A major artery in the heart of Chicago is undergoing double-deck surgery, bolstered by a special concrete mix designed to keep Wacker Drive from crumbling for the next generation.

The two-mile-long, double-deck viaduct section being rebuilt carries about 60,000 vehicles a day. Eight bascule bridges intersect with the upper roadway, as do two elevated transit lines–and 60,000 pedestrians on the Loop. Hammered by Chicago's brutal weather, the original road "was falling apart" from freeze-thaw cycles, road salts and wear, says Denise Casalino, assistant project director for the city's Dept. of Transportation. Despite a 1970s patch-up, the deck continued to show failures, forcing a 15-ton limit on the roadway.