Let’s start with the obvious: not all additives do the same thing. Even when injected mid-stream to an asphalt pavement mix to create warm-mix asphalt (WMA), different additives do different things. They’re not necessarily bringing temperatures down. That’s not what they’re created to do; that’s not what they’re doing. Various additives used to create WMA bind elements or foam asphalt cement, or perform another chemical or mechanical reaction to effect a change on the behavior of the binder with the aggregate. The end result means your plant operator can run the procedure at a lower temperature, and you can realize all the benefits thereof.
Focus on two additives in particular. I’ve selected these two based completely on the kind availability of the two companies represented. In alphabetical order:
Evotherm® WMA technology from MeadWestvaco Corporation, North Charleston, S.C., is manufactured in Charleston and available around the world to promote adhesion of mix elements. In simple terms, the Evotherm WMA is a liquid that you inject into the binder stream. The Evotherm then creates a chemical reaction between the aggregate and the binder in the asphalt mix. Think of this chemical reaction as forever, all-over attraction that the manufacturer calls “compaction insurance.”
Ben Bredenkamp explained that Evotherm creates its own chemical reaction in place of the thermal reaction that happens in the typical HMA process. Once Evotherm is mixed into the binder, either at the terminal or asphalt plant, the plant operator can lower the production temperature and still see full coating of aggregate because of this chemical reaction.
Among the low-temperature benefits of WMA that plant operators have read repeatedly in this and other publications, Bredenkamp discussed the compaction benefits this additive offers. “Evotherm lengthens the compaction window,” he explained. What happens with the liquid additive is a chemical reaction that allows the asphalt binder to adhere to the aggregate fully at lower production temperatures than previously used. The lower production temperature equates to a lower paving temperature in the field, which gives the roller operators a longer window to achieve density behind the screed.
FORTA-FI represents three fiber reinforcement blends that Forta® Corporation, Grove City, Pa., formulated to reinforce HMA, WMA and hot/cold patch, respectively. Each of the three products is a proprietary blend that contains aramid and polyolefin fibers of ¾-inch length and other inert materials to provide reinforcement to the asphalt mix. Their purpose is to enhance the current mix designs.
At this time, Lane Construction Corporation in Bridgeville, Pa., is a certified producer of the fiber blends. The fibers have been used on projects in 14 states and 5 other countries. In the test of the fibers at Evergreen Drive at Arizona State University, researchers found, “The flexural strength and corrected flexural strength are increased by 14% and 25% respectively with the addition of the 1 lb/Ton dosage of the FORTA-FI blend. While various ratios of materials in the FORTA-FI blend were tested to determine the optimum mixture across a wide range of applications, it was determined that the 1 lb/Ton dosage of the FORTA-FI blend would be best, or optimum, in terms of flexural properties.”
Another parameter that proved the fibers’ mettle in the ASU study was the permanent deformation testing. From the study’s authors: “Two important characteristics were observed for the FORTA-FI mixture when compared to the control mixture. One was the endurance of the secondary stage, and the second was gradual/less accumulation of permanent strain. Both were attributed to the presence of the reinforcing fibers in the mixture, as this behavior is not typically observed in conventional mixtures. The fiber-reinforced mixture had higher Flow Time values than the control mixture (over 900% higher), and 700% lower slope values. These indicate that the FORTA-FI mixture has a much higher potential to resist permanent deformation than the control mixture.”
This study with other information Forta Corp has gathered in the field shows a family of fiber-based additives that offer a strengthening property to asphalt mix design. Regional salesman Scott Nazar shared that this property is an ideal offering for those breaking into the Thinlay® market. “It’s a no-brainer,” Nazar said. “The thinner lifts that asphalt professionals are being asked to put down can have this added strength to hold up longer and resist reflective cracking if the letting agency doesn’t allow a mill, profile or crack-stabilizing process for the underlying pavement beforehand.”
The resistance to cracking that Nazar mentions was also proved in the ASU study with crack propagation testing. From the study’s authors: “After the relationships between crack growth rates as well as slopes of crack growth rates for the 1 lb/Ton dosage of FORTA-FI mixture and the control mixture were developed, it was noted that the fiber-reinforced mixture had higher C* and slope values than the control mixture. This means that the fiber-reinforced asphalt mixture has a higher potential to resist crack propagation because of the reinforcement effect provided by the reinforcing fibers.”
It’s All About that F
Whether we’re discussing cracking potential or strength, temperature plays a role in the evaluation of an additive’s benefits. How high does a temperature have to go to melt recycled material or how low can a temperature go and still get all the rocks coated with bitumen?
The ASU study authors wrote: “The viscosity-temperature susceptibility relationship at lower temperatures showed no changes from virgin binder, which is positive and desirable, but at high temperatures, improved properties were observed to have higher viscosities. The modified binder is far less susceptible to viscosity change with increased temperatures…”
With the liquid additive Evotherm functioning as an anti-strip and adhesion-promoter, it allows lower production temperatures for asphalt mixes. That in turn allows longer cooling time behind the screed during which the rolling team can achieve bonus-worthy densities.