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Optimization of Tack Coat for HMA Placement (2012)

Chapter: Section 2 - State of Practice

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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
×
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
×
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
×
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
×
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
×
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Suggested Citation:"Section 2 - State of Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press. doi: 10.17226/13652.
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7 A review of the existing state of practice was conducted to identify factors related to the use of tack coats for both new HMA pavements and overlays on new, old, milled HMA and for PCC pavements. This review involved an extensive search of all published materials and ongoing research projects to obtain the latest information on the research of the bonding mechanisms of tack coat in pavement structure. A worldwide survey on current tack coat practices was conducted to bet- ter understand the current state of tack coat practices and assist in designing an ensuing research experiment. Results of the survey provided the basis for the experimental factorial design that was used in Phase II of the NCHRP Project 9-40 research project. 2.1 Tack Coat Materials According to ASTM D8, Standard Terminology Relating to Materials for Roads and Pavements, “Tack coat (bond coat) is an application of bituminous material to an existing relatively non absorptive surface to provide a thorough bond between old and new surfacing” (1). Generally, hot paving asphalt cement, cut- back asphalt, and emulsified asphalt have all been used as tack coat materials, but cutback asphalts (asphalts dissolved in sol- vents such as kerosene or diesel) are not typically used for tack coat applications today due to environmental concerns. The most widely used tack coat material in the world is emulsified asphalt. Emulsified asphalt, or asphalt emulsion, is a nonflam- mable liquid substance that is produced by combining asphalt and water with an emulsifying agent such as soap, dust, or cer- tain colloidal clays (2). The most common types of emulsions used for tack coats include slow-setting grades of emulsion such as SS-1, SS-1h, CSS-1, and CSS-1h and the rapid-setting grades of emulsion such as RS-1, RS-2, CRS-1, CRS-2, CRS-2P (polymer-modified), and CRS-2L (latex-modified). According to the Construction Procedure Bulletin (CPB) of the California DOT, several basic terms used in an asphalt emulsion tack coat application are as follows (3): • Original emulsion—an emulsion of paving-grade asphalt and water that contains a small amount of emulsifying agent. Original slow-setting grade emulsions contain up to 43% water, and original rapid-setting grade emulsions contain up to 35% water. • Diluted emulsion—an original emulsion that has been diluted by adding an amount of water equal to or less than the total volume of original emulsion. • Residual asphalt content—the amount of paving asphalt remaining on a tacked pavement surface after the emulsion has broken and set (i.e., after all water has evaporated). • Tack coat break—water separates from the emulsion and the color of the tack coat changes from brown to black. A worldwide survey on tack coat application was con- ducted by the International Bitumen Emulsion Federation (IBEF) (4, 5). Seven countries—Spain, France, Italy, Japan, the Netherlands, the United Kingdom, and the United States— responded through their professional associations. The sur- vey results indicated that the most frequently used tack coat material is cationic emulsion. Paul and Scherocman (6) con- ducted a survey of tack coat practices in the United States. This survey received responses from 42 state DOTs and the District of Columbia. They found that almost all the state DOTs use slow-setting emulsions for tack coats. The emul- sions mostly used are SS-1, SS-1h, CSS-1, and CSS-1h. Only one responding state (Georgia) routinely used hot asphalts (AC-20 and AC-30) as tack coats. A recent phone survey conducted by Cross and Shrestha (7) in 13 mid-western and western U.S. states indicated that slow-setting emulsions are the primary materials for tack coat, except for California, where the AR-4000 was the most common tack coat material followed by either SS-1 or CSS-1. The Kansas DOT was the only agency that reported occasionally using cutback asphalts as tack coat. New Mexico DOT and Texas DOT reported that performance-grade (PG) binders (asphalt cement) were occasionally used as tack coat materials. S e c t i o n 2 State of Practice

