Tack coat is an application of asphalt, usually an asphalt emulsion, onto an existing 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 accelerated pavement deterioration.
There has been a pronounced increase in interest regarding tack coat specifications, materials, and construction practices in the past few years, primarily due to:
• Release of NCHRP Report 712, “Optimization of Tack Coat for HMA Placement” in 2012;
• Implementation of Tack Coat workshops in virtually every state in 2015 and 2016 by the Federal Highway Administration (FHWA) and the Asphalt Institute (AI) and the resulting technical brief;
• Documentation of premature pavement failures; and
• Development and marketing of new tack coat products.
As a result of these factors, state agencies across the United States are reevaluating their tack coat specifications, construction manuals, training needs, the materials they use, and the practices by which the tack coats are placed and accepted.
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 aid state agencies as they review their current practices regarding tack coats, and assess what changes to their current specifications and inspection practices should be implemented in order to improve pavement performance.
Information will be gathered by literature review, a survey of the state DOTs and Canadian Provinces voting members of AASHTO Standing Subcommittee on Construction and AASHTO Standing Subcommittee on Materials, and a survey of the National Asphalt Pavement Association (NAPA). Current FHWA and AI guidance will also be reviewed. Further, case examples illustrating the effects of tack coat specifications, materials, and construction on pavement performance shall be included. Topics to be studied include research and practice on the following items (not inclusive list):
Specifications on the use of tack coats-
• Documentation of terminologies used;
• Method of acceptance for the tack coat materials such as testing of material samples, certification, performance testing (i.e. bond strength, etc.);
• 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;
• 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;
• QC and QA processes (including performance specifications);
• Application rate ( shot rate) based on residual asphalt binder or total emulsion;
• The application rate based on material type and surface upon which it is applied;
• Tack coat types (cold applied, hot applied, non-tracking, etc.);
• What tack coat products are available?
• How each state chooses the best product for their agencies (based on performance, environment, cost, and availability)?
• Field sampling and lab testing and/or source sampling.
• For states testing tack coat bond strength, is the test monotonic or cyclic? Is the application of stress in tensile, shear, or torsion mode?
• Practices of surface preparation (milling, cleanliness, etc.) prior to tack coat application;
• Effects of environmental factors (weather limitations, surface temperatures, seasonal limitations, etc.);
• How each state verifies that the appropriate application rate and surface coverage of tack coat is achieved?
• Application practices and rates of tack coat materials to longitudinal joint surfaces;
• Training activities (e.g. Communication between office, field and contractor)
• Communications (e.g. preconstruction meeting, )
• Special handling issues such as specification verbiage regarding short construction windows, nighttime application, limitations on distances allowed to be tacked in front of the paver, etc.;
• How to handle vehicle tracking of tack coat;
• The number of states that use spray pavers, and under what conditions they are used;
• Equipment inspection and calibration (conventional distributor trucks or spray pavers);
• Material storage and transport (short or long term, heating/agitation requirements).
• Mohammad, Elseifi, Bae, Patel, Button, and Scherocman. NCHRP Report 712: Optimization of Tack Coat for HMA Placement. Transportation Research Board of the National Academies, Washington, D.C., 2012.
• Dietz, Jason and Johnson, David. Notes from tack coat workshop meetings with state agencies during 2015 and 2016.
• Multiple states DOT Standard Specifications for Highway Construction.
• FHWA HIPA nation-wide workshops
• NAPA 2005 users guide
• FHWA Technical Brief on the Importance of Tack Coats for Asphalt Pavements
First Panel: September 9, 2016, Washington, DC
Teleconference with Consultant: September 29, 2016, 2:30 p.m., EST
Second Panel: June 1, 2017, Irvine, CA
Lyndi D. Blackburn, Alabama DOT
Steven Brakeall, Maryland State Highway Administration
Paul Burch, Arizona DOT
Steve Hefel, Wisconsin Department of Transportation
Louay N. Mohammad, Louisiana State University-Dept of Civil & Environ. Eng.
Gerald D. Peterson, Texas DOT- Construction Division
Jason M. Dietz, FHWA Resource Center
Morgan Kessler, Federal Highway Administration