Tack Coat Specifications, Materials, and Construction Practices (2018)

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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): Undiluted Emulsion—an emulsion which consists primarily of a paving grade asphalt binder, water, and an emulsifying agent. Diluted Emulsion—an emulsion with additional water added to it. The most common dilution rate is 1:1 (one part undiluted emulsion and one part additional water). Residual Asphalt—the remaining asphalt after an emulsion has set, typically 57% to 70% of the undiluted emulsion. Tack Coat Break—the moment when water separates enough from the asphalt to show a color change from brown to black. Tack Coat Set—when all the water has evaporated, leaving only the residual asphalt. Some refer to this as “completely broken.” It is important that specifications be clear in their language as they relate to tack coats (FHWA 2016). For example, a specification may read simply, “Apply the tack coat at a rate of 0.05 gallons per square yard (gsy).” This statement does not provide enough information. It could be interpreted to mean 0.05 gsy residual asphalt, 0.05 gsy undiluted emulsion, or 0.05 gsy diluted emulsion. Actual residual asphalt values will vary widely from each interpretation. If the specifier intended 0.05 gsy of residual asphalt and instead 0.05 gsy of undiluted emulsion with 60% residual asphalt were applied, the roadway would only have 0.03 gsy of residual asphalt. This amount is 40% less than intended. If 0.05 gsy of emulsion diluted 1:1 with water were applied instead, the roadway would have 0.015 gsy of residual asphalt. This amount is a full 70% less than intended. The FHWA/AI Tack Coat Workshop recommended that all application specifications be in terms of residual asphalt. Other products used as tack coats to a lesser degree are asphalt binders and asphalt cutbacks. ASTM D8, Standard Terminology Relating to Materials for Roads and Pavements, defines these products as follows: Asphalt Binder (bitumen)—a class of black or dark-colored (solid, semisolid, or viscous) cementi- tious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, of which asphalts, tars, pitches, and asphaltites are typical. Cutback Asphalt—petroleum residuum (asphalt) which has been blended with petroleum distillates. Figure 7. Cores from Missouri project showing debonded lifts.

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. When used as tack coat material, the distillates must evaporate or cure out, leaving the residual asphalt as the bonding agent in the tack coat. Tack Coat Specifications Nationally, an AASHTO standard practice for asphalt tack coat design is currently being balloted. At this time, the draft copy of the standard practice includes guidance regarding dilution, tack coat application and application rates, tack coat material based on anticipated traffic and time of day (daytime or nighttime), and how to convert residual asphalt binder rates to emulsified asphalt rates. The final version of the AASHTO standard practice may end up with different or additional guidance. A review of U.S. agency documents revealed that tack coat specifications are generally arranged into seven areas: • Approved materials • Acceptance of tack coat materials • Tack coat material handling • Surface preparation • Tack coat application • Tack coat acceptance • Method of payment Issues related to how agencies approve, accept, and pay for tack coat materials will be discussed in this section. Agency specifications regarding tack coat materials, construction, and testing will be included in the corresponding sections for those topics. Approved Materials All state agencies have some type of list or table that shows the allowable types of tack coat materials for that agency. The materials are often differentiated by type—asphalt binder, emulsion, cutback, reduced-tracking material, or some other specialized material. There are many instances of agency material specifications that are exclusively tailored to the particular agency. However, most tack coat materials used by agencies in the United States and Canada specify properties meeting one of the following AASHTO specifications: Asphalt Binders: AASHTO M 320 (performance-graded) or M 332 [multiple stress creep recovery (MSCR)] Asphalt Emulsions: AASHTO M 140 (anionic emulsions), AASHTO M 208 (cationic emulsions), AASHTO M 316 (polymer-modified cationic emulsions) Asphalt Cutbacks: AASHTO M 81 (rapid-curing), AASHTO M 82 (medium-curing), AASHTO M 316 (polymer-modified cationic emulsions) Reduced-tracking materials do not currently have an AASHTO specification. The AI maintains a database of current asphalt binder specifications for each U.S. state and sev- eral Canadian provinces and territories. The site also maintains a database of current emulsion specifications for each U.S. state (http://www.asphaltinstitute.org/specification-databases/).

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. The procedures typically define the following: • Overall acceptance type, usually one or more of the following: – Approved source list – Supplier certification of the product – Acceptance testing of field samples on a project basis – Acceptance testing of supplier samples on a material type basis • What will be sampled • Where the samples will be obtained • How the samples will be obtained • Who will obtain the samples • How many samples will be obtained • How often the samples will be obtained • How the samples will be handled • What tests will be performed on the samples • Testing pass/fail criteria • How the test results will be documented and shared • Ramifications of out-of-spec materials Sometimes the procedure is designated in the agency’s standard specifications. For example, West Virginia Department of Transportation (WVDOT) specifies their material acceptance requirements in the Standard Specifications, section 408.2.1.1. It states, “Approval of asphalt emulsions used for tack coat material will be handled by the Materials Control, Soils and Test- ing (MCS&T) Division. MCS&T maintains a list of all approved asphalt emulsion sources and grades. The local District Materials Section can provide a copy of the latest list. The list is also posted on the MCS&T webpage under the heading Approved Source/Product Listing. The use of non-approved material without prior testing by MCS&T may result in nonpayment of the item” (WVDOT 2017). Sometimes the procedure is designated in some other type of agency document. For example, Oklahoma DOT maintains a document entitled “SiteManager Sampling Frequency Report.” The document not only specifies sampling frequency information, but also designates whether a material is accepted based upon a document of certification or whether it is accepted based upon a passing test result of a physical sample. The type of certification document required or test method is also specified in the report. Tack Coat Acceptance Once a tack coat has been placed, specifications are needed to assess its acceptability. This acceptance is for the in-place tack coat, which is distinct from the acceptance of the tack coat material discussed previously. This type of acceptance covers (1) assessment of tack coat cover- age and (2) assessment of tack coat bond strength. These two topics will be discussed in further detail in upcoming sections. A significant portion of the FHWA/AI Tack Coat Workshop was spent discussing how to verify that the tack coat application rate received was the same as the tack coat application rate required. In addition to the lecture portion, attendees were allotted workshop time to

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. For example, Virginia DOT released a construction memorandum (TL-143) in 2016 that requires the use of a form (Figure 8) to verify tack coat coverage. The form allows verification by either a plate method or a yield calculation method and provides inspectors with an easy way to verify tack coat coverage in the field. A smartphone app, TackCoat, has been developed to make calculating tack coat application rates easier in the field. The app allows the user to verify the application rate based on the quantity of tack coat used and coverage area, estimate the coverage area given an applica- tion rate and quantity, or estimate the quantity of tack coat needed to cover a given area at a given application rate. The app allows the user to take into account the type of tack coat material (emulsion or straight binder), whether the emulsion is diluted or undiluted, the rate of dilution, and the percentage of residual asphalt. The app is available for iOS and Android. When search- ing for the app, do not include a space between the words, or the correct information will not be returned (i.e., type “TackCoat” not “Tack Coat”). The assessment of tack coat bond can be performed on laboratory-prepared samples or roadway specimens in the lab or on the roadway itself. Laboratory assessment would be more Figure 8. Virginia DOT field tack coverage verification.

Literature Review 17 of a material assessment, while testing roadway specimens would evaluate the in-place bond strength. Kansas Department of Transportation (KDOT) in Special Provision 15-06003 specifies a bond strength test on roadway specimens. KDOT uses their own tensile strength test, detailed in KT-78 Method for Determining the Tensile Adhesive Strength of Asphalt Pavement Tack Coat. Specimens must meet the criteria shown in Table 1. Agency survey responses regarding tack coat acceptance will be discussed in more detail in Chapter 3. Method of Payment Tack coat payment generally falls into one of two categories: 1. The 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. The tack coat is paid for as its own bid item. Tack Coat Materials and Products To be applied as a tack coat, asphalt binder must be in a liquid form. There are three ways to make asphalt binder a liquid: heating it, mixing it with solvent to make a cutback, or mixing it with water and an emulsifying agent (soap) to make an asphalt emulsion (NAPA 2013). Asphalt Binders ASTM defines asphalt as a class of black or dark-colored (solid, semisolid, or viscous) cementi- tious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, of which asphalts, tars, pitches, and asphaltites are typical (ASTM 2017). The terms bitumen and asphaltic bitumen are used in Europe and are synonymous with the term asphalt used in North America. Outside North America, the term asphalt is used to describe mixtures of bitumen with mineral aggregates. To avoid confusion, in North America liquid asphalt is often also referred to as “asphalt cement” or, more generically, as “asphalt binder” (Asphalt Institute 2011). Generally speaking, North America uses the Performance Grade (PG) binder system to clas- sify asphalt binders, while most of the rest of the world uses some form of the penetration grade system. The PG system (AASHTO M 320) describes asphalt binders based on the pavement temperatures under which the binder is expected to perform (Asphalt Institute 2001). The tem- peratures in the PG system are specified in 6°C increments. For example, a PG 64-22 grade meets all high-temperature requirements at least up to 64°C but lower than 70°C. It also meets all low- temperature requirements down to –22°C but higher than –28°C. TABLE 602-19: RECOMMENDED BOND TEST FREQUENCY Tensile Stress (psi) Bond Condition Recommended Test Frequency 70 good 1 test per week 35 - 69 fair 2 tests per week 35 poor Test each day Table 1. KDOT criteria for bond strength.

