Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/358505
RoadScience 6 August 2014 Better Roads However, the majority of asphalt pavement work is of the mill-and-fill variety, he says. In these situations, the synthetic fibers can be used to extend the life of the thin overlay, be- cause decreasing thickness is usually not an option. "Not all fibers are created equal, and the difference mat- ters for our roads," Nazar adds. "Before contractors make the decision to use fiber technology to reinforce roads, they need to know the differences between types of available fibers that could dramatically impact the life cycle of a project." Engineered fibers for asphalt may include asphalt stabiliza- tion additive fibers, asphalt reinforcement fibers and fiberglass reinforcement. "Historically, discussions about using fibers for asphalt mixes have centered on cellulose fibers to prevent drain down that occurs when using stone matrix asphalt mixes (SMA) or other high asphalt-content mixes," Nazar says. "The use of cellulose allows designers to put more asphalt cement in the mix without having a soupy mess." Another such mix is the polymer-modified, open-graded friction course (OGFC), which minimizes fines that could inhibit movement of surface water through the lift. Today's OGFCs utilize cellulose fibers to forestall drain-down of liquid asphalt, although warm mix asphalt used for OGFCs can elimi- nate cellulose fibers in those mixes. One of the first technologies using fibers was stress-absorb- ing membrane interlayer (SAMI). "Constructing SAMIs in- volves applying emulsion and spraying on a layer of fiberglass fibers, then adding on another coat of emulsion and aggregate chips," Nazar says. "The SAMI provides a two-dimensional mat for preventing cracks from migrating through the pavement structure." However, he adds, while the SAMI layer itself may not crack, cracking can occur in layers above and below the SAMI. Until now, most fibers were derived from plastic and often melted in the mix- ing process. A portion of the melted fiber did help to modify the asphalt, increasing rut resistance and also cold weather crack- ing resistance, he says. "We're in a different age of blended fiber use today," Nazar told Better Roads. "High-tensile strength fibers can be added to the asphalt mix during production at the plant, providing crack resistance throughout the entire depth of the pave- ment layer in which it is placed. High- tensile strength fibers include aramid fibers able to withstand temperatures from -320 degrees Fahrenheit to 800 deg Fahrenheit." A blend of fibers works well for the road agency, he says. "I've found a blend of aramid and polyolefin fibers is the way to go," Nazar says. "It's best of both worlds; the blend reduces both high temperature rutting potential and thermal cracking. I have even noticed improved densities of fiber mixes because the fibers keep the material from spreading out under the roll- ers during breakdown compaction. These high-tensile fibers have also shown to provide the best benefit when tested at high-strain values equal to or greater than 250 micro strains." Fibers enhance thin overlays Thin-asphalt overlays are a sustainable, low-cost alternative to conventional hot mix asphalt overlays, but their performance can be enhanced with the optimal design, including additives like fibers, say Songsu Son, Ph.D. candidate, and Imad L. Al- Qadi, professor of engineering, and director, Illinois Center for Transportation, University of Illinois at Urbana-Champaign, in their 2014 Transportation Research Board paper, Engineering Cost-Benefit Analysis of Thin Durable Asphalt Overlays. "Significant improvement of pavement performances has been achieved with regard to material selection and modifi- cation, mix design, and construction technology," they say. "However, most of these improvements require high-quality aggregate and expensive modified asphalt binder or special equipment for construction." In Washoe County, Nevada, FiberMat crack-resistant, stress-absorbing membrane puts a network of fiber- glass strands between two layers of polymer modified emulsion – all placed simultaneously – then topped with a layer of chips which is compacted with pneumatic rollers. Photo courtesy of Colas Solutions Inc.