Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/307110
RoadScience 8 May 2014 Better Roads of geosynthetic interlayers is the bond that is established between the old pavement and the new overlay, they say. "This interfacial bond is directly affected by the type and amount of tack coat," Putnam and Bolger write. "Paving fabric interlayer systems are typically constructed beneath hot-mix-asphalt overlays having a minimum thickness of 1-1/2 inch to ensure enough heat to draw the asphalt tack coat up into the paving fabric." It's generally recommended the overlay temperature does not exceed 325 degrees Fahrenheit to prevent thermal damage to the paving fabric, the authors say, adding it's also recommended the overlay temperature is not below 250 degrees Fahrenheit because lower temper- atures may not generate sufficient heat to warm up the tack coat enough to saturate the overlying paving fabric to create a well-bonded interlayer that will be a moisture barrier and provide stress-absorbing benefits. In view of the increasing use of warm-mix asphalt, which may not reach those temperatures, Putnam and Bolger evalu- ated the feasibility of using paving fabric interlayers with the lower compaction temperatures encountered with WMA overlays. Using Petromat 4598, they studied paving fabric in- terlayers made with four different tack coat binder grades and asphalt overlays ranging from 200 to 300 degrees Fahrenheit using a melt-through test and an interlayer shear strength test to determine the minimum WMA overlay compaction temperature to ensure the interlayer system will perform properly. "The results indicated that the saturation of the paving fabric in an interlayer system is dependent on multiple fac- tors: temperature of the overlay, compaction effort used to compact the overlay and the grade of binder used for the tack coat," they conclude. "It's recommended that the compaction temperature of an asphalt overlay be limited to a minimum of 250 degrees Fahrenheit when using conventional interlayer construction practices." Geogrids reinforce structures While planar or sheet-like geotextiles provide separation be- tween pavement layers, 2D and 3D geogrids provide stabiliza- tion in addition to separation. Their benefits are such that they not only physically rein- force a weak subgrade, but also can allow reduction in asphalt and aggregate layers above, saving time and money. "Geogrids stabilize subgrades, providing a construction platform over a soft subgrade," Tensar's Schlessinger says. "But they also facilate design optimization of pavement sections, where the thicknesses of the structural layers of a pavement section may be minimized, particularly over competent subgrades... Both asphalt and aggregate base layers may be reduced in depth by stabilizing them with a geogrid. Because the geogrid stabilizes the sections, less material is required to achieve a certain performance criteria." The resulting mechanically stabilized layer is a composite layer of a defined thickness made up of unbound granular materials combined with one or more layers of geogrid. "The combination of the two materials creates an enhanced composite layer that has improved pavement properties and performance," Caltrans says. Geogrids may be biaxial or triaxial in design. A biaxial geogrid can be a punched-and-drawn polypropylene geogrid with rectangular or square apertures, resembling wire cloth with square openings. A triaxial geogrid has triangular Geocomposites promote drainage in pavement structures, from moisture moving both upward and downward in a base; here, geocomposite layers connect to longitudinal side drains to expedite discharge. Photo courtesy of Tensar