APSC AE WRD 2019 PYQ: Cement Paste Requirement for Aggregates

Cement Paste Requirement for Aggregates Multiple Choice Question

Question: Which type of coarse aggregate requires the minimum amount of cement paste in concrete?
(A) Rounded
(B) Angular
(C) Irregular
(D) Flaky

Answer: (A) Rounded

Explanation

The amount of cement paste required in concrete depends on the surface area and inter-particle friction of the coarse aggregates. Rounded aggregates have smooth surfaces and a spherical shape, resulting in the least surface area and lowest friction, thus requiring the minimum amount of cement paste to coat particles and fill voids.

Characteristics of Aggregate Types

Aggregate Type	Surface Area	Cement Paste Requirement	Workability	Additional Notes
Rounded	Low	Minimum	High	Smooth surfaces, spherical shape, best for workability.
Angular	High	More	Medium	Rough, jagged edges, better interlocking for stronger concrete.
Irregular	Medium	Moderate	Medium	Shape between rounded and angular, moderate properties.
Flaky	Very High	Maximum	Low	Thin, flat shape, poor packing, high paste demand.

Key Notes

• Rounded aggregates are preferred for high-workability concrete due to their low cement paste requirement and ease of mixing.
• Angular aggregates provide better strength due to improved interlocking but require more cement paste, reducing workability.
• Flaky aggregates are least desirable for concrete due to their high surface area and poor packing characteristics.
• Selecting the appropriate aggregate type is critical in concrete mix design to balance strength, workability, and cost.

Note: The choice of rounded aggregates optimizes cement paste usage and enhances workability, making them ideal for many concrete applications in civil engineering.

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