The Colosseum and the Invention of Concrete

The arch became the essential element in Roman architecture, emphasizing the strength and massiveness of the masonry structure as if to symbolize the sustaining power of the empire itself - most obviously in the triumphal arches erected in honour of emperors (see pp. 214-16). Arches enframed by engaged columns and entablatures -used from early in the first century BC - were a dominant motif in the imperial period. The exterior of the great Flavian amphitheatre, known since the eighth century as the Colosseum, is entirely composed of them in arcades which integrate the units of the design by rhythmic horizontal and vertical repetition (5,43; 44). The orders follow the ascending sequence established by the Romans for multi-story buildings - Doric-Ionic-Corinthian. (The sequence is purely aesthetic, the Doric being visually the heaviest and strongest and the Corinthian the lightest.) Although several such permanent arenas for gladiatorial combats and other spectacles had been built elsewhere, somewhat surprisingly this was the first in the city of Rome, where 'games' had previously been held in the forum or in temporary structures. With a seating capacity estimated between 45,000 and 55,000, the rapid

entrance-exit problem for filling and emptying the vast seating space was formidable. It was brilliantly solved by an ingenious arrangement of stairways and corridors all leading down to the continuous ground-floor arched openings (5,45). The Colosseum was begun as a shrewd bid for popularity by Vespasian, the first emperor of the Flavian family, who came to power in AD 69 as the result of a mass uprising against Nero, the last of the dynasty established by Augustus. To fulfil its purpose it had to be built quickly: the enormous structure, which is on an elliptical plan measuring 615 by 510 feet (188 by 155m) externally and 159 feet (48m) high, was completed in no more than a decade. Various materials were used: concrete for the 25-foot-deep (7.5m) foundations, travertine (a fine local limestone lighter in weight and less strong than marble, easily cut when first quarried, but hardening with exposure to air) for the framework of load-bearing piers, tufa and brick-faced concrete for radial walls between the piers, travertine for the exposed dry-jointed stonework held together by metal clamps (most of which have gone) and marble (of which no trace remains today) for the interior. A giant awning to protect spectators from the sun was supported on wooden poles projecting inwards from the top and manipulated by ropes tied to bollards on the pavement surrounding the building.

The Colosseum is an outstanding work of Roman engineering as well as of architecture. In both design and structure it was, however, conservative. Concrete was used simply for foundations and walls, as it had been in many earlier buildings, including, for example, the sanctuary at Praeneste. Roman concrete (opus caementicum) was a combination of mortar and pieces of aggregate (caementa) laid in courses - unlike modern concrete, which can be mixed and poured. Its unique strength and durability derived from the binding agent, a mortar made of lime and volcanic sand, first found at Pozzuoli near Naples and thus called pozzolano, used as early as the third century BC. Exposed walls of this concrete were usually faced with another material, an irregular patchwork or neatly squared pattern of stone and later, under the empire, brickwork. The full potentialities of the material were, however, only gradually discovered. In early examples the cement dried out quickly so that each layer formed a single horizontal band like an enormous stone slab. But the development, about the time of Augustus, of a slow-drying mortar, probably made with volcanic sand found near Rome, produced a concrete core that hardened into an inert homo-geneous mass. This revolutionized architecture for, when combined with the arch and vault, it enabled the Romans to cover, without any interior support, spaces far larger and of far greater flexibility of form than had ever been possible before. Ancient Egyptian, Mesopotamian and Greek architecture had been essentially an art of composition in mass. Space was simply what was left over or left between the solids. This negative conception was now replaced by that of an architecture of space. A building was conceived as a shell molding space into whatever shape the architect or his patron desired.

