5 ms·

Maintenance of steel reinforced concrete is an all-or-nothing affair. You either get it right and seal everything perfectly and monitor everything perfectly and

by ohazi 5y ago

Maintenance of steel reinforced concrete is an all-or-nothing affair. You either get it right and seal everything perfectly and monitor everything perfectly and repair tiny cracks before they allow moisture to get in, or you're sunk. If surveillance is lax for a few years you can easily end up with damage that can't be repaired. Wood construction, on the other hand, is extremely easy to repair. You can cut away sections and replace them piecemeal, essentially forever. Wood gives you the ability to continuously build a Ship of Theseus, while structurally compromised concrete structures often require you to tear them down and build them again from scratch.

ak217 5y ago
It's relatively straightforward to overengineer concrete to the point where it will take many decades, not just years, of neglect for damage to become unrepairable. A lot of this discussion is comparing apples to oranges. Reinforced concrete often serves applications that wood is simply unable to perform in (bridges, heavy duty foundations, dams, retaining walls). And yes, poorly maintained, especially prestressed concrete in those applications will deteriorate and fail - but the structure would be impossible with wood in the first place. In more light duty applications wood and concrete can both serve well, but good luck protecting a foundation made of wood from water intrusion. For light duty applications there is no argument that concrete can be overkill and wood can be very appealing.
- ohazi 5y ago
  > A lot of this discussion is comparing apples to oranges. I agree that wooden houses are not comparable to reinforced concrete bridges. But the part I'm trying to highlight is that being able to easily inspect a structure is a critically important aspect of maintenance, and that most reinforced concrete is inherently difficult or impossible to inspect. The fact that exterior waterproofing is relatively cheap is only incidental. It's not enough to overengineer a bridge and say it'll last a hundred years if you don't have a reliable way to determine when the bridge is no longer safe beyond year 70. Kicking the can down the road is not a viable long-term strategy. We're about 100 years into widespread use of reinforced concrete and are now starting to see the occasional catastrophic results. Potting steel in concrete is done because it's cheap and easy, not because it's particularly maintainable. It's inherently difficult to inspect the structure when you build things this way. > especially prestressed concrete Agree. Unbonded, post-tensioned concrete (where you can replace individual strands) seems like the only reasonable approach to me, but building this way and doing all of the inspection and maintenance is way more expensive than the "do almost nothing" approach for rebar concrete. But the benefits are only realized after 100 years, so nobody has the incentive to design this way.
  - ak217 5y ago
    > you don't have a reliable way to determine when the bridge is no longer safe I have a hard time believing this. Despite the ridiculous claims of another poster in this thread (that rebar just spontaneously and completely turns into rust) properly formulated and surfaced concrete and steel don't just rot on the inside with no external signs of decay. If you have repeated water intrusion, you will see seeping and cracks. If you have increasing stress, you'll see spalling. You can core concrete, drill out and patch rebar if you have spot damage. In general I find the sentiments in this thread bizarre since the vast majority of long-lasting buildings now and in the past century are made of concrete (many of the more modern ones have a concrete foundation and wood or steel superstructure). You don't see many old wood structures because most of them decayed and were torn down - there's a reason why you don't see many wooden bridges standing. Concrete is the closest thing that we have structurally to something that can last forever, and we're learning more about how to make it last longer all the time. Let me know if you have links that discuss maintainability of concrete in this context. Would be glad to learn more.
fsloth 5y ago
"...or you're sunk" I believe there is a very unfortunate example of this in Miami in the recent condominium collapse. A bridge recently collapsed in Italy. Etc. So the problem of failing concrete structures is not 'theoretical' but a very real issue. Even though most concrete construction does seem to last pretty well.
- afiori 5y ago
  Both of those building would have been almost impossible to build with wood
  - Retric 5y ago
    Miami building was only 12 stories tall, the comparable wooden structures have been built for an 18 story example: https://en.wikipedia.org/wiki/Mjøstårnet https://en.wikipedia.org/wiki/Mjøstårnet
  - fsloth 5y ago
    Sure - my main point was that reinforced concrete will fail concretely and not only theoretically unless maintained. I was not suggesting it could be replaces.
- deleted 5y ago
  [deleted]

by ohazi 5y ago

ak217 5y ago

It's relatively straightforward to overengineer concrete to the point where it will take many decades, not just years, of neglect for damage to become unrepairable. A lot of this discussion is comparing apples to oranges. Reinforced concrete often serves applications that wood is simply unable to perform in (bridges, heavy duty foundations, dams, retaining walls). And yes, poorly maintained, especially prestressed concrete in those applications will deteriorate and fail - but the structure would be impossible with wood in the first place. In more light duty applications wood and concrete can both serve well, but good luck protecting a foundation made of wood from water intrusion. For light duty applications there is no argument that concrete can be overkill and wood can be very appealing.

ohazi 5y ago
> A lot of this discussion is comparing apples to oranges. I agree that wooden houses are not comparable to reinforced concrete bridges. But the part I'm trying to highlight is that being able to easily inspect a structure is a critically important aspect of maintenance, and that most reinforced concrete is inherently difficult or impossible to inspect. The fact that exterior waterproofing is relatively cheap is only incidental. It's not enough to overengineer a bridge and say it'll last a hundred years if you don't have a reliable way to determine when the bridge is no longer safe beyond year 70. Kicking the can down the road is not a viable long-term strategy. We're about 100 years into widespread use of reinforced concrete and are now starting to see the occasional catastrophic results. Potting steel in concrete is done because it's cheap and easy, not because it's particularly maintainable. It's inherently difficult to inspect the structure when you build things this way. > especially prestressed concrete Agree. Unbonded, post-tensioned concrete (where you can replace individual strands) seems like the only reasonable approach to me, but building this way and doing all of the inspection and maintenance is way more expensive than the "do almost nothing" approach for rebar concrete. But the benefits are only realized after 100 years, so nobody has the incentive to design this way.
- ak217 5y ago
  > you don't have a reliable way to determine when the bridge is no longer safe I have a hard time believing this. Despite the ridiculous claims of another poster in this thread (that rebar just spontaneously and completely turns into rust) properly formulated and surfaced concrete and steel don't just rot on the inside with no external signs of decay. If you have repeated water intrusion, you will see seeping and cracks. If you have increasing stress, you'll see spalling. You can core concrete, drill out and patch rebar if you have spot damage. In general I find the sentiments in this thread bizarre since the vast majority of long-lasting buildings now and in the past century are made of concrete (many of the more modern ones have a concrete foundation and wood or steel superstructure). You don't see many old wood structures because most of them decayed and were torn down - there's a reason why you don't see many wooden bridges standing. Concrete is the closest thing that we have structurally to something that can last forever, and we're learning more about how to make it last longer all the time. Let me know if you have links that discuss maintainability of concrete in this context. Would be glad to learn more.

fsloth 5y ago

"...or you're sunk" I believe there is a very unfortunate example of this in Miami in the recent condominium collapse. A bridge recently collapsed in Italy. Etc. So the problem of failing concrete structures is not 'theoretical' but a very real issue. Even though most concrete construction does seem to last pretty well.

afiori 5y ago
Both of those building would have been almost impossible to build with wood
- Retric 5y ago
  Miami building was only 12 stories tall, the comparable wooden structures have been built for an 18 story example: https://en.wikipedia.org/wiki/Mjøstårnet https://en.wikipedia.org/wiki/Mjøstårnet
- fsloth 5y ago
  Sure - my main point was that reinforced concrete will fail concretely and not only theoretically unless maintained. I was not suggesting it could be replaces.
deleted 5y ago
[deleted]