Optimization of Tack Coat for HMA Placement (2012)

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4This report presents the results of NCHRP Project 9-40, “Optimization of Tack Coat for HMA Placement.” This sec- tion 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). It is used to ensure adequate bond between the pavement being placed and the existing surface. A tack coat provides necessary bond- ing between pavement layers to ensure that they behave as a single system to withstand traffic and environmen- tal stresses. Tack coat is normally applied to an existing pavement surface before a new layer of asphalt concrete is placed. It may also be applied to the surface of a new hot- mix asphalt (HMA) pavement layer before the next layer is placed, such as between an HMA leveling course and an HMA surface course. Selection of an optimum tack coat material and application rate is crucial in the development of proper bond strength between pavement layers. Pavement surfaces with differ- ent conditions (e.g., new, old, milled, grooved, or cracked) require different tack coat application rates to achieve proper interface bond strength. In most paving operations, tack coat covers less than 90% of the existing surface. On the other hand, excessive tack coat may promote shear slippage at the interface. Most importantly, it is the residual amount of asphalt—not the quantity of diluted asphalt emulsion—that should be specified in tack coat applications. Few guidelines are available for the selection of tack coat material type, application rate, placement, and evaluation. In general, selection of tack coats has been mainly based on experience, convenience, and/or empirical judgment. In addition, quality-control and quality-assurance testing of the tack coat construction process is rarely conducted, resulting in the possibility of unacceptable performance and even pre- mature pavement failure. 1.2 Research Objective The research objective, as stated in the project descrip- tion, is “to determine optimum application methods, equip- ment type and calibration procedures, application rates, and asphalt binder materials for the various uses of tack coats and to recommend revisions to relevant AASHTO methods and practices related to tack coats.” 1.3 Research Scope Research tasks in this project were organized into two phases. In Phase I, a literature review was conducted to assess the current state of practice on the type of tack coat materi- als, application rates, application methods, and equipment calibration along with methods of measurement of tack coat quality, interface bond strength, and pavement performance related to tack coats. In Phase II, the research team conducted the necessary laboratory and field experiments to achieve the objective of this study. Variables and their ranges were care- fully selected in the experimental program through a world- wide survey on the state of the practice on the use of tack coats conducted in Phase I. The experimental program con- sidered emulsified tack coats and asphalt binder. In addition, the interface shear strength was evaluated for different types of pavement surfaces including old HMA, new HMA, milled HMA, and grooved portland cement concrete (PCC). The findings of this report, presented in Section 4, are expected to be applicable to different climatic and traffic con- ditions across the United States; however, use of the recom- mended test methods and construction guidelines should be demonstrated and validated in different projects with differ- ent traffic and climatic conditions. While the demonstration phase was part of the original project description (in Task 6), S e c t i o n 1 Introduction and Research Approach

5 the Panel elected to extend the experimental program con- ducted in Task 4 in order to consider additional variables and to conduct the validation process in a future stand-alone study. 1.4 Research Approach The research approach followed the one described in the project description (see Table 1). In Phase I, the research team conducted a review of the worldwide state of practice for the use of tack coats for both new HMA layers and HMA overlays on new, old, and milled HMA and for PCC pavements (Task 1). This review involved an extensive literature search of all pub- lished materials and ongoing research projects to obtain the lat- est information on the research of the bonding mechanisms of tack coat in pavement structure. Databases of TRB, the Trans- portation Research Information Service (TRIS), and COM- PENDIX were searched. In addition, researchers conducted a worldwide survey on the state of practice of tack coats. Based on the results of the literature review and the worldwide sur- vey, a statistically based test factorial was developed in Task 2 to (1) evaluate the bonding characteristics of tack coats; (2) select the tack coat material type and residual asphalt binder appli- cation rate required for optimum performance in new HMA pavement and HMA overlay construction, rehabilitation, and reconstruction; (3) calibrate application equipment; and (4) maintain field quality control and quality assurance. In Task 3, 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) and Task 2 (i.e., the design of comprehensive laboratory and field experiments) was submitted for review and approval by the Project Panel. In Phase II, the research team conducted the laboratory and field experiments approved in Task 3. In Task 4, the research team developed the Louisiana Tack Coat Quality Tester (LTCQT) to evaluate the quality of the bond strength of tack coat in the field and the Louisiana Interlayer Shear Strength Tester (LISST) for the characterization of interface shear strength of cylindrical specimens in the laboratory. Dur- ing the course of the experimental program, the research team constructed full-scale asphalt overlays at the Louisiana Trans- portation Research Center’s (LTRC’s) Pavement Research Facility (PRF). The overlays included different tack coat appli- cation rates between a new HMA overlay installed over several types of pavement surfaces including old HMA, new HMA, milled HMA, and PCC. Five types of tack coat materials were each applied at three application rates. Quality of tack coat application was evaluated using the LTCQT, specimens were *Project Panel recommended that Task 6 be conducted as a separate, stand-alone project. Task ID Task Description Task 1 Literature Review Task 2 Design a Comprehensive Experiment to Study Tack Coat Variables Task 3 Develop Field and Laboratory Devices for Evaluation of Tack Coat Bond Performance Subtask 3-1: Develop Laboratory Experiment to Evaluate Tack Coats Subtask 3-2: Develop Field Experiment to Evaluate Tack Coat Task 4 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 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 Table 1. Research approach in NCHRP Project 9-40.

6cored from the test overlays, and interface shear strength (ISS) was measured in the laboratory using the LISST device. Labo- ratory testing of extracted cores and the effects of installation and design variables on the ISS were evaluated. Based on the results of the experimental program, the research team adopted a finite element (FE) approach to relate laboratory-measured interface bond characteristics to field stresses in the pavement structure when subjected to vehicu- lar loading (Task 5). Results of the FE approach ascertained how each tack coat material type and application rate will perform in pavements and examined the main failure mecha- nisms at the interface. Recommendations were provided on the following: (1) the candidate test methods to measure the performance of tack coats and (2) minimum laboratory- measured interface shear strength to provide acceptable field performance. In Task 7, the research team developed instructional mate- rials for a training course for agency and contractor personnel on the function of tack coats and how their proper selection and application affects pavement performance.