Where would you put expansion joint?

[1justin]

Here is a picture. (hover on my thumbnail). We have three slabs 1973, 1978 and 2010, all coming together, and Travertine to lay.

slab problem.jpg
I have got Durabase decoupling arriving soon, along with some 10mm Schluter expansion joints.

My approximate tile locations are determined in the Y axis (as you view the picture) by a planned and very visible expansion joint between old + new slab which is out of shot to the left.

(Ignore the pale area which is a reinforced concrete cover over a air duct for a woodstove. >The reinforced concrete goes right down to the slab at the sides of the duct, and I'm confident it's unlikely to be a problem area)

I have four potential problem areas in this pic. I am faced with placing an expansion joint which does not coincide with any of them exactly. This is not great. Belt and braces approach (but would look very poor) would be cut the tile pattern and have four joints. This is nasty. I would like to use just the one., and it can't easily coincide with the orange dashed lines which are locations of discontinities.

But where exactly? closest to A B C or D ?
Do I chop it up and add two/four joints? Yuk.

[jay]

Your best approach to this problem is to place expansion joints were each different slab meets (old new young) sorry you may have issues with the ignore area depending on depth of cut out . other opinions may differ

[1justin]

Yes I was hoping that wouldn't be suggested!
The "ignore" area goes through the screed down to the new slab (about 60mm down). The screed came away cleanly when I cut it out. The replacement over the duct was a very strong concrete with steel mesh also down the edges to the slab. I also bonded to the screed on sides. I doesn't seem to have shrunk at all.

[jay]

probably hasnt shrunk but there is a weak point either side were cracking may happen

[1justin]

Hi Jay. Maybe so, but that would make six joints within 2M! The others all go right through the slab(s), not just the screed so are my current preocupation!

[jay]

The screed is your substrate if you cut it you create a movement area like a new slab you run a cut in it to create a expansion joint a point were the slab can move rather than breaking . by increasing the strength of the material to fill the void you are introducing a harder area which will move (expand and contract)different to the existing screed you could risk it its up to you,
it is a shame to require so many expansion joint in such a short run. in hindsight it may of been better to remove the other slabs as to create just one expansion joint .
good luck

[1justin]

Hi Jay,
I believe what you say overall is correct. The reaons for not removing the old slab were several. Economics being the first. Secondly the original intent at build was to floor with wood or cork which wouldn't care about the junctions. However the new idea is the Trav and I want to make it work.

So I don't want to dwell on the duct cut-out. I agree it will probably shift a little but it's a very narrow width, so it can't go far, and it can't go vertically because it all sits on the same reinforced slab. The Durabase decoupling sheet should take care if it cracks along the interface.

So my main issue is what I see as the far bigger risk of the other joints, especially in the vertical.

Advice from this forum has been very useful for me, but it tends towards "belt and braces". I'd like to see if a tiler would seriously fit six expansion joints within the 2M width shown. I'd say probably not or they might not have many customers. Making the most of what I have now is what I'm trying to do. I can't 100% eliminate all risk of cracks, but want to reduce it where I reasonably can.

I believe I should be OK without six (!) joints, and because of my decoupling, it's the vertical movement over the old/new slabs which worries me most.

The joint most likely to give vertical movement is "D" since this is new slab abutting 1973 foundation. A and B and C have both been there since 1978 and show no sign of vertical movement. (slight shrinkage crack horizontally but I think this is the purpose of the decoupling sheet).

Have I Just talked my way into adding a joint only at D? Feels risky. I do need at least one anyway because this breaks an otherwise ~11M run. It's a a shame we covered "D" with the screed. Perhaps I should cut a slot which would coincide with the original foundation edge along line D to define the place where it could move, and place the expansion exactly in this line.

[jay]

Your call i can only offer advice

[1justin]

That's fine. Talking about it helps - to anyone!
Rgds