Romans’ concrete way to building strong walls
TWO thousand years ago, Roman builders constructed vast sea walls and harbour piers. The concrete they used outlasted the empire – and still holds lessons for modern engineers, scientists say.
A bunch of half-sunken structures off the Italian coast might sound less impressive than a gladiatorial stadium. But underwater, the marvel is in the material. The harbour concrete, a mixture of volcanic ash and quicklime, has withstood the sea for two millennia and counting. What’s more, it is stronger than when it was first mixed.
The Roman stuff is “an extraordinarily rich material in terms of scientific possibility”, said Philip Brune, a research scientist at DuPont Pioneer who has studied the engineering properties of Roman monuments. “It’s the most durable building material in history, and I say that as an engineer not prone to hyperbole.”
Modern concrete exposed to salt water corrodes within decades.
The mystery has been why the ancient material endured. “Archaeologists will say they have the recipe,” said Marie Jackson, an expert in ancient Roman concrete at the University of Utah. But it’s not the complete picture: it’s one thing to assemble the ingredients, another to know how to bake the cake.
To that end, Jackson and her colleagues peered into the microscopic structures of concrete samples, extracted from the sea walls and piers as part of a project called the Roman Maritime Concrete Study. “This rock-like concrete is behaving, in many ways, like volcanic deposits in submarine environments.”
Where modern concrete is designed to ignore the environment, Roman concrete embraces it. As the scientists report in a study published on Monday in the journal American Mineralogist, Roman concrete is filled with tiny growing crystals. The crystals, like armour plates, may keep the concrete from fracturing.
The scientists subjected the samples to a battery of advanced imaging techniques and spectroscopic tests. The tests revealed a rare chemical reaction, with aluminous tobermorite crystals growing out of another mineral called phillipsite. Brune, who was not involved with the study, called the work a “significant accomplishment”.
The key ingredient proved to be seawater. As it percolated within the tiny cracks in the Roman concrete, Jackson said, it reacted with the phillipsite naturally found in the volcanic rock and created the tobermorite crystals.
If Jackson’s effort is successful, the concrete could have a role to play in human history – “if one was interested in making sea walls” and “forced to protect shoreline environments”.
Modern sea walls require steel reinforcements; a future in which “large relic walls of twisted steel” dot the coast would be “troubling”, said Jackson. The Romans didn’t use steel. Their reactive concrete was strong enough on its own.
“IT’S THE MOST DURABLE BUILDING MATERIAL IN HISTORY, AND I SAY THAT AS AN ENGINEER NOT PRONE TO HYPERBOLE