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). The penetration test determines the dis- tance a standard needle penetrates into a conditioned binder sample at 25°C (77°F). The higher the penetration number, the softer the binder. For example, a 200–300 penetration grade is softer than a 40–50 penetration grade (Asphalt Institute 2001). Asphalt binders are sometimes used as tack coat materials. The main advantage of using a straight asphalt binder is that no time is required for the material to break. The main dis- advantage of using straight binders is that they must be maintained at higher temperatures than emulsions or cutbacks in order to remain fluid enough to spray through a distributor. This is a safety concern with spraying high-temperature asphalt binder in close proximity to workers, and it also takes more energy to heat the binders. Asphalt Cutbacks An asphalt binder that is liquefied by diluting it with selected petroleum solvents is known as a cutback asphalt (Asphalt Institute 2011). When used as a tack coat, the cutback is sprayed onto the pavement surface. The solvent (also called distillate, diluent, or cutter stock) will evaporate, leaving the asphalt binder residue to perform its function as a bonding agent. The relative speed at which the solvent evaporates is primarily a function of the solvent type but is also a function of the percentage of residual asphalt in the cutback. The evaporation of petroleum solvents into the atmosphere may be restricted or prohibited by environmental regulations. Consequently, cutbacks are now used far less often than they have been historically. Cutback asphalts are divided into the following three types, based on the relative speed of evaporation: • Rapid-curing (RC): asphalt binder combined with a light diluent of high volatility, generally with a boiling point similar to gasoline or naphtha. Grades include RC-70, RC-250, RC-800, and RC-3000. RC specifications are detailed in AASHTO M 81. • Medium-curing (MC): asphalt binder combined with a medium diluent of intermediate volatility, generally with a boiling point similar to kerosene. Grades include MC-30, MC-70, MC-250, MC-800, and MC-3000. MC specifications are detailed in AASHTO M 82. • Slow-curing (SC): asphalt binder combined with oils of low volatility. Grades include SC-70, SC-250, SC-800, and SC-3000. AASHTO no longer maintains specifications for SC cutbacks, but SC specifications are still detailed in ASTM D 2026. Although several agencies have used cutback asphalt as a tack coat material in the relatively recent past, only one of the agencies (Illinois) surveyed as a part of this synthesis reported using cutback asphalts (RC-70) as a tack coat. A similar survey from 2012 performed as part of NCHRP Project 9-40 reported Kansas as the only state which used cutback asphalt as tack coats. In the current synthesis survey, Kansas reported using only slow-setting emulsions and one specialty product, Emulsion Bonding Liquid (EBL) as tack coat materials. Asphalt Emulsions Asphalt emulsion is a combination of three basic ingredients: asphalt, water, and small amount of an emulsifying agent. In the same process, these components are introduced into a mechanism known as a colloid mill, which shears the asphalt into tiny droplets. The emulsifier, which is a surface-active agent, keeps the asphalt droplets in a stable suspension and controls the breaking time. The result is a liquid product with a consistency ranging from that of milk to heavy cream, which can be used in cold processes for road construction and maintenance (Asphalt Emulsion Manufacturers Association 2009).

Literature Review 19 Asphalt emulsions are classified into three categories, based on the electronic charge sur- rounding 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. Anionic and cationic asphalt emulsions are by far the most commonly used materials for tack coats. Emulsions are further classified on the basis of how quickly the asphalt droplets will coalesce (i.e., revert to asphalt binder). The relative terms rapid-set (RS), medium-set (MS), slow set (SS), and quick set (QS) have been adopted to simplify and standardize this classification. The terms as written refer to anionic (negatively charged) emulsions. The letter “C” in front of the term, for example, “CSS,” indicates that the emulsion is cationic (positively charged). Emulsions are further identified by a series of numbers and letters related to the viscosity of the emulsions, the hardness of the base asphalt binders, and the presence of polymer modification. The numbers following the letter classification indicate the relative viscosity of the emulsion. For example, a CRS-2 is more viscous than a CRS-1. In certain grades, an “h” follows the number, indicating that a harder base binder was used in the formulation of the emulsion. An “HF” preceding some of the anionic grades (e.g., HFMS-1) indicates “high float.” High float emulsions have a gel-like quality, imparted by the addition of certain chemicals that help keep the tack coat emulsion from flowing off of the roadway. Table 2 shows the AASHTO-standard classifications of emulsions and indicates whether any surveyed agency uses the standard emulsion as a tack coat. It should be noted that many agencies have designated asphalt emulsions in addition to those seen in AASHTO. The survey associated with this synthesis showed that agency respondents have designated a total of 13 categories of emulsions outside of AASHTO standards. Anionic Asphalt Emulsion (AASHTO M 140) Used by Any Agency as Tack Coat? Cationic Asphalt Emulsion (AASHTO M 208) Used by Any Agency as Tack Coat? Polymer-Modified Cationic Asphalt Emulsion (AASHTO M 316) Used by Any Agency as Tack Coat? RS-1 CRS-1 - RS-2 CRS-2 CRS-2P, CRS-2L HFRS-2 - - MS-1 - - MS-2 CMS-2 - MS-2h CMS-2h - HFMS-1 - - HFMS-2 - - HFMS-2h - - HFMS-2s - - SS-1 CSS-1 - SS-1h CSS-1h - QS-1h CQS-1h - Table 2. AASHTO-standard emulsions used as tack coat in U.S. and Canadian agencies.

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. When a hot lift of asphalt is subsequently placed over the tack, the hardened tack is reactivated by the heat, and bonds the new overlay with the existing surface (Seo 2016). A significant variety of proprietary emulsions or additives are available and marketed as non-tracking. However, it is more accurate to call these emulsions “reduced-tracking,” because they still require at least some time to set, even though the time is reduced from traditional emulsions. It is not only the speed at which these products set that reduces tracking. They typically use a harder base asphalt binder that leaves a less tacky finish at ambient temperatures for reduced tracking while achieving good bond strengths when reactivated at overlay temperatures. Although there has been no official standardization of the nomenclature for the “non-tracking” products to date, many state specifications are using the initials NT or TT to designate these materials. In the survey for this synthesis, a total of 16 different designations were used by 15 U.S. agencies to classify reduced-tracking emulsions. Additionally, one Canadian province reported using a reduced-tracking tack supplied by a Canadian manufacturer. NCHRP Project 9-40 conducted considerable research into bond strength testing, amongst other tack-related research. The findings showed that trackless tack exhibited the highest shear strength of the emulsions tested and CRS-1 resulted in the lowest strength. These results related directly to the viscosity of the residual asphalt binders at the test temperature of 25°C (77°F). Texas Transportation Institute (TTI), in report No. FHWA/TX-16/0-6814-1, found in lab test- ing that the reactivation temperature [the average temperature between the existing surface and the loose hot mix asphalt (HMA)] significantly affected bond performance. As the reactivation temperature increased, so did the bond energy. The stiff-residue tack samples had higher bond energy than soft-residue tack samples. The paper “Effects of Temperature on Interface Shear Strength of Emulsified Tack Coats and Its Relationship to Rheological Properties” (Bae et al. 2014) was based off of NCHRP Project 9-40 work. In the conclusions, they stated, “Within the evaluated temperature range, the interface shear strength (ISS) of the tacked interface increased with the decrease in tem- perature. In general, the bonding performance, as measured by ISS, of the trackless emulsion was superior to that of the CRS-1 emulsion, especially at temperatures greater than 40°C.” Another conclusion was, “The binder grade for the residue of CRS-1 emulsion was PG 58-28. The high temperature grade for the residue of the trackless emulsion was PG 82. Trackless material was brittle at low temperature, and the low temperature PG binder grade could not be determined.” Storage and Handling of Emulsions Documents in the literature review that discussed storage and handling indicated that tack coat materials of any type must be stored and handled properly to optimize their effec- tiveness. A literature search of state specifications revealed that tack coat material storage and handling guidance is typically minimal, and often refers the user to the manufacturer’s guidance. Most manufacturers have technical support on hand to provide guidance, and many provide written guidance. Although many product guides include similar information, a specific example is Blacklidge Emulsions’ document named “UltraTack®—Anionic NTSS-1HM Trackless Tack® Product Guide.”

