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

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Survey Results 51 for tack coats, but did not estimate the percentage used and was therefore not included in the weighted percentages. The same methodology was applied to weight the Canadian survey information, which tracked similarly to U.S. usage (Figure 39). The Canadian agencies that provided percentage estimates reported no usage of straight binders or cutbacks. New Brunswick reported using PG 58-28, PG 64-28, and SS-1 as tack coat material, but did not provide estimated percentages of use, and could not be included with the numerical estimates. Nova Scotia estimated that only 1% of their tack coats used reduced-tracking tack, a product called Clean Bond Coat. Therefore, Canadian respondents providing numerical estimates of tack coat usage reported that virtually 100% of tack coat materials were regular emulsions. By breaking down the responses into the individual products in each category, regardless of estimated percentages used, all the responses were able to be used. Three agencies: Nebraska, 0.4% 79.4% 0.0% 20.2% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Straight Binders Emulsions Cutbacks Reduced-Tracking Figure 38. Percentage of tack coat materials used in the United States. Straight Binders Emulsions Cutbacks Reduced-Tracking 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0% Figure 39. Percentage of tack coat materials used in Canada.

52 Tack Coat Specifications, Materials, and Construction Practices Manitoba, and Yukon reported using one product exclusively (CSS-1h, SS-1, and EP-2000, respectively). The rest allow a number of different tack coat materials. Figures 40 through 42 show the number of U.S. and Canadian agencies reporting to use certain tack coat materials. The agencies were asked whether they required a specific tack coat material for nighttime paving. The question was asked because emulsions break more slowly at the lower temperatures associated with nighttime work versus daytime work. Humidity can also be higher at night, further slowing the breaking process. Lane closure times are often limited, making it undesirable to spend extra time waiting for emulsion to break. Four agencies specified something other than the materials they also use in daytime projects. Pennsylvania responded that it was left to the individual districts to determine whether or not reduced-tracking materials would be specified for nighttime paving. Delaware specifically calls for reduced-tracking materials on nighttime paving work. Florida allows straight binders to be used as tack coat material for nighttime work. Nebraska, a state that normally dilutes their emulsion for tack coat, reported that they sometimes use undiluted emulsion for nighttime work. PG 67-22 PG 64-28 PG 64-22 PG 64-16 PG 58-28 PG 58-22 0 1 2 3 4 5 6 7 Figure 40. Number of U.S. and Canadian agencies that use PG binders as tack coat material. 100 5 15 20 25 30 Non-Standard Emulsion SS-1h SS-1 RS-2 RS-1 MS-1 HFMS-2h HFMS-1 CSS-1h CSS-1 CRS-2p CRS-2 CRS-1 CQS-1h Figure 41. Number of U.S. and Canadian agencies that use emulsions as tack coat material.

Survey Results 53 Agencies were asked whether or not they specified different application rates for different surface types, and if so, what the rates were in terms of residual asphalt. U.S. and Canadian agency responses were nearly identical, so they have been graphed together. Figure 43 shows the average application rate of the responding U.S. and Canadian agencies for various surface types. For the states that provided a range of application rates, the error bars indicate the low and high averages of the reported ranges. The histogram indicates that the agencies, on average, are using very close to the recommended application rate for tack coat on new asphalt from Table 4. The reported application rates are slightly less than Table 4’s recommended rates for tack coat on existing asphalt and milled asphalt. The agencies report using a little more than the recommended amount for PCC pavements. 0 2 4 6 8 10 12 14 UltraTack UltraFuse PennDOT Non-Tracking PATT NTSS-1HM NTSS NTQS-1HH NTHAP NTCRS-1HSP NTCRS-1HM NTCQS-1hM LTBC EM-50-TT CRS-1h CNTTC CBC-1H CBC-1HT AE-NT Figure 42. Number of U.S. and Canadian agencies that use reduced-tracking emulsions as tack coat material. Figure 43. Average specified residual asphalt binder application rates and ranges for various surface types.

