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35 This section presents the main findings of NCHRP Project 9-40. It includes the main findings of the worldwide survey, the results of the experimental program described in the pre- vious chapter, and the theoretical investigation that was used to relate laboratory-measured interface bond characteristics to the field stresses in the pavement structure when subjected to vehicular loading. 4.1 Findings of the Worldwide Survey A total of 72 responses were identified as having met the cri- teria for inclusion in this study. Where more than one response was received from the same state, the data were combined, and only one respondent was counted in the analysis. Most of the survey results were presented in terms of percentage of respondents. Two questions on the questionnaire consid- ered the weight of each option (i.e., percentage of use for each option/the importance index of each option). Accordingly, two methods were employed to analyze the data: 1. Data were presented by means of summing the products of the percentage of use and the number of responses, Percentage of use number of responsesÃâ 2. Weighted average was used to show the overall importance of each option, Importance index number of responses Number of r Ãâ esponsesâ 4.1.1 Types and Grades of Commonly Used Tack Coat Materials Figure 25 shows that 100% of the responding agencies indi- cated that asphalt emulsions are permitted by their agency. The percentage of respondents that use asphalt cement and cutback asphalts are 27% and 20%, respectively. Figure 26 lists the asphalt cements or cutbacks used by the different agencies. Sixty percent of the 15 agencies indicated their use of PG 64-22 asphalt cement. Eleven agencies reported the use of eight cutback asphalts for tack coat. MC-70 had the high- est rate of use with 55% of those respondents using cutback asphalts as tack coat. The most commonly used emulsions were slow-setting SS-1 (41%), SS-1h (39%), CSS-1 (37%), and CSS-1h (41%) (see Figure 27). The asphalt content of the emulsions gener- ally ranged between 50% and 65%. A few extremely diluted emulsions were used by some respondents. The residual rate reported for the cutback asphalt materials ranged from 50% to 87%. The survey questions focused on tack coat materials used in recent construction projects and the percentage of their use. Figure 28 was developed by multiplying the percentage of use of tack coat materials by the number of responses in order to evaluate the recent usage. Emulsified asphalts were, by far, the most commonly used tack coat material followed by asphalt cement and then cutback asphalts. SS-1, CSS- 1h, SS-1h, and CSS-1 ranked as the most used emulsified asphalts (see Figure 29). PG 64-22 was the most commonly used asphalt cement. RC-70 ranked as the most commonly used cutback asphalt (Figure 29). 4.1.2 Types of Tack Coat Applied to Different Pavement Surfaces Most of the respondents indicated that their agencies monitor the application of tack coats and specify ranges for dilution rates as well as application rates. Of the respondents, 4% indicated that the dilution rate is determined by the con- tractor, 2% stated that they do not monitor the application rates, and 2% stated that the application rates are monitored visually. Figure 30 shows the most common materials used as tack between new HMA layers. Tack coat materials used on old HMA surfaces and milled HMA surfaces are listed in S e c t i o n 4 Findings
Figure 25. Tack coat material types. 0 20 40 60 80 100 Asphalt Cement Cutback Asphalt Emulsion Tack Coat Material Type R es po nd en ts, % Figure 26. Asphalt cements and cutbacks used as tack coats. 0 20 40 60 80 100 Pe n 85 -1 00 A C- 5 PG 52 -2 2 PG 58 -1 6 PG 5 8- 22 PG 58 -2 8 PG 64 -1 6 PG 6 4- 22 PG 64 -2 8 PG 6 7- 22 PG 70 -1 0 PG 70 -2 2 PG 70 -2 8 PG 7 6- 22 PG 8 2- 22 R C 30 R C 70 R C 25 0 R C 80 0 M C- 3 0 M C- 7 0 M C 25 0 M C- 80 0 R es po nd en ts, % Asphalt Cements Cutback asphalts Figure 27. Emulsions used as tack coats. 0 5 10 15 20 25 30 35 40 45 R S- 1 R S- 1h R S- 2 H FM S- 1 H FM S- 2 H FM S- 2h M S- 2 M S- 1 SS -1 SS -1 h SS -1 P SS -1 L CR S- 1 CR S- 1h CR S- 2 CR S- 2h CR S- 2P CR S- 3 CM S- 2 CS S- 1 CS S- 1h ST -1 P TS T- 1P EA P& T H FE -9 0 ST E- 1 O D O T7 02 .1 3 CQ S- 1H CQ S- 1H P Tack Coat Material Type R es po nd en ts, % Figure 28. Weighted use for the material used as tack coat. 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Asphalt Cutback Emulsified Asphalt Tack Coat Material Type U sa ge % X N o. o f R es po ns es
Figures 31 and 32, respectively. Of the agencies, 4% indicated that they do not require tack coats between new HMA lay- ers, while 2% indicated that no tack is required on old HMA surfaces. For tack coats applied between new, old, and milled HMA layers, the commonly used tack coat materials were CSS-1H (32%â34%), SS-1 (30%â32%), SS-1h (29%â32%), and CSS-1 (21%â27%). PG 64-22 was the most used asphalt cement with an average of 11%, and RC-70 was the most commonly used cutback asphalt with a usage range of 5% to 7%. The residual application rates for most of the emulsions were within the range of 0.03 to 0.05 gal/yd2. Asphalt cement application rates ranged from 0.04 to 0.10 gal/yd2. The range of residual rates for cutback asphalts was 0.03 to 0.05 gal/yd2. Only 27% of the respondents gave feedback for tack coat materials used on top of surface treatments or seal coats, as well as asphalt-treated base courses (see Figures 33 and 34, respectively). These two surface conditions yielded similar results as the first three surfaces, with CSS-1h, CSS-1, SS-1h, and SS-1 being the most used tack coat materials. Figures 35 and 36 list the materials used on PCC pave- ments or diamond-ground PCC pavements, respectively. Again, SS-1, SS-1h, CSS-1, and CSS-1h were the most used materials with the high-float emulsions ranking highest among the emulsions. 4.1.3 Findings Related to Tack Coat Application Methods The Dilution Process Location Several agencies allow dilution at multiple locations: 49% reported that the dilution process occurs while the material is in the supplierâs tank (see Figure 37). Another 45% allow 37 Figure 29. Commonly used tack coat materials. 0 200 400 600 800 1000 1200 1400 PG 58 -2 8 PG 5 8- 22 PG 64 -1 6 PG PG 70 -1 0 A C- 5 R C 30 R C 70 M C- 30 M C- 70 M C 25 0 M C- 80 0 R S- 1 R S- 1h R S- 2 H FM S- 1 H FM S- 2h M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CR S- 2h CR S- 3 CM S- 2 CS S- 1 CS S- 1h ST -1 P EA P TS T- 1P ST E- 1 O D O T7 0 CQ S- 1H CQ S- 1H P H FE -9 0 Tack Coat Material Type U sa ge % x N o. o f R es po ns e s Asphalt Cement Cutback Asphalt Emulsified Asphalt Figure 30. Tack coat materials placed between new HMA layers. 0 5 10 15 20 25 30 35 40 R S- 1 R S- 1h R S- 2 H FM S- 1 H FM S- 2h M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CM S- 2 CS S- 1 CS S- 1h ST - 1P TS T- 1P ST E- 1 H FE -9 0 EA P PG 7 0- 10 PG 5 8- 22 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 PG 5 8- 16 A C- 5 R C 30 R C 70 R C 25 0 M C- 70 M C- 25 0 M C- 80 0 % o f R es po nd en ts
Figure 31. Tack coat materials placed on old HMA. 0 5 10 15 20 25 30 35 R S- 1 R S- 1h R S- 2 H FM S- 1 H FM S- 2h M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CM S- 2 CS S- 1 CS S- 1h ST -1 P TS T- 1P ST E- 1 H FE -9 0 EA P PG 7 0- 10 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 PG 5 8- 16 A C- 5 R C 30 R C 70 R C 25 0 M C- 70 M C- 25 0 M C- 80 0 R es po nd en ts, % Figure 32. Tack coat materials placed on milled HMA surfaces. 0 5 10 15 20 25 30 35 40 R S- 1 R S- 1h R S- 2 H FM S- 1 H FM S- 2h M S- 1 SS -1 SS - 1h CR S- 1 CR S- 2 CM S- 2 CS S- 1 CS S- 1h TS T- 1P ST E- 1 H FE -9 0 EA P PG 7 0- 10 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 PG 5 8- 16 A C- 5 R C 30 R C 70 R C 25 0 M C- 70 M C- 25 0 M C- 80 0 % o f R es po nd en ts Figure 33. Tack coat materials used on surface treatments, seal coats, or chip seals. 0 5 10 15 20 25 30 R S- 1 H FM S- 1 M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CR S- 2P CM S- 2 CS S- 1 CS S- 1h ST -1 P H FE -9 0 H F- 15 0S EA P PG 7 0- 10 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 PG 5 8- 16 R C 30 R C 70 R C 25 0 M C- 70 M C- 25 0 M C- 80 0 % o f R es po nd en ts
Figure 34. Tack coat materials used on asphalt-treated base. 0 5 10 15 20 25 30 R S- 1 R S- 2 M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CM S- 2 CS S- 1 CS S- 1h TS T- 1P ST E- 1 H FE -9 0 EA P PG 7 0- 10 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 PG 5 8- 16 R C 70 R C 25 0 M C- 70 M C- 25 0 % o f R es po nd en ts Figure 35. Tack coat materials used on PCC surfaces. 0 5 10 15 20 25 30 35 R S- 1 R S- 1h R S- 2 H FM S- 1 H FM S- 2h M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CM S- 2 CS S- 1 CS S- 1h TS T- 1P H FE -9 0 70 2. 13 EA P PG 7 0- 10 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 PG 5 8- 16 A C- 5 R C 70 R C 25 0 M C- 25 0 M C- 80 0 Type % o f R es po nd en ts Figure 36. Tack coats used on milled or diamond-ground PCC. 0 5 10 15 20 25 30 R S- 1 RS - 1h R S- 2 H FM S- 2h M S- 1 SS -1 SS -1 h CR S- 1 CR S- 2 CM S- 2 CS S- 1 CS S- 1h TS T- 1P ST -1 P H FE -9 0 EA P PG 7 0- 10 PG 6 4- 16 PG 6 4- 22 PG 5 8- 22 A C- 5 RC 7 0 R C 25 0 M C- 25 0 M C- 80 0 Type % o f R es po nd en ts
40 dilution to occur in the distributor tank. Only 15% of the respondents do not allow dilution of emulsified asphalt for tack. Verification of Asphalt Emulsion Dilution Rate Half of the respondents stated that emulsion was sampled from the distributor and tested for verification. Another 39% of the respondents required certification from the supplier. Only 29% allowed the certification to be performed by the contractor (see Figure 38). Frequency of Verification of Dilution Rate Of the respondents, 36% verify the dilution rate of an asphalt emulsion (see Figure 39); 26% indicated that the dilution rate is not checked; another 26% indicated criteria different from those queried in the questionnaire. Some of these different criteria included the following: verify the dilu- tion rate for every delivery unit, leave it up to the contractor, verify every 2 weeks, verify every 43,000 ft2, and periodically test. Only 10% verify the dilution rate daily. Traffic on Tacked Surfaces The majority of respondentsâ78%âstated that highway traffic is not allowed on tack coat materials prior to HMA placement. Of the respondents who do allow traffic on the tack materials, most stated that the tack coats should be cured first. Some reported a time of 1 to 2 hours before traffic is allowed onto the tacked pavement. All of the respondents who allowed traffic prior to placement of HMA indicated that surface type did not affect the time required before traffic was allowed. Of the respondents, 47% allow highway traffic for a max- imum of 24 hours before placing the covering HMA layer; 18% do not allow highway traffic prior to the placement of the subsequent HMA layer; and 6% allow 5 days of trafficking before the tack coat must be covered (see Figure 40). Tack Coat Application Equipment By far, most agencies (98%) indicated that an asphalt distributor with spray bar was the most common specified application method (see Figure 41); 42% allow an asphalt Figure 37. Potential sites for dilution of emulsified asphalt. 0 10 20 30 40 50 60 Supplier's Terminal Contractor's Storage Tank In the Distributor Tank Other None Allowed Responses % o f R es po nd en ts Figure 38. Verification of the dilution process. 0 10 20 30 40 50 60 Asphalt Supplier Certificate Contractor Certificate Tested from the Contractor's Storage Tank Tested from the Distributor Visual Observation ASTM D2995 None Other Responses % o f R es po nd en ts
41 Figure 39. Frequency of verification of dilution. 0 5 10 15 20 25 30 35 40 Daily Monthly Project by Project Not Checked Other Responses % o f R es po nd en ts Figure 40. Time that tack coat can be exposed to traffic before covering with HMA. 0 5 10 15 20 25 30 35 40 45 50 0 4 12 24 120 Hours R es po n de nt s, % Figure 41. Method for applying tack coat materials. 0 20 40 60 80 100 Hand Wand Distributor Spray Bar Paver Spray Bar Other Responses % o f R es po nd en ts
