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1 S U M M A R Y Selection of an optimum tack coat material and application rate is critical in the develop- ment of proper bond strength between pavement layers. Historically, selection of tack coats has been based mainly on experience, convenience, and empirical judgment. In addition, quality control and quality assurance testing of tack coat construction processes are rarely conducted, with the possibility of unacceptable performance at the tacked interface and even premature pavement failure as a result. As a part of the research activities 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. With this new test setup and proposed test method, the effects of pavement surface types, tack coat material types, as well as 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 recommended as part of NCHRP Project 09-40 to ensure adequate interface bonding. Main Objective The main objective of this project was to evaluate the effects of tack coat material type, pavement surface type, and application rate on the interface bond strength with the test method developed in NCHRP Project 09-40 in actual field projects to support its imple- mentation. The 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. The experimental program consisted of 10 field projects in six states that included 33 in-service test sections to quantify the effects of the selected variables. Field projects included four pavement surface types, namely, new hot-mix asphalt (HMA), existing HMA, milled HMA, and grooved portland cement concrete (PCC) pavements. Each field project used 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 purposes. During the course of the experimental program, tack coat distributor truck calibration and pavement surface texture measurement were performed in each field project before application of the overlays. Moreover, falling weight deflectometer (FWD) tests and manual pavement distress surveys were carried out both before and after the 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 materials were evaluated. Quality and rate of tack coat application were evaluated during construc- tion. Cores were drilled from the test sections to measure ISS, and tack coat materials were collected to characterize in the laboratory. These measurements were used to validate the Validation of the Louisiana Interlayer Shear Strength Test for Tack Coat
2 proposed test method and criterion and to relate tack coat application quality with the observed field performance. Further, an ISS predictive model was developed for use by practitioners during the planning of overlay construction activities, selection of tack coat material type, and calculation of application rate, given the project conditions. On the basis of the findings of this project, the following conclusions were drawn regarding interface bonding as determined by AASHTO TP 114, Standard Method of Test for Determining the Interlayer Shear Strength (ISS) of Asphalt Pavement Layers: ⢠For the effect of emulsified tack coat material type, nontracking RS tack coats with stiff base asphalt cements exhibited the highest ISS, and SS resulted in the lowest. However, nontracking RS tack coats with soft asphalt cements showed similar interface bonding when compared with SS tack coats because of similar rheological properties of the residual asphalt binders. ⢠For the effect of pavement surface type, the ISS was dependent largely on the type of pave- ment surface receiving tack coat (i.e., HMA versus PCC) and pavement surface texture (e.g., milled versus nonmilled). In general, milled HMA surface yielded the highest inter- face shear strength, followed by new HMA, existing HMA, and PCC surface types. ⢠For the effect of residual application rate, within the evaluated residual application rate range, ISS improved with the increase in the residual application rate for all tack coat types and for all pavement surface types. ⢠For the effect of service time, interface bonding strength increased with service time in all field projects and for all pavement surface types. This phenomenon was primarily attributed to tack coat curing, which was more pronounced with nontracking RS with stiff asphalt cement and SS tack coat materials on new HMA pavement surface. Further, the curing effect of tack coats increased with the increase in the residual application rate. ⢠Regarding FWD test results, mean center deflection decreased with service time in all field projects and for all pavement surface types. This phenomenon was primarily attributed to the densification effect of HMA overlays because of in-service trafficking and improved interface bonding with service time. ⢠Regarding short-term pavement performance, laboratory measured ISS values correlated well with short-term cracking performance of field pavements. Test sections in all field projects performed satisfactorily with regard to rutting and surface cracking. A few test sections did not meet the minimum recommended ISS threshold value of 40 psi. ⢠Regarding the nonlinear ISS predictive model, good agreement was noted between the measured and predicted ISS values. Recommendations ⢠This study confirms that the AASHTO TP 114 is a valid test method to determine ISS between pavement layers. With this test setup, the effects of pavement surface types, tack coat material types, tack coat application rates and methods, and service time can be accurately evaluated. Hence, this test method is proposed for quality control and quality assurance testing of tack coat construction processes and for evaluation of interface- bonding condition of in-service pavements. ⢠The minimum recommended ISS threshold value of 40 psi was found to be an appropriate criterion for satisfactory pavement performance. This study proposes using 40 psi as a specified value to avoid premature debonding between pavement layers and according to AASHTO TP 114 test method.
