Aggregate shape and packing: Among aggregates of the same nominal size, which particle shape generally yields the least void content when compacted, thus improving workability and reducing paste demand?

Introduction / Context:
Aggregate shape controls packing density, workability, and required paste content. For a given gradation and nominal size, shapes that pack tightly minimize voids, cutting down on cement paste demand and shrinkage.

Given Data / Assumptions:

• Aggregates have the same nominal size and similar gradation.
• Compaction is performed to typical site standards.
• We compare rounded, irregular (angular), flaky, and elongated shapes.

Concept / Approach:
Rounded particles roll and rearrange under compaction to occupy less void space. Angular irregular particles interlock, increasing voids and requiring more paste for the same workability. Flaky and elongated particles stack poorly, creating high void ratios and poor flow. Therefore, rounded shapes achieve the lowest void content.

Step-by-Step Solution:
Target: minimize voids under compaction.Rounded particles → best packing → least paste demand.Irregular/Flaky/Elongated → higher voids and harsher mixes.Choose the rounded option.

Verification / Alternative check:
Workability tests and mix design charts reflect lower sand/ paste needs when rounded sands and gravels are used.

Why Other Options Are Wrong:
Irregular, flaky, and elongated particles typically increase voids and reduce workability.

Common Pitfalls:
Ignoring the trade-off: angular particles may improve bond and strength but at the cost of higher paste demand.

Final Answer:
Rounded (spherical/ellipsoidal) particles.