Edit- After reading most of the comments the general consensus is listed below:
1. The unique composite matrix of the concrete used gives it a self-healing property. When cracks form in the concrete, it will naturally seal them.
2. The Pantheon was a very significant structure which led to meticulous maintenance and restorations
3. The Romans didn’t have modern engineering. So they didn’t know exactly how strong they’d have to build the Pantheon to make it last. Their solution was to overbuild the hell out of it.
4. Survivorship bias. There were thousands of buildings constructed by the Romans but very few remain which are the ones we marvel at.
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In: 564
There was a scientific test just done on Pantheon concrete. The Romans mixed calcium carbonate pebbles into it. So when water seeps in cracks the carbonate reacts and seals the crack. Self repairing. Just heard this on BBC World News podcast
https://open.spotify.com/episode/3Zn8M9HeflTMSSXJIk7fGd?
People don’t really know why.
I read an article earlier in the week that reported on some new research which suggested that Roman concrete had an additional type of lime mixed into it, which when exposed to water (through cracks forming in the concrete) will expand and fill the crack “healing” the concrete. The researchers hypothesised that the healing lime was formed when hot mixing the concrete, and the article went on to suggest that further research into this could dramatically reduce the embodied carbon impact of building in concrete.
ELI5:
Roman concrete has a fancy type of lime in it
The lime heals the concrete when cracks form
Without cracks concrete lasts a lot longer.
Edit: someone below reads the same thing I do: https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
To add to everyone else.
The pantheon actually did collapse partially after about 100 years- it was rebuilt.
It’s also been restored a *bunch* of times, by Kings and popes and emperors and Il duces and prime ministers.
They recently discovered that they had lime clasts in their concrete that gave the material a sort of self mending effect. Interesting and could be why some of these structures still remain [an article on it](https://greekcitytimes.com/2023/01/07/ancient-roman-concrete-could-self-heal-thanks-to-hot-mixing-with-quicklime/)
One key reason that Roman concrete lasts millennia but modern concrete lasts 50-100 years typically is that most modern concrete is reinforced. Reinforcing concrete dramatically reduces its lifespan because as the iron corrodes it warps and expands which compromises the structural integrity of the concrete
The Romans built *lots* of buildings. Some of those were important to them, so they repaired them when they needed it. Buildings like the Pantheon (literally “temple of all gods”) and the Coliseum were maintained regularly. Other buildings were not, but we don’t see those today, because they wore out and were demolished for materials to build other stuff.
The Romans made really good concrete, and that helped, but the biggest thing is the regular maintenance. We could probably keep a building in pretty good shape for thousands of years if we wanted to. I would imagine the US Capitol and the White House will be in pretty good shape 2000 years from now, if the US still uses them as important government buildings. Other buildings, like any random house, will not be in such good shape.
Survivorship bias – plenty of big beautiful ancient buildings didn’t survive. Only some did. There are plenty of modern buildings that would also last thousands of years.
Maintenance – A lot of work has gone into restoring/conserving important structures like that over time.
I think a bit of it is survivorship bias, if I have built 1000 buildings, and 10 of them are left standing 2000 years from now, that means 1% are still standing and 99% fell, doesnt seem that unrealistic. Some buildings will leave a better foundation by chance, some will not get hit by an earthquake etc. But we look at the 10 buildings that are still standing and be amazed.
Nobody seems to mention this.
The biggest component is Rebar.
Our modern concrete is *significantly* better than Roman concrete because of reinforcement. The cost of this is that the iron rusts, and as it rusts, it expands, and as it expands, it cracks the concrete, which lets it rust more, which causes the death spiral.
Modern concrete is only supposed to last ~50 years because of a design decision. We would be more than capable of building a building that will last 2000 years, that’s just never a design requirement.
The Romans also tended to over-engineer the hell out of things (so did we until computer modeling allowed us to get a better idea of how strong things actually needed to be)
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