A pantheon of Roman engineers

Wednesday, 3 October 2018

Civil engineers all agree that Roman concrete and its 2000-year-old construction techniques continue to stand the test of time for durability and strength, structural design and ingenuity.

The Pantheon, a Roman temple (now church) in Italy's capital, remains in continuous use despite being completed in 128 C.E. under the emperor Hadrian. It is a fine example of work by engineers of the Roman Empire and is barrel-like in design, with a dome and an entrance portico. The structure was made from concrete without steel reinforcement and so large is the dome that it was the longest span constructed up until the 19th century.

The rotunda and dome are 43.4 metres in diameter and the same distance is from floor to an eight-metre-wide oculus in the dome. Roman engineers were aware the centre of a dome is not under load, however, the oculus reduced its weight and allowed sunlight into the temple. It also permitted rain to fall onto the marble floor which has holes carved into it to allow the water to drain into the Roman sewers.

The Pantheon’s concrete dome remains stable two millennia after its construction due to the concrete being a lighter density near the oculus and heavier near the base, made so with the use of brickwork built up on top of it as a counterbalance. The Roman engineers achieved this by variously using lightweight volcanic stones and heavy granite stones as the aggregate in the concrete, similar to modern methods. The coffering, the square sections, serves the purpose of lightening the weight of the dome while maintaining its rigidity. And while cracks have been noted in the dome it remains fully functional.

The late civil engineer, David Moore, who wrote much on Roman concrete, theorised its durability was due to its ingredients and placement being similar to roller-compacted concrete used in modern-day dams.

                              “The building was built entirely without steel reinforcing rods to resist tensile cracking,
                                so necessary in concrete members, and for this concrete dome with a long span to last
                                centuries is incredible.”

In 1995, Moore said no engineer would dare build this structure without steel rods and that modern codes of engineering practice wouldn’t allow it to be built. Being constructed over a 10-year period from 118 to 128 C.E., he said, was fortuitous as it gave the pozzolan concrete ample time to cure and strengthen. The inside walls and floor of The Pantheon are highly ornate and decorated in multi-coloured marble. It offers civil, structural, and heritage engineers an invaluable opportunity to examine Roman engineering up close.

 

Image: The dome of The Pantheon. Source: Desi Corbett.

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