“For the most part the project went pretty well,” Waldera notes. “There were a few things we had to get used to. The only modification we made to our plant was installing a vent in our drum to blow the cellulose fiber into it. The blower was leased from Hi-Tech Asphalt Solutions. The fiber is pretty much engulfed by the oil when it enters the drum. We used flyash as an added filler to meet SDDOT Class S gradation. The flyash slowed our production because of the high volume of dust it brings to the baghouse. We ran at about 60 percent of our normal hourly output.”
Hi-Tech’s fiber machine was fully compatible with the Commercial Asphalt plant computer and operated as part of the plant. The blower device operates on a gravimetric principle, dispensing fiber based on weight depletion from a known weight. The machine breaks down and fluffs the baled fiber in six distinct phases.
Fiber requires hotter temperature
Drum temperature for producing Class S ran between 325 and 350 degrees, slightly hotter than the typical 300 degrees of a standard asphalt mix. Waldera explains that Class S has to be hotter in order to keep the mix pliable between the time it leaves the plant and gets to the paver.
“This mix really hardens once it cools,” Waldera says. “There’s a narrower window for getting it transported while it’s hot enough. If it cools too much before it’s compacted, it’s almost unworkable.”
Because Class S hardens so quickly, SDDOT enforces a seasonal limit on producing the mix, stipulating it can only be used between June 1 and September 15. Ambient temperatures must be at least 60 degrees farenheit before the mix can be used.
“If it’s hauled too far, that becomes an issue too,” Costello points out. “Even when it’s hot, Class S is really sticky, what I call nasty. The nastier the mix is, the better quality the end product is. But it isn’t easy to do handwork with it. Think of it like a Rice Krispie bar, that’s the kind of texture you have. It’s pliable when it’s hot, but once it cools it’s very unforgiving.”
Cost is weighed against benefits
Before his company secured the Highway 81 project, they had some knowledge of Class S and what they would be required to do in order to produce the mix. They used Internet research as well as SDDOT expertise to learn more about Class S and what to expect during the project.
“We’ll probably see more Class S on some of South Dakota’s high traffic areas,” Waldera says. “The biggest adjustment for us was productivity. Our baghouse was the limiting factor. But we won’t hesitate to pursue another project that utilizes Class S.”
High polymer oils, such as those used in Class S, are becoming more common in asphalt mixes. Jebro, Inc. in Sioux City, IA, provided the high polymer oil blended into the Class S mix.
“The oil’s rubber-band-like qualities have proven to extend the life of all types of asphalt surfaces,” Noel Schulz, Jebro Marketing Manager, says. “Polymer oil is very common in asphalt mixes used on roads. We’re seeing more city, county and state government agencies using polymer modified liquid asphalt binders.”
An added benefit of Class S is the residual effect of liquid deicing products when it's applied to Class S roadways.
“Pores in the Class S hold the chemical. It doesn’t run off like it would on a standard asphalt surface,” Costello tells. “There’s also no need to chip seal Class S surfaces. The open nature of Class S is something we want to retain.”
While South Dakota’s DOT enjoys numerous benefits of Class S, Costello says the cost of producing and applying the mix is carefully considered in every state road project.
“It’s our premium mix,” Costello says. “We started using Class S in the eastern part of the state in 2004. We learned a lot on that first project, what worked and what didn’t. We tried to tailor the mix design to obtain the best possible effective and economic product. We’re fortunate to have high quality aggregate in both the eastern and western part of South Dakota. We don’t have to do anything special to make Class S work. With our freeze/thaw cycles, which affect the entire state, cracking road surfaces is probably our biggest challenge. We’re using Class S as a wearing course on interstate and high traffic roadways. We haven’t used a mat thicker than 1.25 inches and we haven’t used the mix long enough to draw conclusions about its overall longevity.