×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Openings jack hammered into existing 2-way flat plate slab.

Openings jack hammered into existing 2-way flat plate slab.

Openings jack hammered into existing 2-way flat plate slab.

(OP)
In an old (1966) existing 2-way 6" thick 4000 psi reinforced concrete flat plate slab spanning 10'-8 in each direction, 5 openings each about 16" square were jack hammered thru the floor to install the piping connections etc. for 5 new replacement urinals. The floor is supported on 20" diameter drilled cast-in-place concrete piers that extend down to good soil. The soil at the underside of the slab is quite weak in this area and this is why a suspended structural slab and not slab-on-grade was used, back in 1966.

The recent work of breaking thru and patching the slab seems to have been carried out under the direction of an architect who basically issued nothing, and no professional engineer was consulted. They say the contractor was careful not to cut the rebar, and the one photo that I was provided after I learned about it and it had been all filled in, does seem to show the rebar in tact and in good condition.

None of the usual details about how to prepare the slab edges, the strength of the concrete, curing, etc. were issued so it seems the contractor may have been left to his own devices.

The last slab opening in the row is close to the supporting drilled pier.

Question: Assuming they did everything right, does a thru-slab opening that is filled in with concrete restore the full punching shear capacity of the slab at the supporting pier? It seems to me that is would not, because once there is a cold joint here, the only way shear can be transferred across the cold joint is by shear-friction or a similar mechanism (strut and tie?), but not by the inherent shear strength of the concrete. What do you think? The shear strength without the hole is around 75% of the Code capacity, assuming the top rebar was placed 1" below the top of the slab (which it may not be since there was no inspection of the 1966 construction).

RE: Openings jack hammered into existing 2-way flat plate slab.

You couldn't count on the infill to restore the punching shear capacity, but at the same time, this doesn't sound like much of a problem. Very short spans, large pier supports, semi-supported on soil, etc. It won't fall far, so life safety issues which dictate some of our conservatism are alleviated to an extent. You can't prove it by reference to any Code, so I suppose engineering judgment is the fallback.

RE: Openings jack hammered into existing 2-way flat plate slab.

I'll take this opportunity to piggy-back a related question. My company has a structural restoration group. I'm not a part of it but they visit my desk often to discuss things. Many of their details require removing some concrete and replacing it, either with regular concrete or a Sika product ect. Based on the details and our discussion, it is apparent to me that they treat patched concrete as though it were structurally monolithic.

Is that ever true? Always true? Sometimes true? These are always partial depth concrete repairs. A recent example is an eight inch suspended slab that was accidentally chipped down to four inches in some locations. The area was prepped and refilled with concrete. Following that, it was assumed that all eight inches of slab thickness could be mobilized for bending and shear. For some reason, shear bothers me more than bending.

@AJK: obviously I don't know the answer to your question but, for what it's worth, my gut is in agreement with Hokie. And I hope that you consider my addition to your question as a contribution to the discussion rather than a wholesale thread hijacking. That was my intent.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Openings jack hammered into existing 2-way flat plate slab.

(OP)
Hi Kootk - your comment is addressing a rather different situation. For the case that you raise, I believe that it is widely accepted that if it is done correctly, that the restored slab will have similar strength to the original. I believe that there have been tests done showing this. I also believe though that it is often not done correctly, either because a "bruised layer" is created by the jack hammers, or the cement slurry bonding agent is not prepared or applied properly, or not all loose particles in the substrate are removed, etc. But I have not heard of any collapses of repaired concrete slabs, except one where the top rebar was found down much too low and the testing company managing the repair failed to realize that corrective action was required. Had a structural engineer been managing the repair, he would most probably have recognized that. It scares me that most repairs are managed by people who are not structural engineers experienced in structural design.

RE: Openings jack hammered into existing 2-way flat plate slab.

I agree with everything that you've said AJK with one exception: I still contend that the situations are intimately related. My patch requires the transfer of horizontal shear. Your patch requires the transfer of vertical shear. Really, what's the difference? The way that I see it, either you can reconstitute concrete for real... or you can't.

Stresses that are locked in pre-patch also cause me some concern but that's a separate matter.

As a mental experiment, imagine chipping away four inches of an eight inch concrete slab within a big circle drawn around one of your columns. Now prep it properly and fill it back in with concrete. Is it an eight inch slab for the sake of punching shear near the column? How about for a punching perimeter coincident with the perimeter of the circle? As you mentioned in your last post, common engineering dogma says that we should be able to count on eight inches of slab after the repair. However, for that to be true, there needs to be capacity for vertical shear transfer at the interface between original concrete and patched concrete. And that, as I see it, is the same as your problem.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Openings jack hammered into existing 2-way flat plate slab.

Back to the original problem. Suppose that you filled your holes with non-shrink concrete or, better still, expanding concrete. Due to the restraint provided by the surrounding concrete, you'd have a very high effective clamping force for use in shear friction calcs. If the shear friction capacity could be made to exceed the diagonal tension capacity appropriate for punching shear, couldn't you assume the concrete to be monolithic for punching shear?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Openings jack hammered into existing 2-way flat plate slab.

(OP)
I agree that there is room for discomfort about relying on a delamination repair restoring the "real" strength of the original slab, but it may restore the "design" strength. In any event, there have been so many billions of square feet of delamination repair carried out, without any subsequent collapse or visible signs of distress, that it is a little hard to argue that it is significantly in error, especially when lab experiments have, I think, been done showing its effectiveness. Although the hole thru slab may involve similar mechansims to some extent, the difference is that I don't know that shear at support type hole thru slab repairs have been much carried out.

If the slab fails, although it may not drop down that far, it could cause the block partition wall to topple over, so I think it is not as benign a failure as might be expected.

I agree is perhaps possible to show by calculations that the design shear strength at the support is restored, but in the case I am looking at, there is no documentation of what was done and no calculations of that nature have been made. The purpose of my question was to assess whether such calculations are needed or whether it can be assumed that the strength is restored (as would be the case of filling a hole that is not near a support).

RE: Openings jack hammered into existing 2-way flat plate slab.

Partial depth and full depth repairs are done all the time on bridge decks. I have never heard of anyone reducing the design capacity of the slab for this. But this is done under the supervision of an engineer who inspects the openings, ensures that all the rebar is properly spliced, reviews the mix design, etc. What bothers me about your situation is that documentation proving that the openings were repaired properly is incomplete. You can hope everything was done right, but you really cannot be sure.

RE: Openings jack hammered into existing 2-way flat plate slab.

(OP)
As far as I know, bridge decks are not the same as what I have described. The situation I describe is the very particular case of 2-way punching shear at a pier support. Bridge decks do not generally involve 2-way punching shear.

RE: Openings jack hammered into existing 2-way flat plate slab.

@AJK: agreed, there is a big difference between our too condition when it comes to historical performance data. It would be interesting to conduct a simple test. One could install a through bolt into the patch and yank on it to simulate the expected punching shear stresses. This will be a good future retirement project for me to conduct in my garage.

@Gray: on full depth bridge repairs, how are the vertical surfaces typically prepared? Also, are non-shrink mixes specified?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources