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Question

Compression test on 4 samples of 150 mm cubes of concrete gave following results:
25 MPa, 28 MPa, 32 MPa, 31 MPa
The characteristic strength of concrete (in MPa, rounded off to two decimal places) is _____.
  1. 23.6

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Solution

The correct option is A 23.6
Number of cubes (n)=4
Mean strength, fm=25+28+32+314=29 MPa
Here, individual variation is with in ±15 % of average.
Hence, okay
Standard deviation, σ=Σ(fmfi)2n1
σ=(2925)2+(2928)2+(2932)2+(2931)23=3.16MPa
Characteristic strength of concrete=fm1.65σ
=29(1.65×3.16)
=23.78 MPa

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