Investigation of Optimum Hot-Mix Asphalt Pavement Overlay Tack Coat Rate via Full-Scale Accelerated Testing

Tack coat is a thin, bituminous material layer which may be applied to an existing pavement surface to promote interface bonding between new and existing pavement components. This ensures a monolithic structure that carries traffic loads and releases thermal stress. Insufficient tack coat application can result in inadequate interface bonding, delamination, or debonding within pavement layers; conversely, excessive tack coat application can cause weak interface bonding, rutting, or bleeding. A slow setting anionic polymer-modified emulsion (SS-1hP) with a standard application rate of 0.23 L/m(2) is commonly used as the tack coat for hot-mix asphalt (HMA) pavement overlay. To investigate optimum tack coat rates, a 6.1 m x 4.3 m. HMA section was divided in this study into four test sections and coated with 50%, 100%, 160%, and 240% of 0.23 L/m(2) tack rate and overlay. The test sections were then subjected to 2 million load repetitions on an accelerated pavement testing machine with an 89 kN single-axle load. The transverse rutting profile and strain at the overlay interface were measured at intervals of 100,000 load repetitions. Laboratory and in-situ pull-off tests were conducted after 1.5 million load repetitions. The results indicate that the test section with a tack rate of 240% has the smallest rut depth, while the 160% sample shows the greatest strain. However, the test section with a tack rate of 100% (0.23 L/m(2)) showed optimal performance based on in-situ strain and in-situ bond strength tests.