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19 3.1 Introduction Based on the results of Task 1, a comprehensive experi- mental plan was designed to identify consistent, reliable, and practical methods for (1) evalutracating the bonding charac- teristics of tack coats; (2) selecting the tack coat material type and residual asphalt binder application rate required for opti- mum performance in new HMA pavement and HMA overlay construction, rehabilitation, and reconstruction; (3) calibrat- ing application equipment; and (4) maintaining field quality control and quality assurance. Findings reported in Chapter 2 also identified a number of factors that were reported to influ- ence interface bond strength including tack coat type, tack coat application rate, tack coat curing time, surface condi- tion, and pavement temperature. Responses from the world- wide survey indicated that the residual application rates of emulsions typically vary from 0.02 to 0.08 gal/yd2, depending on the type of pavement surface. As pavement temperature increases, laboratory bond strength significantly decreases for all tack coat types and application rates. The most common types of emulsions used for tack coats include slow-setting and rapid-setting grades of emulsions. In the United States, most states use slow-setting grades of emulsions. To this end, the experimental plan investigated the influence of a number of factors on the interface shear strength: HMA and PCC sur- face type and properties (e.g., texture, air voids content, and permeability), surface cleanliness, tack coat material type, and application rate and method. The majority of the research activities conducted in this project were based on tack coat experiments conducted in a field environment. Field experiments were complemented with a number of laboratory experiments to assess the influ- ence of variables such as laboratory compaction, rheological properties of tack coat materials, and test temperature. The experimental program was divided into experimental test matrices, which answered specific objectives of the experi- mental program. Since all experiments made use of full-scale test lanes, a description of the construction process and the test variables in the field experiment is presented in the fol- lowing section. 3.2 Tack Coat and Overlay Construction at the Test Site Table 4 presents the test matrix simulated in the LTRC PRF field experiment, which used conventional paving equipment and a computerized tack coat distributor truck. Four types of pavement surfaces and five tack coat materials were evaluated, but only one emulsion (SS-1h) was used on the new HMA surface, and two emulsion grades (SS-1h and SS-1) were used on the milled surface. Four residual application rates were selected including zero (no-tack). Effects of wet and dusty conditions during construction operations were simulated for the different surface types as part of the experimental pro- gram. To evaluate variation in the results, triplicate samples were tested for each condition; 375 samples were tested as part of the test matrix. Laboratory specimens (cores) were obtained from the pavement test sections. 3.2.1 HMA Pavement Surface Preparation Figure 11 presents a plan view of the five test lanes con- structed at the LTRC PRF. Each lane was a total of 215 ft in length and 12 ft in width. It is noted that each lane contained test and distributor truck access areas. Each test section had a length of 15 ft and a width of 6.5 ft. The lengths of the adja- cent (access) areas were selected to ensure that the distributor truck could attain the required speed in order to achieve the correct tack coat application rate. All test lanes selected for this experiment contained a similar old HMA surface type. Surface texture values for each lane were measured using a laser type device (DYNATEST 5051 Mark III road surface profiler), according to ASTM E 1845, Standard Practice for Calculating Pavement Macrotexture Mean Profile Depth. The S e c t i o n 3 Experimental Program
20 roughness value for Lane 1, Lane 2, and Lane 3 were 0.042, 0.043, and 0.043 in, respectively. Lane 4 was not utilized for the experiment because the surface texture was different from that of Lanes 1, 2, 3, and 5. Figure 12 presents the mea- surements and markings of the grid on the actual test lanes, which correspond to the layout presented in Figure 11. The markings were used to assist the tack coat distributor truck in determining the correct location for each type of material and the corresponding application rate. 3.2.2 Dusty and Wet Conditions Simulation The effects of dusty or wet (rainfall) conditions of the exist- ing pavement surface were investigated. In order to simulate dusty conditions, a silty-clay-type soil, classified as A4 based on the AASHTO soil