Open-Graded

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 two types of open-graded mixes typically used in the U.S.:

• Open-graded friction course (OGFC). Typically 15 percent air voids, no minimum air voids specified, lower aggregate standards than PEM. See Figure 1.
• Asphalt treated permeable bases (ATPB). Less stringent specifications than OGFC since it is used only under dense-graded HMA, SMA or PCC for drainage. See Figure 2.

Purpose

Open-Graded Friction Course (OGFC)

Used as a surface course only. Reduces tire splash/spray in wet weather and typically results in smoother surfaces than dense-graded HMA. The high air voids trap road noise and thus reduce tire-road noise by up to 50-percent (10 dBA) (NAPA, 1995¹).

Asphalt Treated Permeable Base (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 Information

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; therefore, anything that tends to clog these pores, such as low-speed traffic, excessive dirt on the roadway or deicing sand, should be avoided.

Performance History

Open-graded friction courses have been built in the U.S. since the 1950s. Agency experience varies widely: in 2011, reported average service lives before maintenance ranged from about 6 years in some states to more than 12 years in others, and states were divided over whether to keep building OGFC. Construction quality, mix design and climate all affect performance, and service life is generally satisfactory when the mix is carefully designed and used in suitable conditions.

Where OGFC Fits

OGFC is most appropriate where its particular benefits are a project priority:

• Heavy rainfall. Water drains through the surface, reducing splash and spray and the risk of hydroplaning. Runoff from OGFC is also typically cleaner than runoff from dense-graded pavements.
• High friction needs. The open surface provides high friction, including in wet weather.
• Noise-sensitive areas. OGFC is generally quieter than dense-graded HMA, which suits residential areas. See Pavement Noise.
• High-volume, high-speed traffic. Fast traffic helps keep the pores clear, and these are often the places where noise reduction matters most.

Warm climates with heavy rainfall have had some of the best results, because the pavement is not exposed to snow and ice.

Limitations

The open aggregate structure makes OGFC susceptible to raveling. Tire chains, studded tires and snowplows all aggravate raveling, so OGFC is generally avoided where snow and ice are common. Sand and salt used for winter maintenance can clog the pores, and snowplows tend to damage pavement markings on OGFC. OGFC also performs poorly where turning traffic imposes high stresses.

States that use OGFC successfully commonly add one or more of the following:

• Polymer modifiers, used by most states that continue to build OGFC.
• Hydrated lime, with favorable results in relatively dry areas that reach freezing temperatures.
• Rubberized asphalt, which allows a higher binder content and improves durability.
• Mineral fibers, which help the binder coat the aggregate.

Deficiencies in the existing pavement should be corrected first, and the layer beneath the OGFC should be dense-graded.

Maintenance

Repairs must preserve the surface's permeability:

• Low-severity, localized distresses such as small cracks and potholes can be sealed or patched with dense-graded mix.
• Medium-severity, localized distresses should be patched with open-graded mix, because a dense-graded patch acts as a dam that blocks the flow of water through the layer.
• High-severity or widespread distress, such as extensive raveling, calls for milling and replacing the OGFC layer, since OGFC performs poorly when simply overlaid. The milled material can be recycled into the new layer.

@misc{pavint-open-graded,
  author = {{Pavement Interactive}},
  title = {Open-Graded},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/pavement-types-and-history/pavement-types/open-graded-hma/}},
  note = {Accessed [date]}
}