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Cement: c.200 BC

Builders in Greek cities on the coast of Turkey (and in particular Pergamum) evolve cement in about 200 BC as a structural material, in place of weaker mortars such as gypsum plaster (used in Egypt) or bitumen (in Mesopotamia). The secret of the new material is the lime which binds sand, water and clay.

The Romans subsequently use finely ground volcanic lava in place of clay, deriving it mainly from the region of Pozzuoli. Their cement, known for this reason as pozzolanic, is the strongest mortar in history until the development of Portland cement. When small fragments of volcanic rubble are included, the result is concrete - making possible the great arches and aqueducts of Roman architecture, and playing its part in Roman roads.

Vitruvius: late 1st century BC

A Roman architect sets out the principles of his craft in ten volumes. He deals with all aspects, from general principles to materials, and from the Orders of architecture to stucco work, painting, aqueducts and machinery. Written well before the greatest achievements of the Roman builders, this treatise is the most influential text in the entire history of architecture.

The architect is Vitruvius and the book De architectura ('On architecture'). Its precepts subsequently guide the classical revival in the Renaissance. Since then the proportions and theories of Greek and Roman architecture - as enshrined by Vitruvius - have remained the basis of architectural tradition.

Arch, vault and dome: from the 1st century BC

The greatest achievement of Roman architecture and technology lies in the development of these three architectural forms. The dome has long been a familiar concept (appearing dramatically in the passage grave on the Île Longue or in the tholos at Mycenae), but nothing has been made of it in the major architectural traditions. The spectacular temples of Egypt or Greece are exclusively trabeate, using flat horizontal lintels to span open spaces.

The arch has far greater capabilities than the lintel, for it can combine many smaller units (of stone or brick) to make a greater whole. In Greek architecture a single vast stone lintel can reach between columns at most 7 yards apart. A Roman brick arch can span 50 yards.

The arch, the vault and the dome are all applications of the same concept. The vault, or open-ended tunnel, is only an exceptionally deep arch. The dome is in effect a collection of arches all sharing the same centre. In each case the pressure of gravity on the material forming the arch will hold it together as long as the outward thrust is contained by buttresses.

The Roman achievement in all these forms is greatly assisted by their development of concrete. An arch or dome bonded into solid form by a strong inner layer of concrete sits as one unit, exerting its weight downwards rather than outwards. This makes possible such miracles as the 1st-century Pont du Gard or the 2nd-century dome of the Pantheon.

The Pont du Gard: AD c.20

The scale of Roman architectural ambition is superbly seen in the great aqueduct at Nîmes, known as the Pont du Gard ('bridge of the Gard'). Constructed in about AD 20, this gigantic structure is purely practical. It is a section of a channel bringing water from the river Eure to the new Roman town of Nîmes.

The water flows gently downhill for a distance of almost 50 km. The Pont du Gard, with its three towering tiers of arches, carries it over the deep valley of the river Gard - in itself a source of water nearer to Nîmes, but too low-lying to reach the town by gravity.

Roman bridges: 1st - 2nd century AD

Bridges are as much part of the Roman architectural achievement as aqueducts, and they present even greater constructional problems.

Some of the most impressive Roman bridges are over ravines. A fine surviving example, built for Trajan in AD 105, spans the Tagus in Spain, at Alcántara. Its two massive central arches, 110 feet wide and 210 feet above the normal level of the river, are made of uncemented granite. Each wedge-shaped block weighs 8 tons. During construction these blocks are winched into place by a system of pulleys, powered perhaps by slave labour on a treadmill. They are supported on a huge timber structure standing on the rocks below - to be removed when the arch is complete.

An equally remarkable feat of Roman construction is the building of bridges across rivers where no rock or island emerges from the water to carry the piers. An example survives in Rome - the Sant'Angelo bridge, built for Hadrian in AD 134 as an approach to his great circular mausoleum, now the Castel Sant'Angelo.

The building of such bridges is made possible by the Roman perfection of cement and concrete, and by their invention of the cofferdam.

The Pantheon: AD c.120

The roof of the Pantheon in Rome is the most remarkable example of the Roman genius in the most impressive of architectural forms, the span of a large dome. In an extra touch of flamboyance, the centre of this one is open to the sky - a detail which adds no great architectural complication but provides a visual thrill. The interior of the building is circular (placing round dome on square base is the next stage of sophistication).

The Pantheon, built by Hadrian in about AD 120 (demolishing an earlier pantheon, or temple to all the gods, on the site), has been in continuous use as a place of worship for nearly 2000 years. For most of that time it has been a Christian church, dedicated in 609 as Santa Maria Rotunda.

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