As per IS 456:2000, the strength of concrete achieved in structure is taken as :
- A(2/3)rd times the strength of the cube cast at laboratories.Correct
- B(1/3)rd times the strength of the cube cast at laboratories.
- C(1/6)th times the strength of the cube cast at laboratories.
- D(1/4)th times the strength of the cube cast at laboratories.
Solution
Concept
Strength of concrete in the structure versus the cube
Formula
fstructure = 0.67 fck (design stress block uses 0.45 fck with γm = 1.5)
Step-by-step solution
1. IS 456 (clause 38.1) takes the strength of concrete in the member as 0.67 times the cube strength. 2. 0.67 ≈ 2/3.
Explanation
Concrete in the structure is placed, compacted and cured under conditions inferior to the laboratory. The code therefore reduces the cube strength by 0.67 to get the strength of concrete in the structure; dividing further by the partial safety factor 1.5 gives 0.447 fck.
Why the other options are wrong
1/3, 1/6 and 1/4 are not the factors adopted in the code.
Shortcut
0.67 fck / 1.5 = 0.446 fck.
Common mistake
Choosing 0.45 as the in-structure factor itself.
Correct answer: A) (2/3)rd times the strength of the cube cast at laboratories.