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fearghusq (Structural)
11 Sep 12 22:48
Hello guys.

These slabs they are externally exposed in a soffit of a loading dock in a chopping centre, around 3km (approx 2 miles) from the Indian Ocean.

They have been in place since approx 2004 (8 Years).

Several, but not all of the units exhibit similar staining. It runs in line with direction of rebar. However no spalling is noted.

I have only visually inspected from ground level 5m (16ft)

Have a look at the image - I have a felling the rust staining is a sign of the beginning of chloride corrosion of rebar, however there is no rust staining.
fearghusq (Structural)
11 Sep 12 22:49
dik (Structural)
12 Sep 12 6:33
Do you have a close up photo?

Dik
TXStructural (Structural)
12 Sep 12 16:07
Yes, it looks like rusting of the embedded reinforcement. It looks like the reinforcement may have not been supported at sufficient depth. We see this same rusty "ghosting" in cast in place with shallow cover. Two miles from a coastline is very close, and you will be getting salt exposure.
fearghusq (Structural)
13 Sep 12 3:04
Dik, Thats as close a picture as I have. The soffit pictured was about 5m over head and only a walk through survey for now.

It's most strange that, given the location and proximity to the ocean, all external concrete was not specified with some kind of coating. I remember once reading, or hearing in a lecture, about different types of sealant to reduce porosity/permeability of concrete.

Am i correct in saying that whilst it will not eliminate corrosion, applying a sealant will reduce transport of water - and chloride ions dissolved within - to the rebar?

Do you have any suggestions as to what an appropriate sealant may be (either chemical class or a proprietary/trade name).

Many Thanks for all your help guys
Helpful Member!  rowingengineer (Structural)
13 Sep 12 8:31
I'm not sold yet.

i need more investigation than a photo from 5m's away. I would check the cover to the steel before starting with any works, maybe even expose a bar? I have viewed similar staining on freshly pour concrete when the form work has reflected some rust from rebar left in the rain. Not suggesting this is the cause in this case, just suggesting that I would want more information.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

fearghusq (Structural)
13 Sep 12 23:05
I know where you are coming from Ron. I have a lot of experience in the precast industry and have seen the same staining from moulds left exposed outdoors when not in use.

Unfortunately the survey we have been commissioned to undertake allows for only walk-through and overview style reporting for this centre to prepare a technical due diligence report for an investment company.

I will simply suggest further investigation is necessary with a cover-meter or electrical resistivity meter to determine cover to rebar.

A profile of the transport characteristics [sorptivity, absorption, permeability, ion transport] to determine the quality of that covering concrete would also be useful.

Im thinking of suggesting the use of Autoclam/Permit (Amphora Non Destructive Testing) or the Germann Apparatus.

I know these are available and used in UK/Ireland - and have inclusion in British/Euro standards. I am currently in Australia.

Does anyone know of any similar apparatus used in USA that they may be more familiar with here in Australia/NZ?

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