GGHMA discussed in this chapter is similar to SMA in
many ways, but there are some differences, so to avoid confusion or arguments over whether or
not the mix design presented in this chapter is truly an "SMA," the term GGHMA is used instead.
In Europe, SMA mixtures have primarily been designed by recipes. It was not until 1994 that
a formalized mix design procedure was available in the United States This mix design procedure
was developed by a Technical Working Group established by the FHWA. This procedure was based
on the Marshall mix design method, since this was the method used to design SMA in Europe.
In 1994, the National Center for Asphalt Technology (NCAT) began a 4-year study to develop a mix
design system to design SMA using the concepts and methods of the Superpave mix design system.
Results from this research project were published in 1998, and, along with more recent experience
and research, they are the basis for the GGHMA mix design system described in this chapter.
The philosophy of GGHMA mix design is not complicated. The first principle is that a gap-graded
blend of aggregate is needed so that the coarse particles will have stone-on-stone contact. The
second principle is that the voids within the coarse aggregate skeleton must be filled with fine
aggregate, mineral filler, and asphalt binder. In order to provide increased durability, GGHMA
has a relatively high asphalt binder content. This leads to the third principle of GGHMA mix
design, which is that the aggregate must have a high VMA value--typically 18 to 20%. This relatively
high asphalt binder content can result in an increased potential for draindown if not properly taken
into account. Draindown can be a common problem in GGHMA; it occurs when the asphalt binder
and fine aggregate separate from the coarse aggregate during storage, transport, or placement.
The fourth and final principle of GGHMA mix design is that small amounts of stabilizing additives,
such as mineral fibers or cellulose fibers, are usually needed to prevent draindown. The sections
below describe in detail how to design GGHMA to achieve the unique properties and excellent
performance for which this mix type is known.
Overview of GGHMA Mix Design Procedure
The mix design procedure for GGHMA contains five primary steps (Figure 10-1). First, suitable
materials must be selected to compose the GGHMA. Materials needed include coarse aggregates,
fine aggregates, mineral fillers, asphalt binder and stabilizing additives. The second step is to blend
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OCR for page 175
Design of Gap-Graded HMA Mixtures 175
Design of GGHMA
Mixtures
Identify Materials
Step 1
Select Aggregates
Stabilizer
Materials
Asphalt Binder
Determine VCA of
Select Trial coarse aggregate in dry -
Gradations rodded condition
Step 2
Low % passing Break Medium % passing Break High % passing Break
Point Sieve Point Sieve Point Sieve
Add 6.0% - 6.5% Within
asphalt binder Specifications
and compact
Analyze data
Step 3
and select
optimum gradation
Fix gradation
and vary asphalt
binder content
Step 4
Adjust asphalt
binder content as
needed
Conduct draindown, moisture
susceptibility, and performance testing.
Step 5
No
Meet all specifications ?
Yes
End
Figure 10-1. Flow diagram illustrating GGHMA mix design methodology.
three trial gradations. For each trial gradation, asphalt binder is added and the mixture compacted.
After each trial gradation has been compacted, mixture volumetric data for each trial mixture is
evaluated as the third step in order to select the best gradation. The fourth step is to fix the selected
gradation and compact mixtures with varying asphalt binder contents. The asphalt binder content
that produces 4% air voids is selected as optimum asphalt binder content. The final step in the
mix design procedure is to evaluate the moisture susceptibility, draindown sensitivity, and rut
resistance of the designed mixture.
