Soaring above the Mississippi River just east of downtown Minneapolis is one remarkable concrete job. There on Interstate 35W, the St Anthony Falls Bridge carries 10 lanes of traffic on box girders borne by massive arching piers, which are supported, in turn, by footings and deep pilings. The bridge, built to replace one that collapsed in 2007, killing 13 people, is built almost entirely of concrete embedded with steel reinforcing bars, or rebar. But it is hardly a monolithic structure: the components are made from different concrete mixes for specific strength and durability and to reduce the impact on the environment. The project, built for more than $230 million,