National Academies Press: OpenBook

Tack Coat Specifications, Materials, and Construction Practices (2018)

Chapter: Chapter 1 - Introduction

« Previous: Summary
Page 5
Suggested Citation:"Chapter 1 - Introduction." 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.
×
Page 5
Page 6
Suggested Citation:"Chapter 1 - Introduction." 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.
×
Page 6
Page 7
Suggested Citation:"Chapter 1 - Introduction." 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.
×
Page 7
Page 8
Suggested Citation:"Chapter 1 - Introduction." 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.
×
Page 8
Page 9
Suggested Citation:"Chapter 1 - Introduction." 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.
×
Page 9

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

8 Tack Coat Specifications, Materials, and Construction Practices • Chapter 2—Literature Review – This chapter summarizes the information from the extensive literature review. The follow- ing outlines the subsections of the literature review chapter: � Importance of Tack Coats: This section discusses the reasons why a good tack coat is important. It reviews the typical pavement distresses that occur as a result of poor bonding between pavement layers. It also examines some of the probable reasons why poor bonding results in the reported distress. � Tack Coat Definitions: This section examines some key definitions related to tack coats. The information serves as background for concepts and terminology needed in subsequent chapters. � Tack Coat Specifications: This section discusses ways agencies have specified issues related to tack coats. It provides information about methods of material acceptance, how tack is paid for by agencies, a discussion of tack coat material dilution, and specified tack coat application rates. This information serves as background for concepts and terminology needed in subsequent chapters and is based on both the literature review and the surveys. � Tack Coat Materials and Products: This section provides information on the materials specified by agencies for use as tack coats. It includes basic information about emulsions, cutbacks, and asphalt binders, including which ones are specified as tack coat material and why. It also includes information about reduced-tracking materials and their usage. It discusses temperatures at which materials are applied, and how agencies choose what they believe to be the best product for their use. � Tack Coat Construction Practices: This section provides detailed findings on agencies’ and contractors’ best practices regarding the application of tack coat in the construction phase. Among the topics discussed are material storage and transport, equipment inspection and calibration, handling issues, surface preparation, the effects of environmental factors, longitudinal joint surfaces, and how agencies and contractors handle vehicle tracking of tack coat. � Testing and Acceptance of Tack Coats: This section discusses tack coat quality assurance (QA) processes. These are planned and systematic actions necessary to provide con- fidence that the product will perform satisfactorily in service or ensure the quality of the product is what it should be. Issues that are discussed include whether tack coat material acceptance is based on field sampling or source sampling, what types of tests agencies specify for acceptance of tack coats, and how agencies verify that the appropriate rate and surface coverage of tack coat is achieved. • Chapter 3—Survey Results – This chapter presents the results of the synthesis survey. The survey responses are shown in different ways. Sometimes graphs or charts summarize the U.S. and Canadian responses separately. Sometimes the U.S. and Canadian responses are combined and represented by a single chart or graph. Sometimes the responses are discussed in paragraph form, and not represented by a chart or graph. • Chapter 4—Case Examples – This chapter presents the results of more in-depth communication with three states that were identified as candidates for further study by the NCHRP Project 20-05/Topic 48-02 panel and consultants. Information is presented regarding their histories with interface bond testing, a summary of their testing methods, and discussion of how the additional focus on tack coats has progressed and been received in their state. • Chapter 5—Conclusions – This chapter provides a summary of the key findings and conclusions of the synthesis project including (a) the current and evolving state of practice and (b) a compilation of common agency recommendations and specifications for tack coat materials, construction, and practices for testing and acceptance.

