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11 This chapter presents the experimental program of NCHRP Project 09-40A. The chapter includes a brief description of each pavement project, overlay construction activities, field experiment, and the laboratory characterization program. 3.1 Introduction The LISST was developed as part of NCHRP Project 09-40 (1) for characterizing the interface bond strength with the use of cylindrical specimens in the laboratory. The objective of this follow-up study (09-40A) was to evaluate the LISST method in order to support field implementation of this device by state DOTs. On the basis of the results of the recently completed project, NCHRP Project 09-40 (1), a comprehen- sive experimental plan was designed to evaluate the effects of various factors affecting interface bonding between new HMA overlay and underlying pavement layers. Review of the existing state of practice suggested that several factors influ- enced the interface bond strength between pavement layers including tack coat material type, application rate, curing period, pavement surface type, pavement surface condition, and pavement temperature. Responses from a worldwide survey indicated that the residual application rate of emul- sions typically varied from 0.02 to 0.08 gsy, depending on the type of pavement surface receiving the tack coat material. As pavement temperature increases, laboratory measured bond strength significantly decreases for all types of tack coat materials and application rates. The most common types of emulsions used in practice for tack coat application are SS and RS grades of emulsions. Most states in the United States use SS grades of emulsion for tack coat application. The experimental program investigated the influence of a num- ber of factors on the interface bond strength: HMA and PCC surface types, surface characteristics, tack coat material type, and various tack coat residual application rates from current state DOT practices to the rates recommended in NCHRP Project 09-40 (1). The effect of interface bonding on short- term pavement performance was also investigated. Findings were used to evaluate the LISST method, residual application rates, and the minimum ISS threshold criterion, as recom- mended in NCHRP Project 09-40 (1). The majority of research activities in this project were based on experiments conducted in the field, such as calibration of the tack coat distributor truck, measurement of pavement surface texture and actual tack application rate, collection of roadway cores and tack coat samples, nondestructive testing, and visual pavement distress survey. To assess the effects of the selected test variables, field experiments were conducted along with several laboratory experiments, including charac- terization of tack coat residues and measurement of ISS using the LISST device. Since all experiments were based on pavement test sections, descriptions of each project, including overlay construction activity, field experiment, and laboratory char- acterization, are presented in the following sections. 3.2 Project Identification The primary objective of the field study was to evaluate the effects of tack coat material type, pavement surface type, and application rate on the ISS and pavement performance during the first 12 months of service. To quantify the effects of the selected test variables, 10 ongoing field rehabilitation projects, including 33 in-service test sections, were selected in six states on the basis of different climatic zones and traffic conditions; see Figure 3.2-1. Further, a detailed checklist was developed and communicated to the DOT staff, material suppliers, and general contractors to ensure both uniform and efficient exe- cution of the experimental tasks in the field; see Figure A-1 in Appendix A. 3.3 Project Description The following sections present a description of each field project evaluated to identify the effects of the selected test variables in different traffic and climatic conditions. C H A P T E R 3 Experimental Program
