With the success of multiple test sections incorporating tack for bonding purposes at the National Center for Asphalt Technology (NCAT) Test Track in Auburn, Ala., contractors are likely to see an uptick in the call for tack between pavement layers. From emulsion-based products you’ll want to spray during daylight hours for best chance of break to trackless tacks that incorporate careful suspensions, the products available perform their best when shot with best practices. We’ve covered the routine care and maintenance of the tack truck in the Keep It Up department recently; now let’s take a look at the truck’s use in different situations to avoid a tacky mess.
The National Center for Highway Research Program (NCHRP) Report 712: Optimization of Tack Coat for HMA Placement suggests that if you have an older, highly oxidized pavement, the surface may require a higher rate of tack application. This makes sense. Consider what happens to a 15-year-old asphalt road that’s never seen preventive maintenance. If a community decides to add a protective sealcoat to the pavement, depending on the level of oxidation the roadway sucks in that first coat of sealer. A second coat is vital to providing a good seal of microscopic voids. The same holds true when shooting tack for a good bond. You’re not out to make emulsion soup, but you want a coat of material that doesn’t immediately get absorbed, leaving only traces of adhesiveness behind.
The Florida Department of Transportation (FDOT) specifies tack will be sprayed only when ambient temperatures are at 40 degrees F and rising, but it has different rates of application depending on the age/oxidation and type of pavement to be sprayed. In the state of Florida, you’re more apt to find a pavement made of an open-graded friction course (OGFC) than you are in Minnesota. When the underlying pavement surface is a newly constructed OGFC, the target tack rate is 0.05 gallons per square yard (gal/yd2), give or take 0.01 gal/yd2. When it’s a milled OGFC, the target tack rate is 0.07 gal/yd2.
Not every pavement in Florida is open-graded, so the DOT has other application rate guidelines. For example, if the underlying pavement surface is newly constructed base course, structural course or dense-graded friction course, the target tack rate is 0.03 gal/yd2. If the underlying pavement surface is a milled surface or oxidized and cracked, the target tack rate is 0.06 gal/yd2. The state also recommends a target tack rate of 0.08 gal/yd2 when shooting on a concrete pavement, but this editor’s opinion is that gray mess probably needs to be milled off and replaced.*1
Next look at Michigan (MDOT) specs for tack shooting. Ambient temperatures there must be 60 degrees F and rising, and the rate of application is fairly uniform at 0.05 to 0.15 gal/yd2 for the products it approves. The differentiation comes in the guidance for product temperature. For example, MDOT Spec P-603 requires a product temp of 75 to 130 degrees for Emulsified Asphalt SS-1h and Emulsified Asphalt CSS-1h; and a product temp of 120 to 160 degrees for Cutback Asphalt RC-70.*2
“Each state has its own spec,” John Ball said. He’s the proprietor of Top Quality Paving, Manchester, N.H., and regularly consults with contractors to help them get tack coverage right. What he pointed out is not every contractor is following a state spec for a state job. When you’re working on a commercial or county project, you have to do your own homework and quality control. “You follow the tack manufacturer’s guidelines for ambient and product temperatures. If the ambient temperature is too cold, the tack coating will cool too fast and the emulsion won’t break. The ground temperature matters.”
What Ball’s referencing is the second law of thermodynamics. The hot tack will transfer its heat to the relatively cooler surface pavement—and to the air above—because it must. It’s the law.
“When we talk about heat transfer, the hotter mass is giving some of its heat to the cooler mass,” Bruce Wilson, O’Fallon, Mo., explained. He’s on the board of the American National Standards Institute and wrote the book on Design Dimensioning and Tolerancing used in college engineering courses around the United States. “We say that the hotter mass is becoming cooler by transferring some of its heat. In a perfect world, the two materials would eventually reach the same temperature and be in a state of equilibrium.”
“When it comes to the product temperature, typically you’re looking at 120 degrees for emulsion-based products,” Ball said. “There’s water in these products that has to evaporate off so it becomes adhesive. For asphalt-based products in a state like Georgia, you’re looking at temperatures of 400 degrees.”
Your tack truck driver doesn’t have to understand the laws of thermodynamics, but he does have to know when his product is ready to spray. Depending on the equipment you use, there will be a number of thermometers located in different places on the truck and tank. “There’s a dial on the outside of the tank that measures from zero to 300 to give the reading of the inside of the tank,” Ball said. “A glass stick thermometer goes inside through the manhole on top or through a port on the side to measure the material temperature.”
The Etnyre Centennial II tack truck pictured at right gives temperature readings in the control panel on the driver’s side, back of the tank. Brian Horner shared that the Centennial II has a temperature sensor located between the jacketing and the tank. It sends the temperature information to the control panel where the operator can read it, but what seems more important is the burner controls “sense” off the thermometer on the belly. The instructions posted right on the equipment state the operator is not to light the lower burner if the material within the tank is less than 650 gallons and not to light the upper burner if the material in the tank is less than 900 gallons, no matter what the temperature sensor is reading. Just a quick glance to the large dial on the left side of the back end of the tank will let the operator see how much product he has—it has a range of 64 to 2,000 U.S. gallons.
With temperatures and application rates in mind, there are some basic best practices that the authors of the NCHRP Report 712 discovered during their experiments. At this time, many state DOTs have adopted these practices/ideas. Here are some tips to make these ideas work for you.*3
- Dusty conditions will negatively affect your tack adhesion. Clean and sweep the existing pavement surface. The Michigan DOT directs contractors, “Immediately before applying the tack coat, the full width of surface to be treated shall be swept with a power broom and/or airblast to remove all loose dirt and other objectionable material.”
- Water will negatively affect the bonding interface. Clean and dry the existing pavement surface. For materials that require dilution with water, Michigan DOT spells out its expected practice. “Emulsified asphalt shall be diluted by the addition of water when directed by the Engineer and shall be applied a sufficient time in advance of the paver to ensure that all water has evaporated before any of the overlying mixture is placed on the tacked surface.”
- A milled surface provides a better/stronger interface than a smooth surface. Consider mill-and-fills rather than merely tacking and paving a thin layer on existing surfaces.
- For the tack products tested in the NCHRP research, each one reached its optimum ISS at a peak temperature, and then began to lose efficiency as the temperature continued to increase. Watch your product’s recommended temperature range(s) and stay within that range during application.
*1 Source: dot.state.fl.us
*2 Source: Michigan.gov
*3 Ideas 1 through 4 sourced from: Button, Joe; Elseifi, Mostafa A; Scherocman, James A; Mohammad, Louay N; Bae, Abraham; Patel, Nachiketa. Transportation Research Board “Summary .” NCHRP Report 712: Optimization of Tack Coat for HMA Placement. Washington, DC: The National Academies Press, 2012.