Concrete Is Remixed With Environment in Mind

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 constructed 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, the recipes tweaked, as a chef would, for specific strength and durability requirements and to reduce the impact on the environment. One mix, incorporated in wavy sculptures at both ends of the bridge, is designed to stay gleaming white by scrubbing stain-causing pollutants from the air.

The project, built for more than $230 million and finished in September, three months ahead of schedule, “might have been the most demanding concrete job in the United States in 2008,” said Richard D. Stehly, principal of American Engineering Testing, a Minneapolis firm that was involved in the project. It is a prime example of major changes in concrete production and use — changes that make use of basic research and are grounded, in part, in the need to reduce concrete’s carbon footprint.

Concrete may seem an unlikely material for scientific advances. At its most basic, a block of concrete is something like a fruitcake, but even more leaden and often just as unloved. The fruit in the mix is coarse aggregate, usually crushed rock. Fine aggregate, usually sand, is a major component as well. Add water and something to help bind it all together — eggs in a fruitcake, Portland cement in concrete — mix well, pour into a form and let sit for decades.

more from the NY Times