42 distributor with hand wand, and 4% require a spray bar attached to the paving machine. However, the average per- centage of use for the asphalt distributor with spray bar was 97% compared with 6% asphalt distributor with hand wand. Half of the agencies that use a spray bar attached to a paver use it 100% of the time, while the remaining half used it for 1% of their mainline paving areas. Not all agencies provided a percentage of use with their survey response. Breaking/Setting of Emulsified Asphalts Of the respondents, 26% permit haul trucks to drive on unbroken emulsion. The majority of respondents, 70%, allow haul trucks on an unset emulsion after it breaks. Out of 53 responses, 74% of the responding agencies allow paving to begin immediately after the tack coat material breaks, whereas 26% do not allow paving until the emul- sion sets. In ranking the factors that affect the break and set times for an emulsified asphalt, respondents indicated that ambi- ent temperature and pavement temperature were the most important factors. Other factors that were reported were road surface condition, solar effect, and emulsion temperature. Application rate, dilution rate, wind velocity, and humidity were considered essentially equivalent in level of importance. The break/set factors are listed below in order of importance, from highest to lowest: 1. Ambient temperature, 2. Pavement surface temperature, 3. Dilution rate, 4. Application rate, 5. Humidity, 6. Wind velocity, and 7. Others. Pickup of Tack Material by Truck Tires Of the respondents, 67% indicated that pickup of tack coat material is a continuing problem; 38% indicated that the tack material is required to be completely set before haul trucks are allowed on it. Few respondents, 13%, allow haul trucks to drive on the tack coat material before breaking (see Figure 42). Other methods specified to reduce pickup include the fol- lowing: tack coat is required to break before haul trucks are allowed, reduce the application rate, clean the surface before applying tack coat, and minimize the distance that haul trucks are allowed to drive on the tack coat. Percentage of Tack Coat Coverage Tack coat coverage is defined herein as the percentage of the pavement surface area coated by asphalt tack. Most agen- cies, 64%, responded that the coverage area is typically above 90%. The percentages of responses are as follows: 1. 100% coverage (37%), 2. 90%â100% coverage (27%), 3. 70%â90% coverage (18%), 4. 50%â70% coverage (9%), and 5. Less than 50% coverage (9%). A majority of the agencies, 73%, indicated that no specific requirement was used to regulate the application of tack coat material; 25% reported that the amount of spray overlap between adjacent nozzles on the distributor spray bar is a specified requirement. Out of the 13 agencies that reported a requirement for overlap, 46% use a double-overlap configu- ration, while 23% use single- and triple-lap. The remaining 8% did not mention which degree of overlap was used. That the angle of the nozzles to the axis of the spray bar is a speci- Figure 42. Methods used to prevent pickup of tack coat by haul trucks. 0 10 20 30 40 50 60 70 80 90 100 Completely Set Sanded Before Break Ongoing Problem Other Responses % o f R es po nd en ts
43 fied requirement was reported by 12% of respondents. Of those who indicated the angle of the nozzles as a requirement, the average minimum angle was 23Â° and the average maxi- mum angle was 32Â°. That the height of the spray bar above the pavement surface is a requirement was reported by 10% of respondents. Figure 43 presents the percent of responses for each requirement. Environmental Restrictions In discussing the environmental restrictions placed on the application of the tack coat material, almost half of the respondents, 43%, reported a minimum ambient tempera- ture. The average minimum ambient temperature was 6Â°C. Less than 2% reported a maximum allowable ambient tem- perature of 65Â°C; 38% reported that a minimum pavement surface temperature was a restriction. The average minimum pavement surface temperature was 3Â°C. No agency reported a maximum pavement for surface temperature as a restriction. Impending rainfall was an environmental restriction for 55% of respondents. More than 75% of the respondents reported that a wet pavement surface was a restriction, whereas 38% indicated that a damp pavement surface was a restriction. Approximately the same number reported that time of year (i.e., paving season) was a restriction. The percentages of responses are given in Figure 44. Some additional common restrictions for application of tack coat were as follows: 1. Surfaces must be free of standing water or contamination, 2. Manufacturerâs recommendations, 3. Do not apply tack coat unless HMA will be immediately placed, and 4. Cannot apply tack coat materials in foggy conditions Application Rates and Residual Tack Coat Rate Verification Of the responses, 51% indicated that measuring the change in the amount of material in the distributor tank after apply- ing a given section was the best way to check the application rates (see Figure 45). Less than 2% of the agencies reported that ASTM D 2995 (19) is used. The differences in the weight of the asphalt distributor over a given area were about 27%. Some of the common methods specified by the respondents, but not queried in the questionnaire, are as follows: meter on the distributor, visually, and dipstick reading before and after an application on a pavement segment. Figure 43. Specified requirements for tack coat application. Figure 44. Environmental restrictions on tack application. 0 20 40 60 80 100 Min. Air Temp. Max. Air Temp. Min Surface Temp. Max. Surface Temp. Impending Weather Wet Surface Damp Surface Time of Year Other Responses % o f R es po nd en ts
44 Uniformity of the Applied Tack Coat Most of the respondents, 66%, indicated that the require- ment to have the entire surface covered with tack coat material was the main specification to check for uniformity (see Fig- ure 46). The second most-used requirement was to ensure that no nozzles are completely or partially blocked, 34%. The remaining options ranged from 13% to 26%. More than half of the respondents (56%) reported that they do not change their application rate due to any factor. Almost all of the remaining 44% of respondents who change their application rate changed them based on the condition of the pavement surface. The remaining of the conditions ranged between 0% and 10% (see Figure 47). Remedy for Non-uniform Tack Coat Application Out of the responses compiled, 70% require the contractor to reapply the tack coat material. Of those responses, 70% require a lower application rate for the reapplication. The remaining respondents who require reapplication of the tack coat material either applied the same rate, or they did not specify which approach was taken. Two percent asked the contractor not to do it on the next pass (and no other action to fix non-uniformity is taken); 17% do nothing. The results are illustrated in Figure 48. 4.1.4 Findings Related to Tack Coat Application Pavement Failures Related to Improper Tack Rate/Material The respondents reported slippage and delamination of the pavement surface layer as approximately equal to results from poor tack coat type or application, 89% and 87%, respec- tively. Fatigue cracking was the only other type of failure that received over 25% of the responses. Other types of failures included shoving, bottom up stripping due to water intrusion, and flushing/bleeding due to excessive tacking (see Figure 49). Lab/Field Test Methods to Determine the Interface Bond Strength The vast majority of the respondents, 92%, indicated that no testing is performed to measure the bond strength between pavement layers. Eight percent of the agencies indicated that testing is performed on the pavement interface. The traction Figure 45. Methods to verify tack application rate. 0 20 40 60 80 100 ASTM D2995 Weight Diff. Volume Diff. Not Checked Other Responses % o f R es po nd en ts Figure 46. Methods for assurance of tack coverage. 0 20 40 60 80 100 Pavement Covered Percentage Covered No Blocked Nozzles Proper Nozzle Angle Proper Height Not Checked Responses % o f R es po nd en ts
45 Figure 48. Steps to correct poor application of tack coat. 0 20 40 60 80 100 Remove Reapply Price Deduction Ask Contractor not to Repeat Require Improved Next Pass Nothing Responses % o f R es po nd en ts Figure 47. Reason for tack coat rate change. 0 20 40 60 80 Time of Year Roadway Type Roadway Condition Ambient Temp. Day vs. Night Traffic Use Subsequent Overlay Thickness % o f Y es R es po nd en ts Responses test, Texas pull-off test, and Florida shear test are some of the laboratory and field test methods being used to quantify interface bond strength between pavement layers. Quality of Tack Coat Materials Only 18% of the responses indicated they use a field or laboratory test to evaluate tack coat material quality. Some of the procedures listed for testing the quality of tack coat materials are residual percentage test; traction test; penetra- tion test on the residual asphalt; AASHTO M 208, Cationic Emulsified Asphalt (39); and oil distillate test. Current Research Related to Performance of Tack Materials Of the respondents, 37% reported that their state or coun- try is conducting or has recently conducted research on tack coat performance. 4.2 Experiment I: Development of a Test Device to Evaluate the Quality of the Bond Strength of Tack Coat Spray Application in the Field The Louisiana Transportation Research Center (LTRC) and InstroTek, Inc., manufacturer of the ATackerâ¢, part- nered to develop the Louisiana Tack Coat Quality Tester (LTCQT), which was developed in this project to evaluate the Figure 49. Failures attributed to improper tack application or type. 0 20 40 60 80 100 Slippage Delamination Fatigue Cracking Top-down Cracking Rutting Other Responses % o f R es po nd en ts
46 quality of the bond strength of tack coat in the field. LTCQT is a modification of the ATacker. The following sections describe details of the development process and evaluation of the LTCQT. 4.2.1 First Generation of LTCQT Figure 50 presents the first generation of the LTCQT that was used to measure the quality of tack coat applications in the field. The modifications included automated operation of the device and installation of electronic sensors for the mea- surement of load and deformation. Subsequent to the initial evaluation of this version of the ATacker, it was determined that additional fine-tuning items needed to be incorporated, such as fixing the flap plates to hold the device firmly in place during testing, increasing the travel distance of the actuator, and additional modifications to the software to make it more user-friendly. Distinctive features of the first generation of LTCQT included: â¢ Automated operation by installation of electronic sen- sors for load and displacement measurements. This led to improved reliability and repeatability of the measurements and minimized operator error. â¢ Incorporation of user-friendly software. 4.2.2 Second Generation of LTCQT Figure 51 shows the second generation of the LTCQT. Several modifications were introduced to the first genera- tion of LTCQT to improve the reliability of the results. The modifications introduced in this version addressed several issues observed in the first generation (details of these modifications are discussed in the following sections): â¢ Improved sensitivity/reliability of the load cell sensor, â¢ Improved sensitivity/reliability of the actuator rate of loading, and â¢ Improved adhesion of the LTCQT test plate to tacked surface. Improved Sensitivity/Reliability of the Load Cell Sensor Several experiments using the first generation of LTCQT were conducted to examine the sensitivity and reliability of the load cell sensor. During this evaluation, it was observed that the load cell had a high noise level (approximately 10% of the load cell capacity), which exceeded the specification value set by ASTM E 74, Standard Practice of Calibration of Force-Measuring Instruments for Verifying the Force Indica- tion of Testing Machines (40). Therefore, a new load cell with a maximum capacity of Â±100 lbs and a signal conditioner were installed. The aforementioned changes yielded a stable load cell signal that met ASTM E 74 standards. In addition, the LTCQT acquisition software was updated to match the new device. Improved Sensitivity/Reliability of the Actuator Rate of Loading Loading rate of the actuator was examined. Several experi- ments were conducted to verify the rate of loading using two tack coat materials with contrasting bond strengths: CRS-1 and PG 64-22. Results from these experiments showed that the device could not maintain the specified displacement rate during testing. Results of this evaluation are presented in Appendix B. The displacement rate changed depending on the strength of the material; therefore, a new actuator and driving motor (closed loop, servo-controlled) with improved control of the displacement rate were installed. It is noted Figure 50. First version of the LTCQT. Electric Sensor for Load and Displacement Flat Plate Incorporated Software
47 that the displacement of the actuator was measured using a position transducer that has a total travel of 3.94 in. The max- imum loading rate was 0.30 in/sec. Experiments were then conducted with the improved device to verify the loading rate. It was observed that the âsetâ and âmeasuredâ displacement rates of loading were in good agreement in these experiments, indicating that the second generation of LTCQT can provide a consistent and reliable displacement rate of loading. Improved Adhesion of Test Plate to Tacked Surface Most of the laboratory research that was performed to evaluate tack coat quality using the ATacker test device was performed with the tack coat applied between two metal plates. During the LTCQT tack coat field evaluation tests, poor adhesion (i.e., not measurable) was observed between the metal plate and the tacked pavement surface. Several types of flexible materials (to better conform to a textured surface) that attach to the metal plate were evaluated. Rub- ber, insulation foam, sill foam, and polyethylene foam are among the materials evaluated. Rubber and the insulation foam showed poor adhesion to the pavement surface; how- ever, polyethylene foam yielded good adhesion. Therefore, polyethylene foam was used to ensure adequate adhesion. The foam can be easily attached onto the metal plate with double-sided tape. Figure 51. Second generation of LTCQT. LTCQT Load Cell and Signal Conditioner Weights Contact Plate: 5.71 in Diameter (a) LTCQT Tester (b) Software Incorporated in LTCQT
