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46 quality of the bond strength of tack coat in the field. LTCQT Improved sensitivity/reliability of the load cell sensor, is a modification of the ATacker. The following sections Improved sensitivity/reliability of the actuator rate of describe details of the development process and evaluation loading, and of the LTCQT. Improved adhesion of the LTCQT test plate to tacked surface. 4.2.1 First Generation of LTCQT Improved Sensitivity/Reliability Figure 50 presents the first generation of the LTCQT that of the Load Cell Sensor was used to measure the quality of tack coat applications in Several experiments using the first generation of LTCQT the field. The modifications included automated operation of were conducted to examine the sensitivity and reliability of the device and installation of electronic sensors for the mea- the load cell sensor. During this evaluation, it was observed surement of load and deformation. Subsequent to the initial that the load cell had a high noise level (approximately 10% evaluation of this version of the ATacker, it was determined of the load cell capacity), which exceeded the specification that additional fine-tuning items needed to be incorporated, value set by ASTM E 74, Standard Practice of Calibration of such as fixing the flap plates to hold the device firmly in place Force-Measuring Instruments for Verifying the Force Indica- during testing, increasing the travel distance of the actuator, tion of Testing Machines (40). Therefore, a new load cell with and additional modifications to the software to make it more a maximum capacity of 100 lbs and a signal conditioner user-friendly. Distinctive features of the first generation of were installed. The aforementioned changes yielded a stable LTCQT included: load cell signal that met ASTM E 74 standards. In addition, the LTCQT acquisition software was updated to match the Automated operation by installation of electronic sen- new device. sors for load and displacement measurements. This led to improved reliability and repeatability of the measurements Improved Sensitivity/Reliability and minimized operator error. of the Actuator Rate of Loading Incorporation of user-friendly software. Loading rate of the actuator was examined. Several experi- ments were conducted to verify the rate of loading using two 4.2.2 Second Generation of LTCQT tack coat materials with contrasting bond strengths: CRS-1 and PG 64-22. Results from these experiments showed that Figure 51 shows the second generation of the LTCQT. the device could not maintain the specified displacement Several modifications were introduced to the first genera- rate during testing. Results of this evaluation are presented tion of LTCQT to improve the reliability of the results. The in Appendix B. The displacement rate changed depending modifications introduced in this version addressed several on the strength of the material; therefore, a new actuator and issues observed in the first generation (details of these driving motor (closed loop, servo-controlled) with improved modifications are discussed in the following sections): control of the displacement rate were installed. It is noted Electric Sensor for Load and Displacement Incorporated Software Flat Plate Figure 50. First version of the LTCQT.

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47 Weights Load Cell and Signal Conditioner Contact Plate: 5.71 in Diameter (a) LTCQT Tester LTCQT (b) Software Incorporated in LTCQT Figure 51. Second generation of LTCQT. that the displacement of the actuator was measured using a performed with the tack coat applied between two metal position transducer that has a total travel of 3.94 in. The max- plates. During the LTCQT tack coat field evaluation tests, imum loading rate was 0.30 in/sec. Experiments were then poor adhesion (i.e., not measurable) was observed between conducted with the improved device to verify the loading rate. the metal plate and the tacked pavement surface. Several It was observed that the "set" and "measured" displacement types of flexible materials (to better conform to a textured rates of loading were in good agreement in these experiments, surface) that attach to the metal plate were evaluated. Rub- indicating that the second generation of LTCQT can provide ber, insulation foam, sill foam, and polyethylene foam are a consistent and reliable displacement rate of loading. among the materials evaluated. Rubber and the insulation foam showed poor adhesion to the pavement surface; how- ever, polyethylene foam yielded good adhesion. Therefore, Improved Adhesion of Test Plate to Tacked Surface polyethylene foam was used to ensure adequate adhesion. Most of the laboratory research that was performed to The foam can be easily attached onto the metal plate with evaluate tack coat quality using the ATacker test device was double-sided tape.