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  1. #241
    Super Star Member EddieWalker's Avatar
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    Default Re: New Home Begins

    Quote Originally Posted by sdkubota View Post
    The minimum concerte cover for rebar is 3". That is to protect it from salt, water, etc. Also, the only time rebar does any good for structural strength is when it is placed in the bottom of the concrete slab as steel has great tensile strength where concrete has viturally none. Rebar only begins to take a load wonce the concrete actually cracks. In school we tested concrete beams 6"x6"x24" for flexural strength. Without fiber mesh they would break within a few cycles...with it they could continually take flexural forces indefinately. If you have a good base you will probably not have flexural forces. The number one problem with concrete failing is excessive water added on site...hands down. Concrete strength is solely dependent on the water to cement ratio.


    I agree with some of what you say, but disagree with some of it too. Are you in the trades? what school took a concrete beam and found it stronger with fiber? I don't want to disagree with you, but I find this just about impossible to believe? Same with rebar requireing 3 inches of concrete over it. That would be a failure of placement in a 3 1/2 inch pad and result in excessive cracking.

    I agree that too much water will destroy the PSI of concrete, but other factors also have a huge impact on how strong it is. How much Cement is added to it, the size of the agrigate, the temperature when it poured and how long it has to cure.

    Eddie

  2. #242
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    Default Re: New Home Begins

    Quote Originally Posted by EddieWalker View Post
    I agree with some of what you say, but disagree with some of it too. Are you in the trades? what school took a concrete beam and found it stronger with fiber? I don't want to disagree with you, but I find this just about impossible to believe? Same with rebar requireing 3 inches of concrete over it. That would be a failure of placement in a 3 1/2 inch pad and result in excessive cracking.

    I agree that too much water will destroy the PSI of concrete, but other factors also have a huge impact on how strong it is. How much Cement is added to it, the size of the agrigate, the temperature when it poured and how long it has to cure.

    Eddie
    I am a Civil Engineer and the flexural testing was done when I was in school in the 1980s. The total flex to the concrete beam was probably along the order of 1/8" but the difference between un-reinforced and poly-reinforced was incredible. That said poly reinforcement does not take the place of structural steel although it can/does stop some microscopic cracking. My statements regarding rebar are also correct but the only time rebar has any benefit is a structural piece of concrete. Rebar can "pin" to sections together but most cracking causes a rough enough face between sections to hold them in place. Most concrete interstate highways will omit rebar because of the extra costs (+20%) unless they are trying to deal with poor or underlying expansive soils. If you have a good, well prepared and compacted sub-base not involving expansive soils, concrete will perform very well. I will say steel or wire mesh will not hurt anything but I deal in cost effectiveness and limited budgets so that is how I am wired.

  3. #243
    Super Star Member EddieWalker's Avatar
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    Default Re: New Home Begins

    Interstate 20 was done in concrete a few years ago. They where laying and tying rebar on that for months in my area. It was all on chairs and in the middle of the pour. Just guessing, I think it was a six inch pour. The rebar was bigger then half inch, but I don't know for sure what they used. At the seams, it was huge where one pad stoped, and then they poured the next and tied into the exposed rebar.

    Areas of FL and LA have made #5 or 5/8 rebar the minimum for house pads in a lot of areas because of Hurricanes and extreme weather.

    CA keeps adding more and more rebar to their codes to the point that you now have to tile the straping from the rebar to over the tops of the rafters. on two stories, they want it to be continuous strapping, and the inspector will check to see that its tied to the rebar. he will measure how far apart the rebar is in the footings, the flat and if required, the post holes that are sometimes required there.

    I've never worked in SD, so I'll defer to your credentials for that part of the country. I still stand by my statement that I would only use rebar in a cement slab, and that fiber will never replace rebar. Is it code to build a house in SD without rebar if you use fiber?

    Eddie

  4. #244
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    Default Re: New Home Begins

    Quote Originally Posted by EddieWalker View Post
    Interstate 20 was done in concrete a few years ago. They where laying and tying rebar on that for months in my area. It was all on chairs and in the middle of the pour. Just guessing, I think it was a six inch pour. The rebar was bigger then half inch, but I don't know for sure what they used. At the seams, it was huge where one pad stoped, and then they poured the next and tied into the exposed rebar.