classification, was reasonably uniformly applied at a rate of 0.070 lb/ft2 onto the old HMA surface prior to tack coat application (see Figure 13a and b). Wet condition was simulated by uniformly spraying water at a rate of 0.06 gal/yd2 on tacked surfaces and prior to placement of the HMA mixture (see Figure 13c and d). Wet condition was considered only for the SS-1h tack coat due to the limited number of test lanes that could be constructed at the PRF. 3.2.3 Tack Coat Application Prior to the day of tack coat application, several calibration trials were performed over a 3-month period for the comput- erized distributor truck to ensure that the selected applica- tion rates might be installed successfully given the restrictions at the site. During these trials, the application rate was found to be in error as much as 40%. Both the owner of the truck and the manufacturer of the equipment worked to identify the sources of the problems and correct them. Several repair and maintenance actions were completed on the distributor truck prior to tack coat application. Application of tack coat materials was performed directly after testing and preparation of the existing surface was com- pleted. Tack materials were SS-1, SS-1h, CRS-1, PG 64-22, and trackless (NTSS-1HM). The truck speed and spray tip used for each tack coat material along with the correspond- ing application rate are provided in Figure 14. As indicated in this figure, 10 passes by the distributor truck were made. A pass is completed when the distributor truck has traversed a given lane. An Etnyre computerized tack coat distributor truck Model 2000 was used to apply the tack coat materials. The truck had a heated tank for holding tack coat materi- als at the desired application temperature. While the track- less tack coat was applied at 82°C, the SS-1h and CRS-1 tack coat materials were applied at 68°C. Tack coats were applied in the undiluted state. Mounted on the back of the truck, a spray bar fitted with nozzles distributed tack coat material at the specified application rate. The total width of the spray bar was extended to 13 ft in order to provide full coverage of a single lane. Application rate was adjusted by altering the truck speed and nozzle type and size. Distribution of tack coat materials was coordinated so that the wheels of the distributor truck never came in contact with the tack coat material (see Figure 15). The application of SS-1h for 100% and 50% coverage were conducted in the same man- ner. For application of the residual rate of 0.031 gal/yd2, the distributor truck drove the entire span of Lanes 1 and 2 at the specified speed to deposit the tack coat at the end of each lane. The residual application rate of 0.062 gal/yd2 was applied at the specified speed in the same manner as the previous appli- cation rate. The residual application rate of 0.155 gal/yd2 was Variables* Content Levels Pavement surface type Old HMA, New HMA, PCC, Milled HMA 4 Tack coat material SS-1h, SS-1, CRS-1, Trackless, PG 64-22 5 Residual application rate 0- (No-Tack), 0.031, 0.062, 0.155-gal/yd2 4 Wet (rain) condition Wet, Dry 2 Dusty condition Dusty, Clean 2 Test temperature~ 25°C 1 Confinement pressure (psi) 0, 20 2 Tack coat coverage 50%, 100% 2 Number of replicates 3 3 Total Number of Samples 474 *Some variables were partially evaluated according to the test factorial; ~ test temperature was varied in the sub-matrix that evaluated the effect of temperature on ISS. Table 4. Test factorial for field-prepared samples.
Wet 0.031 gsy Wet 0.031 gsy Wet 0.031 gsy Wet 0.031 gsy Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Wet 0.062 gsy Adjacent Area Wet No Tack Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Wet 0.062 gsy Adjacent Area Wet No Tack 4.6 ft 4.6 ft 4.6 ft 50 ft 4.6 ft 4.6 ft 4.6 ft 35.1 ft 4.5 ft 4.5 ft 50 ft 4.6 ft 4.6 ft 35 ft Dry No Tack Dry 0.031 gsy 2 Adjacent Area Dry 0.062 gsy Wet 0.062 gsy Adjacent Area Wet No Tack Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Wet 0.062 gsy Adjacent Area Wet No Tack 24.9 ft 4.6 ft 4.6 ft Dry 0.155 gsy Wet 0.155 gsy Dry 0.155 gsy Wet 0.155 gsy Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Adjacent Area Dry 0.155 gsy Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Adjacent Area Dry 0.155 gsy Adjacent Area Adjacent Area Adjacent Area Adjacent Area Adjacent Area Adjacent Area Adjacent Area 4.6 ft Dry 0.155 gsy Wet 0.155 gsy Dry 0.155 gsy Wet 0.155 gsy Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Adjacent Area Dry 0.155 gsy Dry No Tack Dry 0.031 gsy Adjacent Area Dry 0.062 gsy Adjacent Area Dry 0.155 gsy Adjacent Area Adjacent Area Adjacent Area Adjacent Area 40 ft Lane 1 Lane 4 Lane 2 Lane 3 Lane 5 215 ft Adjacent Area Clean Dirt Clean Dirt Clean Dirt Clean Dirt Figure 11. Layout of test lanes in the field experiment.
