The characteristic compressive strength of concrete required in a project is 25 MPa and the standard deviation in the observed compressive strength expected at site is 4 MPa

Q. The characteristic compressive strength of concrete required in a project is 25 MPa and the 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

Solution:

fck=25;σ=4MPa
fm=fck+1.65σ=31.6 MPa
Using interpolation
w/c req=50−(50−45)/(35−25)×(31.6−25)
= 46.7 (Rounded off to the lowest integer).