UFH pipe literally sandwiched between mesh...a problem?

[mvincentd]

So for better or worse i've ended up with a scenario where my ufh pipe must zip tie to the top mat of a393 mesh but then have a142 mesh as anti-crack sit directly on top of the pipe.

 

My concern is that during the concrete pour a workmans welly on the a142 (6mm) mesh becomes somewhat more of a point load on the pipe.....certainly on a test piece not under any water pressure it leaves a dimple depression.

 

One supplier isn't concerned for the robustness of their pert-al-pert pipe during the pour and its inevitable foot traffic (but is concerned about expansion/contraction having a wearing effect on pipe against mesh over time....i've seen enough counter-argument to that to not be concerned).

 

 Nuheat feel a pipe without the metal content such as  PE-Xc would suffer less 'dimples'.

 

Wunda feel their hdpe-al-pex would be a better solution than their standard pert-al-pert pipe but aren't saying with certainly that its adequate.

 

Does anyone have any real-life experience of this mesh-pipe-mesh sandwich situation?

 

Many thanks

 

 

[Nickfromwales]

Can you fix to the underside of the top 393?

[Barney12]

It would be a ball ache to fit but could you put spacers between the two meshes? I know you wouldn't be able to fit enough to stop the 6mm flexing but it would at least help with reducing point loads? 

[Nickfromwales]

That would take the rebar to close to the surface wouldn't it?

[PeterW]

Change the spec. 

 

Bin the A146 as it's only anti crack and have fibres in the concrete mix. Your A146 is acting as a non structural component so the SE doesn't need to sign it off either. 

[Jeremy Harris]

I'm with @PeterW on this, there's no merit at all in the top mesh, all it does is add complication and there's not really any significant cracking risk anyway.  We have a 100mm slab, with mesh in the centre and UFH pipes tied to it, no fibres in the concrete, no addition mesh anywhere, and it's not a problem.  There are lots of similar slabs around, and I'm not aware of any that have had cracking problems.  If you want a belt-and-braces approach, then add fibres, as Peter says, but I'm not convinced they are needed.

Edited July 8, 2017 by JSHarris

[mvincentd]

The concrete contractor, SE and the ufh people are all ok with the cover of 30mm.

 

I originally proposed fixing to bottom a393 temporarily then lifting to underside of top a393 but reinforced concrete contractor felt it would be a practical nightmare to achieve....it's an absolute maze, as per attached photo (the a393 is double layered in places and lapped to close the grids from 200 to 100mm)...the top layer is now fixed so I can't take that approach anyway.

 

Spacers such as 20mm rebar alongside the pipe to give 4mm seems a decent idea yes, except for that flex.....i can't realistically follow all 950 metres of pipe but doing some maybe is viable.  This was our first thought but with so much bar in there already i'm wondering where the concrete fits...I hoped to gain enough confidence in the ufh pipe strength to render this unnecessary.

 

Binning the anti crack is a risk in the view of the RC contractor as its an exposed power floated aesthetic so i don't want to have any cracks of significance on show.  I don't think fibres in the mix is acceptable to the Sikaproof-A waterproof membrane which mechanically bonds to the concrete.

 

It's reassuring to hear though from @JSHarris that perhaps people are over-stating the crack risk.

 

Thank you.

 

[PeterW]

Is that going to be a heated aircraft hanger or do you have some exceptionally heavy furniture ...??!!!!

[Jeremy Harris]

I'm a bit taken aback by the amount of steel in that slab, as well as the thickness.

 

@jack has a similar slab to ours, with UFH pipes tied to the single thickness steel, and a polished concrete finish that looks really nice.  AFAIK he didn't have an cracking problems, and his slab is very similar to ours, I believe, so 100mm thick in the centre.

[mvincentd]

I'm encouraged to pursue the theory of doing away with the anti crack mesh so thanks for the comments.

 

I too was, and continue to be, shocked by the engineering on the slab (and house in general) but the RC guys say it doesn't look out of the ordinary for the scenario (see pic)...the house recedes fully underground so the slab is supporting and tied into  2.7m high retaining walls....hence it has similarly wide edge thickening to 300mm with 3 layers of a393.

 

How well the ufh will work in such a volume of concrete remains to be seen but my mind is more on summer cooling anyway...the plot is essentially a chalk pit hell hole in the hot weather.

[Onoff]

7 minutes ago, mvincentd said:

I'm encouraged to pursue the theory of doing away with the anti crack mesh so thanks for the comments.

 

I too was, and continue to be, shocked by the engineering on the slab (and house in general) but the RC guys say it doesn't look out of the ordinary for the scenario (see pic)...the house recedes fully underground so the slab is supporting and tied into  2.7m high retaining walls....hence it has similarly wide edge thickening to 300mm with 3 layers of a393.

 

How well the ufh will work in such a volume of concrete remains to be seen but my mind is more on summer cooling anyway...the plot is essentially a chalk pit hell hole in the hot weather.

 

Those kids next door would have a long drop if they over bounce on that trampoline!

[mvincentd]

Further research is now making me think the fact i'll not be firing up the ufh for probably a year after the slab is poured, that anti crack really isn't necessary.

Thank you.

 

[Jeremy Harris]

3 minutes ago, mvincentd said:

Further research is now making me think the fact i'll not be firing up the ufh for probably a year after the slab is poured, that anti crack really isn't necessary.

Thank you.

