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Comment by neals | Hacker News Reader

Is there no proper care for concrete though?

If concrete is reinforced, even with proper care, physics will get you eventually. It'll happen faster if you're near a coast. It'll take longer if you're inland. But eventually, the inevitable will happen. Our reinforced concrete structures will never last as long as, say, the Pyramids.

Wood will last longer [than reinforced concrete], again, with proper care. But in the end, entropy wins against wood as well. [And again, wood will never outlast the Pyramids].

In civil engineering, there really is no such thing as a free lunch. All materials come with drawbacks.

EDIT: To clarify wording.

As iconic as the pyramids are, they're not very useful. A better comparison may be the Pantheon in Rome. Nearly two thousand years old and in continuous use throughout its life. There are a number of other Roman structures that have seen continuous, heavy use for over a thousand years.

There are actually quite a number of ancient temples, churches, bridges, castles, and roads still in use throughout Europe, Asia, and the Middle East. Hōryū-ji [1] in Japan is a 1300-year-old wooden Buddhist temple that's still in use today!

[1] https://en.wikipedia.org/wiki/Hōryū-ji

> Our reinforced concrete structures will never last as long as, say, the Pyramids. > Wood will last longer, again, with proper care.

I'm curious to understand why as this seems completely counter-intuitive to me (someone with no expertise in materials science or building things!). Can you elaborate?

What does the caveat of "with proper care" actually mean? Isolation from all the elements? Does routine maintenance count (replacing deteriorating materials? replacing fasteners? reinforcing?)?

A wood structure properly taken care of does not seem like it would last longer than a pyramid or a reinforced concrete structure, if each of those is taken proper care of. But this is a hunch, not data, and based on nothing remotely scientific. I'm fascinated by this kind of thing; I appreciate any tidbits you can share!

Concrete is strong in compression, but weak in tension or shear forces. To solve that you reinforce it with rebar (basically steel bars). But concrete is porous to water, and steel rusts. When it rusts it expands, which damages the concrete. The only ways to escape that is with coating the rebar (hard to do well) or reinforcing with something else (not many options we know of). You can try to prevent or patch cracks in the concrete to slow down the process, and you can use more rust resistant steel. Both of them prolong the lifetime a lot, but they only delay the inevitable.

Wood is simpler, because it isn't a composite material. You have to prevent it from rotting and from being eaten, but on a timescale of a couple decades we can do that quite well. Also with wood structures it's often easier to replace small parts as soon as damage occurs, which prolongs the overall lifetime (similar to steel structures, but unlike reinforced concrete).

Sorry. Perhaps my wording was poor.

I meant that wood will last longer than reinforced concrete, even when you are properly caring for both.

Neither will outlast the Pyramids.

I'll reword my comment so that people understand it better.

What timespans are we talking about here? 30 years until reinforced concrete starts failing? 100?

As usual "it depends".

Basically rust is the problem, so if you are scrupulous on upkeep (read, spend money) it will last much longer than if not. How long is also enviroment dependents (temperature swings, salt air, emiisions etc. can make it worse)

That bridge in Italy collapsed after 51 years.

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

51 years and criminal care.

But if we are comparing concrete and wood as construction material we should use buildings that can be built with either.

About 100 years is the max life span of reinforced concrete. If not done to a high standard half that.

What about prefabricated houses in post-communist countries? You definitely can't accuse them of using highest quality materials nor spending loads of money on maintenance, but I haven't heard about major failures and there are probably millions of such blocks of flats out there, built mostly between '60-'80 using a couple of different variations of the technology (some involving pre fabricated pieces of reinforced concrete connected with steel joints). I think they were estimated to last 50 years or so, but there is still large proportion of the population living in such houses in some countries nowadays.

Sorry, I only got curious and started looking into it after the condo collapse in Florida. Every reference I can find on the subject says 100 years is the max life span of reinforced concrete. Not being a structural engineer I took it at face value.

Rebar reinforced concrete rusts.

Non-reinforced concrete has to be built bigger, but can last a long time. The coliseum is still up - Hoover Dam will also probably last awhile. And that's without much maintenance.

Smaller non-reinforced structures are harder to build. You have to use the concrete to hold itself up which requires a larger structure.

So you're left with reinforcement. There are options for non-rusting rebar, and non-rusting coatings, but I believe they're expensive. Also, any nick or scratch in a coating ruins it. There's also unique and novel research for non-rebar reinforcement - again expensive.

Here in US most buildings are built to last X years. So long as the rebar lasts longer than X, they'll use rebar. Increase X via regulations, and the quality (and cost) of the buildings in that area increases. Otherwise the bidder with the lowest cost of materials is most competitive. Anyone know what it's like in China?

Small nit not to take away from your larger point - I'm not sure the colosseum is a good example of a concrete structure for comparison to the concrete buildings of current age being discussed. The colosseum did use concrete for the arches, but was primarily built of limestone block secured with metal clamping. Then plenty of brickwork was used. Concrete was integral to the colosseum, but (I believe) a relative minority of the material on the whole.

Skyscrapers use a great deal of steel, but their still reinforced concrete structures.

> There are options for non-rusting rebar, and non-rusting coatings, but I believe they're expensive.

Galvanised rebar is omnipresent around the world. I believe even mandatory in some countries.

> Otherwise the bidder with the lowest cost of materials is most competitive. Anyone know what it's like in China?

Just as you said. Construction companies save on everything. GFRP rebar got adopted in China not so much because of advantages, but because of code allowing for lower concrete cover with it, as I heard.

Carrying strucuture is mainly reinforced steel(classic pillar + concrete floor/ceilings design), walls are usually bricks.

In Marina Del Rey, the rebar in the concrete sides of the marina are fed a trickle of electricity to prevent corrosion. I'm not sure how common this is though outside of marine environments.

Maybe not electricity, but sacrificial anodes are otherwise a thing on bridges, boats, water heaters…

It's called cathodic protection and it is common.

Keeping it dry and sealing large cracks helps, but reinforced concrete breaks down on contact with air so not really. Best option would actually be keeping it in a vacuum, but that’s just not cost effective.

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