To maintain quality control of concrete, representative test samples should be taken, cured under controlled conditions and tested for compressive strength after the appropriate 3- , 7- or 28-day period. Steel cylinder and cube moulds are filled in layers with either hand or mechanical vibration. For hand tamping, a 100 mm cube would be filled in two equal layers, each tamped 25 times with a 25 mm square-end standard compacting bar; mechanical vibration would normally be with a vibrating table or pneumatic vibrator. The mix is then trowelled off level with the mould. Test samples are cured under controlled moisture and temperature conditions for 24 hours, then stripped and cured under water at 18–20°C until required for testing.
The European Standard BS EN 12390–1: 2012 lists dimensions of cubes, cylinders and prisms ranging in critical dimension from 100 to 300 mm. The critical mould dimension must be at least 3.5 times the size of the largest aggregate. The appropriate method of filling test samples is described in BS EN 12390–2: 2009. Generally, in Europe, cylinders 150 mm in diameter and 300 mm high rather than cubes are used, as they tend to give more uniform results for nominally similar concrete specimens. For a particular concrete, the characteristic compressive strength as determined by the cylinder test is lower than that obtained from the equivalent cube test. The compressive strength classes , therefore, have a two-number notation (e.g. C 20/25). The first number, which is used in the European structural design codes, refers to the characteristic cylinder compressive strength, and the second number is the characteristic 150 mm cube compressive strength.