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Old 07-03-2010, 01:56 AM   #16
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Some questions about a concrete slab


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Originally Posted by Daniel Holzman View Post
OK, back to the rebar. The claim that rebar can't hurt is nonsense. If you use a #4 bar, that is 1/2 inch diameter. This means the MAXIMUM possible cover is 1-3/4 inch, assuming you get the bars EXACTLY in the middle, which is not going to happen. 1-3/4 inch is less than required minimum cover per ACI.

The reason this is a problem is that with less than minimum cover, the bars are prone to rusting. When they rust, they expand. When they expand, they crack the slab. This is serious trouble, easily avoided by NOT USING bars.

I challenge anyone who thinks bars are useful in a 4 inch slab to find some independent, knowlegeable organization that supports the practice. Or at least offer a well thought out explanation as to why you think steel bars IN THIS application are useful.

Bars are used in structural applications because they provide tensile strength, which concrete lacks, but ONLY when they are placed in the tension side of the slab. When placed in the middle, they are in the neutral axis, which by definition carries no stress or strain. So all they do is sit there and rust, and in the long run they crack your slab.
Would most city inspectors allow a 4" slab to be laid? i'm not suggesting any city codes office is a "knowledgeable organization" but they should be enforcing by ASTM or similar guidelines.

I think i get the rest of what you write and agree. i've also heard rebar as thick as 1/2" will cause damage. if he/she used the "little red rebar holder-uppers" bought at Lowes and pulled rebar up after it was poured but before the finishing started it'd be close enough. and it give more cohesion in the way of tensile strength during temperature changes. i think i am reading the same thing you are off the ACI website. heck, the easiest way to do this is for him to pay a certifier to show up for an hour sometime before he starts. the certifier would be familiar with the local soil/temp/moisture and not be biased (don't use a certifier that is recommended or even connected with the concrete company).

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Last edited by rosco; 07-03-2010 at 02:42 AM. Reason: i read more on the ACI website
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Old 07-03-2010, 02:18 AM   #17
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Some questions about a concrete slab


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Originally Posted by kwikfishron View Post
Wow, I learn something new here everyday. Thatís why I love this place.

Iím confused on one thing though, what 2x4ís from HD should I use for forms, the plastic or cardboard?
you must be a ku "foghorn." look at my thread in trying to figure this out for my first time laying concrete. it is a good summary of that thread and you were part of it if i remember.
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Old 07-03-2010, 06:17 AM   #18
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Some questions about a concrete slab


misspoke,,, yes, dan's absolutely correct - steel can hurt,,, in my own d/w, we placed #4 bar at the throat for added flexural strength HOWEVER that area's 6" thick,,, there's no mesh/steel in the 4" section,,, conc ' cover ' is extremely important since crete isn't wtrproof til it pass's 5,000psi ( if memory serves )

mesh adds strength during the ' green ' stage to resist random cracking,,, after the crete's cured, it holds the broken pcs together
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Old 07-03-2010, 10:21 AM   #19
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Some questions about a concrete slab


Quote:
Originally Posted by Daniel Holzman View Post
OK, back to the rebar. The claim that rebar can't hurt is nonsense. If you use a #4 bar, that is 1/2 inch diameter. This means the MAXIMUM possible cover is 1-3/4 inch, assuming you get the bars EXACTLY in the middle, which is not going to happen. 1-3/4 inch is less than required minimum cover per ACI.

Actually Dan, I believe "reccommended" minimum coverage is 1.5" according to ACI 318 for non-marine applications.

The reason this is a problem is that with less than minimum cover, the bars are prone to rusting. When they rust, they expand. When they expand, they crack the slab. This is serious trouble, easily avoided by NOT USING bars.

I really don't think this theory is as apllicable to residential flatwork as it is to roads, bridges, parking structures, etc... that see large amounts of aggressive de-icing products. I say this because in the thousands of yards of residential concrete I've torn out through the years, the steel is in better shape than the steel I can buy new nowadays to install. I've seen WWM that had zero rust already, yet was within an inch of the bottom. Same with rebar. This are some of the best slabs stucturally IMO that I've removed, but they failed because they were placed on poor soils. I have seen plenty of WWM sticking out of the bottom of the slab at tear-out, & yes, of course it was rusty, but I sure woulnd't say it was ultimately responsible for the one crack in a 200 sq. foot slab. Improper placement of control joints & pizz-poor soil is often the culprit.

I challenge anyone who thinks bars are useful in a 4 inch slab to find some independent, knowlegeable organization that supports the practice. Or at least offer a well thought out explanation as to why you think steel bars IN THIS application are useful.

Why?????? Because I've witnessed it work successfully far more often than not. This is witnessing with my eyes, not by book..............


http://www.aecinfo.com/1/resourcefil...1/crsiee01.htm

Bars are used in structural applications because they provide tensile strength, which concrete lacks, but ONLY when they are placed in the tension side of the slab. When placed in the middle, they are in the neutral axis, which by definition carries no stress or strain. So all they do is sit there and rust, and in the long run they crack your slab.

Dan, I have an idea: I'll pour 2 slabs about 4' by 10'. One will have steel, & one won't. I'll get the two slabs elevated (free-spanning off the ground) I'll sit under the one with steel, & let you have one swing at it with a sledge hammer. Then, you can have a seat under the non reinforced slab, & I'll take a swing. Afterwards, you can explain your definition of zero support in the nuetral axis.


Oh, and when you say steel is "overkill", the cost for the steel that I mentioned to the OP was $14 my price, maybe it would cost him $20. The whole project with his forms, nails, concrete, tools, etc.. will probably cost him $2-300, so the steel would probably equate to 5-10% of the total cost. I find it fairly ironic that any engineer would think that a few bucks like this would be considered "overkill." My expereinces with engineers & "overkill" have been a little more drastic than a few bucks, or even a few percent of the job.

Last edited by jomama45; 07-03-2010 at 12:18 PM.
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Old 07-04-2010, 02:08 AM   #20
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Some questions about a concrete slab


Eeks!

I hope Tyrell hasn't been scared off. Heck, he probably laid the concrete July 2nd and we've been here discussing rebar.

Hope the pour goes well! post pic's and let us know what you do and how it goes!
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Old 07-04-2010, 12:19 PM   #21
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Some questions about a concrete slab


[quote=jomama45;464729]Dan, I have an idea: I'll pour 2 slabs about 4' by 10'. One will have steel, & one won't. I'll get the two slabs elevated (free-spanning off the ground) I'll sit under the one with steel, & let you have one swing at it with a sledge hammer. Then, you can have a seat under the non reinforced slab, & I'll take a swing. Afterwards, you can explain your definition of zero support in the nuetral axis.



i got my money on the re-enforeced slab!

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