6 ms·

Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinf

Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts (and in doing so expands, damaging more concrete and hastening further water inclusion)?

oakwhiz 4y ago
The rusting and getting wet is cyclical in nature and part of what accelerates that is the formation of cracks in the first place. Part of the maintenance of concrete is to fill any cracks that you can find to prevent it from getting worse. The cracks typically start small, and they might form in places that are hard to get to as well as being too small to see, so if the concrete is self-healing it will prevent some of these small cracks from getting worse on their own which should reduce the effect of expanding water (ice) as well as expanding rust. The reinforcement material (rebar) already has a very close thermal coefficient of expansion to concrete and this can be thought of as kind of a lucky coincidence.
- masklinn 4y ago
  > as well as expanding rust. Ideally they’d seal before the rebar is exposed and can rust. As long as the rebar is encased, unless you fucked up dramatically it should be safe.
  - moloch-hai 4y ago
    The rust is essential to bind it to the cement. Without, it adds little strength. Modern concrete isn't expected or designed to last more than a century. It could have been made to last longer, if they wanted, but that would cost a little more. Making no provision for when it will predictably fall apart is a modern failing. That major construction with concrete is about a century old should worry everyone. We have a very great deal of infrastructure that will fail on a predictable schedule, with nothing budgeted to replace it all.
    - saalweachter 4y ago
      On the plus side it makes for some wicked post-apocalyptic ruins. Remove maintenance, add 20 years, and you get the most photogenic wasteland you could ask for.
    - oakwhiz 4y ago
      Yes although it is true what the grandparent comment said about sealing - that keeping things dry, using coatings or topper materials, dewatering, etc. will all help extend the longevity and reduce maintenance requirements a bit.
    - LegitShady 4y ago
      > The rust is essential to bind it to the cement. Without, it adds little strength. Do you have a citation for this? As far as I know this is entirely not true. Rust is an expansionary product - when steel rusts its volume increases something like 10x, which reduces the connection between the concrete and the steel and causes issues like cracking and spalling. Minimizing rust improves the long term performance of rebar.
      - moloch-hai 4y ago
        They leave the rebar out to rust a bit, on purpose, before they pour. Surface rust is good, but more is bad. Like many things in life.
        zoklet-enjoyer 4y ago
        I read your comment as I finished eating my third slice of pie and wondering why I cut myself 3 slices.
        PakG1 4y ago
        3 is the limit. More than 3 is bad.
        4y ago
  - anonuser123456 4y ago
    Steel gives concrete tensile strength. Unfortunately, defects in the epoxy coating become a focal point of oxidation, often pitting that single point more than otherwise uncoated steel. The result is, the pitted area fails quickly and the integrity of the entire member is compromised.
  - LegitShady 4y ago
    this isn't really possible. For the rebar to take up meaningful tensile stresses, the concrete nearby must have cracked. Reinforced concrete ALWAYS cracks. If you want better corrosion resistance, you do other things - lower water ratios, increased concrete cover, sulfate resistant concrete with lower permeability, different reinforcing or epoxy coated reinforcing, etc.
    - dima_vm 4y ago
      Not always, if rebars are pre-tensioned before concrete solidifies. So the rebar is always under tension. Pretty common in prefab parts, like bridge bars.
      - LegitShady 4y ago
        pre-stressed concrete is great, and has many advantages over regular reinforced concrete and is more efficient in terms of cross section so cracks less. It still cracks, though, and in different locations like on the ends horizontally instead of at high moment/deflection areas vertically. Pre-stressed does experience fewer shrinkage cracks, which is nice. It's more expensive to build and repair. And while you absolutely have pretensioned girders on bridges commonly, the bridge decks generally aren't, although they're almost always completely in compression and the steel is there for shrinkage cracks etc.
      - sporkland 4y ago
