Testing of High-Resistivity Concrete

The resistivity of concrete is often used in defining the corrosivity of concrete.  This is often performed using variations on the Wenner four-pin test method.  This paper discusses the application of the State of Florida Test Method FM 5-578, a variation on the traditional Wenner method that was developed to measure soil resistivity in situ.  In work performed on a number of latex modified rapid setting mortars, which incorporated supplemental cementitious materials (SCM), it was noted that changes in mortar resistivity were related to changes in mortar strength.  At early stage, portland cement and SCM hydration makes the mortar pore structure less permeable and more resistive.  This continued hydration is also reflected in strength gains. It was also noted that changes in concrete resistivity proceeded expansive cracking.  Expansive cracking follows several stages that include development of internal stress, breaking of chemical bonds in the cement phase, microcracking, and finally macrocracking.  In relatively moist conditions microcracks can provide more open paths for conducting electricity and hence lower the mortar resistivity.  Expansive microcracking also leads to a reduction in mortar strength.  Resistivity testing provided indications of both changes in strength and the development of cracks.