Binders, quality aggregate and performance-related mix design procedures are all important factors in dictating how long a new road surface will last. For many years, researchers have traditionally focused on these rather than identifying the root cause of early surface failures, which is often construction-related. Researchers at the Texas A&M Transportation Institute (TTI) are now concentrating on eliminating these built-in defects. TTI has come up with a combination of procedures and new technologies that might be called “The Texas Three-Step.” Tom Scullion, manager of the Flexible Pavement Program at TTI, prefers to call it the Comprehensive Compaction Monitoring System.
Best of three
Scullion and fellow TTI researchers Stephen Sebesta and Wenting Liu have developed a three-step process using technologies developed at the institute over the last five to 10 years. Individually, each device measures one of the three components necessary to achieve proper compaction of the asphalt mat: uniform lay-down temperatures, correct rolling patterns and uniform density. Collectively, the comprehensive compaction monitoring system gives a highway agency confidence that, from start to finish, it is getting a quality mat and that every precaution has been taken to make sure the new surface will provide a long service life.
TTI’s research into better paving methods began more than 10 years ago with the Texas Department of Transportation’s (TxDOT) Cooperative Research Program, where the concept of a paver-mounted thermal imaging system was first tested. During these early studies, it was concluded that the current method of localized density measurements or field coring at randomly selected locations was not identifying the problem areas on an asphalt mat.
“What we and others have found are periodic defects caused by cold spots, which are created during the construction operation. You may only have one of these problem areas in every 100-150 ft of paving; for example, one 4-ft x 4-ft defect, which cannot be caught with random sampling,” Scullion said. “Yet these localized defects often lead to future potholes.”
Large swings in asphalt mat temperature are often caused by truck-end segregation. It’s hard to pinpoint the source of the problem. Scullion noted that there are several different issues causing it, including how the mix is loaded into the trucks, the haul distance and how the material is transferred into the laydown machine. Delays, where the paver stops between truckloads, also are a big issue. That’s why the spots occur about every 150 ft, the average distance a single truckload of asphalt covers.
“The current way we test new asphalt layers is by using a random number generator to select a random location and measuring density at that spot. That’s a reading in an area approximately 1 sq ft.
We take one reading right there, and then move several hundred feet down the road, and take another one. No way can you catch these small localized problems with that kind of sampling scheme,” Scullion said. To address these sample-size issues, the three technologies described below have the potential for monitoring close to 100% of the new paved surface area.
Step 1: Pave IR
Pave IR is an infrared imaging system consisting of 12 temperature sensors mounted on the rear of the paver. A lane is normally 12 ft wide. Pave IR produces 12 individual spot readings every 6 in. TTI-designed software converts that information into a color plot. The laydown operator sees a color display of the most recently placed 450 ft of a new overlay. The measured temperatures are color-coded. Blue color represents temperatures substantially below the target lay-down temperature. Localized blue spots at regular spacings are the signature of truck-end segregation.
Previously, an inspector with an infrared temperature gun took random temperature readings of a mat. The problem with this method is, Scullion noted, “The inspector has other things to do besides take the occasional temperature reading. He’s counting tickets. He’s making sure the rollers are operating properly. He doesn’t have time to focus on one function.”
In Texas, the current test procedure requires that the paving section be broken into 150-ft sections. If there is more than a 50°F difference within each section, this is classified as severe thermal segregation. If this occurs two or three times in a row, the agency inspector has to notify the contractor to correct the problem. If the problem persists, current TxDOT specifications allow the inspector to suspend operations.
The question is: How do you fix it? According to Scullion, Texas contractors have several options to improve their operations. One common comment from contractors is that they can go a long way towards solving the problem by better scheduling of their haul trucks.
“If all the trucks arrive at one time and then the paver sits still for 20 minutes before the next trucks arrive, you often see severe thermal segregation problems. Contractors try to schedule their fleet so the paver doesn’t stop. That’s the key. Keep that paving train moving, and you don’t have some of those cold spots,” Scullion said.
Eliminating cold spots is one key to longer-lasting road surfaces. With so many paving jobs now occurring at night, it becomes even harder to detect areas that are not being compacted correctly.
“It’s a dark night and inspectors are looking at a dark surface. Good luck,” Scullion pointed out. Pave IR can minimize the risk that low-density areas are being built into the mat.
Step 2: GPS-instrumented roller
The next key to getting a long-lasting surface layer is uniform compaction. The rollers must get on the mat before the temperature drops and apply a selected roller pattern. The solution is equipping rollers with a combination of a global positioning system (GPS) and temperature sensors. Viewing a screen, operators can see exactly where they have compacted and if the required rolling pattern has been achieved. A color-coded map is produced showing the number of passes for each segment of the mat, and when the required rolling patterns have been achieved. It also generates a map showing the temperature of the mat when the first pass of the roller is applied.
