Even today, there are few spaces more impressive
than the rotunda of Rome’s Pantheon. 43 meters in diameter, and 43 meters
from floor to oculus – and everything besides the columns and marble veneers – the whole
structure of the massive walls and astonishing dome – was made of the versatile and awesomely
durable material that we call Roman concrete. Concrete was everywhere in imperial Rome,
from the walls of tenements to the soaring vaults of the great baths. But in late antiquity,
after centuries of intensive use in cities across the Empire, concrete virtually disappeared.
We’ll explore how and why in this video, which is sponsored by Squarespace.
Roman concrete began with a stroke of geological good luck. The city of Rome was located near
deposits of the fine volcanic powder known as pozzolana [Poe-tsa-lana]. By trial and error,
Roman masons discovered that adding pozzolana to lime and water created a remarkably strong mortar,
which could be mixed with rubble to create durable foundations or walls.
Unlike modern concrete, Roman concrete was not poured, but spread in courses. First, retaining
walls of brick or stone were constructed as a sort of form. A layer of coarse aggregate – usually
chunks of limestone or broken tile – was laid down between these walls, and concrete was troweled
over the rubble bed, almost dry. The mixture was then pounded firm with wooden mallets.
For centuries, Roman concrete was primarily a means of building walls and foundations
quickly and cheaply. Ancient architects had no way of mathematically modeling forces or stresses
or other things likely to cause the collapse of buildings and careers.
They tended to be correspondingly conservative in their use of building materials.
Eventually, however, it was discovered that concrete made with pozzolana could
harden underwater - in fact, thanks to chemical reactions that the Romans knew nothing about,
saltwater actually strengthened the material, forming nearly unbreakable mineral bonds.
So in the waning days of the Republic, the Romans began to build breakwaters and piers
from concrete. Perhaps the most famous example is the artificial harbor of Caesarea Maritima,
pictured here, which was built by Herod the Great with pozzolana concrete imported from Italy.
But it was only in the first century that Roman concrete really came into its own.
Building on earlier experiments, the architects Severus and Celer created a dazzling series
of concrete domes and vaults for Nero’s Golden House. After this, the creative floodgates opened.
The next century and a half witnessed the golden age of Roman architecture,
whose greatest achievements were made possible by concrete. The Colosseum,
built in a marshy valley, was prevented from collapsing by gargantuan concrete foundations.
The cathedral-like concrete halls of the great Roman baths, filled with the sound of fountains
and the glow of sunlight on glass mosaics, were walled and vaulted with concrete.
And the rotunda of the Pantheon, as we have seen, is basically a gigantic concrete cylinder,
crowned by the famous dome. It would be hard to imagine a more spectacular demonstration
of concrete’s possibilities. Yet within a few centuries of the Pantheon’s completion,
the Romans would virtually abandon concrete as a monumental building material.
Before we explore that process, and while we’re still on the theme of building, I’d like to talk
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Back to the show. So what happened to Roman concrete? How could a building material that was
so widespread and so useful just vanish? It should be said, first, that the use of concrete was never
widespread outside of Italy. This partly reflected the fact that – although Roman concrete could be
made with crushed terracotta or volcanic dust from other sources – pozzolana made the best concrete,
and pozzolana was only found in Italy. The reach of concrete was also limited by the persistence of
local building traditions in the provinces, which left little room for new construction methods.
The ultimate reason for the decline of concrete in late antiquity, however, was straightforward:
the buildings for which concrete was best suited stopped being constructed. Sometimes,
this reflected a decline or reorientation of imperial patronage.
In Rome, for example, the last truly spectacular concrete building was the Basilica of Maxentius,
pictured here, which was finished by Constantine. Afterward, with the emperors based elsewhere,
no benefactor had the means or motivation to sponsor anything on a comparable scale.
Monumental buildings were still being constructed in late antiquity,
but the most prestigious were now churches; and especially in the Western Roman Empire,
the default church design was the basilica. Basilicas like Old St. Peter’s – shown
here in cross-section – were long and narrow, with window-filled masonry walls
and wood-framed roofs. There was little need for concrete in such buildings.
Even in the Eastern Roman Empire, where Justinian’s Hagia Sophia made domed churches
popular, concrete was seldom used. Concrete had never been popular in the culturally Greek cities
of the eastern Mediterranean, where walls, vaults, and domes were traditionally made of brick.
In keeping with that practice, even the great dome of Hagia Sophia itself was not made of concrete,
but of brick bedded in thick layers of mortar. Roman concrete was still being used by
the later emperors, at least in specialized contexts. We know that Justinian, for example,
used it to build a new harbor at Constantinople in the mid-sixth century. But after Justinian,
the political turmoil that had already destroyed the Western Empire overtook the Eastern Empire
as well, effectively ending large-scale construction projects for two hundred years.
Not all concrete buildings, of course, were monumental.
Even after the emperors stopped commissioning large projects, concrete was still sometimes used,
as it had been since the beginning, in the walls and foundations of residential buildings.
But the buildings that were best-suited for cement – the big multi-unit insulae
built by professional construction gangs – were no longer in demand. With the general collapse
of urban populations across the Roman Empire, in fact, many cities now had more houses than people.
During the early middle ages, in short, when the few new buildings being constructed
were almost invariably modest structures with rubble walls, there was little demand
for concrete. Concrete was not actually forgotten, since – for the literate minority – descriptions
and recipes were still available in the texts of Pliny the Elder and Vitruvius.
The seventh-century scholar Isidore of Seville, likewise,
mentioned Roman concrete in his widely circulated etymological encyclopedia. By his time, however,
concrete, like so many other aspects of the Roman world, was little more a curiosity – intriguing,
perhaps inspiring, but ultimately irrelevant. If you enjoyed this video, please consider
supporting toldinstone on Patreon. You might also be interested in my book,
Naked Statues, Fat Gladiators, and War Elephants. Thanks for watching.