According to the Unified Facilities Guide Specification (UFGS) 02744N (8), the advantage of the slow-setting grades over the rapid-setting grades is that they can be diluted. Diluted emulsions are reported to give better results because (1) diluted emulsion provides the additional volume needed for the distributor to function at normal speed when lower application rates are used and (2) diluted emulsion flows eas- ily from the distributor at ambient temperatures allowing for a more uniform application (9, 10). On the other hand, diluted slow-setting emulsions may take several hours to break or even several days to completely set. In addition, an overlay tacked with slow-setting emulsion may be vulnerable to slippage dur- ing its early life (8). Such an overlay exposed to heavy traf- fic immediately after construction could experience excessive slippage in a short period of time. 2.2 Tack Coat Application Rate A proper bond between pavement layers is essential in order to provide a monolithic pavement structure. Selection of an optimum tack coat material and application rate is cru- cial in the development of this bond. Pavement surfaces with different conditions (e.g., new, old, or milled) require differ- ent tack application rates to achieve a proper interface bond. Excessive tack coats may promote shear slippage at the inter- face. Most importantly, it is the residual amount of asphalt cement, not the application rate of diluted asphalt emulsion, that should be specified. From their survey, Paul and Scherocman (6) found that the residual application rates of the emulsions varied between 0.01 and 0.06 gal/yd2, depending on the type of surface for application. The IBEF survey (4) indicated that the residual asphalt content ranged from 0.02 to 0.09 gal/yd2 for tack coats applied on conventional asphalt surfaces. The Asphalt Institute (AI) specifications on tack coats reported that the application rates ranged from 0.05 to 0.15 gal/yd2 for an emulsion diluted with one part water to one part emul- sion (11), which is equivalent to residual application rates between 0.02 to 0.05 gal/yd2. The lower application rates are recommended for new or subsequent layers, while the inter- mediate range is for normal surface conditions on an exist- ing relatively smooth pavement. The upper limit is for old, oxidized, cracked, pocked, or milled asphalt pavement and PCC pavements. The residual asphalt contents, as specified in the Hot-Mix Asphalt Paving Handbook 2000 (12), should range from 0.04 to 0.06 gal/yd2. Open-textured surfaces require more tack coat than surfaces that are tight or dense. Dry, aged surfaces require more tack coat than surfaces that are “fat” or flushed. A milled surface would require even more residual asphalt because of the increased specific surface area, as much as 0.08 gal/yd2. Only half as much residual asphalt is typically required for new HMA layers, 0.02 gal/yd2 (7, 12). Recently, Ohio published typical tack coat application rates for various pavement types using slow-setting asphalt emulsions (SS1, SS1-h) (13). As shown in Table 2, the over- all residual rates vary from 0.03 to 0.08 gal/yd2 for different pavement types. 2.3 Tack Coat Breaking and Setting Time Before asphalt emulsion breaks, it is brown in color because it contains both asphalt cement and water. After broken, the water separates from the emulsion and the color of the emul- sion changes from brown to black. Once all water is evaporated, the emulsion is said to have “set.” Under most circumstances, an emulsion will set in 1 to 2 hours (12), but the literature generally lacks complete agreement concerning how long a tack coat should remain uncovered before placing the subse- quent asphalt layer. The IBEF survey indicated that the lapse of time required between the application of the tack coat and the application of the next asphalt layer ranges from 20 minutes for a broken or cold binder to several hours for a “dry” binder (after all water has evaporated or set) (4). Paul and Scherocman (6) found that many state DOTs specified a minimum time between tack coat application and place- ment of HMA to provide adequate curing time for the emul- sion to break and set. Three state DOTs had a maximum time that a tack coat could be left before placement of the asphalt concrete: Alaska DOT specified a maximum setting period of 2 hours for CSS-1; Arkansas DOT specified a maximum setting period of 72 hours for SS-1; and Texas DOT specified 8 Table 2. Typical tack coat application rates (13). Pavement Condition Application Rate (gal/yd2) Residual Undiluted Diluted (1:1) New HMA 0.03 ~ 0.04 0.05 ~ 0.07 0.10 ~ 0.13 Oxidized HMA 0.04 ~ 0.06 0.07 ~ 0.10 0.13 ~ 0.20 Milled Surface (HMA) 0.06 ~ 0.08 0.10 ~ 0.13 0.20 ~ 0.27 Milled Surface (PCC) 0.06 ~ 0.08 0.10 ~ 0.13 0.20 ~ 0.27 Portland Cement Concrete 0.04 ~0.06 0.07 ~ 0.10 0.13 ~ 0.20

a maximum setting period of 45 minutes for SS-1 or MS-2. Four states indicated that paving was required the same day the tack coat was applied. It is generally recognized that an emulsion should be completely set before new mix is placed on top of the tack coat material. Laboratory studies (14, 15) agreed with this assumption showing that greater interface shear strengths are achieved with longer curing times for the tack coat prior to testing. This was true for both laboratory-fabricated samples (14) and field cores (15). However, experience has also shown that new HMA can usually be placed on top of unset tack coat and even over an unbroken tack coat emulsion with no detrimental effect on pavement performance (12). Indeed, in Europe, emulsified tack coat is often applied to the pavement surface underneath the paver just before the HMA in front of the paver screed. Some European firms have used this tacking process with conventional dense-graded HMA mixtures and normal emulsified asphalt tack rates without negative conse- quences, but there may be concerns with water vapor passing through a dense-graded mat. In the United States, this emul- sion spray method is used in the Novachip™ construction process, as reported by Estakhri and Button (16, 17). 2.4 Tack Coat Application Methods 2.4.1 Equipment Two types of tack coat application methods are shown in Figure 1: (a) a conventional tack coat distribution truck and (b) a special paver with tack coat tank and spray bar. Generally, the best tack coat application results from a “double lap” or “triple lap” coverage. As shown in Figure 2, good “double/triple lap” means that the nozzle spray patterns overlap one another such that every portion of the pavement surface receives spray from two or three nozzles. Several vehicle-related adjustments and settings are crucial to achieving uniform tack coat placement. Essentially, the nozzle patterns, spray bar height, and distribution pressure must work together to produce uniform tack coat application (14, 19). Specific guidance is summarized as follows: • Nozzle spray patterns should be identical to one another along a distributor spray bar. To prevent the spray of liq- uid asphalt from interfering with adjacent spray nozzles, all nozzles should be set at the same angle (about 30°) to the axis of the spray bar (see Figure 3). Lack of a uniform angle 9 Figure 1. Application equipment of tack coat. (a) Tack coat distributor truck (b) Paver with tack coat tank and spray bar Figure 2. Uniform tack coat application with double and triple overlapping (18).