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. The UltraTack handling guidance from Blacklidge Emulsions for filling the distributor includes information about the minimum depth of material, heating and circulation requirements, calibrating temperatures, cleaning the distributor, end-of-day procedures, and temperature limitations. For general emulsion storage, handling, and sampling guidelines, the Asphalt Institute’s MS-19 Basic Emulsion Manual provides thorough information that has been established through a long history of successful use. The following sections provide an overview of these general emulsion storage, handling, and sampling guidelines drawn from MS-19, except where otherwise noted. Keep in mind that specially formulated emulsions may have procedures that differ from the following general procedures. Storage of Asphalt Emulsions Storage tanks should be insulated for protection from freezing and for efficient use of heat. A skin of asphalt can form on the surface of emulsions when they are exposed to air. Because tall, vertical tanks expose the least amount of surface area to the air, they are generally thought to be preferable to horizontal tanks. However, horizontal tanks can be kept full to minimize the area exposed to air. Propellers are turned slowly (approximately 60 RPM) to gently circulate the material. Agitating the emulsion at higher rates will typically increase the chances that the emulsion will lose its homogenous consistency and separate in the tank. Forced air should never be used to agitate the emulsion, because it may also cause the emulsion to break in the tank, separating the asphalt from the water. Pumps, valves, and lines should all be well insulated to help maintain proper temperatures. Maintaining the proper storage temperature, which will be different for different types of emulsions, is critical. Recommended storage temperatures for standard emulsion grades are presented in Table 3. In general, emulsions should be stored between 10°C (50°F) and 85°C (185°F), depending on the product. Water evaporates more quickly at elevated temperatures, changing the characteristics of the asphalt emulsion. The emulsion should never be allowed to Grades Temperature Minimum Maximum QS-1h, CQS-1h 10°C (50°F) 50°C (125°F) RS-2, CRS-1, CRS-2, HFRS-2, CMS-2, CMS-2h, MS-2, MS-2h, HFMS-2, HFMS-2h 50°C (125°F) 85°C (185°F) RS-1, SS-1, SS-1h, CSS-1, CSS-1h, MS-1 10°C (50°F) 60°C (140°F) Table 3. Recommended storage temperatures for standard emulsion grades.

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. Handling of Asphalt Emulsions Asphalt emulsion handling guidelines tend to be centered around safe handling, protection of equipment, and maintaining the quality characteristics of the emulsion. For worker safety, neither the asphalt emulsion nor the air above should ever be subjected to an open flame, heat, or strong oxidant. There should be adequate ventilation within the emul- sion handling area to avoid overexposure to fumes, vapors, and mists. Copies of the emulsions’ material safety information should be reviewed and kept readily available. Properly sized pumps should be used when handling asphalt emulsions. Tightly fitting pumps with inadequate clearances can bind and seize. Lines should be cleared and drain plugs opened when not in service. Follow all temperature, agitation, storage, and handling guidelines to keep the emulsion from forming two distinct layers in the tank. Tanks with separated emulsion material (asphalt on bottom with water floating on top) are difficult to clean and restore to their intended function. In addition to these storage guidelines, there are several other recommendations that are intended to preserve the quality characteristics of the emulsion. When filling tanks, different classes, types, and grades of emulsified asphalt should not be mixed together because of the risk of separation. Even emulsions within the same grade designation can be very different chemically and in performance. Always consult the emulsion supplier for compatibility information before mixing different emulsions within the same grade. Avoid repeated pumping and recirculating, as the viscosity may drop and air may become entrained, causing the emulsion to become unstable. Pumping from the bottom of the tank will minimize contamination from any skin formation present in the tank. Sampling of Asphalt Emulsions The purpose of any sampling method is to maintain the representative nature of the sample to the larger quantity. AASHTO R 66 details the Standard Practice for Sampling Asphalt Materials. A few of the requirements regarding the sampling of asphalt emulsions are described below. The type of container used to collect the sample is important to maintain the stability of the emulsion. According to AASHTO R 66, the minimum 1 L (1 qt) container should always be new, and not washed or rinsed. Containers for emulsified asphalt samples need to be plastic wide-mouth jars or bottles with screw caps. Worker safety is an important consideration when obtaining samples. Gloves should be worn and long sleeves rolled down and fastened over the wrist while sampling and sealing containers. A face shield should be worn while sampling. Smoking is prohibited while sampling bituminous materials. Prolonged exposure to fumes, vapors, and mists should be avoided. The sample should not be transferred from one sample container to another. The filled sample container should be tightly and positively sealed immediately after the sample is taken. The con- tainer should not be submerged in a solvent in an attempt to clean the outside of the container, nor should it be wiped with a solvent-saturated cloth. Any residual material on the outside of the container should be wiped with a clean, dry cloth immediately after the container is sealed. During sealing and wiping, the container should be placed on a firm level surface to prevent splashing, dropping, or spilling the sample.

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 sam- ples should be packaged and shipped or delivered to the laboratory the same day they were taken. The sample containers should not be placed on a dashboard or in direct sunlight in a truck. They should be secured to keep them from rolling around in the transport vehicle. The samples should be tested no more than 2 weeks from the date of sampling. Specialty emulsions may need to be tested sooner. Testing of Emulsions Asphalt emulsions are generally tested according to the requirements of one or more of the following AASHTO specifications: • Anionic emulsions: AASHTO M 140 • Cationic emulsions: AASHTO M 208 • Polymer-modified cationic emulsions: AASHTO M 316 The prescribed tests are run either on the emulsion or the asphalt binder residue. The following section briefly describes each test specified. The emulsion tests described in this section are all contained within AASHTO T 59, Standard Method of Test for Emulsified Asphalts. Tests on the Emulsion These tests are intended to characterize the emulsion as a whole. Saybolt Furol Viscosity The Saybolt Viscosity test is used to measure the consistency (rate of flow) of asphalt emulsions. The material must be thin enough to be uniformly applied through the spray bar of distributor, yet thick enough so that it will not flow from the crown or grade of the road. The test is performed at one of two temperatures, 25°C (77°F) or 50°C (122°F), depending on the specification requirement. Basically, the emulsion is heated to the test temperature, then poured through a strainer into the viscometer. The test is started by withdrawing a stopper and determining the time in seconds it takes for 60 mL of emulsion to flow through an orifice. The more viscous the sample, the longer time it takes to flow through the orifice. A longer efflux time is indicative of a higher sample viscosity. Storage Stability The purpose of the storage stability test is to detect the tendency of asphalt particles to “settle out” during storage. This test method determines the difference in percent residue of samples taken from the top and bottom of material placed in undisturbed simulated storage for 24 hr. The result is expressed as the average of the two individual values obtained by determining the difference between the percent residue of the top and bottom samples for each storage cylinder. Demulsibility The demulsibility test is applicable only to rapid-setting emulsions and indicates the rela- tive rate at which the colloidal asphalt particles will break when spread in thin films on soil or aggregate. Calcium chloride causes the tiny asphalt particles in the emulsion to coalesce. In the test, a solution of calcium chloride and water is thoroughly mixed with emulsified asphalt and

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. Coating This test method is applicable to those emulsions intended for use by mixing with aggregate. It is not applicable to rapid-setting types or diluted materials used for tack coats, priming, or mulch treatments, so it will not be discussed in this synthesis. Particle Charge Test This test method is used to identify cationic emulsions. Positively charged particles are classi- fied as cationic. The test is conducted by heating a sample emulsion to 50°C (122°F) and pouring it into a 250-mL beaker. A positive electrode (anode) and a negative electrode (cathode) are then immersed in the sample and connected to a controlled direct current electrical source of at least 8 mA. After 30 min, or after the current has dropped to 2 mA, the two electrodes are examined. An asphalt deposit on the cathode that is clearly discernible (compared with the anode) indicates that the sample is a cationic asphalt emulsion. Cement Mixing Test This test method for slow-setting emulsions, similar to the demulsibility test for rapid- setting emulsions, is intended to ensure that the products will not rapidly break when it comes in contact with fine-grained soils or dusty aggregates. To simulate the fine-grained soils, the test uses a finely divided, high surface area material, high early strength Type III Portland cement. Sieve Test This test method is used to quantify the percentage of oversized asphalt binder particles in the emulsion. Prepared emulsions are poured through an 850-micron (No. 20) sieve, then the sieve and any residue are rinsed with a sodium oleate solution. The retention of an excessive number of particles on the sieve indicates that problems may occur in handling and application of the material. Particles of asphalt retained on the sieve are often caused by agglomeration of the dis- persed phase. Storage, pumping, handling, and temperature can all contribute to the formation of particles. Contamination from the tank, transport, or hose are other factors affecting particle formation. However, this test may be waived if successful application of the material has been achieved in the field. Residue by Distillation and Residue by Evaporation These test methods are used to quantify the constituent percentages of residue and any oils present in the emulsion. The procedures also provide asphalt residue for further testing and clas- sification. The distillation test requires heating the emulsions to temperatures higher than they are likely to be used in the field, potentially altering the properties of the residue such as elastic properties provided by polymer modification. Therefore, residue by distillation is specifically used for regular anionic and cationic emulsions, while residue by evaporation is called for when testing polymer-modified cationic emulsions. Tests on the Residue These tests are intended to characterize only the asphalt binder portion of the emulsion and are not covered in the AASHTO T 59 standardized test procedures for emulsions.

