Slab/stem wall cold joint sealant?

**kfc510** · 12-04-2012, 03:03 PM

Replacing a failing foundation (see photo 1). After demo of the existing footing, a ragged edge of the slab is left (photo 2). There is some kind of pond liner/bitumen product under the slab.

The site has a very high water table, and a general funk has been noticed in the unit.

Would you seal the joint between the (e) slab and the (n) stem wall? Or would you just pour to it and accept the cold joint? If you were going to seal it, what product would you use?

thanks,
kevin

**Roger P** · 12-04-2012, 08:24 PM

I would dampen the ragged edge, and then brush on neat cement just before placing the new concrete.

**kfc510** · 12-05-2012, 08:42 AM

Originally Posted by Roger P

I would dampen the ragged edge, and then brush on neat cement just before placing the new concrete.

Ok, thanks Roger, one vote for no sealant. I usually wash the old concrete and use a latex bonding agent for (e) to (n) joints, but a cement wash might be as good.

Still pondering using some bituthene (and a heat gun) or gooing some asphalt in there pre-pour.

Anyone else?

**MarkMc** · 12-05-2012, 09:40 AM

Nothing you can do to ameliorate the water pressure? If a high water table is an ongoing concern now would be a time to address it since you have one side open to work with. Can you adequately clean the underside of the existing slab? How sure ar you of the condition of the existing membrane both in terms of being able to attach/adhere to it and that no water is getting above it now?

Be 'nice' to be able to drop down with some Grace 4000 or similar to footing base if it could be reliably placed and nicer if you could drain/pump so water above the footing base isn't a worry. Topside a sill gasket bedded in some lovely Tremco acoustical would add a bit more protection, maybe. Assuming the plate covers, strike or cut a ~1/4x1/4 at the joint and [over]fill with healthy bead and a compatible gasket material that is not adhered to the slab outside the acoustic sealant [JIC ;].

**kfc510** · 12-05-2012, 11:44 AM

Originally Posted by MarkMc

Nothing you can do to ameliorate the water pressure? If a high water table is an ongoing concern now would be a time to address it since you have one side open to work with. Can you adequately clean the underside of the existing slab? How sure ar you of the condition of the existing membrane both in terms of being able to attach/adhere to it and that no water is getting above it now?

Be 'nice' to be able to drop down with some Grace 4000 or similar to footing base if it could be reliably placed and nicer if you could drain/pump so water above the footing base isn't a worry. Topside a sill gasket bedded in some lovely Tremco acoustical would add a bit more protection, maybe. Assuming the plate covers, strike or cut a ~1/4x1/4 at the joint and [over]fill with healthy bead and a compatible gasket material that is not adhered to the slab outside the acoustic sealant [JIC ;].

I have tried and failed (so far) to talk the client into any drainage under the slab. There is a drain package going in on the outside of the new foundation (albeit partially above the footing).

I am not sure at all about the condition of the existing membrane. I was surprised to see it, frankly. The original assumption was that there was a slab from 1915 or so there.

The plate will be on top of the stem wall, which will be higher than the slab.

Thanks for the input.
k

**MarkMc** · 12-05-2012, 01:12 PM

Originally Posted by kfc510

The plate will be on top of the stem wall, which will be higher than the slab.

Dam. Shame to miss an opportunity.

**kfc510** · 12-05-2012, 03:27 PM

I think the opportunity to have framing more than a couple of inches above grade trumps the opportunity to use the plate as a seal ;)

**MarkMc** · 12-05-2012, 03:43 PM

No doubt, but I was referring to the bigger opportunity of possibly relieving more pressure from under the slab. Dam. Up or down... and generally the up part isn't a concern unless the down part is. Of course their effectiveness is dam well dependent on water flow source/direction and the new drains may take care of the problems.

Saw a fella once use a soaker hose wrapped in filter fabric and tied in to some pvc through the found as the poor mans cheap solution in a very similar situation. It did appear to work at least until the back fill went in....

**kfc510** · 12-05-2012, 03:55 PM

The exterior drainage should help quite a bit- the solid pipe will carry the discharge of the two downspouts on that side to the curb, and the perf. pipe will catch much of the surface and sub-surface water that comes down the gentle slope from the neighbor.

I also spec'd 3000 psi concrete for the footing/stem wall, partly to tighten up the porosity a bit.

But, you're right, those details won't do much for rising water pressure under the slab. I don't think the HO is going to go for any legit drain system under the slab (since she has rebuffed me on it at least twice already). But I would at a minimum like to do what I can to eliminate direct channels (especially ones directly associated with my work) for water vapor to rise up into the interior.

k

**NW Architect** · 12-05-2012, 08:25 PM

What is the material of the old under-slab membrane, and can you seal a new membrane to it in any fashion? If so, do so, and drape the new material down and under your new pour. Then set a bentonite waterstop on top of the new membrane and pour.

**stamcon** · 12-05-2012, 08:40 PM

Kevin, how are you going to pour(place) the concrete, in reference with the previous thread on preinstalling the sill or ???.

steve

**kfc510** · 12-05-2012, 10:21 PM

Ah, good memory, Steve. I'm going to pre-install the sill and build a wide form so the hose can fill it from the side. 9" form for min. 6" stem wall. I'm going to do a little bevel on the edge.

The funny thing is though, that detail (which was submitted to the city) was based on not having the walls open from sill to top plate. As it turns out, the client opted for opening up the interior walls in two rooms anyway, so I could do something different in those rooms.

But the client doesn't want an inconsistent profile to the foundation, and there is still one section which has to be done retrofit style. So it looks like we'll stick with the permitted detail of a 9" stem wall for the whole length.

(There is another issue with the varying splay of the existing studs that makes a wide stem wall and sill plate a good idea. The wall will be snaking 1-2" along the plate. The wall thickness varies substantially too.)

kevin

**kfc510** · 12-05-2012, 10:41 PM

Originally Posted by NW Architect

What is the material of the old under-slab membrane, and can you seal a new membrane to it in any fashion? If so, do so, and drape the new material down and under your new pour. Then set a bentonite waterstop on top of the new membrane and pour.

I haven't seen any markings on it, so I don't really know what it is. It does appear to be adhered to the slab.

I imagine I could seal to it with some effectiveness, but less than 100%. Even if there were some wrinkles or gaps it would be on the positive pressure side of the slab, so I think it would be a good idea.

kevin

**NW Architect** · 12-06-2012, 01:22 AM

When in doubt, overlap new onto (or under) old and put bentonite between.

BTW - do you need to insulate the stem wall on inside or out?

**kfc510** · 12-06-2012, 09:43 AM

Originally Posted by NW Architect

When in doubt, overlap new onto (or under) old and put bentonite between.

BTW - do you need to insulate the stem wall on inside or out?

I think that's exactly what I'll do.

No insulation of stem wall required.
k

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