Suitability of the measurement techniques of oxygen permeability in order to predict corrosion rates of concrete rebars

Synopsis: The air or oxygen permeability of concrete is usually measured by means of techniques which utilize mechanical driving forces. Thus, air or oxygen is forced to pass through a piece of concrete using different mechanical pressures. The flow of gas so measured is used as an indication of concrete permeability and sometimes is also used to predict the durability of concrete reinforcements based in the relationship between anodic corrosion rate and amount of oxygen which may be reduced in the cathodic areas. However, this extrapolation may lead to erroneous conclusions because a dry concrete allows a higher amount of oxygen to pass through it than a wet one, although the corrosion rate should be much lower in dry than in wet concrete. In the present paper, comparisons between flow of oxygen measured in paste, mortar and concrete specimens held at different relative humidities using electrochemical driving forces (polarization at about -750 mV SCE) and corrosion rates (measured by means of the Polarization Resistance) are presented in order to discuss the relationship between both. The results show that the oxygen permeability is only dependent of the amount of electrolyte inside the pores, but the corrosion rate is also dependent of the concrete resistivity which is fixed by the amount of pore water content.