Modern structures usually last for up to 60 years and sometimes need a lot of maintenance and reconstruction. Studies have even found that rising global temperatures can actually lead to shorter lifespans for concrete structures, so it is a matter of great interest that some Roman structures that were built as early as the 2nd century BC are still standing today.
What are Roman Structures made of?
To understand how these structures withstand the wear of time, we need to know what they are made of. Romans have been making concrete for over 2000 years with a recipe they came up with after 150 years of experimentation. According to 1st century BC architect and engineer Vitruvius’s writings, limestone was burned to produce quicklime which was then added to three parts volcanic ash. This created a paste that was combined with fist-sized chunks of bricks or volcanic rocks to be packed into place to form walls. By second century BC, this concrete was being used in large-scale construction projects.
Researchers think Romans favored these ash deposits because it made the concrete durable. They mostly used ash from Pozzolane Rosse, a deposit created by an ash flow over 456,000 years ago when the Alban Hills volcano erupted 12 miles southeast of Rome. For concrete harbor structures in the Mediterranean, they used ash from Pulvis Puteolanus, tonnes of which they shipped from near the Bay of Naples.
Why is this recipe so durable?
In a 2014 study headed by Marie Jackson, geologist and research engineer at Univeristy of California, Berkeley, the concrete mix was recreated to build the Great Hall of Trajan’s Market (built in 110 CE). Pieces of volcanic rock (the “aggregate” in concrete, in place of sand that is used today) were combined with a mortar made from volcanic ash and quicklime and mixed with water.
They analyzed the concrete as it hardened to find that within 90 days the lime had been used up in chemical reactions. It was found that it had been replaced by stratlingite- a mineral that slowly grows in a crystal structure- that lead to the conclusion that at this point, Roman concrete begins to behave like a rock.
“[Roman concrete] can keep growing new crystals,” says Jackson. “There are self-healing properties in this material.”
To deal with the eventual cracking, the Roman concrete relies on the growth of the crystals along interfacial zones that binds the aggregate and the mortar together- a process that continues to this day in the 2000 year old Roman structures. This process goes a little differently in modern concrete, where the sand never really joins with the mortar, making it easy for cracks to propagate along the interfacial zones.
Sadly for this near-immortal material, its usage did not have the same age-resistant properties, and it disappeared along with the Roman Empire.
Some Roman structures that have withstood the test of time:
1) Arch of Constantine
Built on the order of the Roman Emperor Constantine the Great, the Arch of Constantine was erected as tribute to the victory of Constantine in 312 AD.
2)Basilica of Maxentius and Constantine
Basilica of Maxentius and Constantine was built in 313 AD, during the reign of Constantine the Great. It was used as a meeting house for the judicial council.
3) Castel Sant Angelo
Built in 139 AD originally as a mausoleum for Roman Emperor Hadrian, the Castel Sant Angelo was often a hiding place for popes who felt like their lives were in danger. It later also became a prison.
4) Catacombs of San Callisto
Built in 150 AD, the Catacombs of San Callisto were a burial for Christians. They have five floors and 500,000 bodies.
5) Mausoleum of Augustus
The Mausoleum of Augustus was built around 28 BC as a burial for Augusts, his wife, Livia, and later also for other Roman emperors and public figures.
6)Palatine Hill & Stadium
Built on the birthplace of Augustus, the first Emperor of the Roman Empire, the Palantine Hill & Stadium is believed to have been a private garden for Emperor Domitian.
7) The Pantheon
One of the most celebrated structures, the Pantheon of Rome was built in 25 BC by Marcus Agrippa as a temple for the most important Gods. It was rebuilt in 125 AD after it was consumed by fire in 80 AD.