22 applied at the end of Lanes 3 and 5 for 100% and 50% cover- age, respectively. Figure 16 illustrates 50% coverage of SS-1h for each application rate compared with a typical section with 100% coverage. Both trackless and CRS-1 emulsions were applied to Lanes 3 and 5 in a similar manner at three application rates (0.031, 0.062, and 0.155 gal/yd2). 3.2.4 Overlay Construction A 12.5-mm NMAS HMA mixture was placed on top of the tacked surfaces at a thickness of approximately 3 in. A mate- rial transfer device was used to transfer the mixture from the haul trucks to the hopper of the paver (see Figure 17b) in Figure 12. Preparation of test lanes for tack coat application. (a) Dust Application (c) Water Spraying Using a Hose (b) Tack Coat Application on Dusty Surface (d) Overlaying on Wet Surface Figure 13. Simulation of dusty and wet conditions.
Figure 14. Spraying process plan for four lanes at the test site. Figure 15. Distributor truck spray application at 50% coverage.
24 order to eliminate construction traffic on tacked surfaces. Subsequent to completion of HMA overlay placement, each lane was marked based on previously documented reference points identifying the various test sections within each lane (see Figure 17f). 3.2.5 Quality Testing of Tack Coat Application The calibration of the distributor truck was a lengthy pro- cess in this project and required multiple calibration runs to ensure the accuracy and uniformity of tack coat application. This difficulty highlights the importance of regularly check- ing the calibration of the distributor in practice. The proce- dure outlined in Test Method A of ASTM D 2995, Standard Practice for Estimating Application Rate of Bituminous Distribu- tors, was followed. The surface of each pavement was initially cleaned. Square (1 ft by 1 ft) textile pads were attached to the surface of the pavement using a two-sided adhesive tape. The geometrical layout of the pads is illustrated in Figure 18. Two pads were aligned in the transverse direction relative to the lane. At least 2 ft were given to accommodate the space needed for the wheels of the truck during the spray process. Once the pads were positioned correctly, the tack coat distributor truck applied the material to the section. For emulsion tack coats, the pads were allowed to remain in position for 3 hours to ensure that all water had evapo- rated. After this period, the weight of each pad was measured. The final weight, minus the initial weight of the pads with no tack, represented the residual asphalt cement and was used in the computation of the residual application rate. Table 5 presents the results of these measurements for the tack coat distribution conducted at the test site. For SS-1h with 100% coverage, trackless, and CRS-1, target application rates of 0.062- and 0.155- gal/yd2 were achieved with relatively low errors, although errors for trackless and SS-1h exceeded the 10% error limitation specified by ASTM 2995D. For the 0.031 gal/yd2 target application rate, errors were relatively higher than those of other application rates, but it is noted that coef- ficient of variation (COV) values for 0.031 gal/yd2 rate were relatively low and showed high consistency. In summary, it is noted that the measured application rates were slightly differ- ent than the target values; however, the measured rates met the objectives of the test matrix to simulate low, medium, and high levels. On the other hand, for SS-1h with 50% coverage, it was observed that high errors occurred at all application rates. Figure 19 shows a comparison of 50% to 100% cover- age from two cores extracted from the test facility. 3.2.6 Specimen Coring and Conditioning A minimum of six test specimens were obtained from each test section using a Simco® 255 Pavement Test Core Drill. The core barrel was positioned over the area in which a sample was to be extracted, and water was allowed to flow down the inside of the barrel in order to reduce friction (see Figure 20). The core barrel was then driven to the bottom- most layer in order to remove the sample undisturbed. Sam- ples were cored all the way through to avoid pre-stressing of the samples. The sample was then removed from the core barrel, labeled, and packaged for transportation. It is noted that a manual corer (i.e., Milwaukee Dynodrill B-1000) was used for weaker samples that required smaller amounts of torque. (a) (b) Figure 16. Tack coat application (a) 0.155 gal/yd2 with 100% coverage and (b) 0.031 gal/yd2 with 50% coverage (SS-1h).
