Count test ages from the day and time the specimens were cast. Cubes cast at 10:00 on Monday 2 November 2026 are tested at 7 days on Monday 9 November (04:00–16:00 under ASTM C39) and at 28 days on Monday 30 November (± 20 hours). For C30 concrete, expect about 30 MPa at 7 days and 38 MPa mean at 28.
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Test ages count from casting, with ASTM C39 tolerances: 3 d ± 2 h, 7 d ± 6 h, 28 d ± 20 h, 56 d ± 2 days
Expected mean strength = (fck + 8 MPa) × exp{s[1 − √(28/t)]}, s = 0.25 (EN 1992-1-1), for specimens cured at the standard 20–23 °C
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Formulas
Test date = casting date + age (days), at the casting timeASTM C39 window = ± 2 h (3 d), ± 6 h (7 d), ± 20 h (28 d), ± 2 days (56/90 d)Expected mean = (fck + 8 MPa) × exp{ s × [1 − √(28 ÷ t)] }
Worked Example
Cast Monday 2 November 2026 at 10:00, fck 30 MPa (fcm 38 MPa), class N cement:
Age
Date
Window
Expected mean
3 days
Thu 5 Nov
08:00–12:00
22.7 MPa
7 days
Mon 9 Nov
04:00–16:00
29.6 MPa
14 days
Mon 16 Nov
± 20 h
34.3 MPa
28 days
Mon 30 Nov
± 20 h
38.0 MPa
56 days
Mon 28 Dec
± 2 days
40.9 MPa
Strength at Each Age as % of 28-day (standard curing)
Cement
3 d
7 d
14 d
28 d
56 d
Class R
66%
82%
92%
100%
106%
Class N
60%
78%
90%
100%
108%
Class S
46%
68%
85%
100%
112%
Frequently Asked Questions
How do you calculate cube test dates?
Add the test age in days to the casting date and test at about the same time of day. A cube cast on 2 November is tested at 7 days on 9 November and at 28 days on 30 November.
What should I expect from the 7-day cube test?
About 78% of the 28-day mean strength for class N cement under standard curing, and less for slow-gaining blends. For C30 concrete that is about 30 MPa at 7 days, against a 38 MPa mean at 28 days.
What if a test date falls on a weekend?
Test within the permitted tolerance window if possible, or arrange weekend lab cover. Record the actual test age on the report.
References
ASTM C39: permissible tolerances on test age (24 h ± 0.5 h, 3 d ± 2 h, 7 d ± 6 h, 28 d ± 20 h, 90 d ± 2 d)
EN 1992-1-1 Table 3.1 (fcm = fck + 8 MPa) and Eq. 3.2
