Question on concrete slab

[Hash123]

Hello,

I had a 12m x 8m concrete slab laid in my garden on which to build a timber outbuilding. To make it easier to pour and level the concrete, my builder divided the area into 3 smaller sections by using the 6x2 timers to section it off - i.e they made 3 8x4 sections. At the time they said they would remove the timbers once the concrete had set but now they are saying to leave the timber in as it would be too difficult to remove them as they will need to break the concrete either side of the timber.

My question is, is there any downside by leaving the timber in? It's treated timber so the builder is saying that there is no issue with rot or anything else?

Thanks in advance

 

[Notch7]

yes its fine in regards ot leaving the timber in

as for having 3 separate sections, unconnected, not sure thats so good -what was the contruction used:
was it
reinforced with mesh
have a bigger toe
use type 3 hardcore underneath

etc etc

thats a big building -it should be done under building regs as over 30sq metres

 

[RonnyRaygun]

It’ll rot. They should have removed the timber before pouring the next section. Usually it would be suitably dowelled and lined with a compressible material to allow for contraction / expansion.
What thickness is it? I assume it’s reinforced?

 

[Hash123]

Thanks Notch7,

The actual structure will be 6m x 4m (made from SIP panels) - I needed to leave a minimum of 2m on all sides to the boundary as part of planning approval so will just be laying patio slabs on that part of the slabs. Each section of slab was reinforced with steel mesh

 

[RonnyRaygun]

yes its fine in regards ot leaving the timber in

Is it? How do you join the sections with the timber still there?

 

[RonnyRaygun]

Thanks Notch7,

The actual structure will be 6m x 4m (made from SIP panels) - I needed to leave a minimum of 2m on all sides to the boundary as part of planning approval so will just be laying patio slabs on that part of the slabs. Each section of slab was reinforced with steel mesh

So the building will only sit on one section? Why such a large slab then?

 

[Hash123]

What thickness is it? I assume it’s reinforced?

Hi RonnyRaygun,

The slab is around 150mm thick and each section is reinforced with steel mesh

 

[Hash123]

So the building will only sit on one section? Why such a large slab then?

The building will be roughly in the middle (crossing 2 of the 3 sections) - as mentioned, I needed to leave a minimum of 2m from the back and from the 2 side boundaries so decided on make 1 big slab and lay patio slabs on these sections around the outbuilding

 

[RonnyRaygun]

What happens to the building when the slabs move independently of one another?

 

[Hash123]

Thanks for the replies, so if they were to break the concrete around the timber, remove the timber and then fill the gap with screed/concrete - would that be better?

 

[RonnyRaygun]

Thanks for the replies, so if they were to break the concrete around the timber, remove the timber and then fill the gap with screed/concrete - would that be better?

Yes, that would be better for the joint that doesn’t have the building over it.

Ideally, you wouldn’t have a joint under the building - but too late for that unless your builders go to the trouble of breaking back the concrete by a few hundred mm either side and adding in some connecting reinforcement before filling back up with concrete. The risk is differential movement of the two sections. Timber buildings are at least inherently flexible so can accommodate a bit of differential movement.

If the slab is on non-shrinkable soil, (or on shrinkable soil but there are no trees nearby [and there haven’t been for many years, and there won’t be in the future]), and he laid the slab on a couple of hundred mm of well compacted type 1, differential movement is likely to be minimal.

Hope this helps!