Concrete Curing Calculator — Strength Gain & Formwork Striking

Strength gain follows the Eurocode 2 curve βcc(t) = exp{s·[1−(28/t)^0.5]} (BS EN 1992-1-1 cl.3.1.2). For standard CEM I (Class N, s=0.25) at 20°C this gives about 34% of 28-day strength at 1 day, 60% at 3 days, 78% at 7 days and — by definition — 100% of the characteristic strength at 28 days. Reaching 70% therefore takes around 5 days at 20°C, not 7. Curing is slower at lower temperatures: this calculator converts real time to an equivalent age at 20°C using the Nurse–Saul maturity method, so at 5°C the same 70% point takes roughly twice as long. High-GGBS cement (CEM III/B, Class S, s=0.38) is slower still in the first weeks but often overtakes CEM I beyond 28 days.

The maturity method estimates concrete strength from its cumulative temperature-time history. The Nurse–Saul function (ASTM C1074 / CIRIA R136) sums temperature above a datum (taken as −10°C) multiplied by time, expressing curing as an equivalent age at 20°C. Higher temperatures accelerate hydration and earn maturity faster; near freezing, very little is earned. This lets early-age strength be estimated without destructive testing and is the recognised UK basis for deciding when to strike formwork or load a slab. This calculator uses Nurse–Saul to convert your ambient temperature into equivalent age, then applies the Eurocode 2 βcc curve.

Formwork can be struck when the concrete has gained sufficient strength to support its own weight plus any imposed loads. For vertical surfaces (walls, columns), this is typically 12–18 hours at 20°C. For soffits (beam/slab undersides), wait until the concrete reaches 70% of its design strength — about 5 days at 20°C with CEM I, and longer in cold weather or with slow cements (CIRIA R136). Always follow the structural engineer’s striking schedule. See our Concrete Mix Calculator for mix design and Concrete Volume Calculator for pour volumes.

Below 5°C, curing slows dramatically. Below 0°C, water in the concrete can freeze, causing permanent damage to the microstructure and significantly reducing final strength. BS EN 206 / BS EN 13670 require fresh concrete to be delivered at no less than 5°C and the concrete temperature not to fall below 5°C at any point until it has reached 5 N/mm² (about 24–48 hours for CEM I, longer with GGBS). The practical trade trigger is "5°C and falling" — protect from that point with insulating blankets, heated enclosures or accelerating admixtures, not the looser 2°C sometimes quoted.

Yes. CEM III (GGBS) gains strength more slowly than CEM I (OPC), especially in the first 7 days. The Eurocode 2 slow Class S coefficient (s=0.38) may be assumed only where GGBS exceeds 65% — that is CEM III/B (66–80% GGBS) or CEM III/C, not CEM III/A (36–65% GGBS), which is Class N like CEM I (BS EN 197-1 + BS EN 1992-1-1 cl.3.1.2). Using the βcc curve at 20°C, a Class S CEM III/B (s=0.38) reaches roughly 68% of its 28-day strength at 7 days versus about 78% for Class N CEM I (s=0.25). However GGBS concrete keeps gaining strength beyond 28 days and often exceeds OPC at 56+ days, and it offers better sulphate resistance and lower embodied carbon.