28 Transportation Research Record: Journal of the Transportation Research Board, No. 2444, Transportation Research Board of the National Academies, Washington, D.C., 2014, pp. 28–37. DOI: 10.3141/2444-04 The failure of tack coat on orthotropic steel bridge decks is of concern because the loss of bonding ability between layers jeopardizes the deck overlay. This study investigated tack coat failure on an orthotropic steel bridge deck overlay. A field survey on overlay distress was first conducted on a regular double-layered epoxy asphalt concrete overlay to identify the major distresses. A follow-up survey was conducted by using infrared spectroscopy to detect the changes in chemical functional groups that may respond to distress. Contact analysis in a finite ele- ment platform was used to analyze the influence of the tack coat on the mechanistic characteristics of the deck overlay. Finally, shear clamps were used to evaluate the shear resistance performance of the tack coat. Comparisons of tack coat materials were made to support maintenance strategies for deck overlays. Long-span bridges often employ orthotropic steel plates for the bridge decks because of the considerable lightweight and efficient design ability of the plates. Orthotropic steel plates usually serve as decks in cable-stayed bridges and suspension bridges. The bridge deck overlay provides riding comfort and prevents corrosion (1). The overlay plays an important role in ensuring service quality and extension of service life for the steel structure (2). Furthermore, deck overlays potentially contribute to increased fatigue life of the deck plate because the overlays reduce stress levels in the steel plate (3). However, there is still difficulty in the design and con- struction of overlays for orthotropic steel bridge decks because of the complexity of the girder structure of the bridges (4, 5). Ortho- tropic steel decks consist of a steel plate supported by stiffeners and crossbeams in the longitudinal and transverse directions, respec- tively. The stiffeners and crossbeams are welded to steel plate (3). This unique structure exhibits different stresses and strains in the deck overlay than those traditionally found in pavements. Efforts are still being made to enhance the design and construction of orthotropic steel deck overlays (6–11). One of the concerns in overlay design is the possibility of poor bond strength between the layers. The purpose of the application of tack coat is to provide high bond strength so the layers will act as a monolithic structure and withstand traffic loads. Compared with roadway design, the requirements for bridge deck overlays are extremely stringent. As a result, unfortunately, the tack coats used in roadway pavement are no longer valid in bridge deck overlays. Hence, upgraded tack coat materials were introduced or developed to enhance the bonding abil- ity at interface, including polymer modified asphalt (12), epoxy resin (13, 14), rubber asphalt mastics (15), and the Eliminator system (16), among others. An efficient method to enhance the shear resistance at the interface is to weld steel shear studs to the steel plate as shear connectors between the overlay and the deck (17 ). Some premature distresses found in steel bridge deck overlays seem to be associated with the failure of the tack coat. Separation between the overlay and steel plate is the result of loss of bond- ing capacity in the interface. Subsequently, shoving and U-shaped cracks will be generated as a result of horizontal forces caused by vehicles. Moisture will then infiltrate through the overlay, result- ing in damage to the paving materials and corrosion of the steel plate. Therefore, more research needs to be conducted to enhance the analysis and evaluation of the tack coat. OBJECTIVE The objective of this study was to investigate tack coat failure in orthotropic steel bridge deck overlays. The investigation included (a) a field survey of premature distresses of bridge deck overlay and use of Fourier transform infrared spectroscopy (FTIR) to detect chemical functional groups in the binder, (b) numerical analysis of the effect of the tack coat on the mechanistic characteristics of the deck overlay, and (c) lab evaluation of the performance of tack coat for overlay maintenance. In this study, the investigated project rep- resented a common overlay system in orthotropic steel bridge decks in China. The mechanical analysis was conducted on a platform of finite element software. The shear test was performed on a composite specimen with two types of tack coat and asphalt binders. SURVEY OF TACK COAT FAILURE IN DECK OVERLAY Field Survey The investigated steel bridge, which connects Changxing Island with north Chongming county in Shanghai, China, is a cable-stayed bridge with a span of about 700 m (765.9 yd). It opened to the public in late 2011. The estimated traffic is about 13,000 to 18,000 pas- senger car units per day (pcu per day). There are three driving lanes and one emergency lane in each direction with approximate width of 3.5 m (11.5 ft) and 3.0 m (9.8 ft), respectively. Investigation of Tack Coat Failure in Orthotropic Steel Bridge Deck Overlay Survey, Analysis, and Evaluation Xiaoyang Jia, Baoshan Huang, Benjamin F. Bowers, and Tyler E. Rutherford Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, 851 Neyland Drive, Knoxville, TN 37996. Current affiliation for B. F. Bowers: Virginia Center for Transportation Innovation and Research, 530 Edgemont Road, Charlottesville, VA 22903. Corresponding author: B. Huang, bhuang@utk.edu.