I-85 collapse in Atlanta: What caused it?

Fire still smolders a day after a massive fire took out part of Atlanta's Interstate 85 -- a major artery that supports an estimated 250,000 cars per day.

A major question remained Friday in the hours after a fire caused the collapse of a section of Interstate 85 in Atlanta: What caused the flames?

The fire started Thursday evening under an I-85 overpass in north-central Atlanta, north of the highway’s split with I-75. It grew into a massive fireball, sending thick smoke onto the highway above.

Firefighters responded, but the fire weakened the structure, causing the northbound section to fall and severely damaging the southbound side. No one was injured, and officials said they didn’t immediately know what caused the fire.

Here’s what we know about what led to it:

Started in a construction-supplies storage area

The flames ignited in a fenced-in area where the state stores construction materials and supplies near the overpass, state Department of Transportation Commissioner Russell McMurry said.

Gov. Nathan Deal said Thursday that he had heard speculation it was caused by some “PVC products that caught fire.”

McMurry initially said the materials stored under the highway were PVC pipes, but later said they were HDPE — high-density polyethylene — pipes. He said the conduits are used in the “traffic management, cabling, fiber-optic and wire network.”

The material had been stored there “for some time, probably since 2006 or -7,” McMurry said.

“We’re as eager to learn the cause of this fire as anyone,” McMurry said.

Why did the fire ruin the overpass?

Most structural materials lose strength when subjected to high temperature, so the concrete could have been compromised by the heat, said Reginald DesRoches, a professor at Georgia Tech.

It is too early to say how long it will take to repair the highway, he said.

“It certainly can take anywhere from several weeks to several months,” he said. “The surrounding sections of the highway will be evaluated to determine if any damage was sustained from the heat. It is probably prudent to check both sides of the adjacent sections (northbound and southbound).”