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). The farther the needle penetrates into the residue, the higher the penetration number, and the softer the binder. For example, regular anionic emulsions are softer than those with an “h” designation. Regular anionic grades have a specification range for penetration of 100–200, while the grades with the hard base binder have a specification range for penetration of 40–90. Reduced-tracking tack materials tend to have very low penetrations. Ductility Ductility is a material’s ability to deform under tensile stress. The ductility test (AASHTO T 51) measures the distance a small briquette of asphalt binder residue will elongate before breaking when pulled apart at 5 cm/min at 25°C (77°F). Solubility The solubility test (AASHTO T 44) checks for contaminants in the asphalt binder residue by measuring the purity of the asphalt residue dissolved in a solvent. Many agencies and suppliers no longer measure and report solubility because the test has historically used a potentially dangerous solvent. Additionally, many polymers and other additives may not be soluble in this solvent. Float Test The float test (AASHTO T 50) is performed on residue from high float emulsions. High float emulsion residue is designed to stay in place at higher temperatures so the float test is run at 60°C (140°F). The residue sample is poured so that it forms a plug in a brass collar designed to be screwed into the base of a semicircular float. The float is then placed in a 60°C (140°F) hot water bath and the time is measured in seconds for the hot water to break through the asphalt plug. Tack Coat Construction Practices This section provides detailed findings on agencies’ and contractors’ best practices regard- ing the application of tack coat in the construction phase. The findings are a result of both the literature review and the synthesis survey. The findings are grouped into the following general topics: • Training activities and communication • Tack coat application rates • Dilution of emulsion • Cleaning the existing pavement surface • Tack coat application • Tack coat tracking • Spray pavers Training Activities and Communication A training initiative specifically targeting tack coat best practices was sponsored by the FHWA from 2014 through 2016. The Asphalt Institute and FHWA engineers put together a 4-hour workshop plus several versions of shorter presentations suitable for conferences. During this period, the workshop was given 47 times in 46 states, and in Puerto Rico and the District of Columbia, reaching 2,700 attendees (see Figure 9). An additional 35 presentations were made

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/) that includes a video of the workshop, handouts from the workshop, a webinar on tack coats, and links to influential tack coat documents on the internet. Several states maintain dedicated training courses or guidance documents. Caltrans has a thorough 42-page guideline document covering all aspects of tack coat terminology, materials, application, sampling and testing, and measurement and payment. Likewise, North Carolina DOT has a Best Practices Field Guide geared toward inspectors that is a valuable reference tool (Figure 10). Florida DOT includes a 32-slide module on tack coats, application, and spread rates in their Construction Training Qualification Program (CTQP). Each of these examples can be reviewed online. Appendices D and E contain copies of checklists from KDOT and Louisiana Department of Transportation and Development, respectively, which were developed to help asphalt inspectors verify various aspects of tack coat usage and construction in the field. Industry has also partnered with agencies in putting together solid training documents. For example, the Colorado Asphalt Pavement Association has put together a 34-page document, “Best Practices for Applying Undiluted Emulsified Tack Coats.” This document focuses on undiluted tack coats, providing best practices for application and measurement. It gives practical information regarding cleaning equipment, a troubleshooting guide, and a handy checklist. Figure 9. FHWA/AI Tack Coat Workshop locations.

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. This 28-page appendix covers tack coat materials, how to deal with vari- ous pavement surface conditions, application rates, asphalt distributors, break and set times, and common types of tack coat failures. It also discusses common issues encountered during the construction phase when using tack coats, measuring tack coat material, and characterizing interface shear strength. This appendix can be found on the TRB website at http://www.trb.org/ main/blurbs/166969.aspx. Tack Coat Application Rates NCHRP Report 712, FHWA Tech Brief on Tack Coats, and NAPA’s QIP 128 all agree that the proper tack coat application rate is dependent upon the surface on which it is applied. Figure 10. Examples of state tack coat guidance documents.

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. Table 408.11 of the WVDOT Standard Specifications provides a good example of application rates matched to existing surface condi- tions (shown herein as Table 5). Dilution of Emulsion Diluted emulsion is an emulsion with additional water added to it. The most common dilu- tion rate is 1:1 (one part undiluted emulsion and one part additional water). The practice of dilution has several purported benefits, but also several disadvantages. The potential benefits include providing the additional volume needed for the distributor to function at normal speed when lower application rates are used. Additionally, diluted emulsion flows easily from the distributor at ambient temperatures allowing for a more uniform application (Asphalt Institute 2001). Condition of Existing Pavement Application Rate (gsy) / (L/m2) Undiluted Diluted (1:1) New HMA 0.04 – 0.05 / (0.18 – 0.23) 0.08 – 0.10 / (0.36 – 0.45) Oxidized HMA 0.07 – 0.10 / (0.32 – 0.45) 0.13 – 0.20 / (0.59 – 0.90) Milled Surface 0.10 – 0.13 / (0.45 – 0.59) 0.20 – 0.27 / (0.90 – 1.22) PC Concrete 0.07 – 0.10 / (0.32 – 0.45) 0.13 – 0.20 / (0.59 – 0.90) Table 5. WVDOT application rates for different existing surface conditions. FHWA Tech Brief on Tack Coats, Table 1 Surface Type Residual Rate (gsy) Approximate Bar Rate Undiluted (gsy) Approximate Bar Rate Diluted 1:1 (gsy) New Asphalt 0.02 – 0.05 0.03 – 0.07 0.06 – 0.14 Existing Asphalt 0.04 – 0.07 0.06 – 0.11 0.12 – 0.22 Milled Surface 0.04 – 0.08 0.06 – 0.12 0.12 – 0.24 Portland Cement Concrete 0.03 – 0.05 0.05 – 0.08 0.10 – 0.16 NCHRP Report 712, Table 31 Surface Type Residual Application Rate (gsy) Approximate Bar Rate Undiluted* (gsy) Approximate Bar Rate Diluted 1:1* (gsy) New Asphalt Mixture 0.035 0.058 0.12 Old Asphalt Mixture 0.055 0.09 0.18 Milled Asphalt Mixture 0.055 0.09 0.18 Portland Cement Concrete 0.045 0.08 0.15 * NCHRP Report 712, Table 31, includes residual application rates only. Values in italics were supplied using an example emulsion with 60% residual asphalt binder for comparison purposes only. NAPA QIP 128 Best Practices for Emulsion Tack Coats, Table 4-1 Existing Condition Residual Asphalt Binder (gsy) Applied Undiluted Emulsion (gsy) Applied Diluted Emulsion (gsy) Dusty or Dirty Clean the surface Clean the surface Clean the surface New Asphalt 0.03 – 0.04 0.04 – 0.06 0.09 – 0.12 Old, Aged Asphalt 0.04 – 0.06 0.06 – 0.09 0.12 – 0.18 Milled Asphalt 0.03 – 0.05 0.04 – 0.07 0.09 – 0.15 PCC 0.04 – 0.06 0.06 – 0.09 0.12 – 0.18 Table 4. Examples of nationally recommended tack coat application rates.

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. Terminals will not only control the quantity of water added, but also the necessary quality of water added. However, other agencies allow dilution at the project site, where control of how much water is added to the emulsion and the quality and temperature of the water are sometimes more difficult to maintain. Diluted emulsion also requires additional time to break, which may not be practical on every project. The FHWA/AI Tack Coat Workshop recommended that emulsions used for tack coat should not be diluted. However, according to the synthesis survey, about half of the agencies allow dilution. Therefore, several guidelines regarding dilution should be kept in mind. The workshop recommended that if dilution is allowed, that it be done only one time, at the original supplier’s terminal, by the emulsion supplier, using the same filtered and treated water used to manufacture the emulsion. If field dilution is allowed, several of the guidelines from the literature review recommended to first check the compatibility of the water with the emulsion by testing a small quantity. If possible, the manufacturers suggest using warm water for diluting, always adding the water to the emulsion and not the emulsion to the water. Medium- and slow-setting grades can be diluted without harming the emulsion, but rapid-setting grades should not be diluted. Cleaning the Existing Pavement Surface Virtually all of the literature agreed that tack coat must be applied to a clean surface. If the pavement surface is dusty or dirty, it must be cleaned to prevent the new asphalt pavement surface from sliding or delaminating. Cleaning operations can be achieved either through mechanical brooming (Figure 11), flushing the surface with water, or blowing off debris using high-pressure air (NAPA 2013). When tack coat is applied to a dusty or dirty surface, the tack clings to the dust and dirt, not to the roadway. It therefore sticks very easily tracked onto tires, as shown in Figure 12. The tack coat sticking to the tires is then tracked onto other areas as the dust wears off of the moving tires. Sometimes verbiage is included regarding the protection of nearby appurtenances from unintentional tack coat spray. An example of this is found in Florida DOT’s Standard Specifica- tions, section 300-5: “Before applying any bituminous material, remove all loose material, dust, Figure 11. Power brooming.

