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
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
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