Heat the Longitudinal Joint for Improved Density
Paving Consultant Bryce Wuori of Wuori Consulting, Bismarck, North Dakota, estimates that roughly half the jobs he consults on face a longitudinal joint density specification. “If you include the airport jobs I’m on, it’s up to 70 percent,” he said.
Wuori said that the majority of states in which he works now have incentives for density on the longitudinal joint, anywhere from $0.60 to $1 per linear foot. Not only can that add up to quite a bit of incentive for contractors, Wuori said, but improving density on the longitudinal joint also produces a better road for the owner. “The longitudinal joint is typically the first place that fails,” he said, “so if you can make that last as long as the rest of the road, you’re going to get longer lasting roads.”
“Pavement deterioration always starts with cracking,” said Michael Blake, marketing manager at KM International, North Branch, Michigan. “And the main cause of cracking is water penetration, which usually starts at the centerline joint.”
Density issues at the longitudinal joint are nothing new, Wuori said. What is new is the growing number of agencies specifying longitudinal joint density. In the next three to five years, Wuori believes almost all jobs will specify longitudinal joint density.
“In the past, we didn’t have the tools to achieve better density at the longitudinal joint,” Wuori said. “Now that contractors have the tools available to improve joint density, agencies are feeling more comfortable implementing density specifications because contractors have a good chance of achieving them.”
Methods to Improve Joint Density
Although longitudinal joint density specifications are becoming more common, Wuori said the process by which a contractor achieves density on the longitudinal joint often isn’t specified. One solution is the process of heating the longitudinal joint during construction.
“When you reheat the joint, you’re making the bitumen and aggregates thermal again so their ability to bond increases,” Wuori said, which decreases air voids at the joint.
“By heating the joint, you are ensuring the joint is around 300 degrees Fahrenheit at compaction, instead of 80 to 100,” Blake said. With an infrared heater, he added, contractors can install hot asphalt against hot asphalt, “which is going to create a seamless weld between the two lanes.”
Although Wuori most often recommends to his clients the use of a notched wedge to improve joint density, he said heating the joint is also a good option.
“The notched wedge is the cheapest and easiest option and one that almost always allows the contractor to hit that joint density,” Wuori said. However, he added, “from what I’ve seen, if you can reheat the joint you’re going to see better, more consistent density than with a notched wedge.”
Wuori gave this theoretical comparison: If a contractor paved a 2-inch lift with a standard dense-graded Superpave mix with a standard vertical joint, Wuori estimates they might hit the average longitudinal joint density spec of 90.5 percent density half the time. All else equal, if the contractor added a double shot of tack or J-Band at the joint (which Wuori requires on all his projects), he estimates they could hit that density 70 percent of the time. If they used a notched wedge, he estimates they could hit density 90 percent of the time.
If they use both a double shot of tack or J-Band plus a notched wedge joint, Wuori estimates the contractor could be successful 100 percent of the time. He estimates the use of a double shot of tack or J-Band plus reheating would be similarly successful. And, if they used a double shot of tack or J-Band, plus a notched wedge joint, and reheating, “they will max out the incentive for longitudinal joint density every time,” he said.
Deciding when to use which option comes down to the incentives available on the job, Wuori added, “because it’s quite a bit more expensive to reheat per foot than to do a notched wedge.”
However, Wuori has been on a number of projects—all airports—that have used both a notched wedge and heat on the joint. “I think we might see the use of both across other jobs in our industry in a decade,” he said. “It’s only a matter of time before contractors will have to use several tools to be successful as specs are getting increasingly more difficult.”
Best Practices of Heating the Joint
Wuori has seen all types of technologies used to heat the longitudinal joint, with paver-mounted infrared heaters being one of the most effective, particularly for mainline highway paving. At airports and jobs with smaller pulls, he’s seen other types of heat sources like those mounted on trucks.
Blake estimates that the industry is probably five to seven years from widespread adoption of paver-mounted infrared heaters. KM International’s infrared heaters are usually pulled around by hand since they are most often used to reheat a longitudinal joint after construction.
“In most cases, a contractor will take our infrared heater to the job site after the job has been fully completed and then reheat and recompact the longitudinal joint to get better density,” Blake said. “A common scenario we see for pavers is a customer holding a check for a project because there is an issue with the longitudinal joint.”
With KM International’s infrared heater, the contractor can reheat and re-compact that joint, which Blake said should create a thermal bond between the two lanes and reduce the likelihood of any water penetration and premature cracking, if properly performed.
Whether heating or reheating the joint, it’s important to ensure you don’t burn the asphalt. “You don’t typically see issues with burning the asphalt unless the paver is moving really slowly and it is an extremely hot day,” Wuori said, but added that a good rule of thumb is never to exceed the mixing temperature for the particular mix design being laid. He also recommends never heating the joint above the temperature of what’s coming out of the paver.
Wuori also said it’s important to be patient with new technologies and to take the time to properly train whomever you’re going to put in charge of monitoring these new technologies. “They need to know how to set it up and how to use it correctly, but also how it works,” he said. “Training that person in such a way that they understand how important this is will help them to respect it a bit more.”
Wuori stresses the importance of setting up the system correctly and having someone knowledgeable monitoring the heating system with a temperature gun to manage the process correctly.
Wuori also reminds us that it will be a learning experience. “More goes into it than just getting the tools,” he said. “But once you figure it out, you can easily use those tools on one project to the next.”
Here’s How J-Band Can Help
Void reducing asphalt membranes (VRAM) beneath the longitudinal joint of a course of hot-mix asphalt (HMA) pavement are designed to assist in joint construction. One such VRAM is J-Band, a hot polymer-modified asphalt product from Asphalt Materials Inc., Indianapolis.
“VRAM is a materials solution to centerline permeability and joint density issues,” said Jeffrey Ball, executive director of marketing and communications for Asphalt Materials. “It’s like an insurance policy to minimize the need for agencies to have to repair these joints because of longitudinal cracking, potholes, raveling, etc.”
J-Band is applied by a distributor truck immediately before paving, usually as an 18-inch-wide strip under the eventual location of the longitudinal joint along the final lift. As HMA is applied, J-Band will migrate upward through the new HMA to fill voids up to 75 percent of the overlay height to fend off future water infiltration.
“The heat of the hot mix activates the VRAM,” Ball said. “The heat along with the pressure of the rollers helps the VRAM migrate up into the hot mix. If you core the longitudinal joint area and test the permeability, you’ll see that it’s much lower because the VRAM has filled those air voids.” This process can help roads last up to five years longer, Ball added.