The Glue That Holds it Together

Back of a truck applying tack coat

In my previous days as a home builder, I once saw a plumber break through a roof: shingles, roof paper, and a single layer of a ⅜ inch fiberboard. His leg went through the material to well above his knee and, after much ado, they took him off in an ambulance with a severely broken leg. The man’s weight, with his tools, were too much for the materials on the roof to support. The shingles and the roof paper contributed little to the strength of the roof itself; leaving only the ⅜ inch layer of fiberboard. The strength of the fiberboard sheathing was limited due to its uniform structure and thickness. Typically, you can get more strength out of a multi-layered material like plywood.

Plywood is typically stronger than the uniform fiberboard because the bonding of the thin layers of wood allows the layers to act as a single unit. This is done by the use of glue designed to hold the thin layers of wood together. If this glue does not hold, the sheet becomes weak, as there is no connection between the layers, and they act independently.

The lesson here—without the bonding between layers, the strength of the independent layers do not have the strength of the glued layers working as a single unit.

In the highway world, this layer bonding principle can also be applied between layers of asphalt. Each layer of asphalt in a road is equivalent to the multiple layers in a sheet of plywood. With asphalt, the layers do work together in a limited way without any bonding between them; by the force of weight and friction between the layers. However, if they have not been “glued” together they are probably not providing the greatest strength possible for the pavement surface. When this occurs, a couple things can happen.

Portion of a road showing delanination and slipping

The use of a tack coat can reduce this type of surface layer damage by improving the bond to the lower layer.

To provide the greatest strength and durability to an asphalt pavement surface, use a tack coat. Tack coat is applied on the existing pavement surface, just prior to paving another asphalt layer over it. The tack coat acts like the glue in the plywood and holds the two asphalt layers together so they can work as a single unit, providing additional strength to the load capacity and increasing the durability of the road surface.

What should we know about Tack Coat?

As with plywood, bonding is important to the strength and durability of multiple layers of materials. Some things to keep in mind to improve the tack coat bonding is to keep the surface free and clear of debris and dirt, and make certain it is dry before application. Milling the surface has also been found to improve the bonding by increasing the roughness of the bonding surface. It also improves shear, or sliding, resistance between layers. However, if you are milling and you have sidewalk/road crossings, be advised that you are required to upgrade your pedestrian crossings for compliance with the ADA at the same time, which can add an additional 10–30% to the cost of the project.

When you are applying tack coat, use the proper application rate and make sure the application is consistent. Too much tack coat and you will get bleeding in your asphalt. The excess asphalt will work itself to the surface, particularly in the hot weather causing slick patches of gooey asphalt. Too little tack coat and you will fail to get the bonding strength and the layers will act independently. When you tack coat, apply it to all bonding surfaces, including longitudinal and transverse joints, concrete curbs and concrete infrastructure. This will allow the asphalt material to bond reducing the potential for cracking at these joints.

Proper application rates, measured in gallons per square yard, will often depend on the type of surface that is to be covered. The smoother the surface the less you will need. The amount used will vary from a new, existing, milled, or Portland cement concrete surface.

Close up of a tack coat sprayer

 

Typical Application Rates

Surface Type

Residual Application Rates (gal/yd2)

New Asphalt

0.020–0.045

Existing Asphalt

0.040–0.070

Milled Surface

0.040–0.080

Portland Cement Concrete

0.030–0.050

The application will need to be consistent, to allow the full attachment of the layers. In addition, it is important to limit travel over the tack coat once applied. Too much traffic, both regular traffic and construction vehicle traffic, can remove the coating, or introduce and hold dirt to the surface, defeating its ability to adhere and bond.

According to the NCHPR Report 712, emulsified asphalts are the most widely used tack coat material. An emulsion is a mixture of asphalt cement injected with water and soaps at high pressure. Over time the water and the soaps evaporate out of the mixture leaving only the asphalt. The use of an emulsion allows for a more fluid material that provides an even and consistent application. These emulsions can be either anionic, negatively charged, or cationic, positively charged. Each of these types are classified into three categories: rapid setting (RS), medium setting (MS), and slow setting (SS), depending on how quick the emulsion “breaks” or returns to only asphalt from the emulsion state. The selection of the proper tack coat material will depend on the existing and proposed materials to be bonded, and the application rate. Installation of the asphalt overlay should be done soon after the tack coat has broken. This can be seen when the material turns from a dirty brown to black.

View down a road that has been freshly tack coated

 

Construction Best Practices

Is it worth the extra?

The question of whether or not to tack comes up occasionally. Often it is an item that is removed from a contract to save money. As we have seen, there is a benefit to using a tack coat when paving to improve the bonding of the asphalt layers and, in turn, strengthen and provide more durability to the asphalt investment. When the bonding does not occur, the damage, whether it is delamination, slipping, or some other distress, comes at the cost of the previous project, which includes the planning, materials, and the labor used to install the asphalt pavement. When this type of damage occurs, the options are limited and typically result in the surface being milled off. So the total cost of not using a tack coat could not only include the initial expense of the project, but also could include the expense of removing the previous failed surface. This could run into the millions on larger projects. In studies conducted by the Asphalt Institute, the cost of applying a tack coat in new or reconstruction projects can be in the range of 0.1–0.2 percent of the total project costs, and about 1.0–1.5 percent of the total pavement costs. In a mill and overlay project, a tack coat will be approximately 1.0–2.0 percent of the project cost and 1.0–2.5 percent of the total pavement costs. Maybe the question should be “how can I afford not to use a tack coat?”

Resources

 

Fall 2015

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This work by the Cornell Local Roads Program (CLRP) is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.