Strength gain, example set EX-063

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

C35/45 · mix EX-NBC45B · Northbank Concrete Ltd (fictional) · cast 2026-09-21

EX-063A at 7 days: 36.2 N/mm2. 36.2 against about 35.0 expected at 7 days for Class N, normal early strength. Inside the expectation band for a set heading for its specified strength.

Specified, fck45.0 N/mm2

Ideal at 28 days45.0 N/mm2

Likely at 28 days50.4 N/mm2

Likely at 56 days54.3 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	15.4	n/a		not tested
3 days	26.9	30.2	27.1	on or above
7 days	35.0	39.3	36.2	on or above
14 days	40.6	45.5		not tested
28 days	45.0	50.4	54.4	on or above
56 days	48.4	54.3		not tested
90 days	50.3	56.3		not tested

At 7 days a Class N, normal early strength mix would be expected around 35.0 N/mm2, which is 78 per cent of a 28 day target of 45.0. This set is on the expected curve.

Curve basis: Class N, normal early strength, s = 0.25, from BS EN 1992-1-1 expression 3.2. CEM III/A 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.