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Tack Coat

Tack coat is important to constructing durable long-lasting asphalt pavements. Tack coat adheres to the new layer of asphalt to the underlying layer. When layers have properly adhered, they act as one cohesive unit. If adhesion is not successful, the layers act independently. For example, two two-inch lifts of asphalt should deliver the structural value of a full four inches. If not properly adhered to, the structural value will be compromised.

What are the three things you need to know when working with a tack coat?

  1. Cleanliness is next to godliness. When applying a tack coat, if the surface isn't clean and dry, the tack will adhere to the dirt and dust. In a milling situation, the best way to ensure a clean surface is to let vehicle traffic displace the dust and dirt. Clear the milled surface of rocks and other debris before allowing vehicle traffic. After applying tack coat, ensure that dirt and dust do not get onto the tack coat before it is overlaid. Using less tracking or trackless tack can help prevent construction traffic from degrading the tack before it is overlaid.
  2. Tack coat must be applied uniformly using either a distributor or a spray paver. If using a distributor, make sure it is outfitted with the correct nozzles and that they are adjusted to the proper pressure. Likewise, ensure proper pressure and correct nozzles when applying tack coat as part of the laydown operation using a spray paver. An uneven distribution such as drizzling the tack coat on the surface will not create proper adhesion and will likely result in premature failure.
  3. Too much tack coat is just as bad as not enough tack coat. The proper tack coat coverage creates a uniformly thin layer. If the tack coat is pooling or running there is too much. If the surface is a seal coat or the structure is bleeding or fleshing, the tack coat rate should be reduced. On an underseal project, tack coat is already present, therefore it is important to know how much more is needed. For best accuracy, run a test strip to determine shear strengths. This will reveal whether there is proper adhesion and therefore the right amount of tack.

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