Slab-wall interface detail help

[HeavyCivil]

Please see attached detail.

I am getting back into a project that has been on the shelf for some time. One room of a water supply building has six large filter tanks that will periodically be drained. We have a trench system and 8" D.I. floor drain but the area of floor one which the tanks sit is flat.

The frost walls extend 2' above the finished floor in this part of the building so there can be no overpour or shelf. I think it makes sense to take advantage of this and reduce the restraint on the slab by using smooth dowels instead of deformed to provide transfer of vertical forces but not lateral, like a cold joint within the slab. I have not seen this done but I do not see why it could not be.

The question is whether or not a fiberboard or similar iso joint is necessary at the perimeter. While I want to prevent bond, I think the slab will only contract away from the wall. Also - I need a strip of bentonite as backwashing those filters will result in a huge amount of water on that floor - fiberboard, or even poly sheet, will prevent that from working. Has anyone seen a simple chemical bond breaker or release agent used effectively at cold joints? It would be cleaner (no gap- architect would be happy), allow waterstop to do its job, unless there is a chemical rxn issue (not to mention not interfere with dowels). I know this detail is a little unusual, but I dont see why it will not perform very well. Let me know if you have concerns or issues.

Thanks

 

[Splitrings]

VTEIT,
I have not seen a slab that bonded to a wall. Maybe as an added measure you could require a release agent be applied to the wall before slab placement.

I would make sure you follow the bentonite manufacturers concrete cover requirements. I have seen a 3/4" x 1" bentonite waterstop spall off a large piece of a concrete slab at a wall/slab interface. Most waterstops of that size require a min of 3" of cover. One manufacturer produces a smaller half circle section that requires only 2" of cover. If you want to completely prevent water from entering the interface I think you would need to use a polyurethane type product at the top of the slab and the waterstop below.

Is there a reason for the slab to be doweled to the wall? The slab looks to be a slab on grade based on the reinforcing shown in your detail. Shouldn't the subgrade provide all the support necessary for the slab? If for some reason the slab does settle won't it crack on the bottom tension face of the slab from the moment induced by the dowel?

 

[HeavyCivil]

Splitrings,

Thanks- will look into cover reqs for bentonite.

I think you're right and that release agent is enough to prevent that. I have never seen it either, but I suppose it could happen.

It is a S.O.G.. I dont expect settlement from either the building or slab. The idea of a true floating slab has always troubled me - it seams logical to tie bldg components together if possible. Although unlikely, if there is settlement or heave, I'd like to be able to prevent independent movement. I think compaction at the perimeter of slab subbase is often less than that at the interior due to poor practices/neglect by contractor. Dowels also prevent slab curl - right?

I'm not sure I understand how the dowel will actually induce a moment. With plenty of cover and small (if any) settlement anticipated as worst case scenario, I dont see a problem - please explain.

 

[Splitrings]

VTEIT,
I would agree that unless clean stone is used to backfill the frost wall on the slab side, there could be settlement of the backfill. I would also agree that the modulus of rupture of concrete is likely sufficient to span this fill area without failure.

 

[Teguci]

A couple of considerations:
1. Use minimal rebar between wall and slab. If subsurface settles allow slab to be supported at wall and rotate down to first CJ about 5 ft away (rebar yields)
2. Consider using pipe insulation around rebar for last 6" against wall. This will create a compressible void that will allow the slab to deflect down while the rebar double-bends.