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The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire.The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome.Modern structural concrete differs from Roman concrete in two important details.First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble.They kept the cisterns secret as these enabled the Nabataea to thrive in the desert.
It enabled revolutionary new designs in terms of both structural complexity and dimension.
German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400–1200 BC.
During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana and an aggregate of pumice.
Many Roman aqueducts and bridges, such as the magnificent Pont du Gard in southern France, have masonry cladding on a concrete core, as does the dome of the Pantheon.
After the Roman Empire, the use of burned lime and pozzolana was greatly reduced until the technique was all but forgotten between 500 and the 14th century.
Today, large concrete structures (for example, dams and multi-storey car parks) are usually made with reinforced concrete.