I think asphalt tack is often an overlooked part of asphalt paving. Bad mistake! A good tack or bond coat is critical for optimal performance. “Tack” is the traditional term, but some people prefer “bond;” the terms are interchangeable.
Why is tack important? The short answer is “pavement performance.” Most people recognize slippage failures where tack was applied poorly or the pavement was dirty, but tack influences the load-carrying capacity of the overall pavement structure. Research has shown that a good tack or bond coat can significantly influence the pavement performance. A poor tack coat results in less bonding between pavement layers and can decrease the structural capacity. When the pavement layers separate (de-bond) due to tack failure, fatigue cracking can occur. One study found that a 10 percent decrease in bonding strength between two pavement layers could cause an approximate 50 percent loss in fatigue life.
Tack basics
There are some basic considerations for tack coats, including:
Materials
Asphalt emulsions are widely used for tack coats, with slow setting (SS) grades most commonly specified. Emulsions are used because they can be diluted (one part water to one part emulsion) for uniform spray application. Some agencies choose paving grade asphalt binders.
Application rate
A tack coat should be applied to a clean, dry surface and is typically recommended for all overlays. The appropriate application rate depends on the type and condition of the old surface. A thin, uniform coating should be applied to 90 to 100 percent of the old surface. For emulsions, the normal application rate ranges from 0.5 to 0.15 gallons per square yard. The rate may need to be adjusted for old (versus new) surfaces, for milled surfaces, and when mixes include highly absorptive aggregates or high RAP contents. Also, be aware that too much tack can lead to slippage or bleeding.
Application technique
Most tack failures are the result of poor application technique. A thin, uniform coating is critical. Having clean spray nozzles set at the correct spacing and setting the spray bar at the proper height are basic – but key points.
Innovations
A couple of recent innovations can help with getting good bonding. These include:
Trackless tack
This relatively new, proprietary material is a specially formulated asphalt emulsion that dries to a drivable, no pick-up condition, in as little as 10 minutes. Some producers claim that their trackless tack can help achieve higher density with less compactive effort.
Spray pavers
These specialty pavers spray the tack and apply the overlay mix in a single pass, so no vehicles travel on the emulsion, thus eliminating tack pick-up and damage to the bond coat. A spray bar in front of the paver’s auger distributes the tack material and a built-in microprocessor controls the application rate. In addition to a better bond, these pavers can save work time and minimize construction delays.
While many agencies have made tack “incidental” to the bid price, it is an essential consideration for optimal pavement performance.
You can contact Dwight to talk tack.
