Bond Expectations for Milled Surfaces and Typical Tack Coat Materials Used in Virginia

About the report:

The ultimate purpose of the program of research of which this study was a part is to identify a test method and acceptance criteria for bonding of HMA layers.  In this study, three tasks were performed to help achieve that purpose:

1. A laboratory comparison of the bond strength of typical tack materials
2. A field study of the effect of tack on bond strength between a new HMA overlay and a milled surface
3. A laboratory investigation of a torque-shear field test to measure bond performance.

The findings from these tasks led to several recommendations.  The most substantial of the recommendations was that the Virginia Department of Transportation (VDOT) should no longer require the practice of tacking primary horizontal surfaces when placing a new overlay on a milled surface.  Further, field engineers and inspectors should remain diligent when it comes to establishing a clean, sound construction platform for any overlay work.  Finally, although laboratory tests were inclusive, the torque-shear device should be field tested, with particular emphasis on exposing the test to more realistic conditions.

An analysis of the costs associated with conventional tack use found that the material cost per lane-mile is between $572 and $836.  A review of the typical “mill and fill” paving activity (maintenance) for the 2008 season found that VDOT would save between $488,000 and $650,000 per year by foregoing conventional tacking on milled horizontal surfaces.  If the tacking material is of the non-tracking variety, which is becoming more and more common, the savings could be as much as $950,000 per year.

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