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. When applying prime or tack coat adjacent to curb and gutter, valley gutter, or any other concrete surfaces, cover such concrete surfaces, except where they are to be covered with a bituminous wearing course, with heavy paper or otherwise protect them as approved by the Engineer, while applying prime or tack coat. Remove any bituminous material deposited on such concrete surfaces” (Florida DOT 2017). Surfaces Need to Be Dry or Not Before Tack Coat Application? The other major area of surface preparation regards whether surfaces need to be dry or not. NCHRP Report 712 determined that, for the effect of water on tacked surfaces in the majority of cases, there was no statistically significant difference between wet and dry conditions. However, due to the limitations of the study, the authors suggested that the surface be clean and dry to avoid the negative effects of water on the bonding at the interface (Mohammad et al. 2012). The FHWA Tech Brief on Tack Coats states, “Surface preparation is vital to provide the best opportunity to achieve a high bond strength. The goal of surface preparation is to produce a clean, dry surface” (FHWA 2016). NAPA’s QIP 128 softens the stance on the necessity of a dry pavement surface prior to tack coat application. It states, “A small amount of moisture on the pavement surface should not be detrimental to long-term tack coat performance, although a damp pavement will slow the cure and break time of the tack coat emulsion. If the pavement surface layer is saturated with water and the existing pavement surface is damp or has standing water, the ability of the tack coat emulsion to provide adequate bond between the existing and the new pavement layers will be significantly compromised” (NAPA 2013). State agency specifications reflect both viewpoints. For example, Delaware DOT’s 2016 Standard Specification states in section 401.03.H, “Apply on all dry and broom cleaned surfaces” (Delaware DOT 2016). South Dakota’s Standard Specifications in section 330.3.A.2.b allow for a small amount of moisture on the surface, “emulsified asphalt may be applied when the surface is slightly damp” (South Dakota DOT 2015). Of course, tack coats comprised of straight binder or cutbacks, which have no constituent water, should be applied to dry surfaces only (Mohammad et al. 2012). Figure 12. Tack coat sticking to tires as a result of a dusty or dirty roadway surface.

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) or higher, or when the surface temperature is 35°F (2°C) and rising” (Montana DOT 2014). The specifications further set the same weather limitations on the tack application as are stipulated for asphalt paving. Tack Coat Application Once the surface has been properly prepared, application of tack coat can proceed. Tack coat applications should be uniform and consistent both transversely and longitudinally, as shown in Figure 13. According to the FHWA/AI Tack Coat Workshop, proper tack coat application involves several elements: • Proper temperature of the tack coat during application • Proper application equipment, typically a distributor (except for small areas of hand work) – Proper distributor speed – Proper distributor bar height – Proper distributor bar pressure – Proper nozzle type – Proper nozzle angle – Proper nozzle configuration • Proper application rate, matched to pavement surface type and condition • Avoiding tacking too far ahead of the paver The literature review indicated that different tack coat materials have different temperature ranges at which they should be applied. NAPA QIP 128 gives general application temperatures and storage temperatures in its Table 3.1, reprinted here in Table 6. NCHRP Report 712 noted that excessive heating may cause the emulsion to break while still in the distributor. Figure 13. Properly tacked surface. Type and Grade Spraying Temperature, °F Storage Temperature, °F RS-1, SS-1, SS-1h, CRS-1, CSS-1, CSS-1h 70 - 160 70 - 140 RS-2, CRS-2 140 - 185 125 - 185 Non-Tracking Tack 160 - 180 120 - 130 Polymer-Modified Emulsion 140 - 180 120 - 130 Table 6. Guideline temperatures for asphalt emulsions from NAPA’s QIP 128.

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-polymer- modified emulsions used as tack coat, 70°F to 160°F, according to Table 40.4.3-1 in their Standard Specifications (Wyoming DOT 2010). Asphalt Distributors The literature review documents are in agreement that asphalt distributors (Figure 14) are required for satisfactory coverage. The asphalt distributor is composed of eight major components (Etnyre 2017): 1. The tank is used for storing the liquid which will be sprayed onto the roadway. 2. The heating system is used to safely maintain the liquid at the proper spraying temperature. 3. The asphalt pump and circulating system are used for several actions including circulating the liquid to maintain homogeneity, circulating the material in the bar to prevent clogging, providing the force necessary to spray the liquid, sucking back the liquid in the spray bar to prevent clogging, and flushing the system during cleaning. 4. The spray bar evenly distributes the liquid across the roadway. 5. The flushing and cleanout system are necessary to keep the distributor unclogged, functional, and delivering only the product intended, without regard to previous loads. 6. The power system properly operates all pumps, drives, and controls. 7. The metering and control system allows the operator to spray at different application rates. 8. The chassis provides the gross vehicle weight rating adequate to support the loaded distributor. Arizona DOT has a section in their Standard Specifications which details requirements regard- ing the asphalt distributor. Section 404-3.02(A) discusses spread capabilities, specific items with which the distributor is to be equipped, circulation requirements, and certification requirements (Arizona DOT 2008). Florida DOT’s Standard Specifications contain general verbiage warning not to tack too far ahead of the paver, “do not apply tack coat so far in advance that it might lose its adhesiveness as a result of being covered with dust or other foreign material” (Florida DOT 2017). Other DOTs are more specific in their requirements. For example, Washington DOT’s specifications state, “For Roadways open to traffic, limit the application of tack coat to surfaces that will be paved during the same working shift” (Washington DOT 2016). Figure 14. Asphalt distributor.

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. King and May (2003) estimated that a mere 10% loss in bond strength reduced the pavement life by 50%. Roffe and Chaignon (2002) more conservatively found that no bond at all resulted in a 60% loss of life. Brown and Brunton (1984) estimated that a 30% bond loss was almost as bad as having no bond at all. They found that in the range of 0% to 30% of full bond strength, the pavement life decreased by 70% to 75%. Frequently, tack coat applications can be observed that are streaky or striped in appearance. Some refer to this as “zebra tack” or “corn rows” and it does not produce good bond strengths (FHWA 2016). There are several possible reasons why a distributor might produce a tack coat with a non-uniform appearance. Clogged Nozzles. When emulsions are sprayed at temperatures higher than ambient, the asphalt binder residue often clogs the distributor nozzles as it cools after the distributor work is done. If the nozzles are not properly cleaned and flushed, some individual nozzles can become clogged, resulting in streaks on the pavement surface without tack. Incorrect Nozzle Size. Distributors are used to spray liquids of varying viscosities. Thicker liquids, like the emulsions typically used for chips seals, use nozzles with larger openings. Thinner liquids, like the emulsions typically used for tack coats, require nozzles with smaller openings. Also, there many different opening configurations used by the various distributor manufacturers. Figure 16 shows differently sized openings, all in the “coin slot” configuration. Using a distributor with incorrectly sized nozzles for tack coat can result in a non-uniform appearance. Figure 15. Non-uniform and light tack coat coverage. Figure 16. Different distributor nozzle sizes in the coin slot configuration.

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. Incorrect Nozzle Orientation. If the nozzles are oriented perfectly parallel with the spray bar, emulsion from the nozzle spray will intersect and cause a splashing effect which will result in a non-uniform surface. If the nozzles are angled closer to perpendicular with the spray bar, the fans will not overlap, but will instead leave uncoated streaks. The proper nozzle orientation is 15° to 30° from the spray bar (Asphalt Institute 2001). This will keep the spray fans of emulsion from interfering with each other, plus provide overlap to prevent streaking, as shown in Figure 17. Lack of Proper Pressure in Spray Bar. The spray bar pressure works along with the properly sized and oriented nozzles to provide the triangular-shaped fan of sprayed tack coat emulsion. If the distributor does not maintain the proper pressure in the spray bar, the emulsion will simply dribble out of the nozzles, resulting in a zebra-stripe pattern similar to Figure 10. One of the respondents to the survey said their agency’s specifications call for uniform temperature of the emulsion and uniform pressure in the spray bar. Incorrect Spray Bar Height. If the spray bar is too low to the ground, there may be no overlap between fan-shaped spray patterns, resulting in a streaked appearance. The spray bar height could be raised to a position that would result in single coverage, which would theoretically result in a fully covered surface. However, if anything is wrong with even one individual nozzle, a bare streak would result. Therefore, single coverage is not recommended. As the spray bar height is increased, the fan-shaped spray patterns will overlap, greatly increasing the chances of complete coverage even if a nozzle becomes clogged, as seen in Figure 18. Figure 19, from the Figure 17. Distributor nozzle orientation. Figure 18. Proper overlap mitigating clogged nozzle.

