Taking a blowtorch to the slab!

[Tony K]

My concrete slab now has the walls around it but no roof. I need to finish a fiddly bit of damp detailing where the walls and slab meet. Back when the weather was dry and warm the primer and membrane I have bought worked just fine, but it is now neither dry nor warm, and the products don't like damp slabs.

 

I have the option of buying in special primer and special membrane for damp conditions, but seeing as I've already bought a general primer and membrane and cut the latter to size and shape, I would like to at least try and get the slab dry and use what I have. 

 

It has been suggested that I could hire a three-headed gas torch to dry out the area of the slab I'm working in. Has anyone ever tried this, and if so, does it work?

 

Thanks

[Conor]

That's exactly what my builders did when applying the tanking membrane to our slab. It was just a strip along the fillet between the slab and wall, so they used a handheld butane torch.

[Tony K]

33 minutes ago, Conor said:

That's exactly what my builders did when applying the tanking membrane to our slab. It was just a strip along the fillet between the slab and wall, so they used a handheld butane torch.

 

Do you recall what kind of distance they held the torch from the slab? Did you spot any special techniques, tricks or things to avoid? 

I shouldn't think even I could manage to set a damp concrete slab on fire, but I'd rather not test the theory if there is some 'best practise' I can adhere to!

[Conor]

5 hours ago, Tony K said:

 

Do you recall what kind of distance they held the torch from the slab? Did you spot any special techniques, tricks or things to avoid? 

I shouldn't think even I could manage to set a damp concrete slab on fire, but I'd rather not test the theory if there is some 'best practise' I can adhere to!

 

Fairly close..they weren't shy!

 

[hendriQ]

Would the same work to speed up the curing of screed. Mine was poured over the UFH 5 weeks ago and really would like to start tiling it in another 5 weeks' time, but that's only 70 days which isn't enough for 70mm of screed I think.

[Tony K]

1 hour ago, hendriQ said:

Would the same work to speed up the curing of screed. Mine was poured over the UFH 5 weeks ago and really would like to start tiling it in another 5 weeks' time, but that's only 70 days which isn't enough for 70mm of screed I think.

 

Did you get a sample section of the screed? It's common enough to do so for concrete slabs and have them strength tested at 14 - 28 days. Perhaps a similar process might exist for screed. 

 

Interestingly, when I searched on line (YouTube especially) for advice on using a torch to dry the slab (meaning get the damp surface dry) all the results I found were to do with accelerated curing, which is similar to your question. Maybe give that a go? 

[TonyT]

Get a humidity meter taped to the slab to see the RH of the slab.

 

We used one with a yellow Tupperware type container that the normal humidity sensor slots into to read ground readings.

[Tony K]

Interesting (mixed) results regarding my attempt to dry my slab.

 

I hired a three headed torch (slightly terrifying to be honest, think Lord of the Rings), and it certainly has the ability to dry out the slab. The issue is that I have had to build my outside leaf of blockwork first (due to tightness to the boundaries), and am now drying the space where the inside leaf will go, once the damp detail is in place. This process requires that the DPC is left flapping loose inside the building, above the area I am drying. Even when pinned up out of the way, the heat from the torch is such that it seems likely to melt the DPC, which becomes noticeably more floppy during the drying process. I can't have a melted/perished DPC!

 

I will experiment again with a smaller torch, but otherwise will admit defeat and shell out for a primer and membrane that work in the damp.