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HMA Mix Types
The most common type of flexible pavement surfacing in the U.S. is hot mix
asphalt (HMA). Hot mix asphalt is known by many different names such as
hot mix, asphalt
concrete (AC or ACP), asphalt, blacktop or bitumen. For clarity, this
Guide makes a conscious effort to consistently refer to this material as HMA.
HMA is distinguished by its design and
production
methods (as described in this Guide) and includes traditional
dense-graded mixes as well as stone matrix
asphalt (SMA) and various open-graded HMAs. Typically agencies consider other types of asphalt-based
pavement surfaces such as
fog seals,
slurry seals and
BSTs to be maintenance
treatments and are therefore covered in the Maintenance & Rehabilitation
section. Reclaimed asphalt pavement (RAP) is generally considered a
material within HMA, while forms of in-place recycling are considered
separately.
A dense-graded mix is a
well-graded HMA intended for general use. When properly designed and
constructed, a dense-graded mix is relatively impermeable. Dense-graded mixes
are generally referred to by their
nominal maximum aggregate size. They can further be classified as either
fine-graded or coarse-graded. Fine-graded mixes have more fine and sand sized
particles than coarse-graded mixes. Dense-graded mixes are used extensively in Washington State for all purposes.
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| Figure 1: Dense-Graded HMA |
Figure 2: Dense-Graded Cores |
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Purpose: |
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Suitable
for all pavement layers and for all traffic conditions. They work well for
structural, friction,
leveling and
patching needs. |
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Materials: |
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Well-graded aggregate, asphalt binder (with or without modifiers),
RAP |
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Other
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Particulars about dense-graded HMA are covered by the rest of this Guide. |
Stone matrix asphalt (SMA), sometimes called stone mastic asphalt, is a
gap-graded HMA originally developed in Europe to maximize
rutting resistance and
durability. The mix goal is to create stone-on-stone contact.
Since aggregates do not deform as much as asphalt binder under load, this
stone-on-stone contact greatly reduces rutting. SMA is generally more expensive
than a typical dense-graded HMA because it requires more durable aggregates,
higher asphalt content, modified asphalt binder and fibers. In the right
situations it should be cost-effective because of its increased rut resistance
and improved durability. SMA, has been used in the U.S. since about 1990,
although it has only begun to be used in Washington State recently on several
pilot projects.
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| Figure 1: SMA Surface |
Figure 2: SMA Lab Sample |
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Purpose: |
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Improved
rut resistance and durability. SMA is almost exclusively used for
surface courses on high volume interstates and U.S. roads. |
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Materials: |
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Gap-graded aggregate,
modified asphalt binder, fiber filler |
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Other
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Other
reported SMA benefits include wet weather friction (due to a coarser surface
texture), lower tire noise (due to a coarser surface texture) and less
severe reflective cracking.
Mineral fillers and additives are used to minimize asphalt binder
drain-down during construction, increase the amount of asphalt binder used
in the mix and to improve mix durability. |
Unlike dense-graded mixes and SMA, an
open-graded HMA mixture is designed to be water permeable. Open-graded
mixes use only crushed stone (or gravel) and a small percentage of manufactured
sands. The two most typical open-graded mixes are:
- Open-graded friction course (OGFC). Typically 15 percent
air
voids and no maximum air voids specified.
- Asphalt treated permeable bases (ATPB). Less stringent
specifications than OGFC since it is used only under dense-graded HMA, SMA or
PCC for drainage.
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| Figure 5: OGFC Surface |
Figure 6: OGFC Lab Samples |
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Purpose: |
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OGFC -
Used for surface courses only. They reduce tire splash/spray in wet weather and
typically result in smoother surfaces than dense-graded HMA. Their high air
voids reduce tire-road noise by up to 50-percent (10 dBA) (NAPA, 1995).
ATPB - Used as a drainage layer below dense-graded HMA, SMA or PCC.
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Materials: |
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Aggregate
(crushed stone or gravel and manufactured sands), asphalt binder (with
modifiers) |
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Other
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OGFC is
more expensive per ton than dense-graded HMA, but the unit weight of the mix
when in-place is lower, which partially offsets the higher per-ton cost.
The open gradation creates pores in the mix, which are essential to the
mix's proper function. Anything that tends to clog these pores,
such as low-speed traffic, excessive dirt on the roadway or deicing sand,
can degrade performance.
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