Firstly, the reference C32/40 relates to the characteristic strength of the concrete. In this case to confidently achieve 40N/mm2 from standard cubes tested at 28days, (32N/mm2 for standard cylinders) with a statistical 95% confidence limit, the mean strength of the population will be approximately 7N/mm2 above the specified characteristic i.e. 47N/mm2, depending on the standard deviation of the population.
Secondly, due to the variability of aggregates (sands and gravels) throughout the UK and the variability of cements and additions, it is not possible to provide a generic mix design that will give a characteristic C32/40 strength class concrete (or other strength) to cover all the material variables. Further variability arrises with water content and the use of admixtures to achieve the required consistence (workability) i.e. slump.
To achieve a true characteristic strength, assessment of the mix design must be carried out pre-contract.
If a large quantity (several cubic metres or more) of concrete of specified characteristic strength is required, then it is suggested that ready-mixed concrete is used. For a smaller quantity, on-site batching (volumetric) suppliers can supply a concrete to a specified strength although this is usually only accredited to lower strengths i.e. C20/25.
Note it is recommended that a BSi or QSRMC accredited supplier is used.
Another option for very small quantities is to use the pre-batched bags of materials (cement, aggregates and admixture) that are available from builder’s merchants.