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. Using a Spray Wand on the Main Roadway. The spray wand is a very useful tool for areas that are inaccessible to the distributor spray bar. However, it should never be used in areas where the distributor can freely drive, as shown in Figure 20. Even careful applications with the spray wand will inevitably have areas that are heavy or light on the desired tack coat emulsion. Alabama DOT specifies that “an asphalt distributor shall be provided for use on all acces- sible areas; inaccessible areas such as around manholes, etc., may be coated by other approved methods” (Alabama DOT 2012). Distributor Calibration “Distributor truck calibration is vital to the application of a proper tack coat. Periodically, a trial tack coat application should be placed over a test area to verify correct nozzle operation and configuration. Distributors should be calibrated annually as a minimum” (FHWA 2016). Figure 19. Overlap based on spray bar height. Figure 20. Applying tack using a spray wand.

36 Tack Coat Specifications, Materials, and Construction Practices ASTM D 2995 records two methods to calibrate asphalt distributors (see Figure 21). Both methods involve fastening geotextile fabric pads to the roadway. The distributor tacks over the pads, which are weighed before and after to determine the application rate in l/m2 (gsy). Method A estimates the transverse and longitudinal application rate and variability in application rate of the emulsion applied to a pavement surface. Method B estimates the transverse and longitudinal application rate and variability in residual application rate from emulsified asphalt applications to a pavement surface using a bituminous distributor. Other calibration methods involve measuring the volume of tack sprayed into containers over a specified period of time by a stationary distributor (see Figure 22). This method allows the users to measure the transverse uniformity of spray by each nozzle on the spray bar. The longitudinal application rate can be estimated by assuming a velocity of the distributor at the given rate of spray. Figure 21. Calibrating distributor using ASTM D 2995. Figure 22. Calibrating distributor volumetrically.

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. 2012). In areas where the construction zone must allow private vehicles from driveways and side streets, substantial tracking can occur as well. Tack coat tracking can be mitigated in several ways. In traditional construction, trucks back over the tack coat to reach the paver, then pull forward again over the tack coat as they leave. Delivery truck drivers should be trained how to safely exit and enter active traffic in construction zones and cautioned to avoid driving on freshly applied tack coats as much as possible. If the work zone allows, material transfer vehicles (MTVs) can be used. They can drive in the adjacent lane and convey the material over to a hopper insert on the asphalt paver, as shown in Figure 24. This prevents a great deal of truck traffic driving over the tack coat. Another way to mitigate tack coat tracking is through the use of reduced-tracking tack materials. This material is typically manufactured to harden quickly and adhere minimally to tires. When a hot lift of asphalt is subsequently placed over the tack, the hardened tack is reactivated by the heat, and bonds the new overlay with the existing surface. As described in Chapter 3, currently about 20% of state DOTs use some amount of reduced-tracking materials. Another method is through the use of a spray paver, which will be discussed more thoroughly in the next section. Often, the inspector or paving crew superintendent has to make a judgment call regarding tracking. If too much tack has been judged to have been removed from the roadway surface, Figure 23. Tack coat tracking in the work zone.

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. Many agencies have specifications or policies limiting how far the roadway can be tacked ahead of the paver. This limits the time the tack coat is exposed to dust and debris blown by the wind or adjacent traffic. It also limits the amount of tack coat that is exposed to traffic, which can often track dirt or mud onto the tack coat, rendering it ineffective. Spray Pavers Unlike traditional paving, where a separate distributor applies the tack to the roadway, and the paving crew paves over it after it breaks, a spray paver (Figure 25) contains an emulsion tank on the paver itself. A spray paver has dual functions: to act as an asphalt distributor and to act as an asphalt paver, all in one piece of equipment. The tack coat is stored in the tank of the spray paver, which can range from 550 to 2,100 gal, depending on the manufacturer and model number. The spray paver requires the same heating, circulating, pumping, spraying, and metering systems as a typical asphalt distributor. Figure 24. MTV use at the National Center for Asphalt Technology (NCAT) test track. Graphic courtesy of Vögele Figure 25. Example of a spray paver.

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. Specialized tack coat material formulations, often polymer modified, are often used with spray pavers. There are some potential advantages of a spray paver over typical construction practices. Since the tack is sprayed directly in front of the screed, there is no time for dust and other debris to settle onto the tack coat surface, which might cause a reduced bonding ability. Additionally, it removes the issue of vehicle pickup and tracking of the tack coat. Potential disadvantages include the lack of time for even specialized emulsions to break prop- erly before being overlaid with asphalt. Spray pavers rely on the heat of the asphalt to evaporate the water from the emulsion, which then travels through the open voids in the asphalt mixture (Vögele 2017). Although specialized tack formulations can be used to mitigate this effect to some degree, almost half of the U.S. and Canadian agencies surveyed said that they use the same tack coat materials with spray pavers as they do with conventional paving. Another concern is that agencies must rely solely on proper spray calibration because roadway inspectors cannot clearly see whether or not the tack coat is being sprayed uniformly. Although the spray paver can be calibrated like any other asphalt distributor, some inspectors prefer the visual reassurance afforded by conventional distributors. Testing and Acceptance of Tack Coats The primary goal of an application of tack coat is to ensure the bonding of all asphalt concrete layers into a single monolithic structure. Layer bonding is a common assumption in structural design of flexible pavements, as it results in significantly better performance (FHWA 2016). This section provides detailed findings on agencies’ and contractors’ best practices regarding the testing of tack coat interlayer bond strengths. The findings were a result of both the literature review and the synthesis survey. Background information is also supplied on the testing of inter- layer (or interface) bond strength. Testing of Tack Coat Bond Strength A variety of researchers have investigated and reported on the measurement of interlayer bond strengths of pavements. The procedures that have been developed can generally be classified based on the method in which the load is applied. The loading may be applied in shear, which is further subdivided into direct shear or torsional shear, or in tension. Testing may also be described as destructive versus nondestructive, laboratory versus field, or cyclic versus a monotonic loading to failure. A review of these combinations of will be made based on how the loading is applied. Bond Strength Measurement via Direct Shear Laboratory linear shear testing is a popular option for both researchers of bond character- istics as well as agencies. The attraction to this methodology may be attributed to the relative simplicity of the procedures and the utilization of existing laboratory equipment. The majority of states using bond strength tests are using the linear shear methodology on Marshall or Universal Testing Machine equipment. During the FHWA/AI Tack Coat Workshop, instructors offered the following on direct shear testing: • Lab test • Quick

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. Perhaps the oldest commonly referenced research on interlayer bond strengths was conducted by Dr. Jacob Uzan and his associates in 1978 (Uzan et al. 1978). They used a direct shear test to test a single paving grade binder (60–70 pen), which was used as a tack coat between layers of dense-graded asphalt mixtures. Their protocol included five application rates (0.0–0.4 lb/yd2 [0.0–2.0 kg/m2]), two test temperatures (77°F and 131°F [25°C and 55°C]), and five vertical confining pres- sures (0–71 psi [0–490 kPa]). A constant horizontal displacement of 0.1 in./min (2.5 mm/min) was used to shear specimens. They found that the optimum tack application was doubled at the higher temperature than the optimum rate found for the lower test temperature. Investigation of the Behavior of Asphalt Tack Interface Layer. The Superpave Shear Tester (SST) was the tool used by Dr. Louay Mohammad and his fellow researchers to evaluate six emul- sions and two paving grade binders used as tack coats (Mohammad et al. 2005). Five residual application rates (0.00–0.20 gsy [0.00–0.90 L/m2]), and two test temperatures (77°F and 131°F [25°C and 55°C]) were investigated. A constant loading rate of 50 lb/min (222.5 N/min) was applied to the specimen until shear failure. Optimum test performance was seen with CRS-2P emul- sion at the lower test temperature, and CRS-2L gave the best performance at the higher tempera- ture. For both of these combinations, the optimum application rate was 0.02 gsy (0.09 L/m2). Evaluation of Bond Strength Between Pavement Layers. An NCAT research team lead by Dr. Randy West developed a shear testing apparatus to be used on a Marshall testing frame for the Alabama DOT (West et al. 2005). Bond strengths of two emulsions and one paving grade binder were measured with a the device at three temperatures 50°F, 77°F, and 140°F (10°C, 25°C, and 60°C); at three normal pressures of 0, 10, and 20 psi (0, 69, and 138 kPa); and three residual application rates 0.04, 0.08, and 0.12 gsy (0.18, 0.36, and 0.54 L/m2) for the emulsions, and 0.02, 0.05, and 0.08 gsy (0.09, 0.23, and 0.36 L/m2) for the paving grade binder. In addition to these factors, two dense-graded mixtures’ bonding characteristics were evaluated. They were a 19-mm and a 4.75-mm Superpave mixture. Based on statistical analyses, the NCAT team found that mix type, tack coat type, temperature, tack coat application rate, and normal pressure were significant factors affecting bond strength values. The 4.75-mm mixtures offered greater shear interface strength than did the 19-mm mix- tures. The paving grade binder likewise had higher values for shear compared to the emulsions. Regarding temperature, as the temperature went up, the shear strength was reduced. They also reported generally higher strengths at lower tack coat applications, and only saw confining pres- sure to be a factor at their top test temperature. The Alabama DOT Test Method ALDOT-430, “Standard Test Method for Determining the Bond Strength Between Layers of an Asphalt Pave- ment,” was a product of this research (Figure 26). ALDOT-430 is performed on 6-in. diameter (150-mm) samples whose layer thickness is between 2 and 6 in. The gap between the specimen holder is to be 0.25 in. (6.35 mm). The test is performed at a constant displacement rate of 2 in./min (50.8 mm/min) at 77°F (25°C). Minimum shear strength was set at 100 psi, but the survey response for this project indicated that it is now 70 psi.

