Concrete Slab Unbonded Overlay

[mnmavic]

Hi everyone,

I have a 5" monolithic slab with 12" thickened edges which was poured last fall. This spring, we built a 28'x32' garage on the slab. After the building was completed, tragically blisters were found to exist on the slab and began to break open.

The concrete contractor has offered to jack up the building and pour a 4" slab overlay (with rebar at my request). The building would be lifted using bolts screwed into the existing slab and then the bolts would be left in place and serve as the new bottom plate concrete anchors. The slab overlay would not be bonded to the original slab beyond the new anchors. The original slab has no cracking or other flaws beyond the surface spalling and blisters.

What does everyone think of this plan? As the original slab is structurally sound, is the unbonded slab overlay ok? I have concerns about it being able to transfer the load of the walls down to the existing 12" thickened slab edge.

Any input would be appreciated.

Thanks!

 

[Retrograde]

What do you mean by blisters? Photos?

 

[mnmavic]

Concrete blisters are a surface defect caused by too much air in the mix, or improper finishing of the concrete. They appear as small raised areas on the surface of the concrete that eventually break open under load. They are similar to spalling or chipping of the surface.

 

[Retrograde]

Would you actually need to jack up the building in order to pour the overlay? Is there a head height issue?

 

[mnmavic]

The building must be jacked up so the overlay covers the entire bottom slab, that way the building height is raised uniformly and it will not affect the height of the door openings.

 

[jayrod12]

Can you chip down at the doors, and slope the overlay to maintain the existing door height? We've done this successfully at many locations. The lifting of the building is likely the most costly item of this repair. If there's a way around it, it's worth investigating to save the client some money.

 

[mnmavic]

Thanks for the replies. I guess my biggest question is the ability of the new unbonded 4” slab to support the weight of the walls. The slab will extend over the entire surface of the original slab including the 12” thickened edge footer. Thus the walls will now sit on a 4” slab unbonded to an existing 12” thickened edge footer.

I am nervous about the load transfer through the unbonded concrete.

 

[dauwerda]

jayrod12, I can't imagine that jacking a 28' x 32' garage up 4" is that much of an expense.

I don't see any issues with this, the previous slab essentially becomes a very stout subgrade for the new slab. As far as the new slab transferring the loads into the 12" thickened edge foundation, bearing and shear won't be an issue. The only issue may be uplift. It's not entirely clear to me what the "bolts screwed into the existing slab" will be, but you will want to make sure that these can develop any uplift forces into the original thickened edge foundation (similar to the original anchor bolts) or, you could extend the original anchor bolts using coupling nuts,

https://www.portlandbolt.com/products/nuts/hex-coupling/.

 

[jayrod12]

Although you're correct, jacking a garage is significantly easier than a house or other building, but again if you can save the client a couple of grand in shoring and labour costs, why not at least entertain the idea.

And what about anchorage for your walls. I can only assume there's uplift to deal with, so now you've got to cut off the old anchor bolts or install extensions. All added costs that no one tallies at the beginning.

 

[dauwerda]

jayrod, I agree with your assessment. However, it sounds to me like the solution was proposed by the contractor at the contractor's expense due to poor quality of initial install (I certainly may have that wrong though). If this is the contractor's preferred method I see no issues other than anchorage which we both hit on above.
If this were my garage I would prefer the proposed method as opposed to losing headroom height. If I am incorrect, and the owner is also paying for some of the repair, I would be exploring other less expensive options as well.

 

[mnmavic]

This repair will be fully covered by the contractor. The bolts used for lifting the garage will be left in place during the pour and will become the new bottom plate anchors.

Thanks much for your replys!

 

[jayrod12]

The bolts used for lifting the garage will be left in place during the pour and will become the new bottom plate anchors

I'd be leery of this. I imagine them wanting to use expansion anchors or something like that, and just having the wall anchored to the bonded overlay might not be enough depending on the wall uplifts. Generally in my experience garages are under-designed for this. Granted nothing bad has happened yet, but if you actually run the numbers the uplift is significant.

 

[MotorCity]

Jacking up the building seems a little extreme to fix this dilemma. Why not grind the slab to remove the blisters then apply a self leveling cementitious or epoxy coating? Sika makes several products for this.

 

[BridgeSmith]

I'll second Motorcity's suggestion. A quality epoxy overlay will produce a durable, impervious surface, and it's only about 3/8" thick. We use it on our highway bridge decks all the time.