Hard research delivers "concrete" results

February 12, 2001

Arthur Nead

An elegant snow-white bridge spanning a canal in southeastern Louisiana represents the advances of four decades of cutting-edge research by Tulane's Department of Civil Engineering. The Charenton Canal Bridge, on the banks of picturesque Bayou Teche, is the first bridge in the state and among the first in the nation to be built using high-performance concrete, a state-of-the-art material for civil engineering projects requiring concrete.

Tulane, together with its partners, was recently awarded the American Concrete Institute's Award of Technical Excellence for the bridge project. Those partners include the Louisiana Department of Transportation and Development, the Louisiana Transportation Research Center and Gulf Coast Prestress.

"Tulane was involved in prestressed concrete from the very beginning," says Robert Bruce Jr., the Catherine and Henry Boh Professor of Civil Engineering and an expert on concrete structures. "The Charenton bridge is a culmination of an almost 40-year effort. I honestly think that if it were not for the research at Tulane, this bridge would not have been built."

According to Bruce, Tulane initiated research projects beginning in the 1940s, with sponsorship from the Federal Highway Administration and the State of Louisiana. "I think we were the first American university to get involved in prestressed concrete."

What is concrete? Colloquial speech creates some confusion about the nature of the material. "The layman often fails to distinguish between 'cement' and 'concrete,'" says Bruce. "Especially in New Orleans, people talk about 'cement sidewalks.'

The sidewalks aren't cement, they're concrete. Concrete is made of cement, water, gravel and sand." When water and the solid ingredients are mixed, the cement-water mixture undergoes a chemical reaction and the whole mixture hardens into concrete. Reinforced concrete has steel rods running through it to give it additional strength. Prestressed concrete takes this idea a step further.

To manufacture a prestressed concrete component, such as a pile or girder, steel rods are laid through the concrete casting form, and then put under tension. The concrete is then cast around the steel. After the concrete has hardened, the tension is released on the steel.

"It's like a rubber band," says Bruce. "It wants to go back, but it can't, because the concrete has surrounded it."

The stress compresses the concrete, increasing its strength. Other developments have revolutionized concrete and its possible applications. High-performance concrete reflects a change in the composition of the mixture. A key new ingredient is silica fume, a byproduct of specialized blast furnace operations. The Charenton bridge was built entirely of high-performance concrete components, all manufactured at the Gulf Coast Prestress plant in Pass Christian, Miss.

Tulane and the plant have had a close working relationship for several decades, a relationship that Bruce credits with much progress in the development of high-performance concrete technology. "It's not easy to produce high-performance concrete," says Bruce. Extra care must be taken in both its fabrication and construction.

"We had to do quite a bit of experimentation at Gulf Coast Prestress before we could achieve it. "The Charenton bridge was the culmination of an awful lot of research. For the first time, we took all this research and put it into a real structure," says Bruce.

These advances in concrete technology promise to greatly increase the life expectancy of highway bridges, which will result in saving public money for maintenance and replacement costs.

"The Federal Highway Administration has mandated that by 2005, all federal projects will use high-strength concrete," says Bruce. "As we look at infrastructure and sustainability, we want to make these structures last longer. Some time ago a bridge was expected to last 30 years. Now there's no reason a bridge couldn't last 100 or 150 years."

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