Hello all,

I'm going to be pouring the first half of a 120mm thick concrete floor slab for my extension on Tuesday (weather permitting). This document seems to give a lot of handy info for getting good results:

http://www.ccanz.org.nz/files/documents/fad37ed6-e276-45d2-a594-e1a8fa6eb5b3/MS%2005.pdf

- I'm planning on having a thin top layer added later (epoxy bonded), and then polished, so I'm keen on getting a good result on the underlying slab.

The CCANZ document insists on the use of a vibrating poker, buy my main contractor has been warned off this by a friend - who says that there is a high potential for damage to the DPM. If the poker is dipped at the pattern shown, and not allowed to contact the DPM, is this likely at all?

Thanks,

Tim.

Posted By: tonyHow about a power tamp, or hammer drill fixed to a plank?

I did think of that, but all the hammer drills I've got run at about 60Hz for the vibration rate, whereas all the poker seem to be around 180Hz (which they state is the optimum). 60Hz is likely to be better than nothing tho I suppose.

Pokers are more for deep pours.

So I understand, but a couple of document I've seen (including that NZ one) are fairly insistent on their necessity. I looked at a vibrating tamp, but it said max compaction depth 75mm (and it's a 120mm slab). I'm also going be using a relatively low slump concrete (as it's arriving on a tipper lorry - since that's the only economical way I could get a 70% GGBS mix), which will make the vibration-compaction more important I would have thought?

You may well need to power float it (upside down helicopter) if you want it polished

Powerfloating is also going to be awkward, and my builder is reluctant to do-so (it's also quite a small slab). Instead, the plan is to manually tamp it flat with guides. After the second half of the slab is poured, (can't be done at the moment - there's a wall in the way), a 10 to 20mm epoxy-bonded top will be added, and polished (if I can afford it).

Posted By: TimSmall
So I understand, but a couple of document I've seen (including that NZ one) are fairly insistent on their necessity. I looked at a vibrating tamp, but it said max compaction depth 75mm (and it's a 120mm slab). I'm also going be using a relatively low slump concrete (as it's arriving on a tipper lorry - since that's the only economical way I could get a 70% GGBS mix), which will make the vibration-compaction more important I would have thought?

How far is it going to be travelling in the tipper, I'd be worried about the mix separating out if it is going far or over poor roads and you getting a mess at the end. Poker especially useful if there is any rebar to get good compaction around the rebar. The poker itself won't damage the DPM as it has a blunt end.

I'm looking forward one day to using this self-levelling concrete, that MikeGeorge posted a video of some time back. Actually I'm campaigning to not use conc at all.

Well, we did go ahead with it and poured yesterday, I got a 70% GGBS / 30% Portland Cement blend from Hanson in the end (based on their normal RC20/25 mix, but with more GGBS). The first load was a bit dry, but workable - the poker was useful for that load, and did seem to compact it, and drive out air pockets. The second load was too wet, and had separated out a bit - so needed turning over - and pokering that didn't seem to do anything much to it. Final load water content was about right (couldn't see the 3 bears tho') I think the poker did bring a bit of air out of that.

The pipework and the temperature sensor cables seem to have survived intact - I don't see there being much chance of powerfloating causing any trouble - I believe this is pretty standard practise in other countries, e.g. Aus/NZ, where they tend to have slab, no-screed.

This says that with high GGBS concretes, it's possible to pour at the end of one day, and powerfloat at the start of the next:

http://www.ecocem.ie/technical,working.htm

I've covered the slab with insulation at the moment, and the UFH temperature sensor in the concrete which I've got connected up to this PC says 42.6 Celsius (up from 41.5 a few hours ago) - however it's quite cool at the edges, so I should probably go outside now, and get the edges insulated better to reduce any differential thermal cracking, and ensure the edge is as strong as the middle...

I covered the slab with polythene dust sheet, and then insulated the top and sides of the slab with 150mm EPS (it already has 200mm insulation under it) to try and get the early strength up - since http://www.ecocem.ie/technical,strength.htm says:

"A temperature increase from 20 to 40°C, commonly reached or exceeded in the curing of in-situ and precast concrete, is sufficient to increase the early strength GGBS concrete by a factor of two to four, (LERM, 2001). However, the same increase in temperature only increases the early strength of Portland cement concrete by a factor of 1.5 (Klieger, 1958)."

... as it's going to need to have some (very carefully placed with the load very well spread!) acro props on it in a few days time.