rust on rebar
rust on rebar
(OP)
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.
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.






RE: rust on rebar
RE: rust on rebar
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.
RE: rust on rebar
Is there such a terminology as "mill rust", or is someone confusing "mill scale" with "mill rust"?
RE: rust on rebar
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.
RE: rust on rebar
RE: rust on rebar
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.
RE: rust on rebar
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.
RE: rust on rebar
RE: rust on rebar
RE: rust on rebar
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.
RE: rust on rebar
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.
RE: rust on rebar
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.