The Pantheon
About 128 ADdome completed
142 ftdome width, about 43 m
#1largest unreinforced concrete dome on Earth
0steel bars inside
Many modern concrete structures are designed to last 50 to 100 years.
The Roman recipe
- Volcanic ashCalled pozzolana, from near Naples
- LimeMade by baking limestone
- Mixed hotQuicklime added straight in, heating the mix
- Leaves little lime lumpsOnce thought to be careless mixing
Why it lasts
Self-healing cracks
- Water seeps into a tiny crack
- It dissolves a nearby lime lump
- The lime re-hardens and seals the crack
- Shown by MIT researchers in 2023
Stronger in the sea
- Seawater reacts with the ash
- Rare minerals grow in the gaps
- Roman harbor walls still stand
- Modern concrete has steel that rusts and cracks
Remember
Sometimes the old way really was better.
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Answer key
- What is special about the Pantheon's dome?It's the largest unreinforced concrete domeAbout 142 feet across with no steel inside.
- What volcanic ingredient did Romans use?Pozzolana ashVolcanic ash from near Naples.
- How do Roman concrete cracks heal?Water dissolves lime lumps that re-harden in the crackShown by MIT researchers in 2023.
- What happens to Roman sea walls in salt water?Minerals grow and strengthen themSeawater reacts with the ash to grow new minerals.
- Why does much modern concrete crack in decades?Steel inside rusts and expandsRusting rebar swells and cracks the concrete.