48 4.2.3 Development of Tack Coat Test Procedure Using LTCQT A procedure for evaluation of tack coat quality in the field was developed based on the second generation of the LTCQT test device. Loading rate, time required for breaking of emul- sified tack coat, contact pressure, and contact time between contact plate and tacked surface were examined. Based on the results of this evaluation, a test procedure was written in AASHTO format. Loading Rate Since the loading rate significantly affects the test results, it is essential to select an appropriate rate that can distinguish between the tensile strength of different tack coat materials. Experiments for determining appropriate loading rate were conducted in the laboratory. The tack coat materials used were SS-1h, CRS-1, trackless, and PG 64-22. Tack coat ten- sile strength was measured in the laboratory using LTCQT at 50Â°C and at two loading rates (i.e., 004 and 0.008 in/sec). Based on the applied loading rates, it was found that LTCQT is able to differentiate between different tack coat materials in terms of the measured tensile strengths. Since this trend was consistent at both displacement rates, and to ensure prompt evaluation of tack coats in the field, a 0.008-in/sec loading rate was selected for the test procedure. Evaluation of Cure Time and Accelerating Devices The LTCQT was developed to evaluate the quality of the bond strength of tack coat in tension in the field. For emul- sions or cutbacks, tack coat quality must be evaluated based on the residual material (i.e., material remaining after the emulsion/cutback has cured) and not the total emulsion. Thus, the set or cure time (i.e., the time required for water to evaporate) for tack coat materials needs to be determined prior to the LTCQT testing. This was achieved by continu- ously measuring the weight of a tacked specimen until a constant weight was obtained. Three emulsion types were evaluated, namely, CRS-1, SS-1h, and trackless. Each one of these emulsions was applied to the surface of a HMA specimen with dimensions of 5.9 in in diameter and 2.2 in in height. The weight of the tacked specimen was measured to 1/100th of a gram at several time intervals subsequent to the application of the emulsion on the specimen. It was observed that complete curing of the emulsions was achieved after approximately 12 hours. This time period needed to be shortened in order to permit same-day measurements in field tack coat construction. Three devices were evalu- ated in order to accelerate emulsion curing time: a portable fan/heater, a heat gun, and an infrared reflective heating (IRH) lamp. The IRH source device used in the first, second, and fourth test setup was positioned 2.95 in above the surface of the sam- ple (see Figure 52). SS-1h emulsion was applied to the surface of the sample specimen at 43.3Â°C with a residual application rate of 0.05 gal/yd2. To avoid evaporation of light oil compo- nents during the heating process, the surface temperature of the specimen was not allowed to exceed 135Â°C for any device. Results of these experiments are presented in Figure 52b. It was noted that the target residual asphalt weight (i.e., the weight after approximately 12 hours of evaporation at room tempera- ture) was achieved after approximately 1 hour for each of the four test setups considered. Based on these results, the IRH lamp was selected for use in accelerating water evaporation time and was subsequently adopted in the field experiments. The IRH device provided the most uniform heat distribution on the sample among the four devices evaluated. Furthermore, this device was comparatively simple to setup and use. Contact Time and Pressure A contact pressure, compressive preload, is applied to the contact plate for a preset period of time as a part of the LTCQT. A contact pressure of 1.57 psi for 3 minutes was found to be adequate to provide uniform adhesion between the tacked surface and the loading plate of the LTCQT. Figure 52. Determination of heat source for accelerating water evaporation in emulsions. 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 0 10 20 30 40 50 60 70 80 Time (min) W ei gh t ( g) IRH Lamp IRH Lamp + Fan Heat Gun IRH Lamp + Heater (a) Infrared reflective heating (IRH) lamp. (b) Breaking time of tack coat SS-1h with different heating conditions.
49 Summary of Test Parameters A summary of the test parameters is shown in Table 14. Field test results presented in the following sections were evaluated based on these test parameters. 4.2.4 LTCQT Test Procedure The existing pavement surface at the LTRC PRF facility was thoroughly cleaned (see Figure 53a). An area of 6 in Ã 6 in was used for each test (see Figure 53b). The tack coat material was then applied with a paint brush at the prescribed residual application rate and application temperature (see Figure 53c). Subsequent to the application of the tack coat material, the IRH device was positioned above the test area (for emulsion only) for one hour to accelerate the curing time (see Figure 53d). Surface temperature was allowed to cool to the testing tem- perature, and then the cured surface was ready to test for tack coat quality using the LTCQT. The LTCQT was positioned on the surface (see Figure 53g). A compressive preload of 1.57 psi was applied to the surface via the LTCQT foot, load- ing plate, which has the polyethylene foam for 3 minutes. Then, a tensile force was applied at a displacement rate of 0.008 in/sec until failure. The tensile force was continuously recorded. The ultimate load (PULT) was measured, and the tensile strength (SULT) was computed and used in the analysis. Four tack coat materialsâtrackless, CRS-1, SS-1h, and PG 64-22âwere tested in the field. A minimum of three replicate tests were performed for each condition. A test procedure for assessing tack coat installation quality in the field using the LTCQT device is presented in Appendix C. 4.2.5 Effect of Tack Coat Temperature on the Ultimate Tensile Strength Testing temperature plays a vital role in the response of the tack coat material as measured by the LTCQT. A series of LTCQT tests were conducted in the field at intervals of Table 14. Test parameters in the test method. Category Level Loading Rate 0.5 in/min Contact Time 3 minutes Contact Pressure 1.5 psi Tack Coat Evaporation Time 1 hour Tack Coat Evaporation Device Infrared reflected heating (IRH) source (a) Surface cleaning (c) Tack coat application (b) Drawing test area (d) Water evaporation (curing) for 1 hour Figure 53. Main steps in the LTCQT test procedure.
50 (e) Foam material attachment on loading plate (f) Foam material attachment on loading plate II (g) Placement of LTCQT (h) Pull-off testing and data recording Figure 53. (Continued). Figure 54. Variation of the mean tensile strength with temperature. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 30 to 40 Â°C 40 to 50 Â°C 50 to 60 Â°C 60 to 70 Â°C 70 to 80 Â°C M ea n Te ns ile S tre ng th (p si) Temperature (Â°C) PG 64-22 Trackless SS-1h CRS-1 approximately 10Â°C ranging from 30Â° to 90Â°C on the afore- mentioned four tack coat materials at a residual application rate of 0.05 gal/yd2. Three replicates were tested for each tack coat material. Figure 54 presents the variation of the ultimate mean tensile strength (i.e., SULT, average of three replicates) of the tack coat materials considered in this experiment along with the test temperatures. The tempera- tures presented in these graphs are the ones measured at the end of the test. It is believed that these temperatures are the closest ones at the point where the tensile strengths were measured. In general, the variation in temperature between the start and end of each test was controlled to within 5Â°C. Tensile strength of each tack coat material increased, reached a peak, and then decreased as the temperature increased (see Figure 54); however, the tensile behavior of each tack coat material was different between the asphalt cement and emul- sions in the post-peak region. PG 64-22 exhibited a rapid soft- ening with increasing temperature, whereas the emulsions had
51 Figure 55. Rheological test results of tack coat materials. 1. E+ 00 1. E+ 01 1. E+ 02 1. E+ 03 1. E+ 04 1. E+ 05 1. E+ 06 Tr ackl es s S S-1h PG 64-22 CRS-1 A bs ol ut e V i sc o sit y at 6 0 ÂºC (p ois es) a 0 10 20 30 40 50 60 70 80 Tr ack le ss SS-1h PG 64-22 CRS-1 So fte ni ng P oi nt (Âº C) a 0 20 40 60 80 100 120 Tr ackl ess SS-1 h P G 64-22 CRS- 1 Pe ne tra tio n at 2 5 ÂºC (10 -1 m m ) 0. 0 0. 5 1. 0 1. 5 2. 0 2. 5 3. 0 Tr ackl es s SS-1h PG (b) Absolute viscosity test (d) Softening point test (c) Rotational viscosity test (a) Penetration test 64-22 CRS- 1 R ot at io na l V isc os ity a t 1 35 ÂºC (P a-s ) Table 15. Maximum tensile strength and optimum temperature. Trackless SS-1h PG 64-22 CRS-1 Maximum Tensile Strength (psi) 1.84 2.51 4.34 1.84 Optimum Temperature (Â°C) 60 52 43 42 a lower drop in the tensile strength from the peak value as the temperature increased. Furthermore, trackless emulsion maintained its tensile strength in the post peak region with the increase in temperature. Results shown in Figure 54 indi- cate that each tack coat material exhibits its maximum ten- sile strength at a distinct temperature. This temperature was referred to as the optimum temperature, TOPT. At a tempera- ture higher or lower than TOPT, the tensile strength normally decreased. To determine the peak tensile strength (SMAX) and the optimum temperature (TOPT), polynomial regression lines were fitted for each tack coat. The peak strength from the trend lines was then set to SMAX, and the temperature cor- responding to SMAX was set to TOPT. Trackless material had the highest optimum temperature of 60Â°C. SS-1h, CRS-1, and PG 64-22 had a TOPT of 54, 43, 42Â°C, respectively. PG 64-22 material showed the highest maximum tensile strength of 4.3 psi. Table 15 summarizes the measured TOPT and SMAX for the four tack coat materials evaluated. 4.3 Experiment II: Rheological Properties of Tack Coat Materials and Its Relationship to Bond Strength Four consistency tests were conducted on PG 64-22 binder and the residuals of SS-1h, CRS-1, and trackless emulsions (see Figure 55). The residual asphalts were obtained accord- ing to ASTM D 244, Residue by Evaporation. Trackless, SS-1h, and CRS-1 are emulsified asphalts with residual percentages of 55.3%, 63.0%, and 58.2%, respectively. On the other hand, PG 64-22 has 100% residual. The tests performed were pen- etration, absolute viscosity, rotational viscosity, and soften- ing point. Two replicates of each test were conducted. As shown in Figure 55a, trackless material was the hardest fol- lowed by SS-1h, PG 64-22, and CRS-1. Ranking of viscosity of the materials from this test was consistent with the results of the penetration test (see Figure 55b). In addition, trackless
52 residues exhibited the highest rotational viscosity, whereas CRS-1 residues had the lowest rotational viscosity (see Fig- ure 55c). Furthermore, the ranking of the softening point test results was similar to the ranking of the results of the penetra- tion test, absolute viscosity test, and rotational viscosity test (see Figure 55d). The ranking of the materials from hardest to softest was trackless residual, the SS-1h residual, PG 64-22 binder, and the CRS-1 residual. 4.3.1 Superpave Grading of Emulsified Tack Coats Emulsified tack coats are composed of three basic ingre- dients: asphalt, water, and emulsifying agent. The asphalt binder residues were obtained according to AASHTO D 244, Residue by Evaporation. Table 16 presents the results of tests performed on these residues. It is noted that the residues of CRS-1 and SS-1h emulsions were graded as PG 58-22 and 70-22, respectively. The trackless material, how- ever, failed the intermediate- and low-temperature perfor- mance criteria. This response was expected since trackless is a polymer-modified emulsion with a hard base asphalt cement. To establish sound correlations between the rheologi- cal properties of emulsified tack coat materials and ISS, trackless and CRS-1 were tested using the dynamic shear rheo meter at temperatures ranging from -10Â° to 60Â°C with a 10Â°C interval. Testing was conducted using an AR2000 rheo meter in the dynamic shear mode. Two sample sizes were used, depending on the testing temperature: a sample with a 25-mm diameter and 1-mm thickness was used at high temperatures (from 40Â° to 60Â°C), and 8-mm diam- eter and 2-mm thickness was used at low and intermediate temperatures (from -10Â° to 30Â°C). Figure 56 presents the dynamic shear rheometer (DSR) test results for both tack coat materials. As shown in this figure, the complex shear modulus (G*) increased linearly for both tack coat materi- als on a semi-logarithmic scale. As expected, the trackless materials produced higher G* values than did CRS-1. 4.3.2 Relationship Between LTCQT Test Results and Tack Coat Rheological Properties The LTCQT test was performed on four tack coat materi- als: trackless, CRS-1, SS-1h, and PG 64-22. Based on these Table 16. Rheological test results of emulsified tack coat residues. Aging Status Test Property AASHTO Method Spec. PG 64-22 SS-1h residual SS-1 residual CRS-1 residual Trackless residual Original Binder Rotational viscosity, Pa.s 135Â°C T 316 3.0â 0.5 0.6 0.3 0.3 2.5 Dynamic shear 10 rad/s G*/sin , kPa T 315 1.0+ 1.86 (64Â°C) 15.4 (52Â°C) 2.5 (52Â°C) 3.0 (52Â°C) 19.0 (64Â°C) 6.5 (58Â°C) 1.3 (58Â°C) 1.3 (58Â°C) 7.6 (70Â°C) 2.9 (64Â°C) 0.8 (64Â°C) 0.6 (64Â°C) 3.4 (76Â°C) 1.4 (70Â°C) 1.5 (82Â°C) 0.7 (76Â°C) 0.7 (88Â°C) Softening Point 53Â°C 42.5Â°C 76Â°C Rolling Thin- Film Oven Residue Mass change, % T 240 1.0â 0.009 0.1 0.1 0.1 NA Dynamic shear 10 rad/s, G*/sin , kPa T 315 2.20+ 4.4 (64Â°C) 2.8 (70Â°C) 2.2 (58Â°C) 2.9 (58Â°C) 16.9 (70Â°C) 7.4 (76Â°C) 3.4 (82Â°C) 1.5 (88Â°C) Pressure Aging Vessel Residue 100Â°C Dynamic shear, 10 rad/s , G*sin , kPa T 315 5000â 3,177 (25Â°C) 3,239 (25Â°C) 2,411 (19Â°C) 3,306 (19Â°C) 10,907 (25Â°C) Bending Beam Creep stiffness, S, MPa 60s T 313 300â 210 (â12Â°C) 165.0 ( â12Â°C) 84.5 (â12Â°C) 86.8 (â12Â°C) * 174 (â18Â°C) 187.0 (â18Â°C) Bending Beam Creep stiffness, m-value 60s T 313 0.300+ 0.285(â12Â°C) 0.320 (â12Â°C) 0.42 (â12Â°C) 0.340 (â12Â°C) * 0.34 (â18Â°C) 0.310 (â18Â°C) Direct tension 1.0 mm/min, % T 314 1.0+ 1.2 (â12Â°C) 1.6 (â12Â°C) 1.1 (â18Â°C) 1.1 (â18Â°C) * PG Grading PG 64-22 PG 70-22 PG 58-28 PG 58-28 â *Sample was brittle and failed.
