Philosopher Alfred North Whitehead
wrote, "The Roman Empire existed by virtue of the grandest
of technology that the world had hitherto seen." This technology
included engineering innovations
like the arch, vault, and dome. For the first time, Roman builders
huge volumes of interior space, shaping an architecture of
not supporting mass. With the invention of concrete, they built
increasingly daring forms, from baths to basilicas, on a vast
The essential ingredient of Roman building was the arch. Although Romans were smitten by "superior" Greek style and plastered conspicuous columns on facades, they abandoned the column as an actual structural support.
The arch and its progeny - the vault and the dome - revolutionized architecture. A stone lintel atop two columns rarely spans a distance as wide as 15 feet, but an arch can span 150 feet. Additionally, when its keystone is locked into place, the arch supports itself as well as immense loads on top. Combined with concrete, which could be cast in molds of any shape and scale, the arch allowed Romans to enclose enormous spaces and fully exploit the potential of these new forms and materials.
When an arch is extended in a straight line, or multiplied in depth, it becomes a barrel (or tunnel) vault. Such vaults provide a curved ceiling over two parallel walls and may be combined to form arcades (as in the Coliseum) supporting multiple tiers of superstructure. When two barrel vaults intersect at a right angle, the juncture forms a groin or cross-vault, which provides lunette windows for lighting at either end. An arch, rotated 360 degrees, creates a dome. By the first century B.C.E., the arch and vault were pervasive in Roman buildings.
Ancient concrete was not liquid but a viscous mixture of sand, lime, water, and aggregate. It was laid down in layers inside wooden or brick form work and solidified into a dense artificial stone that was light, strong, fireproof, and monolithic. Roman concrete walls and shells were always lined on both the exterior and interior with brick or a veneer of decorative stucco, fresco, mosaic, or marble. Purely ornamental columns, like olives dressing up a plain salad, adorned arches for a touch of Greek zest. The columns were generally engaged, or partially embedded in walls. When flattened and squared off, they are known as pilasters.