Question

The approximate ratio between the strengths of cement concrete at 7 days and at 28 days is

• A. 3/4
• B. 2/3
• C. 1/2
• D. 1/3

Answer 1

The correct option is B 2/3

The strength of concrete at any age, can be expressed by the relation

$f_T=\frac{T}{a+bT}{f}_{28}$
The 7-day and 28 day strength are related as
$f_{28}=k_2(f_7{)}^{k_1}$
According to IS code, the ratio $f_7/f_{28}$ is approximately 2/3.

Grade of concrete	7 day strength $\left(N/m{m}^2\right)$
M10 M15 M20 M25 M30 M35 M40	7 10 13.5 17.0 20.0 23.5 27.0