The C1⁻/OH⁻ ratio in chloride-contaminated concrete — a most important criterion

Chloride-contaminated concretes were produced either by admixing sodium chloride with original mixes or as a result of exposure to external sodium chloride solutions. Free chloride and hydroxyl ion concentrations in the pore solutions of the concretes were determined. It was found that the value of free chloride content by itself cannot provide a meaningful indication of the susceptibility of the reinforcement to chloride-initiated corrosion. The ratio of free chloride to hydroxyl ion concentrations in the pore solution and differentiation between admixed and externally applied chloride are found to be factors that must be known if indicative values to predict corrosion are sought. Charts giving safe regions of C1⁻/OH⁻ values for various concretes are suggested. The use of a superplasticizer in concrete making was found to increase the concentration of free chloride and to raise the value of Cl⁻/OH⁻ ratio in all the concretes tested. The use of fly ash was found beneficial in immobilizing the chloride ions in non-superplasticized concrete. However, in concrete made with a superplasticizer the inclusion of fly ash was found to cause a greater release of chloride ions into the pore solution.