VOA科学技术2023--Scientists Study Long-lasting Ancient Structures（在线收听）

Scientists Study Long-lasting Ancient Structures

Ancient builders across the world created structures that are still standing today, thousands of years later.

Roman engineers built thick concrete sea barriers, for example. Mayan builders created sculptures to their gods, and Chinese builders raised walls against invaders.

But there are many recent structures that are already starting to fail. The concrete that makes up much of our modern world lasts around 50 to 100 years.

A growing number of scientists have been studying materials from long-ago. They are breaking apart pieces of buildings and reading historical texts hoping to learn how they have stood for thousands of years.

The research has turned up a surprising list of materials that were mixed into old buildings. They include tree bark, volcanic ash, rice, beer and even urine. These unexpected materials could have the ability to get stronger over time and "heal" cracks when they form.

Figuring out how to copy these features could have real impacts today. While our modern concrete has the strength to hold up very tall buildings and heavy infrastructure, it cannot compete with the endurance of these ancient materials.

With the rising threats of climate change, there is a growing call to make construction more sustainable. A recent UN report estimates that the built environment is responsible for more than a third of carbon dioxide emissions worldwide. Cement production alone makes up more than 7 percent of those emissions.

Is ancient Roman concrete better?

Many researchers have turned to the Romans. Starting around 200 BCE, the Roman Empire was building concrete structures that have stood the test of time. That includes the Pantheon to the aqueducts that still carry water today.

Even in places where seawater has been hitting structures for ages, you will find concrete "basically the way it was when it was poured 2,000 years ago," said John Oleson. He is an archaeologist at the University of Victoria in Canada.

Most modern concrete starts with Portland cement, a powder made by heating limestone and clay to super-high temperatures and grinding them up. That cement is mixed with water to create a paste. Then, materials like rock and gravel are added.

Records from ancient builders show the Roman process was similar. The ancient builders mixed materials like burnt limestone and volcanic sand with water and gravel, creating chemical reactions to tie everything together.

Now, scientists think they have found an important reason why some Roman concrete has held up structures for thousands of years. The ancient material has an unusual power to repair itself. Exactly how is not yet clear, but scientists are starting to find reasons why.

Admir Masic is a civil and environmental engineer at the Massachusetts Institute of Technology. In a study, he said this power comes from chunks of limestone throughout the Roman material that is not mixed in evenly. Researchers used to think these chunks were a sign that the Romans were not mixing up their materials well enough.

Instead, the scientists found that the chunks could fuel the material's "self-healing" abilities. When cracks form, water enters the concrete, Masic explained. That water activates the leftover lime, creating new chemical reactions that can fill in the damaged sections.

Ancient Roman ... skyscrapers?

Today's builders cannot just copy the ancient processes. Even though Roman concrete lasted a long time, it could not hold up heavy buildings. "You couldn't build a modern skyscraper with Roman concrete," Oleson said. "It would collapse when you got to the third story."

Instead, researchers are trying to take some of the ancient materials and add them into modern mixes. Masic is part of a company that is trying to build new projects using "self-healing" concrete.

People do not need to make things last quite as long as the Romans did to have an impact, Masic said. If we add 50 or 100 years to concrete's lifespan, he said, "we will require less demolition, less maintenance and less material in the long run."

Words in This Story

concrete — n. a hard, strong material that is used for building and made by mixing cement, sand, and broken rocks with water

infrastructure — n. the basic equipment and structures that are needed for a country, region, or organization to function properly

endurance — n. the ability to do something difficult for a long time

sustainable — adj. able to be used without being completely used up or destroyed

emissions — n. the act of producing or sending out something from a source

aqueduct — n. a structure that looks like a bridge and that is used to carry water over a valley

paste — n. a soft, wet mixture of usually a powder and a liquid

chunk — n. a thick piece of something

demolition — n. deliberate destruction of a building or other structure