Ancient Concrete Lasts Longer than Modern Concrete
I looked out of the concrete military battery. It was slowly decaying and being reclaimed by nature. This battery at Fort Washington Park was built circa 1900, and was built to withstand a military bombardment, yet less than 100 years later it was so structurally unsafe that a chain link fence had been put around it.
How was the Colosseum in Rome, itself a building made of ancient concrete, still going strong 2000 years after construction, and this concrete bunker might be lucky to make it to 200?
The answer is actually quite interesting, while the concrete the Romans used is 10 times weaker than that of modern concrete it is somehow much more resistant to wear over time than modern concrete.
While the exact formula for Roman Concrete is unknown, it did have an element in it that modern concrete does not have: volcanic ash. Due to the properties of the volcanic ash, Roman concrete resists chemical degradation over time, and also stands up to sea water better than modern concrete.
Not only is modern concrete prone to degradation over time, one other thing we do with concrete today makes the structures incredibly durable in the short term (50-100 years), but not in the long term (1000-2000 years). Most concrete buildings are poured with an inner skeleton of rebar to add structural support.
The rebar makes the buildings much tougher and stronger and that is a good thing. Yet, over time micro cracks can form in the concrete. This can be due to a few things: vibrations (City traffic, foot traffic, bad storms year after year) and heat (Metal expands and contracts, the rebar in a concrete structure can expand slightly on a hot day and contract ever so slightly on a cold day, and after years cracks can form from the inside of the structure.). Once a crack appears in the concrete and if it reaches to the rebar, that means water can start getting through to the metal causing rust through oxidation. The oxidation causes gas to expand making the cracks bigger and now breaking the building down from the inside. After a long enough period, the rebar will have rusted away into nothing, and now we have a building with no internal skeleton. Again, to emphasize, this takes a very long time to happen, and the structure by this time would have been long demolished and replaced given the pace and rate of modern construction.
In the short term (let’s say a human life time), any concrete building built today will be able to take much more punishment than an ancient concrete building. But fascinatingly, in 1000 more years, that ancient concrete structure is much more likely to still be around than the concrete structure of today.
Sources:
https://www.smithsonianmag.com/history/the-secrets-of-ancient-romes-buildings-234992/
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