54 Tack Coat Specifications, Materials, and Construction Practices The survey results show that most often, the agency decides both what the tack coat material will be, and at what rate it will be applied. This was the case in all but one of the responding Canadian provinces and territories, and 62% (31/50) of the states. However, the survey results also showed that 10% (5/50) of the state agencies allow the contractor to decide which tack coat material to use, but the agency still specifies the application rate. Another 16% (8/50) of the agencies leave both decisions to the contractor. In the remaining agencies, other scenarios are employed, such as requiring the contractor to provide adequate coverage, as approved by an agency representative or allowing the contractor the freedom to make adjustments in the field. Synthesis Survey: Tack Coat Application Power brooming was the most common method of cleaning the existing roadway. Although no replies were given for solely vacuuming or air blowing, the combination of brooming and vacuuming or air blowing was specified by several agencies. Thirty-five percent (20/57) of agencies simply specify that the pavement must be clean, but do not specify the manner by which it should be cleaned. Again, the U.S. and Canadian agencies’ answers tracked very closely to each other, so Figure 44 shows the combined response. A too-frequent source of confusion are specifications that do not clearly state whether the required application rates are for the residual asphalt binder, the undiluted emulsion, or diluted emulsion. As discussed in the FHWA/AI Tack Coat Workshop, the agency could receive up to 70% less residual asphalt if the application rate was written with residual binder in mind but was interpreted in the field as a rate for diluted emulsion. The bar labeled “multiple” in Figure 45 indicates the percentage of agencies that specify application rates for more than one material. The survey revealed that 21% (10/48) of agencies in the U.S. and 14% (1/7) in Canada specify tack coat application rates in terms of the residual asphalt. The majority of agencies specify tack coat application rates in terms of undiluted tack coat, 52% (25/48) in the U.S. and 29% (2/7) in Canada. Application rates in terms of diluted tack coat are specified in 27% (13/48) U.S. agen- cies and 29% (2/7) Canadian agencies. Several agencies provide application rates for multiple scenarios, 15% (7/48) in the U.S. and 29% (2/7) in Canada. The survey revealed that 17% (8/48) of U.S. agencies and 14% (1/7) of Canadian agencies currently provide tack coat application rates, but do not clearly specify whether the rate is for residual, undiluted, or diluted emulsion. Power brooming: 46% (25/57) Both brooming and air blowing or vacuum: 15% (15/57) Any of multiple specified methods: 2% (1/57) Clean, but method not specified: 35% (20/57) Figure 44. Specified pavement cleaning methods.

Survey Results 55 Another question about agency tack coat applications is whether or not dilution of the tack coat emulsion is allowed. The survey results showed that the number of agencies allowing dilution was fairly evenly split, with slightly less than half of U.S. agencies allowing dilution and slightly more than half of Canadian agencies allowing dilution, as shown in Figure 46. It is interesting to contrast the survey results from this synthesis to the worldwide survey performed as part of NCHRP Project 9-40. NCHRP Report 712 (Mohammad et al. 2012) reported that 85% of responding agencies allowed dilution according to the 2012 survey, which is much higher than the approximate 50% rate reported by the agencies in 2017. The 2015–2016 FHWA/ AI Tack Coat Workshops discussed the potential problems of maintaining control over tack coat dilution and recommended that tack coat emulsions not be diluted. There could be a drop in allowable dilution due to the extensive training from the workshop. It should be noted that the 2012 survey included more international agencies, so part of the drop could be that international agencies allow dilution more than agencies in United States and Canada, thereby skewing the percentages. The location of the dilution can be important. Figure 47 shows the percentage allowing dilution at each location, considering only the 24 U.S. agencies that allow dilution. The percentages Figure 45. How agencies specify tack coat application rate. Figure 46. Percentage of agencies allowing dilution.

56 Tack Coat Specifications, Materials, and Construction Practices for Canadian agencies tracked very closely with U.S. agencies. Note that the percentages do not add up to 100%, because several agencies allow dilution only at the terminal, and others allow dilution at multiple locations. If dilution is allowed, another important consideration is the process of verifying the amount of dilution. Without some formal verification process, the owner cannot be assured that the diluted emulsion has the targeted amount of additional water. Figure 48 shows the percentage of each type of verification, considering only the 24 U.S. agencies that allow dilution. Figure 49 shows the percentage of each type of verification, considering only the four Canadian agencies that allow dilution. Again, note that the U.S. percentages do not add up to 100%, because several agencies allow more than one verification process. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% At the asphalt supplier’s terminal At the contractor’s storage tank In the contractor’s distributor Figure 47. Percentage of agencies that allow dilution at given locations. 0% 10% 20% 30% 40% 50% 60% certification by asphalt supplier certification by contractor sample from contractor’s storage tank sample from distributor visual observation during dilution process dilution rate not verified Figure 48. Percentage of U.S. agencies that specify given types of dilution verification.

