In-service pavement cores also will exhibit unexpected irregularities. “A few years ago we had an asphalt paving job with whole pine cones in the asphalt mix,” he says. “We never were sure how they got there, but we were called to determine how extensive the pine cones were; were they just in one location or distributed throughout the pavement. We went out and took cores, and also made a visual observation of the pavement surface.”

Because Minnesota DOT has a spec that permits up to a certain level of organic materials in a pavement, MnROAD’s forensic evaluation had to determine whether the pavement met the specification, and would the pine cones pose a long-term performance problem. The verdict: The contractor was at fault and took a large deduct.

“A concrete pavement from a few years ago had clumps in the concrete, small balls of unmixed material, aggregates and cement that had not been fully mixed at the plant,” Clyne says. “You could see them behind the paver, small clumps that did not look the way fresh concrete should look. We hired a consultant to evaluate the project by covering every square inch of pavement with ground penetrating radar (GPR). The GPR was able to locate the unmixed clumps in the concrete, and it also found areas where tie bars were missing. The contractor was held responsible to the tune of $1 million.”

Forensic studies don’t just involve tests on fresh pavements or failed pavements; they can involve material samples taken at the time a pavement was produced, and stored. “Pavement forensics for rutting can include in-depth testing of quality control and ‘bag’ samples taken at the time of production,” says Chris Huner, P.E., assistant division engineer – materials, Alabama DOT 7th Division. “There we run volumetric tests and do Abson recovery tests on the liquid binder. We recover the liquid binder from the sample and determine the percent polymer in it if applicable. Then we cut cores from the roadway where the rutting is most severe and compare with the stored samples.”

Closer Look at Materials

Once visual examination of cores is concluded, lab analysis of the asphalt binder, cement paste or aggregates may be necessary to see if the materials confirm the results. A suite of sophisticated laboratory testing devices is available for this analysis.

Lab examination of core displays thermal crack, a problem of HMA mixes in cold-weather regions

Some asphalt paving projects may begin flushing or bleeding. In this case asphalt will rise to the surface and make slick spots on the driving course. Cores will be taken and asphalt extracted to establish the stiffness of the binder and see if the material placed matches specifications.

For this analysis, a chemical lab will extract a pavement sample from the core, heat it, crumble it and put it through a solvent extraction method using toluene, which strips the asphalt from the aggregate.

That asphalt is recovered by “washing” the toluene out of the liquidasphalt via a vacuum distillation process. This liquid asphalt then is tested in various machines to see if it met the spec.

If that binder is too “soft” it will be revealed by the dynamic shear rheometer(DSR) in the course of a lab investigation. The DSR has two parallel plates, in which one is stationary and the other rotates at a certain amount of strain and frequency (speed). This application is useful in revealing the PG spec of the binder.

The DSR is not the same as the machine used in the dynamic modulus test. This test is used to evaluate mix stiffness at different temperatures and loading speeds, and is sensitive to changes in binder grades, presence of RAP, production temperatures, or anything else that would influence stiffness.

The DSR is a test on binder; the dynamic modulus is a test on the whole mixture, including binder and aggregate. “The dynamic modulus is tested in compression, in which you push or squeeze the material together,” Clyne says. “The dynamic shear rheometer tests shear, in which the sample is twisted. If the DSR gives us the PG rating of the binder, the dynamic modulus gives us the overall stiffness of the asphalt mixture, including aggregate.

GPR vs. core data (from University of California-Davis)

“In general we’ve been trying to move away from just testing the binder,” he adds. “We want to test the whole mixture, including the aggregate, because that’s what’s happening on the road, that’s what the traffic is rolling on. Binder typically makes up just 5 percent of the mixture – an important part to be sure – but it’s not the only part.”