Qualitative determination of the penetration of chloride ions

Chloride occurs in sea water or in de-icing salts. The structures that can be subject to the degradation action produced by chloride are therefore marine structures and road, motorway and airport infrastructures, and paving in general. To determine the thickness of concrete penetrated by chloride, a colorimetric method is used, based on the use of two solutions: one based on fluorescein (1 g/l of 95% alcohol solution) and the other containing silver nitrate in aqueous concentration equal to 17 g/l. By alternately spraying the fluorescein solution and the silver nitrate solution on a plain surface parallel to the axis of the extracted concrete core (split under a compression machine), the penetration front of the chloride ions is identified. In the areas penetrated by the chloride, the silver nitrate transforms into silver chloride and thanks to the presence of fluorescein it gives the conglomerate a variable colour, depending on the case, from pale pink to very light brown. In the areas of concrete not penetrated by the chloride, however, the silver nitrate oxidizes and takes on a colour varying from dark brown to black.

Reference standards: UNI 9944:1992; UNI 11747:2019