Thermae be applied to those magnificent edifices which grew up under the empire, in place of the simple balneae of the republic, and which comprised within their range of buildings all the appurtenances belonging to the Greek gymnasia, as well as a regular establishment appropriated for bathing. Writers, however, use these terms without distinction. Thus the baths erected by Claudius Etruscus, the freedman of the Emperor Claudius, are styled by Statius "balnea", and by Martial "Etrusci thermulae". In an epigram by "Martial"—"subice balneum thermis"—the terms are not applied to the whole building, but to two different chambers in the same edifice.
Ancient Roman architecture of these developments are described by Vitruvius, writing in the first century AD in his work De Architectura. Although concrete had been used on a minor scale in Mesopotamia, Roman architects perfected Roman concrete and used it in buildings where it could stand on its own and support a great deal of weight. The first use of concrete by the Romans was in the town of Cosa sometime after 273 BC. Ancient Roman concrete was a mixture of lime mortar, aggregate, pozzolana, water, and stones, and was stronger than previously-used concretes. The ancient builders placed these ingredients in wooden frames