        For someone totally outside this space I had assumed concrete is rigid and brittle and rebar reinforcement allows for it to bend and flex without breaking. But your comment makes it feel like it keeps the concrete in tension with compression forces to make it sturdier. Can you ELI5 what the rebar does to help?
        walty 4y ago
        You're going to love this playlist from Practical Engineering [1]. Grady explains all of this much better than I could. The second video explains reinforcement and the fifth explains pre-stressing. [1] https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKfW90PdaBFt70BLTbz1bTF6Mn https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKf...
- PicassoCTs 4y ago
  Another method is to connect all the rebar and add a https://en.wikipedia.org/wiki/Galvanic_anode https://en.wikipedia.org/wiki/Galvanic_anode
- tuatoru 4y ago
  > expanding rust The alkaline environment inside concrete "passivates" the reinforcing bar, greatly reducing the rate of oxidation and changing its form. The problem exists only in concrete that is badly made with poorly chosen aggregate, and in corrosive environments. 1. https://en.wikipedia.org/wiki/Passivation_(chemistry) https://en.wikipedia.org/wiki/Passivation_(chemistry)
prometheus76 4y ago
And being driven over by semi trucks full of heavy cargo 24/7.
riversflow 4y ago
I'm just going to throw it out there to spread the word; GFRP Rebar (Glass Fiber Reinforced Polymer) is a thing, and you can buy it today at your local hardware store. I really wish I would have known about it on my last project, probably the last time I will buy steel rebar to put in concrete. I'm not an ME, but my understanding is that it is a comp in terms of structural reinforcement. I'd be really interested to do a deep dive on how these technologies stack up against each other, it seems like a win for construction longevity, it would be pretty cool if the ecological impact of production penciled out as a win too. My understanding is that there are some trade off when being used in large projects. My cursory research has indicated that this stuff first hit production in the 80's and was used in a bridge in the US in 96. Still haven't figured out why it hasn't taken over.[2] Is it just for a lack of evangelists? [1]https://www.lowes.com/pd/Owens-Corning/5013333093 https://www.lowes.com/pd/Owens-Corning/5013333093 [2] https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning-in-the-usa/ https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning...
- iamgopal 4y ago
  Any comparison in tensile strength?
- scarier 4y ago
  The main reason is stiffness—off the top of my head, it’s roughly a quarter as stiff as typical steel, so it needs significant engineering and often much more reinforcement to keep the concrete within allowable deformation limits. Basalt and carbon fiber are more promising replacements, but are significantly more expensive.
- _jsnk 4y ago
  I recommend Tyler Ley's youtube channel[0] entirely devoted to concrete. He has an episode[1] addressing GFRP that might interest you. [0] https://www.youtube.com/@TylerLey https://www.youtube.com/@TylerLey [1] https://www.youtube.com/watch?v=thUZImUTZn0 https://www.youtube.com/watch?v=thUZImUTZn0
- xattt 4y ago
  Mike Holmes, a home reno personality in Canada, had been touting its benefits in about the mid-2000s for home foundation re-pours.
tuatoru 4y ago
> Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts No. It's a consequence of designing to requirements of minimum cost and a 50-year lifespan. It'd be easy to make concrete with much longer lifespan, but it costs more.
- raspberry1337 4y ago
  It's a nice thought that all these depressive brutalistic/modernist builduings will all soon crumbly by the same mechanism that spawned them!
  - stephencanon 4y ago
    Most brutalist buildings are already 60+ ish years old and doing great (actual brutalism, rather than “concrete buildings I don’t like,” was mostly a style of the 1950s and 60s in the west, though it lasted quite a bit longer in Eastern Europe). Even the Boston government services center, everyone’s most hated brutalist building in the US is already past 50.
    - bpbp-mango 4y ago
      for many people, myself included, actual brutalism is "concrete buildings I don’t like"
      - stephencanon 4y ago
        Eh, there’s no accounting for taste, but there have always been shitty buildings in every vernacular. We tend to not see the worst buildings in older styles because they’ve long since been torn down.
- ithkuil 4y ago
  “Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.” - Unknown

