Concrete Mix (Current Margin)

Current Margin

The current margin of the proposed designed-mix must be supported by previous results of similar mix proportions and analysed for compliance with Cl. 3.2.1- Section D

1. Margin = 1.64 x s or not less than 3.75 N/mm2
   
   From the results of 100 separate batches produced within 12 months from the same plant for similar proportion of similar materials

2. Margin = 1.64 x s or not less than 7.5N/mm2
   
   From the results of 40 separate batches produced within 12 months from the same plant for similar proportion of similar materials.

Otherwise, the S.O shall decide that the current margin adopted is 12 N/mm2 and the adopted target mean strength shall be derived from this value until sufficient data are available to justify for lower current margin, but not less than 7.5 N/mm2.

Characteristic Strength of Concrete

It is the strength of material below which not more than 5% of the results are expected to fall. That is, at least 95% of the samples will have a strength more than the characteristic value.

Target Strength of Concrete

In order to ensure that the specified minimum strength at 28 days is attained in 95% of all test samples, the concrete mix must be proportioned to give a mean strength at 28 days exceeding the specified strength by 1.64 times the standard deviation.

Standard Deviation (SD)

Initially, the value of the standard deviation may be taken based on past data or experience or judgement and subsequently from the accumulated results of the test samples. The standard deviation must be recalculated from the test results obtained after any change in the source or quality of materials and the mix shall be adjusted as necessary to comply with the requirements.