12 3.3.1 Missouri Project The Missouri DOT (MoDOT) project was located in Cape Girardeau County and consisted of four types of pavement surfaces: milled HMA, new HMA, existing HMA, and PCC on US 61, northbound MO 177, westbound US 61, and south- bound MO 177, respectively. All these tack coat test sections were constructed at night on September 19, 2014, and were categorized as collector roads with an average annual daily traffic (AADT) of 3,000. The weather was cool and cloudy on the day of tack coat application and HMA overlay con- struction, and the air temperature was between 45Â°F and 55Â°F. For the overlay construction, two types of HMA mixturesâ PG 64-22 and PG 58-28âwere used with 9.5-mm (0.37-in.) and 12.5-mm (0.49-in.) NMASs. The HMA overlay was com- pacted to a thickness of 1.75 in., and the measured compac- tion temperature was 165Â°F. The application rates specified by MoDOT and those recommended by NCHRP Project 09-40 were the same for each surface type. A power sweeper vehicle was utilized to clean the pavement surface before tack coat application. Table 3.3-1 presents the experimental factorial. Two types of emulsified tack coatsânontracking RS NTSS-1HM and SS SS-1Hâwere used on each surface type, were applied at a residual application rate of 0.05 gallons per square yard (gsy), and thereby yielded a total of eight test sections. Each section was approximately 1,000 ft. in length and 12 ft in width. The application temperatures for NTSS-1HM and SS-1H tack coats were 160Â°F and 140Â°F, respectively. 3.3.2 Louisiana Project The Louisiana Department of Transportation and Develop- ment (LaDOTD) project consisted of two types of pavement surface: milled HMA and new HMA on LA 30 and LA 1053, respectively. A description of each project is provided. The Louisiana LA 30 project was located in Lafourche Parish and was categorized as a state highway with an AADT of 11,000. Construction of the test sections was performed at night on November 21, 2014. Construction involved 1.25-in. milling DryâNo Freeze DryâFreeze WetâFreeze WetâNo Freeze Figure 3.2-1. Field project selection based on climatic zones. Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy Milled HMA SS-1H1 0.05 NTSS-1HM2 0.05 New HMA SS-1H1 0.05 NTSS-1HM2 0.05 Existing HMA SS-1H1 0.05 NTSS-1HM2 0.05 PCC SS-1H1 0.05 NTSS-1HM2 0.05 Note: 1 gsy = 4.53 l/m2. 1SS. 2Nontracking RS tack coat material. Table 3.3-1. Experimental factorial: Missouri project.
13 of an existing asphalt layer followed by placement of a thin leveling course (less than 0.25 in.) and then a 1.5-in. HMA overlay using a 12.5-mm (0.49-in.) NMAS asphalt mixture containing PG 76-22M polymer-modified binder. It was expected that the leveling layer would have an influence on the interface bonding behavior; this is a realistic field scenario that would provide insight for such a condition. The experimental factorial is shown in Table 3.3-2. The LaDOTD specified application rate and NCHRP 09-40 recommended rate were the same for the milled HMA surface. Therefore, both types of emulsified tack coats, nontracking RS NTSS-1HM and SS SS-1, were applied in the undiluted state with a residual rate of 0.06 gsy. The LaDOTD LA 1053 project was located in Tangipahoa Parish between US 51 and the Mississippi state line and was categorized as a state highway with an AADT of 9,000. Con- struction of all test sections was performed during daytime on July 22, 2015. This project included a 1.5-in. HMA overlay on top of a 2-in. binder course. The overlay was constructed with a 12.5-mm (0.49-in.) NMAS asphalt mixture that con- tained 14.3% recycled asphalt pavement (RAP) and a PG 64-22 asphalt binder. Four types of tack coat materialsâthree nontracking RS (two NTSS-1HM and one CBC-1H) and one SS (SS-1H)âwere used on a new HMA pavement surface. Table 3.3-3 presents the experimental factorial. Each type of tack coat material was sprayed at two target residual appli- cation rates for new HMA surface, one specified by LaDOTD and one recommended by NCHRP Project 09-40 (1). Thus, a total of eight test sections were evaluated. 