The fourth research cycle at the NCAT Pavement Test Track is nearing completion, as trafficking reached 10 million equivalent single axle loads (ESALs) in September. The current cycle began in 2009, when 17 of the track’s 200-ft. test sections were either reconstructed or rehabilitated, while the remaining sections were left in place for traffic continuation from previous testing cycles. Evaluations address a variety of structural and surface performance objectives. Following the conclusion of trafficking, forensic investigations will be performed l, and research findings presented at the 2012 Pavement Test Track Conference scheduled for February.
Test Sections Being Evaluated
The six-section group experiment (GE) forms the core of the structural research effort, which relies on embedded instrumentation to measure pavement response to traffic loading. All GE sections have a total asphalt thickness of seven inches, with six inches of dense-graded aggregate base and a stiff subgrade underneath. Sustainable technologies being evaluated within the GE include full-depth warm-mix asphalt (MeadWestvaco’s Evotherm® DAT technology and the Astec Double-Barrel Green® foaming process), high reclaimed asphalt pavement (RAP)content (50 percent) and a porous friction course (PFC). Sponsors of the GE are Departments of Transportation from Alabama, Florida, North Carolina, Oklahoma, South Carolina and Tennessee, as well as the FHWA.
As a complement to the GE, there are also four privately sponsored structural sections. The Kraton section contains highly polymer-modified mixes, allowing the section to be designed with a reduced total asphalt thickness of 5.75 inches. Two Shell Thiopave® sections use a sulfur technology to replace a percentage of the liquid asphalt in the binder and base layers. In the Trinidad Lake Asphalt section, pelletized natural asphalt makes up 25 percent of the total binder in the mix.
Along with the GE and complementary sections, there are six additional structural sections sponsored by Alabama, Florida and Oklahoma. These sections are being used to evaluate perpetual pavement design, rehabilitation strategies and methods to reduce surface cracking. The two Alabama sections have been in place since 2003, while the Oklahoma sections were built in 2006. The Florida sections were placed in 2009, with a goal of improving the quality of PFC surfaces by studying tack coat methods.
All surface performance sections are built on perpetual foundations to ensure that surface distresses are materials-related. FHWA, Georgia, Mississippi, Missouri, Cargill and Oldcastle Materials Group currently sponsor a total of nine surface performance sections, four of which have been left in place from previous testing cycles. Evaluations include binder modification with ground tire rubber (GTR), stone-matrix asphalt (SMA) aggregate specifications, long-term performance of an all-gravel PFC and long-term durability of 45 percent RAP mixes.
Pavement performance is tracked weekly using an inertial profiler to measure the International Roughness Index (IRI), macrotexture and rut depth. Also, sections are visually inspected for signs of cracking, and any observable cracks are carefully mapped. Performance data for each test section is available at www.pavetrack.com.
One of the biggest surprises of the 2009 research cycle is the excellent performance of the GE test sections. Previous experience on the track has shown that sections with the same pavement layer thicknesses had fatigue cracking well before 10 million ESALs; however, none of the new GE sections have shown any forms of distress to date. . Although the GE section with the PFC surface does appear to make a structural contribution, higher strain levels in the field may be indicative of a value that is lower than that of a conventional mix. As a rehabilitation surface in another experiment, PFC mix was found to extend the performance life of underlying dense mix with a history of cracking susceptibility (especially when placed with a spray paver). Another significant rehabilitation finding resulted from the use of the thinner Kraton mix as a maintenance treatment in a nearby section that had failed rapidly following conventional rehabilitation.
Test track research is also contributing to further understanding of laboratory performance tests and modeling predictions. The NCAT lab has conducted a battery of tests on the mixes from the test sections, and researchers have carefully analyzed data using 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.
Inertial Profiler Certification
During the interim period between the end of trafficking this fall and the beginning of construction next spring, the track will be used as the test bed for an inertial profiler certification program sponsored by the Alabama Department of Transportation (ALDOT). Validations of inertial profilers from various manufacturers will be made using measurements on the inside lane at the track. Several nearby states have indicated that they will also accept certification results from this program, which will be used by ALDOT to build a qualified products list. Contractors may also take advantage of a corresponding inertial profiler serial number and technician certification program at the test track this winter.
Fifth Cycle to Begin Summer 2012
Plans are being made for the next research cycle at the test track. Removal of discontinued sections is planned for next spring, and construction of new sections should be complete by the end of summer, allowing trafficking to begin in August 2012.
The focus of research for the test track’s fifth cycle will be exploring ways to help transportation dollars go further. Pavement preservation techniques will likely be a key component of the research plan, including thin overlays and inlays, microsurfacing, chip seals, and other surface treatments. Alternative binders and binder modifiers, such as GTR, are also expected to be a research focus. Another area of concern is the durability of drainable surface courses, specifically improving joint performance, eliminating raveling and evaluating tack coats. High percentages of recycling—both RAP and (recycled asphalt shingles) RAS—will also be a major focus of the 2012 track.
Since its original construction in 2000, the NCAT Pavement Test Track has been providing practical solutions for building and maintaining long-lasting, cost-effective roads. Federal and state agencies, as well as private sector organizations, benefit from the timely, real-world research conducted at the track. For more information on becoming a test track sponsor, contact Buzz Powell at 334.844.6857.