OGFC: Learning from the Past

Open-graded friction course (OGFC) mixes have been around longer than many contractors or department of transportation officials may recall. As early as the 1930, Oregon was using a type of OGFC; by the 1940s, many other Western states had employed similar types of friction coats. In the 1970s, interest in OGFCs skyrocketed, with 30 states experimenting with some variation of the mix.

However, the increased number of OGFC miles began to reveal some of the problems inherent with existing mixes, especially stripping, raveling and voids that closed over time. In some cases, the damage was severe. For example, Maryland experienced an alarmingly rapid deterioration of many OGFC surfaces during the winters of 1989-1990 and 1990-1991; in some spots, raveling was so severe that entire sections of the friction course were lost. Soon, more than half of the states using OGFC had suspended its use.

At the same time, however, there was a renewed interest in this type of mix. According to Byron Lord, chief of the office of technology applications for the Federal Highway Administration (FHWA), 26 states were again looking at OGFCs in the late '80s. Today, that number is increasing. Because of the nature of the material, most states using it today are located in the South or Southwest (cold-weather exceptions include Illinois, New York and Ohio).

Lord says investigations into enhancements of the mix are still underway, but he emphasizes that the problems encountered in the past have been addressed. In particular, the use of modified binders may hold the key to making today's OGFCs more versatile and practical than earlier versions of the mix. He notes that one of the biggest stumbling blocks to the use of OGFC in this country today is the often disappointing performance of such pavements in the past.

As in Europe, where OGFC is comparable-though significantly different-to porous European mixes (PEMs), the reasons for using OGFC in the U.S. are twofold:

• Increased safety: increased skid resistance, as well as longer-lasting skid resistance; and increased drainage capacity, which reduces tire spray and thus increases drivers' visibility, and
• Quieter ride than conventional pavements.

While Europe tends to emphasize each of these qualities equally (and uses heavier lifts of the porous pavement mix than are normally found in the U.S. to help quiet traffic), U.S. industry leaders are more concerned with the safety characteristics of the mix than its ability to mute tire noise.

OGFC drawbacks include the normal "higher material cost" that often accompanies newer mixes, but there also is the problem of mix deterioration and an increased difficulty in removing snow and ice from the heavily voided surface. (The problem with snow and ice removal helps explain why few cold-weather states are aggressively experimenting with the mix.)

In the U.S., Lord says industry officials have two OGFC goals: Optimize OGFC production and investigate modified binders to maximize the mix's performance. He notes that 57,000 miles of OGFC have been placed in the U.S., 11,000 in California alone.

While there have been problems in the past with this mix, those problems have been, for the most part, rectified. Use of the mix has been thoroughly examined and reports detailing specifications and methods of use are available from a variety of sources, including the American Association of State Highway & Transportation Officials, FHWA and the National Asphalt Pavement Association.