Yes, No or Maybe.
By Brian Hall
Since the main thrust of our articles have been heavily focused on “Best Practices” (Whatever that is), I thought we would spend some time discussing one of the items that gets more overlooked, but gets most of the blame for failure. Emulsions, or tack as we like to call it, is the bond that adheres the old asphalt to the new. How much, or how little to use seems to be a topic of debate on the job site. While many requirements call for a bonding coat, rarely do we see anything more specific than that, especially on commercial jobs. Today’s asphalt jobs are created in layers, much like a strong sheet of plywood. Like plywood, asphalt is built up in layers to create a strong final product and need a layer of “glue” between each layer to ensure its strength and durability.
Let’s start with the makeup of tack. In simple terms, you are looking at two insoluble components, oil and water. The water surrounds the oil and allows the expensive asphalt to sprayed thinner and at a lower temperature than hotter liquid asphalts. As the paving process ensues, the water in the mixture evaporates, leaving only the sticky asphalt that will bond the two surfaces together. You may be familiar with the term “break”. Breaking refers to the point where water is evaporating from the mixture. I have heard some experts contend that you need to wait until the emulsion breaks to pave, but I disagree. Take for example the paving machines that spray tack one foot in front of the screed.
Now, what kind of equipment do we need? I have seen everything from pine tops to watering cans to disperse emulsion. Not only is this bad practice, but it is quite unsafe. Either a tack distributor or a truck mounted asphalt distributor will work perfectly. The truck mounted unit employs an onboard computer that will automatically control your usage as well as heat the material quickly. It will also allow you to spread tack over a wider surface and typically will include a spray wand to reach difficult areas. These units range in capacity up to 4000 gallons and can be used for many liquid asphalt products that need to be heated in excess of 400 degrees F. Trailer mounted “tack tanks” are easier to manage for smaller jobs and usually range from 150 to 500 gallons. The limitations to these units are that they can only spray emulsion. Units are equipped with a handspray wand and a spraybar, if desired. The main consideration is that you will never heat your emulsion to over 212 degrees, since it is water based. Typical spray temps are between 150 and 175 degrees. Above 180 degrees the material starts to lose water and if the material falls to under 120 degrees, you’ll have trouble moving the material through the pumps and nozzles.
The most misunderstood concept of tack coating is how much to use. Everyone agrees that it is an essential practice but the question of how much and when to use still elude contractors. I have been on countless jobsites where the crew sprinkles just enough to say that they tacked the surface. Months go by and the surface is raveling and slipping and the contractor is scratching their head and losing profit due to warranty repairs. Your rule of thumb should be that 95% of your surface must be tack coated for maximum efficiency. Significantly less than that and you risk failure through raveling, slippage or potholes. Excessive amounts of tack will bleed through the asphalt layer and compromise your friction surface. When covering old, oxidized asphalt, your coverage rate should be .05 gallons per square yard. At this application rate you are looking at around 6 ounces of tack over a 3 foot by 3 foot area. If you are required to tack between new lifts of asphalt, you can cut back to .03 gallons per square yard, keeping your 95% coverage area. In case you were wondering, this compares to 40 or 50 ounces per square yard for a chip seal application.
I know it’s hard to make tack coating interesting, but considering the cost of re-visiting the jobsite months down the road and repairing failed asphalt, the cost of that little bit of emulsion certainly is a great insurance policy.
Brian Hall, LeeBoy Territory Manager. He can be reached via email at firstname.lastname@example.org