Which Tack Coat material should I use for an asphalt overlay?

The standard specifications addressing tack coat will be modified as per Engineering Instruction (EI) 12-022 Revised Standard Specifications for Section 407 - Tack Coat and 702 - Bituminous Materials, dated 9/11/2012. As of January 10, 2013, all projects conducted by the state or funded by State or Federal funds let out after this date will be required to comply with these changes.

The traditional tack coat has been diluted asphalt emulsions for conventional HMA paving and non-diluted asphalt emulsions for Polymer-Modified 6.3 mm HMA paving having been the norm for state projects. Section 407 - Tack Coat now consists of 2 items: "diluted tack coat" and "straight tack coat", indicating how they will be supplied as an emulsion.

Some of the changes in the EI include the increase in the maximum residual asphalt content to 50% in the diluted tack coat asphalt emulsion; and the elimination of the dilution of Rapid Set emulsions. Straight tack coat is a rapid set emulsion that is not further diluted and will use either RS-1h or CRS-1h with a minimum residual asphalt content of 55%-60%, respectively. A straight tack coat is required when applying a Polymer-Modified 6.3 mm HMA overlay, and is recommended for conventional overlays as well.

Additionally, application rates have been modified to reflect the use of a diluted or non-diluted tack coat and are included in Table 407-1 - Tack Coat Application Rates:

407-1-Tack Coat Application Rates
Surface Type Application Rate (gal/yd2)
Diluted Tack Coat
Straight Tack Coat
New Hot Mix Asphalt
0.05-0.06
0.03-0.04
Milled Surfaces and Existing Hot Mix Asphalt
0.05-0.06
0.03-0.04
Portland Cement Concrete
0.08-0.10
0.05-0.06
Vertical Surfaces (curbs, drainage structures, and appurtenances)
0.09-0.11
0.06-0.07

 

One very important point is that a tack coat should ALWAYS be used with an asphalt overlay.

Sources

EI 12-022 Revised Standard Specifications for Section 407 - Tack Coat and 702 Bituminous Materials, dated 9/11/2012

Cover of the Asphalt Paving Principles workbook

Asphalt Paving Principles (pdf)
CLRP workshop manual

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