25 (a) (c) (e) (b) (d) (f) Figure 17. Overlay construction at the test site.
Le ft W he el o f T ru ck R ig ht W he el o f T ru ck 6 5 17 2.0 ft 3.0 ft 1.0 ft 13.0 ft 34 2 Figure 18. Tack coat rate measurement pad layout for each section. Tack Coat Target Residual Application Rate (gal/yd2) Measured Residual Application Rate Average (gal/yd2) Standard Deviation (gal/yd2) COV Error (%) SS-1h 50 % 0.031 0.062 0.007 11.8 102.3 0.062 0.071 0.009 10.8 16.0 0.155 0.166 0.099 59.6 45.8 SS-1h 100 % 0.031 0.044 0.004 8.8 42.0 0.062 0.073 0.007 9.1 19.6 0.155 0.139 0.022 16.6 10.8 Trackless* 0.031 0.040 0.002 6.1 28.7 0.062 0.068 0.004 7.7 10.8 0.155 0.177 0.011 5.9 14.9 CRS-1* 0.031 0.035 0.004 15.3 20.8 0.062 0.062 0.004 5.7 3.9 0.155 0.152 0.007 4.5 3.6 * Trackless and CRS-1 were distributed with 100% coverage. Table 5. Tack coat distribution test results at the PRF site. (a) 100% Coverage Surface (b) 50% Coverage Surface Figure 19. Typical tack coat surface coverage on old HMA surface (0.155 gal/yd2 residual application rate).
27 Prior to testing, cored samples were cut to a height of 6.0 in, avoiding disturbances to the interface and the top layer. Because water was used as a coring lubricant, the sam- ples were placed in an oven at 40°C to dry for a minimum of 24 hours. The dried samples were placed in a conditioning chamber at 25°C for a minimum of 4 hours. This conditioning period was adequate as determined through experimentation. A hole was drilled through a dummy core to its interface in which a temperature probe was inserted. The core was heated to 40°C and then placed inside the conditioning chamber at 25°C to determine how long it would take the core to reach 40°C at the center. After conditioning the sample for a mini- mum of 4 hours, the sample was ready for interface shear strength testing. 3.3 Experiment Plan I: Development of a Test Device to Evaluate the Quality of the Bond Strength of Tack Coat Spray Application in the Field The objective of Experiment I was to develop a consistent and reliable test method to evaluate the bonding characteris- tics of tack coat spray application in the field. Developing a consistent and reliable test method to evaluate the bonding characteristics of tack coat in the field was achieved in three main phases. In the first phase, a comprehensive review of current interface bond strength test devices was conducted (see Task 1). After careful evaluation of these test methods, a test method known as the ATackerâ¢, was selected for further evaluation and possible improvement (37). After several mod- ifications were introduced to the original ATacker test setup, a new pull-off test deviceâthe Louisiana Tack Coat Quality Tester (LTCQT)âwas developed. Details of the development of this test device are presented in Section 4 of this report. The efficiency of this device was evaluated in the field, and a consistent and reliable test procedure was developed. Sub- sequent to the application of the tack coat materials described in the previous section, tack coat material quality testing was conducted using the LTCQT device. The sections tested were those of high cleanliness with no water present. The materials tested for tack coat quality were SS-1h, CRS-1, and trackless. The sections for which tack coat quality was measured are pre- sented in Table 6. A minimum of three locations were tested for each section. Material Residual Application Rate (gal/yd2) SS-1h 50 % Coverage 0.031 0.155 SS-1h 100 % Coverage 0.031 0.062 0.155 Trackless 0.031 0.062 0.155 CRS-1 0.031 0.062 0.155 (a) (b) Figure 20. Description of the coring procedure. Table 6. LTCQT test sections.
