(Please click on the images for larger and higher res reproductions)
Aesthetically, the Romans didn’t think much of concrete, but once they got the hang of mixing it, they were quick to realise its potential, – and they used it to build some of the largest and most innovative structures of the Classical World. The Pantheon stands intact to this day, and the Coliseum (Colosseum) is perhaps the most iconic building of Ancient Rome.
The Pantheon painted by Ippolito Caffi 1809-1866. Image in the Public Domain
View of the Square from the Porch of the Pantheon. The Porch was added by the Emperor Hadrian
The Coliseum showing the use of stone, concrete and brick. The concrete was usually faced with brick or marble
The Romans began using mixes of a form of ‘concrete’ around 500BC, at the beginning of the Republican Era. Like the other great civilisations of the Ancient World, especially the Greeks and Etruscans, they had, for centuries, been using various mixtures of mortar. Mortar is a combination of filling and setting agents – sand or rubble for the fill, and some mix of mud for the binding and filling between stones or bricks. Roman concrete, however – ‘opus caementicium’ – was made of aggregate filler – stones and rubble, and broken bricks and pottery – almost anything – and ‘pozzolana’, a particular volcanic dust found in the central regions of Italy, for the principle binding agent . This was the great discovery: that pozzolana made an exceptionally strong bond with the aggregate, and that the concrete would even set under water. In areas where pozzolana was not available, they used lime or gypsum. Roman concrete became the great stalwart – the backbone and foundation – of Roman building and constructional engineering.
Aggregate filler – almost anything was used as the basis
How did this come about? The peoples of the ancient Middle East built their homes and their fortifications from mud or clay, and had found – probably by chance – that applying a moist layer of burnt limestone to the surface of the mud, formed a hard protecting covering that permeated the top layer and made it impervious to water. So perhaps the Romans – some two hundred years before Christ – found the same thing when applying a similar coating to a wall built of the local volcanic stone in the town of Pozzuoli in Italy? And then, that by mixing powdered volcanic ash from that stone – with moist lime – they could make a much harder mix that would set under water. We know of this in detail, because – around 20BC – Vitruvius, one of the most famous of Roman architects, wrote about this in his book of procedures and formulas for his ‘opus caementicium’.
The arch of Septimus Severus
Concrete had huge technical and practical advantages over the traditional methods of building with cut-stone and post-and-beam techniques. It was much stronger, and safer, and – when it was shaped into vaults, arches, and domes – could be used to span greater distances. Largely because of its plasticity, it required less skilled labour for the basic construction, and allowed much faster building. Thus it cut costs.
Concrete made bridge-building easier and safer. This bridge – Ponte del Quattro Capi – across the Tiber, is the oldest bridge in Roma – in its original state – dating from 62BC.
In 55BC, Pompey Magnus – then Consul of the Republic – began building his great theatre in Rome. Although only little of it remains, Pompey’s theatre is thought to have been the largest theatre ever built – in ancient or modern times ∼ and remains infamous as the place of Julius Caesar’s murder. Pompey modeled his theatre in the Greek style, as most Roman buildings were based on Greek models, and whilst it was in use – for more than 600 years – it remained known as ‘Pompey’s Theatre’. It was the principle monument of Ancient Rome, and – free-standing as it was – (not built into earth works or the topography – it could never have been built without the use of Roman Concrete.
An artist’s impression of the interior of Pompey’s Theatre. Image in the Public Domain.
Here – as in all great buildings using concrete, the material was faced with stone and exotic marbles, and decorated with frescoes and scenes in stucco relief. The concrete itself, unsightly as it was considered, was not to be seen.
Ostia – Ostia Antica – at the mouth of the river Tiber – was the port of Ancient Rome. Here, concrete had been used to build the port’s wharves and their underwater foundations, – and throughout the city in its urban and commercial areas. The site has well preserved buildings with magnificent frescoes and mosaics, and the use of Roman Concrete is evident throughout.
In 68BC Ostia was sacked and burned by pirates: the consular war- fleet was destroyed, and two senators were kidnapped ;– all this only 30 kilometres from Rome itself. In response, Pompey Magnus raised an army and destroyed the pirates and their fleet – and Ostia was rebuilt with surrounding protecting walls based on concrete. Here, too, is a free-standing theatre reminiscent of Pompey’s Theatre.
Our civilzation as we know it may not have developed as it did without the thrust, genius, and brilliance of the Italian Renaissance, and the cities of our Modern World would not be as the are without the use of concrete: concrete devised and used by the architects and builders of Ancient Rome.
Copyright Robert Weatherburn 2013