Why should I use Tack Coat and is it worth the extra cost?
The use of tack coat allows the existing asphalt or Portland cement concrete pavement material to bond with the new asphalt concrete material. It should be used whether the existing surface has been milled or not.
Applying a tack coat between the layers of existing asphalt, Portland cement concrete pavement or fresh asphalt concrete and a new layer of asphalt concrete serves to “glue” the layers together much like the layers of sheathing in a sheet of plywood. By gluing the layers together it is possible to increase the strength of the multiple materials and allow them to act as one. This increases the load capacity of the road surface and extends the life of the road.
If the multiple surfaces are not bonded there is often an increase in the fatigue failure (alligator cracking), rutting, slippage and shoving. Another common failure is delamination, where the top coat separates and “pops out” in pieces creating shallow holes in the pavement surface.
Is it worth the cost?
Information from the Asphalt Institute indicates that the benefits from the use of tack coat far outweigh the costs of not using it. From their studies, the use of tack coat in a new or reconstruction project is approximately 0.1–0.2% of the total project cost and about 1.0–1.5% of the total pavement costs. In Mill and Overlay projects, the use of a tack coat is approximately 1.0–2.0% of the total project cost and 1.0–2.5% of the total pavement cost. While the estimated cost of bond failure from not using a tack coat in the top layer only can range from 30–100% of the original pavement costs.