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Waffle Slab Repair

Waffle Slab Repair

Waffle Slab Repair

(OP)
Was it customary to reinforce the 2.5" slab in a waffle slab system with mesh?

How can we place a new slab so that shear is transferred across the cold joint that will be created between the new and old concrete? Would the attached be satisfactory?

Anyone ever repaired a waffle slab?

RE: Waffle Slab Repair

WWF was customary...

What is structurally wrong with the slab that you have to repair it now?

Mike McCann, PE, SE (WA)


RE: Waffle Slab Repair

(OP)
ok, thanks msquared.
It is a below grade parking garage built > 50 years ago. The corrosive deicing chemical got to the top and bottom bars in the waffle slab and they corroded (most likely failure to replace the membrane waterproofing system after it had exceeded its expected life, but also possibly was used for some years before the membrane was installed), the expansive corrosion products caused delamination of the concrete. The top bars extend into the 2½" slab of the waffle system and caused it to delaminate.

Any comment on my preliminary repair detail for the slab?
I will prepare several details for diffferent conditions that will be encountered, but the one I posted would start me off.

RE: Waffle Slab Repair

(OP)
Can anyone comment on the proposed repair of the 2.5" slab, with respect to the cold joint between the new and existing concrete, that was shown on the attached sketch?

RE: Waffle Slab Repair

Fully develop reinforcement on both side of the shear/sliding plane. Roughen the old concrete sufficiently to engage the aggregates OR provide a horizontal ledge upon which the new slab will reside. Make sure the new concrete has low shrinkage characteristics. And be sure it gets sealed properly - these repairs often fail because salty water gets into the cold joints.

RE: Waffle Slab Repair

(OP)
to TXStructural - thanks. I just realized my little calculation that I showed on the sketch is wrong. The calculation should say "cot" of the angle, not "tan" of the angle, but anyway that calculation does not seem to get me anywhere. I agree with your comments on roughening etc. I am reluctant to create a ledge because then the amount of concrete remaining in the narrow rib to take compression (for rib mid-span moment) is being reduced, unless rely on the new 2.5" slab to act compositely with the rib, which seems dicey. I am not sure how I can produce a calculation showing that the concentrated wheel load can be transmitted across the cold joint, if there is no mesh in the 2.5" slab, although I suppose the horizontal restraint provided by the adjacent surrounding slab can provide a horizontal force equivalent to what mesh would have provided.

RE: Waffle Slab Repair

ajk1,

I have completed several repairs to waffle slabs in the past. All the ones I have dealt with have mesh in the top. Some guidance from past experience:

1. If the repair includes adjacent units, also remove the top of the rib and incorporate that into your repair patch. Good patch geometry and clearance around rebar in top or bottom of ribs is important.

2. I would extend the edge of your repair detail and form a 1" key over the rib at the perimeter of the thin slab areas.

3. Ask around, we were lucky on two projects to have found spare waffle slab forms in a storage room. Made the forming of the repair areas very easy. Some contractors may also have some stockpiled.

4. Don't forget to specify vertical sawcuts around the perimeter of the repair patch area at the top of slab, to prevent feather edging.

5. Bonding agent, waterproofing membrane, etc ....



RE: Waffle Slab Repair

(OP)
To Canuck67:

Thanks for all the good points. Much appreciated.

One of the problems on this job is very tiny (3 mm) side cover to bottom bars in some locations. So we think a wood form would be better than the metal form in our particular case, so that we can form a wider rib and achieve better side cover. This was perhaps not an issue in your project. Our job has #7 bottom bars, fitted into a 5" wide rib...not the best situation.

The contractor says that if we are going to take the slab out where it runs over the beam, we should take out the whole rib depth too. Seems like wise counsel because I cannot reliably make the new slab and old rib act together (too narrow to install significant mechanical fasteners), and in any event it will be more trouble to try to retain the old rib below the slab than to just remove it at the same time as the 2.5" slab is removed.


RE: Waffle Slab Repair

If there is shear reinforcing in the ribs, rely on that to help create a composite section. Once they start demo, it will be obvious what direction to take. Do not let them demo any reinforcement (no saws or core drills until you know.)
My real concern is that the replacement rib will not have adequate shear connection with the intersecting ribs for high point loads.

RE: Waffle Slab Repair

(OP)
Yes agreed. We will have to check the cross rib intersection carefully and decide how to detail that. Have not got quite that far yet in the thought process.

RE: Waffle Slab Repair

(OP)
Regarding my question about mesh. I see that the 1965 CRSI design handbook does say to use 6"x6" 6/6 welded steel wire mesh in 2½" slab. So far we have found no mesh in the slab at holes made thru it, but perhaps they will find it when more extensive demolition begins.

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