10 will result in some areas of the pavement having thicker or thinner coverage and possible interference between noz- zles. Differing coverage will result in streaks and gaps in the tack coat (see Figure 4). • The size of the nozzles needed to apply an asphalt emul- sion material for a surface treatment, chip seal, or seal coat is significantly larger than the size of the nozzles needed to apply a tack coat. Using a nozzle that is too small with too much pressure results in a surface that has a spider web coating of tack coat material (see Figure 5). • Spray bar height should remain constant. As tack coat is applied, the vehicle will become lighter, causing the spray bar to rise. The tack coat application vehicle should be able to compensate for this. Excessively low spray bars result in streaks (see Figure 4), while excessively high spray bars cause non-uniform transverse coverage. • Pressure within the distributor must be capable of forc- ing the tack coat material out of the spray nozzles at a con- stant rate. Inconsistent pressure will result in non-uniform application rates. • Tack distributors must be capable of maintaining tem- perature of the asphalt cement material to ensure the material will adequately flow. For slow-setting asphalt emulsions such as SS-1, the spraying temperature within the distributor should be maintained between about 24°C and 54°C. Excessive heating may cause the emulsion to break while still in the distributor. 2.4.2 Proper Tack Coat Application Proper application of tack coat is a key component in high- quality asphalt pavement rehabilitation. Proper tack coat appli- cation begins with properly calibrated application equipment. If the distributor has not been used for some period of time, the operator should place a trial tack coat application over some convenient, unused area to ensure that all of the nozzles are open and operating properly. In addition, the distributor application rate needs to be calibrated, both in the transverse direction and in the longitudinal direction, using the proce- dure described in ASTM Method D 2995 (19). Spray bar height depends on truck speed, nozzle configuration, and application pressure. Operators should adjust the spray bar height through- out the day depending on the amount of emulsion in the tank. As a summary, the literature suggests the fundamental aspects of achieving tack coat success are • Having a thoroughly clean roadway surface, • Ensuring all the equipment functions properly and is set up correctly, • Choosing the proper application rate for the tack material used and the existing surface conditions, • Applying the materials uniformly, and • Allowing the tack to set prior to paving to ensure the best possible bond between layers. One perpetual problem often associated 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 Figure 3. Proper nozzle angle setting (14). Figure 4. Non-uniform tack coat: streaks. Figure 5. Small nozzle opening (19).

11 shown in Figure 6. Currently, there are many methods for addressing the haul truck pickup problem. One method is to apply the tack coat to the pavement surface underneath the paver just ahead of the screed. This can be done by using a special paver fitted with a tack coat spray bar, as shown in Figure 1(b). A material transfer vehicle (MTV) may also be used to address the haul truck pickup problem. A third solution is to use modified tack coat materials without the stickiness or pick-up problem. An example of such a tack coat material is a patented procedure called COLNET, devel- oped by Colas in France (20). The COLNET procedure was reported to allow immediate trafficking after the spraying by employing a clean-bond cationic asphalt emulsion—called Colacid R 70 C—with very fast, controlled breaking agents (see Figure 7). 2.5 Characterization of Tack Coat Application 2.5.1 Laboratory Characterization of Tack Coats As illustrated in Figure 8 and under traffic loading, pave- ment interface failure can be attributed to both shear and tension distress modes. In general, two test modes—shear and tension—are often used in laboratory testing to charac- terize the interface bond strengths of tack coats. Many studies have reported using different performance-related test tools to assess the bonding characteristics of tack coats (14, 15, and 21–29). Sangiorgi et al. (21) conducted a laboratory assessment of bond conditions using the Leutner shear test with speci- mens cored from laboratory-compacted slabs. Two surfacing materials [0.4-in stone mastic asphalt (SMA) and 1.2-in hot rolled asphalt (HRA)], one binder course (0.8-in dense bitu- men macadams), and one asphalt-stabilized base material (0.8-in dense bitumen macadams) were used to simulate sur- facing over binder and binder over base interfaces. Three dif- ferent interface treatments were considered to simulate actual conditions: (1) with tack coat emulsion, (2) contaminated by dirt and without tack coat emulsion, and (3) with tack coat emulsion and a thin film of dirt. Results indicated that the best bond strength was achieved with an interface treat- ment prepared using an emulsified tack coat, while the poor- est bond conditions were observed from binder course/base interfaces. SMA and HRA surfacings showed similar results. Uzan et al. (22) studied the interface adhesion proper- ties of asphalt layers based on a laboratory shear test. Test specimens were prepared using a 0.512-in Marshall mixture. A 60-70 penetration binder was used both in the mixture design and for the tack coat application. Tests were conducted on two asphalt binders at two different test temperatures, five tack coat application rates, and five vertical pressures. They concluded that (1) shear resistance of the interface increased significantly with increasing vertical pressure and decreased with increasing temperature and (2) shear resis- tance peaked at an optimum tack coat application rate that is Figure 6. Pick-up by haul truck tires. Figure 8. Distress modes at pavement interface under service conditions (30).Figure 7. COLNET application in Paris.

