How to add expansion joints in concrete slab?

[Ben Weston]

Hi all,

 

Our ground floor build-up will have the concrete slab at the top (poured over UFH pipes). The area of the downstairs is 105m² and the slab going down is 135mm fibre reinforced.

 

I'm conscious we should probably be adding expansion joints in all doorways and also at intervals in the big rear section which is a 60m² room.

 

Question is: what do we use for the expansion joints (I'd seen Fillaboard?) and how do we add these so that we can pour the full 105m² in one hit?

[Declan52]

Why is your screed 135mm thick. That seems very very thick compared to what normally would be used. A flow screed you can go down to 50mm and a sand cement based screed 75mm. What kind of screed are you using as this will impact how you put a joint in??

[Ben Weston]

It's not screed, it's a concrete slab. Build up is sand, DPM, 200mm PIR, DPM with UFH pipes, slab. So it's a traditional C25 slab.

Edited November 6, 2023 by Ben Weston

[IanR]

18 minutes ago, Ben Weston said:

Our ground floor build-up will have the concrete slab at the top (poured over UFH pipes). The area of the downstairs is 105m² and the slab going down is 135mm fibre reinforced.

 

I'm conscious we should probably be adding expansion joints in all doorways and also at intervals in the big rear section which is a 60m² room.

 

Question is: what do we use for the expansion joints (I'd seen Fillaboard?) and how do we add these so that we can pour the full 105m² in one hit?

 

A little more info please. Are the external and load bearing walls sitting on the slab/raft, or do they go down to a traditional foundation with the slab poured in between them, leading to narrow sections at doorways.

 

There's no issue pouring 105m² in one go, my own is 465m² without any expansion gaps and done in a single pour, but all walls come down on top of the raft.

[Ben Weston]

No, not a structural slab. No load-bearing walls on it. As you say: traditional foundations and slab poured between. House is already built — floor is one of the last things to go in!

[JohnMo]

Our floor build similar to yours, but with an additional 150 to 200mm concrete below the PIR. Ours is 192m2, 25m long, we had 1 crack, with no expansion joints.

 

If you are pouring this time of the year the curing time should be quite slow, so cracks less likely.

[IanR]

Just now, Ben Weston said:

No, not a structural slab. No load-bearing walls on it. As you say: traditional foundations and slab poured between. House is already built — floor is one of the last things to go in!

Then I'd put an upstand of perimeter PIR around the edges, more for a thermal break, but it will also allow some movement. My guess is you haven't got much room for this, at best the thickness of the plasterboard and skirting.

 

There's a chance it will crack at the internal corners of the doorways, where, if you need to control that, you could put a cut in at each door way about 25% the overall depth (30mm - 35mm), as soon as it has cured enough to walk on. 

[Kelvin]

My concrete slab has two cuts made in it the full width of the slab. For the screed I put some thin expansion joint board I bought from the local merchant in all the doorways/cupboards just jammed between the frame. I also fitted expansion foam strip around all the perimeter areas. No cracks. 

Edited November 6, 2023 by Kelvin

[Ben Weston]

Thanks all. We're not going for a screed on top — the concrete slab will be floated up and any minor bits sorted with a self leveling filler (though obviously hoping not to have any!) Flooring to be laid on top of that.

 

We were going to mesh the doorways but otherwise leave the fibres to do the reinforcement.

 

Cutting down the slab might be a bad idea for us as we'd also go through the UFH pipes 😆

Edited November 6, 2023 by Ben Weston

[IanR]

2 minutes ago, Ben Weston said:

Cutting down the slab might be a bad idea for us as we'd also go through the UFH pipes 😆

 

With no steel in the slab, I'd assumed your UFH pipes would be stapled to the PIR, well away from the top surface. 

[Ben Weston]

10 minutes ago, IanR said:

 

With no steel in the slab, I'd assumed your UFH pipes would be stapled to the PIR, well away from the top surface. 

 

Yes that's true, they are. Doesn't that cut need to go all the way down or is it just to encourage any cracking in pre-defined places? 

[IanR]

A cut 25% of the overall depth of the slab is enough to act as an initiator so that if it does crack, then it will (should) crack in the cut.

 

But, do you need to control the cracking. You've mentioned a float finish and a self-levelling filler if needed, so there's no plans to polish the slab. It's probably not important where it cracks (if indeed it does). Just make sure you use a Ditra mat type un-coupler if you are putting tiles down.

Edited November 6, 2023 by IanR

[Ben Weston]

Brilliant, thanks all. This is very helpful. 

 

On your polished point: I'd actually love to polish it and have that as the finished floor (not mirror finish, just matte and flat) but think that's beyond my skillset.

[saveasteading]

Firstly. All slabs crack but we can control it to be microscopic.

Using fibres helps a lot and just about prevents any big cracks if the slab is less than 3.5m.

 

Not expansion joints. Contraction joints.

As @IanRsays. 25% cut depth will make the slab crack where you tell it to. The crack will go right to the bottom in a very jagged fashion (which keeps it very strong) but you only see the straight joint.

In a normal sized room you will only need a contraction  joint at doors, (and a release surface at the perimeter.)

You can cut this next day with a grinder or more simply, position a shutter across the openings.

 

I believe there are cradles for ufh pipes to lift them more centrally in slabs, for a faster heating effect.

 

Are you pouring this yourself?

[syne]

That was also my thought, why are you not suspending the ufh pipes closer to the surface

I'm doing this almost identically on Friday but have the ufh at about 40mm below the surface on (an otherwise pointless) mesh sat on 90mm chairs. 

 

[Chanmenie]

As @IanR suggested, I used a large angle grinder and cut 30mm deep channels.

 

I would question why you are pouring the concrete so thick ?

if it’s fibre reinforced 135mm would be strong enough to park trucks on.