Synthesis of Information Related to Highway Practices. Topic 48-02. Tack Coat Specifications, Materials, and Construction Practices

Tack coat is a light application of asphalt, usually an asphalt emulsion, onto an existing relatively non-absorptive pavement surface. Tack coat is normally applied between two asphaltic concrete pavement layers or between a new asphalt pavement layer that is placed over an existing Portland cement concrete (PCC) surface. It is used to provide an adequate bond between the pavement being placed and the existing surface, achieving a monolithic system capable of withstanding traffic and environmental stresses. Insufficient bonding between pavement layers decreases the pavement bearing capacity and may cause slippage. In addition, insufficient bonding may cause tensile stresses to be concentrated at the bottom of the wearing course. There has been a pronounced increase in interest regarding tack coat specifications, materials, and construction practices in the past few years, primarily due to: (1) Release of National Cooperative Highway Research Program (NCHRP) Report 712, “Optimization of Tack Coat for Hot Mix Asphalt (HMA) Placement” in 2012; (2) Creation and marketing of several new reduced-tracking tack coat products; and (3) Implementation of Tack Coat workshops in virtually every state in 2015 and 2016 by the Federal Highway Administration (FHWA) and the Asphalt Institute (AI). 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. As states review their tack coat related activities, the work being done in the area by other states becomes highly interesting. The previously mentioned FHWA/AI Tack Coat Workshops in each state included both pre-workshop and post-workshop meetings between personnel from state agencies, FHWA, AI, and other industry partners. The meetings established each state’s current tack coat specifications, materials, and construction practices, plus a discussion of what might be changed as a result of the information provided in the workshop. The goal of this study is to provide an overview of the current state of practice regarding specifications, materials, and construction practices. This information will aide state agencies as they review their current practices regarding tack coats, and assess what changes to their current specifications should be implemented. In addition this study will help agencies identify gaps in their current specifications and practices, and provide them with research information necessary to ensure a sufficient bond between subsequent pavement lifts. Specific goals include: (1) state of the practice on the use of tack coat materials and application rates for the various types of pavement surfaces; (2) state practice of quality control (QC) and quality assurance (QA) for tack coat material; and (3) state specifications for methods of payment of tack coat materials. Information will be gathered by literature review and a survey of the state departments of transportation (DOTs). Current FHWA guidance and AI guidance will also be reviewed. Topics to be studied include research and practice on the following items(not inclusive list): (1) Whether the tack coat is paid for as a separate bid item, or whether the cost of tack coat is included in the bid price for asphalt mixtures; (2) What tack coat products are available; (3) The method of acceptance for the tack coat materials (testing of material samples, certification, testing of bond strength, etc.); (4) For states testing tack coat bond strength, is the test monotonic or cyclic? Is the application of stress in tensile, shear, or torsion mode; (5) How each state chooses the best product for their agencies. Either based on performance, environment, cost, and availability; (6) The proper application rate based on the material type and surface upon which it is applied; (7) How each state verifies that the appropriate application rate of tack coat was applied; (8) Whether or not the tack coat is allowed to be diluted, and if so, whether it is allowed to be diluted in the field or whether it is required to be diluted at the supplier’s terminal; (9) Construction-related issues such as specification verbiage regarding pavement cleanliness, allowable surface temperatures, spraying vertical surfaces, nighttime application, verification of uniform application, limitations on distances allowed to be tacked in front of the paver, how to handle vehicle tracking of tack, etc.; and (10) The number of states that use spray pavers, and under what conditions they are used.


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  • Status: Proposed
  • Funding: $45000
  • Contract Numbers:

    Project 20-05, Topic 48-02

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Garcia-Colberg, Mariela

  • Start Date: 20160509
  • Expected Completion Date: 0
  • Actual Completion Date: 0
  • Source Data: RiP Project 40812

Subject/Index Terms

Filing Info

  • Accession Number: 01598967
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 20-05, Topic 48-02
  • Files: TRB, RiP
  • Created Date: May 11 2016 1:00AM