Concrete is considered structural lightweight concrete if it has a 28-day compressive strength of over 2500 psi and an air-dried unit weight of not over 115 pounds per cubic foot. Because use of structural lightweight concrete reduces the dead load on a structure, the size of beams, columns and foundations can be reduced, which in turn cuts the amount of concrete and reinforcing steel required. Lightweight concrete may cost more per cubic yard than normal weight concrete, but because less concrete and less steel are required the total cost of materials can be reduced significantly. Structural lightweight concrete has been used in several ways: to build new bridges or repair or widen old bridges, to add extra floors to existing buildings, to cut the amount of concrete and reinforcing steel required in new buildings, and to reduce the dead weight of large roofs. A few examples are illustrated in this article.
THE PARROTTS FERRY BRIDGE
Lightweight concrete has played a prominent role in bridge construction since the 1930s. Built in the late 70s over the Stanislaus River near Vallecito, California, the Parrotts Ferry Bridge is a prime example of advanced bridge design and construction using lightweight concrete. Lightweight concrete was used in the precast, post-tensioned single cell box girders of the bridge.
CALGARY SADDLEDOME STADIUM
Built for the 1988 Winter Olympics in Canada, this 20,000-seat coliseum was made almost entirely of lightweight concrete, much of it precast.
THE LONGEST DOUBLE-Ts EVER MADE
At 145 1/2 feet, 18 prestressed concrete double-Ts precast for the roof of a Denver automobile showroom are believed by the manufacturer to be the longest double-T panels ever made.