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

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Case Examples 75 test specimen shall not exceed 3 inches. Specimens shall be trimmed to meet this requirement if necessary. Test specimens are to be collected within 3 days of field placement. They are to be oven-dried to a constant weight for no more than 24 h. A standard test temperature of 77 ± 2°F shall be reached by placing the dried specimens into an oven for a minimum of 2 h. After the specimen has reached its test temperature, it is placed in the shear head so that the interface is centered in the gap between the head’s rigid and sliding sleeves, and the direction of traffic is vertical. This gap is normally 0.25 in., but it may be adjusted up if so desired. The shearing load is applied at a rate of 0.2 in. per min until it has reached its peak value. The peak load and the location of specimen failure, at the bond interface or within a particular layer, are recorded. The stress is calculated in pounds per square inch from the peak load and the cross sectional area of each of three specimens. Texas’ Experience While Texas has the longest history of investigation of tack coat quality and bond strength of the three case examples, they are the only one that does not have an official target value for their testing. They continue to collect data via shear testing for informational purposes and unofficial recommendations on criteria have been offered. Moreover, development of Tex-249-F contin- ues and it appears that it will continue to be used by Texas in the near future. Texas has utilized tension testing, Tex-243-F, in the past, and they did at least some limited torsional testing which never progressed appreciably. Neither of these testing options seem to have a future in Texas. Texas has successfully used interlayer bond testing to evaluate new products and procedures. For both reduced tracking tack coats and spray pavers, TxDOT used shear testing as a tool when they were assessing these options. The Texas experience seems to offer other states much in terms of information on bond characteristics especially over time. Texas noted that bond strengths can change considerably over time. Significant increases in strength have been observed from 1 day to the next early in an overlay’s life. Texas also noted that high bond strength does not always equate to the best pavement per- formance. They offered an example of an overlay on a cracked pavement. Crack propagation was said to have been more severe with higher bonding strengths compared with what were considered moderate values. It was further noted by Texas that they found that simply adjusting tack coat application rates were not the only items which needed to be considered when seeking to improve bonding performance. Some other key items they noted included the tack coat materials, the cleanliness of the surface to be paved (especially if it was milled), the overlay and surface temperatures, the compactive effort, and even the aggregate mineral type were all found to affect bonding characteristics. Lastly, Texas seems to be unique in that they sometimes pay for tack separately, and some- times it is considered incidental to the paving operations. The distinction between when it is a separate pay item or not was not clear. It was said that when the tack membrane is light, it is inci- dental, and when it is heavy, it is a pay item. While they reported that discussions have occurred on making tack a constant individual pay item, it has not yet been adopted.

76 Tack Coat Specifications, Materials, and Construction Practices West Virginia Background The state of West Virginia has the fewest years of experience using bond testing of these three states. They began using it during the 2013 construction season. 2013 was also when they went to a PWL evaluation process on some of their projects. PWL projects include all of their interstate, major U.S., and state four-lane highways. By their estimation, this equates to 5% to 10% of all DOT paving projects. West Virginia also indicated that the percentage of projects using PWL, and thus bond testing, is slowly increasing annually. Testing Methodology West Virginia modeled their interface bond testing after the state of Alabama’s method (ALDOT-430). A summary of the West Virginia method (MP 401.07.23) is as follows. Six-inch cores are obtained from roadway. A layer thickness of at least 2 inches is required. If a layer is greater than 3 in. thick, it is to be trimmed to 3 in. with a wet masonry saw. The tech- nician shall mark the interface of the layers to be tested and the direction of traffic. Standard test temperature is 75 ± 5°F (24 ± 2°C). A minimum of 120 min in a water bath or oven is used to stabilize the specimens to the test temperature. The specimens are tested on a Marshall Stability test apparatus or another mechanical device that can provide the required 2 in. per min (50.8 mm per min) movement. The prescribed gap between the shearing frame and the loading head is 0.25 in. ± 1/32 in. The test specimen is oriented in the test frame so that the interface is centered in the gap and the traffic direction is facing up. No confining normal load is allowed. The load is applied at the prescribed rate until failure occurs. The maximum value in pounds is recorded, as is the temperature at the interface via an infrared temperature gun. The peak bond shear strength in pounds per square inch is calculated by dividing the maximum value recorded dur- ing the test by the cross sectional area of the test specimen. The lower specification limit is 100 psi. Reporting for each test specimen includes the following: • Specimen ID • Sample and test dates • Failure location (interface, existing layer, or new material) • Interface appearance (contaminants, milling striations, stripping, tack coat streaks, or other observations) • Specimen dimensions • Maximum load applied • Interface temperature • Bond shear strength West Virginia’s Experience At least 90% of West Virginia’s paving projects do not use interlayer bond testing, but the percent of those that do is increasing. Currently, West Virginia does not have a penalty or bonus system in place for the test results, even though they have the 100 psi lower specification limit in their PWL system. As would be expected with the data being collected without penalties, it was reported that the contractors do not seem to mind the testing. For their non-PWL projects it was reported that data is rarely gathered on this topic and none was evaluated herein. West Virginia has been tracking the test results for those projects that have incorporated interlayer bond testing. As can be seen in Figure 75, there is increasingly positive results coming