53 measurements, the relationship between tack coat bonding characteristics and the rheology of the material was estab- lished. Figure 57b shows the relationship between the tensile strength and the corresponding absolute viscosity, both at 60Â°C, for each tack coat material (i.e., residual from emul- sion). As expected, the increase in viscosity (i.e., resistance to flow) is associated with an increase in tensile strength. Figure 57a presents the relationship between the optimum temperature (TOPT), at which SMAX occurs, and the corre- sponding softening point for each tack coat material. At the softening point, an applied tack coat is in a rheological state that provides sufficient adhesion to the LTCQT loading plate for tensile testing. As the temperature is increased, tack coat consistency is not sufficient to provide full adhesion in the LTCQT loading plate. Based on these results, it is recom- mended to conduct the LTCQT test at the tack coat material softening point, which is a property that is readily available and can be easily specified. 4.3.3 Measurements of Tack Coat Bond Strength at the Softening Point Additional LTCQT tests were conducted in the field to eval- uate the repeatability of the ultimate tensile load, PULT, of the four tack coat materials (CRS-1, SS-1h, trackless, PG 64-22) at the softening point. For each tack coat material, at least three LTCQT tests were performed. Table 17 presents the mea- sured tensile strength at the softening point for the four tack coat materials. Test temperature was controlled within Â±5Â°C from the material softening point. Test results show that PG 64-22 and CRS-1 had the highest and lowest tensile strengths (or ultimate tensile loads), respectively. Tensile strengths for both SS-1h and trackless were similar, and they were ranked between those of PG 64-22 and CRS-1. Figure 58 presents the ultimate tensile loads for the four tack coat materials. The ranking of tensile strength is in good agreement with those presented in Table 17; therefore, it may be concluded that conducting the tack coat pull-off test at the softening point can successfully and consistently evaluate the quality of tack coat application in the field. Following the recommended testing procedure, the LTCQT has shown acceptable repeat- ability for all of the tested tack coat materials. For all four tack materials tested, the repeatability of the results was reasonable with an average coefficient of variation less than 11%. 4.4 Experiment III: Development of a Laboratory Test Procedure to Measure the Interface Bond Strength A direct shear device was developed for the character- ization of ISS of cylindrical specimens (see Figure 59). The device, which was developed through an iterative process, is Figure 56. Relationship between complex modulus (G*) for unaged residues of trackless and CRS-1 and temperature. y = 430032 e -0 . 159 x RÂ² = 0.99 y = 30875e -0 .1 68 x RÂ² = 0.99 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 -10 0 10 20 30 40 50 60 70 C om pl ex M od ul us , G * (k Pa ) Temperature (ÂºC) Trackless CRS-1 Figure 57. Relationship between absolute viscosity and softening point and the optimum test temperature. (a) (b) CRS-1 PG 64-22 SS-1h Trackless TOPT = 0.55 x (Softening Point) + 19.21 R2 = 0.89 30 40 50 60 70 80 30 40 50 60 70 80 Softening Point (ÂºC) Te m pe ra tu re (T O PT ) a t S M A X (ÂºC ) CRS-1 PG 64-22 SS-1h Trackless R2 = 0.46 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 0 5 10 15 20 Tensile Strength at 60 ÂºC (kPa) A bs ol ut e V isc os ity at 6 0 ÂºC (p ois ses ) a
54 referred to as the Louisiana Interlayer Shear Strength Tester (LISST). It consists of two main partsâa shearing frame and a reaction frame. Only the shearing frame is allowed to move while the reaction frame is stationary. A cylindrical speci- men is placed inside the shearing and reaction frames and is locked in place with collars. Loading is then applied to the shearing frame. As the vertical load is gradually increased, shear failure occurs at the interface. The LISST device was designed such that it will fit into any universal testing machine. It has a nearly frictionless linear bearing to maintain vertical travel and can accommodate sensors that measure vertical and horizontal displacements. The device provides a specimen-locking adjustment, applies a constant normal load up to 100 psi, and accommodates a specimen with 4-in or 6-in diameters. The gap between the shearing and the reaction frame is 0.5 in. A wide range of experiments was conducted in order to evaluate the ruggedness and reliability of the LISST. Experiments were conducted comparing the results from this device with those of the Superpave Shear Tester (SST). ISSs of the LISST and SST were similar when dilation was allowed; however, those results were significantly different when dilation was not allowed or was limited in the SST device. Details of these experiments are described in Appendix D. Three shear displacement rates of loading were evalu- ated (i.e., 2 in/min, 0.1 in/min, and 0.02 in/min). Based on these evaluations, a rate of loading of 0.1 in/min was recommended in the testing procedure to simulate the slow rate of loading encountered at the interface in the field. A test procedure for measuring interface bond strength in the laboratory using the LISST device, written in AASHTO format, is presented in Appendix E. Figure 60 presents a typical test result of shear stress versus displacement curve. The ISS is computed as follows: ISS = P AULT ( )1 where, ISS = interface shear strength (ksi); PULT = ultimate load applied to specimen (lb); and A = cross-sectional area of test specimen (in2). Table 17. Tensile strength at softening point for four tack coat materials. Material Type Softening Point (Â°C) Ultimate Tensile Load PULT, (lb) Tensile Strength SULT, (psi) Mean (PULT/SULT) Standard Deviation (PULT/SULT) COV (%) CRS-1 42.5 30.9 1.6 37.7 / 1.9 4.9 / 0.2 13.0/12.4 43.8 2.2 35.5 1.8 40.4 2.1 38.1 1.9 PG 64-22 48.5 66.9 3.4 62.8 / 3.2 6.0 / 0.3 9.6/10.2 65.6 3.3 55.9 2.8 SS-1h 53.0 40.3 2.0 44.7 / 2.3 3.9 / 0.2 8.7/9.1 49.8 2.5 44.4 2.3 44.3 2.3 Trackless 76.0 44.7 2.3 44.5 / 2.3 5.4 / 0.3 12.0/11.149.8 2.5 39.1 2.0 Figure 58. Ultimate tensile load (PULT) for tack coat materials at the softening point. 0 10 20 30 40 50 60 70 80 CRS-1 SS-1h Trackless PG 64-22 U lti m at e Te ns ile L oa d (P U LT , lb )
55 Ï Î¾= k ( )2 where, t = interlayer shear stress (ksi); x = interlayer displacement within the interface (in); and k = interlayer tangential modulus (lb/ft3). The k-modulus is computed by dividing the peak stress by the displacement at failure from the stress versus displacement curve (see Figure 60). 4.4.1 Effects of Tack Coat Characteristics on Interface Shear Strength Tables 18 and 19 present the mean ISS test results along with their standard deviations and coefficient of variations for SS-1h, CRS-1, and trackless tack coats, respectively. Triplicate specimens were tested for each test condition defined by tack coat type, residual application rate, confin- ing pressure, and dusty and wet conditions. All tests were performed at a temperature of 25Â°C. In general, the COVs in the test results were less than 10%. As shown in the following sections, results were analyzed to investigate the effects of variables considered in the test factorial on the ISS. Effect of Emulsified Tack Coat Types and Residual Application Rates Tables 20 and 21 present the statistical analyses of the effects of application rates and tack coat types on ISS based on a two-tailed t-test at a 95% confidence level. As shown in these Figure 59. General description of the Louisiana Interlayer Shear Strength Tester. 0 10 20 30 40 50 60 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 In te rfa ce S he ar S tre ss (p si) Displacement (in) Peak stress k-modulus (lb/ft3) Figure 60. Typical interface shear stress versus displacement for trackless at 0.06 gal/yd2.
56 Table 18. ISS of SS-1h emulsified tack coat. Confinement Pressure (psi) Tack Coat SS-1h Residual Appl. Rate (gal/yd2) 0.031 0.062 0.155 0 Surface Condition D1 D W2 W D D W W D D W W H3 L4 H L H L H L H L H L ISS (psi) 11.1 13.5 13.6 12.7 14.6 7.5 16.3 12.7 36.8 32.9 44.0 34.0 9.7 12.9 13.1 13.2 12.0 8.0 18.1 11.7 41.7 40.1 37.7 34.5 12.9 14.5 15.5 15.0 13.2 7.8 16.8 13.6 40.3 34.9 37.8 34.8 Mean 11.2 13.6 14.0 13.6 13.3 7.8 17.1 12.7 39.6 36.0 39.8 34.4 S.D. 1.6 0.8 1.3 1.2 1.3 0.3 0.9 0.9 2.5 3.7 3.7 0.4 COV 14.2 5.9 9.0 8.8 9.8 3.7 5.4 7.3 6.4 10.3 9.2 1.2 20 ISS (psi) 16.5 20.9 13.7 24.5 18.6 10.4 15.2 15.5 40.4 51.9 38.5 47.7 15.7 20.2 16.1 22.4 14.9 11.6 17.8 16.6 43.6 44.0 38.5 45.4 18.8 26.1 15.9 19.3 17.6 12.9 19.1 16.3 40.7 43.9 41.2 43.4 Mean 17.0 22.4 15.2 22.1 17.0 11.6 17.4 16.2 41.6 46.6 39.4 45.5 S.D. 1.6 3.2 1.4 2.6 1.9 1.2 1.9 0.6 1.8 4.6 1.6 2.1 COV 9.5 14.3 9.0 11.7 11.3 10.6 11.2 3.4 4.2 9.8 4.0 4.7 1 Dry Condition, 2 Wet Condition, 3Clean Condition, 4 Dusty Condition. Confinement Pressure (psi) Tack Coat CRS-1 Trackless Residual Appl. Rate (gal/yd2) 0.031 0.062 0.155 0.031 0.062 0.155 0 Surface Condition D1 D D D D D D D D D D D H2 L3 H L H L H L H L H L ISS (psi) 6.9 10.3 12.3 10.3 12.6 24.6 13.3 20.1 28.4 51.3 58.0 60.2 6.6 10.0 11.4 13.1 15.6 23.9 16.0 22.4 24.6 49.7 61.0 60.5 8.5 7.8 13.3 12.2 14.3 24.1 13.9 23.6 22.2 61.2 68.1 65.1 Mean 7.3 9.4 12.4 11.8 14.2 24.2 14.4 22.0 25.1 54.1 62.4 62.0 S.D. 1.0 1.3 0.9 1.4 1.5 0.4 1.5 1.8 3.1 6.3 5.2 2.7 COV 14.1 14.2 7.6 11.9 10.9 1.5 10.2 8.1 12.3 11.6 8.3 4.4 20 ISS (psi) 10.2 16.9 14.0 18.8 12.9 34.2 18.0 34.3 35.2 75.2 65.2 79.6 13.3 13.6 11.5 20.3 15.6 33.4 22.6 26.4 34.0 76.9 55.8 74.9 11.0 15.4 11.7 17.5 16.8 34.2 22.2 31.9 38.6 69.7 70.2 75.3 Mean 11.5 15.3 12.4 18.9 15.1 33.9 20.9 30.8 35.9 73.9 63.7 76.6 S.D. 1.6 1.6 1.4 1.4 2.0 0.5 2.5 4.0 2.4 3.8 7.3 2.6 COV 14.3 10.5 11.2 7.3 13.1 1.4 12.0 13.1 6.7 5.1 11.5 3.4 1 Dry Condition, 2 Clean Condition, 3 Dusty Condition. Table 19. ISS of CRS-1 and trackless emulsified tack coat.