28 3.4 Experiment Plan II: Rheological Properties and Superpave PG of Tack Coat Materials Performance-graded and softening point tests were per- formed on the asphalt binder residues according to ASTM D 6373, Standard Specification for Performance Graded Asphalt Binder, and ASTM D 36, Standard Test Method for Soften- ing Point of Bitumen (Ring-and-Ball Apparatus) respectively (38). All asphalt binder residues were obtained according to AASHTO D 244, Residue by Evaporation. While this study assumed the applicability of the binder-aging protocol for tack coat emulsions, validation of this assumption was neces- sary in order to understand the aging mechanism for emulsi- fied tack coats. To establish sound correlations between the rheological properties of emulsified tack coat materials and the shear strength at the interface, two tack coat materials (trackless and CRS-1) were tested using the dynamic shear rheometer at temperatures ranging from -10 to 60°C with a 10°C inter- val. This was the same temperature range used in interface shear testing. Testing was conducted using an AR2000 rheo- meter that was set up to work in the dynamic shear mode. Two sample sizes were used, depending on the testing tem- perature: a sample with a 25-mm diameter and a thickness of 1 mm was used at high temperatures (from 40 to 60°C) and a sample with an 8-mm diameter and a thickness of 2 mm was used at low and intermediate temperatures (from -0 to 30°C). 3.5 Experiment Plan III: Development of a Laboratory Test Procedure to Measure the Interface Bond Strength A direct shear device was developed in Experiment III for the characterization of interface shear strength of cylindrical specimens in the laboratory. The device is referred to as the Louisiana Interlayer Shear Strength Tester (LISST). Details of the development and evaluation of this device are presented in Section 5. The LISST device consists of two main parts: a shearing frame and a reaction frame (see Figure 21). Only the shearing frame is allowed to move while the reaction frame is stationary. A cylindrical specimen is placed inside the shearing and reaction frames and is locked in place with collars. The shearing frame is then loaded. As the vertical load is gradually increased, shear failure occurs at the interface. The LISST device was evaluated in a wide range of test conditions (see Table 4). Test specimens were obtained from pavement test sections described in the previous section. As shown in Table 4, direct shear tests were performed at 25°C under two confinement conditions, 0- and 20-psi. To assess the variation in the results, triplicate samples were tested. A number of experiments were conducted in order to evaluate the ruggedness and reliability of the LISST. Experiments were also conducted comparing the results from this device with those of the Superpave Shear Tester. 3.6 Experiment Plan IV: Effects of Test Temperature and Its Relationship with Tack Coat Rheology Experiment IV was designed to test the effects of tempera- ture, emulsified tack coat type, and residual application rate on interface shear bond strength (see Table 7). The factorial matrix consisted of 8 temperatures, 2 emulsified tack coats, and 3 residual application rates resulting in a total of 48 test conditions. Each test condition had two replicates to mini- mize variation due to experimental errors, resulting in a total of 96 interface shear tests. Test temperatures ranged from -10 to 60°C with a 10°C interval. Three residual application rates were considered: 0.031, 0.062, and 0.155 gal/yd2. The experimental program was designed to evaluate performance of tack coats between two HMA layers, a new overlay on an existing pavement. Tack coat was uniformly distributed with 100% coverage. The old HMA surface condition was dry and clean before distributing tack coat in the field. Two emulsi- fied tack coats were tested in this part of the study, a cat- ionic rapid setting (CRS-1) and a trackless tack coat, which consists of a polymer-modified emulsion with a hard base asphalt cement. The test procedure was as follows. The LISST was used to measure ISS at different temperatures. The loading system was a universal testing machine (manufactured by Cox & Sons Company). This machine had a temperature chamber that can control the test temperature from -20°C to 80°C. The maxi- mum load capacity of the actuator was 25,000 lb. Temperature conditioning and interface shear testing were conducted inside the test chamber. Figure 22 illustrates the followed test pro- cedure. As shown in Figure 22a, two replicate samples were Normal Load Actuator Horizontal Sensor Vertical Sensors Shearing Frame Reaction Frame Figure 21. General description of the LISST device.
