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OCR for page 80
80 Section 5 Conclusions The main objectives of this project were to determine opti- lights the importance of regularly checking the accuracy of the mum application methods, equipment type and calibration distributor in practice. Quality of tack coat application was procedures, residual application rates, and asphalt binder evaluated using the LTCQT, samples were cored from the test materials for the various uses of tack coats and to recom- pavements, and ISS was measured in the laboratory using the mend revisions to relevant AASHTO test methods and prac- LISST device. Based on the findings of this project, the follow- tices related to tack coats. During the course of this project, ing conclusions were drawn with respect to both the ISS and the research team developed the Louisiana Tack Coat Qual- the tack coat spray application quality in the field. ity Tester (LTCQT) to evaluate the quality of tack coat spray With respect to ISS in the field: application in the field. The LTCQT and associated test pro- cedure were demonstrated to be viable methods for evaluat- 1. For the effect of emulsified tack coat type, trackless tack ing tack coat quality in the field. The LTCQT could serve as coat exhibited the highest shear strength and CRS-1 re- a valuable tool for highway agencies to perform comparative sulted in the lowest strength. These results relate directly to evaluations of various tack coat materials and application the viscosity of the residual binders at the test temperature methods and rates in the field. Repeatability of measurements (25C). using the LTCQT was acceptable, with an average coefficient 2. For the effect of application rate, all tack coat materials of variation of less than 11%. Research in this project also showed the highest shear strength at a residual applica- resulted in the development of a training manual (which is tion rate of 0.155 gsy. Within the tested residual applica- presented in Appendix F). The training manual provides a tion rate range, it was difficult to determine the optimum comprehensive presentation of the recommended construc- residual application rate. This may be attributed to the tion and testing procedures for tack coat materials. highly oxidized HMA surface at the LTRC site, which The Louisiana Interlayer Shear Strength Tester (LISST) required greater optimum tack coat rates than expected. was developed for characterization of interface shear strength It may also indicate that, under actual field conditions, (ISS) of cylindrical specimens in the laboratory. The LISST optimum residual application rates are greater than what device was designed such that it will fit into any universal test- is commonly predicted from laboratory-based experi- ing machine. The average coefficient of variation in the LISST ments. It is noted, however, that while higher residual test results was less than 10%. As part of the experimental application rates may increase ISS, excessive tack coat program, the research team constructed full-scale test over- may migrate into the new asphalt mat during compac- lays at the Louisiana Transportation Research Center Pave- tion causing a decrease in the air void content of the mix. ment Research Facility. The overlays included different tack 3. For the effect of confinement, the ratio of ISS between coat residual application rates beneath a new HMA overlay confined and no-confinement test conditions was always installed over several types of pavement surfaces including old greater than 1. This ratio increased as the residual appli- hot-mix asphalt, new HMA, milled HMA, and grooved port- cation rate decreased; therefore, a specification devel- land cement concrete. Five types of tack coat materials were oped based on no-confinement testing conditions would applied at three residual application rates. The calibration yield a conservative estimate of the ISS values. of the distributor truck was a lengthy process in this project 4. For the effect of dust, the majority of the cases showed a and required multiple calibration runs to ensure the accuracy statistically significant difference between clean and dusty and uniformity of tack coat application. This difficulty high- conditions. It appears from these results that dusty con-

OCR for page 80
81 ditions exhibited greater ISS than did clean conditions, 7. For the effect of preparation method, laboratory- especially when tested with a confining pressure. This prepared specimens grossly overestimated the ISS when likely resulted when the dust combined with the asphalt compared with pavement cores. In addition, when increas- and formed mastic with a resultant viscosity higher than ing tack residual application rate, a decreasing trend in that of the neat residual asphalt, plus the sand particles ISS was observed for laboratory-prepared specimens, may have provided grit at the interface to further increase while an increasing trend was observed in the field. the ISS. However, one should note that these results are 8. For the effect of temperature (from 10 to 60C), ISS based on using a uniform and clean sand to simulate increased with the decrease in temperature. In addition, dusty conditions--therefore, cleaning and sweeping of the bonding performance--as measured by the ISS of the the existing pavement surface is recommended to avoid trackless emulsion--was superior to that of the CRS-1 negative effects of dusty conditions. emulsion, especially at temperatures greater than 40C. 5. For the effect of water on the tacked interface, the 9. Based on the results of the FE analysis, the minimum majority of the cases showed no statistically significant laboratory-measured ISS obtained from the LISST difference between dry and wet conditions. This data device, tested at 25C, that provides acceptable perfor- indicates that a small amount of water can be flashed mance is 40 psi. away by the hot HMA mat and, thus, have inconsequen- tial effects on the quality of the tack coat. This study used With respect to the tack coat spray application quality in only hot mix as the overlay material; the use of warm the field: mix may change this finding. In addition, these results are based on using a small quantity of water to simulate 10. For pavement cores, tensile strength of each tack coat rainy conditions--therefore, a dry and clean surface is material increased, reached a peak, and then decreased as recommended to avoid the negative effects of water on the temperature increased. The tack coat materials tested the bonding at the interface. using LTCQT exhibited a maximum tensile strength, 6. For the effect of surface type, a direct relationship was SMAX, at a distinct temperature, TOPT. Thus, the response observed between the roughness of the existing surface of tack coat material in tension was characterized using and the shear strength at the interface; therefore, the SMAX at TOPT. milled HMA surface provided the greatest ISS followed 11. For the tack coat materials evaluated, a good correla- by PCC, old HMA, and new HMA surfaces. Table 31 tion was observed between the tensile strength and abso- presents the recommended tack coat residual application lute viscosity. Within the range studied, an increase in rates for different surface types. viscosity (i.e., resistance to flow) was associated with an increase in tensile strength. Table 31. Recommended tack coat 12. For the tack coat materials evaluated, a good relation- residual application rates. ship was observed between the maximum tensile strength Surface Type Residual Application and the corresponding softening point. An increase in Rate (gsy) the material softening point was correlated to an increase New Asphalt Mixture 0.035 in the maximum tensile strength. Old Asphalt Mixture 0.055 13. Based on the results of this study, it is recommended Milled Asphalt Mixture 0.055 to conduct the LTCQT test at the tack coat base asphalt softening point, which is a quantity that can be easily Portland Cement Concrete 0.045 measured and specified.