 

 

 

Worth bearing in mind that the UFH won't get the concrete hot - it will get far hotter just sat in the sun I expect.  Before we fitted the external reflective film to our front glazing the floor would easily get to around 35 deg C on a sunny day, whereas the hottest it ever gets with the UFH running is maybe 23 deg C.

 

I think there is probably a fair bit of unwarranted concern when it comes to the thermal effects that UFH may have, probably based on decades old problems from back when UFH systems needed to run at 30 deg C plus in order to keep a house warm.

[Temp]

On ‎7‎/‎8‎/‎2017 at 11:39, mvincentd said:

One supplier isn't concerned for the robustness of their pert-al-pert pipe during the pour and its inevitable foot traffic (but is concerned about expansion/contraction having a wearing effect on pipe against mesh over time

 

I've no idea if this is a real issue but some 12 years ago one or two UFH suppliers also told us there was a potential issue when cable tying pipe to the mesh. They were concerned about the cut ends of the rods that make up the mesh so perhaps it would be better to run the pipe as per the green rather than the red ?

 

 

 

 

 

[Nickfromwales]

Isn't this only ever going to be an issue if there's a gap between the pipe and the concrete where the pipe is free to move in ? 

I've dug pipes out of rebar bound concrete before, and there is zero gap for movement or expansion afaics . 

Good praxtice would surely see folk at least trying tying the pipes on as per the green arrangement anyway though ? Makes sense to me and no extra cost / time etc. 

[mvincentd]

Yes @JSHarris , that makes a lot of sense.

Equally @Temp I did cut and paste into my notes a while back this counter-argument (I think might actually be @Nickfromwales comment originally); Ok - the only way to damage a pipe in this way is to cause the damage during installation - PEX is good to 10 bar, UFH is 1-2 at most and to get PEX to expand more than 0.5mm needs around 4 bar. Pumps are circulation pumps not pressure pumps so there is no way for a pulse to be created unless there is a fast actuating manifold valve and even then, that would be a sub second pulse only when it closes. 

I think in my situation the green layout creates so many more chances for the full weight of a worker to end up effectively tight-rope walking on the pipe (as the mesh will be more open now i'm scrapping the anti-crack layer.

Thanks

 

[Dudda]

Have you any crack induced joints or movements joints in that slab? With a large polished concrete slab you can go two ways. Have it very thick with loads of steel everywhere and try to stop any cracking or movement. The other way is to have crack induced joints every few meters which is what's usually done but with that huge slab half buried the SE might be trying to avoid that in which case I'd be reluctant to remove any steel.

[mvincentd]

There is no planned induced cracking and no control joints or sawcuts.  It's not like a screed bounded by fixed wall structures, which is where I believe these things tend to be more relevant.  I'm told that some cracking is almost guaranteed and the power floated surface will serve to make even a 1/3mm crack visible, but the cracks could easily get up to 2mm thickness....the mesh is merely to limit any cracks spread in a great long fissure.

 

Final conclusion on this has been to stick with the anti crack mesh as the ufh contractors have satisfied themselves that the pipe will hold up to the in-construction scenario ok.  I'll report back after the mesh-pipe-mesh sandwich has been danced on by the concrete gang.

 

Meanwhile thanks all. 

[Nickfromwales]

You have to do what you feel is right . 

Please update accordingly with some as-you-go pics if you don't mind ?

[mvincentd]

Ok, so I went with 16mm pex-al-pex on the assurance of the installer that it could handle the crushing risk of my mesh-pipe-mesh configuration.

It was laid yesterday and pressurised with water to 4bar mid morning in the midrange of a hot day that got progressively hotter until approx 3pm.

At one point I told the installer the first manifold was reading zero pressure, but when he checked we realised the needle had gone off the scale beyond 6 bar and around to hit the underside of the zero stop-bar on the gauge (It's a serious sun trap of a site) ! The other manifold had also climbed to over 6bar. 

This morning following a wet and much cooler night both manifolds were reading below 2bar and have (along with the weather) remained there through to this evening.

Tomorrow is pour day.  I want to add some pressure back but the ufh contractor is reluctant even though tomorrows forecast is cool.

So far as the mesh sandwich is concerned i've a feeling it's almost a good thing - the greatest vulnerability so far has been where a foot can land on pipe that spans a 200mm square void in mesh as per photo here.  The top mesh significantly reduces these instances and does also seem to distribute the weight of foot traffic fairly effectively.  That said we are treading as carefully as viable.

At this stage i'd say pex-al-pex is probably ok tied to mesh that sits directly on insulation but (to my surprise) I'm so glad I didn't ditch the anti crack and leave it tied to suspended mesh...if this were anyones scenario i'd say ensure every inch of the pipe follows directly along the line of mesh bar (including the loop turns....so impossible I think).

So it's a tense day tomorrow but if we get through it the fun really starts; The pic showing the wall of osb....thats single sided shuttering along there which will require around 24 push-pulls to support it in that walls pour which will exert around 20 tons of pressure.  All the push pulls will have a minimum of two thunderbolts 150mm into the slab. So we're drilling circa 50 holes betwixt the pipe maze!!

[mvincentd]

 

[mvincentd]

Ok 1 week on and I still have good pressure in the pipes (varying with weather between 1 & 6 bar) so it seems we got through the pour.

I couldn't find a single visual example online of my configuration so I'm attaching a pic & video here incase useful to anyone in future.

 

[Nickfromwales]

Happy days. . 

Thanks for the update ?