Special Reports

This stuff isn't like your city sidewalks

The new Tacoma Narrows bridge towers will be made of concrete, but the concrete being used is very different from the type sold in bags at the local hardware store.

The tower concrete must be super strong and dense to keep salt spray and acid rain from penetrating to the towers' steel skeleton. It also needs to be relatively fluid when wet so it will flow easily in and around the tightly woven walls of steel reinforcement rods.

More water in the mix would increase its fluidity, but it also would reduce strength. So instead of extra water, the bridge builder is adding "plasticizers" that increase flow but don't reduce strength.

Normal cement, used to bind together particles of sand and aggregate, leaves tiny air spaces when cured. To reduce the air spaces and increase strength, the bridge builder is adding extremely fine particles to the mix. They're using fly ash, a waste product captured in the stacks of coal-fired power plants, and silica, an even finer manufacturing by-product typically thought of as air pollution.

In relative terms, if a grain of cement were the size of a basketball, a grain of fly ash would be the size of a softball. A grain of silica would be the size of a golf ball.

The concrete produced is dense and strong. Concrete used in most construction projects has a strength of about 4,000 pounds per square inch. Bridge tower specifications call for a strength of 7,000 psi, but tests indicate the mix being used will be closer to 11,000 psi.

Rob Carson, The News Tribune

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