Winning the Battle Against Thermal Segregation

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.