Raft Slab - DIY or not?

[LA3222]

1 hour ago, Tony K said:

 

I was wondering if that was an option. That way I can diy the formwork if needed and then hand over to the experts for the pour. 

 

That said, I presume I'll be getting the formwork level all around the top edge, and that when the concrete is pumped in I would just have to slide a long straight timber across the top of the formwork to get a fairly level top to the slab?

 

Or is it not that simple?

Another mistake I made, attaching the UFH pipe to the top of the mesh.  When you use 50mm mesh spacers and add 8mm mesh onto that you end up with just 34mm of cover in a 100mm slab.  Something I didn't factor in - A292 is 8mm diameter, but it overlaps in a criss cross so actually takes up 16mm.

 

Then when you chuck in the laps where mesh has to overlap, suddenly you only have 18mm of cover to the mesh. Put a 16mm UFH pipe there and you're buggered. These are are the sorts of things no one tells you to be careful with so you dont realise until too late.

 

I spotted this issue as I was doing the mesh but it was too late in the game to change the plan. I considered running the UFH pipe through and under the mesh but that would have been way too problematic. So I ran it over the mesh avoiding laps where possible. 

 

Even so (and this is where it is relevant to your comment about running a rule over the edge - I thought this too), the concrete guys were concerned there wasnt enough cover so poured the slab 10mm higher than the upstand all the way across (it didn't really pour over the edge like you would think!). I got them to remove the bit over the upstand so I could then foam 10mm of insulation back in giving me my upstand back. A major pita, not difficult but just an unwanted job. This created another problem, where they had removed the concrete over the upstand it was jagged due to the stones in the concrete, I had to mix up some load bearing grout up and go around the edge pouring it in to give me a clean finish again.

 

It worked, but there was a lot of unnecessary work created due to the way I put it all together. In hindsight, fixing the pipes to the insulation under the mesh would have saved a lot of work.

 

With my job, I organised the boom pump and concrete and powerfloat, the guys came in and did the work. They were busy from about 8am till 12pm then nothing to do until the concrete cured enough. They started power floating at about 14:30 and finished about 20:00. 

 

[Tony K]

@LA3222 Thanks. What you describe sounds like a significant headache!

 

In my case I'm adding the UFH later on, once the slab is set and the walls and roof are in place. It will live in the screed. Hopefully this will save me some of the complications you describe!

 

Another consideration for me is that my slab is L shaped and split level. I intend to cast the first, lower slab, then once it cures build a 700mm block wall in the corner of the L, where the upper slab will later join the lower one. The SE has designed rebar to come up from the lower slab through the hollow blocks. Then I will form and cast the upper slab. This is all fine, but of course I've got to do two separate pours, pay for the pump twice etc.

 

I'm sure there are ways to create vertical formwork in place of the blocks and so allow one pour, but based on the experiences you describe I'll be keeping it simple and sticking with two distinct phases. 

[Miek]

That said, I presume I'll be getting the formwork level all around the top edge, and that when the concrete is pumped in I would just have to slide a long straight timber across the top of the formwork to get a fairly level top to the slab?

 

Or is it not that simple?

 

 

Just make sure the formwork is solid enough to jump on without it wobbling. Depending on your soil you might need steel stakes (my ground is very hard and a wooden stake is not an option) rather than wood. Make sure all  stakes are sawn off so that they aren't in the way when striking it off.

[Tony K]

3 minutes ago, Miek said:

That said, I presume I'll be getting the formwork level all around the top edge, and that when the concrete is pumped in I would just have to slide a long straight timber across the top of the formwork to get a fairly level top to the slab?

 

Or is it not that simple?

 

 

Just make sure the formwork is solid enough to jump on without it wobbling. Depending on your soil you might need steel stakes (my ground is very hard and a wooden stake is not an option) rather than wood. Make sure all  stakes are sawn off so that they aren't in the way when striking it off.

 

Thanks. I'd looked at road planes and they seem designed to work with steel stakes, though have yet to find a place to hire them out.