"Concrete Autopsy" Helps Strengthen the Pentagon

Kerry Hall
for National Geographic Today

July 9, 2002

Steve Wolter is one of the guys people call after disaster strikes.

As president of American Petrography Services in St. Paul, Minnesota, Wolter gets calls from people needing advice on whether they can salvage concrete that has been damaged or involved in a fire.

A geologist by training, Wolter performs autopsies—on concrete chunks. He tests concrete samples to ensure that buildings made out of the stuff will still be safe. But last year, Wolter was hired to study concrete that came from a building hit by the most extreme conditions that Wolter had ever seen.

The Pentagon.

"From a fire-damage standpoint, this was by far the worst, very intense, hot concentrated fire for a prolonged period of time," Wolter said.

Concrete Turned to Mush

The Pentagon was severely damaged on September 11, when the hijacked American Airlines flight 11 slammed into the building, killing all on board. The plane crashed through three of the Pentagon's five outer rings, destroying many structural columns and damaging dozens of others. The plane's jet fuel also exploded, starting an intense fire that burned for days.

American Petrography Services' task was twofold. The first challenge was to test the integrity of the Pentagon's remaining structures and tell engineers what needed to be replaced. The second task had broader implications: determine whether a stronger concrete mix could be made, such as one that could withstand not only pressure but also intense heat—or even a bomb.

Pentagon samples, pieces of Potomac River gravel mixed with sand, cement, and water, started arriving at the company's St. Paul, Minnesota, office almost immediately after Wolter landed the contract 11 days after the attack.

In all, the group received 170 pieces ranging in size from only a couple inches square to more than a foot long. Some samples arrived in such poor condition that scientists had to use Super Glue to piece them back together before they could study them.

"The heat of the fire drove out the water attached to the cement molecules," Wolter said. "It literally disintegrated the paste and turned it to mush."

Red-Tinted Rocks

And there were other effects of 9/11 that could be seen with the naked eye.

For example, some samples had a reddish hue or tints of bright orange. These samples came from columns near the crash site. The red and orange colors come from tiny amounts of iron in the rock that were oxidized in extreme heat. When the airplane slammed into the Pentagon, the plane's jet fuel exploded and the resulting fire cracked the rock in the concrete mix.

Forensic geologist Gerard Moulzolf, also from American Petrography Services, said he learned a lot from the color change.

"Looking at some of the concrete that's been fire-damaged, we can see how the cracking proceeds through the aggregate," Moulzolf said. "This tells us a lot about the temperatures the concrete reached."

Wolter's group eventually concluded that the concrete was irreparably damaged and that instead of working with the Pentagon's remaining columns and beams it would be better for the construction crews to remove it all and start over.

"We decided to say to heck with it and not do a surgical type of repair, which is what we thought we would do initially," Wolter said. "We decided to take out a large section and demolish the whole thing."

The construction crews demolished an area 300 by 150 feet (92 by 46 meters) that was five stories tall.

New, Improved Concrete

But it was the second task that gave Wolter and his company an opportunity to make their own impact—on public safety: concocting a better recipe for concrete.

"It's frustrating because you see [the attacks] happen and the whole country is devastated by it, and everybody wants to do something to help," Wolter said. "It was really great to be able to do something."

Concrete has many qualities, but there are two in particular that seem to occupy most of our attention—strength and durability, Wolter said.

And as far as coming up with a new, sturdier concrete mix, Wolter said his group has come up with some ideas. For example, stronger rock could be used, or high-tech additives could be mixed into the paste.

But he couldn't divulge more detail.

"In the interests of security, I'm not comfortable talking about it," he said.

Wolter's company finished its contract with the Pentagon on December 26. Wolter said the Pentagon may hire him to do additional work. He looks forward to it, but at the same time hopes it will be under different circumstances.

"I don't know if we'll ever see anything else like this again," he said, adding, "hopefully not."

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