The Secrets of Ancient Rome’s Buildings
What is it about Roman concrete that keeps the Pantheon and the Colosseum still standing?
- By Erin Wayman
- Smithsonian.com, November 16, 2011, Subscribe
The Romans started making concrete more than 2,000 years ago, but it wasn’t quite like today’s concrete. They had a different formula, which resulted in a substance that was not as strong as the modern product. Yet structures like the Pantheon and the Colosseum have survived for centuries, often with little to no maintenance. Geologists, archaeologists and engineers are studying the properties of ancient Roman concrete to solve the mystery of its longevity.
“Roman concrete is . . . considerably weaker than modern concretes. It’s approximately ten times weaker,” says Renato Perucchio, a mechanical engineer at the University of Rochester in New York. “What this material is assumed to have is phenomenal resistance over time.”
That resistance, or durability against the elements, may be due to one of the concrete’s key ingredients: volcanic ash. Modern concrete is a mix of a lime-based cement, water, sand and so-called aggregates such as fine gravel. The formula for Roman concrete also starts with limestone: builders burned it to produce quicklime and then added water to create a paste. Next they mixed in volcanic ash—usually three parts volcanic ash to one part lime, according to the writings of Vitruvius, a first-century B.C. architect and engineer. The volcanic ash reacted with the lime paste to create a durable mortar that was combined with fist-size chunks of bricks or volcanic rocks called tuff, and then packed into place to form structures like walls or vaults.
By the beginning of the second century B.C., the Romans were already using this concrete in large-scale construction projects, suggesting their experimentation with the building material began even earlier. Other ancient societies such as the Greeks probably also used lime-based mortars (in ancient China, sticky rice was added for increased strength). But combining a mortar with an aggregate like brick to make concrete was likely a Roman invention, Perucchio says.
In the earliest concretes, Romans mined ash from a variety of ancient volcanic deposits. But builders got picky around the time Augustus became the first Roman emperor, in 27 B.C. At that time, Augustus initiated an extensive citywide program to repair old monuments and erect new ones, and builders exclusively used volcanic ash from a deposit called Pozzolane Rosse, an ash flow that erupted 456,000 years ago from the Alban Hills volcano, 12 miles southeast of Rome.
“Emperor Augustus was the driving force behind the systemization, standardization of mortar mixes with Pozzolane Rosse,” says Marie Jackson, a geologist and research engineer at the University of California at Berkeley. Roman builders likely favored the ash deposit because of the durability of concrete made with it, she adds. “This was the secret to concretes that were very well bonded, coherent, robust materials.”
Jackson and her colleagues have been studying the chemical composition of concretes made with Pozzolane Rosse. The ash’s unique mix of minerals appears to have helped the concrete withstand chemical decay and damage.
The Romans favored another specific volcanic ash when making concrete harbor structures that were submerged in the salty waters of the Mediterranean. Pulvis Puteolanus was mined from deposits near the Bay of Naples. “The Romans shipped thousands and thousands of tons of that volcanic ash around the Mediterranean to build harbors from the coast of Italy to Israel to Alexandria in Egypt to Pompeiopolis in Turkey,” Jackson says.
Seawater is very damaging to modern concrete. But in Roman concrete, the Pulvis Puteolanus “actually plays a role in mitigating deterioration when water percolates through it,” Jackson says. Although the exact mechanism is unknown, it appears that chemical reactions among the lime paste, volcanic ash and seawater created microscopic structures within the concrete that trapped molecules like chlorides and sulfates that harm concrete today.
Despite the success of Roman concrete, the use of the material disappeared along with the Roman Empire. Concrete structures were seldom built during the Middle Ages, suggesting volcanic ash wasn’t the only secret to the durability of Roman concrete, Perucchio says. “These really large projects could only be done with the appropriate bureaucracy, with the proper organization that the Roman Empire would provide.”
Erin Wayman is an assistant editor at Smithsonian and writes the Hominid Hunting blog.
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Comments (22)
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how many materials were needed
Posted by flossie brock on February 18,2013 | 12:27 PM
it is amazingthat you can make concrete from those supstances without having to use stone or other important ingredents of modern concrete.
