Strengthening Joint Between Slab and Footing
I just poured my 750 square feet slab with help from somewhat experienced friends. Everything went well except there is an obvious joint between the footing which was poured first and the slab which was poured last. See the picture.
We poured the footing first and as per the concrete suppliers recommendation we used an accelerator for the footings and then used water reducer for the slab part so we could finish it with time to spare. However it looks like we did not mix the two different pours well along the form boards.
My question is what joint compound or epoxy would folks recommend for me to fill this joint along the outside, and/or is there something else I can do to help these cracks from getting bigger and/or strengthen this area where the walls will be bearing.
Thanks,
Kevin B
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Replies
Kevin,
So this was a mono-pour, but you did the two parts separately? What ties the slab to the footings? If the two are well connected the crack is really just cosmetic. As it is above grade and will be visible you might want to grout the joint, or parge the outside of the slab and footings, but that would just be for looks.
It was one poor but we poured in the footings with the first truck and had to wait probably a little bit too long for the second truck that came to finish out the height of the footings and do the slab. The second truck with the slab mix had fiber mesh in it and water reducer. The first truck had an accelerator added to put less pressure on form boards - recommended by concrete supplier. When pouring slab on top of footings the footing mix was still wet but firming up. The screeding was done to snap lines as form boards were taller than slab height, so wasnt the best of jobs at edges.
Kevin,
How are the two connected? Is there rebar joining them?
Malcolm,
There is rebar in footings but not going through to the top four inches of slab. The concrete was hand pumped some with 2x4's and shovels to mix the layers some before screeding
Generally the footing is one pour it is generally wider than the foundation wall and is set below the frost line for your location. The footing generally has rebar in it along its length and some vertically to key the cold joint with the foundation walls that would be poured later to full height generally a few inches higher than the slab will be. Generally the slab is poured much later often after the walls and roof are up.
Will this be a garage or a house?
The line you see is called a cold joint and there is not much you can do to it now. Cold joints are best avoided whenever possible as they tend to be weak spots.
Walta
Hi Walter, thanks for reply. It's for a house
After researching cold joints it looks like the decisions and circumstances of my pour created one in certain parts of the mono slab. Bad decisions on my part and some bad advice.
I believe the first pour hadn't fully set before the slab top was poured, so there must be bonding happening within the thickened footings. It also seems like cold joints are a problem in tension, the walls are obviously putting the joint in compression. Walter already answered this, but is there anything I can do to mitigate the open joint on the edge where my walls will be placed? Shifting the walls in could perhaps take the weight off of the outside edge. Moreover, the big question is if the cold joint is a weak spot then how so, and what factors come into play for this to be structurally unsound?
Thanks, hoping for some good news!
Kevin,
The cold joint will not appreciably affect the capacity of the slab and footing to take the vertical loading exerted by your framed walls above. Much more likely is that your slab will crack at the interior edge of the footings due to seasonal movement and differential settlement. You may want to consider saw-cutting the slab there to control where that cracking occurs.
Typically a slab-on-grade house is done one of two ways: You either pour monolithically so the footings are really just a thickened slab edge, or you form up stem-walls and pour the slab in-between them. You have ended up with a sort of hybrid, with an independent slab sitting on the footings and extending out right to the exterior. Not ideal, but probably not the end of the world either.
Have you thought out how you are going to insulate the concrete? It seems like you are going to have a pretty big thermal bridge at the footings.
This project might be a good candidate for exterior rigid foam insulating the concrete. This would solve any thermal bridging problems, and would also act as a sort of "visual cladding" over that cold joint, concealing it from view. I'm thinking a layer of XPS and maybe some Densglass over that to provide exterior protection to the foam?
Bill
Bill,
In the absence of a section it's really hard to tell from the photo what's g0ing on. I'm assuming that exposed footing extends down (a bit? a lot?) and is fairly wide to take the loads. So the uninsulated footing is in contact with the slab above for probably 12" to 18". Even if there is exterior insulation and insulation under the slab, that still leaves a fair sized strip along the exterior wall that will be a large thermal bridge.
Picture shows insulation detail. The slab and footing is on average 24 inches in height. Width is 12 - 15 inches at base. Vapor Barrier is present under slab and most of footing.
As my last post shows, Malcolm is right that the bottom of the footing is uninsulated. Im not too concerned living in the Northwest (4C), plus I'd have to have argued with the county to even put insulation under footing.
Kevin,
That's not too bad. if you are adding exterior insulation I wouldn't worry about parging - or doing anything at all to the cold joint.
Thanks Malcolm.
My next step is to waterproof outside of slab. We will be flooring over the slab so want to do a good job with this. After reading BSI article (https://www.buildingscience.com/documents/insights/bsi082-walking-the-plank), it seems acrylic latex paint formulated for concrete is the way to go. Any thoughts? My main question, however, is do you think I need to paint/waterproof the whole outside, even where the xps and flashing will be up against slab?
I would have wrapped the VB up on edges, but needed some concrete to earth connection, as my electrical grounding is the rebar within the concrete footings.
Kevin
Hi,
The cold joint could be cosmetically fixed with a parge coat. Concrete crews do this all the time to make poorly vibrated concrete acceptable.
If you wanted to tie the slab to the perimeter footing in a positive retrofit way- you could install epoxy anchor bolts for your wall sill plates. Add bolts with say a 8 or 10" embedment. Are your shear wall hold down anchor bolts in the lower portion of your footings?
This would possibly double the number of sill plate anchor bolts.
An Engineered solution would be best, also evaluated to see if it is a problem.
Hi Tim,
I like this retrofit idea. Yes the anchor bolts do extend down into footings. We were able to get them in after floating slab, which also makes me think the footing portion couldn't have set up to much, as the bolts were 2-3" embedded in footing. I am also going to reach out to a local engineer to get his thoughts. We are waiting to start framing til spring, so have some time to figure this out.
Thanks!
Kevin