The earliest building in which the possibilities of using concrete for this new 'spatial' conception of architecture are known to have been explored is the Golden House designed for the Emperor Nero (AD 54-68) by an architect named Severus. Of the parts that remain, the most interesting is a group of rooms which, although divested of all their surface decorations apart from some traces of delicate stucco-work, reveal a truly revolutionary originality (5,46). An octagonal space covered by a rather shallow dome is surrounded on five sides by rectangular vaulted rooms, one of which terminated in an ornamental cascade of water. Lighting, unusually bright and even for a Roman interior, was provided by a circular opening or oculus in the centre of the dome and, very ingeniously, by clerestory windows high up on the walls of the radial rooms. The inner surface of the dome was probably decorated with mosaics, which must have given an almost magically insubstantial effect. And the views through the wide square openings of the octagon from one room to another and on to the garden to the south may well have seemed to realize architectural prospects of the type that Pompeiian painters had feigned. For here the walls really had been broken and bent to permit free spatial flow and to give the appearance of an unending series of opulent chambers. From no single point would it have been possible to grasp and resolve the visual complexities and ambiguities of this highly sophisticated interior.

The surviving ruins are no more than a small part of the Golden House, in which, Nero remarked, he could 'at last begin to live like a human being'. His biographer Suetonius (AD 69-140) described the main banqueting hall, which 'constantly revolved, day and night, like the heavens'. The remark is tantalizingly brief, but it is usually assumed that the ceiling - not the room itself - revolved and was constructed in the form of a vast wooden dome decorated with stars or astral symbols, a kind of planetarium beneath which the emperor entertained his guests at the very centre of the cosmos, as it were. The idea of placing such a cosmic canopy over a ruler with pretensions to universal authority probably derived from the royal tents and canopies of Achaemenid Persia. It would certainly have appealed to Nero; and it may well lie behind the domes that became such a prominent feature of imperial Roman architecture. The development of the concrete dome may even have entrance-exit problem for filling and emptying the vast seating space was formidable. It was brilliantly solved by an ingenious arrangement of stairways and corridors all leading down to the continuous ground-floor arched openings (5,45). The Colosseum was begun as a shrewd bid for popularity by Vespasian, the first emperor of the Flavian family, who came to power in AD 69 as the result of a mass uprising against Nero, the last of the dynasty established by Augustus. To fulfil its purpose it had to be built quickly: the enormous structure, which is on an elliptical plan measuring 615 by 510 feet (188 by 155m) externally and 159 feet (48m) high, was completed in no more than a decade. Various materials were used: concrete for the 25-foot-deep (7.5m) foundations, travertine (a fine local limestone lighter in weight and less strong than marble, easily cut when first quarried, but hardening with exposure to air) for the framework of load-bearing piers, tufa and brick-faced concrete for radial walls between the piers, travertine for the exposed dry-jointed stonework held together by metal clamps (most of which have gone) and marble (of which no trace remains today) for the interior. A giant awning to protect spectators from the sun was supported on wooden poles projecting inwards from the top and manipulated by ropes tied to bollards on the pavement surrounding the building.

The Colosseum is an outstanding work of Roman engineering as well as of architecture. In both design and structure it was, however, conservative. Concrete was used simply for foundations and walls, as it had been in many earlier buildings, including, for example, the sanctuary at Praeneste. Roman concrete (opus caementicum) was a combination of mortar and pieces of aggregate (caementa) laid in courses - unlike modern concrete, which can be mixed and poured. Its unique strength and durability derived from the binding agent, a mortar made of lime and volcanic sand, first found at Pozzuoli near Naples and thus called pozzolano, used as early as the third century BC. Exposed walls of this concrete were usually faced with another material, an irregular patchwork or neatly squared pattern of stone and later, under the empire, brickwork. The full potentialities of the material were, however, only gradually discovered. In early examples the cement dried out quickly so that each layer formed a single horizontal band like an enormous stone slab. But the development, about the time of Augustus, of a slow-drying mortar, probably made with volcanic sand found near Rome, produced a concrete core that hardened into an inert homo-geneous mass. This revolutionized architecture for, when combined with the arch and vault, it enabled the Romans to cover, without any interior support, spaces far larger and of far greater flexibility of form than had ever been possible before. Ancient Egyptian, Mesopotamian and Greek architecture had been essentially an art of composition in mass. Space was simply what was left over or left between the solids. This negative conception was now replaced by that of an architecture of space. A building was conceived as a shell molding space into whatever shape the architect or his patron desired.