oakwhiz 4y ago

The rusting and getting wet is cyclical in nature and part of what accelerates that is the formation of cracks in the first place. Part of the maintenance of concrete is to fill any cracks that you can find to prevent it from getting worse. The cracks typically start small, and they might form in places that are hard to get to as well as being too small to see, so if the concrete is self-healing it will prevent some of these small cracks from getting worse on their own which should reduce the effect of expanding water (ice) as well as expanding rust. The reinforcement material (rebar) already has a very close thermal coefficient of expansion to concrete and this can be thought of as kind of a lucky coincidence.

masklinn 4y ago
> as well as expanding rust. Ideally they’d seal before the rebar is exposed and can rust. As long as the rebar is encased, unless you fucked up dramatically it should be safe.
- moloch-hai 4y ago
  The rust is essential to bind it to the cement. Without, it adds little strength. Modern concrete isn't expected or designed to last more than a century. It could have been made to last longer, if they wanted, but that would cost a little more. Making no provision for when it will predictably fall apart is a modern failing. That major construction with concrete is about a century old should worry everyone. We have a very great deal of infrastructure that will fail on a predictable schedule, with nothing budgeted to replace it all.
  - saalweachter 4y ago
    On the plus side it makes for some wicked post-apocalyptic ruins. Remove maintenance, add 20 years, and you get the most photogenic wasteland you could ask for.
  - oakwhiz 4y ago
    Yes although it is true what the grandparent comment said about sealing - that keeping things dry, using coatings or topper materials, dewatering, etc. will all help extend the longevity and reduce maintenance requirements a bit.
  - LegitShady 4y ago
    > The rust is essential to bind it to the cement. Without, it adds little strength. Do you have a citation for this? As far as I know this is entirely not true. Rust is an expansionary product - when steel rusts its volume increases something like 10x, which reduces the connection between the concrete and the steel and causes issues like cracking and spalling. Minimizing rust improves the long term performance of rebar.
    - moloch-hai 4y ago
      They leave the rebar out to rust a bit, on purpose, before they pour. Surface rust is good, but more is bad. Like many things in life.
      - zoklet-enjoyer 4y ago
        I read your comment as I finished eating my third slice of pie and wondering why I cut myself 3 slices.
        PakG1 4y ago
        3 is the limit. More than 3 is bad.
        4y ago
- anonuser123456 4y ago
  Steel gives concrete tensile strength. Unfortunately, defects in the epoxy coating become a focal point of oxidation, often pitting that single point more than otherwise uncoated steel. The result is, the pitted area fails quickly and the integrity of the entire member is compromised.
- LegitShady 4y ago
  this isn't really possible. For the rebar to take up meaningful tensile stresses, the concrete nearby must have cracked. Reinforced concrete ALWAYS cracks. If you want better corrosion resistance, you do other things - lower water ratios, increased concrete cover, sulfate resistant concrete with lower permeability, different reinforcing or epoxy coated reinforcing, etc.
  - dima_vm 4y ago
    Not always, if rebars are pre-tensioned before concrete solidifies. So the rebar is always under tension. Pretty common in prefab parts, like bridge bars.
    - LegitShady 4y ago
      pre-stressed concrete is great, and has many advantages over regular reinforced concrete and is more efficient in terms of cross section so cracks less. It still cracks, though, and in different locations like on the ends horizontally instead of at high moment/deflection areas vertically. Pre-stressed does experience fewer shrinkage cracks, which is nice. It's more expensive to build and repair. And while you absolutely have pretensioned girders on bridges commonly, the bridge decks generally aren't, although they're almost always completely in compression and the steel is there for shrinkage cracks etc.
    - sporkland 4y ago
      For someone totally outside this space I had assumed concrete is rigid and brittle and rebar reinforcement allows for it to bend and flex without breaking. But your comment makes it feel like it keeps the concrete in tension with compression forces to make it sturdier. Can you ELI5 what the rebar does to help?
      - walty 4y ago
        You're going to love this playlist from Practical Engineering [1]. Grady explains all of this much better than I could. The second video explains reinforcement and the fifth explains pre-stressing. [1] https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKfW90PdaBFt70BLTbz1bTF6Mn https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKf...
PicassoCTs 4y ago
Another method is to connect all the rebar and add a https://en.wikipedia.org/wiki/Galvanic_anode https://en.wikipedia.org/wiki/Galvanic_anode
tuatoru 4y ago
> expanding rust The alkaline environment inside concrete "passivates" the reinforcing bar, greatly reducing the rate of oxidation and changing its form. The problem exists only in concrete that is badly made with poorly chosen aggregate, and in corrosive environments. 1. https://en.wikipedia.org/wiki/Passivation_(chemistry) https://en.wikipedia.org/wiki/Passivation_(chemistry)

