Chloride threshold values to depassivate reinforcing bars embedded in a standardized OPC mortar

Abstract

The chloride threshold to develop active corrosion of the reinforcing steel does not seem to be a unique value and it depends on several factors, such as concrete mix proportions, cement type, C3A content of cement, blended materials, water/cement ratio, temperature, relative humidity, steel surface conditions and source of chloride penetration among others. Numerous studies have been already devoted to the study of the chloride threshold value for depassivation of the steel embedded in concrete. One of the reasons found for the scatter is the large number of variables that influence the chloride amount for depassivation. The other reason is the lack of accordance for the definition of the chloride threshold itself, either on the determining parameters (visual observation, corrosion potential or corrosion current) or on the expression of the threshold (as Cl−/OH− ratio or by weight of cement or concrete).The present paper presents chloride thresholds that were studied in mortar based on corrosion current measurements and expressed as total, free and Cl−/OH− ratio. For this study, mainly smoothed bars were used, but also some ribbed bars were tested. Chloride thresholds in the range of 1.24–3.08% and 0.39–1.16%, by weight of cement, for total and free chlorides, respectively, and in the range of 1.17–3.98 for Cl−/OH− ratio were found for chlorides admixed in the mixing water. Active corrosion is considered when, in a small exposed area, the corrosion rate of the rebar is higher than 0.1 μA/cm2. The threshold in the case of Cl−/OH− results a bit higher than that found in a previous work for synthetic pore solution, although the two types of data can be fitted together finding a good correlation.