57 tables, all cases except one indicated that tack coat types and application rates had significant effects on the measured ISS. Figure 61a presents the variation of ISS with emulsified tack coat types and residual application rates. The results were obtained from clean and dry specimens with no confinement at 25Â°C. For each residual application rate, the trackless tack coat exhibited the highest shear strength and CRS-1 exhibited the lowest. Trackless and SS-1h yielded similar and higher ISSs than CRS-1 at the low residual application rateâthat is, 0.031 gal/yd2. All tack coat materials showed the highest strength at a residual application rate of 0.155 gal/yd2. Shear strength of SS-1h and trackless consistently increased as residual appli- cation rate increased. In contrast, measured shear strength for CRS-1 appeared to stabilize at a residual application rate around 0.062 gal/yd2. Similar trends were noted at a confine- ment pressure of 20 psi. For the residual application rates tested, it was not possible to determine the optimum residual application rate. This may be attributed to the highly oxidized HMA surface at the PRF site, which required greater tack coat rates than expected. It may also indicate that, under actual field conditions, optimum residual application rates may be greater than that commonly predicted from laboratory-based experiments. While higher residual application rates may increase ISS, excessive tack coat may migrate into the HMA mat during compaction, causing a decrease in the air void content of the mix. Figure 61b presents the variation of the measured air voids of the overlaid mixture for each residual application rate. As shown in this figure, the increase in residual tack coat application rate was associated with a decrease in air Tack Coat Statistical Test Condition Confinement P-value Results SS-1h Application Rates Clean-Dry Unconfined < 0.0001 Significant Application Rates Clean-Dry Confined < 0.0001 Significant Application Rates Dusty-Dry Unconfined < 0.0001 Significant Application Rates Dusty-Dry Confined < 0.0001 Significant Application Rates Clean-Wet Unconfined < 0.0001 Significant Application Rates Clean-Wet Confined < 0.0001 Significant Application Rates Dusty-Wet Unconfined < 0.0001 Significant Application Rates Dusty-Wet Confined < 0.0001 Significant CRS-1 Application Rates Clean-Dry Unconfined 0.0010 Significant Application Rates Clean-Dry Confined 0.0893 Not Significant Application Rates Dusty-Dry Unconfined < 0.0001 Significant Application Rates Dusty-Dry Confined < 0.0001 Significant Trackless Application Rates Clean-Dry Unconfined < 0.0001 Significant Application Rates Clean-Dry Confined < 0.0001 Significant Application Rates Dusty-Dry Unconfined < 0.0001 Significant Application Rates Dusty-Dry Confined < 0.0001 Significant Table 20. Statistical analysis of the effects of application rates on ISS. Rate Statistical Test Condition Confinement P-value Results 0.031 Tack Coat Type Clean-Dry Unconfined 0.0022 Significant Tack Coat Type Clean-Dry Confined 0.0032 Significant Tack Coat Type Dusty-Dry Unconfined < 0.0001 Significant Tack Coat Type Dusty-Dry Confined 0.0027 Significant 0.062 Tack Coat Type Clean-Dry Unconfined 0.0004 Significant Tack Coat Type Clean-Dry Confined < 0.0001 Significant Tack Coat Type Dusty-Dry Unconfined < 0.0001 Significant Tack Coat Type Dusty-Dry Confined < 0.0001 Significant 0.155 Tack Coat Type Clean-Dry Unconfined < 0.0001 Significant Tack Coat Type Clean-Dry Confined < 0.0001 Significant Tack Coat Type Dusty-Dry Unconfined < 0.0001 Significant Tack Coat Type Dusty-Dry Confined < 0.0001 Significant Table 21. Statistical analysis of the effects of tack coat types on ISS.
58 voids. This may result in negative effects on the overlay perfor- mance, such as the appearance of âfat spotsâ on the pavement surface, which may affect the friction properties of the mat. Several attempts were made to core the no-tack coat test area; however, these specimens failed at the interface during the coring process. It is noted that the best tack coat performerâ trackless at the highest residual application rateâprovided 60% of the monolithic (no interface) mixture shear strength at 25Â°C, which was estimated at 105 psi. The worst tack coat performerâCRS-1âprovided only 15% of the mixture shear strength at the highest residual application rate. This suggests that the construction of flexible pavements in mul- tiple layers introduces weak zones at these interfaces. Figure 62 presents the relationships between rotational viscosity, G*/sin d, and the softening point of the tack base asphalt with ISS at a residual application rate of 0.155 gal/ yd2. In general, good correlations were observed between these rheological properties and the ISS values. The mea- sured interface strength increased as the tack coat viscosity and its resistance to deformation at high temperatures (G*/ sin d) increased. Similar trends were observed at the low and intermediate residual application rates. 4.4.2 Effect of Confining Pressure Table 22 presents the statistical analysis of the effects of con- finement on ISS based on t-tests. As shown in this table, the majority of the cases (17 out of the 24 cases) indicated that confinement has a significant effect on the measured ISS. Fig- ure 63 shows the ratio of ISS between the 0 and 20 psi confine- ment test conditions. The ratio of ISS between these two test conditions increased as the residual application rate decreased. As the residual application rate decreased, increasing the con- fining pressure resulted in a more pronounced contribution of the effect of roughness and aggregate resistance to sliding at the interface; however, at higher residual application rates (i.e., greater lubrication), the effect of aggregate roughness and resistance to sliding was less crucial since most of the ISS was (a) (b) 0 10 20 30 40 50 60 70 0 0.05 0.1 0.15 0.2 In te rf a ce S he ar St re n gt h (p si) Residual Application Rate (gal/yd2) Residual Application Rate (gal/yd2) SS-1h CRS-1 Trackless 0 1 2 3 4 5 6 7 8 9 10 0.031 0.062 0.155 A ir V oi ds SS-1h CRS-1 Trackless Figure 61. Variation of ISS with residual application rates (a) and variation of air voids with residual application rates: clean and dry condition, no confinement, 25Â°C (b).
59 R 2 = 0. 95 0. 0 0. 5 1. 0 1. 5 2. 0 2. 5 3. 0 0 100 200 300 400 500 Interface Shear St reng th at 25 Â°C (kP a) R ot at io na l V isc os ity a t 1 35 Â°C A (P a-s ) a R 2 = 0. 96 0 5 10 15 20 25 0 100 200 300 400 500 Interface Shear St reng (a) Rotational Viscosity (b) Dynamic Shear Rheometer th at 25 Â°C (k Pa ) D y n a m ic S he ar (G */s in ) a t 6 4Â° C (kP a) a R 2 = 0. 96 0 20 40 60 80 100 0 100 200 300 400 500 Interface Shear Streng (c) Softening Point th at 25 Â°C (k Pa ) So fte ni ng P oi nt (Â° C) Figure 62. Relationship between ISS with 0.155 gal/yd2 and rheology test results. Tack Rate Statistical Test Condition P-value Results SS-1h 0.031 Unconfined vs. Confined Clean-Dry 0.0110 Significant Unconfined vs. Confined Dusty-Dry 0.0440 Significant Unconfined vs. Confined Clean-Wet 0.3330 Not Significant Unconfined vs. Confined Dusty-Wet 0.0150 Significant 0.062 Unconfined vs. Confined Clean-Dry 0.0480 Significant Unconfined vs. Confined Dusty-Dry 0.0344 Significant Unconfined vs. Confined Clean-Wet 0.8279 Not Significant Unconfined vs. Confined Dusty-Wet 0.0123 Significant 0.155 Unconfined vs. Confined Clean-Dry 0.3309 Not Significant Unconfined vs. Confined Dusty-Dry 0.0356 Significant Unconfined vs. Confined Clean-Wet 0.8608 Not Significant Unconfined vs. Confined Dusty-Wet 0.0128 Significant CRS-1 0.031 Unconfined vs. Confined Clean-Dry 0.0323 Significant Unconfined vs. Confined Dusty-Dry 0.0087 Significant 0.062 Unconfined vs. Confined Clean-Dry 0.9486 Not Significant Unconfined vs. Confined Dusty-Dry 0.0037 Significant 0.155 Unconfined vs. Confined Clean-Dry 0.5532 Not Significant Unconfined vs. Confined Dusty-Dry < 0.0001 Significant Trackless 0.031 Unconfined vs. Confined Clean-Dry 0.0303 Significant Unconfined vs. Confined Dusty-Dry 0.0407 Significant 0.062 Unconfined vs. Confined Clean-Dry 0.0087 Significant Unconfined vs. Confined Dusty-Dry 0.0179 Significant 0.155 Unconfined vs. Confined Clean-Dry 0.8048 Not Significant Unconfined vs. Confined Dusty-Dry 0.0026 Significant Table 22. Statistical analysis of the effects of confinement on ISS.
60 derived from the tack coat material. The effect of confinement was more pronounced under dusty and dry conditions. 4.4.3 Effect of Dusty Conditions of HMA Surface Table 23 presents the statistical analysis of the effects of dusty conditions on ISS based on a two-tailed t-test at a 95% confidence level. As shown in this table, results were mixed, with 13 out of the 24 cases indicating that dusty conditions had a significant effect on the measured ISS. Figure 64 pre- sents the effects of dust on the ISS values at no confinement and confinement (20 psi) test conditions. As shown in Fig- ure 64, the majority of the cases showed differences between clean and dusty conditions. In general, dusty conditions exhib- ited higher interface strength than clean conditions, especially when tested with a confinement condition. One possible explanation for these results is that a high-viscosity, gritty mastic was formed when tack coat combined with dust and, thus, provided a greater resistance to shear movement. (a) Clean and Dry Conditions (b) Dusty and Dry Conditions 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0.031 0.062 0.155 IS Sc on fin ed /IS Sn o- co nf in ed Residual Application Rate (gal/yd2) CRS-1 SS-1h Trackless 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0.031 0.062 0.155 IS Sc on fin ed /IS S n o- co nf in ed Residual Application Rate (gal/yd2) CRS-1 SS-1h Trackless Figure 63. Ratio of ISS with confinement to no confinement. Tack Statistical Test Rate Condition Confinement P-value Results SS-1h Clean vs. Dusty 0.031 Dry Unconfined 0.1036 Not Significant Clean vs. Dusty 0.031 Dry Confined 0.0806 Not Significant Clean vs. Dusty 0.031 Wet Unconfined 0.6903 Not Significant Clean vs. Dusty 0.031 Wet Confined 0.0274 Significant Clean vs. Dusty 0.062 Dry Unconfined 0.0188 Significant Clean vs. Dusty 0.062 Dry Confined 0.0264 Significant Clean vs. Dusty 0.062 Wet Unconfined 0.0046 Significant Clean vs. Dusty 0.062 Wet Confined 0.4097 Not Significant Clean vs. Dusty 0.155 Dry Unconfined 0.2339 Not Significant Clean vs. Dusty 0.155 Dry Confined 0.1744 Not Significant Clean vs. Dusty 0.155 Wet Unconfined 0.1234 Not Significant Clean vs. Dusty 0.155 Wet Confined 0.0165 Significant CRS-1 Clean vs. Dusty 0.031 Dry Unconfined 0.1078 Not Significant Clean vs. Dusty 0.031 Dry Confined 0.0462 Significant Clean vs. Dusty 0.062 Dry Unconfined 0.6699 Not Significant Clean vs. Dusty 0.062 Dry Confined 0.0048 Significant Clean vs. Dusty 0.155 Dry Unconfined 0.0078 Significant Clean vs. Dusty 0.155 Dry Confined 0.0039 Significant Trackless Clean vs. Dusty 0.031 Dry Unconfined 0.0044 Significant Clean vs. Dusty 0.031 Dry Confined 0.0369 Significant Clean vs. Dusty 0.062 Dry Unconfined 0.0055 Significant Clean vs. Dusty 0.062 Dry Confined 0.0007 Significant Clean vs. Dusty 0.155 Dry Unconfined 0.9063 Not Significant Clean vs. Dusty 0.155 Dry Confined 0.0640 Not Significant Table 23. Statistical analysis of the effects of dusty conditions on ISS.
