Rusty rebars. 2

[marcosnavarro]

Hello,
I am a surveyor that the other day had a problem with other technician. I inspected a significant quantity of rusty rebars that were in that condition because they received salt water.
To test the kind of water they received, I made a salt water test with silver nitrate that gave a negative result with scratched rusty powder from the same.
I would like to know how these rebars are going to work in their future work in civil industry after being used for reinforced concrete.
I am not sure they were rusty because salt water, but in the case they were, does anybody know how this is going to affect to the future structure to be created? Is this kind of corrosion going to be stronger and quicker than corrosion generated by fresh water when iron is used for reinforced concretes?
Thanks a lot!

 

[kenvlach]

Tight, adherent rust is OK.
But not powdery or chloride-contaminated.
It's not practical to test every piece for chloride, so I suggest high pressure washing the entire lot with the cleanest water available. (Preferably, on a nice sunny, windy day for rapid drying, but hard to dictate weather!)

It is recommended to add nitrate inhibitor based upon chloride content of water used in concrete mix; maybe add some in this case.

 

[Metalguy]

As you're probably aware, rebar *is supposed to* remain rust free because of the high pH on the cement in the concrete. Unfortunately the Cl ion didn't get the message and will cause rust regardless of high pH-but only when it's wet! The rust has ~twice the volume of the steel that made it, so it cracks non-ductile concrete easily. Then water can get into the area even easier than thru the somewhat porous concrete itself.

Don't try galvanizing. The Zn oxide is more than twice the volume, so it can actually fail sooner! Also be careful with nitrate inhibitors-they can cause SCC of high-strength steel. With the loose specs. for rebar some of it is so hard it almost can't be cut with a bandsaw.

Try to get the contractor to do as Kenvlach says and then at least paint them.

 

[kenvlach]

Hey Metalguy, get a new bandsaw blade!
High-strength rebar? I don't think so. Even if a crooked contractor got some (melted scrap), the workers would complain bloody murder whenever they had to bend it!
Pretty much all rebar conforms to ASTM or foreign equivalents:
ASTM A615 carbon-steel rebar grades: 40 ksi [280 MPa], 60 [420] and 75 [520] (and I've never seen the last grade).

http://www.astm.org/cgi-bin/SoftCar...EDLINE_PAGES/A615A615M.htm?L+mystore+jwlu9494

ASTM A706 low-alloy rebar, only one grade: 60 ksi [420 MPa].

Agree about corrosion of galvanized, but pretty leery of painting rebar. Fresh concrete is an effective paint remover; only a few alkaline-resistant (& high adhesion) epoxies can be used.

Cheers.

 

[Metalguy]

Ken,
I sometimes visit a local steel place to buy "used steel"--now up to $.40/lb! They have a real nice big bandsaw that they cut everything except rebar with. One day a new worker cut some up for me, and most of it cut real easy. Then he tried to cut 2 bars (~1/2") and it just killed the teeth.

One of the bosses heard the noise and chewed out the worker-they had to put a new blade on.

I guess somehow that bar got water quenched somewhere.

 

[joeswoes]

I've read some research papers on composite rebars. I have no idea if they're commercially available or the cost, but its an idea for salt water.

 

[kenvlach]

For marine exposures, composite rebar sounds like a good idea, but I don't know of any published standards. Might be a long-term problem of alkaline attack of glass fiber if GRP composite?
Other solutions are SS rebar, fusion bonded epoxy-coated rebar, using corrosion inhibitors in the concrete, using extra low permeability concrete, using cathodic protection, using external zinc anodic protection...

marcosnavarro,
I noticed a procedure for measuring residual salt (chloride)contamination on rebar. See ASTM A934/A934M-04 Standard Specification for Epoxy-Coated Prefabricated Steel Reinforcing Bars, Appendix X1.7 "Detection of Salt Contamination."
It's pretty simple, but you need to locate a supplier for Potassium Ferricyanide-coated Paper test strips. Or make your own, with some filter paper cut into strips & some solution:

https://www1.fishersci.com/Search?t=r&key=potassium+ferricyanide+&s.x=13&s.y=4