Used to construct monuments such as the Pantheon and the Coliseum, Roman concrete has withstood two millennia of attacks by time and the elements, including wave and water, since it was also used to create artificial harbors throughout the Mediterranean.
Tiziana Vanorio, professor at the Geophysicist Department, Stanford’s School of Earth, Energy & Environmental Sciences, discovered that a natural process reflecting that of the engineering of the Roman concrete occurs in the subsurface of Campi Flegrei (Phlegraean Fields),a large volcanic area west of Naples.
The rock’s microstructures in these sunken volcanic fields have shown an exceptional strength, able to withstand tremendous strains.
Campi Flegrei lies at the center of a large depression, or caldera, and comprises fissures and craters formed during past eruptions, the last of which occurred in 1538.
The city of Pozzuoli is nestled within the caldera and was founded in 600 B.C. by the Greeks. Known to Italians as the birthplace of movie star Sophia Loren, the city was in Roman times one of the major trading ports of the Mediterranean, called Puteoli.
Beginning in 1982, the ground beneath Pozzuoli began rising at an alarming rate. Within two years, the town was raised by six feet — an amount unprecedented anywhere in the world.
The ground swelling was accompanied by persistent seismic activity, and nearly 40,000 people were evacuated from Pozzuoli.
“Ground swelling occurs at other calderas such as Yellowstone or Long Valley in the United States, but never to this degree, and it usually requires far less uplift to trigger earthquakes at other places,” Vanorio said.
“At Campi Flegrei, the micro-earthquakes were delayed by months despite really large ground deformations,” she added.
To answer the long standing question of why the subsurface of the caldera was able to accommodate the deformation without immediately releasing the stored energy through rock fracturing or cracking, Vanorio and a post-doctoral associate, Waruntorn Kanitpanyacharoen, now at Chulalongkorn University in Thailand, analyzed the rock cores from wells that were drilled in the caldera just before the Pozzuoli uplift.
The results, detailed today in Science, showed impressive similarities with the Roman concrete engineering.
“The Roman concrete was made by mixing slaked lime and pozzolana, the volcanic ash from Campi Flegrei. Similar to this technique, the natural process in Campi Flegrei forms a layer, laying between 3200 and 6500 feet, that is made up of a fibrous substance resulting from the mixture of lime and pozzolana,” Vanorio told Discovery News.
The core samples showed that in the Campi Flegrei caldera, the natural lime comes from the decomposition of deep carbonate rocks due to the presence of high temperature and mineral fluids, a process called decarbonation.
“Once formed, the lime is transported by the geothermal fluids up to shallower depths where it finds the rock layer made of pozzolana ash and reacts with it,” Vanorio said.
“It is this reaction that leads to the formation of fiber minerals, Tobermorite and Ettringite, that are also found in man-made concrete, including Roman concrete,” she added.
Vanorio suspects the ancient Roman builders made their remarkable technological breakthrough after observing interactions between the volcanic ash at Pozzuoli and mineral water seeped through the underground rocks in the region.
The speculation is supported by historical sources. The Roman philosopher Seneca (4 B.C. – 65 A.D.) and before him the 1st century B.C. author, architect and civil engineer Vitruvius, noted that there was something special about the ash at Pozzuoli.
"The dust at Puteoli becomes stone if it touches water," Seneca wrote.
Pulvis Puteolanus, Pozzuoli's highly reactive volcanic ash, was used across the ancient world. Until the submersion of the harbor in the fourth century AD, Puteoli was the major commercial and military port of the Roman Empire, and it was common for ships to use pozzolana as ballast while trading grain from the eastern Mediterranean.
As a result, the use of Roman concrete became widespread. Archeologists have recently found that piers in Alexandria in Egypt, Caesarea in Israel, and Cyprus are all made from pozzolana-based Roman concrete.
From a material science stand point, the existence of a natural process in the subsurface of the Campi Flegrei caldera that produces an impermeable, and fiber-reinforced concrete-like rock is extremely important, says Vanorio.
Rocks are generally more brittle than ductile -- they crack when stressed, thus generating earthquakes. On the contrary, the presence of an intricate network of intertwining fibers throughout the caprock of the Campi Flegrei's caldera clearly created a rock with exceptional properties and high strength.
Read more at Discovery News
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