Research Briefs

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research briefs Glass and ceramics make pavement markings more visible Pavement markings are essential to road safety—they guide travellers along their journey from here to there. Many people get in their cars and drive to work every day, but seldom pay attention to one of the most important parts of the road. Standard pavement marking paints used by the Ohio Department of Transportation, and similar departments in many other states across the United States, incorporate glass beads to achieve that easy-to-spot reflectivity, also called retroreflectivity. ODOT incorporates glass beads into several of their pavement marking paints, with various bead specifications depending on the type of paint used. The beads are silicon dioxide glass (*99 percent) containing some trace metals. They range in diameter from about one to two millimeters depending on the type of paint in which they are used, according to an email from Steven Loeffler of ODOT’s chemical section. The glass beads generally are incorporated into the paint, although they also can be applied on top of the paint. Incorporated beads contain a mixture of silane-coated and noncoated beads so that some sink within the paint and some float to the surface, allowing the paint to maintain reflectivity as the sunken beads are exposed with wear. Loeffler says that, depending on paint type, pavement marking paints contain 30–45 percent glass beads by weight. Those glass beads are manufactured by Potter Industries (Malvern, Pa.), Weissker (Palestine, Texas), and Swarco Industries (Columbia, Tenn.), which mainly recycle cullet to produce the beads, says Loeffler. ODOT’s records cite $35 million in awarded contracts for pavement markings in 2013—a lot of money is spent on those humble lines. Many reports from DOTs across the country indicate the annual expenditures are so high partially because pavement markings The Ohio Department of Transportation mixes glass beads with pavement paints. need reapplication almost yearly, although wear does depend on the road surface, traffic volume, and local weather conditions. Beyond the seemingly more-standard glass beads, some states have also explored ceramic additions to pavement markings. Vermont’s DOT performed an indepth evaluation of how ceramic and glass bead mixtures stack up to solely glass beads in 2007. Although their tests showed that ceramics offer a fivefold increase in retroreflectivity, the effects do not last. And because the ceramic-incorporated paints cost substantially more, Vermont did not see value in using the paints unless, the report says, “ceramic beads can be protected from winter maintenance practices and other similar abrasion.” To that end, however, there may be a solution. Montana’s DOT has just begun an experiment (and Minnesota has dabbled, as well) to assess the performance of pavement markings containing ceramic elements and glass beads incorporated into grooves carved into the road. Montana’s ceramic clusters, produced by 3M and composed of a mixture of glass beads with silica, pigment, and a proprietary polymer, claim Credit: S. Loeffler; ODOT to improve durability and reflectivity. These efforts may be able to protect the more pricey ceramic-containing paints from wear, particularly from harsh scraping by snowplows. The experiment will run through 2018. n Glass energy landscapes have rough fractal basins The thermodynamics of glass states are not described by a simple model. But research from Duke University offers insights that may be help to unlock these mysteries by providing a new energy landscape of glasses. The work shows that the landscape, which maps all possible energy positions of the glass molecules, is much rougher than previously believed. According to a Duke press release, lead author Patrick Charbonneau says, “There have been beautiful mathematical models, but with sometimes tenuous connection to real, structural glasses. Now we have a model that’s much closer to real glasses.” Charbonneau, a professor of chemistry and physics at Duke, and a team of scientists used mathematical models and theory to analyze how glass molecules behave, complete with a new energy landscape and phase diagram. The article, published in Nature Communications, establishes a model in which the molecules within a glass settle into fractal states within basins in the energy landscape. In the paper, the authors compare movement between the energy positions with boating on a system of lakes: “In the liquid, all of the space can be explored. At lower water levels, each basin is a different glass. The free energy barriers hinder passing from one glass to another; the basin width allows for vibrational relaxation. The water level further determines what features of the landscape are experienced. Deep into the glass, the landscape roughness results in intrastate barriers that are associated with secondary relaxations. At very low water levels—deep into the fractal glass—lakes transform into a complex wetland with a hierarchy of small ponds. The very bottom of each of these ponds corresponds to a given realization of the force network, but the identification of the force contacts remains undetermined before the fractal regime is reached.” 14 www.ceramics.org | American Ceramic Society Bulletin, Vol. 93, No. 5


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