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Anyone who has visited Rome, or any other city where old Roman constructions stand tall today, will likely have marveled at the ingenuity of the Ancient Romans.
One of the most ingenious creations of these consummate engineers was opus caementicium, known today as Roman concrete. Up until only three years ago, the recipe for this material had been completely lost to time.
In 2017, however, researchers cracked the secret. Today, modern engineers are still learning from these methods that are over 2000 years old. Here are some of the most fascinating facts about Roman concrete.
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1. It all started with the Roman Architectural Revolution
The Roman Architectural Revolution, also known as the Concrete Revolution, started with the Roman Republic which was established in 509 BC and flourished with the advent of the Roman Empire in 27 BC.
It is around this time that the Ancient Romans adopted the widespread use of constructions of architectural forms such as arches, bridges, aqueducts, domes, and vaults. To build these, they would need an incredibly sturdy material.
2. Concrete was a cornerstone for the revolution
Great ingenuity was needed to design constructions such as the Pantheon dome in Rome (built approx. 113 AD), a former Roman Temple turned into a Catholic church that draws tourists from all around the world. Today, almost 2000 years after it was built, it is still the world's largest unreinforced concrete dome.
However, the development of one specific material was essential in making these constructions possible: Roman concrete. Though historical documents on the material are scarce, we know that it was definitely in widespread use from roughly 150 BC. Some scholars claim it was developed roughly a century before that time.
3. The Romans knew how strong their "impregnable" invention was
As the authors of the 2017 study that analyzed Roman concrete point out, the Romans "spent a tremendous amount of work [on developing Roman concrete] – they were very, very intelligent people.”
When they had developed the concrete to the required strength, they were perfectly aware of their accomplishments. As the study explains, Pliny the Elder wrote in his Natural History that it is “impregnable to the waves and every day stronger”.
4. The exact concrete recipe was lost to history
Though modern engineers and architects have long marveled at the lasting power of Roman architecture, and Roman concrete, the fear is that the exact method the Ancient Romans used may have been completely lost to time — disappearing with the fall of the Roman Empire.
“The recipe was completely lost,” said the University of Utah geologist Marie Jackson in a press release in 2017. Though Jackson has spent years extensively studying ancient Roman texts in search of the incredibly strong original mixture used to make Roman concrete, no text has yet been found with a comprehensive recipe.
5. Some historical documents do give clues
Vitruvius, in around 25 BC, wrote in his Ten Books on Architecture, specific types of aggregate appropriate used for the preparation of lime mortars.
For structural mortars, he recommended pozzolana (pulvis puteolanus in Latin), volcanic sand from the beds of Pozzuoli.
Vitruvius described a ratio of 1 part lime to 3 parts pozzolana for cement used in buildings. For underwater work, he specified a 1:2 ratio of lime to pozzolana for underwater work — practically the same ratio used today for concrete in marine locations.
6. Roman concrete breakwaters are impressively strong
As Pliny the Elder wrote, Roman concrete was "impregnable to the waves." No wonder then that the Ancient Romans were pioneers in the principles of underwater construction.
By the first century, Roman engineers started to build constructions, such as breakwaters, that could withstand the power of the sea — many of which still stand today. The city of Caesarea (built approx. 25 BCE) is the earliest known example of a large-scale underwater Roman concrete building site. Today it is part of a natural park in Israel.
7. Scientists analyzed concrete cores from Roman piers, breakwaters, and harbors
As part of the ROMACONS historical project, Jackson and a team of researchers visited concrete cores that are submerged in water in order to study their properties.
They examined these cores, that were parts of Roman piers, breakwaters, and other constructions, using methods such as microdiffraction and microfluorescence analyses at the Advanced Light Source beamline 12.3.2 at Lawrence Berkeley National Laboratory.
8. Early analysis showed that the concrete contained a rare substance that is very hard to make
In their investigations, Jackson and colleagues found a very rare mineral called aluminous tobermorite in the marine mortar. The mineral crystals were formed in lime particles through a pozzolanic reaction at slightly elevated temperatures.
It was a surprising discovery. “It’s very difficult to make,” Jackson says of aluminous tobermorite. Making it in a laboratory is an arduous task that requires very high temperatures.
8. Researchers found out that a chemical reaction within Roman concrete makes it stronger over time
The team of researchers came to the conclusion that the aluminous tobermorite was formed when seawater would percolate through the concrete in breakwaters and in piers, dissolving components of the volcanic ash, and allowing new minerals to form.
“As geologists, we know that rocks change,” Jackson said. “Change is a constant for earth materials. So how does change influence the durability of Roman structures?”
Aluminous tobermorite has silica-rich compositions, reminiscent of crystals that form in volcanic rocks. The crystals have platy shapes that serve to reinforce the cementing matrix. These interlocking plates, in turn, increase Roman concrete’s resistance to brittle fracture. “We’re looking at a system that thrives in open chemical exchange with seawater,” Jackson explained.
9. The strength of Roman concrete was partially down to luck
The strength of Roman concrete will likely have been made through a culmination of trial and error, experimentation, and luck.
As Jackson explained after conducting her research, "Romans were fortunate in the type of rock they had to work with. They observed that volcanic ash grew cement to produce the tuff. We don’t have those rocks in a lot of the world, so there would have to be substitutions made.”
10. The scientists are now working to re-create Roman concrete using the newly-discovered recipe
Though we might not know the exact recipe for Roman concrete, the scientific community is in agreement that, generally speaking, it is made from a mix of volcanic ash, lime (calcium oxide), seawater, and lumps of volcanic rock.
Since carrying out the research and publishing her findings, Jackson has been working with geological engineer Tom Adams to develop a replacement recipe using materials from the western U.S.
11. It needs time to become stronger than modern concrete
Though Roman concrete is stronger than typical modern concrete, also known as Portland concrete, it takes time to develop this strength, due to the process outlined in point 8.
For this reason, it might not be a viable substitute for the concrete that is in use today. However, it could be useful in certain contexts — such as the one described below.
12. A scientist argued that a large new seawall in the UK should be made of Roman concrete
Jackson recently suggested that a large project — a proposed tidal lagoon to be built in Swansea, United Kingdom — should be built from Roman concrete.
The reason she gave is that the lagoon, which was planned to be built in order to harness tidal power, would need to be operational for 120 years to recoup the project's building costs.
“You can imagine that, with the way we build now, it would be a mass of corroding steel by that time,” Jackson explained. Using Roman concrete, on the other hand, the construction could strengthen over time, meaning it could stand for centuries.
13. Research into Roman concrete continues today
Though the research of Jackson and others has served to answer many questions about the properties of Roman concrete, much is still not yet known about how this material changes over the long periods of time it has survived.
“I think [the research] opens up a completely new perspective for how concrete can be made – that what we consider corrosion processes can actually produce extremely beneficial mineral cement and lead to continued resilience, in fact, enhanced perhaps resilience over time,” Jackson explained.
“The Romans were concerned with this. If we’re going to build in the sea, we should be concerned with it too."
Today, countless aqueducts, bridges, piers, and other old buildings still stand today, showing us the long-lasting legacy of the Ancient Roman Empire — and empire built on the sturdiest of concrete foundations.