Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/605412
T hey're better known as tack coats, but "bond coats" better describes their function. The pur- pose of a bond coat is to "glue" an asphalt overlay to the asphalt layer below. Today – according to the Asphalt Pavement Alliance – some 94 percent of America's roads are surfaced with asphalt. But if a strong bond is not created between the layer on which the asphalt driving course is placed, be it asphalt or concrete, the new asphalt surface is at risk of failure. Bond, or "tack" coats provide the adhesive that binds the asphalt layer to the substrate, add- ing strength the same way fl imsy, thin laminates of lumber are glued together to create strong plywood boards. "Normally, when a new layer of hot- mix or cold-mix asphalt is placed, the surface of every rock and grain of sand is uniformly coated with a fi lm of asphalt," reports the Cornell Local Roads Program in New York State. "The initial fi lm thickness is only a couple thousandths of an inch thick. Over time, sunlight hardens the sur- face fi lm and makes the asphalt less sticky. Traffi c abrades the surface, and water helps to remove the coating. In one or two years the asphalt fi lm is worn off the surface, leaving the ag- gregate exposed. "From that point forward, there is nothing there to bond a new layer of asphalt concrete to the old one," the Cornell report says. "A tack coat replaces the worn-away asphalt and promotes bonding with the old pavement." But many local road agencies don't provide detailed tack coat specs, or don't make them a "pay item." They just leave it up to the contractor, who may see the bond coat as one more expense he doesn't need. Others ap- ply them, but do it imperfectly. Well after the contractor has de- parted the scene, poor bonding between two layers of hot mix asphalt becomes the cause of many pavement headaches. "Pave- ment bonding is essential to quality pavement perfor- mance," says Dave Johnson, regional engineer, The As- phalt Institute. "Se- lection of an ap- propriate tack coat material, applied in the recommended ranges, provides the glue necessary to the pavement for bonding." Like plywood, asphalt pavements are built of layers, and their bonding is critical to good performance. "Building asphalt pave- ments in layers provides a number of advantages, [as] each layer can be engineered to have specifi c charac- teristics and properties," reports the National Center for Asphalt Technol- ogy (NCAT) in spring 2015. For example, surface layers may be designed to provide high levels of friction, be porous to drain water and eliminate hydroplaning, and opti- mized to provide a low levels of tire/ pavement noise, NCAT says. Other EquipmentWorld.com | December 2015 53 Bond or 'tack' coat is applied evenly, with spray nozzle overlap, across the width of the lane to be paved. Why the bond coat? Multiple layers – each with its own function – comprise the modern asphalt pavement; bond or tack coats are the 'glue' that bonds the layers together. Image: Tom Kuennen Image: NCAT