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.

 

[SkiisAndBikes]

ajk1,

I am glad to have been of help. They should be providing you with two resistance numbers, one for the coil of wire they are using for the half-cell equipment and the other for the total resistance. As long as the (total resistance - wire resistance) is less than 5 ohms, things should be good. I am stretching my memory banks but the wire resistance should be less than 1.5 ohms perhaps .... depends on how much wire they are carrying with them and also the condition of the wire. Also read the section regarding the moisture content of the slab, sometimes the slabs are quite dry and the half-cell test locations really really need to be pre-wetted.

In my past I have also had half-cell data submissions for epoxy coated mats. Some people know the rules, some don't.

 

[ajk1]

SkiisAndBikes;

These are not epoxy coated bars. They are from 1957. I will see if they can get the resistance for the coil of wire. The ASTM Standard says to use at least AWG No. 24 wire. My electrical engineering friend says that is not very heavy wire, he says it is about like speaker connection wire.

 

[ajk1]

SkiisAndBikes;

- When you say that the test locations may need to be pre-wetted, do you mean that is in addition to the wet sponge that is placed around the tip of the half-cell probe?

- If the slab is too dry, would that give lower half cell readings? We have lots of half cell readings >350 mV.