    Areas of FL and LA have made #5 or 5/8 rebar the minimum for house pads in a lot of areas because of Hurricanes and extreme weather.

    CA keeps adding more and more rebar to their codes to the point that you now have to tile the straping from the rebar to over the tops of the rafters. on two stories, they want it to be continuous strapping, and the inspector will check to see that its tied to the rebar. he will measure how far apart the rebar is in the footings, the flat and if required, the post holes that are sometimes required there.

    I've never worked in SD, so I'll defer to your credentials for that part of the country. I still stand by my statement that I would only use rebar in a cement slab, and that fiber will never replace rebar. Is it code to build a house in SD without rebar if you use fiber?

    Eddie
    There are no codes for using any rebar in concrete slabs in SD. Interstates are at a minimum 10" in concrete depth around here and given there vehicle loading I would guess similar across the country. Where a pour stops and starts they generally place smooth dowels to transfer the loads across the cold joint.

  5. #245
    Super Member mjncad's Avatar
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    Default Re: New Home Begins

    Every structural engineer I've worked with for 24-years always specified slab reinforcing to be in the middle of the slab. Ideally 3" of cover all around for rebar would be great; but in a light duty 4" slab I would be happy with 2" of cover over #3 rebar.
    Paraphrasing Douglas Adams - So long and thanks for all the bacon.

  6. #246
    Veteran Member dstig1's Avatar
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    Default Re: New Home Begins

    Eddie, note the point SD made about not needing rebar in slabs unless you have expansive soils or other soil issues. Doesn't "expansive soils" pretty much describe Texas? That would explain why it is code there...

    My limited interactions with civil engineers also shows a min 3" rebar cover in concrete specified...
    -Dave

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  7. #247
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    Default Re: New Home Begins

    Quote Originally Posted by mjncad View Post
    Every structural engineer I've worked with for 24-years always specified slab reinforcing to be in the middle of the slab. Ideally 3" of cover all around for rebar would be great; but in a light duty 4" slab I would be happy with 2" of cover over #3 rebar.
    Then you have never worked with a real one. Think about this...pressure placed on a section of concrete will place some concrete in compression, which it is very good at resisting, and some in tension which it isn't good at resisting. Without any rebar or steel the location of neutral forces would be in the exact center of the concrete section. Steel is very good at tension (also compression) but when placed in the lower portion of the concrete provides the necessary tensile resistance ability as well as allows more of the concrete section to work in compression thereby lowering the neutral point. Every structural engineer knows this or they are pretend engineers.

  8. #248
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    Default Re: New Home Begins

    Quote Originally Posted by dstig1 View Post
    Eddie, note the point SD made about not needing rebar in slabs unless you have expansive soils or other soil issues. Doesn't "expansive soils" pretty much describe Texas? That would explain why it is code there...

    My limited interactions with civil engineers also shows a min 3" rebar cover in concrete specified...
    You will not solve problems of expansive soils with any amount of rebar. I have been involved in a couple of hotels where a water line broke that were in very expansive soil areas. Every room on the ground floor was heaved up around 8" to 12" in the center of a 12' wide room. The edges never heaved because of the bearing pressure of the walls placed on the soil. The concrete floors were torn out, excess soils removed, and a structural slab was placed back for the floor. The slab was designed to completely support the floor as the underlying soil would lose moisture and shrink away. In that case, rebar was required but the floor essentially turned into a bridge.

  9. #249
    Veteran Member dstig1's Avatar
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    Default Re: New Home Begins

    They do use post-tensioned slabs a lot in Texas, from what I have read...same basic reason, I assume.
    -Dave

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  10. #250
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    Default Re: New Home Begins

    Quote Originally Posted by dstig1 View Post
    They do use post-tensioned slabs a lot in Texas, from what I have read...same basic reason, I assume.
    Little off topic but post tensioning or pre-tensioning places the concrete in compression immediately and the steel in tension which is different then rebar which requires cracking to occur before the steel takes on any tension. Also steel used in post/pre tensioning is extremely high in tensile strength >100k psi. Prestensioning is generally done on bridge decking or concrete support beams. Post tensioning is done is slabs or floors after it is poured. The tensioning steel is placed in conduits cast into the concrete. The steel is under such high tension it is critical the cables are not cut as a very violent chain reaction will occur. Sorry for the rant...

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