3.3.3 Florida Project The Florida DOT (FDOT) project was located on SR 415 in Volusia County with an AADT of 28,000. The north- bound inside lane of the two-lane roadway was assigned for this research. The existing asphalt pavement (1.5-in. binder course) was constructed in October 2013 over a 12-in. crushed limestone base with a 12.5-mm (0.49-in.) NMAS asphalt mixture. After one year in service, a 1.5-in. wearing course was constructed on March 3, 2015, with a 12.5-mm (0.49-in.) NMAS crumb rubber-modified asphalt mixture. Table 3.3-4 presents the experimental factorial. Two types of tack coat materialânontracking RS CRS-1HBC and SS SS-1Hâwere applied at two residual application rates on the basis of FDOT specifications and NCHRP Project 09-40 rec- ommendations for an existing HMA surface (1) and thereby yielded four test sections. Two types of tack coat distributor trucks were used in the spray application. The distributor truck used for the SS-1H test sections had difficulties in achieving the low application rate of 0.02 gsy because of issues asso- ciated with the nozzle type selected. Therefore, the resulting actual application rates in both SS-1H tack coat sections were similar, 0.04 gsy. A distress survey before overlay construction on the existing surface (binder course) showed no visible dis- tress, that is, rutting or cracking. 3.3.4 Tennessee Project The Tennessee DOT (TDOT) project was located on US 70S at Murfreesboro Pike near Nashville International Airport. The estimated AADT was about 36,500. The far right west- bound lane of the five-lane pavement section was selected for this study. The construction of test sections involved 1.25 in. of milling of the existing asphalt surface, followed by place- ment of a thin leveling layer (an average of less than 0.25 in.) Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy Milled HMA SS-11 0.06 NTSS-1HM2 0.06 1SS. 2Nontracking RS tack coat material. Table 3.3-2. Experimental factorial: Louisiana project (LA 30). Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy New HMA NTSS-1HM1 0.01, 0.02 CBC-1H1 0.02, 0.04 NTSS-1HM1 0.02, 0.03 SS-1H2 0.02, 0.03 1Nontracking RS. 2SS tack coat material. Table 3.3-3. Experimental factorial: Louisiana project (LA 1053). Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy Existing HMA CRS-1HBC1 0.02, 0.04 SS-1H2 0.04, 0.04 Existing HMA CRS-1HBC1 0.02, 0.04 SS-1H2 0.04, 0.04 1Nontracking RS. 2SS tack coat material. Table 3.3-4. Experimental factorial: Florida project.
14 and 1.25-in. HMA overlay. Paving was completed at night on August 23, 2015, with a 12.5-mm (0.49-in.) NMAS lime- stone mixture with PG 70-22 asphalt binder. Table 3.3-5 presents the experimental factorial. The experimental factorial considered the effect of three types of tack coat materialsâ two nontracking RS (CBC-1H and NTSS-1HM) and a SS (CSS-1H)âon a milled HMA pavement surface. The TDOT specified application rate and NCHRP Project 09-40 recom- mended application rate (1) for milled HMA pavement surface were the same. Therefore, three test sections were evaluated; see Table 3.3-5. 3.3.5 Nevada Project The Nevada DOT (NDOT) project was located on US 95, north of I-1075 near Indian Springs, with an estimated AADT of 12,000. The right lane of the two-lane roadway was selected. Construction of all tack coat test sections was performed during daytime on July 15, 2015. The paving involved 1.25 in. of milling of an existing HMA layer and then replacing it with a 3-in. 19-mm (0.75-in.) NMAS HMA overlay containing 15% RAP and PG 76-22 asphalt binder. Table 3.3-6 illus- trates the experimental factorial for this project. The experi- mental factorial considered the effect of two types of tack coat materialsânontracking RS CBC-1H and SS CSS-1Hâ on a milled HMA pavement surface. Each type of tack coat material was applied at two target residual application rates, one specified by NDOT and one recommended by NCHRP Project 09-40 (1) for a milled HMA surface. As a result, four test sections were evaluated; see Table 3.3-6. 3.3.6 Oklahoma Project The Oklahoma DOT (OkDOT) project was located on Classen BoulevardâNorthwest 10th Street in Oklahoma City with an estimated AADT of 8,000. The outer and middle lanes of the three-lane roadway were selected, and construction of all tack coat test sections was performed during daytime on April 22, 2016. This project consisted of placing a 1.5-in. 12.5-mm (0.49-in.) NMAS HMA overlay containing 12% RAP and 3% RAS on top of an existing grooved PCC pavement. Table 3.3-7 presents the experimental factorial. Two types of tack coat materials were utilized: a nontracking RS CBC-1H and an SS CSS-1H. The OkDOT specified rate and NCHRP Project 09-40 recommended rate (1) for PCC surface type were the same. Hence, each type of tack coat was applied on the two test sections at the same rate. Thus, four test sections were evaluated; see Table 3.3-7. 