Posted by Nick Haynes on January 19,2012 | 02:53 PM
Most of 20 years ago now, the military staff on which I served sent an engineering team to evaluate an overland route in the Balkans for its potential for military use. During a video teleconference with the team leader in the field, he sent photos of the bridges on the route, which consisted of concrete arches, some quite large. For the benefit of the participants, one of the senior officers on our end noted that they were built in "the Roman style", at which point the young engineer in the field interjected: "Actually, sir, these aren't Roman style bridges, they are Romans-built bridges." At least for me, it prompted a moment of historical connection with the preeminent military and civil engineers of their (or, for that matter, any) age.
Posted by G. Hovatter on December 31,2011 | 03:12 PM
In researching my novels set in the Roman Republic, I was continually amazed at the sophistication of Roman concrete construction. The first complete poured concrete structure (to the roof) in Rome was apparently a customs building completed around 185 BCE (long before Augustus.) I found a quote in Cicero (I think) that said they'd learned about concrete from the Egyptians, but I can't find it now. In any event I'm glad someone else is writing about this. Check my website: www.judithgeary.com
Posted by Judith Geary on December 23,2011 | 09:19 PM
Just returned from a visit to the colosseum. In response to Eric Vic's question (above), there's quite a bit of stone in the foundations and lower portions of the high structure, but the rest is mostly brick as is most of whats left of the Palace of the Caesars on the nearby Palatine Hill, and the Pantheon (except for the dome, of course). Apparently it was all once covered with a veneer of white travertine, long since stripped off and recycled elsewhere in the city.
Posted by Vince Lee on December 19,2011 | 07:06 PM
As a long time cement finisher I'm always facinated by articles on Roman engineering, especially their concrete. In response to the previous mention of rebar, current bridge construction in the USA requires epoxy coating on the rebar as a barrier to moisture. I'm not aware of any Roman rebar in concrete but have read that some of their stone construction had lead coated iron rods in it, likely due to previous experience of stonework cracking from bare iron rods corroding and swelling internally.
Posted by Mark Hamilton on December 13,2011 | 10:53 PM
As an architect/designer, I find I am most fascinated with not the technical aspects of the article, but with the reactions of the readers from a cultural point of view. Many prople are seemingly shocked that an "ancient" society would know something that we do not or have forgotten technically. It implies that we generally believe that progress has been an ascending, mostly straight line upward. Rather history is rampant with parallel offshoots of development and yes, even grandeur which fades and gives way to others. Signifcance and meaning are relative to the peoples and their ever expanding desire to express themselves that much beyond what is or has been known.
The Romans' ideas echo back and forth through the centuries- both the good and the horrific.
Concrete that can flourish for two millenia being but one of them.
Posted by Larry A. Daum on December 13,2011 | 02:33 PM
I also assume that the Tuff made for a lighter overall weight, which logically speaking also made the structure less detrimental to itself. The volcanic ash would also create some good weather resistance.
Posted by L. Ross-Mansfield on December 10,2011 | 01:37 PM
Very interesting article but I was looking for more details on what makes it durable. For example what is volcanic ash composed of? How is it different then the modern formula with sand etc. Chemically what is going on between the bonds and crystals in the cement?
Posted by Ron Dabkowski on December 9,2011 | 04:24 PM
Unfortunately Roman concrete was not used on bridges for the New York State Thruway. I can remember my Driver Education teacher stopping us under an overpass and showing us, in 1963, the bridge concrete rotting.
Posted by ErnestPayne on December 7,2011 | 07:24 PM
What if everything we built lasted 2000 yrs? Imagine the savings in housing alone.
Matthew Bronsky a restoration architect, gives a detailed consideration of Roman materials and style:
Durability Lessons Learned from In-Situ Diagnostic Studies of Original Construction Details
http://vitruviusfootsteps.wordpress.com/2010/03/30/week-29-%E2%80%93-the-design-of-concrete-for-durability-by-the-ancient-romans-part-1-reinforcement-or-lack-thereof/
Posted by Homefire on December 5,2011 | 11:35 AM
Modern high strength concrete uses pozzalans (spelled incorrectly} These are similar to volcanic ash. Fly ash is mixed with damp soil to dry it for compaction.
Posted by John muth on December 4,2011 | 06:55 PM
Perhaps the Mafia purposely didn't mix concrete used in the United States so they would get the repair contracts!
Posted by Ace on December 2,2011 | 07:18 PM
I'm surprised that there's no mention of stone. I thought the Colosseum and other monumental and engineered structures were made of stone? Just asking.
Posted by Eric Vic on December 2,2011 | 11:18 AM
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