|Major Topics on this Page|
|3.2||Stone Matrix Asphalt Mixes|
|3.4||Mix Selection Guidance|
There are many different types of flexible pavements. This section covers three of the more common types of HMA mix types used in the U.S. Other flexible pavements such as bituminous surface treatments (BSTs) are considered by most agencies to be a form of maintenance and are thus covered under Module 10, Maintenance & Rehabilitation. HMA mix types differ from each other mainly in maximum aggregate size, aggregate gradation and asphalt binder content/type. This Guide focuses on dense-graded HMA in most flexible pavement sections because it is the most common HMA pavement material in the U.S. This section provides a brief exposure to:
- Dense-graded HMA. Flexible pavement information in this Guide is generally concerned with dense-graded HMA. Dense-graded HMA is a versatile, all-around mix making it the most common and well-understood mix type in the U.S.
- Stone matrix asphalt (SMA). SMA, although relatively new in the U.S., has been used in Europe as a surface course for years to support heavy traffic loads and resist studded tire wear.
- Open-graded HMA. This includes both open-graded friction course (OGFC) and asphalt treated permeable materials (ATPM). Open-graded mixes are typically used as wearing courses (OGFC) or underlying drainage layers (ATPM) because of the special advantages offered by their porosity.
This section is taken largely from the NAPA's HMA Pavement Mix Type Selection Guide (2001). In addition to the general information presented here, the HMA Pavement Mix Type Selection Guide provides specific information on minimum lift thicknesses, mix selection criteria, mix materials as well as several informative examples.
WSDOT Standard HMA Mix Classes
WSDOT uses the following standard HMA mixes:
A dense-graded mix is a well-graded HMA mixture 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 (see Table 2.1 for definitions of fine- and coarse-graded mixes).
|Purpose:||Dense-graded mixes are suitable for all pavement layers and for all traffic conditions. They work well for structural, friction, leveling and patching needs.|
|Materials:||Well-graded aggregate, asphalt binder (with or without modifiers), RAP|
|Mix Design:||Superpave, Marshall or Hveem procedures.|
|Other Info:||Particulars about dense-graded HMA are covered by flexible pavement sections in the rest of this Guide.|
|Mixture Nominal Maximum Aggregate Size||Coarse-Graded Mix||Fine-Graded Mix|
|37.5 mm (1.5 inches)||< 35 % passing the 4.75 mm (No. 4 Sieve)||> 35 % passing the 4.75 mm (No. 4 Sieve)|
|25.0 mm (1.0 inch)||< 40 % passing the 4.75 mm (No. 4 Sieve)||> 40 % passing the 4.75 mm (No. 4 Sieve)|
|19.0 mm (0.75 inches)||< 35 % passing the 2.36 mm (No. 8 Sieve)||> 35 % passing the 2.36 mm (No. 8 Sieve)|
|12.5 mm (0.5 inches)||< 40 % passing the 2.36 mm (No. 8 Sieve)||> 40 % passing the 2.36 mm (No. 8 Sieve)|
|9.5 mm (0.375 inches)||< 45 % passing the 2.36 mm (No. 8 Sieve)||> 45 % passing the 2.36 mm (No. 8 Sieve)|
Stone matrix asphalt (SMA) is a gap-graded HMA (see Figure 2.7) that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stone-on-stone contact (see Figures 2.8, through 2.12). Because the aggregates are all in contact, rut resistance relies on aggregate properties rather than asphalt binder properties. 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 (about 20 - 25 percent) because it requires more durable aggregates, higher asphalt content and, typically, a modified asphalt binder and fibers. In the right situations it should be cost-effective because of its increased rut resistance and improved durability. SMA, originally developed in Europe to resist rutting and studded tire wear, has been used in the U.S. since about 1990 (NAPA, 1999).
