Half cell tests - electrical resistance measurement of rebar; distance awto site of active corrosion 1

[ajk1]

1. What should be the maximum electrical resistance of the rebar to allow proper half cell testing to be conducted? I imagine it may say that in the relevant ASTM standard, but I do not have that.

2. Also, within what distance away does the half cell reading apply? For example, if there is a half cell reading of 350 mV on the top surface of the concrete slab, within what radius from the location of the probe location might the active corrosion be located? 100 mm? 200 mm? 300 mm? 500 mm? Farther?

3. For an 8" deep slab, might both the the top and bottom bars be influencing the half cell reading?

4. How reliable are there tests? Has anyone opened up the concrete and correlated the half cell reading with the actual rust observed on the rebar? I have a feeling the half cell tests may not be very reliable. I believe that I read that about 20 years ago.

 

[Ron]

The resistance of the rebar is not relevant. You are measuring electrical potential in V between the rebar and the half-cell (usually copper-copper sulphate). You have to move the half cell around to determine the potential readings and then plot them on an equipotential contour map.

The test does not tell where the corrosion is occurring....just that it will occur.

 

[ajk1]

Hi Ron -

1) Do you agree that if the rebars are not electrically connected, then you will get meaningless half cell readings? If you agree with that, then is it not important to check the electrical resistance of the rebar assembly (I do not mean the electrical resistance of a single rebar)? For example, if the electrical resistance between 2 points where an ohmeter probes are touched to different bare rebars at say 5 m away from each other gives a low ohm reading, then the rebars would be considered as electrically connected, but if the reading were high ohm reading, then they would not be considered electrically connected. Am I wrong on that?

2) If the test does not give any indication of "where" the active corrosion is occurring, then how does it help in determining where the concrete should be chipped out to repair the rebar? I know an engineer who used the half cell results to determine where to break open the concrete to get at the rebar and repair.

Thanks

 

[Ingenuity]

A field test setup using corrosion RATE analysis (using a Geocor 8 unit for example) may be of more significance than relying upon corrosion POTENTIAL readings/mapping only.

Geocor 8 units are capable of corrosion potential AND corrosion rate readings/mapping, along with concrete resistivity. Usually you do potential mapping first, then based upon the 'hot spots' potentials, you do some corrosion rate testing.

However, corrosion RATE tests and test units are expensive, but less destructive than concrete chipping and patching in areas where no corrosion has occurred to date.

 

[ajk1]

To Ingenuity (Structural): I remember Ken Clear telling me 25 years ago that it was possible to do corrosion rate testing. So what you say is interesting. But I am interested in whether you agree with all of what Ron said earlier in the email string.

 

[Ron]

ajk1....

1. No. The potential is only between the concrete and the rebar. The mass of the rebar or its connection has little or no relevance.

2. Where the potential is higher, you would expect a propensity for corrosion to be occurring at a more advanced rate and would therefore look there first. Not always right, but a good first start.

This is consistent with Ingenuity's comments as well. I agree with him that more sophisticated nondestructive testing is probably more beneficial than half-cell testing alone.

 

[ajk1]

I certainly agree that the mass of the rebar has nothing to do with it. But the electrical connectivity of the rebar is very relevant I have been told by an electrical engineer with whom I just now got off the phone, as well as by a CP specialist that I spoke to earlier in the week.

Perhaps I have not been clear in what I am talking about, namely dividing up a bay of say 20 foot square into a grid of say 2 feet x 2 feet and touching the half cell probe to the concrete surface at each node in the grid, and reading the voltage at each node, in the expectation that we are measuring the corrosion activity of the rebar nearest that node.

To clarify, consider the example of a case where all the bars in a bay are NOT in electrical contact with each other. Then irrespective of which node in the grid the half cell probe is at, it will be measuring only the corrosion activity of the bar to which the half cell electrode is attached, and not the corrosion activity of any other bars in the bay. Do you not agree?

Anyway, I wonder what the ASTM Standard says.

 

[Ron]

You should be electrically connected to the rebar that you are measuring potential to. Connection to the rebar is important. Resistance and conductivity of the rebar, less so.

 

[ajk1]

to Ron: I agree.

