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Comment by twelvechairs | Hacker News Reader

There's a lifecycle issue which isn't accounted for here. Steel and concrete last almost forever once built - how long do you trust your glue-laminated, termite-sprayed, fire-sprayed timber floor? Especially after a couple of cases of water or impact damage over time...

Reconstructing towers (particularity residential towers) in-use is an incredibly problematic process.

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Reinforced concrete suffers from carbonation. The concrete is base, but slowly get's neutralized by carbon dioxide. Once this has been done, the concrete no longer chemically protects the rebar inside. Then if we take into account that most concrete is porous and has moisture content, that rebar is going to rust relatively quickly. When it rusts, it expands breaking away the covering cement.

Typically reinforced concrete has lifespan ranging from 60 to 150 years. Wood in colder climates often has similar lifespan.

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

Ordinary buildings are considered to have a lifespan of thirty years. That doesn't mean that they won't be around longer, but that there is often a point of diminishing economic returns in designing for a longer life cycle because maintenance tends to become a primary factor...for example thirty years is a long life for a roof system or the sealant joints upon which modern buildings rely.

Would you happen to have a reference for that 30 year lifespan?

Is that residential, commercial, industrial, all, other?

I'd like to know that too. Never heard anything shorter than 60 years. But my sources are vague. Stuff heard at civil engineering class and my dad who is construction engineer.

Roman concrete lasts forever. Modern concrete reinforced with rebar and exposed to the elements lasts about fifty years.

Is this just survivorship bias?

No, they are chemically different cements. However, Roman cements have a tenth the strength of Portland cements, so we can build much bigger structures now even if they won't last as long.

Also no rebar, so it means big arches, massive vaulted ceilings, domes etc...

Presumably, having not read any, this is what building codes cover. So in many structures the effect of impact damage would be the same. With water you have more of a point.

Reconstructing towers in use seems like a ridiculous thing to add on to the article.

My timber floor just had its 70th birthday and it's got a lot more in it. That said, it burns well and has borer in it.

No material is maintenance free...

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