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ATLANTA -- 11Alive is now learning more about what caught fire underneath I-85 causing it to collapse – and we’re enlisting Georgia Tech for help.

The commissioner of the Georgia Department of Transportation compared the plastic material under the bridge to a plastic cup you may have at your home. So, our 11Alive Investigator Andy Pierrotti put both to the test.

Thick smoke and bright yellow flames: likely, the result of spools of high density conduit piping catching fire a short time before the collapse. The next day, GDOT Commissioner Russell McMurry downplayed the danger of the material.

“It’s no different than having a plastic cup in your cupboard,” he said. “It does not ignite. It takes something like that to burn.”

So, we decided to burn it ourselves. With the help of Georgia Tech Engineering Professor Dr. Karl Jacob and two PhD. students, we tested plastic cups, made from Polystyrene. We also tested blue pipe made from polyethylene like what caught fire under I-85.

We started with the plastic cup and not only did it immediately catch fire, it produced a lot smoke. That yellow-red color flame typically means it’s burning between 1,800 to 2,2200 degrees.

“It’s self-sustainable; once it starts burning, it will continue to burn,” Dr. Jacob said. “It doesn’t stop.”

It burned fast, too. In about 30 seconds, the cup completely melted.

Next, we tested the polyethylene pipe. That was the material stored under the interstate. It took a few seconds to catch fire, but it burned much more slowly.

“It starts slow, but it can continue to burn once it starts burning,” Dr. Jacob said.

Within 50 seconds, the flame turned bright white which means it was burning between 2,500 to 2,900 degrees – hotter than the plastic cup – and produced less smoke. Another difference - it started sparking and dripping fire.
So, could the dripping fire have contributed to more piping under the interstate catching fire?

“It could, yeah, because of sparking and, yes, it can,” Dr. Jacob said.

Jacobs also agreed that, if large quantities burned at the same time, it could compromise a large structure.

GDOT said the pipes under the bridge were there nearly 6 years. Dr. Jacob said the age of the material and its exposure to weather could have made the material more flammable. He also said that if the polyethylene that burned was pure, and nothing else was burning under the bridge, it likely means the smoke wasn’t as dangerous as first thought.