Concrete strength class is a classification value describing the characteristic compressive strength of hardened concrete at 28 days. It is denoted by two numbers, e.g. C20/25 or C50/60, derived from standardized tests on cylinder and cube specimens.
Definition and Notation
The designation Cx/y follows EN 206 and the related national application standards (e.g. DIN 1045-2 in Germany). The first value (x) is the characteristic compressive strength fck,cyl in N/mm² measured on cylinders with 150 mm diameter and 300 mm height. The second value (y) is the characteristic compressive strength fck,cube in N/mm² measured on cubes with 150 mm edge length, tested according to EN 12390-3. Both values refer to the same concrete but differ due to specimen geometry and the resulting stress distribution during testing.
Characteristic Value
The characteristic strength fck is defined as the value below which no more than 5 % of all test results are expected to fall, assuming a normal distribution of individual test results from a defined concrete population.
Standardized Classes
EN 206 (in its current edition, implemented in Germany via DIN 1045-2) defines normal and heavy-weight concrete strength classes from C8/10 up to C100/115. For lightweight aggregate concrete, separate classes LC8/9 up to LC80/88 apply, using the same notation principle with cylinder and cube values. The selection of a strength class depends on exposure class, structural design requirements, and durability provisions set out in the respective design standard, typically EN 1992-1-1 (Eurocode 2).
Ultra-High Performance Concrete
Compressive strengths exceeding C100/115 are achievable with ultra-high performance concrete (UHPC). These mixes are not yet covered by the harmonized strength class system of EN 206. In Germany, UHPC is regulated separately through the DAfStb guideline for ultra-high performance concrete, which defines additional strength classes and testing provisions outside the EN 206 framework. Specifiers must reference the applicable national technical approval or guideline rather than EN 206 when working with UHPC.
Determination and Testing
Compressive strength is determined on specimens cured and tested according to EN 12390-2 (specimen preparation) and EN 12390-3 (compressive strength testing) at an age of 28 days, unless an alternative age is agreed for early- or late-strength-developing mixes (e.g. with fly ash or slag cement). Conformity assessment of concrete production follows EN 206 in conjunction with EN 13791 for strength assessment in structures.
Relevance for Specification
Strength class is one of several concrete performance parameters that must be specified together with exposure class, maximum aggregate size, chloride content class, and consistency class. It does not by itself ensure durability; exposure class requirements (minimum cement content, maximum water/cement ratio) frequently govern the mix design in addition to the strength requirement. Strength class alone is therefore not sufficient as a complete concrete specification.