12 dependent on the test temperature. It was proposed that, for the 60-70 penetration binder used in this study, the opti- mum application rate at 25°C and 55°C were 0.11 gal/yd2 and 0.22 gal/yd2, respectively. Hachiya and Sato (14) performed both simple shear tests and simple tension tests on samples cut from laboratory- compacted asphalt concrete blocks. Three cationic asphalt emulsions and three rubber-modified asphalt emulsions were used in the study. They concluded that, at low-temperature conditions (0°C), the rubber-modified asphalt emulsion (PK-HR2) provided the highest shear strength among the seven emulsions evaluated. At high temperatures (40°C), the rubber-modified asphalt emulsions (PK-R80, PK-HR1, and PK-HR2) were almost equally effective. The optimum appli- cation rate was 0.04 gal/yd2. In Italy, Canestrari and Santagata (26) utilized a direct shear test device named ASTRA (the Ancona Shear Testing Research and Analysis) to evaluate interface bond strength. Their objective was to determine the effects of different variables on the shear behavior of tack coat. They reported that (1) as the normal stress increased, dilatancy decreased (similar effects of reduced dilatancy were observed while decreasing the test temperature); (2) an increase of the applied normal stress caused an increase in the peak shear stress; (3) compared with the sample without tack coat, samples with emulsions as a bonding treatment at the inter- face exhibited higher peak shear stress at failure at all test temperatures and for each level of normal stress; and (4) an increase in shear resistance was observed as a function of decreasing test temperature. In Switzerland, Raab and Partl (30) investigated the influ- ence of tack coats on the interlayer adhesion of gyratory spec- imens in the laboratory using a Layer-Parallel Direct Shear (LPDS) test. Nearly 20 different types of tack coats were used to compare the behavior of specimens with and without tack coats. Two surface conditions (smooth and rough) and two compaction levels (240 and 50 gyrations) were considered to span actual conditions. The influence of moisture, water, and heat on tack coat mechanisms was investigated. Test results showed that all specimens with smooth surfaces sustained higher shear forces than those with rough surfaces because of the larger contact interface between the smooth surfaces. All types of tack coat yielded similar results. Using a certain tack coat, shear adhesion was improved up to 10% for a top-layer compaction at 240 gyrations, while such improvement was not observed for 50 gyrations. In addition, they showed that the use of tack coats led to better adhesion properties in case of a wet surface or oven heating of the specimens before the shear test. Mohammad et al. (23) evaluated the effect of tack coat material types and application rates on bond strength using a direct shear device on the Superpave Shear Tester (SST). Four emulsions—CRS-2P, SS-1, CSS-1, and SS-1h—and two asphalt binders—PG 64-22 and PG 76-22M—were evaluated as tack coat materials. Residual application rates were 0.00, 0.02, 0.05, 0.10, and 0.20 gal/yd2. The study evaluated tack coats through the simple shear test at temperatures of 25°C and 54°C. Test results indicated that CRS-2P yielded the highest interface shear strength among the six tack coat mate- rials evaluated and was identified as the best tack coat type in the study. The optimum application rate was 0.02 gal/yd2. As expected, the mean interface shear strength increased with an increase in normal stress levels at both 25°C and 54°C. In addition, this study indicated that applying certain types of tack coat (e.g., CRS-2P) provided improved bond strength at the interface of the two asphalt concrete layers compared with that without tack coat application. Sholar et al. (15) studied the effects of moisture, appli- cation rate, and aggregate interaction on bonding perfor- mance of tack coat between two pavement layers. A direct shear test apparatus and procedure were developed, and three field projects were constructed and examined over a period of time. Four diluted emulsion application rates were examined: no tack coat, 0.02 gal/yd2, 0.06 gal/yd2, and 0.08 gal/yd2. Two diluted application rates were examined with water applied to the tacked surface to represent rainfall: 0.02 gal/yd2 and 0.08 gal/yd2. Roadway cores were obtained and tested to determine shear strength in the laboratory with the newly developed direct shear test. The test tem- perature was 25°C. Results indicated that (1) water applied to the surface of the tack coat significantly reduced the shear strength of the specimens, and, in long-term testing, speci- mens with water applied to them never developed a shear strength equivalent to the specimens that had remained dry; (2) varying tack coat application rates within the range of 0.02 to 0.08 gal/yd2 had little effect on shear strengths; (3) the use of a tack coat to increase shear strength was less effective for coarse-graded mixtures than for fine-graded mixtures; (4) coarse-graded mixtures achieved significantly higher shear strength than did the fine-graded mixtures; and (5) a milled interface achieved the greatest shear strengths of surfaces tested. Buchanan and Woods (31) conducted a comprehensive study of tack coat. Three emulsions (SS-1, CSS-1, and CRS-2) diluted and undiluted as well as one asphalt binder (PG 67-22) were used as tack coat materials. A prototype device (named ATacker™) was developed to evaluate the tensile and torque-shear strength of tack coat materials at various application temperatures, rates, dilutions, and set times. For non-diluted emulsions, tests were performed at appli- cation rates of 0.05, 0.09, and 0.13 gal/yd2. The diluted emul- sions contained one part water to each one part emulsion. SS-1 and CSS-1 emulsions were evaluated at temperatures of 24, 43, and 65°C, while CRS-1 emulsions were evaluated at

