For M-45 concrete as per IS: 1343-2012, in prestressed concrete structures, which is the correct value for autogenous shrinkage (εca × 10⁻⁶)?
- A45
- B65Correct
- C75
- D95
Solution
Concept
IS 1343:2012 gives autogenous shrinkage strain values that increase with concrete grade; for M45 the value is 65 × 10-6.
Formula
εcs = εcd + εca
Step-by-step solution
1. Total shrinkage = drying shrinkage εcd + autogenous shrinkage εca. 2. IS 1343:2012 tabulates εca by grade: M30 = 40, M35 = 45, M40 = 55, M45 = 65, M50 = 75, M60 = 95 (all × 10-6). 3. For M45, εca = 65 × 10-6. 4. Answer is (B).
Explanation
Autogenous shrinkage happens as cement hydrates and uses internal water, even without drying. It is larger for higher grade concrete because the w/c ratio is lower. For M45, IS 1343:2012 gives 65 × 10-6.
Why the other options are wrong
A: 45 is the value for M35. C: 75 is the value for M50. D: 95 is the value for M60.
Shortcut
Remember the series 40, 45, 55, 65, 75, 95 for M30 to M60.
Common mistake
Mixing up autogenous shrinkage with total or drying shrinkage values.
Correct answer: B) 65