The compressive strength of concrete delivered by a supplier can be modeled by a

Question

The compressive strength of concrete delivered by a supplier can be modeled by a lognormal random variable. Its mean and the coefficient of variation are estimated to be 4.7 ksi and 0.21. respectively. If the 10th percentile value is the design value, calculate the value of the compressive strength to be used in a design. Suppose the COV of the compressive strength is reduced to 0.10 without affecting its mean value by introducing quality control procedures. Calculate the design value of the compressive strength if it is assumed to be the 10^th percentile value. By comparing the results obtained in Parts (a) and (b). discuss whether quality control measures are preferable.

Explanation / Answer

Mean = 4.7

Coeff. of variation = SD / mean = 0.21

SD = 0.21 * 4.7 = 0.987

The 10 percentile value = 31.0348 ksi

now, if the value of COV changes to 0.10, then the SD = 0.47

Thus, 10% value will be = 60.2 ksi

Thus, the quality control procedures almost double the design value.

Hope this helps.

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