Variation in declared cement content
In many ready-mixed concrete terms and conditions they reserve the right to adjust the cement content by up to 20kg/m3 against the mix design certificate. This quite reasonably allows them to maintain the strength in response to their quality control procedure.
This does not appear to be a clause in the current standards for concrete BS EN 206 or BS 8500 yet is widely accepted, so where does it come from?
BS 5328: 1990 Concrete is the likely last source. Part 3 Specification for the procedures to be used in producing and transporting concrete, has the following clause.
3.2.3 Changes in mix composition
"On request, the producer shall provide the purchaser with details of the following changes made during the period of production:
a) Changes greater than 20kg/m3 in cement content from that last declared"
A similar statement can be found in CP 110: part 1: 1972 The structural use of concrete clause 6.5.2 Evidence of suitability of proposed mix proportions