C30/37 Concrete Mix Design
C30/37 is a common EN 206 strength class for reinforced concrete facing more demanding carbonation exposure than C25/30.
Typical exposure classes
Often paired with XC4 (cyclic wet and dry carbonation exposure) and sometimes a moderate freeze-thaw class such as XF1. As with all exposure-class pairings on this page, the specific durability limits shown or implied are EN 206 recommended values, subject to the national annex that applies to your project.
A note on national annexes
The exposure classes above are generic EN 206 associations; the exact maximum water-cement ratio, minimum cement content and minimum strength class figures for each are EN 206 recommended default values, and can be set stricter, or structured differently, by the national annex that actually applies to your project — for example BS 8500 in the UK or DIN 1045-2 in Germany. See why national annexes matter for why this matters, and use this site’s UK or Germany calculator directly if your project sits in one of those countries.
Typical uses
Exposed reinforced concrete elements, some external structural work, and general reinforced concrete in more demanding but not extreme exposure.
Frequently asked questions
Is C30/37 always needed for external concrete?
Not always — it depends on the actual exposure class assessment for the project and the governing national annex; milder, sheltered external elements can sometimes remain at C25/30, while more aggressive exposure needs a higher class again.
Where do I find the exact w/c and cement content limits for C30/37 XC4?
Check the national annex that applies to your project — see why national annexes matter for why the EN 206 default figure is not automatically the governing one.