3.4 Tack Coat Application and Overlay Construction All field projects used computerized tack coat distributor trucks for tack coat spray application and used conventional paving equipment for overlay construction. Six types of tack coat materialsâSS (SS-1H, CSS-1H, and SS-1) and non- tracking RS (CRS-1HBC, CBC-1H, and NTSS-1HM)âwere selected for evaluation on four pavement surface types (new HMA, existing HMA, milled HMA and PCC). However, CRS-1HBC and SS-1 tack coats were used only on existing HMA and milled HMA surfaces. Each type of tack coat was initially planned to be applied at two target residual applica- tion rates on each surface type, one rate as specified by each state DOT and the other one as recommended by NCHRP Project 09-40 (1). Yet, the specified rates were occasionally the same as the ones recommended by NCHRP Project 09-40. The following tasks were performed at each test section before construction of HMA overlays. Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy Milled HMA CBC-1H1 0.06 NTSS-1HM1 0.05 CSS-1H2 0.05 1Nontracking RS. 2SS tack coat material. Table 3.3-5. Experimental factorial: Tennessee project. Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy Milled HMA CSS-1H1 0.05, 0.07 CBC-1H2 0.03, 0.04 1SS. 2Nontracking RS tack coat material. Table 3.3-6. Experimental factorial: Nevada project. Pavement Surface Type Tack Coat Material Type Residual Application Rate, gsy PCC CBC-1H1 0.03, 0.04 CSS-1H2 0.07, 0.08 1Nontracking RS. 2SS coat material. Table 3.3-7. Experimental factorial: Oklahoma project.
15 3.4.1 Distributor Truck Calibration Proper tack coat application is highly dependent on properly calibrated application equipment. Before the day of tack coat application, calibration of the computerized tack coat distrib- utor truck was performed in accordance with ASTM D2995 (Method A) (20), Standard Practice for Estimating Application Rate of Bituminous Distributors, with the use of preweighed geotextile pads (12 in. by 12 in.) in both longitudinal and trans- verse directions. Figure 3.4-1 (a) presents a typical geometric layout of the calibration pads. Multiple trials were required in the calibration process to ensure that the target application rates were achieved, given the restrictions at the construction site. During initial calibration trials, the actual application rate was found to vary in some projects as much as 40% from the target rate. Both the owner and the manufacturer of the dis- tributor truck collaborated to identify the source of the prob- lem, and corrections were made before its use in the project. Application rates were adjusted by altering the truck speed, and nozzle configuration and size. Truck operators also needed to adjust the spray bar height, depending on the amount of emul- sion in the tank. Figure 3.4-1 (b) presents the pad layout used during distributor truck calibration in the Louisiana project. Table 3.4-1 shows a summary of calibration test results for all projects. All distributor trucks used successfully met the Â± 10% variation allowed by ASTM D2995 (Method A) (20), and the average coefficient of variation (COV) within indi- vidual measurement was less than 15%, indicating reasonably uniform tack coat application. In some projects, two distribu- tor trucks were utilized to avoid the use of different emulsion charge (i.e., cationic, anionic) within the same distributor truck and to meet the construction schedule. (a) (b) Figure 3.4-1. Distributor truck calibration pads: (a) geometric layout and (b) field layout. Project Tack Coat Material Type Total Application Rate, gsy |Error|, % COV, % Target Measured Louisiana SS-1H 0.09 0.09 1 12 CRS-2 0.07 0.06 9 6 SS-1H 0.06 0.06 7 15 Missouri NTSS-1HM 0.08 0.07 9 13 Florida CRS-1 0.06 0.05 9 6 SS-1H 0.06 0.07 8 12 Nevada CSS-1H 0.08 0.08 6 13 CBC-1H 0.07 0.07 7 10 Tennessee CBC-1H 0.11 0.11 5 8 CSS-1H 0.06 0.06 7 15 Oklahoma CBC-1H 0.08 0.08 4 11 CSS-1H 0.11 0.10 8 8 Table 3.4-1. Summary of distributor truck calibrations: all projects.
