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48 4.2.3Development of Tack Coat Test The IRH source device used in the first, second, and fourth Procedure Using LTCQT 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 A procedure for evaluation of tack coat quality in the field of the sample specimen at 43.3C with a residual application was developed based on the second generation of the LTCQT rate of 0.05 gal/yd2. To avoid evaporation of light oil compo- test device. Loading rate, time required for breaking of emul- nents during the heating process, the surface temperature of sified tack coat, contact pressure, and contact time between the specimen was not allowed to exceed 135C for any device. contact plate and tacked surface were examined. Based on Results of these experiments are presented in Figure 52b. It was the results of this evaluation, a test procedure was written in noted that the target residual asphalt weight (i.e., the weight AASHTO format. after approximately 12 hours of evaporation at room tempera- ture) was achieved after approximately 1 hour for each of the Loading Rate four test setups considered. Based on these results, the IRH lamp was selected for use in accelerating water evaporation Since the loading rate significantly affects the test results, it time and was subsequently adopted in the field experiments. is essential to select an appropriate rate that can distinguish The IRH device provided the most uniform heat distribution between the tensile strength of different tack coat materials. on the sample among the four devices evaluated. Furthermore, Experiments for determining appropriate loading rate were this device was comparatively simple to setup and use. 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 Contact Time and Pressure at 50C and at two loading rates (i.e., 004 and 0.008 in/sec). A contact pressure, compressive preload, is applied to Based on the applied loading rates, it was found that LTCQT the contact plate for a preset period of time as a part of the is able to differentiate between different tack coat materials in LTCQT. A contact pressure of 1.57 psi for 3 minutes was terms of the measured tensile strengths. Since this trend was found to be adequate to provide uniform adhesion between consistent at both displacement rates, and to ensure prompt the tacked surface and the loading plate of the LTCQT. 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 (a) Infrared reflective heating (IRH) lamp. constant weight was obtained. Three emulsion types were 7.5 evaluated, namely, CRS-1, SS-1h, and trackless. Each one 7.0 IRH Lamp of these emulsions was applied to the surface of a HMA IRH Lamp + Fan 6.5 specimen with dimensions of 5.9 in in diameter and 2.2 in Heat Gun Weight (g) 6.0 IRH Lamp + Heater in height. The weight of the tacked specimen was measured to 1/100th of a gram at several time intervals subsequent 5.5 to the application of the emulsion on the specimen. It was 5.0 observed that complete curing of the emulsions was achieved 4.5 after approximately 12 hours. This time period needed to 4.0 be shortened in order to permit same-day measurements 0 10 20 30 40 50 60 70 80 in field tack coat construction. Three devices were evalu- Time (min) ated in order to accelerate emulsion curing time: a portable (b) Breaking time of tack coat SS-1h with different heating conditions. fan/heater, a heat gun, and an infrared reflective heating Figure 52. Determination of heat source for (IRH) lamp. accelerating water evaporation in emulsions.