Performance of Sustainable Materials, Construction Practices Highlight Fourth Track Cycle
Dr. David Timm, Brasfield and Gorrie Professor of Civil Engineering at Auburn University, gives a tour of the structural sections of the 2009 test track during the NCAT Pavement Test Track Conference.
NCAT hosted the fourth Pavement Test Track Conference in February, highlighting how the test track helps turn theory into practice and enables transportation agencies to do more with less. Along with presentations from NCAT researchers, the conference also featured representatives from sponsoring agencies who discussed how longer-lasting, safer, more economical roads are the direct result of implementing test track research.
The fourth research cycle began in 2009, when 17 of the track’s 200-foot test sections were either reconstructed or rehabilitated. The remaining 29 sections were left in place to allow for additional traffic loading. Trafficking began in August 2009 and ended in September 2011 after 10 million equivalent single axle loads (ESALs) were applied. “The timing of fleet operations was scheduled so that the 25-month testing cycle included three summers,” says Dr. Buzz Powell, NCAT’s assistant director and manager of the Pavement Test Track.
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.
High RAP Mixes
Mixes containing up to 50 percent recycled asphalt pavement (RAP) have performed successfully at the Pavement Test Track, providing excellent rutting resistance and durability. Two structural sections containing 50 percent fractionated RAP were placed in 2009 as part of the Group Experiment—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 all-virgin materials and polymer-modified PG 76-22 binder in the top two layers. After 10 million ESALs, both high-RAP sections performed as well as the control, with minimal rutting, very small changes in smoothness and texture, and no observed cracking. The increased stiffness of the high-RAP mixes resulted in lower critical tensile strains and subgrade pressures relative to the control.
Four sections with 45 percent 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 that included 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 percent 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.
In addition to the warm-mix asphalt (WMA test section with 50 percent RAP, two structural sections comparing WMA technologies—water-injection foaming method and a chemical additive—were also constructed at the test track as part of the 2009 Group Experiment. After the application of 10 million ESALs, rut depths were satisfactory in both WMA sections. They were slightly higher than in the control section, probably due to less binder aging and absorption during production. There were also few practical differences between the WMA sections and the control with regard to structural response, according to Dr. David Timm, Brasfield and Gorrie Professor of Civil Engineering at Auburn University. No cracking was evident in either section and lab test results indicated greater fatigue life expectations for the WMA sections relative to the control.
Drs. Randy West (left) and Dave Timm answer audience questions after a conference presentation.
Alternative Binders and Binder Modifiers
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, a pelletized natural asphalt imported from Trinidad and Tobago—were successfully used as partial replacements for refined liquid asphalt in three test sections.
Kraton Polymers also sponsored a structural section incorporating highly polymer-modified (HPM) mixes that were very stiff but strain-tolerant, allowing the test pavement to be designed with an 18 percent thinner cross-section. The excellent fatigue and rutting resistance observed in this section made HPM the material of choice in rehabilitating a nearby pavement section that was completely failed.
Other experimental sections at the 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.
Porous Friction Courses and Stone Matrix Asphalt
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, since the structural value of PFCs was unknown, some states attributed no structural contribution at all to PFC layers. Embedded instrumentation at the 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.
A 2009 section sponsored by Georgia DOT evaluated the use of alternative aggregate sources for stone matrix asphalt (SMA), a premium mix used on Georgia’s high-volume interstate highways. The SMA test section contained a higher percentage of flat and elongated particles, yet had excellent performance with regard to rutting, cracking and raveling. These results indicate that aggregate sources meeting Superpave specifications perform as well as the higher-cost cubical aggregate currently used for SMA in Georgia.
Perpetual Pavements and Structural Design
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 test track has also 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 percent thinner asphalt cross-sections. Alabama DOT estimates savings of approximately $40 million per year since implementing the revised layer coefficient.
MEPDG Predictions vs. Actual Performance
Performance data from the 2003 and 2006 structural sections at the test track were compared with performance predictions using the Mechanistic-Empirical Pavement Design Guide (MEPDG). Using the national calibration coefficients generally over-predicted rutting. However, newly calibrated coefficients for the unbound layers produced acceptable rutting predictions. Fatigue cracking predictions were unsuccessful, with poor agreement between measured and predicted performance regardless of the coefficients used. Grouping sections with similar characteristics may result in better fatigue calibration results, an approach which may be helpful in analyzing data for the 2009 sections.
Research at the test track is also 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 to observations in the field. This is especially relevant in the characterization of high-RAP content mixes.
Plans for 2012 Research Cycle
The focus of research for the test track’s fifth cycle, scheduled to begin this summer, will be exploring ways to help transportation dollars go further. A number of sections from the fourth cycle, including the WMA and 50 percent 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 will also be applied to a local county road that provides access to a quarry and asphalt production facility. The existing pavement condition varies from good to poor. This study will 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 will also 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.
There are many sponsorship options available at the NCAT Pavement Test Track. For more information, go to www.pooledfund.org/Details/Solicitation/1325 or contact Dr. Buzz Powell at 334.844.6857.