United States Department of Transportation
i
Performance evaluation and specification of trackless tack.
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2016-09-01
[PDF-3.32 MB]
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Abstract:Several trackless tack products have come to market in Texas; however, there are currently no specifications to
ensure the products have trackless properties and adequate bond strength. The objectives of this project were to
(1) evaluate the tracking resistance of different trackless tacks, (2) evaluate bond strength and other construction
parameters of different trackless tacks, (3) construct test sections in the field to evaluate performance, and
(4) develop test procedures and specifications for trackless tack.
For tracking resistance, a track-free time test and a dynamic shear rheometer (DSR) tackiness test both
distinguished between trackless tack and conventional tack. The DSR test further distinguished among stiffresidue
and soft-residue trackless tacks. For bond strength of laboratory samples, all samples had acceptable
bonding, but stiff-residue trackless tack had the highest bond energy, followed by soft-residue trackless tack,
conventional tack, and then no tack. Higher ambient and hot mix asphalt (HMA) compaction temperatures
improved bonding. Bonded trackless tack samples were resistant to fatigue cracking and cold temperature
delamination. Bond strengths from field samples were considerably lower (15–95 psi) than for lab-molded
samples (100–200 psi) and varied among different overlay projects. This was likely due to different project
conditions (e.g., pavement surfaces, HMA overlay designs, compaction temperatures). In most cases tack rate
did not affect the bond strength.
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.
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