Mix masters try to crack code for construction

Researchers are borrowing a million hours of processor time from NASA to analyze how concrete is combined -- and to find the right recipe for building success


What do you do with one of the world's fastest computers? You can forecast complex hurricane patterns. Or simulate how stars form, how nuclear bombs explode, or how a spacecraft handles solar winds.

Or you can learn how to mix concrete.

Don't laugh. Researchers at the National Institute of Standards and Technology in Gaithersburg are using a million hours of processor time awarded to them on NASA's fastest supercomputer to analyze the billions of possibilities created by the collisions of tiny particles of sand, gravel and cement whenever a cement truck pours a sidewalk.

The different size and shape of each particle -- which scientists blow up to the size of weather balloons on their projection screens -- have a profound effect on the strength and durability of concrete and length of time it takes to harden.

FOR THE RECORD - An article about concrete in yesterday's Health & Science section incorrectly reported the concrete industry's total revenue and the number of microprocessors available for research at the National Institute of Standards and Technology. The concrete industry generates $110 billion in revenue and NIST has 300 microprocessors.
The Sun regrets the error.

All of these, in turn, are critical factors when engineers create the right recipe for what has become a prime structural material in some of the world's tallest buildings.

The use of concrete dates to the Roman Empire, but thousands of years later, many of the material's properties remain a mystery.

"Several things about it are not really understood -- the durability, for one thing, is really not known how to predict," said Edward J. Garboczi, a member of the NIST team working on the project.

NIST is trying to create concrete that's more durable and easier to pour and pump at construction sites. Success would reduce the need for costly repairs to projects ranging from a front sidewalk to the reconstruction of the Woodrow Wilson Bridge, researchers say.

For the $10 billion concrete industry, the research is vital. "You'll find steel in buildings, you'll find asphalt in roads and you'll find wood in houses, but you'll find concrete in all of those," said Iyad M. "Ed" Alsamsam, a structural engineer with the Portland Cement Association, an industry group whose members work with concrete and cement.

High winds are less likely to sway skyscrapers that use concrete as a framing material, he said. That stability, along with improved products and pouring techniques, persuaded architects designing two of the world's tallest buildings to use concrete -- as opposed to steel -- as their principal framing material, Alsamsam said.

Those projects are the 92-story Trump International Hotel & Tower being built in Chicago and the 160-story Dubai Tower, planned for the United Arab Emirates, he said.

NIST researchers need NASA's supercomputer because of the nearly incalculable variations that go into making a typical batch of concrete.

Concrete, they note, is basically a mixture of sand, gravel and cement. The cement is made by mixing limestone, clay and other materials and heating them.

There are national guidelines for the ingredients of cement, but they are fairly broad. Moreover, the sand and gravel that go into any concrete mix are locally quarried. So the exact mineral content of any two batches can vary a bit, Garboczi said.

The particles that make up the mix also come in all shapes and sizes, which can affect the durability of the finished concrete. Cement particles can range from 10 to 200 microns across (there are 25,400 microns in an inch). The stones that make up the gravel can be anywhere from a half a millimeter to 2 inches in diameter.

There also are at least 40 types of additive mixtures -- polymers and materials such as corn syrup -- sold commercially to give concrete specific properties, such as strength, durability and curing time.

"Concrete can be different every time you make it, depending on what you're making it from," said William George, the NIST computer scientist who oversees the project.

With 10,240 processors, NASA's supercomputer -- named Columbia -- is the nation's fourth most powerful in industry rankings, said Bryan Biegel, deputy chief of NASA's Advanced Supercomputing Division. It's outranked only by two government computers at the Lawrence Livermore National Laboratory in California and one at IBM's Thomas Watson Research Center in New York.

NASA's $120 million computer takes up 15,000 square feet in a temperature-controlled room at the Ames Research Center at Moffett Field, Calif.

In 18 months of operation, scientists have used it to analyze weather patterns, make sense of data from the Hubble Space Telescope, test the aerodynamics of NASA hardware and redesign the foam and tiling on NASA's shuttle fleet. The shuttle Columbia exploded in 2003 because foam fell from a fuel tank during launch and hit the shuttle's left wing.

"The computer's named in honor of the Columbia crew," Biegel said.

NIST researchers can already use a smaller cluster of the agency's 3,000 computer processors to simulate what happens when small pieces of concrete are mixed, George said.

But with Columbia, NIST will be able to scale up the work, modeling concrete blocks 10 times bigger and using the supercomputer to see -- for the first time -- how the size, distribution and shape of particles affect the flow and durability of concrete.

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