Publication Number: FHWA-SA-97-024
Date: June 1997

Study Confirms Relationship Between Fine Aggregates and Rutting

Looking for a way to prevent permanent deformation-or rutting, as it is commonly known-in asphalt pavements? Heed the Superpave aggregate gradation specifications. That's one of the findings of a study of data from the long-term pavement performance (LTPP) program's general pavement studies (GPS) experiments.

The study, "Rutting Trends in Hot-Mix Asphalt Concrete Pavements," was based on data collected at 575 GPS sites. It looked at full-depth asphalt pavements, asphalt pavements over a granular base, asphalt pavements over a portland cement-treated base, and asphalt overlays on asphalt and portland cement concrete pavements. The pavements ranged in age from newly constructed to more than 20 years old.

The study team focused on the test sections consisting of asphalt pavements over a granular base, which are the most common types of asphalt pavements in the GPS experiments. They found that pavements with high levels of rutting on average were constructed of asphalt mixes containing more fine aggregates-or sand-than recommended by the Superpave aggregate specifications. Pavements with minimal rutting were within the Superpave aggregate specifications.

Of course, sand is not the only culprit in rutting. The study also found that hot weather, thin pavements, soft asphalts, and wet or low-density bases or subgrades can also contribute to rutting. But as Charlie Churilla, head of FHWA's Pavement Performance Division, which manages the LTPP program, says, the study makes it clear that it is well worth the time and effort to consider aggregate blends that meet the Superpave specifications-they will be much more resistant to rutting.

More than half of the asphalt pavements studied had only nominal rutting (less than 1 mm [0.04 in] per year of pavement service), and pavements more than 20 years old had rutting of only 7 mm (.28 in) on average. These findings are evidence that asphalt pavements that are properly designed and constructed can serve for 20 years or more without excessive rutting.

For more information, contact Amy Simpson at Brent Rauhut Engineering (phone: 512-346-0870; fax: 512-346-8750).

The data on rut depths were collected by the PASCO RoadRecon van, a vehicle able to take a photographic record of pavement conditions while traveling at highway speeds. PASCO provided the service under a contract with the Federal Highway Administration.

Two cameras are used. At the front of the vehicle, a 35-mm movie camera records the general condition of an entire stretch of pavement. At the rear of the vehicle, a 35-mm pulse camera-which takes still photographs at preset intervals-records transverse profiles. These photographs are later digitized to produce data on rut depths.

To ensure that lighting conditions are consistent and to minimize the disruption to traffic, data are collected at night, using powerful lights to illuminate the pavement. An array of high-intensity flood lights at the front of the van illuminates the full width of the lane. The rear of the vehicle is equipped with a strobe light that is synchronized to the timing of the pulse camera. When the strobe flashes, a hairline etched on the lens of the strobe light is projected onto the pavement. This hairline, captured on the photographs, follows the pavement's contours, showing where the pavement is rutted.

As the vehicle photographs a pavement, the crew records the location of each pavement surveyed. When the film is developed, labels are added that include this information.

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