Introduction 9 • Appendices A: Copy of the agency survey questionnaire B: Summary of responses from U.S. DOTs C: Summary of responses from Canadian provinces and territories D: Kansas Department of Transportation: Bond Strength Special Provision and Best Man- agement Checklists E: Louisiana Department of Transportation and Development Tack Coat Inspector Checklists

10 Importance of Tack Coats Pavement thickness design assumes that all lifts of a pavement work 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 struc- ture. A tack coat is a sprayed application of asphalt material upon an existing asphalt or PCC pavement prior to an overlay, or between layers of new asphalt concrete (FHWA 2016). If a proper bond is not established between the existing pavement surface and the new asphalt pavement layer, delamination may occur between the layers (NAPA 2013). Delamination, or separation into constituent layers, may manifest in a number of structural distresses. The primary distress types associated with delamination are slippage cracking and fatigue cracking. Figure 3 shows pavement slippage due to a poor bond between pavement layers. The photo on the left shows the typical U-shaped crack formed when vehicles apply the brakes while driving over a poorly bonded pavement, creating shear between the top and underlying lift which sub- sequently pulls the pavement surface apart. The photo on the right shows movement in the top lift due to a poor bond, which moved the top portion of a roadway core hole out of alignment with the rest of the core hole in the underlying lift. Regardless of the methodology employed, an assumption of all thickness design methods is that the pavement layers will be working together as a single monolithic unit. This is achieved only with proper layer bonding. When a traditionally designed asphalt pavement is properly bonded, the principal failure mechanisms are typically bottom-up fatigue cracking or rutting. If the pavement was designed and built as a perpetual pavement, the principal failure mechanism will be top-down cracking at the surface, typically confined to the top layers. If layer bonding is not achieved, neither traditionally designed pavements nor perpetual pave- ments will perform as intended. As shown in Figure 4, a pavement structure with fully bonded layers will behave as a monolithic structure. It will experience maximum compressive loading at the pavement surface and maximum tensile loading at the pavement base, with a neutral axis at some point in between. The wheel loads are distributed downward through the entire support system. A pavement structure with an unbonded layer(s) will behave as separate structures. As shown in Figure 5, the strain profile will not match that of an equally thick, but properly bonded pavement. This non-uniform strain profile in the pavement structure will lead to decreased fatigue life as the strains at the bottom of the debonded layer will quickly become excessive, which can result in fatigue cracking initiating at the bottom of that layer within the asphalt structure. C H A P T E R 2 Literature Review

Literature Review 11 Courtesy of Road ScienceTM Figure 3. Pavement slippage due to poor bond. Figure 4. Load distribution when bonded layers make monolithic pavement structure. In the state of Missouri, it was reported that an interstate pavement experienced fatigue crack- ing early in its life, after 8 to 10 years of service (Figure 6) (Missouri 2014). The state highway agency conducted a forensic analysis which included the collection of cores. As can be seen in Figure 7, bonding failure was found at various locations within this structure. Again, a pavement with unbonded layers will exhibit compromised performance (FHWA 2016). A recent AI investigation showed the cost of tack coats on new or reconstructed facilities was 0.1% to 0.2% of the total project costs. On mill and overlay projects, tack coat was 1.0% to 2.0% of project costs. If a bond failure occurred and the remedial action was needed, the cost to replace just the top lift was found to be between 30% to 100% of the original project costs. The lower end of the range was for new or reconstructed pavements which consist of multiple lifts, and the higher end was for a pavement that had seen a mill and overlay of just a single lift of material. It is clear that the cost of a tack coat application is relatively insignificant, but the cost of repairing a bond failure is very significant. Add to that cost the additional disruption to the users of the facility, and tack is clearly low-cost insurance for achieving good bonding between all layers of the pavement structure and good pavement performance (FHWA 2016).

12 Tack Coat Specifications, Materials, and Construction Practices Tack Coat Definitions “A tack coat is an application of bituminous material to a pavement surface prior to the placement of a subsequent lift of asphalt mixture for the purpose of providing a thorough bond between the two surfaces. A proper bond between pavement layers is essential in order to provide a monolithic pavement structure” (Mohammad et al. 2012). Both the literature review and the survey indicate that asphalt emulsions are the most com- monly used material for tack coats. It is important to use consistent language when discussing Courtesy of NCAT Figure 5. Changed stress profile due to debonding of lifts. Figure 6. Fatigue cracking in Missouri attributed to poor bonding.

Next: Chapter 2 - Literature Review »
Tack Coat Specifications, Materials, and Construction Practices Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!