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.
Example
You enter
- Specified 28-day strength f'c (psi) 4000
- Concrete age (days) 7
- ACI 209 constant a 4
- ACI 209 constant b 0.85
- Target % of f'c (0 = none) 75
You get
- Fraction 0.703518
- Developed strength 2,814 psi
- Age to reach target 8.3 days
Checked against published worked examples from: American Concrete Institute (ACI Committee 209).
Details, formula, and sources
ScopeThe 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 or tablefraction = 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.
Source and editionACI 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.
Free accessThe strength-development model is published in ACI 209R; the arithmetic is public. The constants are the ACI 209 values for the cement type.
What governsEstimate. AHJ and licensed professional govern.
API field namesfc28, age_days, a, b, target_pct, fraction, fc_t, target_age
- Developed fraction t / (a + b t) lands near 46% at 3 days, 70% at 7, and 88% at 14 for Type I moist-curedACI 209R
- Inverse solve the age to a target percent is (a x f) / (1 - b x f) for f = target/100 below the 1/b asymptoteACI 209R
- Estimate only the spec and cylinder breaks govern the strip; cold weather slows the real gainASTM C1074