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 and 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.
Suitable for all pavement layers and for all traffic conditions. Works well for structural, friction, leveling and patching needs.
Particulars about dense-graded HMA are covered by the rest of this Guide.
Stone Matrix Asphalt (SMA)
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 design goal is to create stone-on-stone contact within the mixture. 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 is cost-effective because of its increased rut resistance and improved durability. SMA, has been used in the U.S. since about 1990.
Improved rut resistance and durability. SMA is almost exclusively used for surface courses on high volume interstates and U.S. roads.
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.
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 portland cement concrete for drainage.
- 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.
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 can degrade performance.
National Asphalt Pavement Association (NAPA). (1999). Designing and Constructing SMA Mixtures – State-of-the-Practice, Quality Improvement Series 122. National Asphalt Pavement Association. Landham, MD.
Federal Highway Administration (FHWA). (1998). Assessing the Results of the Strategic Highway Research Program. Publication No. FHWA-SA-98-008. Federal Highway Administration. Washington D.C.
Federal Highway Administration. (2001). Reclaimed Asphalt Pavement User Guideline: Asphalt Concrete (Hot Recycling). Web page on the Turner-Fairbanks Highway Research Center web site. http://www.tfhrc.gov/hnr20/recycle/waste/rap132.htm
Federal Highway Administration (FHWA). (2002a). Highway Statistics 2001. Office of Highway Policy Information, Federal Highway Administration. Washington, D.C. http://www.fhwa.dot.gov/ohim/hs01
Northeast Center for Excellence for Pavement Technology (NECEPT). (2001). Superpave System. Web page on the NECEPT web site. The Pennsylvania Transportation Institute, Pennsylvania State University. University Park, PA. http://www.superpave.psu.edu/superpave/system.html.