Concrete Age-Strength Gain for Form Stripping (ACI 209)
How much of its specified 28-day strength a slab has reached at a given age, the ACI 209R fraction t / (a + b t) times f'c - about 46% at 3 days, 70% at 7, 88% at 14 for Type I moist-cured - plus the inverse solve for the age to hit a target percent (commonly 75% to pull shores). The schedule decision behind every form-strip and shore-removal. An estimate of the mean trend, not a substitute for field-cured cylinder breaks; the engineer of record and the spec govern the actual strip strength.
Formula and source
fraction = age_days / (a + b x age_days); fc_t = fraction x fc28; target_age = (a x target_pct/100) / (1 - b x target_pct/100) when a target percent is given.
ACI 209R strength-development model f'c(t) = [t / (a + b t)] x f'c(28), by name; the constants are for the cement type and curing named.
Audience
This tile is built for trades and the adjacent professions in the Carpentry and Construction group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.