The Pantheon: still the largest reinforced concrete dome in the world

If there was a competition to find the most durable and beautiful concrete structure ever built, the Pantheon in Rome would most certainly win the prize. It also stands as a monument to Roman concrete genius.

Commissioned by Hadrian (who was emperor 117-138 CE) as a temple to all the gods, the Pantheon replaced the earlier temple of Agrippa following a fire. It is still Italy’s most visited site, having withstood centuries of tourists, floods, wars and earthquakes.

Its huge concrete dome – 43.4m in diameter and 21.75m in height – was unrivaled in size until the construction of Florence Cathedral in the 1400s, and is still the largest ever made with unsupported concrete.

“The mastery of building something so daring and having the structure stand essentially without any structural support for over 19 centuries is nothing short of extraordinary,” says Renato Perucchio, Professor of Mechanical Engineering and Program Director of archaeology, technology and historical structures at the university. University of Rochester in the United States.

So how did the Romans do it, what were the secrets of their recipe for concrete, and what lessons can architects and civil engineers learn from its construction today?

As any builder knows, foundations are everything. One of the most overlooked aspects of the Patheon’s remarkable construction is beneath its famous dome. Although Rome is not on one of Italy’s main seismic zones, it has experienced earthquakes. Many historians believe that seismic activity caused damage to the Colosseum. Additionally, until some of the tributaries of the Tiber were buried in the late 19th century, it was also subject to significant flooding.

The foundations of the Pantheon were made of concrete, originally 4.7 m deep and 7.3 m thick. During construction, however, they cracked due to the marshy, clayey earth below. “For this reason, a second reinforcement ring was built, exceeding the original perimeter by three meters”, according to the Archeoroma site. Thick buttress walls were also built to the south of the building anchored to the nearby Basilica of Neptune. “This had the effect of stabilizing the structure by counterbalancing the forces and weights at each end,” Archeoroma writes.

The Romans did not invent concrete. It already existed for hundreds of years before the construction of the Pantheon.

Curiously, this distinction probably goes back to the Bedouin Nabataean tribes of the land that is today southern Syria and northern Jordan, who used it to create hidden underground water cisterns around 700 BC.

The basic recipe for concrete followed by the Romans can be found in Roman architect Vitruvius’ book “De Architectura”, published 100 years before the Pantheon was built. He described how to make concrete from lime and pozzolana sand, a type of volcanic ash found near Naples, all mixed with stone mass.

Different aggregates have been used to give the concrete various densities. Travertine limestone gave the foundations of the Pantheon a density of 2,200 kg per cubic meter, while a lighter rock was chosen for the dome.

Pozzolans, made up of siliceous and aluminous materials, possess little or no cementitious value, but when mixed with water they chemically react with calcium hydroxide at ordinary temperature to form cementitious compounds.

It was the chemistry of this material that formed the basis of the dome’s durability, allowing it to survive two millennia without the steel tension rods used today.

Indeed, the Romans understood that the bigger the structure, the stronger it was, because the easiest way to keep concrete in compression is to put heavy things on it, like more concrete.

This is a trick still used today. Many large concrete dams are gravity or arch structures that rely on their own weight and geometry to resist water.

However, the circular structure of the dome meant that before the ancient engineers could start making its concrete ceiling, they had to figure out how to direct the weight away from the center. If they hadn’t done this and removed the wooden structure holding it in place, the 3,000 tonnes of concrete used to make the dome would have pushed out and the whole edifice would have collapsed. under its own weight.

Even the type of scaffolding frame used to support such a frame is still under discussion. “Think about the design of the scaffolding that supports a structure of that weight,” says Perucchio. “They [the ancient Romans] had a great mastery in the use of the wooden frame in a way that no other previous culture had developed.

Today, when we build in concrete, we introduce a steel tension rod that picks up half the stresses in the concrete. The Romans used their ancient recipe for concrete and an abundance of highly skilled craftsmen, who stuffed the stiff mix into molds and walls, rather than pouring it as is done today.

To construct the dome, the Roman builders constructed a solid base, a six-meter-thick wall in the shape of a rotunda, to serve as the foundation for the ceiling. They then used the vertical walls on either side to buttress the dome itself.

As the ceiling rose towards its peak, the master craftsmen mixed increasingly lightweight aggregates into the concrete.

This principle of using different aggregate weights ranges from the heavy travertine used in the base to the top of the dome.

“It doesn’t look like it from the inside, but from the outside, it’s a very thick, but relatively light dome,” says Norbert Delatte, director of the School of Civil and Environmental Engineering at Oklahoma State University. .

In some areas, ancient builders mixed in small clay vessels, called amphorae, to control weight.

The concrete aggregate used to make the upper dome region consists of alternating layers of lightweight tuff, found in abundance north of Rome, and pumice, the material we use today to file rough skin. The concrete substance at the top of the dome had a density of only 1,350 kg per cubic meter.

To make the ceiling even lighter, masons cast encased concrete waffle-shaped panel bricks called coffers; five layers of these bricks formed the interior ceiling. They hammered the concrete into the molds using some kind of pestle, probably made of wood or iron.

This meant that aesthetically they had allowed an area of ​​the dome to be decorated while simultaneously reducing the amount of concrete needed for the dome itself.

At the top, the crowning glory of the Pantheon is an open oculus 7.8m in diameter, which let in light, adding to the sense of wonder the building still elicits today. But more importantly, it meant that the top of the dome was made of the lightest material of all, air.

Today, engineers around the world are studying the chemical properties of Roman concrete to see if it can be replicated today to make buildings that last longer. Although the basic ingredients were defined by Vetruvius, modern measurement technologies make it easier to determine exact chemical properties from small samples of material.

Yet the clever use of engineering and the unique material of Roman concrete is not the only reason this enduring temple dedicated to all the gods still stands to inspire awe today. History also played a role in its durability. Perhaps the event that most assured its long destiny occurred in AD 609, nearly 400 years after it was built.

Emperor Phocas, the Byzantine emperor in the east, gave the Pantheon to the Catholic Church in Rome. The Vatican has used it as a place of worship ever since, while its formidable structure also serves as a sanctuary for architects and engineers from around the world.