Optimization of Tack Coat for HMA Placement (2012)

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S U M M A R Y Selection of an optimum tack coat material and application rate are crucial in the development of proper bond strength between pavement layers. In general, selection of tack coats has been mainly based on experience, convenience, and empirical judgment. In addition, quality-control and quality-assurance testing of the tack coat construction process is rarely conducted, resulting in the possibility of unacceptable performance at the interface and even premature pavement failure. The main objectives of this project are to determine optimum application methods, equipment type and calibration procedures, application rates, and asphalt binder materials for the various uses of tack coats and to recommend revisions to relevant AASHTO methods and practices related to tack coats. During the course of this project, the research team developed the Louisiana Tack Coat Quality Tester (LTCQT) to evaluate the quality of the bond strength of tack coat 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 pro- vides a comprehensive presentation of the recommended construction and testing procedures for tack coat materials. The Louisiana Interlayer Shear Strength Tester (LISST) was developed for the characteriza- tion of interface shear strength of cylindrical specimens in the laboratory. The LISST device was designed such that it will fit into any universal testing 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 overlays at the Louisiana Transportation Research Center’s (LTRC’s) Pavement Research Facility (PRF). The overlays included different tack coat application rates between a new hot-mix asphalt (HMA) overlay installed over several types of pavement surfaces including old HMA, new HMA, milled HMA, and grooved portland cement concrete (PCC). Five types of tack coat materials were each applied at three application rates. The quality of tack coat application was evaluated using the LTCQT, specimens were cored from the test pavements, and interface shear strength was measured in the laboratory using the LISST device. Based on the findings of this project, the following conclusions were drawn: • With respect to the interface shear strength in the field: – For the effect of emulsified tack coat type, trackless tack coat exhibited the highest inter- face shear strength (ISS), and CRS-1 resulted in the lowest interface shear strength. These results relate directly to the viscosity of the residual binders at the test temperature (25°C). – For the effect of application rate, all tack coat materials showed the highest interface shear strength at an application rate of 0.155 gsy. Within the tested application 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 PRF site, which required greater opti- mum tack coat rates than expected. It may also indicate that, under actual field conditions, 1 Optimization of Tack Coat for HMA Placement

2optimum application rates are greater than what is commonly predicted from laboratory- based experiments. It is noted, however, that while higher application rates may increase interface shear strength, excessive tack coat may migrate into the new asphalt mat during compaction, causing a decrease in the air void content of the mix. – For the effect of confinement, the ratio of interface shear strength between confined and no-confinement test conditions was always greater than 1. This ratio increased as the resid- ual application rate decreased. Therefore, a specification developed based on no-confinement testing conditions would yield a conservative estimate of the ISS values. – 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 conditions exhibited greater ISS than clean conditions, especially when tested with a confining pres- sure. 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 condi- tions. Therefore, cleaning and sweeping of the existing pavement surface is recommended to avoid negative effects of dusty conditions. – For the effect of water on the tacked surface, the majority of the cases showed no statisti- cally 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 inconsequential 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. – 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 interface shear strength followed by PCC, old HMA, and new HMA surface. Table S-1 presents the recommended tack coat residual application rates for different surface types. – For the effect of tack coat coverage, the use of 50% coverage significantly reduced the interface shear strength by a factor ranging from 50% to 70%. In addition, the use of 50% tack coat coverage resulted in inconsistent and non-uniform interface bonding behavior for tacked surfaces. – For the effect of preparation method, laboratory-prepared specimens grossly overestimated the interface shear strength when compared with pavement cores. In addition, when increasing tack application rates, a decreasing trend in ISS was observed in laboratory- prepared specimens, while an increasing trend was observed in the field. – 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 interface shear strength of the trackless emulsion, was superior to that of the CRS-1 emulsion, especially at temperatures greater than 40°C. Table S-1. Recommended tack coat residual application rate. 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

3 – Based on the results of the finite element (FE) analysis, the minimum laboratory-measured interface shear strength in the LISST device that provides acceptable performance is 40 psi. • With respect to the tack coat spray application quality in the field: – 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 maxi- mum tensile strength, SMAX, at a distinct temperature, TOPT. Thus, the response of tack coat material in tension was characterized using SMAX at TOPT. – For the tack coat materials evaluated, a good correlation was observed between the tensile strength and absolute viscosity. Within the range studied, an increase in viscosity (resis- tance to flow) was associated with an increase in tensile strength. – For the tack coat materials evaluated, a good relationship was observed between the maxi- mum tensile strength and the corresponding softening point. An increase in the material softening point was correlated to an increase in the maximum tensile strength. – 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.