Roman concrete

Roman concrete called opus caementicium in Latin was used from the late Roman Republic until the end of the Roman Empire. It was used to build monuments, large buildings and infrastructure such as roads and bridges. The quality of the concrete was excellent and the buildings and monuments still standing today are a testament to the strength of their construction!

Concrete was usually covered as concrete walls were considered unaesthetic. Roman builders covered building walls with stones or small square tuff blocks that would often form beautiful patterns noting that brick faced concrete buildings were common in Rome especially after the great fire of 64 AD.

Roman concrete formula

Roman concrete or opus caementicium was invented in the late 3rd century BC when builders added a volcanic dust called pozzolana to mortar made of a mixture of lime or gypsum, brick or rock pieces and water. Concrete was made by mixing with water: 1) an aggregate which included pieces or rock, ceramic tile, pieces of brick from previously demolished constructions, 2) volcanic dust (called pozzolana) and 3) gypsum or lime. Usually the mix was a ratio of 1 part of lime for 3 parts of volcanic ash. Pozzolana contained both silica and alumina and created a chemical reaction which strengthened the cohesiveness of the mortar.

There were many variations of concrete and Rome even saw the Concrete Revolution which represented advances in the composition of concrete and allowed for the construction of impressive monuments such as the Pantheon. For example, Roman builders discovered that adding crushed terracotta to the mortar created a waterproof material which could be then be used with cisterns and other constructions exposed to rain or water.

Romans mastered underwater concrete by the middle of the 1st century AD. The city of Caesarea gives us an impressive example of Roman construction. The production technique was quite incredible: the mix was one-part lime for two-parts volcanic ash, and it was placed in volcanic tuff or small wooden cases. The seawater would then hydrate the lime and trigger a hot chemical reaction which hardened the concrete.

caesaria roman harbor

Caesarea harbor before and today - Robert Teringo, National Geographic Society

Was Roman concrete better than modern concrete?

Actually it has been argued that the concrete used by the Romans was of better quality than the concrete in use today. Recent research from US and Italian scientists has shown that the concrete used to make Roman harbors in the Mediterranean was more resistant than modern concrete (known as Portland cement).

The production process was dramatically different. Portland cement is made by heating clays and limestone at high temperatures (various additives are also added) while the Romans used volcanic ash and a much smaller amount of lime heated at lower temperatures than modern methods.

For example, Roman harbors remain intact today after 2,000 years of waves breaking on the harbors' breakwaters whereas Portland concrete begins to erode in less than 50 years of sea battering. The concrete from ancient Rome also had bending properties that Portland concrete does not have due to its lime and volcanic ash, which explains why it does not crack after a few decades.

Incredible facts about Roman concrete
  • Reinforced concrete (reinforced with steel rebar) did not exist. As a result, buildings lasted longer as they did not suffer from steel corrosion.
  • Pozzolana (derived from volcanic dust) made the concrete more resistant to salt water than our modern-day concrete.
  • Amazingly, when the Roman Empire fell, the know-how to making concrete was lost. It was rediscovered only many centuries later in 1710 by a French engineer. His formula remains the basic formula used today to make Portland cement concrete.

SOURCES

  • The Oxford Handbook of Engineering and Technology in the Classical World (Oleson J.P., Oxford University Press, 2009).

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