29 Variables Contents Levels Emulsified Tack Coat Trackless, CRS-1 2 Residual Application Rate 0.031, 0.062, and 0.155 gal/yd2 3 Temperature â10, 0, 10, 20, 30, 40, 50, 60°C 8 Replicates Two replicates at each temperature 2 Total Number of Tested Specimens 96 Table 7. Test factorial to evaluate effects of test temperatures. (b) Assemblage of sample and LISST device(a) Sample conditioning (4 hours) (d) Application of shear loading(c) Stabilization of test temperature (30 min.) Figure 22. Illustrations of the test procedure for interface shear testing.
30 conditioned for at least 4 hours at the test temperature. Sam- ples were then placed in the testing chamber while attempt- ing to minimize temperature loss (Figure 22b) and were then conditioned for 30 minutes at the target temperature to com- pensate for temperature loss during specimen placement in the LISST device (Figure 22c). Finally, shear load was applied by the shear loading frame at a loading rate of 2.54 mm/sec until failure, as shown in Figure 22d. 3.7 Experiment Plan V: Effects of Pavement Surface Type and Sample Preparation Method Experiment V was designed to measure and compare the interface shear strength for different surface types and sample preparation methods. For this purpose, samples were pre- pared to simulate different field conditions and were tested using the LISST device. Table 8 presents the field test matrix. Four types of field pavement surfaces and five tack coat materials were evaluated. However, only one emulsion (SS-1h) was used on the new HMA surface and two emulsion grades (SS-1h and SS-1) were used on the milled surface. Four resid- ual application rates were selected including, zero (no tack) application rate. The effects of rainy and dusty conditions during construction operations were simulated for the differ- ent surface types as part of this experiment. Test temperature and the tack coat coverage rate were kept constant at 25°C and 100% coverage, respectively. To assess variation in the results, triplicate samples were tested for each condition; 375 samples were tested as part of the test matrix. To assess the influence of sample preparation methods, laboratory-fabricated specimens were prepared using five tack coat materialsâSS-1h, trackless, locally-used trackless (AUT), PG 64-22, and CRS-1âas tack coat was applied at four residual application ratesâ0 (No Tack), 0.031, 0.062, 0.155 gal/yd2. Field-cored specimens for tack coat applied between new and new HMA surfaces were available for SS-1h tack coat. Sample sizes and other test conditions were the same as field-cored sample testing. Laboratory-fabricated specimens consisted of two layers, with a tack coat at the interface of these layers. The diameter of each specimen was 4.0 in. The bottom half of each specimen was prepared by compacting the mixture to a height of 2.0 in at 150°C using the Superpave Gyratory Compactor (SGC). The compacted specimen was then allowed to cool to room temperature, and its air void content was measured. Compacted bottom halves having an air voids content of 6 ±1 percent were prepared. The asphalt materials used as tack coat were then heated to the specified application temperature. The calculated amount of the preheated tack coat was then uniformly applied on the bottom half of the specimen using a brush. Once application of the tack coat was complete, it was allowed to cool to room temperature and the top half of the sample was compacted by placing the bottom half in the SGC mold and compacting loose mix on top of the tack-coated bottom half. 3.8 Experiment Plan VI: Effects of Surface Texture and Permeability on Interface Shear Strength The objective of this experiment was to evaluate the effects of surface texture and permeability on tack coat interface shear strength using laboratory-prepared specimens. Three mixture types with different texture and permeability com- positions (see Table 9) were considered to use as the layer on which the tack coat was applied. Table 10 presents the mix designs adopted in the preparation of the three mix types. Variables* Content Levels Pavement surface type Old HMA, new HMA, grooved PCC, milled HMA 4 Tack coat material SS-1h, SS-1, CRS-1, Trackless, PG 64-22 5 Residual application rate 0- (No-Tack), 0.031-, 0.062-, 0.155-gal/yd2 4 Wetness (Rain) condition Wet, Dry 2 Cleanliness condition Dusty, Clean 2 Test temperature 25°C 1 Confinement pressure (psi) 0, 20 2 Tack coat coverage 50%, 100% 2 Number of replicates 3 3 Total Number of Samples 474 * Some variables were partially evaluated according to the test factorial. Table 8. Test factorial for field-prepared samples.
