The Sixth Street Viaduct Bridge in Los Angeles—the site of numerous Hollywood movie scenes—has reached the end of its career. City engineers are reviewing a request-for-proposals document for a final cable-stayed design to replace the structure, which consists of two prestressed-concrete viaducts and a double steel-arch center span.
"The bridge is very sick," says Tonya Durrell, an agency spokeswoman. "It has alkali-silica reaction," or ASR. She says ASR can resemble a chronic disease, ongoing and irreversible. In ASR, the silica in the aggregate of a concrete structure breaks down, creating a gel that takes on water and expands. The concrete cracks from the pressure of the expanding gel.