rust on rebar 1

[ajk1]

Is rust that forms on existing old rebar a few days after being sandblasted, in an underground garage under repair, known as "mill rust" as the contractor claims (he also clams that the"mill rust" protects against corrosion)? I know of mill scale, which forms a protection against corrosion, but I have not heard of "mill rust" which protects against corrosion. So is there an accepted terminology "mill rust" or is someone confusing that with "mil scale", and if there is something called "mill rust", does it protect against corrosion?

Also, is it anyone's experience that properly sandblasted rebar forms a coating of brown rust a few days after being sandblasted to remove all rust? The rebar is in a below grade garage.

I know that is not problem to have tightly adhering rust on new rebar in a new structure, but in a garage under repair, we are concerned about it because we are not sure that this rust is not residual rust (including residual corrosive chloride deicing contaminants) that was not properly removed during sandblasting.

 

[sandman21]

Rust wont stop rust. Bare metal exposed to the open will develop light brown rust. If the rust results in a reduce cross section than you have another issue but you should be able to tell the difference between the two.

 

[PicoStruc]

It is not because it is rusted that it behave like Corten

Corten is a weathering steel. This material is a corrosion resistant steel, that left uncoated develops an outer layer patina. This patina protects the steel from additional corrosion.

 

[ajk1]

So what you are telling me is that in your experience bare metal in an underground garage will develop rust after 3 or 4 days, to cover the entire bar surface.

Is there such a terminology as "mill rust", or is someone confusing "mill scale" with "mill rust"?

 

[Ron]

The rust you describe is relatively normal. After sandblasting you expose "new" steel. Since parking garages are notoriously bad for condensation, you are likely seeing minor corrosion as a result of the exposure and moisture. It is not mill scale and the term "mill rust" has no meaning....steel starts to rust as soon as it is made and continues until WE can't tolerate it anymore or until we do something to mitigate it, like coatings.

For your application, light rust should not prevent bond of repair mortar or concrete provided all else is prepared appropriately. If you are concerned, have them wipe the rust off with a clean cloth before placement.

 

[allgoodnamestaken]

The term I have heard for this kind of rust is "Flash Rust" or "Flash Corrosion".

 

[KootK]

I believe that rust can halt further rust. The problem with it is that the rust doesn't stay put. That's essentially the improvement accomplished with corten steel: the corroded layer stays put. Even then, it can be dislodged relatively easily when actively abraded. Link.

Here's some, admittedly biased, reference material: Link.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajk1]

To Ron: I am not concerned in the least about rusty rebar, per se. But I am concerned that it is an indication they never sandblasted the rebar adequately to get all the corrosive deicing chemicals that penetrated the concrete and reached the rebar. If we place new concrete and there are corrosive ions still attached to the rebar, the corrosion may resume, which is what we are trying to have not happen. I agree that parking garages are notoriously bad for condensation, but I have not seen any condensation around this time of year in an enclosed underground garage, but perhaps there is.

To Kootk: thanks for the information. Very interesting description of the mechanism. Sounds logical.

To Sandman21 and Picostruc: thanks for the added info. Much appreciated.

 

[BUGGAR]

I would have the contractor put all that in writing over his signature. And on his letterhead.

 

[Ron]

ajk1....Yes, what you described can happen. For that, I would coat all exposed rebar with rust inhibitor before placing repair concrete.

 

[SkiisAndBikes]

In my specifications for parking garage restoration, the following is specified:

1. The rebar must be sandblasted, typically to an SSPC-6 finish. As this is an important step, a site visit is recommended either during or immediately after sandblasting is complete. If any areas are missed (typically the underside of the bars), the contractor is directed to touch things up. As a side note, this is also when the concrete substrate is reviewed, patch profile, etc..

2. The rebar must be either coated (if using an epoxy coating ... depends on your philosophy ..) or the bonding agent and concrete must be placed within 48 hours of sandblasting. Any corrosion that develops within that period will not measurably affect the long term performance of the repair patch. The 48 hour period is key here. I'm not saying that leaving the rebar open for 5 days or 7 days or whatever will significantly reduce the life span of the repair, it's just good practice to have some reasonable control over the procedures to minimize any future risk to the performance of the repair patches. It takes some contractors time to adjust to these types of clauses in the specifications, however, in my experience, it is easily feasible.

 

[darthsoilsguy2]

i've heard the term "mill rust" many times before.... but it has always been spoken by people who didn't understand what Mill SCALE was. In some cases i've gone so far as to take a coupon of plate metal and point at the scale and scratch it and still sometimes it just doesn't click in their heads.

the term "flash rust" is certainly used in the water tank coating business when there is too long a break between blasting and coating and/or environment conditions are lousy.

any carbon steel that gets blasted to shiny metal is susceptible to flash rust once all that protective mill scale is removed. also, the blast profile makes the same piece of steel have a lot more surface area than it had coming from the mill (similar to how NOAA ranks Louisiana longer than California with regards to longest coastline) and that also accelerates flash rust development. still, i would think flash rust wouldn't be a problem for rebar if you had fresh concrete coming in soon to kill the corrosion process.... assuming the steel cross-sections were acceptable after blasting... and assuming the repair detailing prevents future access of salts to the steel.

 

[ajk1]

To SkiisAndBikes (Structural)- Yes it does seem that a site visit is required during sand blasting. When we come a couple of days later, and see all the rust, we don't know if they really sandblasted it or not. Yes, we have certainly found that the bottom of the bars are missed.

On your second paragraph, we are using galvanic anodes, so coating the bars with epoxy would kill the beneficial effects. But even if we were not using the anodes, the idea that epoxy coating the bars is a significant benefit is something that we abandoned many years ago when we found severe corrosion a few years after the repairs where this procedure was used, even though extraordinary lengths were reportedly gone to by using a "curved brush" to get the underside of top bars coated. My understanding is that most engineers no longer epoxy coat the rebar in repairs, and even for new construction epoxy coated bars are no longer in favour by many authorities, including Ministry of Transportation of Ontario, CSA S413, Ken Clear etc. But your other point about 48 hours is a good one.

To darthsoilsguy2 (Geotechnical): ok, that is very useful information.

It would be interesting to sandblast the bars to white metal in this project and then come back 4 days later and see if it is rusted. I still feel that the rust we are seeing is because they did a poor job of sandblasting.

DOES ANYONE SHARE MY CONCERN THAT IF THEY DID NOT GET ALL THE RUST OFF WHEN THEY SANDBLASTED, THAT THEY MAY NOT HAVE REMOVED ALL THE CORROSIVE CHLORIDE CLINGING TO THE REBAR? MY BELIEF IS THAT CHLORIDE LEFT ON THE REBAR WILL RESULT IN UNCONTROLLED ONGOING CORROSION.