Survey Results 57 Figure 50 shows how often agencies verify the dilution rate. Some of the comments that pro- vided further insights about verifying the dilution rates are as follows: a few states verify by some quantity, either once every given number of tons, or once per tank; Arizona accepts a certification from the supplier once per project, but also takes daily samples. In Ontario, the entire project is split into one to three sublots, typically comprising the quantity of tack material used to cover 40,000 m2. The U.S. and Canadian responses were very similar, so the results were combined for the chart (Figure 50). The survey asked several questions related to tracking and cleanliness of the tack. Figure 51 shows when U.S. and Canadian agencies allow traffic on a tacked surface. When asked when the paver is allowed to place asphalt on the emulsified tack coat, the U.S. and Canadian responses were very similar. The results were combined in Figure 52. 0% 10% 20% 30% 40% 50% 60% certification by asphalt supplier certification by contractor sample from contractor’s storage tank sample from distributor visual observation during dilution process dilution rate not verified Figure 49. Percentage of Canadian agencies that specify given types of dilution verification. 0% 10% 20% 30% 40% 50% 60% other never by project monthly daily Figure 50. How often the dilution rate is checked (U.S. and Canada combined).

58 Tack Coat Specifications, Materials, and Construction Practices Regarding the maximum time tack coat can be exposed before it must be covered by asphalt, the agencies offered comments about their specification. Three agencies (about 5%) limited exposure time to 2 h or less, with one agency allowing a maximum of 30 min. About 20% of the agencies responded that the tack coat must be covered the same day, or the same shift. About 10% of the agencies responded that the tack must be covered within 24 hours, some phrasing it “1 day,” and one agency specifies one-half day. The rest of the agencies responded that there was no specified limit to how long the tack could lay exposed on the roadway. Most of these agencies have specification wording that tack must be kept clean, maintain the bonding ability, and so forth. One agency specifies that if the tack is left exposed more than a day, another light applica- tion of tack must be applied before paving. Almost 60% of respondents in the United States and Canada said that tracking or pickup continues to be a problem. Responses in the “other method” category included keeping traffic 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% does not specify traffic not allowed on fresh tack between distributor and paver after a specific time delay after the emulsion has set after the emulsion breaks before the emulsion breaks Figure 51. When is traffic allowed on the tack coat? 0% 10% 20% 30% 40% 50% 60% does not specify after a specific time delay after the emulsion has set after the emulsion breaks before the emulsion breaks Figure 52. When is the paver allowed to place asphalt over the tack?

Survey Results 59 off the tack, requiring the use of a spray paver, and retacking as needed. Figures 53 and 54 show the survey responses. Agencies have changed their estimate of average tack coat coverage from the 2012 NCHRP Report 712 survey to this 2017 NCHRP synthesis survey. A noticeable shift to lower estimates of coverage can be seen in this current synthesis survey. It is possible that tack coat coverage has actually declined since NCHRP Report 712 was published, in spite of the greater focus nationally through a great number of presentations on the topic and the FHWA/AI Tack Coat Workshops, which reached well over 3,000 attendees. A more likely reason for the shift is that because of the increased focus, agencies are more aware of what they are actually receiving in tack coat coverage. A summary is provided in Table 8. The FHWA/AI Tack Coat Workshop recommended that every surface be tacked prior to being overlaid, regardless of whether the existing surface was milled or recently placed. However, agencies do not always require every surface to be tacked for various reasons. The survey asked 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% tracking or pickup continues to be a problem other method allows wetting/dampening of pavement before application of tack sanding of tack coat materials after application allows the use of spray pavers requires the use of spray pavers allows the use of reduced-tracking tack materials requires the use of reduced-tracking tack materials tack coat must set tack coat must break Figure 53. U.S. responses: methods to minimize pickup of tack coat. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% tracking or pickup continues to be a problem other method allows wetting/dampening of pavement before application of tack sanding of tack coat materials after application allows the use of spray pavers requires the use of spray pavers allows the use of reduced-tracking tack materials requires the use of reduced-tracking tack materials tack coat must set tack coat must break Figure 54. Canadian responses: methods to minimize pickup of tack coat.

