CONCRETE
Carbonation Depth & Carbonation of Concrete
Depth from concrete surface to which atmospheric CO₂ has neutralized the alkalinity of the cement paste — drives rebar corrosion timeline once it reaches the cover.
Also calledcarbonation depthconcrete carbonationcarbonation testphenolphthalein testcarbonation rate
Related on InfraLens
Definition
Carbonation is the slow reaction of atmospheric CO₂ with calcium hydroxide in cement paste, forming CaCO₃ and dropping pH from ~12.5 to ~8.5. Below pH 9.5 the passive layer protecting rebar dissolves, triggering corrosion. Measured by spraying a fresh-broken concrete face with phenolphthalein indicator — pink = uncarbonated (high pH), colourless = carbonated. Rate follows √t law: depth = k × √age, where k ≈ 1-3 mm/√year for good-quality concrete, up to 6-8 mm/√year for porous concrete.
Typical values
Carbonation rate — M40 dense1-2 mm/√year
Carbonation rate — M25 normal2-3 mm/√year
Carbonation rate — porous concrete5-8 mm/√year
Threshold for rebar attackdepth ≥ cover
Typical service life — moderate exposure50-80 years
Where used
- Service-life assessment of existing structures
- Cover specification for new builds
- NDT-based residual-life estimation
- Repair-strategy selection
Acceptance / threshold
Acceptable when carbonation depth < concrete cover and structure is past its design life; not acceptable if carbonation depth ≥ cover (immediate corrosion risk).
Site example
25-year-old building, cover = 30 mm, phenolphthalein test shows 22 mm carbonation. Implied rate k = 22/√25 = 4.4 mm/√yr. Carbonation reaches cover at age (30/4.4)² = 47 years. Plan corrosion-control intervention before age 40.
Frequently asked
What is concrete carbonation?
Slow reaction of atmospheric CO₂ with cement paste, lowering pH from 12.5 to 8.5. When carbonation depth reaches rebar cover, the passive protection breaks down and rebar starts corroding.
How to test carbonation depth?
Spray phenolphthalein indicator on a freshly broken concrete face. Pink zone = uncarbonated (high pH); colourless = carbonated. Measure colourless depth from surface.
Related concrete terms
Concrete Mix Design
Proportioning of cement, water, aggregates per IS 10262
Concrete Grades (M15-M80)
M20=20 N/mm² characteristic strength at 28 days
Water-Cement Ratio (w/c)
Mass of water to mass of cement. 0.40-0.55 typical.
Concrete Cover
Min concrete from rebar to surface (15-50mm per exposure)
Concrete Curing
Min 7 days for OPC, 10 days for PPC, 14 days for sulphate-resistant
Slump Test / Workability
25-150mm slump per application (IS 1199)
Cube Test (Compressive Strength)
150mm cube compressive strength test at 7/28 days (IS 516)
Concrete Admixtures
Plasticizers, accelerators, retarders per IS 9103