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.
Purpose: Suitable for all pavement layers and for all traffic conditions. Works well for structural, friction, leveling and patching needs.
Materials: Well-graded aggregate, asphalt binder (with or without modifiers), RAP
Information: 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.
Purpose: Improved rut resistance and durability. SMA is almost exclusively used for surface courses on high volume interstates and U.S. roads.
Materials: Gap-graded aggregate, modified asphalt binder, fiber filler
Information: 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. Open-graded mixes can be used in these ways:
- Permeable pavement. An open-graded mix is used for the entire pavement hard surface. The intent is to let water drain through the pavement structure itself into the underlying material. Permeable pavements can be hydraulically engineered such that there is a retention volume under the pavement (usually a layer of uniformly graded aggregate so the water can occupy the space between the aggregates) where the water can reside as it infiltrates into the subgrade below. The lower the infiltration rate of the subgrade, the larger the volume required. A simpler design does not involve much, if any, hydraulic design, and just places permeable pavement over the existing subgrade with no storage area. Municipalities that allow this simpler approach typically limit the credit for such a structure in their drainage requirements (e.g., they may still have to include separate drainage infrastructure).
- Open-graded friction course (OGFC). A surface course (usually about an inch thick) placed on top of an impermeable dense-graded HMA structure. This surface course allows water to drain through it and then run off the pavement on top of the underlying layer. If functioning properly, this can reduce water splash/spray and improve tire-pavement friction (assuming the constituent aggregate material does not readily polish).
- 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.
- Permeable pavement. Used for drainage, often in place of traditional drainage infrastructure.
- 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.
Materials: Aggregate (crushed stone or gravel and manufactured sands), asphalt binder (with modifiers)
Information: Open-graded HMA can be 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. Typically, permeable pavements require periodic cleaning to keep their pore structure open. OGFCs may require cleaning, however they are often used for highways where the high highway speeds cause a suction effect on the back side of the tire contact points, which can provide some/all of the required cleaning.
National Asphalt Pavement Association (NAPA). (1995). Thin Hot Mix Asphalt Surfacings, Information Series 110. National Asphalt Pavement Association. Lanham, MD.
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.
National Asphalt Pavement Association (NAPA). (2000). Recycling Practices for HMA, Special Report 187. National Asphalt Pavement Association. Landham, MD.
National Asphalt Pavement Association (NAPA). (2001). HMA Pavement Mix Type Selection Guide, Information Series 128. National Asphalt Pavement Association. Landham, MD.
Asphalt Pavement Alliance (APA). (2001). Recycling Asphalt Pavement Background. Paper posted on the APA web site.http://www.asphaltalliance.com
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.
- National Asphalt Pavement Association (NAPA). (1995). Thin Hot Mix Asphalt Surfacings. National Asphalt Pavement Association. Lanham, MD.↵