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5 the Panel elected to extend the experimental program con- reconstruction; (3) calibrate application equipment; and (4) ducted in Task 4 in order to consider additional variables and maintain field quality control and quality assurance. In Task 3, to conduct the validation process in a future stand-alone study. the research team reported the findings of Phase I to the Project Panel. An interim report that provided a summary of the survey results and key findings of Task 1 (i.e., the literature review) 1.4 Research Approach and Task 2 (i.e., the design of comprehensive laboratory and The research approach followed the one described in the field experiments) was submitted for review and approval by project description (see Table 1). In Phase I, the research team the Project Panel. conducted a review of the worldwide state of practice for the use In Phase II, the research team conducted the laboratory of tack coats for both new HMA layers and HMA overlays on and field experiments approved in Task 3. In Task 4, the new, old, and milled HMA and for PCC pavements (Task 1). research team developed the Louisiana Tack Coat Quality This review involved an extensive literature search of all pub- Tester (LTCQT) to evaluate the quality of the bond strength lished materials and ongoing research projects to obtain the lat- of tack coat in the field and the Louisiana Interlayer Shear est information on the research of the bonding mechanisms of Strength Tester (LISST) for the characterization of interface tack coat in pavement structure. Databases of TRB, the Trans- shear strength of cylindrical specimens in the laboratory. Dur- portation Research Information Service (TRIS), and COM- ing the course of the experimental program, the research team PENDIX were searched. In addition, researchers conducted a constructed full-scale asphalt overlays at the Louisiana Trans- worldwide survey on the state of practice of tack coats. Based portation Research Center's (LTRC's) Pavement Research on the results of the literature review and the worldwide sur- Facility (PRF). The overlays included different tack coat appli- vey, a statistically based test factorial was developed in Task 2 to cation rates between a new HMA overlay installed over several (1) evaluate the bonding characteristics of tack coats; (2) select types of pavement surfaces including old HMA, new HMA, the tack coat material type and residual asphalt binder appli- milled HMA, and PCC. Five types of tack coat materials were cation rate required for optimum performance in new HMA each applied at three application rates. Quality of tack coat pavement and HMA overlay construction, rehabilitation, and application was evaluated using the LTCQT, specimens were Table 1. Research approach in NCHRP Project 9-40. Task ID Task Description Task 1 Literature Review Task 2 Design a Comprehensive Experiment to Study Tack Coat Variables Develop Field and Laboratory Devices for Evaluation of Tack Coat Bond Performance Task 3 Subtask 3-1: Develop Laboratory Experiment to Evaluate Tack Coats Subtask 3-2: Develop Field Experiment to Evaluate Tack Coat Field Tack Coat Application and Overlay Construction Subtask 4-1: Preparation of Test Lane for Tack Coat Field Application Subtask 4-2: Calibration of Tack Coat Application Rate Subtask 4-3: Field Tack Coat Application Subtask 4-4: Field Evaluation of Tack Coat Bond Performance Subtask 4-5: Overlay Construction Task 4 Laboratory Evaluation of Tack Coat Bond Performance Subtask 5-1: ISS Test for Field-Cored Sample Subtask 5-2: ISS Test at Various Temperatures for Field-Cored Sample Subtask 5-3: ISS Test for Lab-Fabricated Sample Subtask 5-4: ISS Test at Various Permeability and Surface Textures for Lab-Fabricated Sample Task 5 Recommend Test Methods, Criteria, and Construction Guidelines Task 6* Demonstrate the Use of Recommended Test Methods and Construction Guidelines Task 7 Prepare Instructional Materials for a Training Course *Project Panel recommended that Task 6 be conducted as a separate, stand-alone project.
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6 cored from the test overlays, and interface shear strength (ISS) perform in pavements and examined the main failure mecha- was measured in the laboratory using the LISST device. Labo- nisms at the interface. Recommendations were provided on ratory testing of extracted cores and the effects of installation the following: (1) the candidate test methods to measure the and design variables on the ISS were evaluated. performance of tack coats and (2) minimum laboratory- Based on the results of the experimental program, the measured interface shear strength to provide acceptable field research team adopted a finite element (FE) approach to relate performance. laboratory-measured interface bond characteristics to field In Task 7, the research team developed instructional mate- stresses in the pavement structure when subjected to vehicu- rials for a training course for agency and contractor personnel lar loading (Task 5). Results of the FE approach ascertained on the function of tack coats and how their proper selection how each tack coat material type and application rate will and application affects pavement performance.