Concrete evidence

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!

Materials

• 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
• Scissors
• Rubber gloves
• Safety goggles
• 1/2 cup measure
• Tablespoon measure

Directions

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).

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