dynamics of prevention of pex expansion

[OIA]

Quite the mouthful of a topic but a rather easy question.

The recent posting by keith and his spray foam issue got me thinking about the physical deformation due to the prevention of full expansion on the tubing. His tube is being encased by foam that is preventing the ends of the loop to be fixed. As the tubing expands it elongates and gets fatter. When tubing cannot expand lengthwise what happens to the tubing? Something has got to give. does the opening narrow and/or the tubing slightly kink? The same thing happens when it's put into concrete yet doesn't seem to cause any real issues.

Good practice is to keep the ends free to allow for free expansion yet good ole cementitious materials don't quite allow that.

I'm questioning the physical dynamics of the tubing rather than the squeaking that can happen.

Thanks
Dan

[RB]

Off the top, the stresses have to show up somewhere so in concrete it has to be internal. Another great topic OIA let me see if I can get a PEX expert in here to shed some light on this topic.

RB

[OIA]

yep, I'm a regular think tank!

If the stresses are internal, the polyethelene's elasticity is a plus. IF, the stresses aren't great enough to damage the pex due to it's flexibility then I would assume copper embedded with no room for expansion can be a bad thing?

I found this paper that gives some good info:

Thermal Expansion and Contraction in Plastics Piping Systems http://www.plasticpipe.org/pdf/pubs/reports/tr21-01.pdf

It still leaves me wondering about copper....for now. is the failure mechanism the joints and the thrust on those joints?

Dan

[MrPEX]

Hi,
RB invited me for a comment. Thanks!
When PEX Tubing is embedded in concrete, or similar, its thermal expansion and contraction is restrained. That is in longitudinal direction, and in radial direction - outwards. At a temperature increase, some expansion -inwards - will happen = a very slight inside diameter decrease. But no movement in longitudinal direction is possible, so instead there will be a compression stress in the material in that direction. The level of the stress is equal to the Youngs Modulus times the relative expansion withheld. I could write more about the stress level, but it is not very high, since the Modulus is low.
So what about that stress? Any harm with it? Well, genarally PEX is very resistant to Stress Cracking, that is formation of a an initial crack, and then a crack propagation. The reason is the molecular network. Stresses in molecular chains are evened out between the different chains at the crosslinkinks. We have a net - instead of individual fibers. The network will even out peak stresses and distribute the stress fairly evenly inside the material, in this way avoiding excessive stresses that could lead to rupture (crack) and also crack propagation.
But there is a difference here for various types of PEX... For PEX processes where the crosslinks are formed when the material is melted (hot), the formation of crosslinks are not hindered by the crystals in the material during the "curing". That is, for example, PEX-a and PEX-d processes. For PEX-b and PEX-c, etc., the crosslinks are restricted to occur outside the existing crystals in the material, leading to a more un-even distribution of crosslinks. And a corresponding lessor ability to even out peak stresses. But this higher degree of imperfection in those processes are not sufficient to cause any problem in normal applications. Only at very high and repeated stress exposure (fatigue), cracks and crack propagation has resulted for those materials (while not at all for "warm crosslinked" materials).
Copper, on the other hand, is proven to develop excessive stresses when expansion/contraction is withheld. Stresses, that lead to decrease in crystal sizes over time, and subsequent embrittleness, that often has lead to failure over time. But mainly at fairly high temperature variations, and after many cycles (long time), and comparatively long straight lengths installed in this way.
Did this shred any light, - or just confuse...??

Best regards,
Mr PEX

[NRT.Rob]

Heh Heh.. gotta love Mr. Bean. Says he'll get a "pex expert" and he gets THE pex expert. Good show

Very nice discussion there Tomas as well, thank you!

[RB]

If one were to take a picture of the industry, there is a single corner piece of the puzzle that anchors and frames the image…and it’s Tomas Lenman.

Thanks Tomas, don’t be a stranger.

You are welcome here anytime.

RB

[OIA]

As far as shedding light on the subject it got a very nice tan. Yes, Tomas, once again I thank you for the exact answer I was going for. Thank you very much. And thank you RB for having him here.


onlyinamerica
Dan