As per IS 8112: 1989, the minimum required compressive strength of 43 grade ordinary portland cement for 72 hours is
- A23 MPaCorrect
- B33 MPa
- C43 MPa
- D53 MPa
Solution
Concept
IS 8112 gives minimum compressive strength of 43 grade OPC at 3, 7 and 28 days.
Step-by-step solution
1. 72 hours = 3 days. 2. IS 8112 minimum strength for 43 grade OPC: 3 days = 23 MPa, 7 days = 33 MPa, 28 days = 43 MPa. 3. So the 72-hour (3-day) strength = 23 MPa.
Explanation
The grade number (43) is the 28-day strength. At 3 days (72 hours), 43 grade OPC must give at least 23 MPa. This is tested on 1:3 cement-sand mortar cubes.
Why the other options are wrong
B: 33 MPa is the 7-day strength. C: 43 MPa is the 28-day strength. D: 53 MPa is the 28-day strength of 53 grade cement.
Shortcut
43 grade: 23-33-43 MPa at 3-7-28 days.
Common mistake
Taking 72 hours as 7 days, or picking the grade value directly.
Correct answer: A) 23 MPa