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11 2.5Characterization of Tack Coat Application 2.5.1Laboratory 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 2129). Sangiorgi et al. (21) conducted a laboratory assessment of bond conditions using the Leutner shear test with speci- Figure 6. Pick-up by haul truck tires. 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- shown in Figure 6. Currently, there are many methods for men macadams), and one asphalt-stabilized base material addressing the haul truck pickup problem. One method is (0.8-in dense bitumen macadams) were used to simulate sur- to apply the tack coat to the pavement surface underneath facing over binder and binder over base interfaces. Three dif- the paver just ahead of the screed. This can be done by using ferent interface treatments were considered to simulate actual a special paver fitted with a tack coat spray bar, as shown in conditions: (1) with tack coat emulsion, (2) contaminated Figure 1(b). A material transfer vehicle (MTV) may also by dirt and without tack coat emulsion, and (3) with tack be used to address the haul truck pickup problem. A third coat emulsion and a thin film of dirt. Results indicated that solution is to use modified tack coat materials without the the best bond strength was achieved with an interface treat- stickiness or pick-up problem. An example of such a tack ment prepared using an emulsified tack coat, while the poor- coat material is a patented procedure called COLNET, devel- est bond conditions were observed from binder course/base oped by Colas in France (20). The COLNET procedure was interfaces. SMA and HRA surfacings showed similar results. reported to allow immediate trafficking after the spraying by Uzan et al. (22) studied the interface adhesion proper- employing a clean-bond cationic asphalt emulsion--called ties of asphalt layers based on a laboratory shear test. Test Colacid R 70 C--with very fast, controlled breaking agents specimens were prepared using a 0.512-in Marshall mixture. (see Figure 7). 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 8. Distress modes at pavement interface Figure 7. COLNET application in Paris. under service conditions (30).

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

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