Raft foundation expansion/movement joint

[Selexus]

Hi All,

 

Now that my UFH pipe placement and other such incidentals have been settled I'm turning my thoughts toward how to create the 2 x 6m long expansion/movement joints in the 250mm raft, that also have 20mm sleeved dowels through at 600mm intervals. 

Never done it before but I'm thinking some aluminum angle supporting 20mm compressible foam, through which the dowels poke and are supported by rebar either side ?

Suggestions very welcome 🤔

 

Thanks

[Russell griffiths]

I’ve never seen a raft foundation for a single dwelling with an expansion joint in the slab. 
what’s your engineers thoughts on this. 

 

[Conor]

Was thinking the same. Ours was 115m² L shaped, no expansion joints. Two layers of mesh and then a ring beam with more steel.

 

Go by your SE's drawings / spec 

Edited August 5 by Conor

[Nickfromwales]

If you want problems, then go for the gaps. If, however, you'd like a solid slab with no cracks or breaks, do NOT do as you state.

 

On a 140m2 L-shaped MBC raft, with only 100mm concrete depth between intermediate beams, we tiled that with 1200x250mm porcelain and not a single issue for the 12 months it was down for whilst we completed the rest of the job. No decoupling membrane etc, just flexible tile adhesive straight down.

 

Just "NO"  

 

10 hours ago, Selexus said:

that also have 20mm sleeved dowels through at 600mm intervals. 

[Gus Potter]

11 hours ago, Selexus said:

Now that my UFH pipe placement and other such incidentals have been settled I'm turning my thoughts toward how to create the 2 x 6m long expansion/movement joints in the 250mm raft, that also have 20mm sleeved dowels through at 600mm intervals. 

Never done it before but I'm thinking some aluminum angle supporting 20mm compressible foam, through which the dowels poke and are supported by rebar either side ?

Suggestions very welcome 

I'm assuming your SE has given you the joint locations and they should have given you a joint detail as there are about 4 different basic kinds of joint and they all behave in a different way. If not don't go out on your own as you could find yourself in very deep shit! If you go off piste your SE will not be pleased. If BC spot it .. and if you over stress part of the slab by putting the joint in the wrong place your house could move.. or more!

 

Once you have clarified what the joint is intended to do, not just movement but also shear transfer then they are loads of proprietry joints on the internet that you can get at a reasonable price. 

5 hours ago, Russell griffiths said:

I’ve never seen a raft foundation for a single dwelling with an expansion joint in the slab.

Sometimes when we have a L or a funny shape it's more cost effective to put in a movement joint. But if you go off and do your own thing..imagine if we put a movement joint under a big sensitive set of glass doors or glazing! It sounds so wrong and is, but that is why design must be coordinated and you need to run it by your SE. 

 

4 hours ago, Conor said:

Was thinking the same. Ours was 115m² L shaped, no expansion joints. Two layers of mesh and then a ring beam with more steel.

That works on smaller slabs at times. The ring beam ties stuff together. 

 

 

 

 

 

[Temp]

Get a drawing from your SE. Are they really expansion joints or contraction joints to ensure cracks form where you want them?

 

I think 20mm foam will get pushed about and ripped up when you flow concrete. May have to use a form board, pour one side, remove board of replace with foam then pour the other? 

 

[Selexus]

Blue dotted lines are the 2 joints specified by the SE.

 

 

I'm happy with the whys and wheres of these, as the slab is 42m long and NEEDS some form of expansion control.

 

What I'm looking for is a recommendation of a product (preformed joint) or method, that would allow simple construction of this joint prior to our single pour (90 cubic meters).

 

Thanks

S

[saveasteading]

The above drawing is for heavy industrial slabs. Even then, they don't expand, ever.

You just need crack control. Ensure the concrete isn't too wet, keep it under polythene for a week, and let millions of tiny cracks appear. It won't move again.

It might benefit from crack inducing joints but that depends on your slab design.

[Selexus]

24 minutes ago, saveasteading said:

The above drawing is for heavy industrial slabs. Even then, they don't expand, ever.

You just need crack control. Ensure the concrete isn't too wet, keep it under polythene for a week, and let millions of tiny cracks appear. It won't move again.

It might benefit from crack inducing joints but that depends on your slab design.

I don't disagree, but how do i get the SE to agree to this ?

[saveasteading]

1 hour ago, Selexus said:

how do i get the SE to agree 

You were discussing your project with a friend. They mentioned that your SE or their draughtsman has mistakenly shown an industrial floor joint,  and assuming shuttered pours*,  instead of a simple domestic crack inducer. Could they alter it please?

That could be the reality anyway.

 

* it assumes you pour the right hand side against a shutter, with the bars sticking out. Then the rest with the bar in a sleeve to let it slide.  Just wrong, and old -fashioned even for industrial loading.

[Nickfromwales]

The temp of the slab is never, imho, going to be high enough to have an amount of movement that a modern decoupling membrane wouldn't deal with simply. And then you'd have to convince me.

 

You can get tile adhesives that allow <10mm of decoupling between both surfaces.

 

What are your intended floor finishes to be?

[Selexus]

On 07/08/2025 at 21:43, Nickfromwales said:

The temp of the slab is never, imho, going to be high enough to have an amount of movement that a modern decoupling membrane wouldn't deal with simply. And then you'd have to convince me.

 

You can get tile adhesives that allow <10mm of decoupling between both surfaces.

 

What are your intended floor finishes to be?

Polished concrete. 

We'll make a feature of the cut joint that the SE has now agreed to. 

Inlay it with a brass strip or something. 

[Nickfromwales]

1 hour ago, Selexus said:

Polished concrete. 

We'll make a feature of the cut joint that the SE has now agreed to. 

Inlay it with a brass strip or something. 

It's supposed to be something flexible.......

[saveasteading]

2 hours ago, Selexus said:

Polished concrete

I hope you are not expecting a beautiful surface that needs no covering. "Polished" is a warehouse finish.

[Selexus]

10 hours ago, saveasteading said:

I hope you are not expecting a beautiful surface that needs no covering. "Polished" is a warehouse finish.

No definitely not. Warehouse is perfect. We'll have rugs and other stuff around that will soften the look and feel of it.

[saveasteading]

51 minutes ago, Selexus said:

Warehouse is perfect.

OK. Paddle lines, colour variations, perhaps some microcracking. It's a matter of taste. But do ensure that the contractor is a specialist ( does nothing else) and is aware this will be exposed and of your expectations.

 

[Selexus]

Thanks  Yes already had that conversation with them.

It's one of the few aspects of this build I'm not doing myself. We also discussed dry shake surface harder powders for colour enhancement. although that decision is yet to be settled.

Still trying to get a delivery date on my pre-fab ground beam cages 😡

[-rick-]

53 minutes ago, Selexus said:

Thanks  Yes already had that conversation with them.

It's one of the few aspects of this build I'm not doing myself. We also discussed dry shake surface harder powders for colour enhancement. although that decision is yet to be settled.

 

A dry shake floor is something I'm very curious about for if I ever get the chance to build. During your research did you look at https://concria.com/ ? They seem to be selling their solution as quicker/easier to lay and a more reliable outcome. Recently Sika bought a big share so there must be something there. Curious on any thoughts/detail you can share. In any case please come back with the outcome when you get that far.