Rebar Corrosion - how to distinguish active or not

[ajk1]

We recently did a large number of half-cell tests on the top of a parking garage floor (the traffic topping and membrane and underlying concrete topping had been removed), and got a large number of measurements more negative than minus 350 mV, even though there were no top bars in the midpanel region of the flat slab (midpanel is the area common to mutually perpendicular middle strips). We felt then that it must be the corrosion activity of the bottom bars in the 7.5" thick slab. So we broke out the entire midpanel in one of the bays so that we could inspect the rebar for corrosion. The attached photo is representative of the rebar condition. The bottom of the slab was hammer tapped before breaking it out and no hollow sound was found that would indicate concrete delamination due to corrosion.

Question:

Some of the bars had the sort of surface rust as shown in the attached photo. How can we distinguish whether it is due to active corrosion due to corrosive chlorides, or whether it was just rust that was on the rebar in the original construction before the concrete was placed?] It was built 55 years ago, but has been protected by a waterproofing membrane and mastic wearing course which we believe was placed not too many years after the garage was built.

 

[ajk1]

Sorry - forgot the photo, so here it is now.

 

[hokie66]

I don't know how to answer your question, but those bars look in excellent condition to me.

 

[yaschief]

Looks like nothing more than average mill scale surface rust and no loss of section is visible.

 

[DamsInc]

I don't know the answer (I doubt there is a way to find this out), but does it matter? Clean them for good measure, then cover them back up with concrete. Once they are in an alkaline environment the corrosion will stop. Although with new concrete you will need to watch out for the halo effect (are you using embedded anodes?).

Half cell potentials readings below -0.35 indicate there is a 90% probability of corrosion occurring, but that doesn't tell you anything about how long it has been in that state or the progress of the corrosion.

 

[Ron]

The observed corrosion is minimal and insignificant. Remember, the half cell tests measure corrosion "potential", not corrosion. Even the potential of corrosion doesn't guarantee that it is occurring in any "measurable form" other that to look at section loss after it occurs.

I do not see the observed corrosion as an issue.

How long has the concrete been in place?

 

[ajk1]

I think some respondents missed the point, perhaps because I was not sufficiently clear, but the reason for breaking out the concrete in one bay was only to evaluate the accuracy of the half-cell survey that was carried out on the whole floor (of course, when we repair this particular area, we will clean up the bars and do the normal repair procedures).


What Hokie66, Yaschief and Ron say was my judgement too. Nice to have some confirmation. Thanks. Much appreciated.

Andy

 

[allgoodnamestaken]

If you have already broken out concrete to access the bars, perhaps taking a representative sample of the concere and sending it for testing to ensure that chemically there is no corrosion issues (carbonation caused passivity, etc) might be a worthwhile use of funds to reinforce the opinion based results with some facts.