53 Transportation Research Record: Journal of the Transportation Research Board, No. 2372, Transportation Research Board of the National Academies, Washington, D.C., 2013, pp. 53–60. DOI: 10.3141/2372-07 A. Salinas, Applied Research Associates, Inc., 100 Trade Centre Drive, Suite 200, Champaign, IL 61820. I. L. Al-Qadi, Illinois Center for Transportation, and K. I. Hasiba, and H. Ozer, Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, MC-250, 205 North Mathews Avenue, Urbana, IL 61801. Z. Leng, Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. D. C. Parish, Illinois Department of Transportation, Division of Highways, District 4, 401 Main Street, Peoria, IL 61602. Corresponding author: A. Salinas, asalinas@ara.com. cement concrete (PCC), existing HMA, and new HMA. Sholar et al. conducted a field study that evaluated parameters affecting bonding between HMA layers, including application rate, surface texture and condition, and mix type (3). The authors recommended an optimum residual application rate of 0.06 gal/yd 2 (0.26 L/m 2 ) and also reported that shear strength increased with tack coat curing time. Tashman et al. analyzed the effects of curing time, application rate, and surface tex- ture through field evaluation (4). Three devices were used: a Florida Department of Transportation shear tester, a torque bond test, and a University of Texas at El Paso pull-off test. The shear and torque test results showed that milling improves bonding between layers. The pull-off test showed greater strength only in the nonmilled sections. A study conducted at the Illinois Center for Transportation at the University of Illinois at Urbana–Champaign investigated the strength characteristics of an HMA-PCC interface by direct shear testing and accelerated pavement testing. Milling was found to provide the high- est shear strength, but tinning direction did not have a significant effect on interface bonding. Moisture conditioning severely decreased inter- layer strength between the HMA and PCC layers (5). Accelerated pavement testing results conformed to the outcome of the labora- tory study. Asphalt emulsions provided lower strains compared with RC-70 (cutback). PG 64-22 provided the highest shear strength at the interface, and surfaces prepared by milling provided better bonding and rutting resistance than tinned and smooth surfaces. Well-cleaned PCC surfaces resulted in lower interface shear rutting. The acceler- ated pavement testing validated the laboratory-determined optimum residual application rate: 0.04 gal/yd 2 (0.18 L/m 2 ) provided the lowest interface strains and shear rutting (6). As part of this research, a laboratory study was conducted to evaluate the bonding characteristics of tack coat when applied between HMA layers (7 ). The study recommended an optimum residual tack coat application rate of 0.04 gal/yd 2 (0.18 L/m 2 ) for unmilled aged and aged nontrafficked surfaces and 0.06 gal/yd 2 (0.27 L/m 2 ) for a milled aged surface. SS-1vh provided the highest shear strength compared with other tack coat materials. In addi- tion, curing time significantly influenced the shear strength at the interface. When curing time was increased from 15 min to 2 h, bonding was significantly improved. Milling the surface improved the interface bonding. The present study continued the previous laboratory study and aimed to validate its findings under field conditions. Twenty-six sections were constructed on I-80 to determine the optimum resid- ual tack coat application and to study the effects of surface texture and surface cleanliness. Two tack coat materials were used (SS-1hp and SS-1vh) and applied over milled HMA and fresh binder stone mastic asphalt (SMA). The milled surface was cleaned by broom- ing and by air-blast cleaning. On Illinois Route 98 (IL-98), three tack coat materials (SS-1h, SS-1hp, and SS-1vh) were applied at the verified residual application rates. Again, the effect of clean- ing was examined by using the broom and air-blast cleaning Interface Layer Tack Coat Optimization Alejandro Salinas, Imad L. Al-Qadi, Khaled I. Hasiba, Hasan Ozer, Zhen Leng, and Derek C. Parish Interface bonding is a key factor affecting pavement performance life. This study focused on optimizing in situ tack coat application rate and field installation. The objective was to validate the laboratory-determined optimum residual application rate and evaluate the field performance of tack coat materials. The parameters analyzed included two cleaning methods (broom and air blast), two paving procedures (spray paver and conventional paving with use of a distributor and a regular paver), tack coat type (SS-1h, SS-1hp, and SS-1vh), and existing pavement surface. Projects were conducted on I-80 and Illinois Route 98. Cores were tested by using the interface shear test device. A life-cycle cost analysis that considered construction materials and paving methods was performed. Results showed similar bond strength for both cleaning methods; how- ever, air-blast cleaning reduced the required optimum residual applica- tion rate. The resulting interface bond strength was similar when using either of the paving procedures considered. SS-1vh, a nontrack tack coat, performed better than any other material studied. Identification of the optimum tack coat application rate will help ensure cost-effective and efficient tack coat application in the field. Sufficient interface bonding between existing and new pavement layers is critical. Structural performance depends not only on the strength of the pavement layers but also on the bonding strength between layers. Poor bonding can lead to various types of distress, including debonding, slippage cracking, compaction difficulties, and early fatigue cracking, all of which lead to reduction in pavement life. Proper interface bonding strength can be achieved by using the appro- priate type, preparation, application rate, and application method of tack coat. Tack coat is a light application of water-diluted asphaltic material applied on an existing pavement to ensure adequate strength between layers and to provide monolithic behavior of the pavement layers (1). Mohammad et al. examined the effect of tack coat type, application rate, surface type, and surface texture with a full-scale test (2). Five tack coat materials [SS-1h, SS-1, CRS-1, trackless, and performance grade (PG) 64-22] were evaluated. The study found an optimum application rate of 0.15 gal/yd 2 (0.7 L/m 2 ). Milled hot-mix asphalt (HMA) provided the highest interface bonding, followed by portland