How the Pantheon, which was built over 2000 years ago, is still standing when buildings made 150 years ago are about to crumble.

Question

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

Answer 1

A lot of concrete issues now are from the steel reinforcement.
The steel makes it stronger, so you can use less concrete, thinner walls etc.

Damp getting into the concrete makes the steel rust, and the expansion of iron into rust can exert massive forces and blow stuff apart. Roman stuff was done before steel reinforcement, so doesn’t have to contend with that, it was just made way thicker, heavier and stronger instead.