Recently completed research at the National Center for Asphalt Technology (NCAT) Pavement Test Track is providing practical and cost-effective solutions to help transportation agencies do more with less.
The 1.7-mile oval track, located in Opelika, Ala., is a one-of-a-kind accelerated pavement testing facility combining real-world pavement construction and live heavy trafficking. Ten million equivalent single axle loads (ESALs)—a design lifetime of traffic loads for most highways—are applied in only two years, thanks to a fleet of heavily loaded trucks that run 16 hours a day, five days a week. This rapid testing and analysis of pavements provides answers to specific pavement-performance questions in a safer, faster manner than conventional highway test sections and allows highway agencies to quickly implement improvements in mix-design specifications, construction practices and pavement-design methods.
The fourth research cycle at the Pavement Test Track began in 2009, when 17 of the track’s 200-ft test sections were either reconstructed or rehabilitated, while the remaining 29 sections were left in place to allow for additional traffic loading. Some test sections were built on thick pavement foundations to ensure that surface distresses would be materials-related; other sections had varied asphalt layer thicknesses with embedded instrumentation to measure pavement response to traffic loading. For all sections, pavement performance was quantified on a weekly basis with regard to smoothness, rutting, raveling and cracking. Objectives for each individual test section and the track as a whole were decided by highway agency and industry sponsors, with economic and environmental sustainability as top priorities.
Just about half
Asphalt pavement has long been America’s most recycled material, and a recent Federal Highway Administration (FHWA) survey confirms that more than 99% of asphalt pavement is recycled or reused. In 2010 alone, 62.1 million tons of recycled asphalt pavement (RAP) was reused in either hot-mix or warm-mix asphalt, conserving over 3 million tons of virgin asphalt binder. Based on a composite of average 2010 asphalt price indices published by 11 state DOTs, this translates into taxpayer savings of more than $1 billion. State highway agencies typically allow mixes containing 15-20% RAP, and research continues to investigate using higher RAP contents to gain greater economic benefits and further conserve natural resources.
At the NCAT Pavement Test Track, mixes containing up to 50% RAP have performed successfully, providing excellent rutting resistance and durability. Two sections containing 50% RAP were placed in 2009—one mix was conventional hot-mix asphalt (HMA) and the other was warm-mix asphalt (WMA) produced using a water-injection foaming process. Both sections used unmodified PG 67-22 binder, whereas the control section contained polymer-modified PG 76-22 binder and all-virgin materials. After 10 million ESALs, both high-RAP sections performed as well as the control, with minimal rutting, very small changes in smoothness and no observed cracking.
Four sections with 45% RAP were left in place from the previous cycle of testing, accruing a total traffic loading of 20 million ESALs. These sections compared different virgin binder grades (PG 52-28, PG 67-22 and PG 76-22). All four sections had exceptional rutting performance, with rut depths less than 5 mm after two cycles of trafficking and some of the hottest recorded summers for the local area. Mixes containing stiffer virgin binder grades exhibited minor cracking at an earlier stage than mixes with softer binders, indicating that a softer virgin binder grade slightly improves the durability of high RAP mixes.
In 2009, the Mississippi DOT also sponsored a section containing 45% RAP. While the mix used PG 67-22, early results indicate that performance is similar to an all-virgin mix with polymer-modified PG 76-22. Significant cost savings can be achieved by using high RAP contents combined with unmodified binder.
Warm and tough
WMA use in the U.S. saw an overwhelming 148% increase from 2009 to 2010, fueled by the environmental and construction benefits associated with using warm-mix technology: WMA is characterized by reduced production temperatures (typically 275°F or less), resulting in less energy, lower emissions and improved working conditions. Enhanced compactability and the ability to incorporate higher percentages of RAP also are potential benefits of using WMA.
In addition to the previously discussed WMA test section with 50% RAP, two sections comparing WMA technologies—water-injection foaming method and a chemical additive—also were constructed at the Pavement Test Track in 2009. After the application of 10 million ESALs, rut depths were satisfactory in both WMA sections, though slightly higher than in the control section, probably due to less binder aging and absorption during production. No cracking was evident in either section, and lab test results indicated greater fatigue life expectations for the WMA sections relative to the control.
These are the alternatives
Several alternative binders and binder modifiers were evaluated during the 2009 research cycle, investigating ways to reduce the quantity of asphalt materials needed for construction. Two options—Shell Thiopave, a warm-mix sulfur technology, and Trinidad Lake Asphalt, pelletized natural asphalt imported from Trinidad and Tobago—were successfully used as partial replacements for refined liquid asphalt in three test sections. According to Dr. Buzz Powell, NCAT’s assistant director and manager of the Pavement Test Track, “These are additional tools in our toolbox to protect against potential price increases in refined liquid asphalt.”
