Concrete gains strength by chemical reaction, and the reaction follows temperature. At 5 degrees C a pour takes roughly twice as long to reach a given strength as it does at 20 degrees C — but it does get there.
Maturity rather than elapsed time is what sets strength, which is why the familiar 70 per cent at seven days is a British answer at around 10 degrees C. At 20 degrees C you reach it in under five days; through a cold snap it can take a fortnight. Striking formwork or loading a slab against the calendar instead of the thermometer is how early-age failures happen.
Below freezing it stops being a delay and becomes damage. Water that freezes before the paste has set expands and disrupts it permanently, and no amount of later warmth recovers that strength. The rule is about cold, not about frost.
At 10 °C the rule says 7 d and the exact answer is 7.132 d — 1.9% low. It holds to within 15% from 7.3 °C to 13.4 °C, and drifts outside that.
"70% at seven days" is a British answer. At 20 °C you are there in under five days; through a cold snap at 5 °C it takes nearly ten. The rule is a temperature reading disguised as a calendar.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Concrete Mix Calculator →Concrete gains strength by chemical reaction, and the reaction follows temperature. At 5 degrees C a pour takes roughly twice as long to reach a given strength as it does at 20 degrees C — but it does get there. Maturity rather than elapsed time is what sets strength, which is why the familiar 70 per cent at seven days is a British answer at around 10 degrees C.