Measurement and Evaluation of Tack Coat Application Rate for 4.75 mm NMAS Superpave Overlay Mix

Bond strength between two layers of hot mix asphalt (HMA) pavement is a major issue. Poor bond may be attributed to the premature fatigue failure, top down cracking, slippage cracks in the surface layer, and also difficulty in compaction due to insufficient shear strength. A thin and uniform application of emulsified asphalt or tack coat ensures a good bond between the existing surface and the overlying course. This paper discusses the evaluation of bond strength between the 4.75 mm Nominal Maximum Aggregate Size (NMAS) thin overlay and the Hot-in-Place Recycling (HIPR) layer for different tack coat application rates. Tack coat measurements were taken on two rehabilitation projects on US 160 and K-25 in Kansas. Three different test sections were constructed on each project with variable rates of tack coat application. After one year in service, two in. diameter field cores were collected for laboratory bond tests. Laboratory pulloff tests show that most cores are fully bonded at the interface of the 4.75 mm NMAS overlay and the HIPR layer regardless of the tack application rate. The failure mode during pull-off tests at the HMA interface is highly dependent on the aggregate source and mix design of the adjacent layer material. This study also confirms that overlay construction with high tack coat application rate may attribute to the bond failure at HMA interface.