The standard strength classes of cement are based on the 28-day compressive strength of mortar prisms, made and tested to the requirements of BS EN 196–1: The test uses specimens which are 40 X 40 X160 mm, cast from a mix of 3 parts of CEN (European Committee for Standardisation) standard sand, 1 part of cement and 0.5 part of water. The sample is vibrated and cured for the appropriate time, then broken into halves and compression tested across the 40 mm face. Three specimens are used to determine a mean value from the six pieces.
Each standard strength class (32.5, 42.5 and 52.5) has sub-classes associated with the high early (R) and the ordinary (N) development of early strength (Table 3.6). The strength classes and sub-classes give production standards for cements, but do not specify how a particular mix of cement, aggregate and admixtures will perform as a concrete; this needs to be determined by