Tack Coat Specifications, Materials, and Construction Practices (2018) / Chapter Skim
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Literature Review 13 tack coat materials. The following definitions regarding the use of emulsions in tack coat have been offered (FHWA 2016)
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14 Tack Coat Specifications, Materials, and Construction Practices It should be noted that asphalt binders, in the terms used with tack coat emulsions, would be considered 100% residual asphalt because there is no added water that would need to separate and evaporate in this material. Asphalt cutbacks use some form of petroleum distillates blended with the asphalt binder.
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Literature Review 15 Agency responses regarding approved materials are presented in Chapter 3 and summarized in Appendices B and C Acceptance of Tack Coat Materials All state agencies have a procedure by which tack coat materials are accepted.
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16 Tack Coat Specifications, Materials, and Construction Practices individually calculate tack coat application rates by volume and by mass. The importance of verification was strongly emphasized.
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Literature Review 17 of a material assessment, while testing roadway specimens would evaluate the in-place bond strength. Kansas Department of Transportation (KDOT)
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18 Tack Coat Specifications, Materials, and Construction Practices The penetration grading system (ASTM D 946) describes the relative hardness of an asphalt binder based on the penetration test (AASHTO T 49)
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Literature Review 19 Asphalt emulsions are classified into three categories, based on the electronic charge surrounding the asphalt binder particles: anionic, cationic, and nonionic. Because like charges repel, the charge helps keep the suspended asphalt particles separated from each other so it will be more difficult for them to coalesce and settle out of suspension.
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20 Tack Coat Specifications, Materials, and Construction Practices Reduced-Tracking Asphalt Emulsions Non-tracking tacks are designed to improve the pavement performance by avoiding the tracking problems associated with traditional tacks. This material is typically manufactured to harden quickly and adhere minimally to tires.
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Literature Review 21 This 18-page document provides guidance in six sections: • Product Overview • DO'S & DON'T'S • Best Practices – Storage • Best Practices – Filling Distributor • Best Practices – Product Application • Troubleshooting Guide The guidance specific to material handling includes several categories of information. Guidance regarding storage includes best tank types, maximum storage times, agitation, storage temperature, compatibility with other liquids, contamination, freezing, and boiling.
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22 Tack Coat Specifications, Materials, and Construction Practices freeze, which will break the emulsion in the tank. Likewise, heating the emulsion to above the boiling point of water will cause premature breakdown of the emulsion on the heating surface.
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Literature Review 23 Immediately after filling, sealing, and cleaning, the sample containers should be properly marked for identification with a permanent marker on the container itself, not the lid. All samples should be packaged and shipped or delivered to the laboratory the same day they were taken.
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24 Tack Coat Specifications, Materials, and Construction Practices then decanted over a sieve to determine how much the asphalt particles coalesce. For cationic emulsions, a solution of sodium dioctyl sulfosuccinate in water is used in place of a calcium chloride solution.
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Literature Review 25 Penetration The penetration test (AASHTO T 49) is an empirical test that evaluates the relative hardness of an asphalt binder based on the distance a standard weighted needle penetrates into a conditioned binder sample at 25°C (77°F)
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26 Tack Coat Specifications, Materials, and Construction Practices at various conferences around the country, reaching an estimated 3,100 attendees. The Asphalt Institute maintains a website (http://www.asphaltinstitute.org/tack-coat-information/)
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Literature Review 27 Flexible Pavements of Ohio maintains a technical bulletin on their website dedicated to proper tack coat application, which also includes information about traffic maintenance and dealing with tracking. Another resource is the tack coat training manual originally published as Appendix F of NCHRP Report 712.
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28 Tack Coat Specifications, Materials, and Construction Practices All three national documents have tables showing recommendations for tack coat application rate on various surfaces, reprinted herein as Table 4. Many agencies have their own requirements for tack coat application rates, apart from any general recommendations made in national publications.
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Literature Review 29 The disadvantages include a potential loss of control of the product. Some agencies allow dilution only at the supplier's terminal, which will uphold the quality of the tack coat material.
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30 Tack Coat Specifications, Materials, and Construction Practices sand, dirt, caked clay, and other foreign material which might prevent proper bond with the existing surface for the full width of the application. Take particular care in cleaning the outer edges of the strip to be treated, to ensure the prime or tack coat will adhere.