13 temperatures of 49, 63, and 77°C. PG 67-22 asphalt binder was evaluated at application rates of 0.04, 0.07 and 0.10 gal/yd2 at an application temperature of 149°C. A laboratory bond interface strength device (LBISD), similar to the direct shear devices, was developed to assess interface shear strength and reaction index (the slope of load-displacement dia- gram) of laboratory-prepared specimens at 25°C. Tensile and torsional-shear test results showed that PG 67-22 yielded the highest overall strengths, while CRS-2 yielded the highest and CSS-1 the lowest strengths of the emulsions. Results indicated that application rate, tack coat type, and emulsion set time affect the tensile and torsional-shear strength. West et al. (32) developed a new test method, the National Center for Asphalt Technology (NCAT) Bond Strength Test. The test results were used for the selection of the best type of tack coat material and optimum application rate. The project included both laboratory and field phases. For the laboratory work, the following were evaluated: two types of emulsion (CRS-2 and CSS-1) and a PG 64-22 asphalt binder; three residual application rates (0.02, 0.05, and 0.08 gal/yd2); and two mix types [0.75-in nominal minimal aggregate size (NMAS) coarse-graded and 0.19-in NMAS fine-graded]. Bond strengths were measured using normal Superpave mix- design specimens at three temperatures (10, 25, and 60°C) and three normal pressure levels (0, 10, and 20 psi). The main conclusions were as follows: 1. As the temperature increased, bond strength decreased significantly for all tack coat types, application rates, and mixture types at all normal pressure levels. 2. PG 64-22 exhibited higher bond strength than the two emulsions, especially for the fine-graded mixture tested at high temperature. 3. For the application rates studied, tack coats with low application rates generally provided high bond strength for the fine-graded mixture; however, for the coarse- graded mixture, bond strength did not change much when application rate varied. 4. At high temperature, when normal pressure increased, bond strength increased, while, at intermediate and low temperatures, bond strength was not sensitive to normal pressure. In phase two of West et al. (32), seven field projects were per- formed to validate the bond strength test results of phase one using the same tack coat material. Tack coat was sprayed on milled or unmilled pavement surface before the HMA over lay was placed and compacted; three to five cores were obtained from each field section, and then bond strength was measured using NCAT Bond Strength Test. For projects using an emul- sified asphalt tack coat material, the residual application rates were 0.03, 0.045, and 0.06 gal/yd2. For projects using a pav- ing grade binder as the tack coat material, the target applica- tion rates were 0.03, 0.05, and 0.07 gal/yd2. Three distribution methods (hand wand sprayer, distributor truck spray bar, and Novachip™ spreader) were employed. A Novachip spreader featured a spray bar attached to the asphalt paver. The main observations of the field study were that 1. Milled HMA surfaces appear to significantly enhance bond strength with a subsequent asphalt pavement layer; 2. Despite the fact that paving-grade asphalt tack coats appeared superior to emulsified asphalt tack coats, the differences were not statistically significant; and 3. Bond strengths in sections that used the Novachip spreader for application of tack coat were significantly higher than similar sections that applied tack coat using a distributor truck. Akhtarhusein et al. (29) evaluated the contribution of prime and tack coat to the interlayer properties in compos- ite asphalt concrete pavement. The project had two main components: experimental and analytical. The experimental part involved determination and comparison of properties of different combinations of materials and test conditions. Some material characteristics were used in the stress-strain- displacement analysis of the analytical part. CMS-2 emul- sion and PG 64-22 asphalt cement were used as tack coat, and three prime coats (EPR-1, CSS-1h, and EA-P) and three composite pavements (AC-AC, AC-PCC, and AC-CTB) were considered in this study. According to North Carolina DOT (NCDOT) specifications, the application rates for tack and prime coats are 0.06 gal/yd2 and 0.24 gal/yd2, respectively. Bond strength was determined on specimens from laboratory- fabricated composite slabs using simple shear test at constant height and axial ramp test. For composite pavements, AC- AC and AC-PCC, the shear tests were conducted at three temperatures—70, 104, and 140°C. For AC-CTB, the test temperatures were 40 and 60°C. Axial ramp tests were per- formed only for AC-AC composite, and test temperatures were 40 and 60°C. The main conclusions based on bond strength tests were as follows: 1. The absence of tack or prime coat severely hinders the development of bond between two layers, causing undue slippage. 2. For AC-AC composites, the strength of PG 64-22 tack coat was comparable with that of CMS-2. 3. For PCC-AC composites, the results confirmed the earlier observation that CMS-2 provided comparable adhesion to PG 64-22. 4. The bond between two similar surfaces (AC-AC) was stronger than the bond between two dissimilar surfaces (AC-PCC).

