by Jon M. Casey
In recent years, highway asphalt paving projects have evolved to include the addition of innovative new mix designs including Superior Performing Asphalt Pavements (Superpave™) and most recently stone matrix asphalt (SMA). With each improvement, increased costs accompany the new products. New binder formulations as well as the need for mix additives have helped to improve pavement durability and longevity in high stress situations. At the same time, these increased costs have caused engineers to question the overall benefits of using the newer, more expensive SMA in place of the familiar Superpave material.
In response to that inquiry, Dr. Randy West and Dr. Fan Yin, both Ph.D., P.E. engineers at the National Center for Asphalt Technology, presented a webinar on Jan. 29, aired by the National Asphalt Pavement Association (NAPA), titled “Performance and life-cycle cost benefits of stone matrix asphalt.” In a nutshell, they explored to determine if it’s worth the extra cost to use SMA over Superpave in high stress, heavy use paving and overlay applications. For those who would like the details of their findings, the entire study (funded by NAPA and the Federal Highway Administration) is obtainable at the National Center for Asphalt Technology (NCAT) website: http://eng.auburn.edu/research/centers/ncat/files/technical-reports/rep18-03.pdf
West began the webinar with a brief history of SMA. SMA is a mix design that was developed in northern Europe as a way to resist studded tire wear. These countries found that this mix was also resistant to rutting and was extremely durable. The design calls for an aggregate gradation that provides stone-on-stone contact. This is what provides the added strength. Additionally, a “rich” mortar binder content made with high concentration of asphalt and stabilizers gives the mix the added durability.
West said SMA was introduced to the U.S. in the early 1990s, just prior to the introduction of Superpave. “It is considered a premium mix that has an extended lifespan for higher risk pavement. It is more expensive than dense-graded mixes containing polymer-modified asphalt binders, however, the higher cost is offset by a longer life expectancy,” he said.
“In the 1990s, there were several studies that evaluated SMA mixtures. In a few cases, they were compared to dense-graded mixes. In the early days, the asphalt binders used in SMA in the U.S. were not polymer-modified. It was pre-Superpave days and it was common to use [existing binders].”
The early tests showed better rutting resistance due to the stone-on-stone aggregate structure and it had better resistance to moisture damage because of the thicker asphalt film between aggregate particles. The testing did not find any consistent trends on stiffness or cracking resistance. Field performance, conducted at NCAT in 86 pavement sections showed excellent results, however the tests were only two to six years old — thus did not offer actual longevity results.
For this presentation, West and Yin did a market analysis to determine which states actually use SMA. They then did a performance analysis and a life-cycle cost analysis. These comparisons of SMA to polymer-modified Superpave dense-graded mix designs were performed on similarly constructed and similarly trafficked highways.
West and Yin found at least 18 states use SMA on a regular basis. Their follow up work included surveys of the state highway agencies to determine such factors as field performance data, cost and tonnage from 2011 to 2015. They also looked at mix design specs and mix selection policies.
Most states have a policy on when SMA is to be used. Six leave it to a pavement engineer or district engineer to make that decision, while 10 have written policies. Nine states follow the AASHTO 46 mix design and two follow the AASHTO 35 design. Five have a state designed method, which adds or modifies either of the two AASHTO designs. During those five years, the lowest state usage was South Dakota at 68,000 tons, while the highest was Maryland, using 1.872 million tons.
Yin said SMA consistently proved to be more expensive than dense-graded mixes — with price variations of between $6 per ton in one state to as much as $31 per ton in another state. He said the higher prices of SMA were possibly due to higher asphalt contents, differing requirements for cubical and durable aggregates and the inclusion of fibers as stabilizers.
Yin noted that fibers as stabilizers are included in SMA because of the higher asphalt content. The fibers are there to help prevent asphalt drain-down during transport and application. He also noted that there was no use of RAP or RAS in the mix.
Yin explained that comparisons of SMA vs. Superpave dense-graded mixes required equivalent roadway categories and equivalent pavement types. When the results were submitted from the participating state DOTs, Yin and West separated the data into two categories — flexible pavement vs. composite pavement. Their data included more than 400 SMA test results and more than 800 Superpave pavement sections. These came from nine state DOTs.
Yin said the performance analysis was done using a Network-Level analysis approach. All the data from each single state were compiled using information from several sites to draw a single conclusion for each state. Since most of the data come from roadways that have not been in place for more than 10 years, Yin and West had to use performance prediction models to calculate the extended performance over the upcoming 10 or so years.
Yin presented a series of slides that summarized the results from states like Michigan, Virginia and Pennsylvania. West then summarized both flexible pavements and composite pavements with data from several additional states, which included Maryland, Illinois, Georgia, Alabama, Colorado and Minnesota.
Overall, SMA life extension generally outperforms Superpave in flexible pavement applications by an average of about 5.7 years. They noted that Alabama, Colorado and Maryland showed no apparent difference in longevity. Composite Pavement predicted service life extension for SMA sites proved much better with increases ranging from 0.9 years (Michigan DOT) to PennDOT’s 13.5 years life expectancy.
Interestingly, PennDOT reported the highest longevity on their non-interstate application at 13.5 years and they also reported the lowest at -0.9 years on interstate pavement. Meanwhile, VADOT reported the next longest extension at 10.3 years of extra life.
In the Virginia deterministic approach, SMA at $114/ton with a 23-year service life (composite pavements) compared to the Superpave at $89/ton with a 13-year service life (composite pavements) and the need for an overlay after 13 years showed a cost benefit of 18.5 percent per lane mile. By comparison, in Michigan, SMA needs a service life of 26 years to make it a cost effective alternative to Superpave.
SMA had equivalent or better performance than Superpave mixes on similarly trafficked highways in most cases. In cases where SMA had better performance, the improvement varied from one to 13 years. In each case the individual application needs to be examined to determine if the increased cost versus the extended life service is worth the difference. That can only be determined by the circumstances of how the roads are used and maintained over time.
In conclusion, Yin and West determined that there was not enough overall consistency in the results to conclude that the extended wear of SMA offsets the added cost per ton of material over that of Superpave on a general basis. Yin said that each state should do their own analysis to determine the benefits of both mixes in their respective locales before making a decision on which material to use. Overall, SMA did have better resistance to rutting and moisture damage over Superpave, but there weren’t consistent results on mixture stiffness and cracking. “In addition to performance benefits, SMA pavements demonstrated functional benefits such as improved visibility, reduced splash and spray, increased frictional resistance and noise reduction” (NCAT Report 18-03; Performance and life-cycle cost benefits of stone matrix asphalt).
For more information on the latest in paving information, visit www.pavementinteractive.org and the NCAT at Auburn University online.