Track Puts New Asphalt Technologies to the Test
The current cycle of the NCAT Pavement Test Track includes the first test sections sponsored by private businesses to validate the benefits of their technologies. Kraton Polymers, Shell and Trinidad Lake Asphalt are taking advantage of the opportunity to prove their products through real-world performance testing on the track, laboratory testing and advanced pavement modeling. The models will allow for the analysis of the unique paving technologies in virtually any pavement loading and environmental condition.
Technologies provided by the above firms were used to build structural test sections that are equipped with high-speed instrumentation arrays. Strain gauges are embedded at the bottom of the asphalt layers to monitor pavement response to traffic loading. Data from these devices are obtained safely and efficiently via a wireless data-collection system.
Performance of the test sections are tracked weekly. An inertial profiler is used to measure the International Roughness Index (IRI), macrotexture and rut depth. Also, sections are visually inspected for signs of cracking. Any cracks that appear will be carefully monitored, with the location and extent of cracking recorded.
Approximately 5 million ESALs have been applied to date—50 percent of the projected traffic loading for the testing cycle—with a scheduled project completion date of September 2011.
The Kraton section contains highly polymer-modified mixes, incorporating 7.5 percent styrene-butadiene-styrene (SBS) in all three lifts. This high level of polymer modification yields mixes that are very stiff but strain-tolerant in the laboratory, allowing the section to be designed with a reduced total asphalt thickness of 5.75 inches. In comparison, other comparable structural sections at the track have 7 inches of asphalt.
This reduction in total asphalt thickness with Kraton's highly polymer-modified mixes is a potential economic benefit for owner agencies, according to Buzz Powell, NCAT's assistant director and manager of the test track. "A potential advantage of this technology is the cost-to-benefit ratio associated with building thinner pavements while achieving comparable performance," says Powell.
Laboratory results and theoretical pavement modeling indicate that mixes with this level of modification provide increased resistance to both rutting and fatigue. As temperatures climbed this summer, the Kraton section experienced extremely high strain levels, yet performance has been excellent to date. There are no signs of fatigue cracking, despite the high deflections measured.
Shell Thiopave is a sulfur technology that replaces a percentage of the liquid asphalt in a mix. The two Thiopave sections have binder layers containing 30 percent sulfur by weight of asphalt and base layers containing 40 percent sulfur. The surface layers on both sections are conventional hot-mix asphalt (HMA). Because the specific gravity of sulfur is different from the specific gravity of liquid asphalt, a mix that contains 40 percent sulfur gravimetrically reduces the liquid asphalt requirement by approximately 25 percent. One section has an extra binder layer, giving it a total asphalt thickness of nine inches, while the other section has seven inches of asphalt, similar to the structural sections within the Group Experiment. Thiopave is a pelletized sulfur product, so no plant modifications are required for its use. In addition, Thiopave mixes are produced at warm-mix temperatures to avoid the emission of toxic H2S fumes.
Replacing a significant portion of liquid asphalt could make Thiopave an economically favorable choice, says Powell. The fact that Thiopave mixes are produced at warm-mix temperatures is a potential economic, as well as environmental, benefit of the technology. Life-cycle cost may also be reduced when factoring in the potential for increased pavement life. Performance to date for both Thiopave sections has been favorable on the track and in the laboratory.
Trinidad Lake Asphalt
Trinidad Lake Asphalt (TLA) is a natural asphalt that was used in the first asphalt pavements built in the U.S. more than 100 years ago. TLA is still used today in high-stress applications around the world, and it is now produced in a pelletized form to facilitate shipping and introduction into modern asphalt plants. To prevent coalescence during transport and storage, the pellets are coated with a small amount of clay material, which should be accounted for in the mix gradation.
The TLA section has a total asphalt thickness of seven inches like the other sections in the Group Experiment. TLA pellets account for 25 percent of the total binder in the mix. The remaining binder is PG 67-28. No plant modifications are required to incorporate the TLA pellets, which are added to the mix as if they were recycled asphalt pavement (RAP) with a high residual asphalt content.
Use of TLA in the U.S. was limited in the past because of the difficulty of introducing bulk material shipped from Trinidad and Tobago into the plant. Hardened material in drums had to be stripped and melted to facilitate mixing. TLA pellets now offer a viable alternative, especially when the cost of refined liquid asphalt is high. Powell says, "It's another tool in the toolbox to protect against potential price increases in liquid asphalt."
Performance of the TLA section has been favorable, with rutting results comparable to the control section, a PG 76-22 mix, within the Group Experiment.
Ongoing testing at the track will provide valuable performance data on these technologies from Kraton Polymers, Shell and Trinidad Lake Asphalt, each of which presents a promising alternative in today's challenging economy.