Better Roads Staff
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.
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