Violet: alkaline concrete
The indicator turns violet where the cement paste is still alkaline. The passive film on the reinforcement is intact at this depth.
Colourless: carbonated concrete
No colour change means carbonation has lowered the pH. The distance from the outer face to the colour transition line is the carbonation depth, in millimetres.
The protocol
Fresh surface. Phenolphthalein at 1% in alcoholic solution, applied to a freshly drilled or cored surface
Cover measurement. A pachometer (cover meter) locates the bars and measures the concrete cover
The comparison. Carbonation depth read against cover depth, per the LNEC E391 / E465 methodology
Carbonation front short of the bars: protection remains, monitor over timeDEPTH < COVER
Front at or beyond the bars: steel already depassivated, corrosion runs whenever moisture reaches itDEPTH ≥ COVER