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FOREWORD Highway administrators, engineers, and researchers often face problems for which information already exists, either in documented form or as undocumented experience and practice. This infor- mation may be fragmented, scattered, and unevaluated. As a consequence, full knowledge of what has been learned about a problem may not be brought to bear on its solution. Costly research findings may go unused, valuable experience may be overlooked, and due consideration may not be given to recommended practices for solving or alleviating the problem. There is information on nearly every subject of concern to highway administrators and engineers. Much of it derives from research or from the work of practitioners faced with problems in their day- to-day work. To provide a systematic means for assembling and evalu ating such useful information and to make it available to the entire highway community, the American Association of State High- way and Transportation Officialsâthrough the mechanism of the National Cooperative Highway Research Programâauthorized the Transportation Research Board to undertake a continuing study. This study, NCHRP Project 20-5, âSynthesis of Information Related to Highway Problems,â searches out and synthesizes useful knowledge from all available sources and prepares concise, documented reports on specific topics. Reports from this endeavor constitute an NCHRP report series, Synthesis of Highway Practice. This synthesis series reports on current knowledge and practice, in a compact format, without the detailed directions usually found in handbooks or design manuals. Each report in the series provides a compendium of the best knowledge available on those measures found to be the most successful in resolving specific problems. PREFACE By Mariela Garcia-Colberg Staff Officer Transportation Research Board In 2017, the state departments of transportation (DOTs) expressed an immediate need for a summary document of state of the practice of tack coat specifications, materials, construction practices, and acceptance procedures. As a result, the NCHRP commissioned this synthesis study that documents the current practice in both the Unites States and Canada. A literature review and detailed survey responses from all 50 DOTs (100% response rate) are provided. Detailed case examples of three states are also included in the report and provide additional insights on the state of the practice, and the importance of the selection of tack coat materials, application rates and methods, and bond testing on pavement performance. The information presented in the report will be extremely useful to 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 to improve pavement performance. This synthesis was prepared by Danny Gierhart and David R. Johnson. The members of the topic panel are acknowledged on page iv. This synthesis is an immediately useful document that records the practices that are acceptable within the limitations of the knowledge available at the time of its preparation. As progress in research and practice continues, new knowledge will need to be added.
1 Summary 5 Chapter 1 Introduction 5 Synthesis Objectives 6 Synthesis Scope 6 Report Organization 10 Chapter 2 Literature Review 10 Importance of Tack Coats 12 Tack Coat Definitions 14 Tack Coat Specifications 17 Tack Coat Materials and Products 20 Storage and Handling of Emulsions 23 Testing of Emulsions 25 Tack Coat Construction Practices 39 Testing and Acceptance of Tack Coats 48 Chapter 3 Survey Results 48 Synthesis Survey: Tack Coat Payment Specifications 50 Synthesis Survey: Tack Coat Materials 54 Synthesis Survey: Tack Coat Application 65 Synthesis Survey: Tack Coat Evaluation 70 Synthesis Survey: General Questions 72 Chapter 4 Case Examples 72 Kansas 74 Texas 76 West Virginia 77 Summary 78 Chapter 5 Conclusions 78 Importance of Tack Coats 78 Tack Coat Specifications 80 Suggested Future Tack Coat Research 82 References 85 Appendix A Survey Questions for U.S. and Canadian Agencies 93 Appendix B Summary of Survey Results (U.S.) 103 Appendix C Summary of Survey Results (Canada) C O N T E N T S
111 Appendix D Kansas Department of Transportation: Bond Strength Special Provision and Best Management Checklists 118 Appendix E Louisiana Department of Transportation & Development Tack Coat Inspector Checklists Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.
1 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: â¢ Release of NCHRP Report 712: Optimization of Tack Coat for HMA Placement (NCHRP Report 712) (Mohammad et al. 2012) â¢ Creation and marketing of many new reduced-tracking tack coat products in the past few years â¢ Implementation in 2015 and 2016 by Federal Highway Administration (FHWA) of FHWA/AI Tack Coat Workshops in virtually every state, put on by the Asphalt Institute (AI) and the FHWA (FHWA/Asphalt Institute 2014â2016) 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. Information for this synthesis was collected through a literature review and an online survey. The literature review relied heavily on the following documents and slide presentations: â¢ NCHRP Report 712 â¢ FHWA-HIF-16-017, FHWA Tech Brief on Tack Coat Best Practices (FHWA Tech Brief on Tack Coats) â¢ National Asphalt Paving Associationâs (NAPAâs) Quality Improvement Publication 128, Best Practices for Emulsion Tack Coats (NAPAâs QIP 128) â¢ FHWA/Asphalt Institute 4-hour workshop: Tack Coat Best Practices However, dozens of research papers, agency specifications, and state tack coat guidelines were also reviewed and referenced. The AASHTO Subcommittee on Materials (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. departments of transportation (DOTs). There were additional responses from Canadian provinces and territories. 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 bond- ing 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. S U M M A R Y Tack Coat Specifications, Materials, and Construction Practices
