Was Roman Concrete Especially Durable or Is Modern Concrete Exceptionally Crappy?

There is the famous scene in Monty Python's "Life of Brian" where Reg asks, "What did the Romans give us?" A commando responds, "The aqueduct?" After some back and forth, Reg asks, "All right, but apart from the sanitation, the medicine, education, wine, public order, irrigation, roads, a freshwater system, and public health, what have the Romans ever done for us?" Apparently, we can add durable concrete to the list.

An MIT study, published in Scientific Advances, claims Romans "hot mixed" highly reactive quicklime (calcium oxide) with slaked lime (calcium hydroxide, made by mixing quicklime and water). This resulted in little lumps called lime clasts, little calcium oxide bombs that, when exposed to water in a crack in the concrete, react to form calcium carbonate, healing the crack. This is of interest to Treehugger because, if you are going to build with concrete, it should be built to last.

“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations," said study author Admir Masic, a professor of civil and environmental engineering at MIT.

It was big news in the mainstream media with headlines like "Mystery of why Roman buildings have survived so long has been unraveled, scientists say," while the MIT press release headline is "Riddle solved: Why was Roman concrete so durable?"

The architectural press has been notably silent about this, probably because everyone who went to architecture school knew that this was the wrong question. It should be, "Why is modern concrete so crappy?"

We were taught that the answer to the question is that modern concrete has steel reinforcing in it, and Roman concrete didn't. Simply put, reinforcing rusts and expands, cracking the concrete, allowing more water in, more rust, and more concrete destruction. 

As my photo from the Colosseum demonstrates, Roman engineers essentially built formwork out of brick, filled it with concrete and rubble, and then covered it in marble, most of which had been stolen. The concrete is different. It is made with lime and pozzolan, or volcanic ash, but concrete is basically durable, and reinforcing isn't.

Looking for validation, I was going to contact Brian Potter of Construction Physics, but he beat me to it with his new post "Roman vs Modern Concrete." He looked at the new research and wrote, "It's an interesting result, but it's important to put this in context." That context is the steel reinforcing in every modern building.

"While reinforcement provides a lot of benefits, it has drawbacks. The primary one is that, over time, the steel in concrete corrodes... The comparatively short lifespan of modern concrete is overwhelmingly the result of corrosion-induced failure. Unchecked, reinforced concrete exposed to the elements will often start to decay in a few decades or even less."

Potter pointed to a pier in Mexico built in 1941 with stainless steel rebar and still in great shape, while one built next to it in 1969 has already disintegrated. But stainless steel reinforcing costs six times as much as conventional rebar. There is also the green epoxy-coated rebar you see now on balconies and bridges, but it slows the process rather than stopping it.

Potter also reminded us that self-healing concrete isn't unique to the Roman stuff. There are mineral and polymer and even bacterial additives being used and tested. Engineers have been adding Xypex to heal concrete cracks for 50 years.

Ultimately, it's all a question of money and us asking how durable we want our concrete to be. How long do we want it to last? This is a financial rather than an engineering question. As Potter noted: 

"Adding cost to a building to potentially extend its lifespan is often tough to make the numbers work for a developer. Well-made reinforced concrete that’s protected from the weather can last over a century, so the net present value of any additional lifespan beyond that is pretty low. It's much more likely that the building will be torn down for other reasons long before the concrete fails."

When Adolf Hitler's architect Albert Speer was redesigning Berlin to be the capital of the world, he did not want to use reinforced concrete. When he built Zeppelinfeld, the stadium seen in the film "Triumph of the Will," he avoided steel or reinforcing because of his "theory of ruin value." He wanted his buildings to leave a good-looking corpse. Speer wrote in his autobiography, "Inside the Third Reich": 

"Hitler liked to say that the purpose of his building was to transmit his time and its spirit to posterity. Ultimately all that remained to remind men of the great epochs of history was their monumental architecture, he would philosophize. What had remained of the emperors of Rome? What would still bear witness to them today, if their buildings had not survived?"

Speer knew how to make buildings last a thousand years, and so do we. We just don't want to pay for it.