Normal (class N) cement at 20 °C reaches 50% of its 28-day strength in about 2 days, 70% in about 4.8 days and 78% at 7 days. In colder weather it slows: at 10 °C, 70% takes about 7.7 days. ACI 308.1 asks for at least 7 days of curing (3 days for high-early cement).
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Minimum moist curing (ACI 308.1)
7 days→ Mon, 9 Nov 2026
50% of 28-day strength
47 hours → Wed, 4 Nov 2026
70% (soffit striking)
4.8 days → Sat, 7 Nov 2026
100% (28-day strength)
28.1 days → Mon, 30 Nov 2026
Strength at 7 days
78 % of 28-day
Estimates only. Confirm striking times with field-cured cubes or maturity sensors and the engineer’s approval.
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Strength gain βcc(t) = exp{s[1 − √(28/t)]}, s = 0.25 for class N (EN 1992-1-1 Eq. 3.2)
Temperature-adjusted age: 0.998 equivalent days per day at 20 °C (Annex B)
Days to ratio r = 28 ÷ (1 − ln r ÷ s)² ÷ temperature factor → 50%: 2.0, 70%: 4.8, 100%: 28.1 days
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Formulas
Strength ratio βcc(t) = exp{ s × [1 − √(28 ÷ t)] } s = 0.20 (R), 0.25 (N), 0.38 (S)Temperature factor = exp{ −(4000 ÷ (273 + T) − 13.65) } equivalent days per real dayDays to ratio r = 28 ÷ (1 − ln r ÷ s)² ÷ temperature factor
Worked Example
Class N cement, average 20 °C, poured Monday 2 November 2026:
50% strength: 28 ÷ (1 + 0.693/0.25)² ≈ 2.0 days → Wednesday 4 November
70% (soffit striking): ≈ 4.8 days → Saturday 7 November
ACI 308.1 minimum curing: 7 days → Monday 9 November
At 7 days the concrete is at about 78% of its 28-day strength
Days to Reach 70% of 28-day Strength
Average temperature
Class R
Class N
Class S
5 °C (41 °F)
7.6
10.0
15.6
10 °C (50 °F)
5.9
7.7
12.1
20 °C (68 °F)
3.6
4.8
7.5
30 °C (86 °F)
2.3
3.0
4.8
Frequently Asked Questions
How long does concrete take to cure?
Concrete keeps gaining strength for months, but most specifications require at least 7 days of moist curing for normal cement (3 days for high-early cement, longer for slag or fly ash blends). It reaches about 70% of its 28-day strength in roughly 5 days at 20 °C.
When can formwork be removed?
Side forms are often struck after about 24 hours. Soffits and props need the concrete to reach a strength set by the engineer (often 60–75% of the 28-day strength) which depends heavily on temperature and cement type.
How does temperature affect curing time?
Strength develops more slowly in the cold. Using the EN 1992-1-1 temperature-adjusted age, a day at 10 °C counts as about 0.62 days at 20 °C, and a day at 30 °C as about 1.57 days.
References
EN 1992-1-1 Eq. 3.2: βcc(t) = exp{s[1 − (28/t)^½]}; Annex B Eq. B.10 (temperature-adjusted age)
ACI 308.1 (Specification for Curing Concrete)
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