Cheating Cover With Galvanized Rebar 1

[KootK]

I'm doing a minor outdoor parking slab in a norther climate where deicing chemicals will be used. The Canadian code for parking structures contains some great stuff with respect to durability for these kinds of structures. In a perfect world, this would lead me to using some great concrete as well as 40mm/30mm top and bottom cover.

Unfortunately, on this project, I am limited to using a very thin slab. And the high covers kill me for that. One option that I'd considered was using galvanized rebar and then relaxing the cover requirements somewhat. However, I've been unable to track down any guidance for what combination of cover + galvanized might produce an equivalent level of durability.

Anybody have any ideas? Also, if anyone feels that this is simply a bad idea, I'd be interested in hearing that as well.

 

[XR250]

Geez, 40mm does not even seem like enough cover to make it durable. Can you get rid of the rebar and make a bunch of construction joints by checkerboarding the placement?

 

[KootK]

It's a suspend slab XR. 5".

 

[Celt83]

For repair/after the fact jobs I've seen the combination of epoxy coated bars + a surface sealer like Sikagard® SN-40. In addition a maintenance schedule coordinated between Sika or other sealer manufacturer and the building owner gets worked out and notes relative to the upkeep added to the contract docs.

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[JAE]

This link:

https://www.azz.com/sites/default/files/Patnaik - AZZ GalvaBar Final Report 01_10_2019.pdf

Suggests that you can reduce the cover (do a search for "cover" in the document). But doesn't go into much detail.

There's a prof that I know who does a lot of testing of various bars in bridge decks - plain, epoxy coated, galvanized, etc. and gave a talk once on the durability of bars in a corrosive environment. I'm not sure if he dealt with bar cover or not. You could possibly contact him. Chingwook Sun :

https://engineering.unl.edu/civil/faculty/chungwook-sim/

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[canwesteng]

Lots of bar at the centre of the slab instead of two mats maybe? That's my preference when I do industrial slabs subject to wash down and chlorides or other corrosive chemicals, because you get plenty of cover and the tight bar spacing should help with crack control, although I haven't been around long enough to see these in action for >10 years. Otherwise welded wire mesh could gain you some extra cover, but might not have the strength and could be a pain to place properly.

 

[WARose]

I wouldn't do it. But if I had to, I'd be checking via the "z" factor in the old ACI 318-95 code. I think some codes still reference it.

 

[KootK]

Thank you all so much for the excellent input. Truly, great stuff.

I wouldn't do it.

Yeah, I sort of accidentally got the thing condemned and demolished earlier in the year. Now I'm on the hook for a replacement that works spatially with the other existing stuff and doesn't add appreciable weight. At the same time, however, the original slab was deteriorated something fierce and I can't see anybody expecting me to just replace it with something similar to what was there before. Granted, my read on the original issue was less to do with the specified cover than the fact that the top steel placement was so erratic that there was essentially no cover in some spots.

Lots of bar at the center of the slab instead of two mats maybe?

Huh, this option wasn't even on my radar screen as a possibility. It is now though.

For repair/after the fact jobs I've seen the combination of

This is a contender too. Thanks for the copious detail.

 

[KootK]

This is my baby this afternoon.

 

[TehMightyEngineer]

I've used galvanized rebar with reduced min. clear cover in low-permeability concrete with corrosion inhibitor admixture (Sika CNI or similar). A surface sealer or maybe a silane coating would definitely help. If you were super worried I'd just go stainless or MMFX rebar and be done with it.

Another "out" is there's usually language in the clear cover sections of most codes that says you can use the reduced clear cover requirements for precast concrete even for field-cast concrete if you can show that it meets similar stringent practices of batching, rebar tolerance, placement, etc. as a precast plant.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[TehMightyEngineer]

Just saw your pictures. Is that thing even sloped properly to drain? If this were my project I'd go 5,000 psi concrete with galvanized WWR, corrosion inhibitor, silane sealer, and call it a day.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[BAretired]

I've never used it myself, but it might be a good idea to investigate galvanic protection for the slab reinforcement. I remember one of my colleagues who planned to repair a badly corroded parkade floor using galvanic protection. Unfortunately, he has passed away and I don't know which parkade he was proposing to repair that way, but there are a few companies who specialize in galvanic protection.

BA

 

[KootK]

Thanks TME. It actually slopes heavily in the long direction where it's been pooling against a wall instead of being chanelled to somewhere deliberate.

 

[MIStructE_IRE]

I would tend to consider designing with “inadequate” cover and then using a concrete protective coating. These guys state that a 2mm film of their cementitious coating 851 is equivalent to 100mm of real cover! YMMV...

http://www.sigca.ie/wp-content/uploads/Longs_Concrete_Repair.pdf

 

[Compositepro]

Galvanizing is not very good protection in a salt environment.

 

[Settingsun]

Are the de-icing chemicals similar to/same as de-icing salts? I understand they get lumped with marine environments for durability if so.

For chloride exposure, galvanising increases the time to corrosion initiation but then has little effect on rate. Chloride concentration at initiation varies from not much improvement to about 4x, with 2x often quoted.

Some options inlcude:
- Simply-supported with no top reo. Deflection problem?

- Stainless reo/MMFX at the top only with normal reo at the bottom. Cost.

- Surface coating: Some good ones but they may wear off due to traffic leaving no protection. Silanes penetrate a few mm.

- Void sealing admixture. Eg Caltite, but Caltite is usually expensive so you may end up being able to afford stainless anyway. Caltite can also make the concrete difficult to work.

- Really good concrete. 50 MPa and up with supplementary cementitious materials. The 50MPa isn't critical itself but you end up with it from the amount of cement in the mix.

I think if you reduce cover and go with anything other than stainless, you're taking a fair risk. But it won't show up for 20 years so probably get away with it.

 

[Huevo]

You could try running some cases in Life 365 to get an idea of relative performance.

 

[BridgeSmith]

Don't have any better suggestions for mild reinforcing than previous comments, but have you considered replacing them with prestressed slabs (solid or hollow-core)?

 

[IDS]

Thinking of some photographs I have seen of car parks in Canada where corrosion has advanced just a little further, and looking at the photograph of the current structure, wouldn't removing the whole slab and replacing it with a thicker one be the best solution?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Agent666]

One issue with galvanising is that once the galvanising is gone/used up locally, the corrosion is greatly accelerated at these locations if its in concrete. So you get improved durability until you don't anymore, then things deteriorate faster than ungalvanised reinforcement that is corroding.

Stainless or frp reinforcing are another option that I don't think have been mentioned.

Consider going to a corrosion consultant and getting some concrete advice.