Two thousand years ago, Roman builders constructed vast sea walls and harbor 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 colosseum. But underwater, the marvel is in the material. The harbor 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 human history, and I say that as an engineer not prone to hyperbole.”
By contrast, modern concrete exposed to saltwater 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. (Pliny the Elder once wrote an ode to concrete “that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass, impregnable to the waves.”) 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 rocklike concrete is behaving, in many ways, like volcanic deposits in submarine environments,” Jackson said.