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. 2012). This time Dr. Nam Tran was the lead investigator. While the previous NCAT work above was strictly a laboratory study, this effort was both lab and field in nature, plus computer simulations. The materials studied for the lab portion included four emulsions (including one non-tracking variety) and one paving grade binder, applied at three different residual levels each. The residual values for the non-tracking tack coat were lower than for the other materials evaluated, and samples were tested without tacking between layers as well. The surfaces for the lab work were milled, micro-milled, and new HMA. All testing was done per ALDOT-430. The field portion of this study utilized 10 sites from around Alabama. Three of these were on the NCAT Test Track, and the other seven were highways. Tack coat materials included three emulsions (including one non-tracking variety) and one paving grade binder. At each of the test sites, materials were placed at three application rates. On the test track, two of the inside lanes were built without tack. Computer simulations were run using Bituminous Structures Analysis in Roads (BISAR) which was developed by Shell Oil Company. BISAR is a multi-layer analysis program. The goal of this portion of the study was to examine the influence of recognized factors (layer thickness, material properties, etc.) that affect pavement bonding when they are altered. Based on the simulations, the factors that had the greatest effect on bond strengths were (a) surface thickness and stiffness and (b) pavement temperatures. Subgrade thickness and total pavement thickness did not have as great an effect. Also, the simulations and the testing of extracted cores from sound and failed areas yielded a determination that ALDOT’s failure criteria for ALDOT-430 of 100 psi minimum interlayer bond strength is a reasonable value. Preliminary Investigation of a Test Method to Evaluate Bond Strength of Bituminous Tack Coats. Another investigation into interlayer binder strengths, which also led to the development of a laboratory shearing device, was performed in the state of Florida by Gregory Sholar and a team from the DOT and the University of Florida (Sholar et al. 2002). This research investigated interlayer bonding of the two most common emulsions used in Florida at the time. Four application rates of what seems to have been undiluted emulsions were made from a low of Figure 26. NCAT shear testing device (West et al. 2005).

42 Tack Coat Specifications, Materials, and Construction Practices 0.00 to 0.08 gsy (0.36 l/m2). Asphalt layers were constructed of 12.5- (both fine- and coarse-graded) and 19.0-mm Superpave mixtures with both milled and unmilled surfaces. Rain was also simu- lated via water spraying, which proved to diminish bond performance. Shear testing in Florida is similar to Alabama. It is also performed on 6-inch diameter (150-mm) specimens. The test is also performed at a constant displacement rate of 2 in./minute (50.8 mm/minute), typically on a Marshall stability test apparatus, at 77°F (25°C). The gap between the specimen holder is adjustable, but typically is set at 3/16 in. (4.75 mm). The minimum cal- culated shear strength must be at least 100 psi. Florida uses the test for the approval of new tack products. Figure 27 shows the Florida device. NCHRP Report 712. The largest research effort to date on the topic of tack coats and inter- layer bond strength was NCHRP Project 9-40. This effort was headed by Dr. Louay Mohammad with a research team and reported in NCHRP Report 712 (Mohammad et al. 2012). As Table 7 shows, this research effort was very complex and complete with a factorial for field-prepared samples yielding 474 test specimens. Some of the key findings from NCHRP Project 9-40 included a recommendation for shear testing. The work developed the Louisiana Interlayer Shear Strength Tester (LISST), which will be described below. The work generally found that stiffer base asphalts performed better in Figure 27. Florida shear device (Sholar et al. 2002). Variables Content Levels Pavement surface type Old HMA, New HMA, PCC, and Milled HMA 4 Tack coat material SS-1h, SS-1, CRS-1, Trackless, PG 64-22 5 Residual application rate 0.00 (No-Tack), 0.031, 0.062, 0.155 gsy 4 Wet (rain) condition Wet, Dry 2 Dusty condition Dusty, Clean 2 Test temperature 77°F (25°C) 1 Confinement pressure (psi) 0, 20 2 Tack coat coverage 50%, 100% 2 Number of replicates 3 3 Total number of samples 474 Table 7. Test factorial for field-produced samples (as modified from NCHRP Report 712).

Literature Review 43 terms of interlayer bond strengths. The best laboratory bonding was commonly seen at 0.155 gsy (residual). Therefore, the question was raised whether current common rates may be too light. Milled surfaces performed better than those that were not milled. A training appendix was also produced, which can be found on the TRB website at http://www.trb.org/main/blurbs/166969.aspx. AASHTO TP 114-16 “Provisional Standard Method of Test for Determining the Interlayer Shear Strength (ISS) of Asphalt Pavement Layers” offers the details of the LISST test. The test frame (Figure 28) consists of two main parts: a free to move shearing frame and a fixed reac- tion frame with a gap of 0.5 in. (12.7 mm) between them. It can test two different sample sizes of either 4-in. (100-mm) or 6-in. (150-mm) diameter. The layers should be 2 in. ± 0.2 in. (50 mm ± 5 mm) and no more than 6 in. (150 mm) of total specimen thickness. A constant displacement of 0.1 in./min (2.54 mm/min) is employed (AASHTO TP 114-16, 2017). Results are reported in either psi or kPa. A minimum value of 40 psi was developed as a bond strength criterion. A recent study of three field projects with 14 in-service test sections using four types of emul- sified tack coats at different residual application rates used the LISST to evaluate bond strength (Das et al. 2017). The ISS from laboratory tests correlated well with the short-term cracking performance of field pavements. All test sections except those that did not meet the minimum ISS threshold of 40 psi performed satisfactorily with regard to cracking. Another product of NCHRP Project 9-40 is the Louisiana Tack Coat Quality Tester (LTCQT), which also has an AASHTO provisional standard, AASHTO TP 115-16, “Determining the Quality of Tack Coat Adhesion to the Surface of an Asphalt Pavement in the Field or Laboratory” (AASHTO TP 115-16, 2017). In the words of TP 115, “This test method cov- ers the determination of tack coat adhesion quality as measured by the tensile strength of tack coat materials on the free surface of asphalt pavement in the field or the laboratory” (AASHTO TP 115-16, 2017). The LTCQT is a device that was developed in conjunction with InstroTek, Inc. It is a newer ATackerTM, which was produced by the same company. A 6-in. (150-mm) specimen has the appropriate amount of tack coat applied, conditioned, and brought to the desired test tempera- ture, typically the asphalt binder’s softening point. It is recommended that the AASHTO T-53 be used to determine the softening point of binder. A 20-lb (89N) compressive load is applied for 3 min to the loading plate on the tacked surface. Immediately following this loading, the plate is pulled from the surface at a rate of 0.008 in./s (0.02 mm/s) until failure is recorded. Virginia Test Method-128 (VTM-128), Test Method for Bond Strength of Asphalt Layers (Laboratory). The Virginia DOT has a unique combination method of testing bond strengths of tack coats. They use both a linear shearing test and a tension test. Details of the Normal Load Actuator Horizontal Sensor Vertical Sensor Shearing Frame Reaction Frame Figure 28. LISST (NCHRP Report 712).

