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.
Choose a class
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.
Strength class, designated concrete and ST mixes: which one are you ordering?
UK concrete can be ordered in three ways, and they are often confused:
- Designed concrete — you state a strength class such as C32/40 together with the exposure classes, cover, working life and other requirements. The producer designs the mix to meet them.
- Designated concrete — BS 8500 packages a set of decisions under names such as GEN1, FND2, RC32/40 or PAV1. These can only be supplied by a producer holding third-party product conformity certification, and they are intended for specific applications.
- Standardized prescribed concrete (ST1–ST5) — fixed proportions intended for small-scale or site-batched work where a certified producer is not available. They are simpler but less flexible, and are not intended for demanding exposure conditions.
Cube testing in the UK
UK compliance testing normally uses cubes rather than cylinders. The strength class records both values — in C32/40 the 40 N/mm² is the characteristic cube strength and 32 N/mm² the characteristic cylinder strength used in structural design to Eurocode 2. Cubes are made and cured to BS EN 12390-2 and crushed to BS EN 12390-3, commonly at 28 days, with early-age cubes used for striking and stressing decisions. Poor site curing of specimens is one of the most common causes of apparently low results, so cube tanks or temperature-controlled curing should be part of the site set-up.
Typical UK strength classes by element
As a starting point only: C20/25–C25/30 for mass fill, blinding and unreinforced foundations in non-aggressive ground; C28/35–C32/40 for general reinforced slabs, beams and walls in XC exposures; C32/40–C40/50 where de-icing salts, chlorides or freeze–thaw apply; and higher classes for precast, post-tensioned or early-stripping work. The exposure class and cover, not the element name, decide the minimum class — check the BS 8500-1 tables for the actual combination.
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.