Relationships between free and water-soluble chloride concentrations in concrete

Corrosion of metals embedded in concrete is pivotal when studying durability of reinforced concrete structures. Chloride ions present in the vicinity of steel reinforcements tend to initiate and/or accelerate their corrosion. Chloride ions are either bound physically or chemically to the cementing material or free in the pore solution, and only the latter can be transported. Free chloride concentration Cf was measured by pore solution expression method however it is found to be very difficult in low water to cement ratios. Water extraction method is used to measure water-soluble chloride concentration Cw in concrete. However, the results are found to be highly dependent on liquid to solid ratio, temperature, size of particles, and extraction method. The aim of this study is to experimentally evaluate current standard test method for acid-soluble chloride (ASTM C1152-04 (2012a)) and water-soluble chloride (ASTM C1218-15 (2017)) in mortar and concrete, and the relationships between Cw and Cf. Six concrete mixes containing Portland-limestone (CSA type GUL) cement with and without supplementary cementing materials including silica fume or ground granulated blast-furnace slag were subjected to a non-steady state migration experiment. The acid-soluble and water-soluble chloride concentrations at different depths within the sample were determined at 84 days. Freundlich isotherm was employed along with total chloride concentration data to determine the concentration of free chloride Cf values. Linear relationship between Cw and Cf was observed with a significant dependency on the binder composition. Measurements of water-soluble chloride concentration are not equivalent to free chloride concentration.