4 ms·

The primary difference between Roman and modern cement is that Roman cement does not have reinforcing steel in it and cannot support loads that put it under ten

The primary difference between Roman and modern cement is that Roman cement does not have reinforcing steel in it and cannot support loads that put it under tension stress. This reinforcing steel is what fails with age - cracks develop, let in air, and corrode the steel. No steel, and what you have is basically a shaped rock, and that can last a really long time. The downside, of course, is that not being able to resist tensile loads severely limits the designs you can make with concrete. Reinforced concrete is significantly more useful than unreinforced concrete.

LegitShady 7y ago
and to add to this, all reinforced concrete cracks. Otherwise it doesnt need to be reinforced.
dustfinger 7y ago
I wonder if it would be feasible to replace the steel reinforcement rods with ceramic? No iron no rust. I also wonder if concrete might bond better with ceramic. My understanding is that concrete does not bond well with steel.
- dustfinger 7y ago
  It looks like some effort was made in 2010 to coat the reinforcing steel with a porcelain enamel [1]. I believe the enamel's primary function was to provide a bond with the encasing concrete. Naturally, it would also help to protect the steel from water and oxygen. There must be a reason why ceramic cannot fully replace steel. Cost perhaps. Maybe these technologies would be better combined. [1] https://www.ceramicindustry.com/articles/90943-improving-steel-reinforced-concrete-structures https://www.ceramicindustry.com/articles/90943-improving-ste...
  - ThrustVectoring 7y ago
    A big part of the problem with coated steel rebar is that rebar is treated very roughly on construction sites, and any nick or failure of the coating is really bad news when the chemical isolation provided by the concrete fails. Instead of the corrosion happening relatively uniformly at a slow rate across the entire length of the reinforcing bar, it'll happen at a few select sites at a significantly higher rate.
    - numakerg 7y ago
      What about using plastic or some flexibly epoxy. Would it have some adverse effect or is there a cost issue?
      - ThrustVectoring 7y ago
        Epoxy-coated steel rebar has the problem I mentioned, as well as being more expensive than standard rebar. You can remove the steel and use non-corroding materials, but these tend to have significantly less stiffness than steel (and reinforcing stiffness determines the load-to-crack-size ratio)
    - dustfinger 7y ago
      That is really interesting. I didn't realize that corrosive oxidation would become concentrated and react at a higher rate if the point of exposure was limited.
    - asdff 7y ago
      Why isn't it galvanized like a nail or a porsche?
      - ThrustVectoring 7y ago
        It can be, but it adds cost to the project, and any galvanized rebar has to be bent before it gets galvanized rather than on-site.
- Gibbon1 7y ago
  I think concrete bonds with steel just fine. Problem is when it rusts or is exposed to chloride the steel corrodes which expands and cracks the concrete. I think massive reinforced concrete sections that are kept dry are stable long term. Some work is being done to find alternatives. Difficult because you need high tensile strength and toughness. And the alkaline environment will wreck other materials. I think ordinary fiber glass turns to gel over time. A lot of reinforcing bar is coated with epoxy. But I think that's not working out very well in practice. Any nick in the coating directs the corrosion to that point. Whole thing is an open problem
  - wahern 7y ago
    Stainless steel seems to be working well, though much too expensive for most projects. Maybe part of the problem is that because stainless steel is a proven alternative where it matters, it's hard for other alternatives to break into the market, either at the low-end where there's zero chance your crews are going to apply the new procedures properly, or at the high-end where conservatism and greater access to cash make stainless viable.
    - imtringued 7y ago
      If corrosion of rebar is limiting the lifespan of concrete structures then using a resistant rebar could increase the lifespan of a building enormously. Modern buildings could last multiple centuries instead of starting to crumble at 80 years.
- ThrustVectoring 7y ago
  It'd be a material engineering feat to get the required plasticity and tensile strength out of ceramics. The plasticity is probably more worrying in practice - the failure mode of rebar rusting is usually visible to inspection and slow enough that action can be taken to ensure that the structure remains safe. The failure mode of a brittle reinforcing member suddenly failing is much more worrying from a safety perspective.
  - ggcdn 7y ago
    In structural engineering we call this plasticity "ductility", but you're exactly right. We want ductile failures not sudden brittle ones, and that can only be achieved with yielding reinforcing. Ceramics generally will fail brittly, or cause the concrete to crush if the member is over-reinforced.
  - dustfinger 7y ago
    > the failure mode of rebar rusting is usually visible to inspection and slow enough that action can be taken to ensure that the structure remains safe. The failure mode of a brittle reinforcing member suddenly failing is much more worrying from a safety perspective. That is an excellent point!
- wahern 7y ago
  Lots of different materials and techniques have been tried, but nothing so far seems to have really stuck. As an observer I think there are two main factors: 1) The types of steel used coincidentally have nearly identical coefficients of thermal expansion as the concrete used. 2) Everybody in the construction industry and the entire supply chain is familiar with and habituated to steel rebar. The inertia is tremendous. That's a very high bar to meet to gain a foothold with new materials or new techniques as the entire system has evolved to fit the pros and cons of steel rebar. Anything that departs from standard practice becomes very costly, very quickly, for a gazillion reasons.
  - dustfinger 7y ago
    I never considered the potential social side to this problem. Thank you for commenting.
- coryrc 7y ago
  Basalt rebar, which is basically like epoxy fiberglass as used in boats, wind generator blades, etc. except basalt fibers and just really long. However, it can't be bent so you have to make joins at every corner and have to order all bent pieces ahead of time.
- asdff 7y ago
  What about wood?
  - dustfinger 7y ago
    Wood absorbs water causing it to warp, swell and decay. Having said that, steel rebar is expensive which has motivated some organizations to use wood, as well as other plant materials, as a reinforcement material in concrete with some success [1]. [1] https://www.quora.com/Was-wood-used-as-a-reinforcement-material-in-reinforced-concrete https://www.quora.com/Was-wood-used-as-a-reinforcement-mater...

