Optimization of Tack Coat for HMA Placement (2012)

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

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