"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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