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Fingerprinting Asphalt: New Handheld Testing Device Ensures Binder Quality

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In recent years, transportation agencies in the United States, including the Texas Department of Transportation (TxDOT), have noticed the premature failures of some newly constructed pavements. Concern that contaminants in recycled engine-oil products might be affecting the binders in asphalt led TxDOT to contract a two-year study of the problem by the Texas A&M Transportation Institute (TTI).

Ryan Barborak, TxDOT's flexible pavements section director, was a key participant on the research team. "We needed to make every effort to ensure that the asphalt binders that TxDOT purchases do not have contaminates in them that may negatively affect the performance of our pavements, and that our specifications (for binder composition) are as accurate as possible," Barborak says.

handheld X-ray fluorescence instruments

Handheld X-ray fluorescence instruments like this one can expedite evaluations to determine the amount of re-refined engine oil bottoms in asphalt and seal-coat binders.


The paving industry routinely uses a variety of materials to produce and modify asphalt and seal-coat binders. A 2016 Asphalt Institute report estimated that nearly 160,000 tons of re-refined engine oil bottoms (REOBs) go into asphalt mixtures in the United States every year. The lab tests done over the course of this research project confirmed that including more than 5 percent REOB (by weight) to a binder can negatively impact the lifespan of asphalt mixes and seal coats.

After extensive laboratory testing, the team developed a simple process to detect the amount of REOB used in both asphalt binders and hot-applied seal-coat binders. The team also confirmed the handheld x-ray fluorescence (XRF) instrument can be used in the field and produce results just as accurate as a traditional benchtop test. Using the handheld XRF makes it possible to check the binder quality on the job site, a more efficient process than testing in the lab. XRF spectrometry can determine the chemical composition of materials, whatever their form (solid, liquid, etc.), and detects the amount of trace metals that confirm the presence of REOB in asphalt binders.

TxDOT chemist Ashley Yunkun

TxDOT Chemist Ashley Yunkun calibrates an XRF unit prior to testing asphalt samples.


"The method is fast, accurate and non-destructive, and usually requires only a minimum of sample preparation," says TTI Research Engineer Fujie Zhou. "The precision … of XRF analysis is very high. It's almost like taking a fingerprint of the asphalt binder. The analysis time after the measurement is only a few seconds."

"We want to recycle any chance we get," says Barborak, "but we want to get the correct performance from the asphalt. The research we did with TTI proved that the handheld model of the XRF is just as accurate as the bench unit we have used for several years, and is faster.

"Due to the findings of this project, we've issued a special provision to TxDOT specifications that limits the REOB content to 5 percent in all asphalt binders, including seal coats, and allows the use of the new handheld XRF units for testing."

This article was originally published in longer form in the Texas Transportation Researcher, Vol. 50, No. 4.

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