| July 03, 2012 |
IIntelligent Compaction (IC) for hot mix asphalt is a work in progress. IC rollers are valuable quality control tools that go a long way toward solving the shortcomings of the conventional process. With some IC systems, a display screen in the operator’s compartment shows real-time mat temperature, roller coverage of the mat, and some indication of mat stiffness. No current IC system, however, will directly read out mat density. We’ll have more on that later.
There are shortcomings in the conventional asphalt compaction process. Today, using a standard vibratory roller, a fixed number of passes are applied to the asphalt. The problem is that a number of critical factors can vary during construction operations. Such factors include support from underlying materials, lift thickness, materials type and asphalt mat temperature. The changes in these critical factors are invisible to the roller operator during conventional compaction. The result is that too little or too much compaction effort may be applied to the asphalt.
The conventional density quality control process also has shortcomings. Typically, a small number of spot tests are run, either with nuclear gauges or core tests. A judgment is made about the density level of the entire roadway based on these spot tests. Unfortunately, densities measured in those spots may not be representative of the entire roadway section.
A Closer Look
Todd Mansell, a training consultant for Caterpillar Global Paving Products, defines an IC roller as one that has:
• GPS location tracking;
• a “stiffness” measuring device, usually an accelerometer;
• temperature measuring device;
• the ability to store and transfer collected data; and
• auto-feedback control of the drum.
Of roller manufacturers offering machines in the United States, only BOMAG, Sakai and Hamm offer IC systems that use an accelerometer to measure asphalt stiffness, not asphalt density. “The contractor wants something that can measure density, but no system can now do that,” says Mansell. “Accelerometers measure the rebound of the drum. As the density increases, the frequency response of the drum changes – the wave that measures the response becomes more irregular.
As the density increases, the frequency response of the drum changes – the wave that measures the response becomes more irregular.
In theory, one can correlate stiffness values from IC systems to laboratory findings and come up with asphalt densities. But it’s not quite that simple, because accelerometers have their shortcomings too. “The accelerometers are responding to ground conditions 3 to 5 feet below the surface, not just to the asphalt mat,” says Mansell.
Mansell says that what differentiates the manufacturers’ IC systems is the variety of ways they analyze the accelerometer data. “After it goes through their formula in a black box they come up with a roller-measured value,” says Mansell. “Each manufacturer has a different one.”
IC can provide a better method of quality control because real-time information is being relayed to the roller operator related to pass count, the in-place material stiffness, and mat temperature. Once laid down, asphalt has an optimum window of temperature during which compaction should be applied. If the mix cools down too much, it stiffens and will not compact as easily. Temperature sensors on IC rollers can indicate mat temperature and feed that back to the operator. If the mat is too cool, the decision can be made to either apply extra compactive effort, or tear out and replace the cold asphalt.
IC systems can provide a color-coded map showing stiffness values, mat surface temperatures and pass counts of the entire roadway. IC provides 100 percent coverage of each of the project sections, which makes it possible to identify areas of low stiffness, weak base or improperly compacted material.
What They Offer
BOMAG’s value for stiffness is called Evib, and is measured in units of meganewtons per square meter. Sakai has a CCV, or compaction control value, and it has no units. “In 2012 through 2014, the Federal Highway Administration is doing a study to try to establish a correlation of density with these roller-measured values,” says Mansell. “You can take cores out and try to correlate the core density to the roller-measured values, but most of the time you don’t get a reliable correlation – you don’t get an accurate prediction of density. That is the purpose of the FHWA study – to see if we can establish a good correlation or not.”
Part of FHWA’s research is directed at determining what level of accuracy is truly needed for Intelligent Compaction.
BOMAG says that its roller automatically adjusts the output energy that the roller drum puts into the work surface, based on stiffness readings that the machine receives. The roller makes this adjustment through a process called vectoring, which involves changing the angle of the energy delivered from the drum. For softer areas, the drum will direct energy at a straight vertical angle. As the material stiffens, the roller directs the drum’s energy at a more horizontal angle until the vector goes totally horizontal when the surface reaches optimum compaction.
Chris Connolly, BOMAG sales manager, fully agrees that his system does not measure density, but measures asphalt stiffness. “But it gives you a quality control tool to help you correlate stiffness to density at a given temperature,” says Connolly.
“Let’s say there’s a QC team working with a nuclear density gauge behind the roller,” says Connolly. “I made four passes with the breakdown roller and I get to 92-percent density based on the nuclear density gauge. And on that day, I see on my BOMAG IC system that the asphalt temperature is 250 degrees and I’m getting 270 meganewtons per square meter as a stiffness value. So now I know, without using the nuclear gauge, that if I give it four passes at that mat temperature and reach 270 as a stiffness value, I will have a pretty good idea that I have met my goal of 92-percent density.”
We asked Connolly about the fact that the accelerometer is measuring stiffness of the earth 2 to 3 to 5 feet down. “I say it sure does,” says Connolly. “Your compaction bubble goes down 10 feet. But I can manually adjust the vector on a BOMAG roller. If I put it in Mode 2, my compaction bubble is shallow; it makes a shallower read. The more vertical amplitude you have, the deeper you are reading. You can manually set the vector to six different positions. As you move the setting from high to low, the reading becomes shallower.”
Caterpillar currently has a color-coded mapping system that is tied in with GPS location and has dual temperature sensors (front and rear). The color mapping shows the number of roller-passes, color coded. The temperature sensors read ahead of the drum in the travel direction. The sensors alternate when the roller changes direction to minimize the influence of water spray on the drums and provide an accurate temperature reading. There is no accelerometer or other “stiffness” measuring technology.