14 In the analytical portion, 3-D stress analysis software was developed to analyze the stress, strain, and displacement of composite pavement. The pavement was modeled primarily as a layered system of linear elastic materials with the pos- sibility of treating the surface asphalt layer as a linear visco- elastic material. Anisotropy and temperature effects were incorporated. Besides vertical load, the development of a 3-D computer program takes into account the horizontal shear stresses induced on the pavement surface due to vehicle brak- ing effects (acceleration and deceleration). Using the software, a detailed parametric study was conducted to investigate the effect of system parameters including layer thickness and stiff- ness on the stress-strain-displacement fields induced in the pavement. For the delaminating problem in layered pave- ments, it was found, through the analysis in this study, that higher loading leads to higher maximum interface shear stress and that increasing overlay thickness is an effective way to reduce maximum interface shear stress. Maximum interface shear stress can be found at the tire edges for a vehicle apply- ing both normal and shear stresses to a pavement surface. After the maximum interface shear stress is available, it can be used to compare with the bond strength obtained through simple direct shearing testing so that an appropriate interface binder can be chosen. The interface bond condition can seriously influence stress and strain distribution in a pavement structure. Hakim et al. (27) used falling weight deflectometer (FWD) deflec- tion data to assess the bonding condition between bitumi- nous layers. They reported that the FWD-backcalculated stiffness was lower than that obtained in the laboratory. This difference was attributed to the fact that the backcalcula- tion procedure of modulus assumes full bonding between bituminous layers. To address this issue, the interface shear bond stiffness was considered in a modified FWD back- calculation method. Several studies derived interface con- stitutive models for characterizing the bonding condition of a pavement structure in a numerical simulation. Among them, the BISAR program considers the Goodman model for the surface and base interface (33). In this model, shear stress is proportional to the difference in the horizontal dis- placements of the bonding layers. Uzan et al. (22) reported that the interface reaction modulus used in the Goodman model is independent of the normal stresses at the interface. Crispino et al. (34) proposed the use of the Kelvin model to predict the viscous-elastic phenomenon of interlayer reac- tion under dynamic loading. Romanoschi and Metcalf (35) reported that, in the direct shear test, the shear stress and displacement were propor- tional until the shear stress equaled the shear strength and the interface failed. Based on this observation, they pro- posed a constitutive model for the asphalt concrete layer interface using three parameters: (1) the interface reaction modulus, which is the slope of the shear stress-displacement curves; (2) the maximum shear strength; and (3) the fric- tion coefficient after failure. They concluded that the values of interface reaction modulus and shear strength were not affected by the normal stress for an interface with a tack coat. They were, however, affected for an interface with- out a tack coat. The study showed that the interface bond might also fail in fatigue and that the permanent shear dis- placement had a linear relationship with the number of load repetitions. 2.5.2 Interface Bond Strength and Tack Coat Film Test Devices Table 3 describes interface bond strength and tack coat film test devices used in the laboratory and in the field to charac- terize tack coat application and performance (see Figure 9). In general, three test modes—shear, tension, and peel—have been used in both the laboratory and the field to characterize interface/bond strengths of tack coat materials. 2.6 Worldwide Survey A worldwide survey on tack coat practices was conducted to better understand the current state of tack coat practices and to design a corresponding research experiment. The pri- mary objective of the survey was to investigate the current tack coat state of practice related to types of materials used for tack coats, dilution rates of tack coat materials, residual application rates, determination of rate for different types of surfaces, methods used for tack coat distribution, and pave- ment failures related to tack coat application. A questionnaire was developed to meet these objectives. The survey was organized into three main sections: tack coat materials, tack coat application methods, and character- ization of tack coat application. In total, 27 questions were included in the questionnaire concerning all aspects of tack coat practices. All questions included in the survey are pre- sented in Appendix A. Figure 9. An interface bond strength test specimen (a) and a tack coat film test specimen (b). (a) (b) Layer 1 Tack Layer

15 Table 3. Available in situ and laboratory bonding tests. appli memb ed geocomposite rane and tack coat. Apparatus Significance and Use Procedure Specimen Test Results Lab or in situ Remark 1. Leutner Shear Test The maximum shear load and corresponding displacement are measured to evaluate the bonding property of interface. The bonding property is used to determine the appropriateness of the material for use as tack coat. A vertical shear load is applied to a double-layered specimen with a strain controlled mode at a constant rate of 2.0 in/min at 21.1°C until failure. 6.0-in-diameter specimen cored from laboratory- compacted composite (12 in × 12 in width by 2.8 in height) (1) Maximum shear load (2) Corresponding maximum displacement Lab No normal load is applied 2. LTRC Direct Shear Test Shear strength of the tack coat interlayer is measured to evaluate the bonding property of tack coat. The bonding property is used to determine the appropriateness of the material for use as tack coat. A horizontal shear load is applied to a dual-layer specimen of asphalt concrete with a stress control mode at a constant rate of 50 lbs/min at a given temperature until the sample is separate. With a climate chamber, the temperature can be set in the range from –20 to 80°C. (1) 5.9-in-diameter dual-layered specimen cored from the pavement or fabricated in laboratory (2) To be trimmed before testing to ensure the two ends are flat to fit the shear mold (3) Gap width between the shearing platens is around 1 in (25.4 mm) Shear stress at failure Lab (1) Normal load is optional (2) Developed by Louisiana Transportation Research Center (LTRC) 3. TTI Torsional Shear Test Plastic shear strength in torsion is measured to evaluate the shear resistance of the interface and the quality of the tack coat. A twisting moment with constant rate of 2.9 E-04 radian/sec and a normal load is applied on the top of a double-layered cylinder specimen at a constant rate until failure. (1) Dual-layered cylinder specimen with diameter of 6- in compacted in laboratory using two half-molds (2) Space between the two halves is 0.08 in (2 mm) (1) Shear strength (2) Construct Mohr-Coulomb failure envelopes to get the cohesion and the tangent of internal friction angle Lab Developed by Texas Transportation Institute (TTI) 4. Florida Direct Shear Test Bond strength of the tack coat interlayer is measured to evaluate the performance of tack coat. A vertical shear load is applied to dual-layer asphalt concrete specimen with strain control mode at a constant rate of 2.0 in/min at 25°C until failure. (1) Dual-layered cylinder specimen with diameter of 6-in (2) Samples can be roadway cores or laboratory-fabricated specimens and do not need to be trimmed to accommodate the device (3) Gap width between shear plates is 0.19 in Shear strength at failure Lab (1) No normal loads can be applied during the test (2) Developed by Florida DOT 5. Virginia Shear Fatigue Test (36) The number of shear loading cycles at failure is used to determine the optimum application rate of asphalt binder tack at interface between two layers. Cyclic shear load [a 0.015- in deflection was applied to the specimen in the form of a 0.10-s half-sine wave, followed by a relaxation period of 0.9 s (the total cycle is 1s)] is applied at the geocomposite membrane interface of dual-layer sample composed of concrete and HMA specimens until failure at ambient temperature. (1) Composite cylinder specimen with diameter of 3.7 is composed of concrete core, geocomposite membrane, HMA, and tack coat applied on the interface. (2) Concrete core is cored from laboratory- prepared concrete slab. (3) The upper HMA layer is gyratory- compacted on the top of concrete core after (1) Maximum shear stress of each cycle (2) Maximum shear stress against the number of cycles of failure (3) Optimal tack coat application rate Lab Developed by Virginia Polytechnic Institute & State University and the Virginia Tech Transportation Institute (continued on next page)