Case Examples 77 out of those projects that are receiving bond testing. In 2013, about 51% of the tests were less than the 100 psi desired threshold. Therefore, about 49% were above it. By 2016, only about 33% were below, and about 67% were above the threshold. Moreover, the number of projects that are evaluated under the PWL system, and thus have interlayer bond testing, has increased by about threefold over the same 4 years. So, there have been an increasing number of projects getting tested, and the results show a positive trend in the bond strengths. When asked about any issues they have seen, it was reported that those projects with milling have had some issues. Namely, on those milled projects that have scabbing in the milled surface, especially when a leveling or scratch course had been used previously, or on composite pave- ments, difficulty in the gathering of test cores was reported. When these projects are milled to collect test specimens for bond testing, they are more prone to having material in the scabs fail while drilling specimen cores. West Virginia was asked if they could note any differences between PWL and non-PWL proj- ects and the application of tack coats. None were noted. Moreover, it was noted that anecdotally, zebra striping and tracking seem to be similar for both types of projects. Summary Three states were further investigated to better ascertain what their experiences has been with interlayer bond testing as it pertains to tack coats. Those states were Kansas, Texas, and West Virginia. These were chosen based on discussions with the NCHRP Project 20-05/Topic 48-02 panel, and their unique characteristics with tack coats and bond testing. Texas has the longest history of interlayer bond strength testing of tack coat, while West Virginia has the shortest. In Kansas, they utilize a tension style of testing, while West Virginia uses a shear test. Texas has experience with both methods as well as a limited amount of testing performed in torsion. All the states reported noticing improvements in their respective tack coats when additional attention is paid to the selection of tack coat materials, their application rates and methods, and bond testing. 0 10 20 30 40 50 60 70 80 <100 psi >100 psi Pe rc en ta ge Shear Strengths 2013 2014 2015 2016 Figure 75. West Virginia interlayer bond testing (pass/fail threshold 100 psi) results by year.

78 Importance of Tack Coats The literature review clearly showed the importance of a strong bond between separate lifts of pavement to allow the pavement to behave as a monolithic structure. Several papers showed a significant loss of service life if the bond was inadequate. The Asphalt Institute pointed out in the FHWA/AI Tack Coat Workshop that while the cost of providing an adequate bond with tack coat was a small percentage of overall project costs, it has a strong influence on pavement service life. Therefore, it is worth the investment to do it properly. Tack Coat Specifications A review of U.S. agency documents revealed that tack coat specifications generally fall into seven areas: • Approved materials – The specifications clearly identify the materials allowed to be used as a tack coat. PG binders account for only 0.4% of the tack coats used in the United States (none in Canada). The most commonly specified materials for tack coat usage in the United States and Canada are asphalt emulsions. The three most commonly specified types of asphalt emulsions are CSS-1h, SS-1h, and SS-1. About 20% of the states allow the use of reduced-tracking emulsions. Currently, the most commonly used reduced-tracking emulsion is NTSS-1HM. • Acceptance of materials – Agencies clearly specify how tack coat materials are accepted and who has authority of acceptance. States typically accept material by some combination of supplier certification, supplier testing, and agency testing. Most agencies maintain an approved list of acceptable tack coat materials. – The most common material acceptance tests are the emulsion tests listed in AASHTO T 59, Standard Method of Test for Emulsified Asphalts. • Tack coat material handling – Most agencies specifications include at least some information about material handling con- siderations. Many agencies refer complete handling considerations to the supplier. Typical handling guidelines include information about safety, storage tank requirements, storage tem- perature requirements, distributor requirements, and material sampling requirements. • Surface preparation – Most agencies include verbiage in their specifications about cleaning the existing surface before the application of a tack coat. About 28% did not specify how the surface was to be cleaned, while almost 70% required a power broom or a combination of power brooming and air blowing the existing surface. C H A P T E R 5 Conclusions

