Rebar in a suspended slab #60 vs. #40
Rebar in a suspended slab #60 vs. #40
(OP)
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.
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.






RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
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
RE: Rebar in a suspended slab #60 vs. #40
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?
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
http://pagasasteel.com/products/guide.html
BA
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
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.
RE: Rebar in a suspended slab #60 vs. #40
I mentioned my concerns to him - just wonder what else might come of it.
RE: Rebar in a suspended slab #60 vs. #40
Dik
RE: Rebar in a suspended slab #60 vs. #40
RE: Rebar in a suspended slab #60 vs. #40
Are you serious?
BA
RE: Rebar in a suspended slab #60 vs. #40
RE: Rebar in a suspended slab #60 vs. #40
BA
RE: Rebar in a suspended slab #60 vs. #40
which I guess it does
RE: Rebar in a suspended slab #60 vs. #40
RE: Rebar in a suspended slab #60 vs. #40
BA
RE: Rebar in a suspended slab #60 vs. #40
Many of the housebuilders around here prefer small diameter bars (10M usually) so that they can bend the bars on site. I have no problem with that, but the fact remains that the effort required to bend a bar is directly proportional to the yield strength of the material.
BA
RE: Rebar in a suspended slab #60 vs. #40
RE: Rebar in a suspended slab #60 vs. #40
No to what? I don't understand your comment. Please elucidate.
BA
RE: Rebar in a suspended slab #60 vs. #40
He was answering dik's query 21/10/09, 1408.
RE: Rebar in a suspended slab #60 vs. #40
Did he also use 6" of concrete instead of 8" because it was easier to pay for?
As already said, contact the original designer. The only way to determine the effect is to have specific knowledge of this particular slab.
If you want generic advice it would be; slab strength is inadequate unless shown otherwise.