Washington State SMA Experience
WSDOT has built several SMA wearing courses both in urban and rural settings. For a brief summary of SMA in Washington State, see the WSDOT SMA Tech Note. The first SMA project in Washington State was a 1999 resurfacing of SR 524 in Lynwood. Experiences on this project are documented in a WSDOT Research Report available at:
|Purpose:||Improved rut resistance and durability. Therefore, SMA is almost exclusively used for surface courses on high volume interstates and U.S. roads.|
|Materials:||Gap-graded aggregate (usually from coarse aggregate, manufactured sands and mineral filler all combined into a final gradation), asphalt binder (typically with a modifier)|
|Mix Design:||Superpave or Marshall procedures with modifications. Refer to NAPA's Designing and Constructing SMA Mixtures: State-of-the-Practice, QIP 122 (1999) publication or NCHRP Report 425: Designing Stone Matrix Asphalt Mixtures for Rut-Resistant Pavements.|
SMA mixes have a high asphalt binder content (on the order of 6 percent), as
the mix sits in the HMA storage silos, transport trucks, and after it is
placed, the asphalt binder has a tendency to drain off the aggregate and
down to the bottom - a phenomenon known as "mix draindown". Mix
draindown is usually combated by adding cellulose or mineral fibers to keep
the asphalt binder in place. Cellulose fibers are typically shredded
newspapers and magazines, while mineral fibers are spun from molten rock.
A laboratory test is run during mix design to ensure the mix is not subject
to excessive draindown.
In mix design a test for voids in the coarse aggregate (AASHTO T 19) is used to ensure there is stone-on-stone contact.
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 usually added to minimize asphalt binder drain-down during construction, increase the amount of asphalt binder used in the mix and to improve mix durability.
|Figure 2.9: SMA Aggregate Structure. Notice the stone-on-stone contact of the larger aggregate particles.||Figure 2.10: Dense-Graded HMA (left) vs. SMA (right). (it is a bit more shiny from the extra asphalt binder)|
|Figure 2.11: Dense-Graded HMA (left) vs. SMA (right). Notice the SMA has a better-defined large aggregate skeleton (from NAPA, 2001)||Figure 2.12: SMA Pavement Surface|
An open-graded HMA mixture is designed to be water permeable (dense-graded and SMA mixes usually are not permeable). Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands. There are three types of open-graded mixes typically used in the U.S.:
Washington State Open-Graded Mix Experience
|For specifics, see the WSDOT Class D HMA discussion.|
OGFC and PEM - Used as for
courses only. They reduce tire splash/spray in
wet weather and typically result in smoother surfaces than dense-graded HMA.
Their high air voids trap road noise and thus 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.
|Materials:||Aggregate (crushed stone or gravel and manufactured sands), asphalt binder (with modifiers)|
|Mix Design:||Less structured than for dense-graded or SMA mixes. Open-graded mix design generally consists of 1) material selection, 2) gradation, 3) compaction and void determination and 4) asphalt binder drain-down evaluation. NCAT Report 99-3: Design of New-Generation Open Graded Friction Courses provides a recommended mix design procedure for OGFCs.|
|Other Info:||Both OGFC and PEM are 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. Therefore anything that tends to clog these pores, such as low-speed traffic, excessive dirt on the roadway or deicing sand, should be avoided.|
Based on the previous information, there are some general rules for HMA mix type use, which are summarized in Table 2.2. Notice that, as discussed, dense-graded HMA is generally appropriate for all uses, SMA and OGFC (and PEM) are typically used as surface courses on high volume roads and ATPB is usually used for base courses on high volume roads. Keep in mind that Table 2.2 is just a summary of general guidance and that there are, as always, case specific exceptions.
|Course||Low Traffic||Medium Traffic||High Traffic|
|(< 300,000 ESALs)||(300,000 - 10 million ESALs)||(> 10 million ESALs)|
Note: Before deciding to use ATPB, the Pavement Research Center's research results should be carefully considered.
|= Moderately Appropriate|
|empty||= Not Appropriate|
Most of this process is taken directly from the NAPA HMA Pavement Mix Type Selection Guide (2001).