60 Tack Coat Specifications, Materials, and Construction Practices each agency if they required tack coat before every lift. If they sometimes waive the requirement for every lift to be tacked, they were asked to estimate what percentage of surfaces were tacked prior to being overlaid with an asphalt mixture. Although the majority of agencies require every surface to be tacked, a significant percentage of agencies leave some surfaces untacked. The U.S. and Canadian percentages were virtually the same, so the responses were combined and are shown in Figure 55. Most agencies (46% [26/57]) do not waive tack coat requirements. The most common reason for waiving the requirement, given by slightly less than half of the agencies (44% [25/57]), was when a second lift was placed on “fresh” material. The second most common reason given by about 14% (8/57) of the respondents was that the environmental conditions were deemed too poor. One state sometimes waives the typical tack coat requirement if the surface to be overlaid is milled. The majority of agencies in the United States and Canada do not have specified intervals when distributors are to be calibrated. Responses other than “never,” “annually,” or “per job” varied. Some agencies indicate to calibrate when the application rate is suspect. Some said that one calibration record is required per distributor, typically when it is new. Some agencies said that distributor calibration is at the agency representative’s discretion. One agency reported calibration was required twice per year. Figure 56 shows the responses for the Unites States and Canada. Of the 32 U.S. agencies that specify calibration of distributors, most (53% [17/32]) use their own in-house calibration method. In the United States, 13% (4/32) of agencies specifying a calibration method require ASTM D 2995, Method A. Another 6% (2/32) require ASTM D 2995, Method B. Eight agencies that responded that they require distributor calibration did not respond to this question about the calibration method. In Canada, all of the agencies that require distributor Agencies’ Self-Estimate of Tack Coat Coverage 2012 Survey Results: NCHRP Report 712 2017 Survey Results: NCHRP Synthesis 516 100% Coverage 37% 11% (6/57) 90% to 100% Coverage 27% 39% (22/57) 70% to 90% Coverage 18% 39% (22/57) 50% to 70% Coverage 9% 12% (7/57) < 50% Coverage 9% 0% (0/57) Table 8. Agency self-estimates of tack coat coverage. 72% 23% 5% 100% of surfaces are required to be tacked before overlay - 72% (41/57) >75% of surfaces are required to be tacked before overlay - 23% (13/57) between 25 and 75% of surfaces are required to be tacked before overlay - 5% (3/57) Figure 55. Agency specifications regarding percentage of surfaces required to be tacked prior to being overlaid.

Survey Results 61 calibration (2/2) use their own method. One requires the contractor to perform the calibration, while the other requires that it be done by the distributor manufacturer. Figure 57 shows the responses of the 32 U.S. agencies that said they require calibration of the distributor. Note that some agencies answered in more than one category, so the percentages do not add up to 100%. In the survey, agencies were asked how they checked the application rate in the field. Figure 58 summarizes the results for the United States and Canada. The results do not always add up to 100%, because some agencies allow verification by more than one method. The survey asked, “Which of the following are specified by your agency for spray application of tack coat materials?” Figure 59 shows the wide variety of responses. Eight agencies reported no requirements. In the “other” category, most agencies that wrote that they require “uniform coverage.” Most agencies 0% 10% 20% 30% 40% 50% 60% 70% 80% never (18/50) annually (7/50) per job (9/50) other (16/50) never (5/7) annually (2/7) per job (0/0) other (0/0) Figure 56. Agency calibration requirements. 0% 10% 20% 30% 40% 50% Other - 3% (1/32) Distributor manufacturer - 16% (5/32) Independent testing firm - 16% (5/32) The agency - 28% (9/32) The contractor - 44% (14/32) Figure 57. Who performs distributor calibration—by % of agencies that require calibration.

62 Tack Coat Specifications, Materials, and Construction Practices did not provide specific numbers for spray bar height or angle of nozzles, stating “per manu- facturer’s recommendations.” Note that most agencies reported more than one requirement. Fourteen of the states surveyed had verbiage in their specifications specifically calling for uniform tack coat coverage. At least one state’s specifications called for 100% coverage. If the tack coat application is not uniform, a decision must be made concerning that. The agencies have multiple remedies at their disposal to address non-uniform coverage. The following were the most common responses, with about 50% of the United States and Canadian agencies responding that they used them: • Make the contractor reapply the tack coat • Make the contractor reapply the tack coat at a reduced rate • Clean the distributor nozzles A combined 13 of the U.S. and Canadian agencies replied that they penalize the contrac- tor for non-uniform coverage. Another 12 replied that they require the contractor to improve 0% 10% 20% 30% 40% 50% 60% Figure 58. Methods used to verify tack coat coverage rate. 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% spray without overlap of emulsion streams double overlap of the emulsion streams triple overlap of the emulsion streams minimum height of the spraybar maximum height of the spraybar min. angle nozzle angle relative to the spraybar max. angle nozzle angle relative to the spraybar clean nozzles uniform nozzles other none Figure 59. Spray requirements for distributors—by percentage of agencies specifying them.

