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Solid Information: Chemical composition can determine concrete's durability

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10:47am, July 26, 2006
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A new analysis reveals how damage progresses in concrete that's exposed to sulfate, a nearly ubiquitous compound. The work could lead to the design of concrete structures with improved durability, the report's author says.

Concrete is made with Portland cement, a powdery mixture of crushed limestone and clay or shale. When the cement is mixed with water and added to gravel and sand, the resulting slurry hardens into concrete.

Portland cement consists primarily of four minerals: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. The chemical reactions among these compounds and water form a glue that binds the gravel and sand.

Paulo J.M. Monteiro, a civil engineer at the University of California, Berkeley, has been studying mechanisms behind concrete damage caused by exposure to sulfate. Sulfate ions, which are common in water and soil either naturally or from inputs such as fertilizer, can penetrate concrete, causing it to crack and expand.

"Many people call this the 'cancer of concrete,'" Monteiro says.

He analyzed U.S. Bureau of Reclamation measurements of various concrete cylinders submerged in sodium sulfate solutions. The data, collected over 40 years, charted the progression of damage. Monteiro developed a model to determine how the chemical composition of each cylinder's cement and the amount of water used to make the cylinder influenced the formation of cracks during sulfate exposure.

Concrete with a high water-to-cement ratio eventually forms a network of cracks. Monteiro found that once that network appears, the rate of damage is influenced only by the chemical composition of the cement—not by the water-to-cement ratio. For instance, those concrete cylinders with the most tricalcium aluminate in their cement incurred structural damage sooner than other samples did.

Monteiro developed an index of "potential for damage" during sulfate exposure of concrete that had been made with a high water-to-cement ratio.

Concrete with a low water-to-cement ratio doesn't develop a network of cracks in response to sulfate solutions. Monteiro reports that when initial cracks occur, the exposed cement reacts with water around it, creating chemical products that "heal" the cracks. He describes his work in the Aug. 1 Proceedings of the National Academy of Sciences.

Monteiro "brings an extraordinary new insight" on how sulfate damage affects concrete, comments Hamlin M. Jennings, a materials scientist at Northwestern University in Evanston, Ill. "To have a fresh look at a complex problem like this can help push the field forward" and improve concrete durability, he says.

Jacques Marchand, a civil engineer at Laval University in Quebec City, says that the work "is certainly a contribution that will help people understand how all these parameters interact with one another."

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