2 Tack Coat Specifications, Materials, and Construction Practices If a proper bond is not established between the existing pavement surface and the new asphalt pavement layer, delamination may occur between the layers. Delamination, or sepa- ration into constituent layers, may manifest in a number of structural distresses. The pri- mary distress types associated with delamination are slippage cracking and fatigue cracking. An important aspect of this synthesis is what U.S. state and Canadian province agencies articulate about tack coats in their respective standard specifications. A review of U.S. agency documents revealed that tack coat specifications are generally arranged into seven areas: â¢ Approved materials â¢ Acceptance of materials â¢ Tack coat material handling â¢ Surface preparation â¢ Tack coat application â¢ Tack coat acceptance â¢ Method of payment Important considerations addressed by agency specifications are how tack coat is paid for and whether or not it is allowed to be diluted. If it is allowed to be diluted, the specifications can specify who is allowed to dilute it to help avoid accidental overdilution. Tack coat payment generally falls into one of two categories: 1. Price of tack coat is considered incidental to construction, that is, the cost of the tack coat and application is absorbed into the price bid for the asphalt mixture; or 2. Tack coat is paid for as its own bid item. The FHWA/AI Tack Coat Workshop recommended that tack be paid for as its own bid item. It was found that 67% of U.S. agencies and 86% of Canadian agencies pay for tack coat as its own pay item. The FHWA/AI Tack Coat Workshop also recommended that if tack is allowed to be diluted, that it should only be done by the tack coat supplier at the terminal, i.e., terminally blended. It was found that 46% of U.S. agencies and 57% of Canadian agencies allow their tack coat material to be diluted with water. Over 80% allowed the dilution to be done at the supplierâs terminal, but a significant percentage also allowed the tack to be diluted at the contractorâs storage tank (32%) and in the contractorâs distributor (48%). Although asphalt cutbacks, emulsions, and binders could all be used as tack coat materi- als, asphalt emulsions are by far the most commonly used. According to the survey, almost 80% of U.S. agencies use some type of standard emulsion, and 100% of Canadian respon- dents use emulsions. In the U.S., only 0.4% of tack coats are Performance Grade binders, and a little over 20% are reduced-tracking emulsions. One U.S. state sometimes uses RC-70 cutback as a tack coat material. A significant variety of proprietary emulsions or additives are available and marketed as non-tracking. However, it is more accurate to call these emulsions âreduced-tracking,â because they still require at least some time to set, even though the time is reduced from traditional emulsions. Reduced-tracking tacks are used to improve pavement performance by avoiding the tracking problems associated with traditional tack coats. This material is typically manufactured to harden quickly and adhere minimally to tires. When a lift of hot asphalt is subsequently placed over the tack coat, the hardened tack is reactivated by the heat, and bonds the new overlay with the existing surface. The survey indicated that U.S. and Canadian agencies on average specified tack coat appli- cation rates within typical recommendations, with the exception of rates for PCC surfaces.
Summary 3 Virtually every document reviewed indicates that the tack coat needs to be applied to a clean surface. Dusty or dirty surfaces could cause the tack coat material to stick to the dust and not the pavement, resulting in an easily tracked surface that will be subject to slipping and delamination. Power brooming is the most commonly specified method of cleaning, followed by a combination of power brooming and air blowing. One problem in the asphalt industry is the specification of tack coat application rates without clearly defining the product to which the rate applies. If the specified application rate is intended to be in reference to the residual asphalt, but is not clearly defined, up to 70% less binder could be applied if the application rate was misunderstood to be intended for a 1:1 diluted emulsion. About 80% of the U.S. agencies reported that they clearly specify whether their specified rate is for residual asphalt, undiluted emulsion, or diluted emulsion. Literature review documents agree that to obtain a sufficient bond between layers, the tack coat needs to be applied at the correct rate. Too little or too much could cause the slip- page issues that tack coat is designed to mitigate. There are several documents which suggest tack coat application rates for various surface types and materials. Table 4 in this synthesis shows the recommendations from a few of the most prominent documents: the FHWA Tech Brief on Tack Coats, NCHRP Report 712, and NAPAâs QIP 128. Tack coats are applied to the pavement surface using an asphalt distributor. For the dis- tributor readouts to be accurate, the distributor must be regularly calibrated. Over 50% of U.S. agencies and over 70% of Canadian agencies reported that they have no requirement to calibrate asphalt distributors. Some agencies require annual calibration, while some require calibration before every project. It is important that the emulsion be uniformly applied to the pavement surface to obtain full coverage. Non-uniform application can lead to a lower bond strength. One study esti- mated that a mere 10% loss in bond strength reduced the pavement life by 50%. Another estimated that a 30% bond loss was almost as bad as having no bond at all. Another found that no bond at all resulted in a 60% loss of life. Several issues were cited as reasons why a distributor might produce a tack coat with a non-uniform appearance: â¢ Clogged nozzles â¢ Incorrect nozzle size â¢ Incorrect nozzle orientation â¢ Incorrect nozzle configuration â¢ Lack of pressure in the spray bar â¢ Incorrect spray bar height â¢ Using a spray wand in areas that are accessible to asphalt distributors The most common agency responses to address non-uniform coverage were to make the contractor reapply the tack, to make the contractor reapply the tack at a reduced rate, and to require the contractor to clean the distributor nozzles. After a tack coat has been applied, the actual application rate is verified by about 80% of U.S. agencies and almost 70% of Canadian agencies. Most of the agencies that verify tack coat application rates use either a volume or mass applied calculation. A perpetual problem with tack coat application using distributor trucks is that haul trucks normally drive on the applied tack coat, remove some of it from the pavement, and track it to other areas of the roadway. Although agencies have methods to mitigate tracking, almost 60% reported that tracking continues to be a problem. According to the survey, the most