Northeast Asphalt runs a Pave IR system in Wisconsin.
Anderson Brothers Construction Co. runs a Pave IR system in Minnesota. The system was specified into this project and others.
Here Granite Construction runs a Pave IR bar with 12 infrared thermometers.
Here Madden Contracting runs a Pave IR in Louisiana.
From Rhode Island to Minnesota and Utah, state transportation departments are taking the first steps to detect and reduce thermal segregation – in real time, not after the fact – in asphalt pavements. Thermal segregation refers to the differential cooling of asphalt mixtures during placement. The result is an asphalt mat with uneven temperature – cool spots. The causes go to the basics of asphalt paving: truck loading, hauling, unloading, an inconsistent flow of mix through the paver, stops and starts of the paving train, and so on.
Because compaction potential is based on mix temperature, cool patches are directly related to density problems in the finished pavement. After the pavement has cooled and stiffened, these patches are largely invisible, but they are weak points in the structure. Research in Washington state and elsewhere shows that thermal segregation can reduce the service life of an asphalt pavement by 40 to 50 percent.
Coring and density gauges have two major drawbacks. First, the only inspected areas are random samples that test a very small fraction of the whole pavement. Second, if a problem is detected, it is likely too late for corrective action.
Now, there’s a way to detect thermal segregation in real time – and take steps to reduce or stop it – during the paving process, before it’s too late. It is a piece of equipment called the Pave IR from MOBA Corp.
The system detects thermal segregation in the asphalt mat – in real time – and displays a thermal profile on a computer screen as the paver moves along. A thermal profile is recorded and can be used to train crews in the off-season.
“I am currently working with 18 different states to initiate demonstrations and pilot projects, and move toward writing a specification,” says Jim Hedderich, technical marketing specialist for MOBA.
Texas has a specification – and contractors there have put some 30 systems into action. Contractors in Minnesota and Washington have used the Pave IR on a case-by-case basis. Louisiana has written a specification. Ohio wrote a spec and then opted to experiment with the system.
Pave-IR’s basic components are a bar mounted across the rear of a paver with 12 or more infrared thermometers attached, a computer, a distance encoder, and a GPS antenna. The infrared thermometers measure mat temperature at 2- to 6-inch intervals as the paver moves along. The computer records those temperatures and matches them to distance measurements taken by the distance encoder and attaches a GPS location to each of them.
In addition to displaying real-time mat temperature data on the computer screen for the operator or QC manager, the system can provide a detailed, color-coded thermal profile, foot by foot, of your asphalt mat. The profile is stored in a memory stick and can be printed out or emailed to the office for review. Reports can be performed to show tabular details of temperatures and paver stops. Thermal segregation is visible by colors shown in the profile.
Spec’d in Texas
The Texas Department of Transportation (TxDOT) has written the system into a specification that incentivizes contractors to use it. Early on, TxDOT was concerned that contractors would fear the system would be used to shut down their operations. Instead, TxDOT is encouraging the use of the system.
Texas specifications do not require contractors to use the Pave-IR system, but if they do, they can pave at temperatures down to 32 degrees F if they can show the absence of thermal segregation. Without the Pave-IR system, contractors typically have to wait for ambient temperatures of 50, 60 or 70 degrees before paving.
When not using the Pave-IR system, TxDOT required a density profile to be performed with either a nuclear gauge or an impedance gauge, says Dale Rand, Director of the Flexible Pavements Branch, Construction Division, TxDOT. Density profiles are required every time the paver stops – a minimum of once per sub-lot and anytime the inspector detects thermal segregation.
“Under our new specifications, the contractor who has a Pave IR system does not have to run any density profiles,” says Rand. “Our standard specification says if they have severe thermal segregation, they automatically waive their production and placement bonus payments. Now, that does not happen if they use the Pave IR bar.
“So they don’t risk automatically waiving their bonus for having segregation or failing a density profile,” says Rand. “If a contractor using the Pave IR bar has recurring severe thermal segregation, he must take corrective action. If that fails, the state may suspend operations. (Texas defines moderate segregation as temperature differences of between 25 and 50 degrees. Severe thermal segregation is for differences of more than 50 degrees.)
“If a contractor is not using the Pave-IR bar, and has severe segregation detected by an infrared temperature gun, they automatically waive bonuses,” says Rand. “They have to run a density profile, and if that fails also, the pavement is subject to removal and replacement at the engineer’s discretion. The bottom line is that if a contractor elects to use the Pave-IR system, we are confident they will do a better job so we are willing to relax some of our other specification requirements,” says Rand.
Most contractors interviewed for this article gave the Pave IR system a thumbs-up review. Inland Asphalt, based in Richland, WA, bought a system that was specified and bid into a mill-and-fill project on SR 124 near Burbank, WA.
“We would definitely use it again,” says Chuck Barnes, general superintendent for Inland. “It gave us really good measurements of how far we went. That was great. And it told us the asphalt temperature everywhere across the mat. We didn’t have to use the temperature gun and constantly shoot everything.
