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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. Tack Coat Specifications, Materials, and Construction Practices. Washington, DC: The National Academies Press. doi: 10.17226/25122.
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Page 1
Page 2
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. Tack Coat Specifications, Materials, and Construction Practices. Washington, DC: The National Academies Press. doi: 10.17226/25122.
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Page 2
Page 3
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. Tack Coat Specifications, Materials, and Construction Practices. Washington, DC: The National Academies Press. doi: 10.17226/25122.
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Page 3
Page 4
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. Tack Coat Specifications, Materials, and Construction Practices. Washington, DC: The National Academies Press. doi: 10.17226/25122.
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4 Tack Coat Specifications, Materials, and Construction Practices common method to mitigate tracking is to allow the tack coat to break before driving or paving over it. The two next most common methods are to use reduced-tracking emulsions and to use spray pavers. Unlike traditional paving, where a separate distributor applies the tack to the roadway and the paving crew paves over it after it breaks, a spray paver contains an emulsion tank on the paver itself. The tack is sprayed beneath the paver, behind the paver tracks and just in front of the screed. This eliminates the possibility of any vehicle driving over the tack, having it stick to the tires, and tracking it elsewhere. Specialized tack coat material formulations, frequently polymer-modified, are often used with spray pavers. According to the survey, bond strength testing is performed most often as part of forensic pavement evaluations. The second most common reason agencies gave for bond strength testing is to evaluate a tack coat material as part of an approval process: 24% of U.S. agencies and 14% of Canadian agencies perform some type of bond strength testing. Only one state uses field bond strength testing routinely. Three states are included as case examples because of their experiences with interlayer bond testing as it pertains to tack coats. Those states were Kansas, Texas, and West Virginia. These were chosen based on discussions with the NCHRP Project 20-05, Topic 48-02 panel, and their unique experiences with tack coats and bond testing. Of the three states, Texas has the longest history of interlayer bond testing pertaining to tack coat, while West Virginia has the shortest. Kansas utilizes a tension style of testing, while West Virginia uses a shear test. Texas has experience with both methods as well as a limited amount of testing performed in torsion. All three states reported noticing improvements in their respective tack coats when additional attention is paid to the selection of tack coat materials, their application rates and methods, and bond testing. Based on the synthesis findings, it seems that agencies can improve tack coat performance in several areas: how they specify items relating to tack coat, proper material selection, thoroughly cleaning the surface before tack coat application, selecting the proper tack coat application rate based on existing surface type, proper asphalt distributor setup and calibra- tion, verifying that the targeted application rates have been applied in the field, and performing some type of bond strength testing. According to the survey, 15 states and 4 provinces and territories have either recently conducted tack coat related research or are currently conducting tack coat-related research. While much of the research seems to be focused on bond strength testing, several agencies are researching the performance of reduced-tracking materials, working to reduce tack coat tracking in the field, and producing general guidance for contractor and agency personnel.

5 There has been a pronounced increase in interest regarding tack coat specifications, materials, and construction practices in the past few years, primarily due to the following: • Release of NCHRP Report 712 (Mohammad et al. 2012) • Creation and marketing of many new reduced-tracking tack coat products in the past few years • Implementation in 2015 and 2016 by FHWA of FHWA/AI Tack Coat Workshops in virtually every state, conducted by the Asphalt Institute (AI) and the FHWA (FHWA/Asphalt Institute 2014–2016) As a result of these factors, state agencies across the United States are reevaluating their tack coat specifications, the materials they use, and the practices by which the tack coats are placed. At the 2016 Annual Meeting of the TRB Standing Committee on Surface Requirements of Asphalt Mixtures (AFK40), many practitioners discussed the immediate need of a summary document regarding the state of the practice of tack coat specifications, materials, construction practices, and acceptance procedures. AFK40 then submitted a synthesis proposal on the topic, resulting in this synthesis of practice. Synthesis Objectives The objectives of this synthesis were to compile information on the following: • Importance of bonding between pavement layers • Definition of tack coat • Purposes of tack coat • Specifications regarding the use of tack coats – Documentation of technologies used by agencies – Methods of acceptance – Whether tack coat is paid for as a separate bid item or included in the bid price for asphalt mixtures – Whether or not the tack coat is allowed to be diluted and, if so, under what circumstances – How application rates are specified • Tack coat material types – Asphalt cutbacks – Asphalt emulsions – Asphalt binders – Proprietary reduced-tracking (often marketed as “non-tracking”) materials C H A P T E R 1 Introduction

6 Tack Coat Specifications, Materials, and Construction Practices • Tack coat construction practices – Material handling – Equipment inspection and calibration – Pavement surface preparation – Effects of environmental factors – Tack coat application rates – Vehicle tracking issues • Testing and acceptance of tack coats – Types of tests used by agencies for acceptance or verification – Whether bond strength test specimens are loaded via creep loading, monotonically increas- ing stress or displacement rate, or cyclical – Whether bond strength tests specimens are stressed in tensile, shear, torsion, or some combination • The current and evolving state of practice • Agency responses regarding specifications, materials, best practices, and testing and acceptance Synthesis Scope Information was collected through a literature review and an online survey. The literature review relied heavily on the following documents and slide presentations: • NCHRP Report 712 (Mohammad et al. 2012) • FHWA-HIF-16-017, FHWA Tech Brief on Tack Coats (FHWA 2016) • NAPA’s QIP 128 (NAPA 2013) • FHWA/AI 4-hour workshop (FHWA/Asphalt Institute 2014–2016) However, dozens of research papers, agency specifications, and state tack coat guidelines were also reviewed and referenced. The AASHTO SOM members were surveyed to determine agency specifications, materials, practices, and acceptance procedures. The survey was also distributed to Canadian provinces and territories through the Transportation Association of Canada. There was a 100% response rate from the U.S. DOTs (Figure 1). There was a 54% (7/13) response rate from Canadian provinces and territories (Figure 2). The survey questions covered tack coat payment specifications, tack coat materials, tack coat application, and tack coat evaluation. The survey questions are presented in Appendix A. Appendix B summarizes the U.S. agency responses, while Appendix C summarizes the Canadian agency responses. Report Organization The information is organized into the following chapters: • Chapter 1—Introduction – The Introduction provides a summary of the background information for the synthesis. An outline of the synthesis objectives is presented, providing a broad overview of the goals of the synthesis. The scope of the work is presented, which summarizes the litera- ture review and survey. The final portion of this chapter shows how the information in the synthesis is organized, giving the title and a concise synopsis of each chapter.

Introduction 7 Figure 1. U.S. DOTs responding to the survey (shaded). Figure 2. Canadian provinces and territories responding to the survey (shaded).

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 516: Tack Coat Specifications, Materials, and Construction Practices provides state agencies across the United States with guidance as they reevaluate their tack coat specifications, the materials they use, and the practices by which the tack coats are placed. Pavement thickness design assumes that pavement works together as one layer, rather than a stack of individual layers working independently. Tack coats function as bonding agents between each lift of a pavement section to create the desired monolithic pavement structure. A tack coat is a sprayed application of asphalt material upon an existing asphalt or Portland cement concrete (PCC) pavement prior to an overlay, or between layers of new asphalt concrete.

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