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4 Section 1 Introduction and Research Approach This report presents the results of NCHRP Project 9-40, in the possibility of unacceptable performance and even pre- "Optimization of Tack Coat for HMA Placement." This sec- mature pavement failure. tion describes the problem statement, objective, scope, and research approach. 1.2 Research Objective The research objective, as stated in the project descrip- 1.1 Problem Statement tion, is "to determine optimum application methods, equip- Tack coat is a light application of asphalt, usually asphalt ment type and calibration procedures, application rates, and emulsion diluted with water, onto an existing relatively asphalt binder materials for the various uses of tack coats non-absorptive pavement surface (1). It is used to ensure and to recommend revisions to relevant AASHTO methods adequate bond between the pavement being placed and and practices related to tack coats." the existing surface. A tack coat provides necessary bond- ing between pavement layers to ensure that they behave 1.3 Research Scope as a single system to withstand traffic and environmen- tal stresses. Tack coat is normally applied to an existing Research tasks in this project were organized into two pavement surface before a new layer of asphalt concrete is phases. In Phase I, a literature review was conducted to assess placed. It may also be applied to the surface of a new hot- the current state of practice on the type of tack coat materi- mix asphalt (HMA) pavement layer before the next layer als, application rates, application methods, and equipment is placed, such as between an HMA leveling course and an calibration along with methods of measurement of tack coat HMA surface course. quality, interface bond strength, and pavement performance Selection of an optimum tack coat material and application related to tack coats. In Phase II, the research team conducted rate is crucial in the development of proper bond strength the necessary laboratory and field experiments to achieve the between pavement layers. Pavement surfaces with differ- objective of this study. Variables and their ranges were care- ent conditions (e.g., new, old, milled, grooved, or cracked) fully selected in the experimental program through a world- require different tack coat application rates to achieve proper wide survey on the state of the practice on the use of tack interface bond strength. In most paving operations, tack coat coats conducted in Phase I. The experimental program con- covers less than 90% of the existing surface. On the other sidered emulsified tack coats and asphalt binder. In addition, hand, excessive tack coat may promote shear slippage at the interface shear strength was evaluated for different types the interface. Most importantly, it is the residual amount of of pavement surfaces including old HMA, new HMA, milled asphalt--not the quantity of diluted asphalt emulsion--that HMA, and grooved portland cement concrete (PCC). should be specified in tack coat applications. The findings of this report, presented in Section 4, are Few guidelines are available for the selection of tack coat expected to be applicable to different climatic and traffic con- material type, application rate, placement, and evaluation. ditions across the United States; however, use of the recom- In general, selection of tack coats has been mainly based mended test methods and construction guidelines should be on experience, convenience, and/or empirical judgment. In demonstrated and validated in different projects with differ- addition, quality-control and quality-assurance testing of the ent traffic and climatic conditions. While the demonstration tack coat construction process is rarely conducted, resulting phase was part of the original project description (in Task 6),