“We ran it with a Roadtec Shuttle Buggy,” says Barnes. “So our delivery of mix to the paver was pretty consistent. I could see where if you didn’t run a Shuttle Buggy that you would really see a lot of variations in your mat.”
Northeast Asphalt, from Greenville, WI, bought two Pave IR systems more than a year ago, even though the state does not specify the Pave IR system.
“We typically don’t keep the system on one job for the entire project,” says John Bartoszek, regional manager for Northeast Asphalt and a sister company, Payne & Dolan. “We move the systems around and use them as quality control devices, to assure us of how our pavers are operating and to make sure we’re not getting thermal segregation. It also helps us make screed adjustments on our pavers.
“The Pave IR can tell us how many times a paver stops in a day and give us our average paving speed,” says Bartoszek. “We can see how consistent the paver speed is, and therefore we can figure how many tons per hour we’re averaging in a day, without looking back through a bunch of paperwork. If you look at one of the reports you get, you will see how many feet per minute the paver is moving. If you’re making a 12-foot pass, and paving three inches deep, you can convert over and see how many tons per hour you’re paving. That gives you a good idea for planning purposes, and also tells you how efficient you are in a certain area.”
Setting the bar high
Ervin Dukatz, PhD, P.E., is vice president of Materials and Research at Mathy Construction Co., Onalaska, WI, and the company bought a Pave IR system last year. “Our purpose was two-fold,” says Dukatz. “One is that we use it as a training tool, to give the crews visual feedback on what they’re doing. The other reason is that for our high visibility jobs, the system gives us that extra edge to help us pay attention to every detail so that we can maximize our compaction and ride values.”
Dukatz lists a series of projects for which Mathy used the system: in Wisconsin, Interstates I39 and I94, highways US 2, US 53, a SMA overlay project, and US 61, a 5-year warranty project, STH 40, a WMA overlay project; in Iowa Central City Cty 13; and Th 16 and 76 in Minnesota.
“The watchword on quality control is consistency,” says Dukatz. “Our goal is to set up the paving train so that when the mix comes out of the paver there is only one color showing on the thermal image display. We try to set our temperature gradients so that we have five degrees between colors. The goal for the crew is to maintain the paving operation so that they have no more than two colors – or no more than a 10-degree temperature differential from one side of the paver to the other side. That’s our goal.
“We have some crews that have taken it as a challenge, to try to get the temperature differential down to one digit!” says Dukatz. “So it is a real-time tool that the crews use to see how they’re doing.”
Just the use of a material transfer device alone does not eliminate thermal segregation, Dukatz says. “If the transfer device is not used properly the mix can become segregated, and that will show up on the Pave IR system very quickly.”
He says the keys to success are paving basics. Maintain a constant paver speed and keep the drag slats full. You should never see the asphalt tunnel on the paver while it is moving. Use the anti-segregation devices built into the paver by the manufacturer.
On Mathy’s U.S. 2 project in Wisconsin, the Pave IR system helped the crew improve IRI (International Roughness Index) ride numbers by nearly 6 inches per mile, Dukatz said.
“As the crew became better trained through the use of the Pave IR system, the total pavement smoothness including exceptions improved from an IRI of 42.3 inches per mile EB to 36.8 inches per mile WB. What was truly exciting was the 14 percent reduction in localized roughness penalties from the east to west bound lanes,” says Dukatz.
In Minnesota, this marks the third year that contractors have used the thermal imaging system, says Greg Johnson, assistant bituminous engineer for the Minnesota Department of Transportation. So far, the system has been used to gather information and do research.
“We have not put it in with a specification at this point,” says Johnson. “We are currently looking at the benefits of it and how we would put it in if we were to specify it at this point. We are evaluating how we want to use this technology in our state.”
Johnson reports that Anderson Brothers Construction Co., Brainerd, MN, used the Pave IR system, took cores in appropriate locations, and made the connection between lower temperatures and lower density.
“We didn’t change our coring pattern of random sampling,” says Johnson. “But the contractor, on their own, had taken some more cores. They made adjustments to the paver, and I would say that they demonstrated to the crew the importance of keeping that paver moving instead of stopping and starting. Now they have a visual indicator on the back that says here is the result. It clicks in a little more when you see a visual instead of someone telling you something.”
To some extent, specifying the use of this thermal image bar becomes a political decision, not an engineering one. In Minnesota, Johnson says, “We need to work through our industry/agency groups before we move ahead with specifying Pave IR on paving projects. As an agency it is difficult to specify a device that is only made by one manufacturer in a low bid system. Developing a specification where this system is included as a standard practice will take time.”
Meanwhile, Texas’ spec works in that state. “Our experience with the systems over the past two years has been very positive,” says Rand. “These systems are a valuable tool for contractors but they also provide an excellent ‘passive inspection’ device for DOT inspectors. Inspectors do not have to watch every detail of the paving operation because the Pave IR system will identify major paving defects.”