You know that city-mouse/country-mouse story? Well, I feel like country-mouse right now. I’ve been interning this quarter in Washington, D.C. – a bustling city 2,388 miles from dear old Davis.
Traveling across this city has made me realize that I take a huge part of modern life for granted. There’s a great invention that makes it possible for me to travel underground on the subway and take an elevator 20 floors above street-level.
This technology is concrete. It’s the gray, coarse, wonderful material we build our lives on.
Think about this: Until 1309, the tallest freestanding structure on Earth was the Great Pyramid of Giza in Egypt. It was 481 feet tall. Today, the tallest building in the world is Burj Khalifa, a skyscraper in Dubai. It’s 2,717 feet tall – more than half a mile. This ridiculous new height is made possible by reinforced concrete.
The Romans had an early version of concrete, but if you’d told the ancients what concrete does for us today, it would have sounded like fantasy. Until 1994, the English Channel could only be crossed by boat or plane. Now, we can take a train underneath the 23.5 miles of ocean between France and the UK. Thank you, concrete.
“It’s the most widely used material in the world,” said Dr. John Bolander, associate professor of structural engineering and mechanics at UC Davis.
Bolander shared with me the recipe for concrete. The basic ingredients are water, cement and aggregates. Aggregates are little pieces of rock and sand. The cement is usually a kind called Portland cement, which is mainly crushed silica and limestone.
“Portland cement is the glue that binds everything together,” Bolander said.
These concrete ingredients can be found around the world, so once engineers perfected steel, concrete buildings reinforced with steel rebar sprang up everywhere. Concrete is stronger than wood, lighter than brick and more portable and shapeable than stone.
From my science-nerd perspective, the behavior of concrete is fascinating. Unlike glue, concrete doesn’t harden through a drying process. Concrete “cures,” which means the water reacts with chemicals in cement to produce crucial binding agents that make concrete strong. The chemical reaction causes concrete to heat up (an exothermic reaction), and it’s important to keep the concrete wet during this process.
Fun fact: When they built the Hoover Dam in the 1930s, engineers realized that heat from the chemical reaction in the concrete would take 125 years to dissipate naturally. The solution was to embed coils of steel pipe in the concrete slabs so cold river water could run through and cool it down.
Sadly, this fabulous technology has a drawback – concrete is environmentally unfriendly.
To make Portland cement, you have to heat up the silica and limestone to 1,400 degrees Celsius. Portland cement production accounts for 5 to 10 percent of our CO2 production, Bolander explained. That’s a lot of energy. Plus, huge concrete structures can screw up natural ecosystems and agriculture by diverting rainwater when it falls.
Bolander was excited to tell me how material scientists are turning concrete green. One advance has been the invention of porous concrete, a material that allows rain to soak through to the soil below. With this technology comes the challenge of making concrete freeways strong enough for cars, but porous enough for water. New-fangled concrete could also be good for the atmosphere.
“They’re designing concrete now to actually remove CO2 from the atmosphere,” Bolander said.
Bolander said material scientists and engineers are looking to fly ash, a waste product of coal combustion, as an alternative to Portland cement. Fly ash is already mucking up the environment, so we should put it to good use.
When I walk up to my office in D.C., the sturdy concrete building feels as permanent as the pyramids. Yet the pyramids are changing, eroding and shrinking over time. Concrete changes, too. Newly poured concrete actually gets stronger after it has “cured.”
“It’s a chemical process that goes on for years,” Bolander said. “It’s alive.”
MADELINE MCCURRY-SCHMIDT doesn’t usually write about engineering, so this column was a challenge. She dedicates this endnote to the inventor of the Cheez-It omelet. E-mail her column ideas at email@example.com.