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30 40 so

Crushing strength, f, (MPa)

Figure 2.25. Results of compressive strength tests on concrete cylinders

© The denominator in this equation is (n - 1) because we are calculating the standard deviation of a sample of the population, rather than of the population itself (in which case we would divide by n).

© This equation is easier to use in hand calculations than the previous one (and gives identical answers).

© Physical properties are inherently variable. By applying a minimum value here, we guard against the possibility of using an unrealistically small value of VX in the subsequent calculations. If the coefficient of variance had been less than 0.1, then we would have needed to assume a higher standard deviation, given by: sx = 0.1 x mX

Example 2.3 Concrete cylinder tests Determination of characteristic cylinder strength

Results

Consider the results of a series of concrete cylinder tests. The measured crushing strengths of the concrete were: 38.6, 36.5, 27.6, 30.3, 37.9, 39.3, 41.4, 38.6, 49.0, 32.4, 37.9, 40.7, 44.1, 40.0, 46.2, 37.2, 34.5, 40.0, 42.7, 38.6,

Statistical analysis of data

The number of test results is n = 25

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