Concrete @ your Fingertips

Can you provide mix proportions for a concrete?

Due to the variability of aggregates (sands and gravels) throughout the UK and the variability of cements and additions, Further variability arrises with water content and the use of admixtures to achieve the required consistence (workability) i.e. slump.

It is not possible to provide a generic mix design that will give a characteristic strength class concrete (or other strength) to cover all the material variables. For example, 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.

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. 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.

Acknowledgement: Concrete Society