T
he history of Roman architecture would be very different had
the Romans been content to use the same building materials as
the Greeks, Etruscans, and other ancient peoples. Instead, the Ro-
mans developed concrete construction, which revolutionized archi-
tectural design. Roman concrete was made from a changing recipe of
lime mortar, volcanic sand, water, and small stones (caementa,from
which the English word cement derives). Builders placed the mixture
in wooden frames and left it to dry. When the concrete hardened
completely, they removed the wooden molds, leaving behind a solid
mass of great strength, though rough in appearance. The Romans of-
ten covered the rough concrete with stucco or with marble revetment
(facing). Despite this lengthy procedure, concrete walls were much
less costly to construct than walls of imported Greek marble or even
local tufa and travertine.
The advantages of concrete go well beyond cost, however. It is
possible to fashion concrete shapes that masonry construction cannot
achieve, especially huge vaulted and domed rooms without internal
supports. Concrete enabled Roman builders to think of architecture
in revolutionary ways. Roman concrete became a vehicle for shaping
architectural space.
The most common types of Roman concrete vaults and domes are
❚Barrel VaultsAlso called the tunnel vault,the barrel vault (FIG.
10-6a) is an extension of a simple arch, creating a semicylindrical
ceiling over parallel walls. Pre-Roman builders constructed barrel
vaults using traditional ashlar masonry (FIG. 2-24), but those earlier
vaults were less stable than concrete barrel vaults. If even a single
block of a cut-stone vault comes loose, the whole vault may col-
lapse. Also,masonry barrel vaults can be illuminated only by light
entering at either end of the tunnel. Using concrete, Roman
builders could place windows at any point in a barrel vault, be-
cause once the concrete hardens, it forms a seamless sheet of “arti-
ficial stone” in which the openings do not lessen the vault’s struc-
tural integrity. Whether made of stone or concrete, barrel vaults
require buttressing (lateral support) of the walls below the vaults
to counteract their downward and outward thrust.
❚Groin VaultsA groin or cross vault (FIG. 10-6b) is formed by the in-
tersection at right angles of two barrel vaults of equal size. Besides ap-
pearing lighter than the barrel vault, the groin vault needs less but-
tressing. The barrel vault’s thrust is concentrated along the entire
length of the supporting wall. The groin vault’s thrust, however, is
concentrated along the groins,the lines at the juncture of the two bar-
rel vaults. Buttressing is needed only at the points where the groins
meet the vault’s vertical supports, usually piers.The system leaves the
area between the supports open, permitting light to enter. Groin
vaults, like barrel vaults, can be built using stone blocks—but with the
same structural limitations when compared with concrete vaulting.
When a series of groin vaults covers an interior hall (FIG. 10-6c;
compare FIG. 10-46), the open lateral arches of the vaults form the
equivalent of a clerestory of a traditional timber-roofed structure
(for example,FIG. 11-10). Such a fenestrated (with openings or win-
dows) sequence of groin vaults has a major advantage over wooden
clerestories. Concrete vaults are relatively fireproof, always an im-
portant consideration given that fires were common occurrences
(see “Timber Roofs and Stone Vaults,” Chapter 17, page 435).
❚Hemispherical DomesThe largest domed space in the ancient world
for more than a millennium was the corbeled, beehive-shaped tholos
(FIG. 4-21) of the Treasury of Atreus at Mycenae. The Romans were
able to surpass the Mycenaeans by using concrete to construct hemi-
spherical domes (FIG. 10-6d), which usually rested on concrete cylin-
drical drums.If a barrel vault is described as a round arch extended in
a line, then a hemispherical dome may be described as a round arch
rotated around the full circumference of a circle. Masonry domes,
like masonry vaults, cannot accommodate windows without threat-
ening their stability. Concrete domes can be opened up even at their
apex with a circular oculus (“eye”), allowing much-needed light to
reach the vast spaces beneath (FIGS. 10-35and 10-51).Roman Concrete Construction
ARCHITECTURAL BASICS10-6Roman concrete construction. (a) barrel vault,
(b) groin vault, (c) fenestrated sequence of groin vaults,
(d) hemispherical dome with oculus (John Burge).
Concrete domes and vaults of varying designs enabled
Roman builders to revolutionize the history of architecture
by shaping interior spaces in novel ways.
a
bRepublic 241c dlos at the peak of an ascending triangle, the layout reflects the new
Republican familiarity with the terraced sanctuaries of the Hellenis-
tic East. The means of construction, however, was distinctly Roman.
The builders used concrete barrel vaults (FIG. 10-6a) of enormous
strength to support the imposing terraces and to cover the great
ramps leading to the grand central staircase, as well as to give shape
to the shops selling food, souvenirs, and the like, aligned on two con-
secutive levels. In this way, the Romans transformed the entire hill-
side, subjecting nature itself to human will and rational order. The
Palestrina sanctuary is emblematic of the growing dominance of
Rome in the Mediterranean world. By the end of the second century
BCE, Romulus’s village of huts belonged to the legendary past.