Cracking the case

March 17, 2009

Every winter, we feel them on the highway. Every summer, the acrid smell of road tar reminds us of their presence: cracks in the road. When the temperatures drop, thermal contraction puts stress on paved roads. When that stress becomes too much, the pavement cracks.

Every winter, we feel them on the highway. Every summer, the acrid smell of road tar reminds us of their presence: cracks in the road. When the temperatures drop, thermal contraction puts stress on paved roads. When that stress becomes too much, the pavement cracks.

Low-temperature cracking is one of the four major failure modes for asphalt pavement, along with high-temperature rutting, fatigue cracking and moisture damage. While easier to repair than concrete, cracked roadway asphalt is inconvenient and costly to repair or maintain. Other types of stress often exacerbate the problem; once cracks appear in road asphalt, it is only a matter of time before it must be replaced. Sang-Soo Kim, associate professor of civil engineering in the Russ College of Engineering and Technology at Ohio University, estimates that the cost of repairing such damage on roads each year is in the billions of dollars. Part of the problem, he thinks, is the lack of a reliable method to test the cracking temperature of the asphalt binder, and in response he has invented an innovative new technology.

The current industry standard for estimating cracking temperatures, the American Association of State Highway & Transportation Officials (AASHTO) M320 tests, relies on two types of equipment measuring different properties of the binder. Creep stiffness is determined by a bending beam rheometer (BBR) used to calculate thermal stress, and a direct tension tester (DTT) is used to find the tensile strength. The problem, said Kim, is that this type of test does not actually measure the cracking temperature of the material.

Kim’s answer to the problem took the shape of a small metal ring, which he dubbed the Asphalt Binder Cracking Device, or ABCD. The ABCD offers much simpler, more accurate and more reliable test results because it directly measures the cracking temperature of the asphalt binder, said Kim.

“What the ABCD does is simulate field conditions. ABCDs can approach the problem a different way and give us the information that we need,” he said.

The device consists of a silicone mold and a metal ring with temperature and strain gauges attached. The ring, 2 in. in diameter and made of the low-thermal-expansion alloy Invar, fits inside the mold, and binder material is poured in between the two. When cooled at a steady rate to simulate field conditions, the asphalt binder will contract at least 100 times more than the ring it surrounds, putting strain on the ring. The strain is measured by a strain gauge. In addition, the binder temperature is measured: When the asphalt binder cracks, the binder immediately ceases to contract, the impact of which is manifested as a sudden change in strain. The temperature at the time of the sudden change in strain is the cracking temperature.

It’s all inside

According to Kim, the ABCD’s method of determining the cracking temperature eliminates many of the assumptions and detailed calculations that are the essence of the current methods of finding binder cracking temperatures.

“We don’t need to know the material’s physical properties. We don’t need to know about the stiffness. We don’t need to know about the strength. We don’t need to know the thermal expansion coefficient. We don’t need to know about the temperature shift function, because all those are automatically factored in the ABCD test process itself,” he said.

The key is the ring’s circular shape. There is no end, as opposed to the typical prismatic and cylindrical column shapes. The circular shape reduces many of the difficulties associated with stress testing, Kim said.

“To perform a test with a prismatic, linear shape of an asphalt binder specimen, one has to have something to grab at both ends. Or, the specimen may be glued to end fixtures. Either case will cause a complicated stress pattern development at the ends of test specimens and may significantly affect test results,” he explained.

Though the ABCD is simple in design, it was not without complexities during development. Early on, Kim had trouble with inconsistent results. The temperatures at which the binder samples would crack varied widely. The location of the failures did, too.

“The locations of cracking always changed, so we could not measure reliably the strength value,” he said. “Even more important was that the variability of the test was so high. You prepare four samples—same binder prepared the same way—you put it in the freezer, you run the test, and some samples crack at minus 30°C, while other samples crack at minus 60°C.”

Zeroing in

To correct this, he added a small cylindrical protrusion onto the inside of the silicone mold, creating a hole in the sample ring of the asphalt binder. This gave the stress a place to concentrate and crack consistently.

