Which of the following statements is INCORRECT?
- ALower water-cement ratio leads to lower workability.
- BGraphically, the relation between the strength of concrete and the water-cement ratio is approximately cubic.Correct
- CLower water-cement ratio is required when the concrete mix is mechanically compacted to achieve higher strength as compared to the hand-compacted concrete mix with higher water-cement ratio.
- DThe lower the water-cement ratio is, the higher will be the strength of concrete.
Solution
Concept
Abrams' law: concrete strength goes down as w/c ratio goes up. The curve is roughly hyperbolic (exponential), not cubic.
Formula
fc = A / Bw/c (Abrams' law, A and B are constants)
Step-by-step solution
1. A is correct: less water gives a stiffer mix, so workability is lower. 2. B is wrong: the strength vs w/c ratio graph is approximately hyperbolic (Abrams' law), not cubic. 3. C is correct: mechanical (vibration) compaction can fully compact a low w/c mix and give higher strength than a hand-compacted mix. 4. D is correct: lower w/c ratio gives higher strength, if full compaction is achieved. 5. So the INCORRECT statement is B.
Explanation
As per Abrams' law, the strength of fully compacted concrete depends inversely on the w/c ratio. The shape of this curve is like a hyperbola. A cubic relation is used for gel/space ratio (Powers' law), not for w/c ratio.
Why the other options are wrong
A, C and D are all true facts about the w/c ratio, so they are not the answer to an 'INCORRECT' question.
Shortcut
Strength vs w/c ratio = hyperbolic (Abrams). Strength vs gel/space ratio = cubic (Powers).
Common mistake
Mixing up Powers' gel/space ratio law (cubic) with Abrams' w/c ratio law (hyperbolic).
Correct answer: B) Graphically, the relation between the strength of concrete and the water-cement ratio is approximately cubic.