concrete were very great. Concrete was cheap, since the best ingredients were readily available in Italy. It was also economical, since it used up the waste generated by the masons. Most importantly, it was "incomparably strong," and it avoided the kinds of difficulties with precise stone cutting that, Robertson says, were incompatible with the Roman temperament (232). Once they discovered its usefulness they rapidly began to improve it in order to expand its potential. By the second century BC the Romans had begun using pozzolano cement which could "fortify and waterproof the mass as it cured" (MacDonald 5). Stone- or brick-faced concrete allowed for the construction of vaults that were single-shelled, fairly light, and increasingly easy to build.
By the first century BC, "vaulting and the arch form are generally pervasive, instead of being the rare exception" (Kostof 194). Baths, warehouses, shops, and other utilitarian sites demonstrate the use of both forms and vaulting became vital in the creation of exceedingly solid substructures for
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