Carbonation depth matters because once carbonation reaches the reinforcement, the steel's protective passive film breaks down and the bars become prone to corrosion. The method is classed as semi-destructive because it requires a freshly broken concrete surface, and it is part of structural inspection (NDE). This article explains the principle, the site procedure, how to read the results and how to use them to assess durability.

Reading a carbonation test with phenolphthalein: comparing carbonation depth with concrete cover The two left panels show freshly broken concrete surfaces after spraying with phenolphthalein. The top edge is the face exposed to air and carbon dioxide. The uncolored band from the surface inward is the carbonation depth dk, and the deeper pink zone is still alkaline. In the first panel dk is less than the cover c, so the reinforcement is still in the pink zone and remains protected. In the second panel dk reaches or exceeds the cover, so the bar sits in the uncolored zone, the passive film breaks down and the steel becomes prone to corrosion. The right panel is a pH scale: normal concrete is about 12 to 13, and phenolphthalein changes color at about pH 9; below that it stays colorless. The bottom strip says to spray on a freshly broken surface, measure dk at several points and compare with the cover, per EN 14630 or RILEM CPC-18. Reading carbonation: compare dₖ with cover Not to scale dₖ short of steel dₖ c CO₂ from air Gray = no color (carbonated) Pink = still alkaline Steel still protected dₖ at steel (corrosion risk) dₖ c CO₂ from air Gray = no color (carbonated) Pink = still alkaline Steel at corrosion risk pH 8 9 10 11 12 13 Normal concrete pH ≈ 12–13 ≈ 9 color change of phenolphthalein Passive film on steel lost 1 Spray fresh break Old surface misleads 2 Measure dₖ at many pts Report mean & max 3 Compare with cover EN 14630 · RILEM CPC-18
Reading principle: areas that are still alkaline turn pink, while carbonated areas do not change color — if the carbonation depth (dₖ) approaches or exceeds the concrete cover, the reinforcement is at risk of corrosion

How does carbonation happen?

Carbonation occurs when carbon dioxide (CO₂) in the air penetrates concrete and reacts with calcium hydroxide to form calcium carbonate, lowering the pH. Normal concrete is highly alkaline (pH about 12–13), which helps form a thin passive film that protects the reinforcement from rusting.

When the pH drops at the level of the reinforcement, the protective film breaks down and the steel corrodes easily in the presence of moisture and oxygen. Carbonation is therefore one of the main causes of deterioration in reinforced concrete, especially in cities with high CO₂ levels.

Carbonation advances slowly from the surface inward and slows down further over time. Porous, poorly cured or cracked concrete carbonates faster than dense concrete, and members with small cover reach the steel level first.

📚 Basic principles

Carbonation does not directly weaken concrete; it destroys the "alkaline condition" that protects the reinforcement. The real risk is corrosion of the steel, not loss of concrete compressive strength.

What are the site steps for a carbonation test?

The site test has 6 steps. The key is to spray the indicator on a freshly broken surface immediately, before that surface has been exposed to air for long.

  1. Scan for rebar and measure cover — use a cover meter to choose breakout points between bars and obtain the cover (c) for comparison
  2. Expose a freshly broken surface — chip out the concrete locally, or split a core from a Concrete Core Test lengthwise
  3. Clean dust from the broken surface — brush or blow the dust off; do not wash with water
  4. Spray phenolphthalein immediately — observe the boundary between the pink and uncolored zones
  5. Measure the depth at several points — measure from the outer surface to the color boundary, record the mean and maximum, and take photos
  6. Repair the test spot — patch neatly with repair mortar

How do you read carbonation test results?

The result is read from the color of the concrete surface after spraying phenolphthalein, as follows:

  • Pink/magenta = concrete is still alkaline (high pH), not yet carbonated
  • No color change (clear) = concrete has carbonated (pH has dropped below about 9, the range where phenolphthalein changes color)

Carbonation depth (dk) is the distance from the concrete surface to the boundary where pink begins. It is measured with a ruler at several points and averaged, then compared with the concrete cover (Cover, c), giving three cases:

Measured resultMeaningGeneral guidance
dk much less than cCarbonation has not reached the steel; passive film intactMonitor at regular inspection intervals
dk close to cApproaching steel level; risk startingPlan surface protection and monitor more frequently
dk greater than cPassive film lost; steel at risk of corrosionFurther corrosion assessment, e.g. Half-Cell Potential and exploratory opening
⚠️ Points to watch

Spray only on a "freshly broken" surface, because an old surface exposed to air for a long time is already carbonated at the face and gives misleading readings. Measure at several points because the depth is not uniform across the section.

The figures below summarize the reading above: a 2D section showing the boundary between the pink and uncolored zones, multi-point measurement and averaging, and the three cases relative to the cover, plus a 3D model of the site procedure from rebar scanning to surface repair.

