Strength gain, example set EX-072

Example data. This is the worked example every new register can load, not anybody's real project.

C30/37 · mix EX-NBC37A · Northbank Concrete Ltd (fictional) · cast 2026-10-11

EX-072A at 7 days: 29.3 N/mm2. 29.3 against about 25.3 expected at 7 days for Class S, slow early strength. Inside the expectation band for a set heading for its specified strength.

Specified, fck37.0 N/mm2

Ideal at 28 days37.0 N/mm2

Likely at 28 days42.8 N/mm2

Likely at 56 days47.9 N/mm2

Ideal gain for this cement class, with a plus or minus 12 per cent band Likely gain for this set, projected from its own results Specified strength, fck Individual identity limit, fck minus 4 Results, coloured red, amber or green

Move along the chart to read the likely strength on any day. The age axis is logarithmic, which is what gives strength gain its S shape. The likely line starts at three days because BS EN 1992-1-1 does not stand behind the expression below that.

Age	Ideal	Likely	Measured	Position
1 day	7.2	n/a		not tested
3 days	16.9	19.6		not tested
7 days	25.3	29.3	29.3	on or above
14 days	31.6	36.6		not tested
28 days	37.0	42.8		not tested
56 days	41.4	47.9		not tested
90 days	43.8	50.7		not tested

At 7 days a Class S, slow early strength mix would be expected around 25.3 N/mm2, which is 68 per cent of a 28 day target of 37.0. This set is ahead of the expected curve.

Curve basis: Class S, slow early strength, s = 0.38, from BS EN 1992-1-1 expression 3.2. CEM III/B recorded against this set.
Likely line basis: the BS EN 1992-1-1 curve scaled through this set's own results.

Ideal and likely are expectations, not results. They come from a standard strength gain curve at 20 C for specimens cured to BS EN 12390-2, and real curing, temperature and mix variation move them. Conformity is decided by the 28 day test and the laboratory certificate is the record. Nothing here is authority to strike formwork: that is the engineer's decision against a striking strength they set.