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Literature Review 31 Some state specifications also provide guidance regarding weather limitations specifically for tack coat application. For example, Montana DOT specifications state, "Apply the tack coat when the ambient temperature is 50°F (10°C)
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32 Tack Coat Specifications, Materials, and Construction Practices State agency specifications typically address these temperatures for the materials they use. For example, Wyoming DOT specifies a wide range of application temperatures for non-polymermodified emulsions used as tack coat, 70°F to 160°F, according to Table 40.4.3-1 in their Standard Specifications (Wyoming DOT 2010)
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Literature Review 33 Tack Coat Coverage According to the FHWA Tech Brief on Tack Coats, "It is extremely important that the emulsion be uniformly applied to the pavement surface to obtain full coverage." Non-uniform application, as shown in Figure 15, can lead to a lower bond strength.
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34 Tack Coat Specifications, Materials, and Construction Practices A little less than half of the responding agencies had verbiage in their specifications regarding the nozzles, almost all of which stated that the nozzles were to be clean and uniform. In practice, the distributor manufacturer can recommend nozzle sizes and configurations that best fit the type of liquid to be sprayed.
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Literature Review 35 Asphalt Institute's MS-22 Construction Manual, shows the recommended spray bar heights to allow double or triple coverage. Of the agencies surveyed, only one had a specification regarding spray bar height, a maximum of 12 in.
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36 Tack Coat Specifications, Materials, and Construction Practices ASTM D 2995 records two methods to calibrate asphalt distributors (see Figure 21)
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Literature Review 37 Tack Coat Tracking One perpetual problem with tack coat application using distributor trucks is that haul trucks normally drive on the applied tack coat, thus tracking the tack coat material and removing it from the pavement, as shown in Figure 23 (Mohammad et al.
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38 Tack Coat Specifications, Materials, and Construction Practices the distributor must be called back to reapply tack, either over the entire roadway surface or in tracked areas only, if they are limited to wheel paths, for example. Another scenario related to tracking is cleanliness of the tack.
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Literature Review 39 The tack is sprayed beneath the paver, behind the paver tracks and just in front of the screed. This eliminates the possibility of any vehicle driving over the tack, having it stick to the tires, and tracking it elsewhere.
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40 Tack Coat Specifications, Materials, and Construction Practices • Repeatable • Most widely promoted • Typically uses common lab equipment • Cleanly ranks materials • Has standard test method The following section gives an overview of some of the most commonly referenced studies. Investigation into Adhesion Properties Between Asphalt-Concrete Layers.
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Literature Review 41 Refinement of the Bond Strength Procedure and Investigation of a Specification. Follow-up work was done by another NCAT team a few years later (Tran et al.
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42 Tack Coat Specifications, Materials, and Construction Practices 0.00 to 0.08 gsy (0.36 l/m2)
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Literature Review 43 terms of interlayer bond strengths. The best laboratory bonding was commonly seen at 0.155 gsy (residual)
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44 Tack Coat Specifications, Materials, and Construction Practices shear test are found here and information on the tension test are be provided in the tension test section. Specifications for both testing styles are found in VTM-128 (VTM-128 undated)
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Literature Review 45 Bond Strength Measurement via Tension Measurement of bond strength via tension is the second most popular method, based on a review of the literature and the survey respondents. Three states (Kansas, Texas, and Virginia)
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46 Tack Coat Specifications, Materials, and Construction Practices Figure 31. Kansas tension tester (KT-78, 2012)
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Literature Review 47 Figure 33. Specimen ready for VTM-128 tension test (VTM-128 undated)
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48 This chapter presents a summary of the results of a 45-question survey. The survey was sent to the AASHTO SOM members and to Canadian provinces through the Transportation Association of Canada.
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Survey Results 49 66% 34% Individual Pay Item Incidental to Paving Figure 34. How is tack paid for on your contracts?
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50 Tack Coat Specifications, Materials, and Construction Practices Synthesis Survey: Tack Coat Materials One important goal of the synthesis survey was to assess the relative usage by agencies of the various tack coat materials available. The survey results indicate that the vast majority of tack coat materials used by agencies are emulsions, around 20% of which fall into the category of "reduced-tracking." The respondents were asked to submit the specific name of each tack coat material their agency uses, and to estimate roughly what percentage of tack coats in their state or province used the product.
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