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3 This report presents the results of NCHRP Project 09-40A, Field Implementation of the Louisiana Interlayer Shear Strength Test (LISST). This chapter describes the problem statement, objective, scope, and research approach. 1.1 Problem Statement Tack coat is a light application of asphalt, usually asphalt emulsion diluted with water, onto an existing, relatively non- absorptive pavement surface (1). Tack coat is applied on a pavement surface before overlay construction to ensure ade- quate bond strength between the overlay being placed and the existing pavement surface. If the interface cannot provide enough strength to resist stresses due to traffic and environ- mental loading, shear failure may occur at the interface. Poor interface bond strength may also accelerate the appearance of other distress, such as slippage and surface cracks. Tack coat is normally applied to an existing pavement surface before a new layer of asphalt concrete is placed. Tack coat may also be applied to the surface of a new hot-mix asphalt (HMA) pave- ment layer before the next layer is placed, such as between an HMA leveling course and surface course. Asphalt cement, cutback asphalt, and emulsified tack coats have been widely favored because they provide additional benefits, such as reduced energy consumption and environmental impacts, and improved personnel safety (2). As a part of the research activi- ties in NCHRP Project 09-40, Optimization of Tack Coat for HMA Placement, a test device referred to as the Louisiana Interlayer Shear Strength Tester was developed to determine the interface shear strength (ISS) of field extracted cores in the laboratory (1). With this new test setup and proposed test method the effects of pavement surface types, tack coat material types, and tack coat application rates and methods can be evaluated. In addition, an ISS threshold criterion and tack coat residual application rates for different surface types were proposed to ensure adequate interface bonding. 1.2 Research Objective The objective of this research was to evaluate the effects of tack coat material type, pavement surface type, and applica- tion rate on the interface bond strength with the test method developed in NCHRP Project 09-40 in actual field projects to support its potential implementation. Effects of interface bonding on short-term pavement performance were also investigated. These measurements were used to validate the proposed test method and criterion and to relate observed tack coat field performance to these test results. 1.3 Research Scope To achieve the objective of this study, a list of candidate HMA rehabilitation projects was identified across the United States to represent different traffic and climatic conditions. The experimental program encompassed 10 field projects in six states to quantify the effects of the selected variables. Field projects included hot-mix asphalt (HMA) overlays on new, existing, and milled HMA pavements and on portland cement concrete (PCC) pavements. Each field project con- tained at least one slow setting (SS) and one rapid setting (RS) nontracking tack coat material and thereby created one or more pairs of tack coats for comparison. Six types of emulsi- fied tack coat materialsâSS (SS-1H, CSS-1H, and SS-1) and nontracking RS (CBC-1H, CRS-1HBC, and NTSS-1HM)â were evaluated on four pavement surface types; however, CRS-1 HBC and SS-1 tack coat materials were used only on existing HMA and milled HMA surfaces. Initially, researchers planned to evaluate each type of tack coat material at two residual application rates: one specified by the state DOTs and the other recommended by NCHRP Project 09-40 (1). Incidentally, in some field projects, the state DOT specified and NCHRP Project 09-40 recommended the same applica- tion rates, with the resulting use of a single application rate. Necessary field experiments and laboratory characterizations C H A P T E R 1 Introduction and Research Approach
4 were conducted to achieve the objectives of this study. A man- ual distress survey was performed to evaluate the short-term pavement performance after one year in service. Further, measured pavement performance was evaluated in relation to the laboratory ISS test results. This study emphasized monitoring field performance of different types of emulsi- fied tack coat materials under different climatic and traffic conditions. 1.4 Research Approach The research approach is presented in Table 1.4-1. To achieve the objective, research activities were divided into six tasks. In Task 1, a comprehensive experimental plan was developed in collaboration with the project panel to evaluate the effects of tack coat material type, pavement surface type, various resid- ual application rates ranging from the current state DOT prac- tices to those recommended in NCHRP Project 09-40, and pavement structural condition on interface bonding charac- teristics in new HMA pavement and HMA overlay construc- tion, rehabilitation, and reconstruction. In the experimental program, variables and their ranges were carefully selected on the basis of the state of practice of using tack coat materials. In Task 2, the research team contacted state DOTs to secure field projects for this study. Ten field projects that included 33 in-service test sections (i.e., 6 existing HMA, 10 new HMA, 11 milled HMA, and 6 grooved PCC) were successfully iden- tified in six states that represented different traffic and cli- matic conditions according to the project objective. In Task 3, the research team conducted necessary laboratory and field experiments, as developed in Task 1. During the course of the experimental program, tack coat distributor truck calibrations and pavement surface texture measurements were performed in each field project before construction of the test sections. Moreover, a falling weight deflectometer (FWD) test, and visual distress surveys were conducted both before and after overlay construction. HMA overlay construction used different types of tack coat materials at various residual application rates between the overlay and underlying pavement layers. In total, six types of emulsified tack coat materialsâSS (SS-1H, CSS-1H, and SS-1) and nontracking RS (CBC-1H, CRS-1, and NTSS-1HM)âwere evaluated. During construction, the qual- ity and the rate of tack coat application were evaluated, and the tack coat materials were sampled to characterize in the laboratory. Laboratory testing of extracted cores and the effects of selected variables on the ISS were evaluated. In Task 4, the research team conducted a visual distress survey during the first year of service to monitor field performance of tack coats in terms of interface bonding. The distress survey included identification of slippage problems as well as rutting and crack- ing measurements. In Task 5, necessary data analyses were performed to validate the LISST and to relate the observed field performance to the laboratory experimental results. An analy- sis of variance (ANOVA) was performed with the measure- ments in Task 3 to determine the most influential factors that dictate the performance of tack coat in the field. Finally, in Task 6, findings of this study were summarized, conclusions were drawn, and recommendations were proposed for the best practice of using tack coat materials to ensure satisfactory pavement performance. Task ID Task Description Task 1 Develop the experimental plan. Task 2 Identify field projects in different climatic and traffic conditions. Task 3 Conduct approved experimental plan. Task 4 Monitor field performance. Task 5 Perform data analysis. Task 6 Prepare and submit final report. Table 1.4-1. Research approach in NCHRP Project 09-40A.