Question

The characteristic compressive strength of concrete is found to be 0.79 times of its mean compressive strength. If variance is 14.5, find the grade of concrete .

• A. 23.6
• B. 29.9
• C. 36.3
• D. 27.6

Answer 1

The correct option is A 23.6

$f_{ck}=0.79$ $f_m=f_m-1.65\sigma$

${\sigma }^2=Variance=14.5$

$\sigma =3.81$

$1.65\sigma =f_m-0.79f_m=0.21f_m$

$f_m=\frac{1.65\sigma }{0.21}=\frac{1.65\times 3.81}{0.21}=29.9$

$f_{ck}=f_m-1.65\sigma =29.9-1.65\times 3.81$

$f_{ck}$= $23.6MPa$