A concrete grade of M25 was used for the construction of a highway bridge for which the standard deviation of the test strength samples obtained was 4.0 N/mm2. Using the Indian standard method of concrete mix design, what is the target mean compressive strength at 28 days?
(Take the risk factor as 1.65 )
1
30 N/mm2
2
28.5 N/mm2
3
25 N/mm2
4
31.6 N/mm2