Working with the Asphalt Pavement Association of Oregon (APAO) and resources at the Texas Department of Transportation, the City of Eugene developed a special specification for use of warm mix asphalt concrete (WMAC) on City projects in 2008. In order to test WMAC on a City project, the City worked with a local asphalt concrete producer, Eugene Sand and Gravel, to place approximately 8,000 tons of WMAC on an existing multi-year paving project on Roosevelt Blvd. This would be the largest WMAC project in Oregon at the time. Eugene Sand and Gravel retrofitted their asphalt plant to produce WMAC using a plant foaming process, which foams the asphalt oil through an injection of steam as it is mixed with the aggregate to create asphalt concrete. This process reduces mix temperatures by approximately 50 degrees Fahrenheit. The City worked closely with Eugene Sand and Gravel, and Lane County testing staff who provided mix design review and field testing to assure that the WMAC product was successful. The required compaction densities were easily achieved.
The foaming process increases the volume of the binder in the mix which results in better coating of the aggregate in the mix. Eugene found that this better coated material acted as if there was more asphalt cement in the mix and was easier to compact than conventional hot mix asphalt pavement. Compaction is a primary factor in the longevity of asphalt pavements, so ensuring better compaction would also ensure a longer life. Preliminary measurements by Eugene Sand and Gravel showed energy savings of approximately 15% during WMAC production.
After the success of the pilot project, the City chose to require the use of WMAC on six pavement preservation projects under two separate contracts, amounting to an additional 20,000 tons of WMAC to be produced. In addition to this, the City allowed the use of WMAC on other paving projects in 2009 at the option of the Contractor. The use of WMAC in Eugene has generated interest across the state for use of this technology and has given municipalities as well as asphalt concrete production and paving companies an example of how this product can be successfully implemented in Oregon. The City has provided specifications and related construction experiences to multiple jurisdictions interested in using WMAC in Oregon including Federal Highways, ODOT, City of Roseburg, City of McMinnville, Douglas County, and Washington County. Between 2009 and 2014, the City of Eugene specified the placement of approximately 361,000 tons of WMAC, reducing greenhouse gas emissions by approximately 8,700 Metric Tons CO2e. The use of WMAC in Eugene is now the standard practice for all City paving projects.
Even though the use of WMAC can allow longer hauling distances, Eugene specifies a haul distance limitation to ensure that the GHG benefits realized during material production are not negated by the longer travel distances.
The use of reclaimed asphalt pavement, or “RAP”, has been used for more than 30 years in Eugene. For many years, it has been standard practice to specify up to 30% RAP in asphalt pavements. Using typical materials properties, the 30% by weight RAP content replaces approximately 25 - 30% of the virgin binder content of a typical asphalt pavement.
In 2013, Eugene began to move beyond 30% RAP content by increasing the overall asphalt binder replacement in the pavement on a test project. After reviewing materials from various agencies studying this concept, staff met with the three asphalt pavement producers in the Eugene area to discuss the possibilities and plant capabilities. Because each producer had different plant capabilities, the City decided to revise the percentage of RAP content specification to call for 35% binder replacement with reclaimed asphalt materials. A benefit to using a specification based on binder replacement is that the mix design is based on the asphalt content of the specific reclaimed materials rather than industry averages that may not be accurate for the specific stockpile.
The contractors were allowed to use a combination of reclaimed shingles or reclaimed asphalt pavement to meet this 35% binder replacement requirement, up to 5% shingles. Adjustments were made to the grade of virgin asphalt cement used to compensate for the stiffer mix, a result of higher RAP content. Not only did this mix design decrease the use of virgin asphalt binder and increase the use of reclaimed materials in the pavement mix, but there is a potential cost savings to the City. In 2013, two streets were repaved using increased reclaimed binder content asphalt pavement.
Based on positive test results from the 2013 projects, in 2014 and 2015, Eugene continued the practice of using the increased the reclaimed binder in asphalt pavements, using the 35% binder replacement mixes on nearly a dozen streets. With over 13,700 tons of RAP used on 2014 projects, the City reduced the need for nearly 800 tons of asphalt cement and 12,900 tons of aggregate to be mined, refined, processed and subsequently shipped to the pavement producers. In 2016, the City will let a project with 40% reclaimed asphalt materials.
Selection of mix design properties is critical for any asphalt pavement material. Considerations include the environmental conditions, traffic loading and local materials that go into the mix design. For example, while RAP may be more resistant to rutting due to a stiffer mix, many local agency roadways do not experience traffic loading such that rutting is a concern. Instead, local agency roadways are typically more susceptible to cracking and a stiffer mix will increase this susceptibility. Eugene balanced this stiffer material by working with APAO to select an asphalt cement grade that would compensate for this characteristic.