5 ms·

I saw a video of somebody poking a stick into a wall and it just came apart like sand/rocks - so cement was too little. The worst was the rebar that you could

I saw a video of somebody poking a stick into a wall and it just came apart like sand/rocks - so cement was too little. The worst was the rebar that you could bend with your hand like toffee.

abduhl 5y ago
Localized defects like you're describing are generally a result of poor placement rather than an issue with the mix. And I think you'd be surprised at how easy it is to bend rebar anchored in concrete. Pretty much anything below a #8 (roughly a 25M in metric; 1 inch diameter) can be bent by hand or, worst case, with a pipe.
- pengaru 5y ago
  Right, and when doing such a large job, at least in the USA, the concrete would be getting delivered by a third-party material services company. From what I recall (my dad had a concrete construction company back in the day) the material services co. played a significant role in selecting appropriate mix ratios and additives for the application and conditions. They're not going to be deliberately sending out a mix with insufficient binders, especially not on such a massive order. The whole system runs on credit; the material services company doesn't get paid for the concrete they delivered until the whole job gets paid, sometimes years later. So they're incentivized to be quite involved and engaged in the whole process and ensuring success to the maximum extent available to them. The last thing they want is some finger-pointing lawsuits over a faulty mix getting in the way of collecting payment, or worse, get stuck with the bill of demolishing and disposing it all for a redo on top of it.
SahAssar 5y ago
Rebar is supposed to help with compression/stretching, not lateral forces, right? Does being able to bend it tell you anything about it's quality?
- pengaru 5y ago
  > Rebar is supposed to help with compression/stretching, not lateral forces, right? Negative, at least WRT compression. Concrete is always stellar in the compression department, that's its strength. Rebar alone is mostly about improving shear strength. I'm not sure how much it really improves tension, maybe when it's a mesh. I think when tension is a priority there's simply steel reinforcement added spanning the same points, unlike rebar which is simply encapsulated in the concrete.
  - SahAssar 5y ago
    I was (very) badly remembering this video https://www.youtube.com/watch?v=cZINeaDjisY https://www.youtube.com/watch?v=cZINeaDjisY Still, does being able to bend rebar tell you anything about it's quality as a reinforcement?
    - pengaru 5y ago
      This isn't my area of expertise, so it's not something I feel qualified to speak authoritatively about. Considering how flimsy stucco reinforcement is (it's basically chickenwire) I doubt it's a red flag. They even used to consider exterior stucco walls structural enough to entirely omit sheathing, though that's not code compliant anymore.
      - abduhl 5y ago
        I had a very long post typed out responding to this thread but I pressed back and it got deleted. The short version: 1. The layperson explanation for how reinforced concrete works is that concrete holds compression and steel holds tension. This isn't really true - steel also contributes to compression. Why not just use steel for everything? Because concrete is easier to work with and cheaper. 2. Shear failures are actually usually either crushing failures (compression) or tension failures. This is because any shear stress state can be transformed into pure tension/compression by a simple rotational transformation (because any given stress state can be represented by a tensor in a chosen basis). 3. Welded wire fabric (or mesh, depending on your supplier) is more robust than chickenwire but you'll find that most crews actually call it chickenwire. 4. Bending rebar by hand means nothing. You can bend anything by hand given a long enough length beyond an anchor point because the moment arm is a function of the distance from the point of fixity. See my other post for sizes that I would not be surprised could be bent "by hand" in the field. 5. It's unclear what's meant by "lateral forces" in the GP post. Lateral forces can be taken at the structure level or the member level. When someone talks about "lateral forces" in the structure context they're usually talking about forces that are not in the direction that gravity works. For buildings this means wind and earthquakes. Lateral forces result in bending and shear (aka tension failures), both of which require rebar to resist for significant structures. From this perspective, lateral forces are usually a primary reason to include rebar in designs.

I saw a video of somebody poking a stick into a wall and it just came apart like sand/rocks - so cement was too little. The worst was the rebar that you could bend with your hand like toffee.

abduhl 5y ago

Localized defects like you're describing are generally a result of poor placement rather than an issue with the mix. And I think you'd be surprised at how easy it is to bend rebar anchored in concrete. Pretty much anything below a #8 (roughly a 25M in metric; 1 inch diameter) can be bent by hand or, worst case, with a pipe.

pengaru 5y ago
Right, and when doing such a large job, at least in the USA, the concrete would be getting delivered by a third-party material services company. From what I recall (my dad had a concrete construction company back in the day) the material services co. played a significant role in selecting appropriate mix ratios and additives for the application and conditions. They're not going to be deliberately sending out a mix with insufficient binders, especially not on such a massive order. The whole system runs on credit; the material services company doesn't get paid for the concrete they delivered until the whole job gets paid, sometimes years later. So they're incentivized to be quite involved and engaged in the whole process and ensuring success to the maximum extent available to them. The last thing they want is some finger-pointing lawsuits over a faulty mix getting in the way of collecting payment, or worse, get stuck with the bill of demolishing and disposing it all for a redo on top of it.

SahAssar 5y ago

Rebar is supposed to help with compression/stretching, not lateral forces, right? Does being able to bend it tell you anything about it's quality?

pengaru 5y ago
> Rebar is supposed to help with compression/stretching, not lateral forces, right? Negative, at least WRT compression. Concrete is always stellar in the compression department, that's its strength. Rebar alone is mostly about improving shear strength. I'm not sure how much it really improves tension, maybe when it's a mesh. I think when tension is a priority there's simply steel reinforcement added spanning the same points, unlike rebar which is simply encapsulated in the concrete.
- SahAssar 5y ago
  I was (very) badly remembering this video https://www.youtube.com/watch?v=cZINeaDjisY https://www.youtube.com/watch?v=cZINeaDjisY Still, does being able to bend rebar tell you anything about it's quality as a reinforcement?
  - pengaru 5y ago
    This isn't my area of expertise, so it's not something I feel qualified to speak authoritatively about. Considering how flimsy stucco reinforcement is (it's basically chickenwire) I doubt it's a red flag. They even used to consider exterior stucco walls structural enough to entirely omit sheathing, though that's not code compliant anymore.
    - abduhl 5y ago
      I had a very long post typed out responding to this thread but I pressed back and it got deleted. The short version: 1. The layperson explanation for how reinforced concrete works is that concrete holds compression and steel holds tension. This isn't really true - steel also contributes to compression. Why not just use steel for everything? Because concrete is easier to work with and cheaper. 2. Shear failures are actually usually either crushing failures (compression) or tension failures. This is because any shear stress state can be transformed into pure tension/compression by a simple rotational transformation (because any given stress state can be represented by a tensor in a chosen basis). 3. Welded wire fabric (or mesh, depending on your supplier) is more robust than chickenwire but you'll find that most crews actually call it chickenwire. 4. Bending rebar by hand means nothing. You can bend anything by hand given a long enough length beyond an anchor point because the moment arm is a function of the distance from the point of fixity. See my other post for sizes that I would not be surprised could be bent "by hand" in the field. 5. It's unclear what's meant by "lateral forces" in the GP post. Lateral forces can be taken at the structure level or the member level. When someone talks about "lateral forces" in the structure context they're usually talking about forces that are not in the direction that gravity works. For buildings this means wind and earthquakes. Lateral forces result in bending and shear (aka tension failures), both of which require rebar to resist for significant structures. From this perspective, lateral forces are usually a primary reason to include rebar in designs.