Reinforced concrete structures constructed according to the required specifications and standards for an intended application usually show good durability and long-term performance throughout its intended their service life. Deterioration of concrete may occur due to a number of mechanisms, especially when concrete is exposed to potentially deleterious chemical agents, e.g., chloride, and carbon dioxide, which may penetrate down to the level of reinforcing steel to cause corrosion of steel. Corrosion of steel in concrete is the most destructive form of damage that affects serviceability and strength of concrete structures. The inherent alkaline environment of Portland cement concrete normally passivates the steel reinforcement in which a tenacious oxide passive film is formed on the surface of embedded reinforcing steel, which protects steel reinforcement against corrosion.
The condition of passivity can be destroyed due to penetration of chloride ions, which are considered as the most aggressive and most widespread corrosive ion since it contributes to corrosion of steel reinforcement. Chloride acquires different forms in concrete which include chemically bound, physically absorbed, and free chloride, of which free chloride is responsible for rebar corrosion in concrete. Chlorides may be incorporated into the concrete through mixing water, chloride contaminated aggregates, or chloride containing admixtures, or, most commonly, from applications of chloride-containing deicing chemicals for concretes exposed to severe weather environments. Therefore, determination of chloride contents in concrete is important for assessment of potential for chloride-induced corrosion of steel in concrete, and subsequent corrosion-related distress.
ASTM C 1152 test for nitric acid-soluble chloride content in concrete, which corresponds to the total chloride content contributed from all the solid and liquid ingredients of concrete as well as chlorides from external sources, and, ASTM C 1218 test for water-soluble chloride content in concrete, which is directly responsible for the chloride-induced corrosion of steel in concrete are two most common standardized test methods that are followed for determination of chloride contents in reinforced concrete structures.
CMC uses both test methods where filtrates from nitric acid or deionized water-digested pulverized concrete samples are subjected to potentiometric titration with silver chloride titrant to determine the chloride contents. Set-ups for chloride content tests are fully automatic where numerous samples can be run simultaneous using advanced titrators from Metrohm. Metrohm has a wide range of titration systems that provide answers to critical chemical parameters such as acid number, chloride content, and total hardness among thousands of others. Metrohm offers a comprehensive range of stand-alone and automated titration products, featuring premium quality sensors and backed by unparalleled applications know-how.
Along with potentiometric titration, chloride contents are also determined by ion chromatography on water-digested filtrates according to the procedures of ASTM D 4327. CMC has used both titration and ion chromatography methods for water-soluble chloride contents on numerous samples for a wide range of chloride contents to find excellent correlations of results from both methods. Ion chromatography has an aided advantage of getting not only chloride but water-soluble sulfate, nitrate, nitrite, and other anion contents as well on the same sample run.