Scullion doesn’t fault the operators. “The guy’s on a roller eight hours a day. He’s got a rolling pattern that he is supposed to keep. With all the distractions, such as equipment break downs and paver stoppages, it is easy to lose track. Things like this happen, and the operator can forget where he’s rolled. At night, it is even more difficult. There may be some areas he just doesn’t see,” he said.
Data collected with instrumented rollers has found that, in most cases, the specified rolling patterns are achieved. In some areas, major variations occur. As the industry shifts to thin overlays, temperature is critical. Thin overlays cool two to three times faster than regular overlays. Temperature sensors on the instrumented rollers show that mats are cooling before they are rolled, leading to density problems.
“With thin overlays, what we’re seeing is a big temperature drop on one side of the mat while the roller is working the other side. We have concluded that for thin-lift mixes, two breakdown rollers working in tandem are needed,” Scullion said.
Step 3: Rolling density meter
After compaction with the instrumented roller is complete, a hand-rolled device—called a rolling density meter—is pushed along the entire mat. This is a custom-designed ground-penetrating radar system that measures an electric property of the pavement surface strongly correlated to mat density. Through a calibration process, this measurement is converted into surface air voids. The system is completely automated. Once calibrated, the system generates mat air voids in real time.
According to Scullion, this is the most critical part of the three-step system. “The way we pay contractors at the moment is by density. That’s what everybody is familiar with. If the density is too low or too high, there are big concerns. There’s normally a window between 3% air voids and 9% air voids where we want to be. Temperature uniformity goes a long way towards getting us there, but it’s not the entire answer,” he pointed out.
The rolling density meter does away with spot-specific measurements. Random tests aren’t capable of catching problem areas. Scullion pointed out that “we put a gauge down, take a one-minute reading that covers about a foot, and then we move somewhere else and take a reading. What our SHRP 2 R-06C project (Pre-Implementation of Infra-red and Ground Penetrating Radar Technologies for Improving Asphalt Mixture Quality) accomplished was development and field testing of a rolling density meter.”
A reading is taken every 6 in. in a continuous stream. Multiple passes across a mat allow researchers close to 100% coverage. Scullion’s team has been working with a single-channel system, which works well. Researchers are in the process of developing a three-channel system to provide better coverage.
According to Scullion, low-density areas are where pavements fail. The road between these spots may be in great shape, but the low-density spots soon become obvious to motorists.
“In severe climates those low-density spots are the start of potholes. That’s where they initiate. Water gets in there, it freezes, the asphalt begins to ravel, and that’s where potholes start. If we can eliminate these low-density spots that fail, we can probably get 50% more life out of the mat,” he said.
Not so random
Scullion observed that most contractors have embraced Pave IR, though some have been hesitant to change ways that have worked for them over the years. TxDOT used some innovative methods to encourage the adoption of Pave IR.
“It was implemented by using a carrot-and-stick method. If a contractor used Pave IR, they were allowed to pave at a cooler temperature, as long as they were able to demonstrate they were getting uniform mat temperatures. Instead of requiring an air temperature of 60°F or 70°F and rising before they could begin work, Pave IR users got a 10° decrease to 50°F and rising. It’s a big deal for contractors because they want to get out there and get the job done.”
That was the carrot. “The stick was if there were more than two 150-ft sections with severe temperature segregation, the operation had to be scrutinized more carefully, perhaps closing it down,” he said.
Input from contractors has resulted in changes to Pave IR. The original system is currently being marketed by Mobile Automation (MOBA) of Germany. MOBA has now developed the next-generation system, which eliminates the bar on the back of the paver and replaces it with a single scanner system. This new Pave IR system improves accuracy as it takes more readings across the mat.
TTI Researcher Stephen Sebesta is currently testing the two systems simultaneously to make sure that the data produced by both systems is comparable. “We have a great working relationship with MOBA Corp. We develop the technology here at TTI, and MOBA does the implementation,” Sebesta said.
The rolling density meter is the newest member of the Comprehensive Compaction Monitoring System. “We have always known that radar would be a great technology to detect air voids. We’ve never had the proper hardware to go out and do it. Now we do,” Scullion stated.
Even though all of these components are being developed independent of each other, the ultimate goal is the same: take the randomness out of asphalt mat testing. “We’re trying to move away from random measurement to a near 100% coverage scan, so we can see the small localized problem areas which eventually become big problems. All of this is to make sure we have a uniform and hopefully longer-lasting product,” said Scullion. The Comprehensive Compaction Monitoring System goes a long way towards accomplishing that goal. AT
