Getting the right type of concrete for your job is important. We have tried to explain the various grades of concrete and when to use them.
What the C Number Means
All grades are defined with a ‘C’ number (e.g. C25, C30). The number following the ‘C’ defines the average compressive strength of the fresh mix when tested on a standard cylinder in N/mm2. The higher the ‘C’ number, the higher the average strength of the hardened concrete and the denser it will be. These designations are UK wide and conform to university Certificate in Concrete Technology– defined classifications.
Common Grades and Their Uses
For non-structural uses such as blinding a foundation, a lean mix C10 would be adequate. For a domestic floor, kerbs and boundary walls a C20 Concrete would be sufficient. A general-purpose grade for drives, paths and lightly loaded slabs is C25 Concrete. Heavier loaded slabs, such as podium decks are best made with C30 Concrete. For structural use, groundbeams and foundations etc. a higher strength such as C35 and C40 Concrete would be used. See also Concrete Tewkesbury.
The Risk of Specifying Too Low
Specifying a grade of concrete that is too low can lead to concrete cracking and/or premature wear. For a driveway, for example, specifying C20 instead of C25 could result in surface breakdown within a few years. For foundations, under-strength concrete may not satisfy the requirements of building regulations.
Standard Mixes Versus Designated Mixes
A Standard Mix is a supply of concrete made up to a fixed recipe (i.e. a number of agreed ingredients), whereas a Designated Mix is a supply of concrete with agreed performance characteristics (i.e. strength) which will allow a simpler ordering process for most projects.
Confirming the Grade Before You Order
It’s also important to ensure the correct grade is specified by the engineer/ architects/building control officer for the pour and this is confirmed with the supplier prior to placing the order as changes to grade after pour cannot be made.
