Tack Coat Rate and Geotextile Coverage Influence on Interlayer Bonding and Rutting Resistance in Flexible Pavements

Abstract

Interlayer bonding in flexible pavements has an essential influence on their structural strength and performance. In this study, the combined outcome of application rate of tack coat and the condition of geotextile coverage on the interlayer shear strength (ISS) and the rutting resistance of double-layer asphalt systems is investigated. Specimens of binder and wearing courses were prepared with CSS-1H (cationic slow-setting asphalt emulsion, low-viscosity grade, with harder asphalt residue) emulsified tack coat at amounts of 0, 0.3, 0.6, 0.9 and 1.2 L/m ² . Three interface patterns of geotextile coverage were tested; unreinforced, 50% and 100%. The bonding between layers was assessed by direct shear bond test, and the rutting resistance was evaluated by wheel tracking test.

Results revealed that the optimal amount of tack coat was highly dependent on the interface condition. Unreinforced specimens achieved the maximum ISS of approximately 1083.89 kPa at 0.3 L/m ² , while the geotextile-reinforced mixes required higher tack coat rates to develop sufficient bonding due to emulsion absorption by the geotextile. In general, control specimens showed higher ISS values compared to specimens compromising geotextile between layers, but there is a significant enhancement in resistance to permanent deformation for specimens modified with geotextile with a sufficient amount of tack coat. The full geotextile coverage with 1.2 L/m ² tack coat resulted in the best rutting performance, reducing the rut depth by approximately 28% when compared with the optimum unreinforced condition. At its optimum tack coat application rate of 0.9 L/m ² , partial geotextile coverage achieved a 17.4% reduction in rut depth relative to the optimum unreinforced condition, indicating a potentially economical alternative. The results emphasize the need to optimize the application rate of tack coat and geotextile coverage to provide balanced interlayer bonding and rutting resistance of asphalt pavements.