NCAT Makes Pavements Fail to Produce Better Designs

By Paul Fournier

Newest test track sections at National Center for Asphalt Technology include one group that researchers hope will fail by cracking

Researchers at the National Center for Asphalt Technology hope that one group of experimental pavement sections just built at their Pavement Test Track in Opelika, Alabama will crack.

“We want them to fail,” said Dr. Raymond “Buzz” Powell, NCAT Assistant Director and Track Manager. NCAT personnel operate and manage the Pavement Test Track under Dr. Powell with key staff including Jason Nelson and Dr. Nam Tran. The track utilizes accelerated performance testing that simulates the effect on pavements from up to 15 years of interstate truck traffic. NCAT Director Dr. Randy West has overall responsibility for the research facility, which is headquartered at Auburn University.

NCAT personnel launched its sixth research cycle at the track in August 2015, with the construction of new test sections by East Alabama Paving of Opelika.  Hot mix asphalt for the experimental pavements is provided by the contractor’s Astec Double-Barrel plant located nearby.  This latest research cycle includes a special focus group on pavement cracking.

Cracking Indicates Fatigue Failure
Cracking in asphalt pavements is generally recognized by researchers as a principal indicator of pavement failure due to fatigue from repeated loading, and is often manifested as alligator type cracking in the wheel paths. One type of fatigue cracking initiates at or near the pavement surface and propagates downward, in contrast with reflective type cracking, which commonly occurs in rehabilitated pavements near the junction with an underlying asphalt layer or stabilized base that is already cracked, and propagates upward to the surface of the overlay.

“Cracking is our biggest concern with asphalt pavement at present,” said Dr. Powell. “Rutting used to be, but that problem was mostly solved with the development of SUPERPAVE. Now we need to have healthy asphalt binder in the mix to ward off cracking,” he said.

But with fewer new roads being built today, and more calls for reusing the asphalt already installed on existing roads, there is growing concern among researchers about the “health” of the aged, oxidized reclaimed asphalt binder and whether its use detracts from a pavement’s ability to resist failure by cracking.

“Everybody wants to use more RAP and more RAS in the mix, but we need to balance that demand against the need for a long service life,” said Dr. Powell. “Dr. West and I have discussed the lack of information on the effects of high RAP and high RAS content on mix performance in resisting cracking,” he said.

“We decided to address this by proposing the formation of a Cracking Group for our sixth research cycle to the 17 current state department of transportation sponsors of test track sections, and nine of them decided to take part, together with the Federal Highway Administration and the Alabama Department of Environmental Management,” he said. The state sponsors of CG test sections include the departments of transportation of Alabama, Florida, Illinois, Michigan, Minnesota, North Carolina, New York, Oklahoma, and Wisconsin.

Controlling Variables
Recent studies show that principal factors influencing the occurrence of load-related fatigue cracks in asphalt pavements are environmental conditions, tire-pavement interaction, mixture characteristics, pavement structure, and construction practices.

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