Concrete evidence

It's an early Wednesday afternoon. Mark Butler is on his second delivery of the day. But he's not bringing milk or delivering mail. Mark drives a huge truck with a large spinning drum on the back. Tumbling around inside it are tons of liquid concrete.

This delivery is for a construction project in Washington, D.C., where workers are building a sewer culvert - one of those big curbside drains in the street. A hole 10 feet deep is filled with wooden forms that will hold and shape the concrete until it "cures" (dries and hardens).

Mark backs up his truck, stops, gets out, and runs around to the back. He positions a long steel chute over the hole, then pulls a handle on the back of the truck. Liquid concrete oozes out of the drum, down the chute, and into the hole.

"This is a general mix of concrete that runs 4,000 p.s.i.," Mark says. He's referring to the strength of the concrete. It will be able to support up to 4,000 pounds per square inch. (High-strength concrete may be five times as strong.)

Mark pushes the handle forward, cutting off the flow of concrete. A worker plunges a thick tube into the concrete. The tube starts to vibrate. The vibrations mix the concrete and push any air pockets up to the surface. It's sort of like burping a baby. Getting rid of the air pockets makes the concrete stronger.

"It's pretty neat to watch the truck being loaded," Mark says. "You've got stone, cement powder [mostly limestone that's been heated and crushed], and sand, all coming off different belts. And water is added as the drum spins, mixing it." After all ingredients are added, the drum keeps spinning so the concrete won't harden.

At the site, the workers motion to Mark. He pulls back on the lever again to send more concrete skidding down the chute. They will repeat this process many times, adding concrete and "burping" it until the wooden molds in the hole are filled. Three days later, the concrete will be cured and the new culvert ready for rainstorms.

Just a few miles down the road, the National Building Museum has a whole exhibition devoted to concrete. "It's just cement, sand, rocks, and water," says designer Martin Moeller. Each ingredient is crucial. The sand and rock provide the strength: The harder the rock, the harder the concrete. The water causes a chemical reaction in the cement, which then glues all the rock and sand together.

The exhibit has pictures of such buildings as the Pantheon in Rome, built more than 2,000 years ago. Its dome was cast out of special concrete made with volcanic pumice to make it lightweight. The foundations of Rome's Coliseum, completed in AD 82, are made of concrete as well. Why concrete? In an amazing twist, Italy's many volcanic eruptions provided Roman builders with tons and tons of powdered, heat-treated lime. In effect, it was natural cement mix - just add gravel, sand, and water to make concrete.

But the show isn't just about the past. "Most people think of concrete as something to make buildings with," Mr. Moeller says, "but it's also a part of experimental design."

Adding different ingredients to concrete can give it different properties. A concrete ball floats in a tub of water. You can see light and shadow through a stack of special concrete blocks. The ball floats because the liquid concrete mixture foams when a metal is added during mixing. The air pockets make the concrete lightweight. And because the air pockets are tiny, they don't affect the strength of the concrete.

"It's just as strong as regular concrete," Moeller says. But don't look for a concrete kickball in your future. "It's probably best used to insulate buildings," he says.

The "see through" blocks are more than a foot thick. They contain fiberoptic filaments that transmit light. Moeller demonstrates by standing on the other side of the wall and waving his hand. The shadows he casts show through the blocks.

"I don't think that anyone has figured out what do with them yet," he says. (Can you can think of a good use for them?)

Metal wire was one of the early additions to concrete. Concrete is very strong "in compression." In other words, you can stack a lot of weight on it and it won't collapse. It's not as strong "in tension" - that is, it doesn't stretch very well. Wire is very stretchy, though. Putting wires in concrete adds stretching strength. The result is a material with the strengths of both concrete and metal. This technique of reinforcing concrete is more than a century old and is often used in construction. That's why you see metal bars ("rebar") sticking out of buildings under construction.

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