Better Roads Staff
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.”
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