Survey Results 63 performance on subsequent applications. Seven agencies responded that they have no specified response to non-uniform tack coat applications. Other responses include “correct non-uniform areas” and “stop paving until the situation is resolved to the satisfaction of the Engineer.” When asked a question about any conditions that might cause the specified residual asphalt tack coat application rate to change, almost half of the respondents replied that the pavement surface condition is a factor. Thirty-nine percent of the respondents said that conditions would not cause the application rate to change. The other choices received little or no response. A few agencies pointed out that the inspector could adjust the application rate as needed within the allowable specification range. Another important consideration regarding tack coat coverage is the width of coverage. In 2016 and 2017, FHWA and the Asphalt Institute put on several 7-hour workshops geared toward obtaining higher roadway densities. The longitudinal joint was identified as the area of the mat which usually experienced the lowest in-place densities. The workshop recommended that tack coat be placed beyond the width of the imminent lift to provide some level of confinement to help minimize lateral movement of the asphalt mixture during compaction. Only a small percentage of U.S. agencies (20% [10/50]) explicitly state to apply the tack coat some distance beyond the planned mat width in their specifications, as shown in Figure 60. One of the methods some agencies use to deal with tack coat tracking issues is through the use of spray pavers. According to the survey, U.S. agencies allow the use of spray pavers (Figure 61) more so than Canadian agencies (Figure 62). Of the agencies that allow the use of spray pavers (28 in the United States and 2 in Canada), most actually use the spray paver on less than 20% of their projects; however, Missouri and Texas estimated that they use spray pavers on 21% to 40% of their projects, and Mississippi, which estimated spray pavers were used on 61% to 80% of their projects. Figure 60. Tack coat sprayed beyond planned mat width at longitudinal joint.

64 Tack Coat Specifications, Materials, and Construction Practices In the United States, slightly more than half of the responding agencies that use spray pavers said that they require a different type of tack coat material when using spray pavers. Canada was 50/50, but it should be noted there were only two provinces responding that spray pavers were an allowable option (Figure 63). Those agencies requiring a different type of tack coat material when using spray pavers used the ones specified in Figure 64. Eighty-one percent (22/27) of the agencies in the United States and two out of two of the agencies in Canada do not require any specific methodology to ensure that the tack coat applied by spray pavers is uniform and at the proper application rate. Most that do have verbiage relating to the subject rely on some type of visual inspection and tracking of quantities used. A notable exception is the state of Nevada. Their methodology, very similar to ASTM D 2995, is documented as follows: “Polymer Modified Membrane (PMM) application rate shall be checked twice per day using random sample location techniques. Determination of the application rate of the PMM shall be as follows: 1. At the location to be sampled, immediately adjacent to the paving area, use 2 pads approximately 15-in. wide by 20 in. long, placed side by side, to determine the PMM application rate based on the average of 2 application rate measurements. 2. Capture the tare weight of each pad to be used prior to capturing the PMM sample. 3. Place the first pad 5 feet in front of spray bar on the spray paver. 4. Place the second pad in front of the first pad farther away in the travel direction. 5. Set the machine in automatic mode; do not use manual mode when calibrating emulsion application rate. 6. Circulate the emulsion through the spray bars for approximately 5 minutes before spray calibration in order to purge the system. 7. Select the Spray Pavers Allowed Spray Pavers Not Allowed 44% (22/50) 56% (28/50) Figure 61. Allowable use of spray pavers in the United States. 71% (5/7) 29% (2/7) Spray Pavers Allowed Spray Pavers Not Allowed Figure 62. Allowable use of spray pavers in Canada.

Survey Results 65 machine ground speed/production rate to be no less than 30 feet per minute. 8. Select the desired emulsion application rate and take a sample at this setting. 9. Weigh each pad that has been sprayed with the PMM. 10. Calculate the net weight of emulsion and convert it into gallons using the PMM weight-per-gallon information provided by the emulsion manufacturer. 11. Divide the gallons of PMM by the pad area and compare with the target application rate in gallon per square yard.” Synthesis Survey: Tack Coat Evaluation In the United States, 24% (12/50) of the survey respondents indicated that they were doing interface bond testing (Figure 65). As can be seen in Figure 66, only one of the seven Canadian respondents, Ontario, reported usage of bond testing. It is worth noting that an additional seven states have recently completed or have ongoing research in this area. Therefore, additional states may be using bond testing in some fashion in the future. Also, one province said that they would potentially initiate bond strength testing in 2017. 50%50% different tack coat material used (1/2) same tack coat material used (1/2) 57% 43% different tack coat material used (16/28) same tack coat material used (12/28) Figure 63. Tack coat materials used with spray pavers in the United States and Canada. CQS-1hp CRS-1P polymer-modified CRS-1s CSS-1 CSS-1h Emulsion Bonding Liquid (EBL) non-tracking tack PG 64-22 polymer-modified emulsion 0 1 2 3 4 5 6 7 8 9 Figure 64. Number of agencies requiring different tack coat materials with spray pavers, by product.

