Evaluation of the Bond Strength of Asphalt Overlays

One of the imperative components contributing to longevity and performance of asphalt concrete overlays is sufficient bond strength between the new overlay and existing surface. If the multi-layered asphalt pavement does not have ample bond strength to endure dynamic loads from traffic, surface distresses may develop. Surface distresses may occur in areas of, but not limited to, stopping, starting, and turning. These surface distresses are, but not limited to, de-bonding, delamination, slippage, or fatigue cracking. Therefore, the objective of this study was to develop a test method and testing device for measuring the bond strength between pavement layers. The research evaluated tack coat materials and HMA mix designs commonly used by West Virginia Division of Highways. The project was conducted in both laboratory and field. The effects of mix design, tack coat type, surface condition, and moisture damage were evaluated. Initial testing concluded that laboratory compaction using Superpave Gyratory Compactor affected bond strength. It was also found that the cut face which represents the worn surface and the new HMA face of the sample exhibit different bond strengths. Increased nominal maximum aggregate size of the mix design increased bond strength, but not a significant amount. Cut faced samples exhibited higher bond strength than milled samples on average. The bond strengths of samples with or without tack coat were not statistically different. For milled and cut faced samples, trackless tack coat was superior to a standard SS-1h emulsion. Set time from 0.25 to 2.0 hours, was statistically significant for both milled and cut faced samples. The difference in bond strength for moisture damage was statistically significant.