ToolsFabric SurveyConcrete Testing

Carbonation Depth Calculator

Free online carbonation depth recorder and service life calculator. Estimates residual service life using BRE IP 6/81 √t model. Built to BS EN 14630.

Last updated 17 March 2026

About this tool

Assess carbonation depth in reinforced concrete using the classic square-root-of-time diffusion model (BRE IP 6/81).

Enter phenolphthalein indicator test readings from site, and the tool calculates the carbonation rate coefficient, estimates when the carbonation front will reach the reinforcement, and gives a pass/fail assessment against the specified cover depth.

Ideal for condition surveys, residual life assessments, and maintenance planning of ageing concrete structures.

How to use this tool

Enter cover depth — the nominal concrete cover to reinforcement in mm.
Enter carbonation readings — comma-separated depths (mm) from phenolphthalein indicator tests on drilled or broken surfaces.
Enter structure age — the age of the structure in years at the time of testing.
Review results — the tool outputs mean and max carbonation depth, rate coefficient (k), estimated time to reach rebar, residual service life, and a pass/fail verdict.

Technical information

The tool uses the well-established square-root-of-time diffusion model from BRE IP 6/81 (Currie, 1986):

x = k × √t where x = carbonation depth (mm), k = rate coefficient (mm/√year), t = time (years).

This relationship derives from Fick's first law of diffusion — CO₂ penetrates an ever-thickening carbonated layer, producing the characteristic square-root relationship.

Assumptions

Constant environmental exposure over the structure's life.
Carbonation front is treated as reaching rebar when mean depth equals cover depth.
Residual life considers the initiation period only — the propagation period before structural damage is not included.

Limitations

The √t model assumes steady-state diffusion and may overestimate carbonation in sheltered or coated concrete.
Phenolphthalein readings can be affected by admixtures, rendering, and surface coatings.
Should be used alongside other condition survey data (e.g. half-cell potential, chloride profiling) for a complete durability assessment.

Revision history

17 March 2026: Initial release

Disclaimer

This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.

Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.

All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.

By using this tool, you accept full responsibility for how you interpret and apply the information provided.

Spotted an error? Let us know.

Cover Depth (mm)

Carbonation Depth Readings (mm)

Enter comma-separated phenolphthalein test readings

Age of Structure (years)