While modern concretes crack within a few decades, the Pantheon of Agrippa has been standing for 2,000 years: myths and realities of Roman concrete
Every year, just after Pentecost Mass and as the choir sings the Veni Creator Spiritus, a handful of altar boys climb the fifteen floors up to the lantern in the dome and throw thousands of red rose petals. And while those red sighs make almocárabes in the air and the Sun sneaks in, carving the vault with its rays, you understand perfectly the description of the Paten that Pedro Torrijos once gave: 2,000 years of concrete supported by a column of light.
It is only a sample, perhaps the most intense and radical, of what the Roman Empire was able to raise two millennia ago. But it is not the only one. It is enough to travel a little further east, to the north of the Samaria region in what is now Israel to find Caesarea Marítima and, above all, with the remains of the port of Sebastos: the one that, with its 100,000 m2, It was for centuries the largest man-made port built on the open sea.
There, but also in the dozens of ports, monuments and civil works that still persist around the Mediterranean, it is impossible to ask what we have done wrong. Above all, because we are fed up with seeing concrete structures cracked, compromised or completely crumbling within a few decades of being built, How is it possible that Rome was able to build these concrete wonders? What’s more, how is it that they have reached us?
The myth of Roman concrete
Still today, the numbers of the port of Sebastos are impressive. To build the two spurs that make up the port, Herod (yes, that Herod) needed to use 44 ships and gather 12,000 m3 of kukar (lithified marine sand) to use as fill, 12,000 of lime and 24,000 of pozzolans, a type of volcanic ash. . And all this almost blind: there were no precedents for the use of concrete under water on this technical, economic and logistical scale.
Rome was a lot of Rome. And, for that very reason, it is logical that these structures cemented the myth of Roman cement. Above all, because with the fall of the Empire, concrete disappeared from civil construction and the idea spread that the secret recipe had fallen with the Eternal City. The myth only grew. However, much of the explanation is on King Herod’s shopping list itself.
A few months ago we spoke with Manuel F. Herrador, professor of Structural Concrete at the School of Roads of the University of Coruña and he explained that, despite the widespread idea that Roman concrete is a great mystery, the truth is that we know perfectly how they worked. ‘Pozolana’ comes from the town of Pozzuoli, near Naples. There a volcanic ash similar to pumice is extracted and with a color that ranges from yellowish brown to gray. A material of the highest quality that allows the manufacture of concrete.
That is the keystone of Roman concrete. Vitruvio, in the year 25 a. C. already explained the whole process in his treatise ‘De architectura’ and it is not precisely a piece of knowledge that was completely lost. What happens is that during the Empire 24,000 m3 of ash could be moved to build a port; but in later years that became much more complicated. Architects and engineers knew how to make concreteThey simply didn’t have what to do it with.
It is not that there are only volcanic ash (or materials that allow the construction of concrete) at the foot of Vesuvius. The Vitruvius himself recommends a reddish brown sand from Rome itself. The problem is that the search, extraction and processing of these materials requires a considerable technical and organizational structure. And, of course, a market in which to sell them (and someone with the resources to consider buildings and engineering works of that caliber). All that is what fell with the Roman Empire.
Wit and luck
The lack of raw materials and political organizations with enough resources to embark on civil works of that size may explain why large concrete structures were stopped. But not why it is so good compared to modern concretes. Why have these structures endured so long and ours collapse within a few years?
The answer to this lies in the “survivor bias”. The idea of the quality of Roman concrete comes from our contact with the best structures that they made, the ones that have been best preserved. To be clear: much of everything the Romans built has disappeared during the last 2,000 years. The Pateon has been one of the few classical buildings that has survived to this day in perfect condition because the Byzantine Emperor Phocas donated it to Pope Boniface IV in 608 and he transformed it into the Church of Santa María de los Mártires.
Obviously the building is wonderful. The great earthquake of 1349 that severely damaged the structure of the Colosseum, could have also destroyed the Pantheon and did not. But precisely it does not seem that it makes sense to speak of Roman concrete in a generic sense; but from the Roman concrete that survived.
In recent years, many research groups have examined different concretes distributed throughout the Mediterranean and have found that their survival is due, in part, to luck. A few years ago, researchers at the Lawrence Berkeley National Laboratory in the US analyzed several concrete samples from the Gulf of Naples. Thus they discovered that the mixture used in these structures in contact with sea water generated “aluminous tobermorite”, a mineral that strengthened the stone.
In the same way, a few days ago, an analysis of the Mausoleum of Cecilia Metela, on the Appian Way, showed that potassium-rich leucite was used. That also strengthened the structure. The interesting thing about all this is that it is something that the builders could not know: that is why I speak of luck. Good luck for the buildings and good luck for us too because all those buildings have been living laboratories that today they will teach us to build better and more sustainably.
Image | Evan Qu