When is a Tack Coat Necessary?
We have all seen areas of spalled pavement, with older layers of asphalt concrete showing through. They may not all be as bad as the example below, but they certainly make it rough going for drivers, driving up vehicle operating expenses. What causes the spalling?
Weather and traffic play a role, but the root cause is the lack of a tack coat prior to construction of the overlay. A tack coat is used to help the overlay adhere to the underlying, weathered pavement surface.
Normally, when a new layer of hot-mix or coldmix asphalt is placed, the surface of every rock and grain of sand is uniformly coated with a film of asphalt. The initial film thickness is only a couple thousandths of an inch thick. Over time, sunlight hardens the surface films and makes the asphalt less sticky. Traffic abrades the surface, and water helps to remove the coating. In one or two years the asphalt film is worn off the surface, leaving the aggregate exposed.
From that point forward, there is nothing there to bond a new layer of asphalt concrete to the old one. A tack coat replaces the worn away asphalt and promotes bonding with the old pavement.
What is a “Tack Coat?”
A tack coat is sprayed on the surface of an existing asphalt or concrete pavement by a distributor truck immediately prior to placing an overlay. The goal is to achieve uniform coverage over the entire surface to be paved. Once the tack coat is in place it should be given time to cure and become tacky. Normally this would only require a few minutes.
Traffic should NOT be allowed on a tack coat, to avoid getting dust and dirt on the surface.
Until a few years ago the most commonly used material for tack coats was MS-30 or MS-70 cutback asphalt. Today, in many areas the environmental agencies no longer allow cutbacks to be used due to a concern about the hydrocarbons they contain, such as kerosene, which evaporate into the atmosphere. A satisfactory substitute for cutbacks was not easy to find. Some companies have developed proprietary compounds, based on resins or less volatile hydrocarbons. Many of these work quite well.
Today most state DOT Standard Specifications have requirements for tack coat materials, their method of application, and payment. These usually involve the use of diluted asphalt emulsions. In Section 702 of the 2008 NYSDOT Standard Specifications, subsection 6, Table 702-9 lists the approved grades of anionic and cationic asphalt emulsions for use in tack coats. Table 702-10 lists the required composition. All of the grades incorporate hard base asphalt. The anionic grades are slightly preferred where limestone and dolomitic aggregates are exposed on the old road surface. Cationic grades would be preferred for all other aggregate types.
To meet the requirements of Table 702-10 the asphalt emulsion is diluted 50-50 with water. The pavement surface should be thoroughly cleaned by brooming prior to the placement of a tack coat. The application rate varies with the type of surface being treated. Recommended application rates and construction details are described in Section 407 of the 2008 NYSDOT Standard Specifications.
This photo shows a four-inch diameter core hole in an older asphalt concrete surface. The black tack coat is clearly visible at the interface between the top layer and an old asphalt concrete layer below. The heat generated during the coring probably melted the tack coat, causing it to smear over the core hole and appear thicker than it really is. The properly applied tack coat has bonded the upper layer to the lower one.
Too much tack coat can cause more problems than it solves. It cures slowly, delaying paving and causing prolonged traffic congestion. It puddles in the low areas. Too much tack can actually foster slippage between the layers that are supposed to be bonded. It is important to use the proper material at the proper application rate for the pavement conditions.
How to Decide Whether a Tack Coat is Needed
Most standard specifications tell you what to do after you have decided that a tack coat is necessary. But how do you make that decision?
Some agencies say that tack coats are cheap insurance and should be used in nearly all circumstances. Others recognize that when a thick overlay is placed (say, four inches or more) it may be possible to skip the tack coat. However, as the layer thickness goes up, the percentage of the job cost that goes into the tack coat becomes inconsequential, so I tend to agree with the “all circumstances” point of view.
The photo above shows a state highway where two one-inch HMAC overlays were placed about 10 years apart. Tack coats were not used for either overlay. A few months after the more recent construction, surface raveling began to appear in the outer wheelpath. Wetness during late spring accelerated the stripping, and both overlays were completely removed in a matter of days.
Needless to say, it was a teeth-jarring experience to hit the defect at highway speed.
How Can You Repair the Raveled Pavement?
An isolated problem, as in the last picture, is easy to repair using proper techniques for a permanent pothole patch. Wait for good weather; square up the hole and dry it out; prime the bottom and sides with asphalt emulsion; fill the hole with hot-mix; and compact it thoroughly. The area is structurally sound, so all you need to do is repair the defect.
For a more extensive problem, as in the first photo, a more costly repair is needed to be effective. A cursory repair may solve the problem for a short period of time, but it will eventually come right back and continue to spread. Careful examination of the photo shows that perhaps as many as three skin patches have been applied, and all of them are raveling.
A permanent repair will require a structural overlay or removal and replacement of all of the asphalt below the depth of the ravelling. This would be two or three inches of hot-mix as a minimum. First the entire area should be thoroughly cleaned with a rotary broom. Then a tack coat is applied. To fill the depressions, a truing and leveling (T&L) course is applied and thoroughly compacted. Then the structural overlay is placed, preferably on the same day as the T&L course. If traffic is allowed on the T&L course, a new tack coat is needed before placing the overlay.
Why use the T&L course? If you only put down the overlay, some areas of the uncompacted mat will be thicker than others. During compaction, the thicker areas will compact more than the thinner areas. This will leave a rippled effect on the surface, mimicking the raveled surface. If the T&L course is applied first, both the uncompacted and the compacted overlay will be of uniform thickness, and the rippling will be avoided.
Note that a tack coat was required during the repair. Had it been used during the original overlay construction, the raveling could have been prevented, and the cost of the more extensive repair might have been avoided entirely.
Tack coats don’t cost much, but they perform a very valuable function. Generally speaking, you don’t save money in the long run when you skip that step. So when you are paving over an old, weathered surface, use a tack coat!