Which one of the following statements is NOT correct with respect to mix design?
- AThe mix must be workable so that it can be placed and finished easily.
- BThe cement is the most costly ingredient in the concrete mix.
- CThe strength and water tightness increase with the density of the mix.
- DIncreasing the water content beyond that required for plasticity causes the strength to increase constantly.Correct
Solution
Concept
Extra water beyond what is needed for workability reduces strength (Abrams' w/c law).
Formula
fc = A / Bw/c
Step-by-step solution
1. A: The mix must be workable for easy placing and finishing. This is correct. 2. B: Cement is the costliest ingredient, so mix design tries to use the minimum needed. This is correct. 3. C: A denser mix has fewer voids, so strength and water tightness increase. This is correct. 4. D: Extra water raises the w/c ratio and leaves voids after it evaporates. So strength decreases, not increases. This is NOT correct. 5. Answer is (D).
Explanation
By Abrams' law, strength goes down as the water-cement ratio goes up. Water above the amount needed for plasticity leaves pores in the concrete and lowers strength. So statement D is wrong.
Why the other options are wrong
A: True, workability is a basic need of a mix. B: True, cement is the most costly material. C: True, a dense mix gives higher strength and less permeability.
Shortcut
More water means a higher w/c ratio, which means lower strength.
Common mistake
Thinking more water helps hydration and so gives more strength.
Correct answer: D) Increasing the water content beyond that required for plasticity causes the strength to increase constantly.