31 Mixtur e Type Texture Roughness Permeability Tack Coat Residual Application Rate (gsy) No. of Tested Specimens Sand Low Low SS-1 0.000 3 0.031 3 0.062 3 0.155 3 SMA High Low SS-1 0.000 3 0.031 3 0.062 3 0.155 3 Open-graded friction course (OGFC) High High SS-1 0.000 3 0.031 3 0.062 3 0.155 3 Table 9. Test matrix to evaluate effects of texture and permeability on SS-1 tack coat. Mixture Type Sand SMA OGFC Binder Type PG 70-22 PG 76-22 PG 76-22 Binder Content (%) 6.0 6.2 6.5 Air Voids (%) 13.2 3.5 21.2 Aggregate Gradation Sieve Size % Passing 37.5 mm (1½ in) 100 100 100 25 mm (1 in) 100 100 100 19 mm (¾ in) 100 100 100 12.5 mm (½ in) 100 93 95 9.5 mm ( in) 100 66 67 4.75 mm (No.4) 97 29 17 2.36 mm (No.8) 90 23 8 1.18 mm (No.16) 81 19 6 0.6 mm (No.30) 66 18 5 0.3 mm (No.50) 25 15 5 0.15 mm (No.100) 8 12 4 0.075 mm (No.200) 4 8.8 3 Table 10. Job mix formula.
32 a Flexible Wall Permeameter. All texture and permeability test results are presented in Table 11 and Table 12. 3.9 Theoretical Investigation The effects of tack coat interface shear bond characteristics, as measured by the LISST, on pavement responses at the interface were investigated using a 2-D FE approach. Six pavement struc- tures typically used in Louisiana were simulated using the com- mercial FE software, ABAQUS Version 6.9-1 (see Figure 23). Structure A consisted of a 1.5-in HMA overlay on top of a 2.0-in old HMA layer and a 4.0-in crushed stone base layer. Struc- ture B consisted of a 2.0-in HMA overlay on top of a 3.0-in old HMA layer and an 8.0-in crushed stone base course. Struc- ture C consisted of a 2.0-in HMA overlay on top of a 6.0-in old HMA layer and a 12-in crushed stone base layer. Structure D consisted of a 2.0-in HMA overlay on top of a 4.0-in old HMA layer and a 12.0-in crushed stone base course. Structure E con- sisted of a 2.0-in HMA overlay on top of a 2.0-in old HMA layer and a 12.0-in crushed stone base course. Structure F consisted of a 2.0-in HMA overlay on top of an 8.0-in old HMA layer and a 12.0-in crushed stone base course. The six structures are constructed on the same subgrade material, A-7-6 clayey soil. For the FE analyses, the tacked interface is located between the HMA overlay and the old HMA layer. Table 13 presents the assumed mechanical properties for the pavement materi- als. As shown in the table, the base and subgrade materials were assumed to respond elastically to the load. On the other hand, the HMA overlay and old HMA layer were simulated as a viscoelastic material using a Generalized Kelvin model. As part of the viscoelastic definition of asphaltic materials, the initial instantaneous moduli, presented in Table 13, were used to define the elastic component of HMA. Elastic element foundations were used to simulate the support provided to the pavement structure by the subgrade. These elements, which act as nonlinear springs to the ground, provide a simple way of including the stiffness effects of the subgrade without fixation of nodes at the bottom of the model. A dual-tire assembly applying a load of 9,000 lbf on the pavement structure over an equivalent rectangular area was simulated with a uniform pressure of 105 psi and for a total loading time of 0.1 sec. The surface interactions between the old HMA and the base layer and between the base and subgrade layers were assumed to be a friction-type contact (MohrâCoulomb theory). Limited sliding was also allowed between the aggregate layers. This formulation assumes that a slave node will interact with the same local area of the mas- ter surface throughout the analysis. The interface conditions between the HMA overlay and the old HMA layer was simulated according to the con- stitutive model adopted by Romanoschi and Metcalf (35) for asphalt