Talk:Electrical resistivity measurement of concrete

Resistivity formula

70.29.111.82 (talk) 18:37, 28 February 2012 (UTC)when the resistivity meter shows that the obtained current is 100% you can call the ratio of V/I resistance, becuse 100% of the applied current obtained so the meaured voltage by two inner probes represnt the resistance of the concrete.

The formula $\rho =2\pi aR$ has been repeatedly inserted by IP editors as the resistivity measured by a Wenner array. This is incorrect and seems to be a conflation of two separate cases. Wenner arrays are intended to be applied to the surface of a bulk substrate (originally from geophyisics). The ratio of V/I in this configuration might have the units of resistance but it does not represent the resistance of any physical object. The current is not flowing through the same "lump" of concrete that the voltage is being measured across. The correct formula for this configuration is $\rho =2\pi aV/I$. On the other hand, the case of four electrodes attached to a uniform cross-section sample in the laboratory will have V/I equal to the resistance of the sample (between the two inner electrodes) but the Wenner array formula no longer applies to this geometry. I would also say it is dubious that this configuration should be called a Wenner array - although it is certainly still a four-electrode measurement. SpinningSpark 09:22, 28 February 2012 (UTC)

The two-probe formula has a similar problem. The formula only applies to a uniform cross-section sample, not to probes placed on a substrate. Authors who talk of a two-probe measurement are usually discussing the latter case. SpinningSpark 18:02, 28 February 2012 (UTC)

70.29.111.82 (talk) 18:17, 28 February 2012 (UTC)Just to let you know two probes is used to measure the electrical resistivity of concret for concrete cylinder or cube, so the the formula works great for this situation.

Ok, but there is still an issue with the four-probe formula. I think the the article will be a lot clearer if the measurements are divided into laboratory measurement and field measurement which are different and have different formulae. SpinningSpark 23:23, 28 February 2012 (UTC)

Dubious tag

Same IP has now placed a [dubious - please discuss] template on the description of substrate resistivity measurement (despite it being cited) without actually opening a discussion. Please state here what you think the problem is. SpinningSpark 18:02, 28 February 2012 (UTC)

70.29.111.82 (talk) 18:31, 28 February 2012 (UTC)Three major problems are related to four prbes measurments (Wenner-array)

1.Probe spacing must be chosen in a way that the specimen becomes semi-infinite.

2.Proper conection of the probes and concrete.

3.Dramatic error will occurs when there are two or more surface layers with different resistivities like carbonation.

Also you shoud not be angry and put the knowldge of other people under question as you did!?! I have contributed to this article and make it better the readers can judge by comparing it now to previous one.

You may well be right that the method has its problems but it is still a method that is used. The {{tl:dubious}} tag is not appropriate for information that is correct and properly cited. The article can discuss the problems of the method but the tag should be removed, unless you have some other objection. SpinningSpark 23:04, 28 February 2012 (UTC)

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