What is Concrete? | Hacker News Reader

What is Concrete?

gizmodo.com

34 pointsmikecarlton10 comments

Cool stuff -- author doesn't mention that Roman cement had a usability life of >2000 years, while Portland cement starts to degrade after 50 years (for rough harbor usages).

http://www.bloomberg.com/bw/articles/2013-06-14/ancient-roma...

One of the really surprising things when you visit the Colosseum in Rome is how modern a structure it feels. Rather than ruins, it feels more like a construction site, a half-finished stadium for some up coming Olympics or World cup.

It's not just two Greek amphitheaters dug into facing hillsides...it's something on a completely different level.

All this despite being left in complete, abandoned ruins for something like a thousand years. Then damaged in earthquakes, stripped of stone (probably also concrete blocks) for other construction, legend has it's been used by monastic orders, as a wool factory and other crazy uses.

This isn't modern reinforced concrete, it's just concrete.

If you wonder around Rome you come across possible the pinnacle in concrete work in human history, the Pantheon. I've heard that we still don't know how they produced the dome, another nonreinforced wunder construction that's survived for millenia. Again, survived earthquakes, wars, and other uses, and again a huge structure that feels amazingly modern in size and scope.

"I've heard that we still don't know how they produced the dome, another nonreinforced wunder construction that's survived for millenia."

We didn't, but we do now. It has recently been fully understood to have been volcanic ash mixed into the concrete that makes it so strong and stable.

https://en.wikipedia.org/wiki/Roman_concrete

http://www.romanconcrete.com/docs/spillway/spillway.htm

my understanding is that the density (and thickness) of the concrete changes as the dome moves inward from the walls to the oculus, and we have no modern method for reproducing such a constant density variance

Incidentally, there is a (small) school of modern architectural thought that attributes the longevity and elegance of Roman architects' work to their tendency to design far below the yield stresses of their materials. That approach is very different from contemporary architecture, which often builds far closer to material limits and relies on the very high strength of structural steel.

FWIW I find this argument very persuasive; John Ochsendorf and Christopher Alexander are two guys who are known for taking this position, and they are both huge fans of the Pantheon. See [0] among many others for more.

[0] http://nautil.us/issue/24/error/ingenious-john-ochsendorf

it still boggles my mind that Rome has working sewers that were constructed more than 2000 years ago and there is every indication that they will last another 2000 years of continuous use

http://www.bbc.co.uk/programmes/b05xxl4t

The book Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World has a chapter on concrete. The self-healing concrete was completely new to me. Definitely worth a read, and not just for the concrete chapter:

http://www.goodreads.com/book/show/18222843-stuff-matters

See the concrete canoe competition:

https://en.wikipedia.org/wiki/Concrete_canoe http://www.asce.org/concrete-canoe-photo-gallery/

Another interesting property of unset concrete is that it can burn your skin due to alkalinity.

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