61 100% coverage. As shown in this figure, using 50% coverage significantly reduced the ISS by a factor ranging from 50% to 70%. Table 25 presents the LTCQT test results for 50% cover- age. The tensile strength test results were highly variable. This may be due to the partial coverage of the tacked surfaces. For actual pavements, this suggests inconsistent interface bond- ing behavior for tacked surfaces with incomplete or non- uniform coverage. 4.5 Experiment IV: Effects of Test Temperature and Its Relationship with Tack Coat Rheology 4.5.1 Interface Bond Strength at Various Temperatures Table 26 presents the ISS test results for trackless and CRS-1 specimens. Each value represents the average of two test specimens. At temperatures over 50Â°C, some specimens collapsed before shearing due to their own weights. This mostly occurred at the low residual application rate and for the CRS-1 emulsion. As shown in Table 26, the trackless material 4.4.4 Effect of Wet (Rainfall) Conditions of Tacked Surface Table 24 presents the statistical analysis of the effects of wet conditions on ISS based on a two-tailed t-test at a 95% confidence level. As shown in this table, the majority of the cases indicated that wet conditions had no significant effect on the measured ISS. Figure 65 presents the effects of water (i.e., light rainfall) of a tacked surface at no confinement and with confinement (20 psi) on ISS. It is noted that the major- ity of the cases showed no significant differences between dry and wet conditions. 4.4.5 Effects of Tack Coat Coverage As previously discussed, 50% tack coat coverage was only investigated for SS1-h. For the application of SS-1h, the residual application rate of 0.031 gal/yd2 was achieved with a high level of error (see Table 5); therefore, this residual appli- cation rate was not considered in the analysis. On the other hand, the residual application rates of 0.062 and 0.155 gal/ yd2 at 50% coverage were comparable with the ones at 100% coverage. Figure 66 compares the measured ISS for 50 and (a) No-Confinement Condition (b) Confinement Condition 0 10 20 30 40 50 60 70 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 In te rfa ce S he ar S tre ng th (p si) Residual Application Rate (gal/yd2) Clean/Dry Dirty/Dry SS-1h Trackless CRS-1 0 10 20 30 40 50 60 70 80 90 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 In te rfa ce S he ar S tre ng th (p si) Residual Application Rate (gal/yd2) clean/dry dusty dry SS-1h Trackless CRS-1 Figure 64. Dust effect on ISS with (a) no-confinement and (b) confinement.
62 Figure 65. Effect of wetness on ISS for SS-1h tack coat with (a) clean and (b) dusty conditions. (a) Clean Condition (b) Dusty Condition 0 5 10 15 20 25 30 35 40 45 50 0.031 0.062 0.155 0.031 0.062 0.155 In te rfa ce S he ar S tre ng th (p si) Residual Application Rate (gal/yd2) Dry/Clean Wet/Clean No-confinement Confinement 0 10 20 30 40 50 60 0.031 0.062 0.155 0.031 0.062 0.155 In te rfa ce S he ar S tre ng th (p si) Residual Application Rate (gal/yd2) Dry/Dirty Wet/Dirty No-confinement Confinement Tack Statistical Test Rate Condition Confinement P-value Results SS-1h Dry vs. Wet 0.031 Clean Unconfined 0.0743 Not Significant Dry vs. Wet 0.031 Clean Confined 0.2168 Not Significant Dry vs. Wet 0.031 Dusty Unconfined 1.0000 Not Significant Dry vs. Wet 0.031 Dusty Confined 0.8961 Not Significant Dry vs. Wet 0.062 Clean Unconfined 0.0147 Significant Dry vs. Wet 0.062 Dusty Confined 0.8444 Not Significant Dry vs. Wet 0.062 Clean Unconfined 0.0132 Significant Dry vs. Wet 0.062 Dusty Confined 0.0108 Significant Dry vs. Wet 0.155 Clean Unconfined 0.9313 Not Significant Dry vs. Wet 0.155 Dusty Confined 0.1865 Not Significant Dry vs. Wet 0.155 Clean Unconfined 0.5511 Not Significant Dry vs. Wet 0.155 Dusty Confined 0.7320 Not Significant Table 24. Statistical analysis of the effects of wet conditions on ISS.
63 0 10 20 30 40 50 60 70 80 90 0.062 0.155 In te rfa ce S he ar S tre ng th (p si) Residual Application Rate (gsy) 100% 50% Figure 66. Effect of tack coat coverage on ISS. Tack Coat Material 1 Residual Application Rate Tes t Temperature (Â°C) Maximum Tensile Load (lb) Maximum Tensile Strength (psi ) Average (PULT/SULT) Standard Deviation (PULT/SULT) COV (%) SS-1h 50% 0.031 51.0 12.3 0.63 8.6/0.44 3.48/0.18 40.4 55.0 5.4 0.28 57.0 8.1 0.41 0.155 52.0 14.3 0.73 12.3/0.62 2.78/0.14 23.0 51.0 9.1 0.46 53.0 13.4 0.68 1 All tack coats were tested at 53Â°C. Table 25. LTCQT test results with 50% coverage surface. Mean ISS (psi) Temperature (ÂºC) Trackless CRS-1 0.031 gal/yd2 0.062 gal/yd2 0.155 gal/yd 2 0.031 gal/yd2 0.062 gal/yd2 0.155 gal/yd2 â10 132.0 255.7 370.4 147.6 196.8 331.7 0 127.1 263.1 401.9 111.7 171.3 216.3 10 88.5 194.1 322.7 85.4 91.1 120.4 20 39.7 101.8 167.5 46.0 46.3 45.5 30 21.9 45.8 75.3 11.0 16.8 21.5 40 3.8 17.8 34.1 2.9 1.9 3.2 50 * 4.4 14.2 * * 2.8 60 * 5.4 18.0 * * * *Specimens collapsed under their own weights before shear loading. Table 26. ISS at various test temperatures.
64 (a) 0.031 gal/yd2 (b) 0.062 gal/yd2 (c) 0.155 gal/yd2 0 20 40 60 80 100 120 140 160 -10 0 10 20 30 40 In te rfa ce S he ar S tre ng th (p si) Temperature (Â°C) Trackless CRS-1 Trackless CRS-1 Trackless CRS-1 0 50 100 150 200 250 300 -10 0 10 20 30 40 50 60 In te rfa ce S he ar S tre ng th (p si) Temperature (Â°C) Temperature (Â°C) 0 50 100 150 200 250 300 350 400 450 -10 100 20 30 40 50 60 In te rfa ce S he ar S tre ng th (p si) Figure 68. Variation of the ISS with test temperature. Figure 67. Variation of the ISS with residual application rate and test temperature. (a) Trackless 0 50 100 150 200 250 300 350 400 450 0.000 0.050 0.100 0.150 0.200 IS S (ps i) Residual Application Rate (gsy) -10C 0C 10C 20C 30C 40C 50C 60C (b) CRS-1 0 50 100 150 200 250 300 350 0.000 0.050 0.100 0.150 0.200 IS S (ps i) Residual Application Rate (gsy) -10C 0C 10C 20C 30C 40C 50C had a greater shear resistance than CRS-1 at high tempera- tures. It is noted that the PG binder used in the asphalt mix- ture was PG 64-22; therefore, test temperatures ranging from 0Â° to 60Â°C did not exceed the associated PG-grading range. Figure 67 (a and b) presents the variation of the ISS with residual application rates and test temperatures. For both tack coat materials, as the residual application rate increased, the ISS increased at all temperatures, and the highest ISS val- ues were measured at the rate of 0.155 gal/yd2; therefore, for the range of residual application rates from 0.031 to 0.155 gal/yd2, there was no optimum tack coat residual application rate as might have been expected. This may be attributed to the highly oxidized and coarse HMA surface at the selected site, which required greater tack coat rates than expected. It is also noted that, for CRS-1, the ISS did not consistently increase with the increase in residual application rates at a temperature of 20Â°C or higher. On the other hand, ISS consistently increased with residual application rate for the trackless material, even at high test temperatures. Variation of the ISS with test temperatures at each residual application rate is presented in Figure 68. As shown in this fig- ure, ISS of the trackless increases from 60Â°C to 0Â°C and then decreases toward -10Â°C. This is due to the low elongation properties of the trackless at low temperatures (see Table 16). In contrast, the ISS for CRS-1 continuously increased as tem- perature decreased. However, the trackless material still pro- duced higher shear strengths than CRS-1 at low temperatures and at residual application rates of 0.062 and 0.155 gal/yd2.
65 4.5.2 Interface Stiffness Characteristics at Various Temperatures Variation of the k-modulus ratio and the ISS ratio between trackless and CRS-1 are shown in Figure 69 (a and b). In Figure 69a, it is observed that the k-modulus of the trackless material was greater or equal to that for the CRS-1 tack coat, except for the residual application rate of 0.062 gal/yd2 at 30Â°C. In addi- tion, the difference between the two tack coats was marginal at a residual application rate of 0.031 gal/yd2, except at a test tem- perature of 30Â°C (see Figure 69b); however, at an application rate of 0.062 and 0.155 gal/yd2, the bonding performance of the trackless was superior to that of the CRS-1 as the tempera- ture increased. The ratio of the k-modulus and ISS was not plotted at a temperature greater than 40Â°C since the bonding resistance of CRS-1 was significantly lower than the trackless material. It is worth noting that the ISS values for the trackless emulsion tested at temperatures greater than 40Â°C were much higher than those of similar specimens with CRS-1 emulsion (see Table 26). Since the temperature at a pavement interface can reach 40Â°C or higher during the summer months, the use of a trackless-type of emulsion would provide greater shear resistance than that of the CRS-1 emulsion. 4.5.3 Relationship Between Interface Shear Strength and Tack Coat Rheology Interface Shear Strength versus G*/sin d The parameter G*/sin d is used as an indicator of the binder susceptibility to permanent deformation in the Superpave binder specification system. It was, however, adopted in this study because it simulates oscillation in a shear mode, which closely resembles the interface shear mode between two layers. The relationships between ISS and G*/sin d and k-modulus and G*/sin d are presented in Figures 70 and 71, respec- tively. Results presented in Figure 70 indicate that as G*/sin d increased, the ISS for both tack coat materials at each residual application rate also increased. On the other hand, interface stiffness did not vary noticeably with the residual application 0.0 0.5 1.0 1.5 2.0 -10 0 10 20 30 k- m od ul us T ra ck le ss /k -m od ul us C R S- 1 Temperature (ÂºC) Temperature (ÂºC) 0.015 gal/yd2 0.031 gal/yd2 0.062 gal/yd2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -10 0 10 20 30 IS S Tr ac kl es s/ IS S C R S- 1 0.015 gal/yd2 0.031 gal/yd2 0.062 gal/yd2 (a) (b) Figure 69. Ratio of trackless to CRS-1 in terms of (a) k-modulus and (b) ISS. (a) Trackless (b) CRS-1 0 500 1000 1500 2000 2500 3000 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 In te rf ac e Sh ea r S tr en gt h (k Pa ) G*/sin (kPa) 0.07 l/m 0.14 l/m 0.28 l/m 0 500 1000 1500 2000 2500 3000 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 In te rf ac e Sh ea r S tr en gt h (k Pa ) G*/sin (kPa) 0.07 l/m 0.14 l/m 0.28 l/m Figure 70. Relationship between ISS and G*/sin d.
