DETERMINATION OF CHLORIDE CONTENTS IN CONCRETE
By Potentiometric Titration
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.
Two complete workstations of 751 GDP Titrino with 730 Sample Processor each equipped with 5 mL exchange units, Ag-electrodes, 728 magnetic stirrers, and 702 rod stirrers for small to large volume chloride titration of up to 24 samples simultaneously. Titration is fully automated and report is generated by Brinkmann Titrino Workcell software.
848 Titrino Plus with 5 mL exchange unit, 801 magnetic stirrer, and pH or metal electrode for small volume potentiometric and acid-base titration with automated data acquisition in tiBase database software
841 Titrando with 803 stirrer stand and 20 mL 800 Dosino for Karl-Fisher Titration and 700 Dosino for potentiometric titration – the entire setup is controlled by the Metrohm's classic tiamo software
916 Ti-Touch compact latest Titrator attached to 5 mL 800 Dosino, and 814 USB Sample processor for single to 12-sample chloride titration with automated data acquisition to the Metrohm's database tiBase platform
HOW WE DO CHLORIDE ANALYSIS
CHLORIDE PROFILE IN A CONCRETE CORE
Acid-Soluble and Water-Soluble Chloride Contents of Concrete According to ASTM C 1152 and C 1218, respectively
Determination of chloride profile at various depths of a drilled concrete core from a bridge deck or parking garage slab is a commonly requested test that is done regularly in the CMC laboratory.
Shown here are: (a) the partial saw-cut sections for chloride analyses taken from the top, mid-depth, and bottom locations of a concrete core, and (b) the potentiometric titration curves of chloride analyses at the respective depths shown beside the core. Chloride contents at all depths are shown both in the data from the Metrohm Titrators as well as in red. Equivalence points of titration are marked as circles on the titration plots.