pavements. In this model, the stiffness penalty These mixtures were used to fabricate the bottom layer of the specimens in the laboratory for interface shear strength test- ing. The top layer of the test specimens used the mix design adopted for preparation of the HMA overlay at the PRF site. A complete specimen consisted of two layers, top and bottom, with a tack coat placed at the interface of the two layers. Each layer was compacted to achieve a 6 ±1 percent air void. The diameter of each specimen was 6.0 in. The bottom half of each specimen was prepared by compacting the mixture to a height of 2.2 in at 165°C using the SGC. Each compacted bottom layer was allowed to cool to room temperature, then its air voids content was measured. The calculated amount of preheated SS-1 tack coat was then applied on the bottom half of the sample. The tack coat was allowed to cure. Once the application and curing of the tack coat was completed, the top half of the specimen was applied by placing the bottom half in the SGC mold and compacting the prescribed mixture on top of the tack coated bottom half. Four-inch-diameter specimens were then cored from the SGC-compacted samples, and the interface shear strength was measured at 25°C. Texture and permeability of the selected three mixtures (see Table 9) were quantitatively measured. Mixture sur- face texture measurements were performed according to ASTM E 965, Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique, which is known as the sand patch test method. Permeability tests were conducted according to ASTM PS-129-01, Measure- ment of Permeability of Bituminous Paving Mixtures using Mixture Type Sand Mix SMA OGFC Texture Category Low High High Texture (in) 0.019 0.039 0.071 COV (%) 1.1 0.4 2.5 Table 11. Texture test results for selected mixtures. Mixture Type Sand Mix SMA OGFC Permeability Category Low Low High Permeability (ft/day) 2.2 1.4 408 2.8 1.3 441 2.8 1.9 401 Average (ft/day) 2.6 1.8 417 Standard Deviation (ft/day) 0.3 1.5 21.1 COV (%) 13.1 21.0 5.1 Table 12. Permeability test results for selected mixtures.
33 Structure A Structure B Structure C Structure D Structure E Structure F Figure 23. Pavement structures simulated in the FE analysis. Material Description Constitutive Behavior Elastic Modulus (psi) Poissonâs ratio HMA Overlay Viscoelastic 650,000 0.25 Old HMA Viscoelastic 500,000 0.25 Base Elastic 40,000 0.30 Subgrade Elastic 6,000 0.35 Table 13. Mechanical properties of pavement materials in the FE analysis.
34 method is used to describe the interface conditions. The pen- alty method allows relative motion between the surfaces as long as the behavior is in the elastic region, as defined by dmax (limiting displacement in the elastic region). While the sur- faces are sticking (i.e., t < tmax), the motion between the sur- faces is elastic and recoverable. However, if the applied shear stress exceeds the interface shear strength, the interface fails and the interface condition is converted to a simple friction model, defined by a friction coefficient (µ = 0.7). Figure 24a presents the general layout of the FE model for Structure A; in total, 7,168 elements were used to simulate the pavement structure. The shear response of the top two layers is presented in Figure 24b. As shown in this figure, the axisymmetric shear response of the pavement structure to the applied tire load is demonstrated. In addition, while the maximum shear stress is located in the middle of the layer, the critical shear stress for the interface is the one calculated at the bottom of the HMA overlay. (a) (b) HMA Overlay Existing HMA Base Layer Subgrade Figure 24. General layout of the FE model (a) and shear stress distribution in the HMA overlay and the old HMA layer (b).
