Resistivity of Repair Materials for Concrete Repair Prior to the Application of a Cathodic Protection System

Suitability of repair materials for local concrete repair in combination with a cathodic protection system depends on the compatibility of their electrical resistivity characteristics with the parent concrete. As data on resistivity of repair mortars is very limited, selection of a compatible product is not always possible. In this research, the electrical resistivity of 10 different structural repair mortars was tested at three different environmental conditions (temperature of 20 degrees C and relative humidity of 60, 80, and 100%) by means of three resistivity test methods: four-point electrode method (Wenner Probe), two-plate electrode method and method with embedded electrodes. In this way, a database is created through which the most suitable repair mortar can be selected for each concrete structure in different environments. The relative humidity (RH) of the environment has a large influence on the resistivity of the repair products: on average, at the age of 91 days, a decrease of the RH of 20% tripled the resistivity of the repair mortars. Furthermore, based on the obtained results it is expected that this effect will be even larger on the long term (e.g. after several years). The two bulk resistivity methods showed comparable results. However, the two-plate electrode method was found to be unsuitable for resistivity measurements of non-saturated mortar specimens. The surface resistivity, measured by the Wenner Probe, was found to be a factor of 4 to 5 times higher compared to the bulk resistivity. The difference can mainly be attributed to the small size of the specimens used in this research.