Similarly, Volvo is developing a different IC system that will be under license from the University of Oklahoma. Bob Marcum, a Volvo product specialist for Road Machinery, says his company is working with Trimble to use its GPS product.
Volvo has a GPS system and a display screen on the roller that monitors the number of passes and the location of the machine. The display screen monitors coverage, and temperature sensors monitor mat temperature, so the Volvo roller operator can tell where he has rolled, how many times he has been there and what the mat temperature was during compaction. “We can identify areas that may be missed and areas that have received the desired degree of coverage,” says Marcum.
“So the conclusion is that when the compaction has been completed by meeting the job conditions, we can have a very high degree of confidence that the mat meets specified density,” says Marcum.
GPS systems offer various levels of accuracy in locating the roller, Marcum says. If a GPS system uses a base station to correct signals sent to the receiver on-board the roller, it will be a more accurate system. Marcum says a portion of FHWA’s research is directed at determining what level of accuracy is truly needed for Intelligent Compaction. More accuracy costs more, and IC systems already cost in the area of 35 percent of the cost of a highway class roller.
Under the title of “Improving Quality Control of Hot Mix Asphalt Paving Using Intelligent Compaction Technology,” four authors submitted a paper to the Transportation Research Board in July 2011.
The paper is based on the findings of the Intelligent Compaction Pooled Fund (ICPF) project that included 16 field demonstrations in 12 participating states.
Sakai is pleased to point out that its IC roller meets FHWA criteria, says Yuki Tsukimoto, Ph.D., director of the Creative Marketing Division for Sakai America. Sakai’s main on-board IC display screen reads the exact location of the drum on the road map. As the roller moves along, an icon colors the map by passes – red means one pass, yellow is two passes, deep blue is three passes, and subsequent passes show green on the road map. Sakai’s IC display is actually a heavy-duty laptop computer driven by Windows. It can provide the same operations as any laptop, such as zoom in/out and data input/export through a USB memory stick.
The system measures compaction control value that Sakai says can be correlated to density and with stiffness and with modulus of elasticity of the pavement. The screen also reads mat surface temperature as measured by an infrared temperature sensor mounted on the front of the roller frame. “If you get a cold truck-load, it will show up on the color-coded display screen,” says Tsukimoto.
“The compaction control value represents the system for 2 to 3 feet in depth,” says Tsukimoto. “It shows not only the asphalt but also the base course and the subbase. If you find a soft spot, it could be that the base is weak. One of the strengths of the CCV system is that you can see the stiffness, or the quality of the pavement, over the whole pavement. You can sample the entire roadway. This is a big difference from conventional QC/QA tests, which estimate the quality of pavement from spot tests.”
Connolly of BOMAG notes that his company’s roller also meets the FHWA criteria.
Hamm, a division of the Wirtgen group, placed its first IC system for asphalt into the field in 2010. Hamm’s system works with an accelerometer and a Trimble GPS receiver. Hamm uses an OmniStar GPS subscription to get an accurate signal. Or you can use a land-based system and base station, says Tim Kowalski, application support manager for Hamm. With OmniStar, a GPS provider based in Houston, there is no need for base stations, and the accuracy is 1 to 5 centimeters, Kowalski says.
“We learned there is more research to be done on the calculation to correlate density to stiffness.”
– Rob Larson, director of equipment, Anderson Brothers
Hamm’s display screen can be split into two sides. You can see two of three parameters at the same time: number of passes, stiffness values, or mat temperature. “We have one temperature sensor in front and one in back,” says Kowalski. “It will always monitor the hottest part; if you go forward the front sensor reads, and if you go backward the back sensor reads the temperature.”
Kowalski says one unique feature of a Hamm IC system is the ability to see two different GPS rollers on one machine’s screen. “You can see both rollers on each machine,” he says. “That way you don’t need to worry about overcompacting. You can see what the other roller has done. You can see both temperature and passes for both machines on both rollers. Or you can see passes and stiffness values for two rollers.”
Dynapac offers its DCA – Dynapac Compaction Analyzer. The system does not have an accelerometer to indicate mat stiffness, but it does indicate mat temperature and the number of passes made. So an operator can see his coverage of the mat, and the system distinguishes between vibratory and static passes made. The Dynpac system comes from the factory with the Trimble GPS receiver attached. With everything wired up, the DCA costs about $30,000 retail.
One application of Intelligent Compaction comes from Anderson Brothers Construction, Brainerd, Minn. The Minnesota DOT approached Anderson Brothers about doing IC on the Trunk Highway 169 project, a 20-mile mill-and-fill job that involved paving back about 2 inches of asphalt.
The contractor accepted the IC task, and fitted up a breakdown roller with a Topcon GPS system. A base station was used to correct the signal to the GPS receiver on the roller. The roller was fitted with an accelerometer to measure Compaction Control Value, or stiffness.
Rob Larson, director of equipment for Anderson Brothers, said density did not correlate very well to the stiffness values measured by the roller. Portions of the mix used for the project were warm mix, and that made it more difficult to correlate stiffness to density, Larson says.
“We learned there is more research to be done on the calculations to correlate density to stiffness,” says Larson. “But my thought process tells me that in order to accurately correlate the stiffness factor, what we need is a roller to do a test roll between the mill and the paver, and then compare that result to your breakdown roller results. You need a baseline number to compare to, and on a mill-and-fill we did not have that.”
Larson says the benefit of intelligent compaction is that it shows roller coverage with a GPS component. “But as for the correlation between stiffness and density, I don’t think we are there yet,” says Larson. “It is going to take some more money and research and development to establish that correlation.”