16 Table 3. (Continued). 6. ASTRA Interface Shear Test Maximum interface shear stress is measured to evaluate the shear resistance property of interface. The shear resistance property is used to evaluate the tack coat properties. Horizontal load is applied along the interface of dual- layered sample at constant rate until failure; meanwhile, a constant normal load is applied on top of the specimen. (1) Dual-layered cylindrical specimen with diameter of 3.94 in (2) Laboratory- fabricated or extracted from pavement Shear stress at failure Lab If carried out at different normal load, a Mohr- Coulomb failure envelope can be obtained. 7. Layer-Parallel Direct Shear (LPDS) Nominal average shear stress and maximum shear stiffness are measured to determine the in-layer and interlayer shear properties of asphalt concrete layers. The in- layer shear properties are used to evaluate the quality of the mixture and the interlayer shear properties are used to evaluate the tack coat properties. Vertical shear load is applied to a composite specimen with strain control mode at constant rate. (1) Cylindrical composite specimen of 3.94-in diameter (2) Laboratory- fabricated sample and pavement core (3) The specimen needs to be glued Tensile strength Lab (1) Shear-plane can be along interface or within the layers (2) Modified by EMPA, Swiss Federal Laboratory for Materials Testing and Research 8. Switzerland Pull-Off Test Tension strength values are measured to evaluate the interlayer shear performance between different asphalt concrete layers. Shear performance is used to evaluate the quality of the tack coat and in A tensile load is applied to asphalt concrete specimen composed of two layers at constant rate. (1) Cylindrical composite specimen of 3.94-in diameter (2) Laboratory- fabricated sample and pavement core (3) The specimen needs Tensile strength Lab Test is carried out according to German testing specification ZTV-SIB 90 comparison of various tack coat materials. to be glued Apparatus Significance and Use Procedure Specimen Test Results Lab or in situ Remark 9. Loboratorio de Caminos de Barcelona Shear Test (LCB) Shear strength of the tack coat interlayer is measured to evaluate the bonding property of tack coat. The bonding property is used to determine the appropriateness of the material for use as tack coat. The dual-layer specimen with tack coat interlay is used as a beam located over two supports and a vertical load is applied to the specimen at a constant deformation speed of 0.05 in/min in the middle of the two supports until failure. (1) Cylindrical composite specimen of 3.94-in diameter and 7.0- in high (2) Laboratory- fabricated sample and/or pavement core (1) Shear strength (2) Shear modulus and the specific cracking energy Lab (1) No normal load can be applied during this test (2) Developed by DOT, Technical University of Catalonia, Spain 10. Wedge-Splitting Test Maximum horizontal force (Fmax) and specific fracture energy (GF) are determined to characterize the fracture- mechanical behavior of layer bonding. The fracture-mechanical behavior is used to determine the appropriateness of the material for use as tack coat. A vertical load is applied through a wedge to a dual- layered specimen with a groove and starter notch along the interface at a constant rate until complete separation of the specimen. (1) Cubic or cylindrical composite specimen with interface in the middle and a start notch in the interface (2) Laboratory- fabricated or cored or cut from pavement (1) Maximum horizontal force (2) Specific fracture energy Lab Developed by Technical University, Austria 11. Dynamic Interaction Test Interlayer reaction complex modulus KI* is determined for the pavement structure analysis. The pavement structure analysis evaluates the capacity of A sinusoidal shear force is applied to dual-layered specimen at particular temperature and given load frequency. Cylindrical composite specimen of 3.94-in diameter, cored from laboratory-compacted twin layer slab or from pavement. The norm of Interlayer reaction complex modulus KI* and phase angle Lab Developed by University of Naples, Italy the pavement and can be used to predict the remaining life of the pavement.

