Definition of characteristic mean strength and target mean strength of concrete

Characteristic Compressive strength

The compressive strength of the concrete is illustrated with regard to characteristic compressive strength of 150mm sized cubes that is checked on 28 days ( fck).

The definition of characteristic compressive strength is given as the strength of the concrete under which not in excess of 5% of the test results are accepted to fall. Even through, the compressive strength of the cube samples demonstrate variations, when projected on a histogram are detected to abide by the shape of a bell shape curve, as provided below.

The curve given above is described as the normal or the Gaussian Distribution. The curve stands for an idealized distribution of the values of compressive strength for a sizeable number of test cubes.

The horizontal axis demonstrates the values of the compressive strength. The vertical axis shows the number of the test samples for a specific compressive strength. This is also defined as the frequency. The average of the values of the compressive strength i.e. the mean strength is demonstrated with fcm.

The characteristic compressive strength fck refers to the value in the X-axis beneath 5% of the total area under the curve falls.

As illusted in the figure-1, the value of fck remains underneath fcm by an amount identical to 1.65𝛔. Where 𝛔 denotes the standard deviation of the normal distribution. So,

fcm = fck + 1.65𝛔

Where, fcm denotes mean strength, fck denotes compressive strength and 𝛔 denotes standard deviation

To design the concrete mix concerning a specified characteristic compressive strength, the design strength or the target strength is measured by the above formula.

As for instance, in order to create the design of M20 concrete mix, the design strength is fcm = 20 +1.65𝛔

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