66 Tack Coat Specifications, Materials, and Construction Practices The number of states that perform bond testing has seen an increase since the worldwide survey conducted as part of NCHRP Project 9-40. NCHRP Report 712 reported that 8% of agencies perform testing to measure the bond strength between pavement layers (Mohammad et al. 2012). Twenty-four percent of U.S. agencies reported that they perform bond strength testing in the 2017 synthesis survey, with several more agencies reporting that they are currently researching interface bond strength. Agencies that use interface bond strength testing were asked to describe details regarding when they use the testing, what style of testing, how the stress is applied, and the criteria they have for passing or failing tests. As can be seen in Figure 67, the two most common reasons offered for bond testing in the United States were for forensic investigations (9) and for product evaluation (6). Reasons beyond the simple measurement of bond strength that were offered by states included the evaluation of non-tracking tack coats, and to assist in the resolution of disputes related to tack coat. Ontario is using interface bond testing for product evaluation and as a part of forensic inves- tigations. They reported that their current work is intended to produce a performance-based specification for tack coats. 24% 12/50 Yes No 76% (38/50) Figure 65. Number of U.S. states that perform interface bond strength testing. 14% 1/7 Yes No 86% (6/7) Figure 66. Number of Canadian provinces and territories that perform interface bond strength testing.

Survey Results 67 Eight out of the 12 responding states indicated that they are using a laboratory shear test- ing apparatus for interface bond testing (Figure 68). The second most popular was found to be field tension testing (4), followed by lab tension testing (2). Both field and laboratory torsion are being used by one state. Both Texas and Virginia are using more than one style of testing. Ontario province relies on the AASHTO TP 114 (LISST test) for their bond strength testing, which uses a lab shear methodology. The most common method of applying the stress is monotonically in shear at a rate of 2 in./min (50.8 mm/min), as reported by six states. Other monotonic rates were 0.8 in./min 0 1 2 3 4 5 6 7 8 9 On every project By special provision For product evaluation As part of forensic investigations Other Figure 67. When does your agency perform interface bond testing (by number of states)? field tension testing (4) lab tension testing (2) field torsion testing (1) lab torsion testing (1) lab shear testing (8) 0 1 2 3 4 5 6 7 8 9 Figure 68. Number of states using bond strength testing method.

68 Tack Coat Specifications, Materials, and Construction Practices (tension) by one state, 0.2 in./min (shear) in one state, and 0.1 in./min (shear) in two states. One state (Virginia) is utilizing tension testing by applying a 1,200 lb/min (5.3 kN/min) load until failure. Employing a normal load during bond strength testing is a recognized optional addition. It was found from the survey that three states require a normal load while testing (Figure 69). Another two states and one province have the use of a normal load as an option while testing. Five states reported that a normal load was not allowed in their testing. Two states that said they do bond strength testing did not provide an answer to this question. As seen in Figure 70, the minimum bond strength requirements differ for shear and tension testing, with minimum shear values being higher. Six states have a minimum shear requirement of 100 psi (690 kPa), and one indicated a minimum of 50 psi (345 kPa). It should be noted that in the case of West Virginia, their 100 psi (690 kPa) requirement is a part of percent within limits (PWL) system as a lower specification Required, 3 Optional, 3 Not allowed, 5 Figure 69. Use of a normal load while bond testing. 6 1 2 1 1 1 100 PSI 50 PSI 40 PSI 40 PSI 35 PSI 30 PSI SH EA R TE N SI O N Figure 70. Minimum test values by number of states specifying.

Survey Results 69 limit. In Virginia on milled surfaces, a 100 psi (690 kPa) minimum average of three tests with no single value less than 50 psi (345 kPa) is required. On unmilled surfaces, a 50 psi (345 kPa) minimum average of three tests with no single value less than 30 psi (207 kPa) is required. Furthermore, Dr. Louay Mohammad reported that two states are using the LISST test method for forensic investigations, with a minimum value of 40 psi (276 kPa). It should also be noted that pass/fail criteria are often different between agencies because the tests used have different rates of loading, resulting in different strengths. Regarding tension testing, Virginia uses a similar approach to their analysis in shear. For milled surfaces they again test three cores with a minimum average of 40 psi (276 kPa), with no single value below 20 psi (138 kPa). If they are testing an unmilled surface, the average from three tested cores must be 30 psi (207 kPa), with no tests falling below 20 psi (138 kPa). The remaining state which tests in tension is Kansas. They are performing tension testing on specimens produced in the field and also testing in the laboratory (KT-78, 2012). A minimum of 35 psi (241 kPa) is their requirement. One state and the Ontario province both reported that they are in the process of determining what they deem to be acceptable values. Two states did not provide an answer to this question on the survey. Twelve states and one province said they require tests, either in the lab or in the field, to evaluate tack coat materials in lieu of or in addition to bond strength, as seen in Figures 71 and 72. Six of the states did not provide enough detail within their answers to be able to comment beyond the statement that they do additional testing. YES - 27% (13/49) NO - 73% (36/49) Figure 71. States that require tests for the evaluation of tack coat materials in lieu of or in addition to bond strength testing. YES - 17% (1/6) NO - 83% 5/6 Figure 72. Provinces and territories that require tests for the evaluation of tack coat materials in lieu of or in addition to bond strength testing.

