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Repairing a concrete slab with missing rebar at bottom

Repairing a concrete slab with missing rebar at bottom

Repairing a concrete slab with missing rebar at bottom

I'm looking for some assistance on how to repair the bottom of a concrete slab in a parking garage. The contractor decided to start repairing the slab by saw-cutting around the spalling area but the area was so large that entire lengths of rebar started to fall. Fortunately he shored the slab but now I have a 180 sf area that needs new rebar and repair mortar. I would like to form the area, core holes and then pour the repair material into the formwork. My problem is I have to support the slab while it is being formed. I have shoring posts at a 4 ft grid. I was going to place a stainless steel pipe at the top of each post and then form around each post while anchoring the formwork to the slab with post installed anchors. After the repair mortar cures I was going to grind the pipes to be level with the slab. Does anyone have experience with this or a better idea? I'd really appreciate it! Thanks!

Clarke Engineering Services, PC
Jobsite Engineering and Consulting

RE: Repairing a concrete slab with missing rebar at bottom

Cut pipe rings from 3" diameter pipes and place these directly above the shoring, but on top of your flat form.  This will support the slab while providing a cavity for forming and placing the repair grout.  The height of the pipe rings should be the expected thickness of the repair grout.  After placement, the rings can be removed and a repair mortar used to repair the void.  If you are using a wood flat form, then weld a plate on the bottom of the ring to distribute the slab load and prevent crushing the wood.

Use a polymer modified repair grout.  Prepare the surface of the existing slab to achieve good bond.  I would consider dowel pins as well.

RE: Repairing a concrete slab with missing rebar at bottom

Whenever we encountered soffit repairs this size, we found it was cheaper$$ and easier to blow through the entire slab depth, i.e. full depth removal to coincide with the soffit delaminations. You will end up with a much better quality repair in the end by going through slab. Forming and pumping such a large soffit repair through core holes in the top surface takes a very skilled contractor. I have been less than satisfied with the results of contractors claiming to be able to pump much smaller overhead repairs. Honeycombing, poor bond, etc....

The perimeter of the through slab repair will have to be shored, however, it is a good idea to also shore the adjacent bays surrounding the repair area.

I also have to think out loud ... why did the concrete on the soffit delaminate??? And if the soffit has significant concrete delaminations, what is the condition of the upper, not as of yet, sections of concrete. Is the 'upper' concrete severely chloride contaminated and waiting to deteriorate in the short term??


RE: Repairing a concrete slab with missing rebar at bottom

Thanks for the replies.
Ron-I was going to use the Sikacrete 211 SCC+ as the repair mortar/grout. My only problem with sitting the 3" pipe on top of the formwork is ensuring they are the same depth in all locations because the surface is not level at all. That's why I was going to have the pipe go through the formwork and sit directly on the shoring. The down side to this is a lot of wood cutting for all of the posts (4'x4' grid). I suppose they could use a laser level to make the bottom flat.
Skiis&Bikes - great point! This building is not that old (1991). I was very surprised with the amount of deterioration. I've done a lot of bridge work and haven't seen this much deterioration. The upper level is not even the closest to the street so all of the water is dripping from cars. The surfaces look like they have been repaired a few times with eaither a sealant or the cracks were routed and sealed. There is one crack above where the crack sealant is no longer there. FYI- the slab is only 8"-10" thick and the rebar is not epoxy coated. The repair depth is approximately 3".

I've attached a very good ACI Repair Bulletin #4 that has good graphics on page 6. It looks like they are drilling the core holes every 2 feet which is much tighter than I was hoping to do. (Any thoughts?). Also, the anchors look like simple drop in (Bang-it) anchors with threaded rod. I'm not in love with these types of anchors. The Sika specs that I downloaded also advised vibrating the formwork from below and testing the system with water first using a vent/plug. Any thoughts on a vent plug? The ACI spec advises a test ACI 503R when done to test the bond which I will add to my spec.

Finally, using shot-crete is sounding like a much better/easier alternative... Any thoughts?

Clarke Engineering Services, PC
Jobsite Engineering and Consulting

RE: Repairing a concrete slab with missing rebar at bottom

anchor...I've been involved with shotcrete repairs in the past.  Make sure the shotcrete company has documented experience in structural repairs...not just a pool builder.

With good design and planning, they can work fine.  A key is bonding the shotcrete to the existing concrete and keeping it bonded.

RE: Repairing a concrete slab with missing rebar at bottom

Thanks Ron. I'm almost done designing and detailing the core and pour method. I don't think these guys have much experience with shotcrete...

Clarke Engineering Services, PC
Jobsite Engineering and Consulting

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