Conclusions 79 • Tack coat application – Whether or not to dilute the emulsion is an important consideration, as well as where the dilution is allowed to occur. A little less than half of U.S. agencies currently allow dilution, while a little more than half of Canadian agencies allow dilution. The FHWA/AI Tack Coat Workshop recommended that if the tack coat emulsion is allowed to be diluted, that it should be done only at the asphalt terminal by the supplier not in the field by the agency or contractor. – A further consideration is the tack coat application rate. The FHWA Tech Brief on Tack Coats discussed the importance of specifications being clear in their language regarding tack coats. It was recommended that the application rates be clearly specified as residual, undiluted, or diluted. The survey indicated that about 80% of agencies clearly specify whether their specified application rates apply to residual, undiluted, or diluted emulsions. – Most agencies specify different application rates for different surfaces. In general, the rougher the surface, the higher the application rate. The reported application rates were generally in line with those recommended by national guidelines, including those set forth by NCHRP Report 712, the FHWA Tech Brief on Tack Coats, and NAPA’s QIP 128. The average low, high, and target application rates of U.S. and Canadian agencies, as indicated by the survey responses, are shown in Table 10. – Over 50% of U.S. agencies and over 70% of Canadian agencies do not specify any require- ments to calibrate the asphalt distributor. – The FHWA/AI Tack Coat Workshop recommended that tack coat be placed beyond the width of the imminent lift to provide some level of confinement at the longitudinal joint to minimize lateral movement of the asphalt mixture during compaction. – The literature review uncovered many reasons why a distributor might not spray the tack coat with a uniform coverage. These included clogged nozzles, incorrect nozzle size, incorrect nozzle orientation, lack of proper pressure in the spray bar, and improper spray bar height. – The literature review revealed that to protect bonding ability, agency specifications should address the maximum time a tack coat should be exposed before it must be covered by asphalt. Dust and debris blown onto the tacked surface will inhibit its ability to act as a bonding agent between pavement surfaces. About 35% of the agencies address this issue in their specifications. Most of the remaining agencies specify that the tack coat must be covered the same day or within 24 h. – Similarly, the literature review revealed that agency specifications should address the maxi- mum distance the tack coat can be applied in front of the paver. Only two of the agencies specified maximum distances, one 500 ft and the other 900 m. – Tack coat tracking occurs when the tack coat sticks to the tires or tracks of a vehicle driven over the tack coat and is then later deposited upon another surface. Agencies have tried several strategies to decrease tack coat tracking. These include the following: � requiring the tack coat to break before vehicles are allowed over it, � requiring the tack coat to set before vehicles are allowed over it, Surface Average Low (gsy) Average High (gsy) Average Target (gsy) New Asphalt 0.027 0.043 0.035 Existing Asphalt 0.034 0.051 0.043 Milled Asphalt 0.041 0.058 0.049 New or Existing PCC 0.042 0.062 0.052 Milled PCC 0.047 0.067 0.057 Table 10. Survey responses regarding tack coat residual application rates.

80 Tack Coat Specifications, Materials, and Construction Practices � requiring the use of reduced-tracking tack coat materials, � allowing the use of reduced-tracking tack coat materials, � sanding of tack coat materials after application, � wetting or dampening of the pavement before the application of tack, and � allowing/requiring the use of a spray pavers. Almost 60% of the respondents in the United States and Canada reported that tracking continues to be a problem. • Tack coat acceptance – After a tack coat has been placed, agencies need a way to assess its in-place acceptability. This type of acceptance tends to cover one or both of the following two methods: (1) assessment of tack coat coverage and (2) assessment of tack coat bond strength. – About 80% of U.S. agencies and over 70% of Canadian agencies responded that they have some methodology to check the tack coat application rate in the field. Most do so by verifying that the volume used from the distributor tank corresponds within the prescribed tolerances the volume required to spray a given width and length of roadway at the required application rate. – Twenty-four percent of U.S. agencies and 14% of Canadian agencies perform some type of bond strength testing. Only one state uses field bond strength testing routinely. According to the survey, bond strength testing is performed most often as part of forensic pavement evaluations. The second most common reason agencies gave for bond strength testing is to evaluate a tack coat material as part of an approval process. • Method of payment – Tack coat payment 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. – In the United States, 66% of states that responded to the survey pay for tack coat as an individual pay item. In Canada, 86% of the responding provinces and territories pay for tack coat as an individual pay item. – Half of both U.S. and Canadian agencies pay for tack coat materials by volume of the undiluted emulsion. The second most common method of measurement for payment, used by about one-third of all agencies, is by mass of the undiluted emulsion. A small number of agencies measure materials for payment by either volume or mass of diluted emulsion or by area covered. Suggested Future Tack Coat Research As a result of many factors, state agencies across the United States are reevaluating their tack coat specifications, the materials they use, the practices by which the tack coats are placed, and the methods by which they ensure acceptable quality. Thirty percent of U.S. agencies reported having ongoing, planned, or recently completed research in this area. Most of these research activities involve assessing the bond strength char- acteristics of tack coats. Four of the seven responding Canadian provinces and territories report that they are now or soon will be conducting tack coat research. Two of the agencies will be evaluating reduced- tracking emulsions, while the other two will be researching methods to evaluate bond strength. There is a need for even more research in the area of tack coat materials. Although the usage of reduced-tracking tack coat materials is becoming more common, many of the materials still have areas in which they could be improved.