U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
|Accelerating Infrastructure Innovations|
Publication Number: FHWA-RD-02-015
Date: October 2002
Nearly 30 million tons of reclaimed asphalt pavement (RAP) are recycled into hot-mix asphalt pavements each year, saving taxpayers more than $300 million annually. The Louisiana Transportation Research Center recently experimented with an alternative use for RAP that holds the promise of pushing those numbers even higher.
The experiment took place during the reconstruction of U.S. Highway 190 near Baton Rouge, Louisiana, in January 2002. The continuously reinforced concrete pavement design called for the removal of the existing pavement and the application of a lime treated subbase and 20 cm (8 in) of stone base. The LTRC experiment was designed to use RAP rather than stone as a base. By itself, RAP is not a qualified base material since its strength is not comparable to other base materials, especially stone. LTRC's solution was to pretreat the RAP using a foamed asphalt design method created by Wirtgen America, Inc. This process combines hot asphalt and a small quantity of water in a mixing chamber to produce an asphalt foam that is then incorporated into the base material. Because the process uses less water than an emulsion-based mix, compaction can be started earlier.
Using a Wirtgen model K-150 pug mill mixer to mix the RAP with foamed asphalt cement and portland cement, LTRC created a foamed RAP with higher cohesion and triaxial strength based on lab tests.
It was important to maintain a minimum asphalt cement temperature of 160 °C (320 °F) at time of delivery and at least 150 °C (302 °F) during production. Maintaining these temperatures assures proper foaming of the asphalt and mixing in the field. The pug mill unloaded directly to the back of the trucks, with the mixing site less than a mile from the roadway. Optimum density was achieved by establishing the rolling pattern with a density gauge. For a State like Louisiana, which imports all its stone from out of State, this onsite delivery method of the RAP saves both time and money. The savings can amount to nearly $2 for each ton of base when using foamed RAP.
The first 10 cm x 10 m x 330 m (4 in x 33 ft x 1000 ft) section of pavement was placed in one day with a conventional asphalt paver. The total thickness desired was 20 cm (8 in). The biggest problem encountered in constructing the project was the contractor's initial difficulty in compacting the longitudinal joints, but the contractor succeeded by placing the material full depth and width with a bulldozer prior to rolling, a method typically used in laying stone.
Although cores could not be obtained, LTRC performed a number of strength tests on both the foamed RAP base and the adjacent stone base. In tests such as the falling weight deflectometer and dynamic cone penetrometer, the results for the foamed RAP equaled or exceeded those of the stone base.
For quality construction, the stockpile RAP moisture needs to be closely watched. The material's moisture content was monitored throughout the production process and the resulting levels were acceptable. Approximately 2.5 cm (1 in) of rain fell on the second day of construction but it did not have a detrimental effect on the compacted foamed RAP base. The effect of long-term exposure to moisture is yet to be determined. LTRC is continuing to monitor these effects.
Christopher Abadie of LTRC says, "Foamed RAP will create savings and reduce construction times in certain areas of the country and on certain projects." The Louisiana Department of Transportation and Development currently allows up to 20 percent use of RAP in its asphalt mixtures. However, since many reconstruction projects generate great quantities of RAP, using it as a base material on the same project can be more efficient than hauling it to a plant for use in asphalt mixtures.
Having successfully concluded the January experiment, LTRC is now looking for more projects that could benefit from the use of foamed RAP. For more information on the LTRC project, contact Christopher Abadie of LTRC, 225-767-9109 (fax: 225-767-9108; email: firstname.lastname@example.org) or Michael Smith at the Federal Highway Administration's Southern Resource Center, 404-562-3694 (email: email@example.com).