prometheus76 4y ago

And being driven over by semi trucks full of heavy cargo 24/7.

riversflow 4y ago

I'm just going to throw it out there to spread the word; GFRP Rebar (Glass Fiber Reinforced Polymer) is a thing, and you can buy it today at your local hardware store. I really wish I would have known about it on my last project, probably the last time I will buy steel rebar to put in concrete. I'm not an ME, but my understanding is that it is a comp in terms of structural reinforcement. I'd be really interested to do a deep dive on how these technologies stack up against each other, it seems like a win for construction longevity, it would be pretty cool if the ecological impact of production penciled out as a win too. My understanding is that there are some trade off when being used in large projects. My cursory research has indicated that this stuff first hit production in the 80's and was used in a bridge in the US in 96. Still haven't figured out why it hasn't taken over.[2] Is it just for a lack of evangelists? [1]https://www.lowes.com/pd/Owens-Corning/5013333093 https://www.lowes.com/pd/Owens-Corning/5013333093 [2] https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning-in-the-usa/ https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning...

iamgopal 4y ago
Any comparison in tensile strength?
scarier 4y ago
The main reason is stiffness—off the top of my head, it’s roughly a quarter as stiff as typical steel, so it needs significant engineering and often much more reinforcement to keep the concrete within allowable deformation limits. Basalt and carbon fiber are more promising replacements, but are significantly more expensive.
_jsnk 4y ago
I recommend Tyler Ley's youtube channel[0] entirely devoted to concrete. He has an episode[1] addressing GFRP that might interest you. [0] https://www.youtube.com/@TylerLey https://www.youtube.com/@TylerLey [1] https://www.youtube.com/watch?v=thUZImUTZn0 https://www.youtube.com/watch?v=thUZImUTZn0
xattt 4y ago
Mike Holmes, a home reno personality in Canada, had been touting its benefits in about the mid-2000s for home foundation re-pours.

tuatoru 4y ago

> Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts No. It's a consequence of designing to requirements of minimum cost and a 50-year lifespan. It'd be easy to make concrete with much longer lifespan, but it costs more.

raspberry1337 4y ago
It's a nice thought that all these depressive brutalistic/modernist builduings will all soon crumbly by the same mechanism that spawned them!
- stephencanon 4y ago
  Most brutalist buildings are already 60+ ish years old and doing great (actual brutalism, rather than “concrete buildings I don’t like,” was mostly a style of the 1950s and 60s in the west, though it lasted quite a bit longer in Eastern Europe). Even the Boston government services center, everyone’s most hated brutalist building in the US is already past 50.
  - bpbp-mango 4y ago
    for many people, myself included, actual brutalism is "concrete buildings I don’t like"
    - stephencanon 4y ago
      Eh, there’s no accounting for taste, but there have always been shitty buildings in every vernacular. We tend to not see the worst buildings in older styles because they’ve long since been torn down.
ithkuil 4y ago
“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.” - Unknown