Concrete Maturity Calculator (Nurse–Saul Equivalent Age)

What this tool does

Concrete gains strength with time and temperature together. The Nurse–Saul function adds up, hour by hour, how far the concrete was above a datum temperature: M = Σ (T − T₀) Δt in °C·h. Dividing by the difference between a reference temperature and the datum gives the equivalent age — the hours at the reference temperature that would have brought the same maturity. A datum of 0 °C is commonly used, but the right value depends on the mix and should come from calibration; the function is also only a rough guide at very low or high temperatures. Real strength still has to be confirmed by testing.

Formula

M = Σ (T − T₀) × Δt (°C·h, Nurse–Saul) · equivalent age = M ÷ (T_ref − T₀)

Variables

Symbol	Meaning	Unit
`a1`	Stage 1 — concrete temperature	°C
`h1`	Stage 1 — duration	h
`a2`	Stage 2 — concrete temperature	°C
`h2`	Stage 2 — duration	h
`a3`	Stage 3 — concrete temperature	°C
`h3`	Stage 3 — duration	h
`t0`	Datum temperature	°C
`tr`	Reference temperature to compare with	°C
`M`	Maturity index	°C·h
`EH`	Equivalent age at the reference temperature	h
`ED`	Equivalent age in days	d
`P28`	Share of a 28-day cure at the reference temperature	%

Worked example

Stage 1 — concrete temperature20 °C
Stage 1 — duration24 h
Stage 2 — concrete temperature12 °C
Stage 2 — duration48 h
Stage 3 — concrete temperature5 °C
Stage 3 — duration24 h
Datum temperature0 °C
Reference temperature to compare with20 °C

Maturity index1176 °C·h
Equivalent age at the reference temperature58.8 h
Equivalent age in days2.45 d
Share of a 28-day cure at the reference temperature8.8 %

Limitations

For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.