Concrete, with a Dash of Green

(Deccan Herald 4-10-09) 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 ten lanes of traffic on box girders borne by massive arching piers, which are supported, in turn, by footings and deep pilings. The bridge 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. Much of the cement used in the project was replaced with two industrial waste products, fly ash, left over from burning coal in power plants, and blast-furnace slag. Both are what are called pozzolans, reactive materials that help make the concrete stronger. Because the CO2 emissions associated with them are accounted for in electricity generation and steel making, they also help reduce the concrete’s carbon footprint. Some engineers and scientists are going further, with the goal of developing concrete that can capture and permanently sequester CO2 from power plants or other sources, so it cannot contribute to the warming of the planet.