United States Department of Transportation
i
Performance evaluation and specification of trackless tack : [project summary].
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2016-09-01
[PDF-609.86 KB]
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Abstract:Researchers compared two tracking resistance tests for tack: a track-free time test and a modified dynamic shear rheometer (DSR) tackiness test.
Researchers compared four bond strength tests: interface shear, pull-off, torque, and Arcan. Then, using the recommended test, the researchers compared the bond strengths and bond energies achieved with different trackless tack types, surface types, reactivation temperatures (average of surface and HMA temperature), and compaction efforts. Researchers also assessed the susceptibility of bonded samples to cracking using the overlay and beam fatigue tests.
On three overlay projects, located on US 183, SH 336, and US 96, test sections were constructed with different tack types, application rates, and surface types. The researchers collected cores and measured bond strength in the lab. The researchers also measured core bond strengths from a wide range of overlay projects in the Laredo District.
The researchers recommend adopting the DSR tackiness test and track-free time test to qualify trackless tack materials. The researchers also recommend adopting the shear bond strength test. Draft test methods and a trackless tack material specification are provided in the full report.
The laboratory results suggest that all the trackless tack products can produce high bond strengths. Several other factors, not just tack type, have an equal or greater impact on overall bond strength. The highest bond strengths are achieved between overlays over new or milled HMA and when compacting at higher temperatures. Overlay designs with higher binder contents may also improve bond strength. Tack application rates did not appear to have a significant effect on bond strength.
This study did not consider long-term performance. Tack type and application rate may be more significant over time, mitigating moisture-related damage and increasing bond strength through age hardening.
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