Question

The characteristic compressive strength of concrete required in a project is 25 MPa and standard deviation in the observed compressive strength expected at site is 4 MPa. The average compressive strength of cubes tested at different water-cement(w/c) ratios using the same material as is used for the project is given in the table.

w/c(%)	45	50	55	60
Average compressive strength of cubes (MPa)	35	25	20	15

The water-cement ratio(in percent, round off to the lower integer) to be used in the mix is________.

• A. 46.7

Answer 1

The correct option is A 46.7
$f_{ck}=25;\sigma =4$MPa
$f_m=f_{ck}+1.65\sigma =31.6$ MPa
Using interpolation
$w/creq=50-\frac{(50-45)}{(35-25)}\times (31.6-25)$
= 46.7 (Rounded off to the lowest integer).