Which of the following statement(s) is/are true regarding maximum aggregate size in mix design? (i) Larger aggregate size reduces cement requirement. (ii) Larger aggregates always improve workability.
- AOnly (i)Correct
- BOnly (ii)
- CBoth (i) and (ii)
- DNeither (i) nor (ii)
Solution
Concept
Maximum aggregate size and mix economics
Formula
NONE
Step-by-step solution
1. As maximum aggregate size increases, specific surface area per unit mass of aggregate decreases. 2. Less cement-water paste is required to coat aggregate particles, reducing cement content for a specified target strength; statement (i) is true. 3. Increasing aggregate size beyond an optimum level can lead to harshness, segregation, and internal bleeding, and does not universally improve workability; statement (ii) is false. 4. Therefore, only statement (i) is true. Answer: A
Explanation
Utilizing larger nominal maximum aggregate sizes reduces the total surface area per unit volume of aggregate, which decreases the amount of water and cement paste needed to coat the particles for a given strength. However, larger aggregates do not always improve workability, as they can induce segregation, internal bleeding, and placement difficulty in narrow formwork or congested reinforcement.
Why the other options are wrong
B) Statement (i) is correct regarding cement economy. C) Statement (ii) is false because oversize aggregates in lean mixes cause harshness and segregation. D) Statement (i) is a standard principle of concrete mix proportioning.
Shortcut
Larger aggregate size -> lower surface area -> less cement paste needed; but excessive size risks segregation (not always better workability).
Common mistake
Assuming larger aggregates unconditionally improve all workability aspects regardless of mix proportioning.
Correct answer: A) Only (i)