How are Rome's monuments still standing?

Nearly 2,000 years on, how are the Colosseum and the Pantheon still standing despite earthquakes, floods and military conflicts?

Inside the Colosseum's stone and mortar bowl, visitors have ample space to picture the roaring crowds of more than 50,000 that once thronged to the arena for events ranging from bloody gladiatorial battles to opulent processions and chariot races. Also known as the Flavian Amphitheatre, the venue's grand opening in 80 AD featured 100 straight days of games and gore that are said to have included the slaughter of some 9,000 animals. At four storeys tall and 188m across at its widest point, the oval structure remains the largest amphitheatre in the world.

Constructed around 40 years later, the Pantheon houses a mind-bending dome that spans 43m of air and culminates in a pupil-like circular window at its apex known as the oculus that floods the interior with natural light. The name Pantheon, which combines the Greek words for "all" and "gods", suggests a religious function, but some historians think the monument was mainly constructed to pay tribute to Roman emperors. Despite the ravages of time, the iconic half-sphere remains intact and is still the world's largest unreinforced concrete dome.

When it came to building big, the Romans clearly knew what they were doing. Nearly 2,000 years after they were constructed, these two enormous and technically astounding structures have withstood earthquakes, floods and military conflicts, long outlasting the empire that spawned them and becoming physical embodiments of the enduring influence of Roman culture across the globe.

But how did ancient Rome accomplish such monumental, long-lasting architecture so long ago?

Engineers and materials scientists are still studying Roman structures today, and they say the secret is the marriage of ingenious design with an innovative recipe for concrete, a supremely durable and adaptable material that is still used the world over. While the Romans didn't invent concrete, they certainly raised the bar for building with it.

Pouring concrete allowed Roman architects to achieve almost any shape they could imagine, limited only by their ability to construct the wooden forms necessary to mould the rocky slurry. But the arches, vaults and domes that are signatures of Roman buildings were not just flights of fancy.

The highest expressions of the Roman Empire's built environment confront modern visitors with an "engineering approach", said Renato Perucchio, a mechanical engineer at the University of Rochester in New York. "The Romans performed sophisticated analyses that led them to these designs, which were then expressed through an extremely careful construction process."

The concrete that held these designs together was also unique and deeply considered. Roman concrete used a different recipe than modern concrete, and researchers studying this ancient material say its ingredients appear to endow the material with phenomenal resistance to degradation.

Today, most concrete is made up of portland cement – a combination of silica sand, limestone, clay, chalk and other minerals that is baked around 2,000C and crushed into fine powder – and pieces of rock or sand called aggregate. Mixing the rocky aggregate, which varies in size from sand to gravel to small chunks of stone, with the cement makes the resulting concrete stronger and saves cement. Finally, adding water to the concrete mix sets off a chemical reaction in the cement that binds these elements together. For the most part, aggregate in modern concrete is carefully chosen to be as chemically inert as possible. The idea is to avoid any unwanted chemistry once this initial reaction concludes, since any additional reactions down the road usually crack or otherwise weaken the concrete.

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Roman concrete, on the other hand, is a simpler mix of quicklime made from baking and crushing limestone rocks and, most importantly, volcanic rock aggregates of various types, which were abundant in the region surrounding Rome. In contrast to the aggregates used in modern concrete, these volcanic materials used by the Romans are highly reactive and the resulting concrete remains chemically active for centuries after it first hardens.

"Portland cements nowadays are not meant to change chemically, and if they do change it's usually going to have a bad effect," said Marie Jackson, a geologist at the University of Utah who has been studying Roman concrete for decades. "Romans wanted their concrete to react. They chose an aggregate that would continue to participate in the concrete processes over time."

In contrast to modern concrete, this ongoing reactivity allows Roman concrete to get stronger over time. These long-term chemical reactions can serve to reinforce small cracks that often form between the pieces of aggregate and the binding cement and prevent them from propagating farther. This regenerative capacity, made possible by the reactive volcanic minerals, is what enables Roman concrete's tremendous capacity to endure.

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