Potentiostatic determination of chloride threshold values for rebar depassivation: Experimental and statistical study

Abstract

The presence of chlorides in concrete is the main cause of corrosion of reinforcements, responsible for most of the damages and cost losses attributed to the degradation of concrete structures. In spite of this, there are not enough data when defining chloride threshold determination. Most experimental work has been devoted to identify the variables involved in depassivation: type of cement, water/cement (w/c) ratio, pore humidity, type of steel, etc. Present situation considers a comparative value, in general, of 0.4% by weight of cement. In the present work, tests on mortar specimens were performed using several types of cements, covering a region of potentials from +250 to −650 mV (SCE) (+150 to −800 mV Mn/MnO 2). Series of identical specimens made with different types of cement were used. A statistical treatment of the results of chloride threshold is made. Results indicate that two regions of potential are identified: (1) The region of potentials more positive to −200 mV (SCE). The mean values of chloride threshold for 95% probability of corrosion vary between 0.497 ± 0.126 and 0.569 ± 0.177 for free chlorides and 0.632 ± 0.112 and 0.771 ± 0.346 for total chlorides. The lowest limits of chlorides for corrosion initiation, from a normal distribution (characteristic values) are 0.283% for free chlorides and 0.316% for total chlorides (wt.% of cement). Differences in chloride threshold with the type of cement are identified after the statistical study of the experimental results. (2) For potentials more negative to −200 mV (SCE) the experimental results fit well with a log-normal function. A mean value for total chlorides of 1.53%, with a variation coefficient of 0.36 is found. A mean potential for protection of −500 mV (SCE) could be identified.