70 Tack Coat Specifications, Materials, and Construction Practices One province and five states are determining the percent of residual asphalt found in their samples. A penetration test is run on the residual in the same province and in two states. Viscosity testing is conducted in two states. Other tests, reported in one state each, included a unit weight, ductility, and specific gravity. Visual inspection for tracking of the applied tack was reported by Florida. Two states did not provide an answer to this question. Synthesis Survey: General Questions Figure 73 shows the specific types of pavement failures agencies believe are caused by poor bond strength, according to survey responses. Agencies were asked if they have any completed, ongoing, or planned research on tack coat materials or performance. A total of 15 states responded in the affirmative. 1. Arkansas completed TRC-0706, “Evaluation of Bond Strength Between Paving Layers for Hot-Mix Asphalt” in 2012 (Hall 2012). 2. Colorado is currently studying bond strength. 3. Illinois reported that they have completed several studies regarding tack coats through the Illinois Center for Transportation. 4. Indiana recently completed SPR-3904, “Tack Coat Installation Performance Guidelines” (Lee and McDaniel 2017). 5. Kansas reported that they have completed research on KDOT’s bond strength test and are willing to provide reports upon request. 6. Maryland reports that they are currently looking into laboratory shear strength testing. 7. Missouri reports that they have ongoing research in the area of tack coats and are currently preparing a final report. 8. Nebraska has recently completed Report SPR-P1(16) M039, “Evaluation of Tack Coating Practices for Asphalt Overlays in Nebraska” (Amelian and Kim 2017). The report is available on Nebraska DOT’s website in the Research section. 9. Ohio reports that they are currently wrapping up a bond test research project. 10. Oklahoma reports that they are just beginning to collect laboratory shear strength data and plan to perform structured research in the near future. 0% 20% 40% 60% 80% 100% Other distress(es) Rutting of the pavement Top-down cracking Fatigue cracking Delamination of pavement layer Slippage of the surface layer Figure 73. Pavement failure types cited by agencies as potentially caused by poor bond strength.

Survey Results 71 11. Oregon recently completed report SPR 782, “HMAC Layer Adhesion Through Tack Coat” (Coleri 2016). 12. Texas has one research project currently in progress, “Comparative Analysis of Tack Coat, Underseal Membrane, and Underseal Technologies,” which is scheduled to be completed in 2018. One report, FHWA/TX-16/0-6814-1, “Performance Evaluation, Specifications, and Implementation of Non-Tracking Tack Coat,” (Wilson et al. 2016) was published in 2016. 13. Virginia reported two research projects: VTRC 09-R21, “Bond Expectations for Milled Surfaces and Typical Tack Coat Materials Used in Virginia” (McGhee and Clark 2009); and 12-R14, “Trackless Tack Coat Materials: A Laboratory Evaluation for Performance Acceptance” (Clark et al. 2012). 14. Wisconsin reports a research project beginning in 2017 regarding tack coats and tracking. 15. West Virginia reported ongoing research into application rate as part of their PWL specification. In Canada, a total of four provinces and territories responded that they have completed, ongoing, or planned research on tack coat materials. 1. Manitoba reported research into trackless tack. 2. New Brunswick is considering research into bond strength testing. 3. Ontario has ongoing research on development of bond strength criteria for tack coat applica- tion using LISST. 4. Saskatchewan is experimenting with new tack coat materials this construction season. Test sections will be monitored for performance.

