History of architecture questions

The temple - 43 meter in diameter - was built by emperor Hadrian. Its dome with 8-meter wide oculus was built of unreinforced concrete that has been a mystery for ages. The weight of Roman concrete used to build the monolitic structure is merely 1/5 of common concrete. Such effect was achieved by mixing in of volcanic tuff and pumice.

The porous light stone was used in the concrete of the dome to reduce its weight. Scientists estimated that, if normal weight concrete had been used throughout, the stresses in the arch would have been some 80% greater. The same technique was for the ceilings of the Colosseum.

All Greek theatres were built on the slopes of a hill. It was not until the Romans invented concrete, when the construction of tall theatres on flat surfaces (such as the Colosseum) became possible.