The Importance of the Tack Coat: The Glue That Makes Milling Work

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 In the process of  asphalt milling services, there is a step that takes only minutes but determines the success of the entire project for the next 20 years. It happens after the milling machine leaves and before the paver arrives. It is the application of the Tack Coat.

Many property owners watch the paving crew and wonder, "Why are they spraying that black liquid on the ground?" That liquid is the glue that holds your driveway together. At Black Diamond Paving & Construction, we never skip or skimp on the tack coat. Here is the science behind this vital component.

What is Tack Coat? Tack coat is a liquefied asphalt emulsion—essentially diluted bitumen. It acts as a bonding agent.

The Function: It bonds the new layer of hot asphalt to the existing (milled) surface below. Without it, the two layers would just sit on top of each other like two slices of bread. With it, they fuse into a single structural unit.
Why Milling + Tack = Strength Milling creates a textured, grooved surface. This increases the surface area. When we spray tack coat into these grooves, it acts like the "teeth" of a zipper.

Shear Strength: Vehicles stopping and starting exert "shear force"—a horizontal push on the pavement. If the layers aren't bonded, the top layer will slide. This creates "shoving" (ripples in the asphalt) or "delamination" (where chunks of the top layer peel off).
Waterproofing: A good tack coat also helps seal the surface below, preventing water from seeping between the layers.
Application Best Practices Applying tack coat is an art.

Cleaning is Key: You cannot glue a dirty surface. Before tacking, we use power sweepers and blowers to remove every speck of dust from the milling process. If there is dust, the tack coat sticks to the dust, not the road.
Coverage Rate: It must be applied uniformly. Too little, and the bond fails. Too much, and the oil bleeds through the new asphalt, creating soft spots. We use calibrated spray trucks to ensure perfect coverage.
Curing Time: The emulsion is brown when sprayed and turns black as the water evaporates ("breaking"). We must wait for it to "break" and become sticky before paving. Paving too soon traps water; paving too late allows dust to settle on the glue.
The "Tracking" Issue One challenge with tack coat is that it is incredibly sticky. If a car drives on it before we pave, it will pick up the oil and track it onto concrete curbs or sidewalks (and it is very hard to remove).

Site Control: This is why we are strict about closing off the work zone. Keeping traffic off the tack coat is essential for the bond quality and the cleanliness of the site.
Conclusion The tack coat is the unsung hero of pavement restoration. It ensures that your milled surface and your new asphalt act as one cohesive slab. At Black Diamond Paving, we pay attention to the sticky details so you don't have to worry about delamination down the road.