Analysis of the relation between water and resistivity isotherms in concrete

Abstract

Concrete is a porous material, that contains an alkaline solution, whose pore network enables mass transport from the exterior. The concrete as composite material is quasi an insulator when it is dry and presents relatively low resistivities, in the order of few kΩ cm when it is fully saturated. Concrete resistivity reflects the degree of saturation and is therefore a useful indicator of the risk of chemical attack and of reinforcement corrosion. The dependence of resistivity on the degree of saturation has been studied, yet this subject has still not been fully elucidated. The present paper studies the evolution of concrete drying from the end of curing in order to analyze the water and resistivity isotherms. Four concrete mixes (w/c = 0.4 and 0.7 after 3 and 7 days of curing wrapped in plastic sheets) have been prepared and submitted to conditions of isothermic water desorption in controlled atmospheres with relative humidity (RH) of 55, 65, 75, 85, 95%. Resistivity measurements were simultaneously carried out which enabled to identify for the four mixes the water content and the RH which gives resistivity values higher than 105 Ω cm. The radii of water-filled pores calculated from Kelvin's law were found to be smaller than those calculated by integrating the mercury intrusion porosimetry (MIP) accumulated intrusion curve with the water lost during conditioning at different RHs. The water content measurement that seems to be most adequate for comparison with resistivity values is the volumetric fraction.