Encountered a first today and thought I’d get some input. The house was built in 1985, the photo shows metal beams under a patio. The patio above is a slate tile type of patio. My thinking is that the patio above isn’t “waterproof” and what I’m seeing is where water has seeped down through the concrete above to the metal beams. So, my next question would be, is this type of mineral deposit relatively harmless to a metal beam? How do I approach this; “Mineral deposits noted on deck beams”? Has anyone else encountered this before?
The concrete was probably never (or minimally) sealed and the concrete has been soaking up water year after year which has caused the corrugated steel decking to rust and form pinholes. What you see is the minerals and salts draining out of the concrete through the pinholes.
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Yep, coming out through the puddle welds of the deck on to the C-channels.
That decking should of been made water tight before pouring that concrete slab.
That steel needs to be cleaned off and repainted to prevent future rusting.
I agree that the rusting has to stop or slow down immensely. But, how does one get between the joists and the low parts of the C channel on the deck to do that?
Not my forte.
Hey guys, thanks for the information. It’s one of the things I love about this job; always something new. Again, I appreciate the info! - Tony
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I thought maybe it had something to do with galvanic corrosion since it only seems to be happening where the deck panels touch the beam. If the panels are Galvalume or some other aluminum alloy it would be lower on the anodic chart than the steel from which the beam is made.
Anybody have input on the how of this?
I’d call that severe corrosion that may have significantly affected the structural integrity of the panels. I’d be passing that photo along to a steel panel manufacturer for their opinion. And disclaiming it.
I don’t think you do, Larry. That deck maybe should have been electrically isolated from the panels when it was installed. Strips of EPDM or similar on top of the beams. Even then the fasteners would connect them.
Thanks, Kenton.
Just my best guess, Larry. I’m studying steel decks now. I think I will send that photo to a panel manufacturer tomorrow.
Thanks for the perspective, Kenton. I didn’t even consider dissimilar metals.
Would the isolation or EPDM been required back in 1985?
I don’t know, Tony. If it is galvanic corrosion (and I think it does tie in with what Stephen and Marcel said), the mfgr. would have had a recommendation to avoid this condition.
I think the mineral deposits are a result of leaching through the concrete deck, which supports the water penetration theory. Galvanic corrosion occurs with dissimilar metals (one metal loses electrons to the other). Both appear to be steel, so I wouldn’t expect galvanic corrosion just regular corrosion.
I’m getting ready to have a large, elevated, redwood deck rebuilt with a steel frame structure. If you have any advice for me, please share it. Looks like it’s going to cost more than my first house.
If you’re contemplating putting together an inspection guide and want me to take photos of the process, I’d be happy to. let me know if there’s anything specific you want photographed. If you want to see it in-process, you’re welcome to come out too.
I would not mind seeing that Chuck.
I agree, it looks like painted B-Deck to me. I know it is not a composite deck, and it is not a corrugated form deck that you would use for an elevated slab. Both, composite and form deck used for concrete slabs are galvanized.
What we see is more of a roof deck which comes in painted steel.
It normally gets welded every 12", and if the welds don’t get painted, contributes to the rust leaching through along with minerals coming out of the concrete.
Not an easy fix at this point. Looks like loose laid stone pavers which would have to be removed and then waterproof the slab and reinstall.