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