16 3.4.2 Pavement Surface Texture Measurement Pavement surface macrotexture depth was measured before construction of the HMA overlays in accordance with ASTM E965, Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique, a test com- monly referred to as the sand patch test (70). Three tests were conducted within each test section. Glass beads of known volume were spread on a clean and dry pavement surface, and four measurements were then taken along the diameter of the formed circular plane (Figure 3.4-2). The mean texture depth (MTD) of pavement macrotexture in mm (in.) was calculated according to the following equation: V D = Ï MTD 4 (1) 2 where V = sample volume, mm3 (in.3), and D = average diameter of the area covered by the material, mm (in.). 3.4.3 Pavement Surface Preparation According to the Basic Asphalt Emulsion Manual (2), asphalt may not adhere to pavement surface unless the surface is com- pletely cleaned. To prevent the HMA overlay from sliding or delaminating, a dusty or dirty existing pavement surface must be cleaned before any type of tack coat material is applied to the surface (1). Further, flushing of water may be necessary when brooms are used to meet the clean air standard (2). Milling of pavement surface generates a considerable amount of dust and loose materials on the existing pavement surface [Figure 3.4-3 (a)]. To clean the existing pavement surface, a power street sweeper or a rotary power broom was used in all field projects before tack coat application to remove dust, any loose particles, or both [Figure 3.4-3 (b)]. 3.4.4 Tack Coat Application Application of tack coat materials was performed directly after calibration of the distributor truck and preparation of the existing surface. Six types of tack materialsâSS (SS-1H, CSS-1H, and SS-1) and nontracking RS (CRS-1HBC, CBC-1H, and NTSS-1HM)âwere utilized. The tack coat distributor trucks had a heated tank for holding tack coat at the desired application temperature. A spray bar was fitted with a specific type of nozzle and mounted on the back of the distributor truck to provide uniform application coverage. The total width of the spray bar was adjusted before tack coat applica- tion to provide full coverage of a single lane. All emulsified tack materials were applied in the undiluted state. Appli- cation rates were adjusted by varying the truck speed and nozzle type and size. Each test section included a distributor truck access area. The lengths of the access areas were selected to ensure that the distributor truck could attain the required speed to apply the target tack coat application rate. Distribu- tion of tack coat materials was coordinated so that the wheels of the distributor truck never came in contact with the tack coat material. As stated earlier, some projects utilized two dis- tributor trucks to avoid the use of different emulsion charge within the same distributor truck and to meet the construction schedule. Figure 3.4-4 (a) presents a typical example of tack coat application. During construction, the total application rate was measured in each field project in accordance with ASTM D2995 (Method A) (20). Figure 3.4-4 (b) presents a (a) (b) Figure 3.4-2. Pavement surface texture measurement: (a) test location and (b) sand patch test.
17 typical calibration pad layout utilized in the field. The resid- ual application rate was estimated for each type of tack coat by Equation 2. Even though the distributor trucks were cali- brated before tack coat application, in some projects the actual residual application rates in the field were different from the target rates because of equipment and construction variabil- ity. Therefore, the ISS test results were analyzed in relation to the actual residual application rates. = Ã residual application rate percent residue total application rate (2) 3.4.5 Overlay Construction After tack coat application and before placement of the HMA mixtures, sufficient time was allowed for tack coat breaking and setting. Construction of the overlay was per- formed with conventional paving equipment in each field project. Then, the overlay was compacted to the specified thick- ness with a compactor at the specified compaction temperature. Figure 3.4-5 shows selected activities during paving operation in the Oklahoma project. Moreover, each lane was marked on the basis of previously documented reference points to identify the test sections for future reference. During construction of (a) (b) Figure 3.4-3. Pavement surface preparation: (a) milling machine and (b) power sweeper. (a) (b) Figure 3.4-4. (a) Field tack coat application and (b) application rate measurement.
