Concrete Carbonation Testing: Methods and Interpretation

Carbonation is the slow chemical process that strips concrete's alkaline protection from reinforcement. Testing reveals how far it's progressed and predicts when reinforcement corrosion will start.

The mechanism

CO₂ in air reacts with calcium hydroxide in cement to form calcium carbonate. Alkalinity drops from pH ~13 to pH ~8. Once pH drops below ~10, embedded steel is no longer passively protected and corrosion begins.

The phenolphthalein test

Simple, on-site, indicative:

1. Fresh concrete surface exposed (break out a section)
2. Spray phenolphthalein indicator
3. Alkaline (uncarbonated) concrete turns pink
4. Carbonated concrete remains uncoloured
5. Measure depth of colourless zone

Carbonation rate

Rule of thumb: depth proportional to √time. A building 20 years old with 5mm carbonation: at 80 years, expect ~10mm. Reach cover depth = start of corrosion problems.

Interpretation

• Carbonation depth less than cover: protection intact
• Carbonation depth approaching cover: imminent corrosion expected
• Carbonation depth equals or exceeds cover: corrosion likely active

Remediation options

• Surface inhibitor treatment (early stage)
• Realkalisation (electrochemical, expensive)
• Patch repair where spalling has started
• Cathodic protection (long-term)

Lab testing alternatives

• Thermogravimetric analysis
• X-ray diffraction
• Used for definitive analysis on critical structures

When to test

• Buildings 30+ years old
• Before refurbishment planning
• When spalling visible
• For life-extension assessments

MCFAR commissions carbonation testing as part of concrete assessment.

Frequently Asked Questions