Geosynthetics in Flexible and Rigid Pavement Overlay Systems to Reduce Reflection Cracking (0-1777-1)

The primary objective of this research project was to evaluate geotextiles placed under or within a hot mix asphalt (HMA) overlay to reduce the severity or delay the appearance of reflection cracks. Researchers evaluated six geosynthetics, representing fabrics, grids, and composites in the laboratory using the TTI Overlay Tester. The overlay tester accommodates a 3x6x20-inch HMA beam and evaluates the relative ability of different beams (with and without a geosynthetic) to resist thermal cracking. Geosynthetics consistently increased number of cycles to failure in laboratory tests. Researchers then used fracture mechanics to evaluate the laboratory results and develop a new evaluation methodology termed the "reinforcing factor. " Comparative field test pavements were established in three regions of Texas (Amarillo, Waco, and Pharr Districts) with widely different climates. Due to unavoidable delays, only the Pharr test pavements were constructed before this report was written. These planned field tests will be constructed and all three will be evaluated for several years particularly regarding reflective cracking. Syntheses of the findings from the literature and personal interviews were prepared. Supplementary benefits of additional tack coat and a leveling course in overlay systems were demonstrated. Significant changes in pertinent construction specifications were recommended. When placing a self-adhesive grid product, particularly on a high-traffic facility, a tack coat should be applied on top of the grid to minimize slippage problems. Generally, the cost effectiveness of fabrics for reducing reflection cracking appears to be marginal. Information on cost effectiveness of grids and composites is not available. An asphalt-impregnated fabric or composite is essentially impermeable to water and thus can provide significant benefits or possibly cause significant problems. The presence of an impermeable layer in an overlay system should be carefully considered during the design stage. Finally, researchers prepared comprehensive guidelines for using geosynthetics with HMA overlays to reduce reflection cracking and recommended they be printed in a handbook.