What this calculator does
Reinforcement corrodes when chloride reaches the steel. Whether it ever does is a diffusion question set by the surface concentration, the diffusivity and most of all the depth of cover. This tells you whether your cover survives the design life, and by how much.
How to use it
- Enter the actual cover to the reinforcement in the member being checked.
- Enter the surface chloride concentration and the diffusivity from your exposure class and mix — neither is looked up here.
- Enter the threshold chloride level for your cementitious system and reinforcement.
- Read the time to corrosion against the design life; the shortfall in cover is reported directly.
Governing formula
C(x,t) = C_s · erfc( x / (2√(D·t)) ) Where:
-
C_s= Surface chloride concentration -
x= Depth to the reinforcement, mm -
D= Chloride diffusivity, mm²/year -
t= Time, years
Derived Equations:
t = x² / (4·D·[erfc⁻¹(C_thr/C_s)]²) Worked example — 45 mm cover, 10 mm²/yr, 50-year life
The figures below were produced by running this page's own engine with the example inputs (Cover to reinforcement (mm), Surface chloride concentration (% by cement weight), Chloride diffusivity (mm²/year), Corrosion threshold (% by cement weight), Design life (years)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Time to corrosion at this cover | 142.94 years |
|---|---|
| Chloride reaches the steel within the design life | — |
| Chloride at the steel after the design life | 0.15 % |
| Cover needed for the design life | 26.61 mm |
| Cover margin (negative = short) | 18.39 mm |
| What this means | At 45 mm the steel stays below the threshold for about 143 years, against a 50-year design life. The cover has roughly 18 mm in hand. |
- After 50 years the steel sees well below the 0.4 % threshold at 45 mm.
- Doubling the cover would quarter the time to corrosion, which is why cover matters so much.
Assumptions & limitations
Engineering calculation. Fickian chloride diffusion from a surface concentration to the reinforcement, reporting the time to corrosion at the cover you have.
- Surface concentration and diffusivity are YOUR inputs. This tool selects no exposure class, mix design, cement content, w/c ratio or curing regime.
- Chloride ingress is dominated by wet/dry cycles. A single constant diffusivity assumes a uniform saturation this tool cannot verify, so the time-to-corrosion is indicative.
- The corrosion threshold you enter depends on the cementitious system and the reinforcing steel. This tool does not claim a code threshold value.
- Diffusion alone ignores carbonation, binder reaction capacity, cracking, and any surface coating or hydrophobic treatment. It is a screening check, not a durability design.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 12390-9 and the fib Model Code — chloride ingress models, as the framework behind the inputs.