The earliest building in which the possibilities of using concrete for this new 'spatial' conception of architecture are known to have been explored is the Golden House designed for the Emperor Nero (AD 54-68) by an architect named Severus. Of the parts that remain, the most interesting is a group of rooms which, although divested of all their surface decorations apart from some traces of delicate stucco-work, reveal a truly revolutionary originality (5,46). An octagonal space covered by a rather shallow dome is surrounded on five sides by rectangular vaulted rooms, one of which terminated in an ornamental cascade of water. Lighting, unusually bright and even for a Roman interior, was provided by a circular opening or oculus in the centre of the dome and, very ingeniously, by clerestory windows high up on the walls of the radial rooms. The inner surface of the dome was probably decorated with mosaics, which must have given an almost magically insubstantial effect. And the views through the wide square openings of the octagon from one room to another and on to the garden to the south may well have seemed to realize architectural prospects of the type that Pompeiian painters had feigned. For here the walls really had been broken and bent to permit free spatial flow and to give the appearance of an unending series of opulent chambers. From no single point would it have been possible to grasp and resolve the visual complexities and ambiguities of this highly sophisticated interior.

The surviving ruins are no more than a small part of the Golden House, in which, Nero remarked, he could 'at last begin to live like a human being'. His biographer Suetonius (AD 69-140) described the main banqueting hall, which 'constantly revolved, day and night, like the heavens'. The remark is tantalizingly brief, but it is usually assumed that the ceiling - not the room itself - revolved and was constructed in the form of a vast wooden dome decorated with stars or astral symbols, a kind of planetarium beneath which the emperor entertained his guests at the very centre of the cosmos, as it were. The idea of placing such a cosmic canopy over a ruler with pretensions to universal authority probably derived from the royal tents and canopies of Achaemenid Persia. It would certainly have appealed to Nero; and it may well lie behind the domes that became such a prominent feature of imperial Roman architecture. The development of the concrete dome may even have been stimulated by the symbolism of the textile or wooden cosmic canopies of the East.

The Emperor Domitian (AD 81-96), whose palace on the Palatine Hill in Rome exhibits similar flexibility of planning, used the new technology to provide an appropriate setting for his imperial rule. Trajan (AD 98-117), who ruled for the 20 years during which the Roman empire reached the peak of its power and its greatest extent, was more concerned with public works. The most notable were public baths, of which unfortunately little survives, and a new commercial quarter known as Trajan's Market, created by cutting away the slope of the Quirinal Hill. Trajan's Market is one of the most fascinating of all surviving Roman structures, at once logical and complex, utilitarian but possessing an austere monumental beauty. One hundred and fifty or more shops and offices on three different levels connected by streets and steps are combined with a great covered market hall (5,47). Built for a city most of whose inhabitants were engaged exclusively in working for, buying from and selling to one another, it had a social importance hard to exaggerate. The concave main facade was articulated with pilasters, but the rest of the exterior and the interiors were severely simple, of brick-faced concrete with travertine surrounds to rectangular doors and windows. A better idea can be obtained from them than from the numerous but much less well preserved remains at Ostia and elsewhere of the architectural form and appearance of Roman multi-story construction. At street level there are shops with small windows above giving light and air to timber-floored mezzanines or garrets approached from within the shops by timber ladders. The main horizontal divisions, however, are concrete barrel vaults between the party walls, and access to the upper stories is by concrete stairways. (Concrete was made compulsory for floors and stairways after the great fire of AD 64.) Great ingenuity was applied to the planning. The architect's aim was eminently practical: to provide the maximum space, well lit and aired, for the various activities connected with buying and selling within a limited area on an extremely awkward sloping site. The result is a rare achievement of volumetric organization, an autonomous structure of interlocking curved corridors, straight streets and passages, and vaulted rooms of different sizes (5,48). Nowhere else - except in the Pantheon - can the characteristically Roman genius for molding space be better experienced.