2D model (section) of reading carbonation depth with phenolphthalein on a freshly broken surface, compared with the concrete cover Section through concrete at an exposed face. Carbon dioxide penetrates from the surface into the concrete. On the freshly broken surface sprayed with phenolphthalein, the carbonated outer layer stays colorless while the still-alkaline core turns pink. The boundary between the two is irregular, so the depth from the surface to the pink line is measured at several points and averaged. Example of five points: 18, 22, 25, 20 and 15 millimeters, average 20 millimeters, compared with a cover of 30 millimeters. The right panel shows carbon dioxide reacting with calcium hydroxide to form calcium carbonate, and a pH scale where normal concrete is about 12 to 13 and the indicator turns pink above about pH 9. The bottom row compares three cases: carbonation depth much less than the cover, close to the cover, and beyond the cover so that the passive film is lost and the steel starts to rust. Values are hypothetical. Concrete surface (2D Section) Not to scale c Numbers = dₖ (mm) CO₂ enters from surface 18 22 25 20 15 Pink = still alkaline No color = carbonated (top layer) Rebar · cover c = 30 mm Mean dₖ = (18+22+25+20+15) ÷ 5 = 20 mm vs c = 30 mm → not at steel (hypothetical) Why lower pH = steel risk CO₂ + Ca(OH)₂ → CaCO₃ + H₂O Ca(OH)₂ consumed → pH drops 7 8 9 10 11 12 13 Changes at ~pH 9 Clear (carbonated) Pink (alkali 12–13) 1 Passive film needs alkalinity Carbonation at steel → film lost 2 Risk is rebar corrosion Not lower concrete strength 3 Spray on “fresh break” only Old face already carbonated dₖ << c Not at steel yet Monitor at regular inspection intervals dₖ ≈ c Near steel level Risk starts · plan protection dₖ > c Passive film lost Corrosion risk · assess more
2D model (section) — carbon dioxide penetrates from the surface; the carbonated outer layer stays colorless while the still-alkaline core turns pink. The boundary is irregular, so measure at several points and average. The example gives a mean dₖ of 20 mm versus a cover of 30 mm. The bottom row compares three cases. Values are hypothetical (image: SPN Soil Engineering)
3D model of an on-site carbonation depth test, from rebar scanning, breaking out a fresh surface, spraying the indicator and multi-point measurement to surface repair 3D view of a reinforced concrete column on a floor slab. The front face is marked with vertical bar and tie locations from a cover meter scan. At test point A, a small pocket is chipped out between the bars to expose a fresh surface; after spraying phenolphthalein the core is pink and the outer rim is uncolored, and the depth is measured with a ruler at several points. Point B on another face of the column has already been repaired. The right panel shows a core split lengthwise and sprayed with indicator, with the surface end uncolored, and the six-step site sequence: scan for rebar and measure cover; chip or split a core to get a fresh surface; spray the indicator immediately; measure at several points and average; compare with the cover; and repair the test spot. 3D model Not to scale Point A: fresh break Between bars · spray at once Rim uncolored · core pink Ruler dₖ in several directions Spray indicator Cover meter Mark bars and measure c first Vertical bars/ties From scan Point B: repaired Mortar after test Split core dₖ Surface end on top Split lengthwise then spray at once Site sequence 1 Scan rebar + measure c Cover meter / GPR 2 Chip or split core for a fresh break 3 Spray indicator before long air exposure 4 Measure dₖ at many pts then average 5 Compare dₖ with c Near or over = risk 6 Repair test spot Patch the hole neatly
3D model — scan for rebar and measure cover first, chip between bars to expose a fresh surface and spray the indicator immediately, or split a core lengthwise; measure at several points, compare with the cover, then repair the test spot (image: SPN Soil Engineering)

What is a carbonation test used to assess?

A carbonation test is used to assess the corrosion risk of reinforcement and the durability of a structure. Common applications include:

Measuring carbonation depth at different times helps estimate the rate of progression, but concluding the remaining service life must be done by an engineer based on engineering principles and the context of the structure.

What are the limitations of a carbonation test?

The main limitations are that it is semi-destructive and only shows the alkaline/non-alkaline boundary. Points to be aware of:

  • Requires a freshly broken surface — so every test spot must be repaired afterwards
  • Does not give an exact pH value — phenolphthalein only shows whether the pH is above or below its color-change range
  • Results are local — depth varies with moisture, surface condition and concrete quality in each area, so measure at several points
  • Does not cover chloride-induced corrosion — coastal structures may have rusting rebar due to chlorides even when carbonation is still shallow, so chlorides must be tested separately
✅ Getting the most out of it

The carbonation test is fast and low-cost, well suited as a first screen for corrosion risk, followed by cover measurement and in-depth corrosion assessment only at locations where risk is found.

SPN provides carbonation depth testing with reports certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number.