Qualitative determination of the penetration of chloride ions

Chloride occurs in seawater or in de-icing salts. The structures that can be affected to the degradation action produced by chloride are therefore marine structures and road, motorway and airport infrastructures, and paving in general.
In order 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 a 95% alcohol solution) and the other containing silver nitrate at an aqueous concentration of 17 g/l.
By spraying the fluorescein solution and the silver nitrate solution alternately onto a flat 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, gives the conglomerate a variable colour, from pale pink to very light brown, depending on the case. In the areas where the chloride has not penetrated, the silver nitrate oxidises and takes on a colour that varies from dark brown to black.

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