66 rate (see Figure 71). Therefore, it may be concluded that the amount of tack coat material influences the ISS but not the interface stiffness. The authors postulate that the interface stiff- ness modulus may be mainly influenced by surface texture. As shown in Figure 70, the ISS values did not exhibit much difference for a G*/sin d value below about 100 kPa (14.5 psi) and 1000 kPa (145.03 psi) for trackless and CRS-1, respectively. At higher G*/sin d values, the difference in ISS between the three residual application rates became more pronounced. Further, the trackless material produced greater ISS differ- ences than did CRS-1 at the same G*/sin d values. The rela- tionship shown in Figure 70 between the ISS and G*/sin d may be used to establish a laboratory design threshold for this parameter in order to ensure that the selected residual appli- cation rate and tack coat material would perform acceptably in the field. However, setting this limit on G*/sin d would require field validation of tack coat performance and that the required ISS be greater than the predicted shear stress at the interface due to traffic and/or thermal loading. The variation of the limit on G*/sin d with surface texture and surface type should also be investigated. The influence of sur- face texture on tack coat ISS has been investigated as part of NCHRP Project 9-40 and is presented in the next section. Results implied a direct relationship between the roughness of the existing surface and the shear strength at the interface; therefore, a milled HMA surface would provide the greatest ISS, followed by PCC, old HMA, and new HMA. 4.6 Experiment V: Effects of Pavement Surface Type and Sample Preparation Method The mean ISSs along with their standard deviations and COVs were obtained for each condition considered in the test factorial. Triplicate samples were tested for each test condi- tion defined by tack coat type, residual application rate, con- fining pressure, dusty surface, and wet conditions. The COVs in the test results were less than 15% for all conditions. As presented in this section, test results were analyzed to investi- gate the effects of the variables considered in the test factorial on ISS. Since the focus of this experiment was on the effects of surface types and preparation methods, the effects of surface cleanliness were presented in Experiment III. 4.6.1 Effects of Tack Coat Type and Residual Application Rate Figure 72 (a through d) presents the variation of the ISS with emulsified tack coat types and residual application rates for the different surface types (i.e., old HMA surface, PCC sur- face, milled HMA surface, and new HMA). As previously men- tioned, only one emulsion (SS-1h) was used on the new HMA surface and two emulsions (SS-1h and SS-1) were applied on the milled HMA surface. These results were obtained for clean and dry samples with no confinement pressure at 25Â°C. As shown in Figure 72, all tack coat materials showed that the ISS increased as the residual application rate increased within the evaluated application-rate range (0.031 to 0.155 gal/ yd2); hence, it was not possible to identify an optimum resid- ual application rate. This may indicate that, under actual field conditions, optimum residual application rates may be greater than that commonly predicted from laboratory-based experi- ments. However, while higher application rates may increase ISS, excessive tack coat may migrate into the HMA mat dur- ing compaction and service, causing a decrease in the air void content of the mix, and may even cause the appearance of fat spots on the HMA surface. One study reported that excess tack might be picked up by hauling trucks and paving equipmentâ causing safety concerns when tracked onto pavement mark- ings in traffic intersections close to the construction area (42). For old HMA and PCC surface types, the trackless tack coat exhibited the highest shear strength at the residual appli- cation rates of 0.031 and 0.062 gal/yd2 for both old HMA and PCC surfaces, and CRS-1 and SS-1 exhibited the lowest. (a) Trackless (b) CRS-1 0 2000 4000 6000 8000 10000 12000 1. 0E +0 1 1 .0E+ 02 1. 0E+ 03 1. 0E +0 4 1 . 0E+ 05 1.0E+0 6 k- m o du lu s (k N /m 3 ) G* /s in (k Pa ) 0. 07 l/m 0. 14 l/m 0. 28 l/m 0 1000 2000 3000 4000 5000 6000 7000 1.0E+0 1 1 .0 E+ 02 1. 0E +0 3 1 .0E+04 1. 0E +0 5 1 .0E+06 k- m o du lu s ( kN /m 3 ) G* /sin (k Pa ) 0. 7 l/ m 0. 14 l/m 0. 28 l/m Figure 71. Relationship between k-modulus and G*/sin d.
(b) 0 10 20 30 40 50 60 70 80 90 100 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) SS-1h CRS-1 Trackless PG 64-22 0 10 20 30 40 50 60 70 80 90 100 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) Trackless SS-1h PG 64-22 SS-1 0 10 20 30 40 50 60 70 80 90 100 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) SS-1h SS-1 (a) (c) 0 10 20 30 40 50 60 70 80 90 100 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) SS-1h (d) Figure 72. Effects of residual application rates and tack coat types on ISS for (a) old HMA surface, (b) PCC surface, (c) milled HMA surface, and (d) new HMA surface.
68 Trackless tack coat consists of a polymer-modified emulsion with hard base asphalt cement. These results relate directly to the viscosity of the residual binders at the test temperature. The influence of tack coat type appears to increase with the increase in the residual application rate. Except for the milled HMA surface, the no-tacked cores failed during extraction due to the poor bonding at the interface. This emphasizes the importance of using a tack coat material at the interface to avoid poor bonding between the layers. To balance the aforementioned factors, one should select a tack coat residual application rate that would ensure that the ISS is greater than the calculated shear stress at the interface due to traffic and thermal loading. 4.6.2 Effects of Surface Type SS-1h emulsified tack coat was evaluated on all four sur- face types. On the other hand, the trackless tack coat and PG 64-22 asphalt binder were evaluated for two surface types: old HMA and grooved PCC surfaces. PCC samples were tested parallel to the direction of the grooves. This test arrange- ment should generate the lowest ISS, which is in the direc- tion of traffic and, therefore, is more conservative. Figure 73 (a through c) presents the variation of ISS with surface types and residual application rates. As shown in these figures and due to its high roughness, the milled HMA surface provided the highest ISSs, followed by the PCC surface. In most cases, the old HMA surface provided greater interface strength than did the new HMA surface. It is noted that differences are more pronounced at low and intermediate residual applica- tion rates and less pronounced at high residual application rates. It is likely that the effects of microstructure features that contribute to the surface roughness or texture are less pronounced when they are filled with tack coat materials. 4.6.3 Effects of Surface Wetness The effects of surface wetness on the ISS were evalu- ated for old HMA, PCC, and milled surfaces. Figure 74 (a through c) presents the effects of surface wetness. Sta- Figure 73. Effects of surface types on ISS for (a) SS-1h tack coat (b) PG 64-22 and (c) trackless tack coat. (a) (b) 0 10 20 30 40 50 60 70 80 90 10 0 0.0 0 0.0 5 0.1 0 0.1 5 0.2 0 In te rf ac e sh ea r Bo nd S tr en gt h (p si) Residual Application Rate (gsy) Mi ll ed HMA PCC Existi ng HMA New HM A 0 10 20 30 40 50 60 70 80 90 10 0 0.0 0 0.0 5 0.1 0 0.1 5 0.2 0 In te rf ac e sh ea r Bo nd S tr en gt h (p si) Residual Application Rate (gsy) Existi ng HMA PCC
69 Figure 73. (Continued). 0 10 20 30 40 50 60 70 80 90 100 0.00 0.05 0.10 0.15 0.20 In te rf ac e sh ea r Bo nd S tr en gt h (p si) Residual Application Rate (gsy) Existing HMA PCC (c) tistically different sets are identified in these figures with an asterisk above the bar. As shown in Figure 74a, statisti- cally significant sets, shown with an asterisk, are often cases where wet conditions provided greater ISS than dry condi- tions. This is probably due to unaccounted-for factors such as the presence of coarse aggregates at the surface (higher/ coarser texture), which increased the friction resistance of the interface at the selected coring locations. This indicates that, even in the presence of light rain, the placement tem- perature of an HMA overlay will cause the water to evapo- rate or infiltrate into the underlying layer with no practical consequence on the interface bond strength. For the PCC surface, SS-1h, SS-1, trackless, and PG 64-22 were evalu- ated (see Figure 74b). The use of PG 64-22 did not generate sufficient bond strength at 0.031 and 0.062 gal/yd2 under wet conditions, indicating possible negative effect of surface wetness at low tack rates. On the other hand, for SS-1 and trackless tack coats, surface wetness did not affect the ISS. Only SS-1h was evaluated for the milled surface in dry and wet conditions. The influence of surface wetness did not follow a consistent trend (see Figure 74c). 4.6.4 Effects of Preparation Methods Figure 75 and Table 27 present the measured ISS for labo- ratory-fabricated specimens. For SS-1h, AUT, and PG 64-22, it was found that the optimum rateâat which the greatest ISS was achievedâis 0.062 gsy. For CRS-1, as the residual appli- cation rate increased, the ISS value decreased. On the other hand, the trackless material showed continuous increase of ISS from 0.031 to 0.155 gsy. To assess the influence of sample preparation methods, Figure 76 compares the ISS of laboratory-fabricated sam- ples with that of field-extracted cores for tack coat SS-1h in the case of the new HMA surface. As shown in this fig- ure, laboratory-prepared samples grossly overestimated the ISS by a factor ranging from 2 to 10 when compared with field-extracted cores. In the laboratory, ISS decreased with tack rate, whereas, in the field, ISS increased with tack rate. A number of factors may cause this discrepancy, includ- ing the difference in mixing and compaction methods and application method for the tack coat materials. Difference in compaction methods may result in differences in air void contents and distributions in the specimen, mix resistance to shear loading, and mix density. The most probable factor appears to be the greater asphalt film thickness at the inter- face of the new HMA and the smoother/flatter surface of the freshly made specimens. 4.7 Experimental VI: Effects of Texture and Permeability on Tack Coat Bond Strength The objective of this laboratory experiment was to evaluate the effects of surface texture and permeability of the exist- ing pavement on tack coat ISS. The details of the mixturesâ design, surface texture and permeability measurements, and specimen fabrication were previously reported. The tack coat material used in this experiment was SS-1 emulsion. ISS tests were conducted for open-graded friction course (OGFC), SMA, and sand mixtures. Table 28 presents the mean ISSs along with their standard deviations and COVs for the three mixtures evaluated. Figure 77 shows the variation of the ISS with the residual application rate. For the SMA mixture, the peak ISS was observed at a resid- ual application rate of 0.031 gsy. The ISS was lower at residual application rate 0.155 gsy than that for the no-tack condition. For the sand mixture, the peak ISS occurred at the no-tacked condition. For smooth interface conditions and a new sur- face (i.e., still coated with asphalt) such as the one simulated
70 0 10 20 30 40 50 60 70 80 90 100 0.031 0 .062 0.155 0 .031 0.062 0 .155 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) Wet and Clean Dry and Clean 0 10 20 30 40 50 60 70 80 90 100 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) Dry and Clean Wet and Clean SS-1h PG 64-22 Trackless SS-1h PG 64-22 SS-1 * * * * * * * * (b) (a) 0 10 20 30 40 50 60 70 80 90 100 0.031 0.062 0.155 In te rf ac e Sh ea r B on d St re ng th (p si) Residual Application Rate (gsy) Dry and Clean Wet and Clean SS-1h * (c) Figure 74. Effects of surface wetness on ISS for (a) old HMA, (b) PCC, and (c) milled surfaces.
71 50 60 70 80 90 100 110 120 130 140 150 0.000 0.050 0.100 0.150 0.200 IS S (p si) Residual Application Rate (gsy) SS-1h Trackless PG 64-22 CRS-1 AUT Figure 75. Effects of residual application rate on ISS for lab-compacted samples. SS-1h Trackless PG 64-22 CRS-1 AUT 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 ISS (psi) 90.6 93.2 83.0 117.9 125.4 129.1 76.4 98.8 82.7 67.0 65.1 60.1 93.5 112.3 106.8 92.5 97.3 84.6 119.3 123.0 131.2 105.2 99.8 85.2 70.4 69.3 62.4 99.2 115.4 109.4 100.1 107.6 87.9 122.6 124.5 131.7 105.6 101.9 88.2 73.1 69.8 66.0 111.6 119.5 115.4 Mean 94.4 99.4 85.1 119.9 124.3 130.7 95.7 100.2 85.4 70.2 68.1 62.8 101.4 115.7 110.5 S.D. 5.0 7.4 2.5 2.4 1.2 1.4 16.8 1.6 2.7 3.1 2.6 3.0 9.3 3.6 4.4 COV (%) 5.3 7.5 2.9 2.0 1.0 1.1 17.5 1.6 3.2 4.4 3.8 4.7 9.1 3.1 4.0 Table 27. ISS test results for lab-compacted samples. 0 20 40 60 80 100 120 140 0 0.05 0.1 0.15 0.2 In te rf ac e Sh ea r S tr en gt h (p si ) Residual Application Rate (gal/yd2) Laboratory-Prepared Field-Prepared Figure 76. Effects of sample preparation methods on the ISS.