18 each overlay, site information was collected with the form pre- sented in Figure A-2 in Appendix A. 3.4.6 FWD Test In all field projects, nondestructive evaluation of each test section was performed with FWD testing equipment. The objective of FWD testing was to assess change in deflection due to a new HMA overlay and in-service trafficking. The FWD test uses an impulse loading mechanism in which a set of weights is dropped from various heights onto a pavement surface by either hydraulic or mechanical means. Typically, the applied maximum force is 16 kips. The applied impact force generates a deflection basin, and the deflection becomes smaller with increasing distance from the loading plate, Figure 3.4-6 (a). The vertical surface deflections in response to the impulse load are measured with a series of sensors attached to the FWD equipment (63). Figure 3.4-6 (b) shows Dynatest FWD equipment used for testing in the Oklahoma project. Four hammer drops were utilized at four load levels (6-, 9-, 13-, and 18-kips) at each test location. Vertical surface deflections were measured with nine geophones located at 0- (D0), 8- (D1), 12- (D2), 16- (D3), 20- (D4), 24- (D5), 28- (D6), 32- (D7,), and 36-in. (D8) from the center of the loading plate. In all field projects, a series of FWD tests was performed throughout the test sections at 50-ft. intervals before and after construction of overlays and at different service times. Pavement surface temperature was (a) (b) Figure 3.4-5. Overlay construction: (a) conventional paver and (b) vibratory compactor. (a) (b) Figure 3.4-6. FWD: (a) deflection basin (63) and (b) test equipment.
19 recorded during FWD testing. Because FWD tests were per- formed in different seasons, the center deflection was corrected to 25Â°C on the basis of the model developed by Park et al. (72). The following equations were utilized to determine the temperature correction factor: w C H T ToÎ» = ( )( )â â10 (3)ac C Ar Co= â + (4) where lw = temperature correction factor; Hac = AC layer thickness, in. (mm); T = pavement surface temperature, Â°C; To = 25Â°C; C = regression constant; r = radial distance from the center of the load plate, in. (mm); Co and A = constants; and Co = 4.65Eâ5. 3.4.7 Sample Collection Triplicate 5.91-in. (150-mm) diameter test specimens were collected at different service times over a period of twelve months; see Figure 3.4-7 (a). Before ISS testing, cored sam- ples were trimmed with a saw with care to avoid any distur- bance at the interface. Because water was used as trimming solution, all samples were dried for a minimum of 48 h at room temperature. The dried samples were then placed in an environmental chamber at 25Â°C for a minimum of 4 h for temperature conditioning before ISS testing. The tack coat materials were sampled from the nozzle of the distributor truck during construction, and transported to laboratory for residue characterization; see Figure 3.4-7 (b). 3.4.8 Distress Survey Before overlay construction, a manual pavement distress survey was performed on the test sections in accordance with the Distress Identification Manual for the Long-Term Pavement Performance Program (50) to assess the existing pavement conditions. The distress survey included rutting and cracking measurements. These measurements assisted in interpreting pavement performance in terms of interface bonding. Further, areas showing significant distresses were excluded from future coring. Figure B-1 in Appendix B shows predistressed areas on an existing PCC pavement in the Oklahoma project. To assess short-term pavement performance of the test sections, another manual distress survey was conducted at 12-monthsâ service of the overlay. 220.127.116.11 Cracking Measurements Each test section was approximately 1,000 ft in length and 12 ft in width. The performance of the test sections in terms of cracking (i.e., longitudinal, transverse, reflective) was mea- sured on the basis of the number of cracks observed within each test section. The crack lengths were recorded in meters, and the observed cracks were classified on the basis of their severity levels, that is, low (mean crack width â¤ 0.24 in.), moderate (mean crack width > 0.24 in. and â¤ 0.75 in.), and high (mean crack width â¥ 0.75 in.) (50). Figure 3.4-8 (a) shows a typical crack distress survey in a chart format. In the figure, the location of a core is noted in the interval between 220 and 230 ft, and a moderate severity crack is noted at 280 ft. (a) (b) Figure 3.4-7. Sample collection: (a) roadway coring and (b) tack coat sampling.