72 As indicated in Chapter 3, interface bond testing is a used by 12 states and one province. Of the 12 states, three were identified by the synthesis consultants and the NCHRP Project 20-05/ Topic 48-02 panel as candidates for case examples. The selected three states were Kansas, Texas, and West Virginia. These were chosen for several reasons. They all have a history long enough to establish trends within their respective states. The testing style is varied among the three with Kansas using a tension methodology, West Virginia using a shearing technique, and Texas having experience with both styles. Their reasons for using bond testing were varied as well. Kansas is using bond testing via special provision on projects; Texas is using it as part of forensic investigations and a referee system between the DOT and contractors; and West Virginia is using bond testing for product evaluation and as a standard part of their PWL projects. Unique questions were asked of each of these states to better ascertain their experiences with, and general impressions of, the benefits they have seen, as well as any advice they may have for others exploring a move into this testing arena. More information regarding each state’s experi- ence with bond testing is detailed in the following sections. Kansas Background Kansas’ experience with bond testing of tack coat materials began in 2011. It their case, bond testing is called for via special provision. Kansas tracks its usage based on centerline miles (Table 9). Kansas reported that they moved to bond testing because they felt complacency had crept into the industry for both DOT personnel and for the contractors. The initiation of bond testing helped to reinforce the importance of tack coats and the bond that it helps to create, ensuring high performance of the pavement. Testing Methodology The tension test employed by the KDOT is an adaptation of ACI 503R, Appendix A, as seen in Figure 74 (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 bot- tom 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 C H A P T E R 4 Case Examples

Case Examples 73 Fiscal Year Centerline Miles Let with Bond Test 2017 345 2016 80 2015 399 2014 559 2013 291 2012 189 2011 67 Table 9. Kansas centerline miles let with bond tests by fiscal year. Figure 74. Kansas Tension Tester (KT-78, 2012). 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. Kansas’ Experience The experience in Kansas was reported to be very positive. Both inspectors and contractors had become complacent in the application of tack coats. With the implementation of testing, that was reported to have changed. Greg Schieber, Bureau Chief for Construction and Materials with the KDOT, explained their impression as follows: “Both our field inspectors and the contractors had grown complacent on tack application. This (bond testing) has reinforced the importance of tack coat both internally and externally to where we feel there has been a

74 Tack Coat Specifications, Materials, and Construction Practices large improvement on the contractors’ practices of applying tack. They are setting an application process that our field inspectors can monitor. We have seen them try to limit the construction traffic on the tacked surface and have seen a better job of cleaning the milled surface with them using multiple brooms on occasion. Also the maintenance of the distributor and ensuring they are getting a uniform coating has improved. For the most part we have seen more attention paid to the tack application and cleaning the milled surface when the specification is on the project as opposed to when it is not on a project, but we have seen some improvements in general on all HMA projects.” When they initially began their testing process they had a penalty associated with low results (<35 psi). The issuance of a penalty was very rare and it has been waived after a couple of seasons of usage. It was reported that the reason for this is that they have not been able to correlate a bond strength to performance. Rather, they indicated that with the improved treatment of tack coats, from application until the placement of asphalt mix, better performance of their pave- ments has resulted. The contracting community in Kansas was reported to have offered some resistance to the testing regimen initially, but that has faded with the elimination of a penalty. It was reported that they too acknowledged that attention to tack coats was not as it should have been prior to the testing regimen. Perhaps most important, even without the penalty, tack coat practices have maintained a high priority by the contractors and the inspectors. Texas Background Of the three case examples, Texas has longest history of interlayer bond testing of tack coat. As early as 2004, the TxDOT sponsored research into tack coat application quality and bonding performance. Additional work has been performed under TxDOT contracts on tack coat materi- als in both laboratory and field settings since that time. There continues to be active research in Texas on tack coats today. Namely, the effect of tack coats on long-term overlay performance is being investigated in conjunction with other potential factors. Two TxDOT test methods have been, or are being, developed to assess tack coat’s quality and bonding properties: Tex-243-F, Tack Coat Adhesion; and Tex-249-F, Shear Bond Strength Test. Tex-243-F was developed in cooperation with the University of Texas El Paso in the mid-2000s. The method is no longer practiced. It was a pull-off or tension test that measured the cohesive properties of an applied tack coat. Tex-249-F is a shear test. Its development is on-going as of the writing of this report. As such, some of what is reported herein may change. The reader is encouraged to obtain the most cur- rent version of this test from TxDOT to determine what, if any, changes have been made since this report was produced. Testing Methodology As Tex-243-F is no longer utilized in Texas, and Tex-249-F is the methodology that seems to be the future for bond testing there, only Tex-249-F will be described in greater detail in this section. Please refer to Chapter 2 for more information on Tex-243-F. The Shear Bond Strength Test is intended to test the interlayer bond strength of 6-in. test specimens that have been cored from a pavement or that were produced within a laboratory. Four-inch specimens are an option, but they are not preferred. The direction of traffic is to be indicated on field collected specimens. The thickness from the layer interface to the end of the