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). The shearing portion of VTM-128 uses a Marshall stability tester with a 4-in. (100-mm) shear- ing head as seen in Figure 29. Standard test temperature is 70°F (21°C), and the test is run mono- tonically at 2 in./min (50.8 mm/min) until failure. Results are reported in psi or kPa. Test criteria differ for milled versus unmilled surfaces. For milled surfaces, the criterion is a 100-psi (690-kPa) minimum average of three tests, with no single value less than 50 psi (345 kPa). For unmilled surfaces, the requirement is a 50-psi (345-kPa) minimum average of three tests, with no single value below 30 psi (207 kPa). Bond Strength Measurement via Torsional Shear A less popular interlayer bond testing method involves the torsional application of a shearing force at the interlayer. The survey of state and provincial transportation departments found only one (Texas) that indicated they were using a torsional test method. Based on this result, only one test method will be expanded upon in this category. That will be the Torque Bond Test, as it was the torsional test most often referenced within the literature. Torque Bond Test (TBT). The TBT originated in Sweden and was subsequently adopted in Great Britain as part of their evaluation of thin bonded overlays (Tashman et al. 2006). The TBT can be run in the field to test in situ bond strengths, or in a laboratory for a more controlled environment as can be seen in Figure 30. The testing process is similar for field or lab testing based upon British Board of Agrément’s (BBA) “Interim Guideline Document for the Assessment and Certification of Thin Surfacing Systems for Highways” approach (BBA 2013). In the case of field testing, 4-in. (100 ± 5 mm) specimens are utilized, and the technician drills to depth 0.8 in. (20 mm) below the interface to be tested. For laboratory testing, they may be the same size or 6-inch (150 mm), and the cores are to be at least 3.15 in. (80 mm) deeper than the layer to be tested. A metal plate is bonded to the specimen and torque is applied in a parallel fashion over a 90° sweep in 30 ± 15 s. The torque is applied until failure occurs, or until the maximum test value of 221 ft•lb (300 N•m) is exceeded. There is no temperature requirement for a field test in the guideline, but it is to be recorded. For a lab test, the standard test temperature is 68 ± 3.5°F (20 ± 2°C), but other temperatures can be evaluated if so desired. The interlayer bond strength is reported in psi or kPa. Figure 29. Virginia shear tester (VTM-128 undated).

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) have established standard test methods for analyzing interlayer bond strength. A variety of researchers have also used tension to evaluate tack coats (Mohammad et al. 2012; Tashman et al. 2006; Deysarkar and Tandon 2004). The FHWA/AI Tack Coat Workshop instructors indicated that tension testing could be done in the field or in a laboratory. It is a reasonably quick test to run, with good repeatability. It has been found to cleanly rank materials. Kansas Test Method KT-78. The tension test employed by the KDOT is an adaptation of ACI 503R, Appendix A, as seen in Figure 31 (KT-78, 2012). A 2-in. (50-mm) core drill is used to drill into the prepared test surface to a depth 0.25 to 0.75 in. (6.35 to 19.0 mm) below the layer to be tested. Three such holes are drilled in a triangular fashion so that a 6-in. (150-mm) core drill can circumscribe them. The larger core drill is used to collect the specimen for laboratory testing by drilling either to the bottom of the pavement or 9 in. (230 mm), whichever is less. In a 77°F (25°C) lab, a standard 1½-in. (37-mm) diameter pipe cap that has been machined flat with the shoulder cut to provide a 2-in. (50-mm) diameter surface for bonding is glued to the surface with an appropriate epoxy resin. A preload of approximately 10 lb (5 kg) is applied. Tension is applied at a rate of 0.8 ± 0.1 in./min (20.3 ± 2.5 mm/min) until failure or the peak capacity of the 500-lb (227-kg) scale is reached. The location of the failure is to be noted by the technician. Tex-243-F, Tack Coat Adhesion. The University of Texas El Paso developed a tension test for Texas DOT (TxDOT) in 2004 (Deysarkar and Tandon 2004). Figure 32 shows the engineered device. This test is intended for field testing. It should be noted that TxDOT has abandoned this test and intends to adopt a shear test soon. Figure 30. Torque Bond Test (Tashman et al. 2006).

46 Tack Coat Specifications, Materials, and Construction Practices Figure 31. Kansas tension tester (KT-78, 2012). Figure 32. Texas Tack Coat Pull-Off Device (Tex-243-F, 2009).

Literature Review 47 Figure 33. Specimen ready for VTM-128 tension test (VTM-128 undated). The procedure for Tex-243-F is as follows. 3MTM or equivalent double-sided tape is attached to the 5-in. (127-mm) diameter contact plate of the device. A moisture barrier sheeting is then attached to the tape. The foot is placed upon the tack coat treated surface and a 40-lb (18-kg) seating load is applied for 10 min. A torque wrench is used to apply the load until separation of the contact plate from the surface and the peak load is recorded. Results are recorded in psi or kPa. VTM-128, Test Method for Bond Strength of Asphalt Layers, Tension Portion (Laboratory). The tension portion of VTM-128 is a laboratory procedure. Like their shear test, it is run at 70°F (21°C) on 4-in. (100-mm) specimens. A loading rate of 1,200 lb/min (5.3 kN/min) until failure is applied within a universal testing machine. The results are reported in psi or kPa. Figure 33 shows a specimen ready for testing under this procedure. As was the case for Virginia’s shear criteria, a distinction is made for milled and unmilled surfaces when testing in tension. A three-test average of 40 psi (276 kPa) with no less than 20 psi (138 kPa) is the milled requirement. For the unmilled, a three-test average of 30 psi (207 kPa) with none below 20 psi (138 kPa) is required.

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. There was a 100% (50/50) response rate from the U.S. DOTs. There was a 54% (7/13) response rate from Canadian provinces. The exact wording of each question is provided in Appendix A. The U.S. responses are provided in Appendix B. The Canadian responses are provided in Appendix C. The survey was split into four main sections: • Tack Coat Payment Specifications (questions 2 through 3) • Tack Coat Materials (questions 4 through 7) • Tack Coat Application (questions 8 through 35) • Tack Coat Evaluation (questions 36 through 42) Synthesis Survey: Tack Coat Payment Specifications The survey asked whether tack coat was paid for by individual bid item, or whether it was considered incidental to paving. The FHWA/AI Tack Coat Workshop recommended that agencies pay for tack coats as an individual pay item. It was thought that if agencies paid for tack coats as an individual pay item, it would remove any incentive to minimize the use of tack coat materials. Of U.S. agencies, 66% (33/50) indicated that they paid for tack coat as an individual pay item, as shown in Figure 34. Of Canadian agencies, 86% (6/7) indicated that they paid for tack coat as an individual pay item, as shown in Figure 35. For the 33 agencies that pay for tack coat as an individual pay item, the two most common methods of payment are by volume of the undiluted emulsion and by mass of the undiluted emulsion (Figure 36). A few of the states that both allow dilution of the tack coat and pay for tack coat as an individual pay item pay by either volume (4/33) or mass of the diluted emulsion (2/33). A small percentage of states pay by area of application (square yards). It should be noted that eight of the 33 U.S. agencies that pay for tack coat as an individual pay item allow two differ- ent methods of measurement. Therefore, the percentages of the individual methods of payment do not add up to 100%. The responses of the six Canadian agencies that pay for tack coat as an individual pay item (Figure 37) were fairly similar to U.S. agencies, except that no Canadian agency pays by the diluted or residual quantities. C H A P T E R 3 Survey Results

Survey Results 49 66% 34% Individual Pay Item Incidental to Paving Figure 34. How is tack paid for on your contracts? (U.S.). 86% 14% Individual Pay Item Incidental to Paving Figure 35. How is tack coat paid for on your contracts? (Canada). 0% 10% 20% 30% 40% 50% 60% 70% By area (square yards) - 6% (2/33) By volume or mass of residual asphalt - 6% (2/33) By volume or mass of diluted emulsion - 18% (6/33) By mass of undiluted emulsion - 36% (12/33) By volume of undiluted emulsion - 58% (19/33) Figure 36. Percentage of U.S. agencies by measurement method used to pay for tack coat.

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. A total of 26 of the 57 respondents offered estimated percentages of tack coat use. The other 31 respondents gave the material types their agency uses, but no estimate regarding the percentage of each material is used by their agency. Figure 38 was generated by grouping each specific material into one of the following four broad categories: • Straight asphalt binders • Emulsions • Cutbacks • Reduced-tracking emulsions A method was used to weight the percentages of materials used within the broad categories for those agencies that provided numerical estimates. Thirty-four of the U.S. agencies provided numerical estimates for 100% of their tack coat usage. One agency provided a numerical estimate for 50% of the specific tack coat products they use. These were summed to give a total of 3,450. The percentages within each broad category were then added together and divided by the total sum of percentages. Using the PG binders as an example, a total of five agencies reported using PG binders as tack and estimated usage percentages of 1, 5, 5, 1, and 2. These were summed to give 14 and divided by the total of 3,450 to give a weighted average of 0.4%. The totals were shown to the nearest 0.1% in order to show a value for straight asphalt binder usage, because 0.4% would have been shown as 0% if it had been rounded to the nearest whole number and therefore not reflected in the histogram. Likewise, the agency estimates of regular emulsion usage totaled 2,740. Dividing by the total of 3,450, a weighted average for emulsion use was 79.4%. The reduced tracking emulsion estimated percentages, totaling 696, resulted in a weighted average of 20.2%. It should be noted that one state, Illinois, indicated that they sometimes use an RC-70 cutback By area (square meters) - 33% (2/6) By volume or mass of residual asphalt - 0% (0/6) By volume or mass of diluted emulsion - 0% (0/6) By mass of undiluted emulsion - 17% (1/6) By volume of undiluted emulsion - 50% (3/6) 0% 10% 20% 30% 40% 50% 60% 70% Figure 37. Percentage of Canadian agencies by measurement method used to pay for tack coat.