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Secrets of Lost Empires

How prestressed concrete is made

1. It all begins at the prestressed concrete plant. 2. This is called a prestressing strand. Made of high strength steel, it will soon be embedded in concrete.





3. The prestressing strand is stretched across the casting bed. About 30,000 pounds of tension will be applied to the cable before it's surrounded by concrete. Why? Hold a rubber band loosely between two fingers. Now stretch your fingers farther apart. See how the rubber band becomes taut and more resistant to downward pressure? The same principle holds true for prestressing strands. 4. Cement, sand, stone, and water make up concrete.

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Photos: (1-3) Rita Seraderian-Precast/Prestressed Concrete Institute; (4) Portland Cement Association.

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