 

[SkiisAndBikes]

ajk1,

I have not done half cell tests in quite a few years, but do remember that we always checked resistance between two core locations, where the cores were taken directly over rebar. We often were 100 ft or more apart. I believe we checked the resistance readings against either the ASTM standard or MTO standard to determine whether or not the connection of the rebar was good enough to validate the test.

 

[ajk1]

To SkiisAndBikes - your memory is good! After posting my question and receiving some answers that seemed illogical to me, I went on the internet and found that the ASTM Standard C 876 is available free on line! I attach that herewith.

You will note that at the bottom of page 3 it states "Electrical continuity of steel components with the reinforcing steel can be established by measuring the resistance between widely separated steel components on the deck".

Unfortunately they do not suggest what an acceptable resistance might be. On my project, the technician reportedly got 30 ohms. I will have to ask my electrical engineering genius friend if he might know.

Also of interest is the last statement in the ASTM Standard that says "...A half cell potential should not be interpreted as indicative of corrosion rate, or even as indicative of corrosion reaction".

After reading the whole Standard, and what is supposed to be done, I question if this is ever really done. I also am coming to further believe the article I read about 30 years ago that said half cell tests were more sensitive to the moisture wetting of the concrete than they were to the corrosion activity, and that they were essentially of little if any value, except perhaps in very particular circumstances.

 

[SkiisAndBikes]

Ajk1,
I will dig a little deeper over the next few days and see if I can come up with something more concrete. Perhaps the OSIM, Ontario Structure Inspection Manual.

I also remember an in house form we used to fill out every half cell survey. That's going back a few years, but I was working out of the head office of the company that I think you now work with. Perhaps an inquiry to some of the structure rehab guys that have worked there since the early 90's.

This brings back old memories of being sent out to Site to troubleshoot some terrible resistance readings ... after checking batteries in the electrical metres, swapping out a new electrical metres, etc... I finally figured out the core locations were either side of an expansion joint.

 

[Ingenuity]

I went on the internet and found that the ASTM Standard C 876 is available free on line!

I think that ASTM will disagree with the fee online availability. Based upon the linked document footer (bottom right side), it is licensed to Purdue University and a little 'digging' reveals that it is from Prof Jan Olek's FTP site which has a bunch of unsecured PDF's of ASTM documents.

 

[ajk1]

To SkiisAndBikes - anything that you can find further would be of interest. There is no expansion joint in this structure, but there is a construction joint where the two buildings butt up, and the rebar does not cross the construction joint...very unusual construction (and would not meet current Codes for seismic design because the 2 buildings would hammer each other). So the lack of rebar continuity across the construction joint is something I will pursue with the guys that did the half cells...thanks for the reminder. After Bickley left, there was not the same expertise.

 

[Ron]

ajk1....yes, ASTM frowns upon pirated versions of its standards....even the outdated ones. The one you posted is the 1991 version, reapproved in 1999. The latest version is 2009.

The document you posted is still under copyright by ASTM and is still for sale on its website. I have flagged the post with the attachment for management to remove the document under copyright as that is not allowed by forum rules and violates ASTM's copyright. Sorry.

 

[ajk1]

Hi Ron: Thanks for picking up on the improper posting of the ASTM document and flagging it. It was certainly not my intention to violate laws or rules. You did not need to say sorry...on the contrary I am grateful to you; I owe you one.

 

[SkiisAndBikes]

ajk1,

Please see attached excerpt from MTO's Structure Rehabilitation Manual. Perhaps this answers your question, if not, I can dig deeper.

 

[ajk1]

Hi SkiisAndBikes: That is exactly what I need! The MTO literature that you sent deals precisely with the question of the acceptable resistance, which they say is 5 ohms. Since the technician on my project measured 30 ohms I believe (I have to go back and check what he said), then it would seem his readings should not be relied upon. Seems odd that the ASTM Standard says that the resistance must be checked, but then gives no guidance on what the acceptable resistance would be. Anyway, it is nice to see on the forum someone like you who takes the time to read the question and understand it, and then provides the answer. You have been very helpful. Much appreciated.

 

[Ron]

Skis and Bikes....good info. Thanks

 

[ajk1]

I do not have the 2009 edition of ASTM C 876. Does anyone know if that is a new edition, or just a re-approved printing of the earlier edition? If it is a new edition, then does it say anything significantly different? Does it give a maximum permissible resistance like the MTO literature?