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How accurate is the 28-day strength prediction from 7-day tests?

For ordinary Portland cement concrete cured at standard conditions, the 7-day to 28-day strength ratio is typically 0.65-0.70. The ACI 209 model provides a reasonable estimate, but actual results vary with cement type, admixtures, and curing temperature. Always confirm with actual 28-day tests.

What is the ACI 209 maturity function?

ACI 209 predicts strength gain as f(t) = f(28) × (t / (4 + 0.85t)) for moist-cured concrete. This model captures the rapid early strength gain and the gradual approach to ultimate strength.

Can I use this for high-performance concrete?

The standard ACI 209 model is calibrated for ordinary Portland cement concrete. For high-performance concrete with silica fume, fly ash, or slag, the strength gain curve differs. Consult ACI 363R for high-strength concrete guidance.

Concrete 28-Day Compressive Strength Predictor Calculator

Early-Age Strength (MPa)

Early Test Age (days)

Target Age (days)

Prediction Method

Uses the ACI 209R maturity model for moist-cured concrete: f(t) = f(28) × (t / (4 + 0.85t)). The calculator back-calculates the expected 28-day strength from the early-age test result by reversing the maturity ratio. Results are estimates — always confirm with actual 28-day cylinder tests.

Limitations

This model assumes standard moist curing at 20-23°C. Accelerated curing, different cement types (Type III, slag, fly ash), or temperature extremes will shift the strength gain curve. For critical decisions, rely on actual 28-day test results per ACI 318 / BS 1881.