72 with the sand mixture, tack coat application reduces inter- face shear bond strength. It appears thatâfor a new, smooth surfaceâtack coat acts as a lubricant and, thus, decreases the shear strength at the interface. For the OGFC mixture, the ISS decreased slightly from the no-tack condition with an increase in the residual applica- tion rate, reached a minimum at residual application rate of 0.062 gsy, and then increased with an increase in the resid- ual application rate at 0.155 gsy. It appears that the higher voids in the surface of the OGFC initially yielded lower shear strength than did that for the sand mix. However, when the voids in the surface of the OGFC are filled with asphalt at the highest residual application rate (0.155 gsy), the shear strength becomes equivalent to that of the sand. One would expect that higher surface texture would yield higher ISS, such as a milled surface or an OGFC; however, it was observed that the surfaces of the laboratory-compacted specimens (which are compressed against a smooth, flat steel plate) were flat but with significant voids in the case of the OGFC. These highly permeable voids likely absorbed the asphalt from the tacked interface and, thus, reduced the ISS of the OGFC to a lower level than that of the relatively smooth, voidless, impermeable surface of the sand mix. Once the voids in the surface of the OGFC were filled with tack coat material, the OGFC showed an increase in ISS to a value that is slightly higher than that of the sand mix. 4.8 Theoretical Investigation Peak values of ISS, k-modulus, and displacement at fail- ure (dmax) were calculated for each tack coat material and are presented in Table 29. As previously noted, all tack coat materials showed the highest strength at a residual applica- tion rate of 0.155 gsy. Within the residual application rate range considered, no optimum residual application rate was determined. This was attributed to the highly oxidized HMA surface at the PRF site, which required greater tack coat rates than expected. The mean profile depth (MPD) for the old HMA surface, which was measured using a road surface pro- filer according to ASTM E 1845, was 0.04 in (1.05 mm). While higher residual application rates may increase ISS, excessive tack coat may migrate into the HMA mat during compac- tion, causing a decrease in the air void content of the mix. It is also observed from the results presented in Table 29 that OGFC Sand SMA Residual application rate (gsy) 0.000 0.031 0.062 0.155 0.000 0.031 0.062 0.155 0.000 0.031 0.062 0.155 ISS (psi) 64.6 60.1 57.6 70.6 83.5 71.6 74.0 63.5 62.4 77.7 66.8 39.5 65.7 64.1 55.3 70.7 87.6 79.5 73.7 68.4 59.4 91.3 73.0 40.4 73.2 66.4 52.3 75.2 89.3 86.4 70.8 70.3 69.4 91.8 73.2 43.7 Mean 67.8 63.6 55.1 72.2 86.8 79.2 72.8 67.4 63.7 86.9 71.0 41.2 SD 4.7 3.2 2.7 2.6 5.0 7.4 1.8 3.5 5.1 8.0 3.6 2.2 COV (%) 6.9 5.0 4.8 3.6 5.8 9.3 2.4 5.2 8.1 9.2 5.1 5.4 Table 28. ISS test results for the OGFC, sand, and SMA mixtures. Figure 77. Mean interface shear bond strengths for the OGFC, sand, and SMA mixtures.
73 the interlayer tangential modulus decreased with the increase in residual application rate, which is indicative of greater deformability and flexibility at the interface. In addition, the trackless tack coat exhibited the highest shear strength, and CRS-1 exhibited the lowest. The effects of tack coat interface shear bond characteris- tics, as measured by the LISST test on pavement responses at the interface, were investigated using the results of the FE model. Figure 78 (a to f) compares the calculated shear stress at the interface between the old and HMA overlay with the ISS for the different tack coat material types and residual appli- cation rates. As shown in Figure 78, only two cases (Struc- ture A and Structure E with CRS-1 at 0.031 gal/yd2 residual application rate) failed due to a single load application. For the other structures, none of the evaluated cases failed at the interface due to a single load application. It is also noted that the calculated shear stress did not substantially change from one tack coat application case to another. However, the cal- culated shear stress changed from one pavement design to another. While the results presented in Figure 78 relate to the shear response of the interface against a single tire load application, pavement structures are typically subjected to repeated fluc- tuating vehicular loads. Such load patterns may cause fatigue failure at the tacked interface through a process of cyclic cumulative damage. To assess the potential for fatigue failure at the interface, the stress ratio (which is the ratio of the pre- dicted shear stress at the interface to the ISS) was calculated. If the stress ratio was less than 0.50, the interface response against fatigue failure was assumed to be acceptable. On the other hand, if the stress ratio was greater than 0.50, the inter- face was expected to experience fatigue failure before the end of its service life. A stress ratio of 0.50 is usually assumed in laboratory fatigue testing of HMA and tacked interface as an indication of failure (35, 43). It is also hypothesized that, at a stress ratio of 0.50 or less, the fatigue life at the tacked inter- face would be infinite (i.e., no fatigue-related distress at the interface). Based on this theoretical approach, Figure 79 (a to f) pres- ents the calculated stress ratio for each tack coat type and residual application rate. For Structure A, it is noted that tracklessâat intermediate and high residual application ratesâand SS-1h and PG 64-22âat a high residual appli- cation rateâpassed this criterion. CRS-1 did not meet this criterion at any of the rates evaluated. For Structure B, the majority of the tack coat types and residual application rates would be expected to perform satisfactorily against fatigue damage at the interface. In this case, only CRS-1 and SSh-1h at the low residual application rate (0.031 gal/yd2) would be expected to experience fatigue damage at the interface. It is evident from these results that the performance of tack coat materials at the interface is primarily dictated by the pave- ment design. In other words, the influence of tack coat type and residual application rate becomes more relevant in thin pavements and less dominant in thick pavements. Based on the results presented in Figures 78 and 79, Fig- ure 80 presents the variation of the predicted shear stress ratio with the ISS for the different tack coat materials and residual application rates. As shown in this figure, a power law model is adequate in describing the relationship between the shear stress ratio and the ISS. Utilizing the presented models, it was determined that the minimum laboratory-measured ISS at the FE Case ID Tack Coat Material Residual Application Rate (gal/y d 2 ) ISS (MPa x 10 3 ) ISS (psi) COV (% ) k (N/mm 3 ) d ma x (mm) 1 CRS-1 0.031 76.5 11.1 14.1 0.1916 0.39 2 0.062 129.6 18.8 7.6 0.1845 0.70 3 0.155 148.9 21.6 10.9 0.1304 1.14 4 SS-1h 0.031 117.9 17.1 14.2 0.2297 0.51 5 0.062 139.3 20.2 9.8 0.2826 0.49 6 0.155 415.7 60.3 6.4 0.2769 1.51 7 Trackless 0.031 150.9 21.9 10.2 0.2688 0.56 8 0.062 263.4 38.2 12.3 0.2642 0.99 9 0.155 655.0 95.0 8.3 0.2456 2.67 10 PG 64-22 0.031 138.6 20.1 13.0 0.1757 0.79 11 0.062 154.4 22.4 12.7 0.1898 0.81 12 0.155 258.5 37.5 7.2 0.1411 1.83 Table 29. Interface shear behaviors for different tack coat types and at three residual application rates.
74 Figure 78. Comparison of the calculated shear stress to the ISS. (a) Structure A (b) Structure B 0 10 20 30 40 50 60 70 80 90 10 0 0. 031 0. 062 0. 15 5 0 .031 0. 062 0. 155 0.031 0. 062 0. 155 0. 031 0. 06 2 0 .1 55 Sh ea r S tr es s (p si) Application Rate (gsy) IS S Shear Stres s SS-1h Trackless PG 64-22 0 10 20 30 40 50 60 70 80 90 10 0 0.031 0. 062 0.155 0.031 0. 062 0.155 0.031 0. 062 0.155 0.031 0.062 0.155 Sh ea r St re ss (p si) Application Rate (gsy) ISS Shear Stres s SS-1h Trackless PG 64-22 CRS-1 (c) Structure C 0 10 20 30 40 50 60 70 80 90 100 0.031 0. 062 0. 15 5 0 .0 31 0.062 0. 155 0. 03 1 0 .0 62 0.155 0. 031 0. 06 2 0 .1 55 Sh ea r S tr es s (p si) Application Rate (gsy) ISS Shear Stres s CRS-1 SS-1h Trackless PG 64-22 CRS-1
75 (e) Structure E 0 10 20 30 40 50 60 70 80 90 100 0.03 1 0 .062 0. 155 0.031 0.062 0.155 0.03 1 0 .062 0. 155 0. 031 0.062 0.155 Sh ea r S tr es s ( ps i) Application Rate (gsy) ISS Shear Stress CRS- 1 SS-1h Trackless PG 64-22 (f) Structure F 0 10 20 30 40 50 60 70 80 90 100 0. 03 1 0 .062 0. 15 5 0 .031 0.062 0. 155 0.031 0. 06 2 0 .155 0.031 0. 062 0.155 Sh ea r S tr es s ( ps i) Application Rate (gsy) ISS Shear Stres s CRS-1 SS-1h Trackless PG 64-2 (d) Structure D 0 10 20 30 40 50 60 70 80 90 100 0.03 1 0 .062 0.15 5 0 .031 0.06 2 0 .155 0.03 1 0 .0 62 0.15 5 0 .0 31 0.06 2 0 .1 55 Sh ea r S tr es s ( ps i) Application Rate (gsy) ISS Shear Stress CRS- 1 SS-1h Trackless PG 64-22 Figure 78. (Continued).
76 Figure 79. Calculated shear stress ratios for different tack coat types and residual application rates. (a) Structure A (b) Structure B 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 Sh ea r St re ss R at io Application Rate (gsy) CRS-1 SS-1h Trackless PG 64-22 Limiting Ratio Application Rate (gsy) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 Sh ea r St re ss R at io CRS-1 SS-1h Trackless PG 64-22 Limiting Ratio (c) Structure C Application Rate (gsy) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 Sh ea r St re ss R at io CRS-1 SS-1h Trackless PG 64-22
77 Figure 79. (Continued). (e) Structure E (f) Structure F 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 Sh ea r St re ss R at io Application Rate (gsy) CRS-1 SS-1h Trackless PG 64-22 Limiting Ratio 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 Sh ea r St re ss R at io Application Rate (gsy) CRS-1 SS-1h Trackless PG 64-22 (d) Structure D 0 0.1 0.2 0.3 0.4 0.5 0.6 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 Sh ea r St re ss R at io Application Rate (gsy) CRS-1 SS-1h Trackless PG 64-22 Limiting Ratio
78 y = 15.514x-1.035 RÂ² = 0.99 y = 10.671x-1.047 RÂ² = 0.99 y = 3.634x-0.977 RÂ² = 0.9984 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 10 20 30 40 50 60 70 80 90 100 Sh ea r S tr es s R at io ISS (psi) Design A Design B Design C y = 7.1991x-1.054 RÂ² = 0.99 y = 12.633x-1.033 RÂ² = 0.99 y = 3.634x-0.977 RÂ² = 0.99 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 10 20 30 40 50 60 70 80 90 100 Sh ea r S tr es s R at io ISS (psi) Design D Design E Design F Figure 80. Relationship between shear stress ratio and laboratory-measured ISS.
79 interface to achieve a shear stress ratio of 0.50 or lower was 28 psi for Structure A, 19 psi for Structure B, and 8 psi for Structure C. Similarly, the minimum laboratory-measured ISS at the interface to achieve a shear stress ratio of 0.50 or lower was 23 psi for Structure D, 13 psi for Structure E, and 8 psi for Structure F. These limits can be used in the selection of tack coat materials and residual application rates based on labora- tory DST results to predict performance at the interface in the field. If a single ISS value needs to be specified to prevent failure at the interface, and considering a safety factor of 1.4 against variability in measurements and in construction, an ISS value of 40 psi is recommended. Based on the results of the FE analysis, findings of the experimental program for different surface types, and dis- Surface Type Residual Application Rate (gsy) New Asphalt Mixture 0.035 Old Asphalt Mixture 0.055 Milled Asphalt Mixture 0.055 Portland Cement Concrete 0.045 Table 30. Recommended tack coat residual application rates. cussions with state dots and industry personnel, Table 30 lists the recommended tack coat residual application rates for the various pavement surfaces.