Cubes - general
It is a standard quality control procedure to make test cubes from the fresh concrete, which are then stored under controlled conditions and tested (generally at an age of 28 days). Procedures are given in relevant parts of BS EN 12390, Testing hardened concrete, which replaces corresponding parts of BS 1881. Cubes are intended to be representative of the concrete quality in the structure. They do not necessarily reflect the in-situ strength where the concrete is cured under different conditions.
Low cube strength test results may be due to the way in which the concrete was sampled or the cube made and not representative of the concrete supplied. It is important to follow the requirements of the relevant British Standards but this can be difficult when the cubes are made on site. Clearly inadequacies in cube production will undermine confidence in what has actually been built.