20 18.104.22.168 Rut Depth Measurements Rut depths within each test section were manually measured with either a straight edge or an A-frame, Figure 3.4-8 (b), at 50-ft intervals in both outer and inner wheel paths. The rut depths were recorded in millimeters in a distress survey chart and were classified on the basis of their severity levels, that is, low (mean rut depth â¤ 0.47 in.), moderate (mean rut depth > 0.47 in. and â¤ 0.98 in.), and high (mean rut depth â¥ 0.98 in.) (69). 3.5 Laboratory Characterization 3.5.1 Rheological Testing and Performance Grading of Tack Coat Residues Asphalt emulsion is composed of three basic ingredients: asphalt, water, and an emulsifying agent. Consistency of emul- sified asphalt was measured with the Saybolt Furol viscometer in accordance with AASHTO T 59, Standard Method of Test for Emulsified Asphalts. Residual asphalt content of the emulsified tack coats was obtained by performing the residue by evapora- tion test according to AASHTO T 59. A suite of physical and mechanical tests using this residue was conducted to deter- mine the rheological properties. Penetration and softening point tests were performed on the original residual asphalt according to AASHTO T 49, Standard Method of Test for Pene- tration of Bituminous Materials, and AASHTO T 53, Standard Method of Test for Softening Point of Bitumen (Ring-and-Ball Apparatus), respectively. The apparent viscosity of the residual asphalt cement was measured at 135Â°C, according to AASHTO T 316, Viscosity Determination of Asphalt Binder Using Rota- tional Viscometer. The PG of the residual asphalt was deter- mined according to AASHTO M 320, Standard Specification for Performance-Graded Asphalt Binder. Three replicates were conducted in each test to allow for statistical comparisons. 3.5.2 ISS Testing The ISS test was conducted in accordance with AASHTO TP 114, Determining the Interlayer Shear Strength of Asphalt Pavement Layers. The LISST direct shear test device, Fig- ure 3.5-1 (a), was employed to characterize the interface bond strength with the use of cylindrical specimens (1). This device consisted of two main parts, a stationary reaction frame, and a shearing frame, which was allowed to move freely under loading. A test sample was placed inside the frames and locked in place with collars. A monotonic shear load was applied on the shearing frame in the actuator displacement-controlled mode at a constant rate of 2.54 mm/min (0.1 in./min) until specimen failure. During testing, the time, displacement, and load data were continuously recorded. A typical test result for the interface shear load versus displacement is shown in Figure 3.5-1 (b). The shear strength imposed on the interface can be calculated as follows: = ÏISS 4 (5) ult 2 P D where ISS = in psi (kPa); Pult = ultimate load applied to the specimen, lb (N); and D = diameter of the test specimen, in. (mm). Distance, ft. L an e W id th , f t. (a) (b) Figure 3.4-8. (a) Typical distress survey in chart format and (b) rutting measurement.
21 Traffic Direction (a) (b) 0 400 800 1200 1600 2000 0.0 0.1 0.2 0.3 0.4 In te rf ac e Sh ea r L oa d, lb . Pult Displacement, in. Figure 3.5-1. ISS test: (a) LISST device and (b) typical testing curve of the interface shear load versus displacement.