Rebar Corrosion

[ajk1]

I recently examined some #8 (1" diameter) rebar in a 60 year old garage slab, where the top surface of the top slab bars appeared to be flattened. I measured with digital calipers and found that the left-to-right dimension was significantly greater than the top to bottom dimension. Although there was rust on the bars (and some of the bars were necked down and completely severed by the corrosion), for the flat topped bars it was not the flaky type of rust and did not look severe. But the only explanation I can think of as to why the top of the bars looks flat is that it has corroded away. See attached photo, although the flattening does not show up that well in the photo. The photo is 16 megapixels so you can enlarge it a lot if you wish, without it losing clarity (it takes a while to load, so please have patience). Has anyone seen anything like this flattened top surface of rebar? If so, is it attributable to corrosion?

 

[Canuck67]

ajk1, I have seen similar many times, where the existing corroded rebar appeared to be flattened. I can only guess at a reason. In some cases, after the concrete spalls, the top of the bar remains exposed for a length period while the sides and underside still have some additional protection (passivation) by being in contact with the concrete. The chloride laden water has more access to the exposed portion of the bar over time. I think this even would occur in delaminatations as the chloride laden water would find it's way into the void and the top of the rebar eventually. Another possibility I have seen in parking garages with exposed corroded rebar getting driven over which continually rubs/wears off the flakey portion of the rust.

 

[CTW]

Being 60 years old, it's not likely to be round bar. Possibly square or flat. See attached.

 

[JAE]

I think Canuck67 is correct - more corrosion and loss of section on the top, exposed surfaces creating a non-round non-symmetrical shape.

Square bars weren't used 60 years ago (i.e. 1955) that I'm aware of. Square bars were used much earlier, which is what CTW's link indicates:

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[ajk1]

It is definitely round bars. I think in Ontario the square bars were mostly used before the war.
What I do not quite understand is why the top surface of the bar does not seem to be disintegrating from the corrosion. It seems quite hard and sound...although perhaps I just did not examine it thoroughly enough. Thanks for the comments. Thanks Canuck67 and JAE for the information that you have seen top flat surface bars caused by corrosion. It is re-assuring that you too have seen that and I am not looking at an illusion. For a #8 bar, a typical measurement that I made was 0.85" vertical x 1.06" horizontal. This means that 16% of the nominal 1" diameter bar area was lost due to the loss of the top part. For my calculations, I am going to assume (perhaps conservatively) that 25% has been lost, since there may be some loss on the remainder of the bar as well.

 

[Canuck67]

I do not have any data from measuring such bars that I have seen in the past. Thinking back I likely have seen some bars with maybe 20% loss of section in the horizontal direction and 75% loss of section in the vertical direction. We generally did not measure as we would A) assume essentially zero cross-section remaining, B) continue to jackhammer concrete along edge until a proper lap length of mostly non-corroded rebar was present, C) sandblast and leave existing bar intact, then D) lap splice new bar of original size into the patch.

 

[ajk1]

To Canuck67 - yes agreed if we were at the repair stage, that is what we would be doing. But we are only at the investigation stage, assessing whether the part of the garage is unsafe and must be closed or not.

 

[Ron]

Have seen similar as well. Chloride permeability occurs in a gradient, with obviously higher concentrations near the top. Further, there is a greater likelihood of carbonation near the surface; but increases with depth as chlorides penetrate. Depending on the speed of chloride permeability, the corrosion pattern will change. With gross penetration in a short period of time, exfoliation of the rebar typically occurs. With slower, long term and low concentration penetration, the corrosion will be slow to progress from the top down.

 

[ajk1]

To Ron - thanks for the information that you have seen the flat top of bar too. Yes I know about the chloride gradient and that is why the bar surface closest to to concrete surface usually sees the most severe corrosion, so I am not at all surprised that the top of the top bar has suffered more than the rest of the bar. In addition to the chloride gradient, there is the "blocking" effect where the bar prevents the chlorides from diffusing past the bar and the chlorides pile up directly above the top bar (as noted in CSA S413). What does surprise me, and no one has commented on yet, is that the top of the bar seems sound ... i.e. there is no "exfoliation" as you so aptly put it (a good word go describe what often happens when there is corrosion loss of metal). Also, it does not have the heavy brown appearance that is often seen where corrosion occurs without exfoliation. In fact, the appearance is as though it were a bar that has been pounded into that shape, although I do not think that is the case. Is that what the sort of appearance that you have seen, i.e. no heavy rust coating and no exfoliation?

 

[Ingenuity]

ajk1,

I just received an email from ACI on an upcoming webinar, that may be of interest to you:

ACI WEBINAR
Rehabilitation of Structure with Reinforcement Section Loss
Tuesday, August 4, 2015
1:00 - 2:00 pm EDT

Link

 

[Ron]

ajk1....I've seen both slow and fast corrosion presentations. What you are seeing is the slow version. The rate of corrosion is low, thus a selective attack on a more pervious area. Slight variations in the rebar metallurgy will enhance this effect.

 

[ajk1]

to Ingenuity - I

​

have made a note of the webinar. Thanks.

to Ron - thank you for the information. Much appreciated.