Concrete slab

[nh26302]

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

[Thorfun]

10 minutes ago, nh26302 said:

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

I can't comment on whether it's ok to leave the timbers in, sorry. but this dividing in to 3 smaller sections just sounds like laziness! sounds like they couldn't be bothered to build the formwork for the 12m x 8m area and didn't want to get their wellies dirty tamping down the concrete and levelling it.

 

please note, I am not a builder so this might be very common but it seems a bit wrong to me!

[saveasteading]

23 minutes ago, nh26302 said:

no issue with rot or anything else

It will rot eventually, even treated, starting at the ends and rotting inwards.

Before the rot, there will be damp ingress.

It is not concrete so they have not completed the job.

It was a silly (or inexpert) idea to cast them in, so let them learn to be less silly and more expert by removing them and infilling with concrete.

It will be difficult to get the timbers out and the concrete will be damaged locally, but they can repair it.

 

Complete the job before payment!

[MikeSharp01]

Put an old blade in the circular saw, set the depth to just less than the thickness of the beam and cut a line in the wood then chisel the timber out using the cut to get the chisel in.

[Thorfun]

11 minutes ago, saveasteading said:

It will rot eventually, even treated, starting at the ends and rotting inwards.

Before the rot, there will be damp ingress.

It is not concrete so they have not completed the job.

It was a silly (or inexpert) idea to cast them in, so let them learn to be less silly and more expert by removing them and infilling with concrete.

It will be difficult to get the timbers out and the concrete will be damaged locally, but they can repair it.

 

Complete the job before payment!

phew! I was right and it is the wrong thing to do. must start believing in myself some more. especially when the chippies turn up and say that my battening isn't bad at all and I should see some battens that carpenters have fitted! and when other people in the industry complement me on how well I'm doing and I should see other sites run by 'professionals'.

 

I generally think they're just being nice but maybe I do have a clue after all. 😂

[Miek]

Usually the 6x2 would have been removed while the concrete was wet after the slab was level, and made good by hand. This is the easy way to do it. Removing it afterwards is the hard way. 

 

Whether you leave it in depends on the use, I would break it out with a pick/bar/lever of some sort and make good with concrete. 

[nh26302]

Thank you all for your answers...I will insist on them removing the timber joists. 

 

What would be the best mix of concrete to fill this? Also, I have a jumbo bag of sharp sand - can that be used in the concrete mix?

 

I'm starting to lose confidence in my builders - therefore,  would like to be sure that they fix it properly as one of them mentioned that if I wanted them to remove the joists then they would fill the void with screed....

 

 

[saveasteading]

Same concrete as the slab. A screed is the easy and cheap option for them, especially if using your bag of sand.

Screed would not be concrete, and I assume you wanted a concrete slab, not a screed one.

They can go to Wickes and get 2 or 3 bags of ballast and a bag of cement.  NOT diluted with your sand.

 

Was there reinforcement in the concrete?

[nh26302]

Steel mesh was laid in each section - if that's what you mean by reinforcement in the concrete?

[nod]

Very difficult to remove afterwards 

Absolutely unnecessary 

 

Ive just laid a similar size slab 9 cube 

2 hours 

[saveasteading]

3 hours ago, nh26302 said:

Steel mesh was laid in each section

As I expected. so you have three slabs now with no continuity of the reinforcement. In theory the slab can swivel at these points.

In practice it shouldn't matter, as it is for a shed and reinforcement may not be necessary.

[nod]

6 hours ago, saveasteading said:

As I expected. so you have three slabs now with no continuity of the reinforcement. In theory the slab can swivel at these points.

In practice it shouldn't matter, as it is for a shed and reinforcement may not be necessary.

The mesh I put in is the smallest grade 30 quid a sheet Just to prevent surface cracking 

[Conor]

11 hours ago, nh26302 said:

Thank you all for your answers...I will insist on them removing the timber joists. 

 

What would be the best mix of concrete to fill this? Also, I have a jumbo bag of sharp sand - can that be used in the concrete mix?

 

I'm starting to lose confidence in my builders - therefore,  would like to be sure that they fix it properly as one of them mentioned that if I wanted them to remove the joists then they would fill the void with screed....

 

 

Surely you are getting a ready mix delivery to site rather than mixing on site? That's a 10m³ total pour. You'll want C25 or C30 mix. Concrete company can advise. You won't get a consistent, graded mix on site.

 

Ask them to dowel through the formwork so the steel is continuous. Even with that, I think differential movement in the slab is eventually inevitable. Whether this matters or not, depends on what you're doing with it.

 

I'm assuming the build up is something like: firm ground/compacted stone, sand/dust binding layer, DPM, 100mm concrete? Are they doing basic things like using a vibrating plate to compact the ground, a later to set the levels of the formwork etc?

Edited May 17, 2022 by Conor

[nod]

3 hours ago, Conor said:

Surely you are getting a ready mix delivery to site rather than mixing on site? That's a 10m³ total pour. You'll want C25 or C30 mix. Concrete company can advise. You won't get a consistent, graded mix on site.

 

Ask them to dowel through the formwork so the steel is continuous. Even with that, I think differential movement in the slab is eventually inevitable. Whether this matters or not, depends on what you're doing with it.

 

I'm assuming the build up is something like: firm ground/compacted stone, sand/dust binding layer, DPM, 100mm concrete? Are they doing basic things like using a vibrating plate to compact the ground, a later to set the levels of the formwork etc?

As above 

Also it would be a mammoth task mixing 8 cube with a mixer 

Mine arrived late in the day at 4pm and 430 and was down before 6pm 

[Dudda]

4 hours ago, Conor said:

Surely you are getting a ready mix delivery to site rather than mixing on site? That's a 10m³ total pour. You'll want C25 or C30 mix. Concrete company can advise. You won't get a consistent, graded mix on site.

 

31 minutes ago, nod said:

As above 

Also it would be a mammoth task mixing 8 cube with a mixer 

The concrete is already poured. It's only a small bit required to replace the two bits of timber he intends to remove?

[nh26302]

Thank you all for your responses...timbers have now been removed and gaps made good with concrete 

[Thorfun]

42 minutes ago, nh26302 said:

Thank you all for your responses...timbers have now been removed and gaps made good with concrete 

did the builders do it? or did you do it yourself? if they did it how grumpy were they? 😂