Rebar in a suspended slab #60 vs. #40 3

[skibum2b]

Hello out there - I have an interesting question based on reality.
Suppose a home was built, with a suspended concrete slab. The architect/engineer specified #60 rebar on 12" centers, two layers offset 6" (not technically correct, but you get the picture).
Suppose the guy pouring the slab used #40 rebar (40ksi).
What are the possible failures, etc?

It is the garage floor of a home.

Thanks.

 

[struct19]

Hi
the defects will depend on how the slab was designed and detailed. For example, if the designer has a good factor of safty in his calculation then nothing will happen immediately. If is was designed marginally for the expected loading, then the steel reinforcing will get yields when loading the slab and may develop cracks before it fails. The grade 40 steel means that yield strength of reinforcing is 40000 psi instead of the designed yield strength of 60000psi.

 

[skibum2b]

Thanks - specifically, where would the potentially fatal cracks first appear? On the edge of the slab where it hooks into the supporting wall? Or in the middle?

 

[cessna98j]

Most likely the cracking will occur at mid-span, on the underside of the slab. Depending on the rigidity of slab connection to the wall, you may also get cracking near the edge of the slab by the wall on the top side of the slab.

Another thing to note, you didn't mention the size of the rebar, just the yield strength. It is possible that 40ksi steel was used, but a larger size than what was spec'd. For example, the flexural capacity of a slab with 40 ksi #5 bars at 12" o.c. is approximately the same as 60 ksi #4 bars at 12" o.c.

 

[beton1]

Other factors to consider: Span (single garage, 1 1/2 or double? (Likely single, but should ask?!) Also, slab thickness. Concrete strength. Testing? How did the difference in steel reinforcing strength become known.
How old is the slab? Is it newly built and untried, or
in service with cracks now appearing? Call in a specialist,
to do a full review if you cannot wade through this alone.

 

[skibum2b]

I heard it from the person building the house. It has haunted me ever since I heard it.
The garage is ~25' wide x 30' deep, and will hold some fairly heavy equipment. As far as I know, the rebar used was the same diameter as was spec'd, just lower yield strength.
The slab is between 4-6" thick, new, with very little service (the home is still under construction).
I am not responsible in any way - just want to know exactly what the potential is for problems and what action I need to take as an outsider.

 

[hokie66]

The owner needs to have his structural engineer analyze the slab to determine if it has adequate strength.

 

[JLNJ]

Is this slab on grade or is it over a basement?

It sounds like it's a slab on grade so I doubt the rebar strength is an issue. Heck, if it's new construction it might be grade 60 rebar even if someone "said" it was 40. 40 is not very common any more.

 

[BAretired]

When you say it is a suspended slab, I assume you mean a structural slab with nothing below it but fresh air. That being the case, if the original design was tight and based on a yield strength of 60 ksi, the structure as built with 40 ksi steel will fall short of what is required by a factor of 40/60 or 0.67.

Failure would be a gradual yielding of the intermediate grade steel. Deflections would become very evident and there would not be a sudden, catastrophic failure.





BA

 

[JAE]

...but....

The actual yield of Grade 40 bars might be higher than 40. Would require sampling and testing if they did. I wasn't aware that 40 grade bars were even still produced.

And I'd repeat the questions above - how do you know it was 40?

 

[Gumpmaster]

I'm with JAE.

I don't think you could buy grade 40 bars if you tried.

In grad school we had to order a special heat of grade 40, and the bars still tested out above 50ksi.

 

[BAretired]

You still can buy intermediate grade reinforcement in Canada. Why you would want to is another matter. See the following link:

http://pagasasteel.com/products/guide.html

BA

 

[skibum2b]

I was told it was 40 - and it was very easy to bend (I was helping him put the rebar into the foundation walls, then bending it over so it could tie into the floor).
I asked who spec'd the rebar, etc. The concrete guy replied that he used #40 instead of #60 'cause the #40 was easier to bend.

 

[CTW]

I am not responsible in any way - just want to know exactly what the potential is for problems and what action I need to take as an outsider.

You need to notify the owner and/or the person who originally spec'd grade 60 about the situation so that it can be investigated.

 

[skibum2b]

The owner knows - he was the guy putting the rebar in.
I mentioned my concerns to him - just wonder what else might come of it.

 

[dik]

Can you see any of the rolling mill marks on the deformation. This generally indicates the grade.

Dik

 

[JAE]

Hmmm....40 ksi rebar and 60 ksi rebar each have the same modulus of elasticity right? So effort to bend doesn't depend on fy but rather E. Unless it goes plastic when bending....but that depends on the radius of bend used.

 

[BAretired]

JAE,

Are you serious?

BA

 

[JAE]

Uh..ya

 

[BAretired]

I was afraid of that. The yield strength of the material is the only factor which determines the effort required to bend the bar. The moment required to bend a bar (or any other shape) beyond yield is Fy*Z where Z is the plastic modulus of the shape.

BA