Optimum Tack Rate for Hot-Mix Asphalt Bonding

Construction of hot-mix asphalt (HMA) pavement involves spraying tack coat on an existing surface in order to promote an acceptable interlayer bond and to ensure that multiple pavement layers behave monolithically. Insufficient tack coat application has been linked to premature cracking failure, a major factor in reduced HMA overlay life in Kansas. The Kansas Department of Transportation (KDOT) currently specifies that a slow-setting polymer-modified emulsion (SS-1hP) tack coat be applied at a rate of 0.23 L/m² (0.05 gal/yd²) for overlays. This study utilized accelerated pavement testing (APT) to determine the optimum rate of SS-1hP tack material. At the APT facility at Kansas State University, an existing pavement pit (20 ft long × 14 ft wide) was milled and cleaned with a broom and compressed air. The pit was then divided into four test sections with varying rates of SS-1hP (50, 100, 160, and 240 % of 0.05 gal/yd²). Inlay paving was done with a 12.5 mm (0.5 in.) Nominal Maximum Aggregate Size Superpave mixture. An 89 kN (20 kip) single-axle load was bi-directionally applied over a 6.1 m (20 ft) long lane at approximately 11.3 km/h (7 mph). Rut depth and strain measurements at the overlay interface were done at periodic intervals during application of 550,000 load repetitions. Results showed that the section with 50 % (of 0.05 gal/yd²) SS-1hP tack coat had higher rutting when compared to other sections.