“Since we introduced the protrusions and created a hole in the test specimen, the average standard deviation went from more than five degrees to less than one degree. So there was enormous improvement in the test,” Kim said.

With the kinks worked out, Kim’s hope is that a simple and reliable test that accurately mirrors field conditions will encourage the development of better road materials, thereby reducing the large amounts of money and man-hours spent each year on repairs.

“Those saved resources can be used for some other benefits for the general public and taxpayers, and also we could reduce some inconvenience caused by construction projects,” he said.

The National Cooperative Highway Research Program (NCHRP) seemed to agree and granted the initial funding for the ABCD concept through their Innovations Deserving Exploratory Analysis (IDEA) program. The success of his design led to a patent in August 2007, and a year later Ohio University’s Technology Transfer Office licensed the technology back to Kim and his new company, EZ Asphalt Technology LLC, for further development and marketing.

Last year, the Federal Highway Administration’s (FHWA) Highways for Life program selected the ABCD to receive support and funding in a technology partnership. With Highways for Life, Kim plans to make improvements to the ABCD system (including a smaller, more efficient cooling chamber and analysis software that will automatically process the raw data) and begin sending his invention out to be tested more thoroughly.

“We’re completing phase one now. Two ABCD units were sent to North Central Superpave Center, and the University of Madison just completed the ruggedness test. Next year, we’ll be looking at interlaboratory studies,” he said.

In addition to EZ Asphalt, more than 30 laboratories, including more than 20 state departments of transportation, have been slated to participate in the studies beginning in early 2009. They will be getting their first look at the ABCD in action and evaluating its suitability for more widespread use.

One key feature the labs will examine is the ABCD’s ability to measure the cracking temperature in chemically or physically modified binders. Many types of asphalt produced today have polymers added to them to enhance their properties. While the tests employed in AASHTO M320 have proved useful for grading unmodified asphalt binder, they have not done as well with these newer types.

Adapting to polymers

Kim said that tests of polymer-modified asphalts using the BBR test showed higher cracking temperatures with increased polymer concentrations despite large amounts of evidence from the field that suggests the modified asphalts perform much better in cold conditions than their unmodified counterparts. The same tests using the ABCD showed results much closer to field data.

Kim said he did not have to look far to affirm that polymer-modified asphalt binders were becoming more widely used.

“A few years ago, the state of Ohio mandated that all the surface courses of the asphalt pavement must be polymer modified,” he said. “The demand for the modified pavement is getting much bigger because of large volumes of traffic.”

More new pavement means more new materials to be tested. Hoping that demand for his ABCD also increases, Kim is still improving and refining his design. This includes not only making the device more portable and efficient, but also allowing a larger number of tests to be conducted simultaneously.

“The models we’re developing can test up to 16 ABCD rings at one time,” he said.

Kim hopes that positive results from the interlaboratory studies may lead to the wider adoption of the ABCD as the industry standard for measuring asphalt binder’s low-temperature cracking potential.

“We hope that this test will be adopted as the specification test,” he said. “I’m convinced that this test will contribute to the savings of billions of dollars annually across the nation . . . and to providing long-lasting pavement. That will be the benefit for the general public and taxpayers.”

David Powers, of the Ohio Department of Transportation’s Office of Materials Management, said the approval of the federal body will be key to the ABCD’s success. The success of such inventions often depends on how much the FHWA and their Experts Task Group (ETG) champion them to standards-setting bodies and the state departments of transportation, he said. But Powers went on to say that the ABCD has “high potential” and credited its basic engineering as the reason why.

“I think this is mainly due to its simplicity of design, sound engineering basis, high correlations with actual field measures and high repeatability,” he said.

With such a propitious outlook for the ABCD, Kim is now looking to take his concept a step further. He is currently developing a larger device that could measure the cracking strength of whole asphalt mixes. He has completed an initial design with encouraging results and is currently working on peer-reviewed papers to widen its exposure.

About The Author: Elliott is an external relations assistant in the Russ College of Engineering and Technology at Ohio University.

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