This test is carried out to assess the quality / uniformity of concrete at various depths. The instrument consists of a four probe device. Electrical current is passed through the outer probes & the potential drop is measured by the inner probes. From the current & voltage drop measurements, the resistivity of concrete can be measured. This resistivity can be related to quality of concrete.
Electrical resistivity = 2(pi)aE/i (in kilo-ohm cm)
where pi = 22 / 7 = 3.14
a = distance between probes
= 5 cm in the 1st set of readings
= 10 cm in the 2nd set of readings
E = potential difference between inner two probes in mV
i = current passed through outer probes in microamp the penetration of current depends upon the distance between the probes. Various sets of readings are taken. The electrical resistivity for a homogeneous material is constant.
When the RCC members to be tested are covered with costly finishes like marble, granite etc. this test can be used without having to remove such finishes.
Apart from quality / strength of concrete it can also give information about the total thickness of the specimen under test & the backing material (in case of tunnel lining or pavements where only one surface is available for testing & no other test can give the said information). The backing material could be concrete, water, air or rock in case of tunnel lining.