What is the Phenolphthalein Test?

The phenolphthalein test is a simple, immediate site test for how deeply concrete has carbonated. Phenolphthalein indicator solution is sprayed onto a freshly exposed concrete face: sound, alkaline concrete turns bright pink or magenta, while carbonated concrete stays colourless. The depth of the colourless zone, measured in from the surface, is the carbonation depth. It is the quickest way to establish whether the protective alkalinity around the reinforcement has been lost. In more detail: new concrete has a pore-water pH of around 12.5 to 13, and at that alkalinity the steel reinforcement carries a passive oxide film that stops it corroding. Atmospheric carbon dioxide reacts with the calcium hydroxide in the cement paste to form calcium carbonate, which drops the pH to below about 9. Phenolphthalein changes colour across roughly pH 9 to 10, so the boundary between pink and colourless is a direct proxy for the carbonation front. Once that front reaches the steel, the passive film breaks down and corrosion begins — which is why the number that actually matters is not the carbonation depth on its own but the carbonation depth compared with the depth of cover. The test is covered by BS EN 14630, 'Products and systems for the protection and repair of concrete structures — determination of carbonation depth in hardened concrete by the phenolphthalein method'. In practice: break out a small area or drill and split a core so you have a genuinely fresh fracture face, brush or blow the dust off (dust carbonates almost immediately and will read as a false positive), spray the indicator from a hand atomiser, and read the colour change within a minute or two before the surface dries. Measure from the concrete face to the colour boundary with a depth gauge or steel rule at several points across the face, because the front is rarely straight — it runs deeper along cracks, at blowholes and through poorly compacted concrete. Record the minimum, mean and maximum. On the same visit, take cover meter readings over the same area so the two depths can be compared directly. The standard indicator solution is around 1% phenolphthalein in ethanol, or in an ethanol-and-water mix. It is an irritant and the solvent is flammable, so it needs gloves, eye protection and a COSHH assessment like any other site chemical. The limits are as important as the result. The test tells you about pH and nothing else — it will not detect chloride contamination, which attacks the passive film at high pH and is the mechanism on car park decks, coastal structures and anything that has been salted. It cannot be read on a weathered or dusty surface, or on a drill-dust sample, and a wet face will bleed the indicator and blur the boundary. It is also a one-off snapshot: carbonation advances roughly in proportion to the square root of time, so one reading plus the age of the structure gives a crude rate from which remaining service life can be estimated, but two readings years apart are far better. Where results matter structurally, phenolphthalein testing is normally paired with a cover meter survey, chloride sampling by profile, half-cell potential mapping, and cores for strength. We use the phenolphthalein test on almost every concrete repair survey, because it costs minutes and it changes the specification: if the carbonation front has passed the steel, the repair has to deal with corroding reinforcement and the surrounding concrete usually needs an anti-carbonation coating to BS EN 1504-2; if it has not, a cosmetic or protective treatment may be all that is justified.