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.
Architects love concrete because it's so easily shaped. They can make buildings imitate natural shapes. The famous Sydney Opera House in Australia looks like a bunch of seashells stacked on top of each other. Another concrete building looks like a sponge. Another, like an eagle. A bridge being built in Millau, France, looks like a sail floating across a valley.
The most common concrete buildings are houses - except in the United States. Americans don't build their homes of concrete. "But in the rest of the world," Moeller says, "it is expected that your house is going to be made out of concrete."
Well over 2,000 years ago, the Romans were using concrete to make aqueducts and a network of roads that ran for more than 5,300 miles. Today, most of the world's houses are made of concrete, although few of these homes are in America. But many Americans do work in concrete offices, partly because architects love shaping 'liquid stone' into a variety of beautiful, sturdy buildings. Below is a timeline of important dates in concrete.
300 BC-AD 476 Romans begin building with a natural cement based on volcanic ash. Animal fat, milk, and blood were added to mixtures to make them stronger. Many of these structures still exist.
1200-1500 The quality of concrete declines as people lose the technology to make good cement.
1779 Bry Higgins of Ireland patents his hydraulic cement, which can harden underwater, for plastering the outside of houses. Today, we call this stucco.
1848 Joseph Louis Lambot of France builds the first concrete-hulled boat.
1867 Frenchman Joseph Monier finds that wire reinforcement makes concrete stronger.
1889 The first bridge made of reinforced concrete is built in America.
1891 George Bartholomew builds America's first concrete street in Bellefontaine, Ohio. (It's still there!)
1908 Thomas Edison builds concrete houses (still standing) in Union, N.J.
1936 The world's largest concrete dams are built in America: the Hoover and Grand Coulee Dams.
October 2003 The Taipei 101 Tower in Taiwan, made of reinforced concrete, is declared to be the world's tallest building (1,667 ft.).
Concrete isn't just for grown-ups. Here's how to make mini-concrete blocks you can use to build a doll house, a mini-skyscraper, or a fort for your toy soldiers. You'll need an adult's permission and help for this. Cover your work area with old newspapers and wear protective goggles and rubber gloves. Do not pour leftover cement down the drain!
• Bag of Quikrete cement mix (Note: It's fairly cheap at hardware stores, but it comes in big heavy bags.)
• Crisco shortening (or equivalent)
• Plastic ice cube tray
• Zipper-closure freezer bag, any size
• Rubber gloves
• Safety goggles
• 1/2 cup measure
• Tablespoon measure
1. Lightly coat the inside of the ice tray with Crisco. This will keep the concrete from sticking.
2. Wearing gloves and goggles, pour 1/2 cup of concrete mix and 5 teaspoons of water into the freezer bag.
3. Seal the bag. Mix the concrete by squishing the bag around. The chemical reaction will make the mix feel warm.
4. Snip off a corner of the bag and squeeze the concrete into the ice tray. (It's more fun to use your hands, but you must wear gloves if you do that.)
5. Let the blocks cure for at least three hours before you turn over the tray and gently pop them out.
You can stick the dried blocks together with white glue or a dab of wet concrete.
Source: American Society of Civil Engineers. Used with permission.
Cement: mixture of fine compounds, including ground limestone and sand, which harden when combined with water.
Concrete: building material of water-hardened cement, sand, and gravel.
Curing time: the amount of time it takes cement to completely harden.
Mortar: a paste of cement and sand used to hold building materials such as bricks or stone together
Tension: the force caused by stretching something.
Yard: unit of concrete measurement, shorthand for a cubic yard (3 feet high by 3 feet wide by 3 feet deep).