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Desiccant in post-tensioning tendon inspection opening

Desiccant in post-tensioning tendon inspection opening

Desiccant in post-tensioning tendon inspection opening

(OP)

Is it worthwhile placing a packet of desiccant in access openings before installing the removable metal cover plates? (The access openings were chipped into the beam soffits at tendon low points to allow inspection of the condition of the post-tensioning tendons in beams).
If so, how to determine amount of desiccant?
Has anyone on this forum done this, or have knowledge of anyone having done this?
Is there any negative aspect to including the desiccant?

RE: Desiccant in post-tensioning tendon inspection opening

Seems pointless as the concrete will doubtlessly provide far more moisture than the desiccant can remove. At 50% relative humidity silica gel will remove about 25% of the mass of water per mass of silica gel (25 grams of water for 100 grams of silica gel). This works well for a sealed environment (food, pills, etc.) but is poor for something where you have a long exposure time and a relatively unlimited amount of moisture.

That said, it's cheap and I can't think of any downside. I wouldn't rely on it for preventing corrosion but if you already have a bunch of desiccant then might as well throw it in.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Desiccant in post-tensioning tendon inspection opening

(OP)
TehMightyEngineer (Structural)- that is very interesting information. Thank you very much. Just some follow-up: Although I did not say it in my post, but the concrete is 40 years old, and the top surface of the floor is completely covered with a waterproofing hot-applied rubberized asphalt membrane overlaid by a 1 inch thick mastic traffic topping. So the vapour ingress would be up from the beam soffit thru the pore structure of the concrete. I am not sure how much vapour travel that would be, but it could perhaps be significant, or perhaps there is always moisture locked in the concrete irrespective of its age. Would I be correct in assuming that you are saying that it would be significant when you say that the desiccant would be pointless?

I was thinking of the desiccant as being perhaps useful in removing moisture from the air at the time when the gasketed cover plate is bolted in place to cover the opening. If it is bolted in place when the ambient air has a high relative humidity, that would not be good, and it was to that aspect that I was thinking of the desiccant.

I would of course not "rely" on it to prevent corrosion. There are other systems such as Densotape and grease on the tendon to address the corrosion issue.

If the desiccant has a low probability of a significant beneficial effect, and could become the subject of ridicule, then I would probably prefer to omit it. What would you do...include it or not?

RE: Desiccant in post-tensioning tendon inspection opening

(OP)

As a further follow up: what is the weight of moisture in 1000 cubic inch volume of air at 80% R.H?

RE: Desiccant in post-tensioning tendon inspection opening

This should help you.



I calculate 20 grams of air at 15°C. At 80% R.H. and 15°C that gives 0.2 grams of water in that volume of air per the graph. So, 100 grams of silica gel should remove about 50 g of water for a good safety factor.

However, the concrete will have far more moisture in it than that even being older concrete in a dry environment. I highly suspect it will quickly equalize the volume of air at some relative humidity (greater than zero) but this will (hopefully) be lower than the 80%.

I imagine the end result that you're going for is to prevent the normal temperature fluctuations from causing water to precipitate on the post-tensioning steel. I would think that the desiccant would likely help with this. Not sure how successful it would be but given the low cost and ease of install I don't think it will be a source of ridicule and the reasoning is at least "it can't hurt". I'd include it if you want to.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Desiccant in post-tensioning tendon inspection opening

Unless you've gone totally out of your way to make a hermetic chamber and seal, a desiccant is likely to get saturate within a very short time, likely, much less than a year. This is why military systems, even those that are supposedly sealed, use desiccant cartridges that can be readily replaced.

a bit of anecdotal history -- we had a system (~3 gal air volume) that wasn't particularly well sealed and in a coastal environment, it could pump an ounce of water out of the air in about a week.

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RE: Desiccant in post-tensioning tendon inspection opening

"Subject of ridicule" sums it up well.

RE: Desiccant in post-tensioning tendon inspection opening

Hokie,

You had better watch out, the PTI will have you red flagged with immigration next time you try to go "home" for a visit!

RE: Desiccant in post-tensioning tendon inspection opening

(OP)
To TehMightyEngineer (Structural)- Thank you again for the information.

To IRstuff: Interesting. Thank you

I will give the matter more thought. Depends on the cost of 100 grams of desiccant and the space it occupies, and how many packs are required per opening. There are about 150 openings so cost may not be negligible. I am dealing with a space of about 20" x 12" x 3" deep (200 mm x 300 mm x 75 mm). I am sort of getting the feeling it may not be worthwhil...

RE: Desiccant in post-tensioning tendon inspection opening

Yeah, that's becoming non-negligible. At lets say $5 per pack that's $750 that I'm sure the owner would like to keep.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Desiccant in post-tensioning tendon inspection opening

The residue left after desiccant exhausts may be as corrosive as water vapour. But I don't know.

RE: Desiccant in post-tensioning tendon inspection opening

(OP)

ok, I have decided not to include desiccant. Thanks everyone for the comments.

RE: Desiccant in post-tensioning tendon inspection opening

hokie66...most dessicants are calcium chloride.....the same material once used as a set accelerator in concrete....highly corrosive to the rebar....thus the reason it is rarely used. Good point!!

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