Polymer-modified asphalt binder has been used for many years to provide enhanced rutting resistance and increased pavement life. A 2009 section sponsored by Kraton Polymers incorporated highly polymer-modified (HPM) mixes that were very stiff but strain-tolerant, allowing the test pavement to be designed with an 18% thinner cross section. “A potential advantage of this technology is the cost-to-benefit ratio associated with building thinner pavements while achieving comparable performance,” said Powell. The excellent fatigue and rutting resistance observed in this section made HPM the material of choice in rehabilitating a nearby pavement section that had completely failed.
Other experimental sections at the Pavement Test Track compared binder modification with ground tire rubber (GTR) and styrene-butadiene-styrene (SBS) polymer. Both laboratory testing and field measurements showed that mixes containing GTR performed comparably to SBS mixes in every way. This allows for the environmentally friendly disposal of used tires in asphalt mixes as opposed to landfills.
The benefits of porous friction courses (PFCs) include improved surface-friction characteristics, reduced tire spray during rain events and reduced noise from tire/pavement interaction. However, the structural value of PFCs was previously unknown. Embedded instrumentation at the Pavement Test Track allowed for the structural characterization of a PFC section, indicating that PFCs do contribute to a pavement’s overall structural integrity. A provisional structural coefficient of 0.15 was determined for PFCs, allowing states to optimize pavement designs and make full use of available resources.
As a rehabilitation surface in another section, PFC mix was found to extend the performance life of underlying dense mix with a history of cracking susceptibility. Performance was further improved when the PFC surface was placed with a heavy tack coat using a spray paver compared with conventional tack methods.
30 million strong
Two sections placed in 2003 that were designed to reach terminal serviceability at 10 million ESALs have survived an impressive 30 million ESALs at the Pavement Test Track. Both sections were designed using the 1993 AASHTO Pavement Design Guide, with an asphalt structural coefficient of 0.44 (the Alabama DOT standard at the time). The sections differ with respect to binder grade—one used PG 67-22, whereas the other used SBS-modified PG 76-22. After 30 million ESALs, both sections exhibited minimal rutting and no fatigue cracking. These results indicate that pavements can withstand higher levels of strain than suggested by lab tests, allowing the design of perpetual pavements with thinner cross sections that are more cost-competitive.
Recent research at the Pavement Test Track also has shown that the asphalt structural coefficient can be increased from 0.44 to 0.54 for flexible pavement designs using the 1993 AASHTO Pavement Design Guide. The coefficient recalibration was based on structural measurements from test sections with a broad range of asphalt thicknesses and mix types, as well as different bases and subgrades. Increasing the coefficient to 0.54 results in approximately 18% thinner asphalt cross sections. This translates directly into annual cost savings and/or more efficient use of material to pave more highway mileage.
A bit unrealistic
Research at the Pavement Test Track also is contributing to further understanding of laboratory performance tests and modeling predictions. The NCAT lab has conducted extensive testing on the mixes from the test sections, and researchers have carefully analyzed data using both the conventional pavement-design approach and mechanistic-based methods. One of the key findings is that some of the tests used to assess cracking performance use unrealistic strain levels that result in different performance rankings compared with observations in the field. This is especially relevant in the characterization of high RAP content mixes.
Future lies with the fifth
The focus of research for the Pavement Test Track’s fifth cycle, scheduled to begin this year, will be exploring ways to help transportation dollars go further. A number of sections from the fourth cycle, including the WMA and 50% RAP sections, will likely remain in place for further trafficking as part of the preservation group experiment. Pavement-preservation treatments (e.g., thin overlays and inlays, microsurfacing, chip seals and other surface treatments) will be applied when a predetermined level of distress is reached. Further performance monitoring will allow researchers to determine the life-cycle cost of various pavement-preservation alternatives relative to pretreatment condition. Preservation treatments also will be applied to a local county road in order to expand the scope of testing on the NCAT Pavement Test Track into a “proactive versus reactive” experiment that defines the relationship between life-cycle performance and pretreatment condition for popular preservation alternatives.
Multiple sponsors also will be participating in the Green Group, which will be constructed this summer using high recycled contents—both RAP and recycled asphalt shingles (RAS)—in addition to unconventional materials and alternative design methodologies. The goal will be to assist states with implementation of these green technologies that have the potential to reduce initial construction cost, pavement thickness and environmental impact.
For more information regarding research at the NCAT Pavement Test Track, visit www.ncat.us or www?.pavetrack.com. R&B