Concrete Strength Classes (EN 206)
Under EN 206, concrete strength is specified by a class such as C25/30 — the class sets a floor on strength, but the exposure class often sets the mix through its own durability limits.
Reading the notation
A strength class is written C(cylinder)/(cube) — for example C25/30 means a characteristic strength of 25 N/mm² measured on a cylinder, and 30 N/mm² measured on a cube, at 28 days. The target mean strength used in mix design, fcm, is set above the characteristic strength fck by a margin of about 8 N/mm² to allow for normal variability, so a design is not made right at the characteristic limit.
Classes at a glance
| Class | Typical uses |
|---|---|
| C20/25 | Mass concrete, blinding, lightly loaded elements |
| C25/30 | General reinforced concrete |
| C30/37 | Common for reinforced concrete in carbonation / moderate exposure |
| C35/45 | More severe exposure — chlorides, freeze-thaw |
| C40/50 | Higher strength and durability requirements |
| C45/55 | Higher strength and durability requirements |
| C50/60 | Higher strength and durability requirements |
The strength class is only part of the picture: the project’s exposure class (carbonation, chlorides, freeze-thaw, chemical attack) sets its own durability limiting values, and those can require a mix stronger or richer than the strength class alone implies. See the method page for how the two are combined.
Frequently asked questions
What does C25/30 mean?
The first number, 25, is the characteristic cylinder strength in N/mm²; the second, 30, is the characteristic cube strength in N/mm². Both describe the same concrete, tested on different specimen shapes.
Does the strength class alone decide the mix?
No. EN 206 also sets durability limiting values by exposure class — maximum water-cement ratio, minimum cement content, minimum strength class and, where relevant, minimum air content. The more demanding of strength or exposure governs.