LegitShady 7y ago

and to add to this, all reinforced concrete cracks. Otherwise it doesnt need to be reinforced.

dustfinger 7y ago

I wonder if it would be feasible to replace the steel reinforcement rods with ceramic? No iron no rust. I also wonder if concrete might bond better with ceramic. My understanding is that concrete does not bond well with steel.

dustfinger 7y ago
It looks like some effort was made in 2010 to coat the reinforcing steel with a porcelain enamel [1]. I believe the enamel's primary function was to provide a bond with the encasing concrete. Naturally, it would also help to protect the steel from water and oxygen. There must be a reason why ceramic cannot fully replace steel. Cost perhaps. Maybe these technologies would be better combined. [1] https://www.ceramicindustry.com/articles/90943-improving-steel-reinforced-concrete-structures https://www.ceramicindustry.com/articles/90943-improving-ste...
- ThrustVectoring 7y ago
  A big part of the problem with coated steel rebar is that rebar is treated very roughly on construction sites, and any nick or failure of the coating is really bad news when the chemical isolation provided by the concrete fails. Instead of the corrosion happening relatively uniformly at a slow rate across the entire length of the reinforcing bar, it'll happen at a few select sites at a significantly higher rate.
  - numakerg 7y ago
    What about using plastic or some flexibly epoxy. Would it have some adverse effect or is there a cost issue?
    - ThrustVectoring 7y ago
      Epoxy-coated steel rebar has the problem I mentioned, as well as being more expensive than standard rebar. You can remove the steel and use non-corroding materials, but these tend to have significantly less stiffness than steel (and reinforcing stiffness determines the load-to-crack-size ratio)
  - dustfinger 7y ago
    That is really interesting. I didn't realize that corrosive oxidation would become concentrated and react at a higher rate if the point of exposure was limited.
  - asdff 7y ago
    Why isn't it galvanized like a nail or a porsche?
    - ThrustVectoring 7y ago
      It can be, but it adds cost to the project, and any galvanized rebar has to be bent before it gets galvanized rather than on-site.
Gibbon1 7y ago
I think concrete bonds with steel just fine. Problem is when it rusts or is exposed to chloride the steel corrodes which expands and cracks the concrete. I think massive reinforced concrete sections that are kept dry are stable long term. Some work is being done to find alternatives. Difficult because you need high tensile strength and toughness. And the alkaline environment will wreck other materials. I think ordinary fiber glass turns to gel over time. A lot of reinforcing bar is coated with epoxy. But I think that's not working out very well in practice. Any nick in the coating directs the corrosion to that point. Whole thing is an open problem
- wahern 7y ago
  Stainless steel seems to be working well, though much too expensive for most projects. Maybe part of the problem is that because stainless steel is a proven alternative where it matters, it's hard for other alternatives to break into the market, either at the low-end where there's zero chance your crews are going to apply the new procedures properly, or at the high-end where conservatism and greater access to cash make stainless viable.
  - imtringued 7y ago
    If corrosion of rebar is limiting the lifespan of concrete structures then using a resistant rebar could increase the lifespan of a building enormously. Modern buildings could last multiple centuries instead of starting to crumble at 80 years.
ThrustVectoring 7y ago
It'd be a material engineering feat to get the required plasticity and tensile strength out of ceramics. The plasticity is probably more worrying in practice - the failure mode of rebar rusting is usually visible to inspection and slow enough that action can be taken to ensure that the structure remains safe. The failure mode of a brittle reinforcing member suddenly failing is much more worrying from a safety perspective.
- ggcdn 7y ago
  In structural engineering we call this plasticity "ductility", but you're exactly right. We want ductile failures not sudden brittle ones, and that can only be achieved with yielding reinforcing. Ceramics generally will fail brittly, or cause the concrete to crush if the member is over-reinforced.
- dustfinger 7y ago
  > the failure mode of rebar rusting is usually visible to inspection and slow enough that action can be taken to ensure that the structure remains safe. The failure mode of a brittle reinforcing member suddenly failing is much more worrying from a safety perspective. That is an excellent point!
wahern 7y ago
Lots of different materials and techniques have been tried, but nothing so far seems to have really stuck. As an observer I think there are two main factors: 1) The types of steel used coincidentally have nearly identical coefficients of thermal expansion as the concrete used. 2) Everybody in the construction industry and the entire supply chain is familiar with and habituated to steel rebar. The inertia is tremendous. That's a very high bar to meet to gain a foothold with new materials or new techniques as the entire system has evolved to fit the pros and cons of steel rebar. Anything that departs from standard practice becomes very costly, very quickly, for a gazillion reasons.
- dustfinger 7y ago
  I never considered the potential social side to this problem. Thank you for commenting.
coryrc 7y ago
Basalt rebar, which is basically like epoxy fiberglass as used in boats, wind generator blades, etc. except basalt fibers and just really long. However, it can't be bent so you have to make joins at every corner and have to order all bent pieces ahead of time.
asdff 7y ago
What about wood?
- dustfinger 7y ago
  Wood absorbs water causing it to warp, swell and decay. Having said that, steel rebar is expensive which has motivated some organizations to use wood, as well as other plant materials, as a reinforcement material in concrete with some success [1]. [1] https://www.quora.com/Was-wood-used-as-a-reinforcement-material-in-reinforced-concrete https://www.quora.com/Was-wood-used-as-a-reinforcement-mater...