Defective structural slab to be removed

[ataman]

Hi,

I have a situation with a structure which has three bays of one way spanning structural slabs. One of the end bays has received consistently low compressive strengths based on results (12 MPa) and as such we have instructed the contractor to demolish the slab. The contractor has confirmed that the concrete supplier added several additional gallons of water to the concrete truck for that bay due to a faulty pump and the time that it took them to replace the pump.

Tests indicate that the other two bays are ok. My question is how much of the good slab shoulf be removed as well. If we were doing a construction joint then I would have recommended that the joint fall in the middle third of the slab where the shear is less. Is it unreasonable to ask the contractor to remove this much of the existing slab? Some engineers have advised that I can ask the contractor to demolish from the face of the beam and that the load will be transfered by shear through the reinforcing steel. The bottom steel in several locations does not continue past the beam so I'm wondering if it wise to use it for shear transfer especially since it is a one way slab and the shear as the face of the beam will be the highest.

Thanks in advance for you comments.

 

[JAE]

I would say that having a vertical shear plane at the face of the supporting beam is a bad idea. Lots of shear across it.

What I would do is have the cut occur over the top of the supporting beam, but only cut deep enough to get a good edge on the concrete, then have the contractor hand chip the remainder, creating a small ledge and leaving the original top bars and any bottom bars that might be continuous into the adjoining slab.

The ledge width doesn't have to be much but we usually kept it to a minimum of 1 1/2" (40mm).

So you are essentially just removing concrete and trying to keep the rebar in place. Your replacement slab will then rest over the top of the beam

 

[ataman]

Thanks JAE,

I can't help but think that part of the beam needs to be removed as well. If that is the case, what is your advice.

Thanks

 

[JAE]

Shore up the remaining slab and have the joint occur on the other side of the "new" beam. Again, if possible, keep as much of the reinforcing that extends from slab to slab in place and intact.

--------------------------------------
New Slab | Exist. Slab
|
------------ --------------
| |
| |
| |
| | <---New Beam
| |
| |
| |
-------------

 

[ataman]

thanks

 

[csd72]

Dont forget to shore the adjacent spans, or they will sag due to the lack of continuity (and you will never get it out).

When I have done this I have called up for the slab to be saw cut at the end of the top bar development length and then hand chipped for the remainder.

I agree with JAE's comments.