Performance of Tack Coat Materials in Saskatchewan Climate

Tack coat materials are used to provide sufficient bond between an existing asphalt concrete layer and a new asphalt concrete overlay or between two lifts of a new asphalt concrete layer. Tack coat materials are typically emulsified bituminous materials. Tack coat performance has not been extensively studied in the harsh climate of Saskatchewan. In practice, tack coat materials are often picked up on the tires of paving equipment, which leaves little tack coat material in the wheel paths where it is needed most. An ideal tack coat material should have a short curing time and a high bond strength to achieve better constructability and performance of the pavement structures. Improper installation of the tack coat material can lead to a poor bond and premature failure of the pavement structures. The objective of this research is to evaluate the strength and performance of several tack coat materials in Saskatchewan climate. Findings from this research will be used by the Saskatchewan Ministry of Highways and Infrastructure to create a recommended tack coat material list and provide guidelines for construction best practices. Currently, most tack coat materials selected for road construction in Saskatchewan are the basic slow-setting emulsion, SS-1. The tack coat materials tested in this research include basic anionic emulsions: SS-1, SS-1h, MS-1, a cationic emulsion: CSS-1h, and three proprietary quick setting/non-tracking emulsions provided by industry partners. Tack coat performance was evaluated through a field study as well as a laboratory-testing program. Ten test sections were constructed in August 2017 on a two-way, two-lane rural highway near Blaine Lake, Saskatchewan. At the time of test section construction, weather conditions, tack coat material curing properties, and application rate were measured. The proprietary products had faster setting and breaking times as well as less pickup of tack coat onto vehicle tires than all other products. All tested products had better setting and breaking times than SS-1. A correlation between tack coat breaking and setting times with temperature and humidity at the time of construction was observed. Two distress surveys were conducted post-construction in September 2017 and after one year in September 2018. The two distress surveys did not show early distresses or deformities in the road due to poor bonding between pavement layers. Laboratory testing was conducted on the cores collected from the test sections to evaluate the interlayer shear strength using a Louisiana Interlayer Shear Strength Tester according to AASHTO TP 114. Cores collected after construction were separated into two groups: baseline cores and freeze-thaw conditioned cores. Baseline cores were tested to measure initial bond strength after construction. Conditioned cores were exposed to three levels of freeze-thaw cycling: 3, 9, and 15 freeze-thaw cycles. Each freeze-thaw cycle consisted of twelve hours of freezing at -25°C and twelve hours of thawing at 15°C. The baseline cores, one year cores, and laboratory conditioned cores were tested to evaluate the change in bond strength over time. The laboratory-performance of tack coat materials was quantified based on interlayer shear strength (ISS), strain, k modulus, energy to peak stress, and the type of failure of core samples. Samples collected after one year showed higher ISS than baseline cores. Lab conditioned cores had higher ISS than baseline cores but lower ISS than year one cores. The increase in bond strength after one year and lab conditioning can be attributed to the continuous curing of tack coat materials. The failure type of bond strength samples was classified into two types according to shape and location of the failure surface: Type A (clean failure at the tack coat surface) and Type B (failure partly in the mix). Failure Type B indicates that the tack coat material can successfully provide enough bond strength to make the two asphalt concrete lifts behave as one thick homogenous layer. Although TackMaxTM and Colasphalt Tack had lower ISS values than SS-1 NB (50-50W) and SS-1 (30-70W), TackMaxTM and Colasphalt Tack showed stronger type of failure (Type B). Therefore, failure mode should be considered when evaluating bond strength of cores in addition to ISS value. The energy required to reach the peak shear stress is a comprehensive parameter that accounts for both the applied stress and the amount of deformation that the sample undergoes before reaching bond failure. Results show that the ranking of tack coat materials varies if the energy values are used as the ranking criterion instead of ISS values. Therefore, the energy required to reach peak shear stress is a significant parameter that should also be considered when evaluating tack coat materials. Overall, SS-1h, MS-1, CSS-1h, and the 3 proprietary products showed better performance than SS-1 emulsion according to the test results of the baseline and year one cores