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evaluating exist. conc slab w/ low f'c

evaluating exist. conc slab w/ low f'c

evaluating exist. conc slab w/ low f'c

This is more of a theoretical - judgment type question. I'm looking for a gut check, I think.

I'm working on a renovation of a 1910 (+/-) library building with a poured-in-place concrete slab supported by a steel WF girder.  A couple of cuts have been made in the slab for mechanical penetrations and that has given me the chance to measure the reinforcing.  

Based on what I saw (diamond mesh at the bottom of the slab supplemented with 1/2" bars) I asked that the steel be tested because it doesn't quite meet current 'library reading room' LL with my assumed fy.  

For good measure, we also tested a couple concrete cores.  These came back at f'c = 700 psi (I did not forget a zero).  The slab exhibits top-side cracking over the girders and significant (L/200) deformation which I presume to be long term creep. This, of course, is well below current code minimum.  It was an acceptable working stress at the time.

With 2000 psi concrete, the calculated capacity was close, depending on the steel strength.  When I know the results of the steel test, I think I'll find that it doesn't work on paper...but it may be within 10%-20%.

So, the question is: when do you leave well enough alone if a structure has performed for 100+ years as a library and when do you say, "this slab never worked on paper and needs to be reinforced."


RE: evaluating exist. conc slab w/ low f'c

You are the man/woman in charge.  Your call.

You do need to at least advise the owner of these concerns.  While it may have worked for over 100 years - it may be right at it's failure point and imminent collapse.

I think a complete analyse is required and updates installed.  OR I couldn't sleep at night!!

RE: evaluating exist. conc slab w/ low f'c

I would not state "...this slab never worked on paper...", unless you have done the calculations per the original code requirements. Having seen the concrete core results, you should IMHO write a letter stating that with this concrete value and under the current codes (going back to whatever the oldest code you have available to check) that it does not work for these codes and/or does works for this old of a code. One question I have is if the concrete cores were taken in place by an acceptable testing means or from the demolition pieces? If from the demolition pieces, I would require correctly taken samples to be tested. You might also want to get the building department involved at some level.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: evaluating exist. conc slab w/ low f'c

I never, never depend upon the historical performance of a floor like that.  For the mere reason that you have no "performance" to look at.  You don't know what the overall loading really was through its history, you don't know whether the concrete has slowly degraded over time, and you never "see" the degree of safety factor in a floor structure.  

The floor could have been loaded to 99% of the collapse load and no one would have ever seen the narrow level of safety that was there.  I suppose you'd see deflections and cracking in some cases, but not always for brittle failures.

700 psi is pretty amazingly low.  I think I heard once that it takes hard work to get concrete to be below 1000 psi.  

You have these options as I see it:
1.  Meeting the CURRENT code by strengthening whatever has to be strengthened.
2.  Meeting the current code by posting load limits (if acceptable to the building official).
3.  Further investigation of concrete strengths (were the first tests done wrong?) and steel properties and analysis/design checks ... if the owner will pay for it.

Item 1 is typical.  Item 2 may not be feasible if the floor still has to serve as a stack room, etc.   Item 3 may be chasing the wind as you might just end up with the same results.


RE: evaluating exist. conc slab w/ low f'c

Another possibility is that the concrete started out as 2500 psi but due to some internal degradation (ASR?), its strength is deteriorating.
But numbers don't lie. If the floor can't be shown to work, you need to notify the owner. But make sure you've exhausted all the potential load paths and approaches. You don't want someone to double check your work (likely to happen before any reinforcing is done) and figure out that it did make it after all.

RE: evaluating exist. conc slab w/ low f'c

I had an experience with similar strength concrete in a building from the 1920's.  Much of this concrete was hand mixed, and there was no actual attempt at quality control or consistency.  If you take a lot of cores and do a statistical evaluation, the strength will probably come in less than 700 psi.

Probably the only way of proving the capacity would be load testing.  If that is not palatable, then adding some supplementary steel beams below the slab may be the way out.

RE: evaluating exist. conc slab w/ low f'c

I appreciate the thoughtful replies.  Definitely a helpful sounding board.

Mike: Once I saw the reinforcing, I did a calculation based on presumed material capacities.  Based on that, I've had a conversation with the owner and architect to discuss my concerns and recommended the testing.  At that point I was concerned about too little steel but wanted the actual material properties to use in my calc.

Woodman: We did not test the pieces that were cut out, but rather went back to take core samples.  Good point about phrasing.  What I may say in a conversation at the site is not necessarily how I'd write it in a letter with my seal on it.

JAE: This is a situation where the intended use of the space is to remain the same so it's not (necessarily) a requirement to reinforce it.  I generally feel that if we're changing the loading or modifying the structure, then I definitely have my finger prints on it and it better work for current code.  If not, then...it depends.  Once I saw the reinforcing in this case, however, I no longer felt comfortable with the idea of continuing use.  In spite of it's long history as a library, I had direct knowledge that it might not be adequate and that's what led to the testing.

For the options:
1. I'm leaning toward reinforcing.
2. I haven't come across a situation where I've posted limits and I'm not sure I'd feel okay doing it knowing that occupants won't follow them.  
3. The testing is being done now and the final reports on the steel and concrete aren't yet issued.  I think we'll go with the values we get rather than go through another round hoping for better values.

I'll admit that I don't know much about alkali-silica reactivity, so I'll read up on that. Thanks.

In this case it's a simple slab with nothing but a basement/crawlspace underneath.  You make a good point that often you come across a structure that doesn't seem to work until you account for the redundancies, two way action, or load bearing 'partitions', etc.

The underside access also means that a potential repair will be simple (not to be confused with easy or cheap).

I've also read that you can use a value higher than the core sample tests because the act of sampling degrades the concrete.  In this case I don't think I'll be overly optimistic with the f'c I use in the analysis.

Thanks, again, for the replies.

RE: evaluating exist. conc slab w/ low f'c

Even with very low strength concrete, the strength would have only a minor impact on the flexural capacity of lightly reinforced sections.  Shear is the main issue, and in one way slabs, that is probably not a big concern.

RE: evaluating exist. conc slab w/ low f'c

Cinder concrete slabs with draped wire mesh (catenary action) with spans 7 to 8 feet on center can carry about 200 psf load.  There are over a million square feet of cinder concrete slabs in NYC alone.  it is also called short-span construction, sometimes goulash concrete.  See if you can find an early version of US Steel welded wire mesh catalogue which shows that low strength concrete around 800 psi was normally used.

RE: evaluating exist. conc slab w/ low f'c

hokie66:  you're right.  It's the steel that drives the capacity in this calculation.  

mfrad:  Thanks for the reply.  I haven't found anything in my Carnegie Steel Pocket Companion.  This slab is a longer span with a triangle mesh made of 1/8" x 1/8" bar.  I'll keep looking for a contemporary reference.

RE: evaluating exist. conc slab w/ low f'c


Thanks for digging through your files.  FYI, what I'm looking at is an expanded mesh.  Likely from the Consolidated Expanded Metal Company, Wheeling WV.  It may help me that this steel was advertised at a higher yield than was typical.  I was assuming 40 ksi and that doesn't cut it.


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