Merlin is used to measure the bulk electrical conductivity, or its inverse, the bulk electrical resistivity, of saturated 100 mm diameter concrete cylinders or cores with lengths up to 200 mm. The test is simple to perform and a measurement is obtained within two seconds. The conductivity of a saturated concrete specimen provides information on the resistance of the concrete to penetration of ionic species by diffusion. The term bulk is used to indicate that the measurement is made through the specimen as opposed to a surface-based measurement.
Merlin can be used for the following purposes:
The electrical resistance R of a conductor of length L and uniform cross-sectional area A is given by the equation shown in the figure to the right. The quantity ρ is the electrical resistivity and is a material property, with units of resistance multiplied by length, such as ohm·m. If the electrical resistance R of a specimen of length L and area A is measured, the resistivity can be calculated from the relationship ρ = R A/L. The inverse of electrical resistivity is the electrical conductivity, σ. The inverse of ohms is a unit called siemens (S). Therefore, electrical conductivity has units of S/m. For concrete, it is convenient to express electrical conductivity in millisiemens per meter or mS/m.
In assessing the ability of a concrete mixture to resist penetration of a particular type of ion, one of the key properties is the diffusivity, which defines how readily the given type of ion will migrate through saturated concrete in the presence of a concentration gradient. For a saturated porous material, such as hardened concrete, the diffusion coefficient of a give type of ion can be related to electrical conductivity through the Nernst-Einstein equation as follows (Snyder et al. 2000; Nokken and Hooton 2006):
where σ = bulk electrical conductivity of the saturated porous material
σp = conductivity of the pore fluid
D = bulk diffusion coefficient of the specific type of ion through the porous material, and
Dw = diffusion coefficient of the specific ion through water (Mills and Lobo 1989).
If the conductivity of the pore fluid is assumed to be similar among different concretes, the measured bulk electrical conductivity is related directly to the bulk diffusion coefficient (Berke and Hicks 1992). Measurement of the bulk diffusion coefficient of a particular type of ion through concrete is a time consuming process, while electrical conductivity can be measured in a matter of seconds.
The electrical conductivity of saturated cement paste is related to the volume of pores and how they are connected within the paste. The paste porosity is related to the water-cementitious materials (w/cm) ratio, the types of supplementary cementitious materials (SCMs), and the degree of hydration. For the same w/cm and degree of hydration, the use SCMs reduces pore size and increases the tortuosity of the pores and, thereby, reduces electrical conductivity and the ease of fluid penetration.
The following is a schematic of the measurement method incorporated in Merlin. The four-point measurement method that is used provides an accurate measure of specimen resistance by minimizing the effects of the conductive sponges and the pressure applied to the electrodes. The specimen must be in a water-saturated condition to obtain a meaningful measurement.