17 Table 3. (Continued). 12. NCAT Shear Test The interface shear strength of core samples is measured to evaluate the bonding property of pavement layers. The bonding property is used to determine the appropriateness of the material for use as tack coat. A vertical shear force is applied to dual - layered specimens along the interface with strain control mode at constant rate until failure. (1) Cylindrical composite specimen with 5.9 in (2) Height of the core above the interface being tested is greater than 3 in. The height of each layer should be greater than 1.97 in, less than 5.9 in . Bond shear strength Lab Developed by National Center for Asphalt Technology (NCAT) 13. HasDell EBSTTM Emulsion Shear Test The bond strength between two layers is measured to determine the appropriateness of the material for use as tack coat. A shear force is applied along the interface until failure. (1) Cylindrical composite specimen with 5.9 in diameter (2) 2.95 - in x 2.95 - in - square composite speci men Bond shear strength Lab or in situ Marketed by R/H Specia lty and Machine, Terre Haute, Indiana 14. Traction Test Tensile strength of the tack coat interlayer is measured to evaluate the bonding property of tack coat. The bonding A tensile force is applied at constant rate of 54 lb/s to a cylindrical sample until failure Cylindrical lab or field sample of 4 - in diameter Bond tensile strength Lab or in situ Develope d by Ministère des Transports du Québec, Canada Apparatus Significance and Use Procedure Specimen Test Results Lab or in situ Remark property is used to determine the appropriateness of the material for use as tack coat. 15. T he ATacker TM Test Shear and/or tensile strength of tack coat material are measured to evaluate its bonding property. The bonding property is used to determine the appropriateness of the material for use as tack coat. A pull and/or torque force is applied to detach the tack - coated plates or detach the contact plate and tack - coated pavement. Tack - coated plates or attach plate to tack - coated pavement Tensile strength and/or shear strengt h Lab or in situ Developed by Introtek , Inc . 16. UTEP Pull - Off Test Tensile strength of tack coat material is measured to determine its bonding property. The bonding property is used to determine the appropriateness of the material for use as tack coat. A torque force is applied to detach the tack - coated plates or detach the contact plate and tack - coated pavement Tack - coated plates or attach plate to tack - coated pavement Tensile stress at the point of failure Lab or in situ Developed by University of Texas at El Paso 17. UTEP Simple Pull - Off Tes t Tensile strength of tack coat material is measured to determine its bonding property. The bonding property is used to determine the appropriateness of the material for use as tack coat. A tensile force is applied directly to pull off the contact plate from the tack - coated surface. Tack - coated plates or attach plate to tack - coated pavement Tensile stress at failure Lab or in situ Developed by University of Texas at El Paso 18. Impulsive Hammer Test The vertical dynamic response of pavement and fractal dimension (FD) are determined to evaluate the bond condition between asphalt layers in field. The bonding condition is used to determine the appropriateness of the material for use as tack coat. An impu lsive loading is applied with a hammer to the pavement surface at particular locations and given loading frequency. Pavement in field FD number In situ Under development at Nottingham University (continued on next page)

18 Table 3. (Continued). Apparatus Significance and Use Procedure Specimen Test Results Lab or in situ Remark 19. Torque Bond Test Torque force at failure is measured to evaluate the in-place bond effectiveness of wearing course system. A torque force is applied to core sample from pavement with a torque wrench to failure. Core sample of 3.94-in or 5.9-in diameter Bond strength In situ Developed by Highway Agency, United Kingdom 20. In situ Shear Stiffness Test The shear strength is measured to evaluate the shear properties of asphalt concrete pavements in the field. Shear properties of pavement relate to the performance of the pavement. A rotational force is applied to the pavement through a test plate, meanwhile a normal weight is provided by the test equipment. Pavement in field Shear strength and shear modulus In situ Developed by Carleton University, Canada In order to facilitate participation in the survey, the questionnaire was converted into a web-based format. Other forms such as PDF, MSWord, and hard-copy were used, based on request of the respondents. Questionnaires were sent to state DOTs, FHWA, the Asphalt Institute, field engineers, contractors, and selected highway agencies in Canada, Europe, and South Africa during the period of August 2005 through January 2006. Follow-up emails and phone calls were made to ensure the respondents under- stood the questions and completed all the questions on the questionnaire. Remarkably, responses were received from 46 state DOTs; from Washington, D.C.; and from Canada (7 responses). Other countries that participated in the survey were Den- mark, Finland, South Africa, and the Netherlands. Figure 10 indicates the state DOTs that responded to the survey. Figure 10. State DOTs that responded to the survey. States that did not respond States that responded

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Optimization of Tack Coat for HMA Placement Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 712: Optimization of Tack Coat for HMA Placement presents proposed test methods for measuring the quality and performance characteristics of tack coat in the laboratory and the field, and includes a training manual presenting proposed construction and testing procedures for tack coat materials.

Links to appendixes B and D to NCHRP Report 712, which are available only in electronic format, are below:

• Appendix B: ATacker™ Displacement Rate Verification Experiment

• Appendix D: Comparison of the LISST Device and the Simple Shear Tester (SST)

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