Optimization of Tack Coat for HMA Placement

A tack coat is typically applied just before a hot mix asphalt (HMA) layer or overlay is placed. A tack coat is a simple, relatively inexpensive process included as an incidental item in many HMA construction specifications. Indeed, a tack coat is so simple in concept that its real importance is often overlooked. An inadequate tack coat product or improper application of a good product can result in costly failure. A review of the extensive body of literature on tack coat materials and their use suggests that tack coats are placed to bond pavement layers together, with the intent of providing a monolithic, impermeable structure or preventing slippage of HMA overlays. However, these views on the function of a tack coat are not universally accepted. Studies conducted on the strength of interfaces in HMA pavements have shown that a strong tack coat bond between pavement layers is critical for transfer of radial tensile and shear stresses into the entire pavement structure. On the other hand, no tack coat bond or an insufficient bond decreases pavement-bearing capacity and may cause slippage. No bond or insufficient bond may also cause tensile stresses to be concentrated at the bottom of the wearing course. Such concentrated stress may accelerate fatigue cracking and lead ultimately to pavement failure. Few guidelines are available for proper selection of tack coat material type, application rate, and placement. Optimum tack coat type and application rate will be determined by the type and condition of the existing pavement surface as well as other factors including material type and permeability of the HMA pavement overlay to be placed, the traffic loading, and the climate. Proper, uniform tack coat application requires close attention to equipment calibration and operating parameters. The objectives of this study are to determine optimum application methods, equipment type and calibration procedures, application rates, and asphalt binder materials for the various uses of tack coats and to recommend revisions to relevant American Association of State Highway and Transportation Officials (AASHTO) methods and practices related to tack coats. In accomplishing these objectives, both present and emerging technology in the United States and worldwide will be evaluated.


  • English


  • Status: Active
  • Contract Numbers:

    Project 9-40

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway & Transportation Officials

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Harrigan, Edward

  • Performing Organizations:

    Louisiana Transportation Research Center

    4101 Gourrier Avenue
    Baton Rouge, LA  United States  70808
  • Principal Investigators:

    Mohammad, Louay

  • Start Date: 20050701
  • Expected Completion Date: 0
  • Actual Completion Date: 20111231
  • Source Data: RiP Project 10564

Subject/Index Terms

Filing Info

  • Accession Number: 01549319
  • Record Type: Research project
  • Source Agency: National Cooperative Highway Research Program
  • Contract Numbers: Project 9-40
  • Files: RiP
  • Created Date: Dec 31 2014 1:01AM