Open Graded Friction Courses – Keeping an Open Mind

Open graded friction courses (OGFCs) are a type of pavement that have been built across the United States since the 1950’s. These asphalt mixes contain only a small portion of fine aggregate, creating a pavement with a relatively large percentage of air voids. They are primarily composed of single size coarse aggregate, and generally have a high asphalt content. In this newsletter we will explore the benefits of OGFCs, the experience of different agencies, and considerations for their use.

Benefits of OGFC

This aggregate structure provides a higher degree of friction as well as permeability to the surface of the pavement. This permeability further improves frictional behavior during wet weather while reducing the dangers of splash and spray and hydroplaning due to increased drainage from the pavement surface. In addition, open graded friction courses are generally quieter than typical pavements. There has been a wide range of experience with OGFC’s in different regions. While a large majority of states have experimented with these pavements, there is a large split over whether they will continue to be built.

Longevity of Open Graded Friction Courses

One important variable that has shaped agency opinion on OGFC’s is the service life that they have provided. A pavement’s longevity, or service life, is a crucial characteristic that affects whether a given technology is economically viable. Of course, ultimate performance is related to a number of factors such as quality of construction, mix design, and the regional climate. Different states have reported average pavement longevities as short as 6 years, while others have reported more than 12 years of service life before needing any maintenance. This represents huge variability, and spans the gap between a viable paving option and one that isn’t practical. However, this variability can be controlled. OGFCs will exhibit a satisfactory service life as long as care is taken with the mix design and it is used in the proper conditions.  In the following sections, we will explain when OGFCs should and should not be used.

When to Use

OGFCs are the most appropriate when their unique benefits are tailored to a specific project. This is the case in areas with:

• Heavy rainfall – The open nature of the pavement will allow water to permeate and prevent splash and spray. This also reduces the risk of vehicle hydroplaning, which can be a serious danger in certain areas where drainage is an issue. In addition, the stormwater runoff from OGFCs is typically cleaner than that of dense graded pavements.
• High Friction Needs – OGFC’s can also be used to increase safety in scenarios where high surface friction is needed to prevent accidents.
• Noise Sensitive Areas – Residential settings and other similar areas well suited to quiet pavements represent a good opportunity for OGFC application
• High Volume/High Speed Traffic – Use of OGFCs in a high traffic volume or high speed setting is also beneficial, as this helps keep the pores of the pavement clear. In addition, these often represent some of the areas where their noise reduction properties will provide the most benefit.

Warm southern climates have had some of the best results with OGFCs, as they help mitigate the heavy rainfall in these areas without being negatively affected by snow and ice, which will be explained in the next section. OGFCs should be used when one or more of the above benefits represent a project priority, but where adverse conditions that will threaten their success are not present.

Concerns

The aggregate structure of open graded friction courses makes them particularly susceptible from raveling. To avoid accelerated raveling, their use should be avoided in areas where snow and ice occur. Chains, studded tires, and snowplows are all known to aggravate raveling in OGFCs. In addition, particles on the roadway, particularly snow removal materials such as sand and salt, can clog up the pores in the pavement, limiting their drainage capability. Snowplows have also shown a tendency to deteriorate traffic markings on such pavements. OGFC’s also do not perform well when high stresses are expected from turning.

Tips for Successful Application

Many state DOTS have recorded their experiences with OGFCs. The states that have had success use a number of different additives to increase performance.

• Polymer additives are used by most states that continue to successfully use OGFCs.
• Hydrated lime can also be used as an additive and has produced favorable results in areas that are relatively dry but reach freezing temperatures.
• Rubberized asphalt has also been successfully used in open graded applications. This allows mixes with higher binder content, improving durability.
• Mineral fibers have been added to mixes to increase the ability of asphalt to coat the aggregate.

As with most paving jobs, deficiencies on any existing pavement structure should be corrected. The base layer used for placement should be dense graded to avoid structural failure.

Maintenance

Some specific maintenance guidelines apply to open graded friction courses. Patching must be done with care to prevent disruption of surface permeability, which is essential to the function of OGFC’s. In general the surface treatment required is dependent on the size and severity of the failure. Let’s take a closer look at the most common solution to address each size of failure:

• Low severity, localized distresses such as small cracks and potholes can be sealed or patched using dense graded mix.
• Medium severity, localized distresses must be filled with open graded material as dense graded fill will create a “dam” which blocks the flow of water.
• High severity, expansive distresses, such as raveling, require the top layer of pavement to be milled and replaced, as these pavements perform poorly when simply overlaid. The milled material can then be recycled for use in the new pavement layer.

The Future of OGFCs

Because of the specific utility of OGFC’s, they are likely to remain a part of our roadway networks. The issues with longevity that have occurred in the past can be managed by using intelligent design practices and only building them in the right scenarios. When implemented correctly, OGFC’s can have meaningful impacts on roadway safety as well as the health of the surrounding environment. As long as transportation departments continue to keep an open mind, open graded friction courses will continue to play a beneficial role in our roadway networks.