Steel Stakes Puncturing Vapor Retarder in Slab-on-Grade 2

[TXEng-USA]

Typical practice in my area is to utilize #4 rebar stakes to chair the post-tensioned tendon in the bottom of the grade beams (3" clr cover). The PTI Construction and Maintenance Manual for Post-Tensioned Slab-on-Ground Foundations (PTI DC 10.2-06) appears to approve the practice in Fig 5.13 (see attached) with the notes "steel or plastic stake or other method of positioning tendons in rib".

My concerns include:
[ul]
[li]Penetrations in the vapor retarder which greatly increase its permeance. [/li]
[/ul]
[ul]
[li]Moisture in the bottom of the slab corroding a steel stake and causing damage in the bottom of the grade beam. [/li]
[/ul]

Since this is standard practice should I not concern myself with it? My thought on the steel corrosion is that it may workout that there is a lack of oxygen below the slab so the corrosion is not extreme. If anyone had a case study or experience with this it would be greatly appreciated because I will need to have a solid reason in order to enforce plastic stakes or not allow stakes at all.

 

[thaidavid40]

In the Deep South, where I practice, corrosion (rust) definitely follows rebar pins from the subgrade into the slab, and usually leads to corrosion of the flexural reinforcing too. The plastic stakes would seem to be the better choice, if your area experiences similar problems. As for the vapor retarder, it is just that: a retarder. It isn't expected to be watertight, just to be a barrier to slow down global moisture passage through the slab. A few stake holes shouldn't be an issue. If they are, then you have a particular and special case which demands extra design provisions to address those issues. You could always cut some squares of the retarder material, punch stakes through them, and tape the pierced holes tight. Then when you drive the stakes through the slab retarder fabric, tape around the four edges of the flaps to seal them to the global retarder sheet.

Thaidavid

 

[Ingenuity]

Penetrations in the vapor retarder which greatly increase its permeance.

The 'infamous' Dr. Joe Lstiburek and Fick's Law have something to say on this subject, here: Link

Okay, so we don’t need the sand layer to handle the “curl” thing. But what about using the sand layer to protect the polyethylene? Hah. The polyethylene does not need protecting. You can poke holes in it, you can puncture it, you can tear it, you can leave gaps in it, and pretty much have your way with it as long as it is in direct contact with the concrete. Huh? But, but, vapor barriers have to be continuous and free from any holes. Actually, no. Air barriers need to be continuous and free from holes, but vapor barriers do not need to be. Lots of vapor moves by air movement, not a heck of a lot of vapor moves by vapor diffusion. The concrete slab is the air barrier, and the ripped and torn and punctured polyethylene sheet is the vapor barrier. It’s that Fick’s Law thing. Diffusion is a direct function of surface area—if I get 95 percent of the surface covered I am pretty much 95 percent effective—and the parts that are left I have filled with concrete which is also pretty good as a vapor barrier. I could wear golf shoes and march around the plastic vapor barrier and not do much damage. But put that sand layer in there and you are doomed.

 

[wannabeSE]

Vapor retarder manufactures also have something to say and it seems to contradicts the 'infamous' Dr. Joe Lstiburek :

http://www.stegoindustries.com/hubf...ncture_the_Vapor_Retarder.pdf?t=1492451225195

https://www.wrmeadows.com/data/Under Slab Vapor Barriers-Retarders White Paper.pdf

 

[Ingenuity]

Suprenant and Malisch's (in the Stego link) CaCl cup testing did not include the concrete slab, so not really a real-world test.

And Brewer's results from 1997 testing (in the Stego link too) had a slab with w/c ratio of 0.7. That is a butt load of water.

 

[XR250]

Vapor retarder manufactures also have something to say and it seems to contradicts the 'infamous' Dr. Joe Lstiburek :

I always take Lstiburek's stuff with a grain of salt. Building science guys always seem to change their tune about things later in life. Of course, it is already too late for the folks who took their earlier advice.

 

[thaidavid40]

Guys, maybe we should remember that we are talking about vapor here - not liquid water under pressure. This isn't a bathtub situation under the slab (if it is, then there are bigger problems than vapor retardation). Put the retarder fabric in, give it reasonable care during rebar and concrete placement, and be done with it. No need to stress out over this subject (in most typical cases).
Dave

Thaidavid