Cover, formwork and curing
Concrete strength gain with age and temperature (Eurocode 2)
- where
- temperature-adjusted age (days)
- age of concrete (days)
- average concrete temperature (°C)
- strength ratio at that age
- cement class coefficient
- mean 28-day strength (MPa)
- 28-day characteristic cylinder strength (MPa)
- mean strength at age t (MPa)
- estimated characteristic strength at age t (MPa)
Explain this equation
Estimates how strong concrete is at a given age and curing temperature, using the Eurocode 2 strength-gain curve. Use it as a guide for striking formwork or loading early.
Concrete gains strength faster when warm and slower when cold. The first equation turns the real age into an equivalent age at 20 °C. The strength ratio β_cc follows a curve that reaches 1 at 28 days; the coefficient s is smaller for rapid-hardening cement. The mean strength is taken as f_ck + 8 MPa; the estimate then takes 8 back off and never exceeds f_ck.
An empirical maturity curve from Eurocode 2.
Sense check: C30 concrete with normal cement at 7 days and 20 °C reaches about 78% of its mean strength, roughly 22 MPa characteristic. At 5 °C it would be noticeably lower.
Watch out: this is an estimate only. Strength for critical decisions, and anything at 3 days or less, should come from cube or cylinder tests.
Estimates how strong concrete is at a given age, allowing for the cement type and the curing temperature, for deciding when to strike formwork or load a slab.
Enter your values to see the result.
Good to know
Eurocode 2 gives the mean strength at age t as a fraction βcc of the 28-day mean strength, with faster cements (class R) gaining strength sooner. Temperature is allowed for by a maturity-type temperature-adjusted age: at 20 °C it equals the real age, while at 10 °C concrete gains strength at roughly two-thirds of the rate. The characteristic strength before 28 days is estimated as the mean minus 8 MPa. Keep concrete moist and protected from frost while it cures (BS EN 13670 sets curing periods).
Common mistake
Striking formwork or backpropping on a calendar date in cold weather, when the concrete may be days behind its 20 °C strength.
Limits
An estimate assuming normal curing and that the concrete meets its 28-day specification. Use cube or cylinder results, or a maturity meter in the pour, for critical decisions such as striking soffits or prestressing. Temperatures 0–80 °C.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.